JP4219519B2

JP4219519B2 - Frequency modulator

Info

Publication number: JP4219519B2
Application number: JP2000010929A
Authority: JP
Inventors: 優布施
Original assignee: Panasonic Corp; Matsushita Electric Industrial Co Ltd
Current assignee: Panasonic Corp; Panasonic Holdings Corp
Priority date: 1999-01-19
Filing date: 2000-01-19
Publication date: 2009-02-04
Anticipated expiration: 2020-01-19
Also published as: JP2000278218A

Description

【０００１】
【発明の属する技術分野】
本発明は、半導体レーザによる光周波数変調と光ヘテロダイン検波とを利用して、広帯域な周波数変調信号（以下「ＦＭ変調信号」という）を発生させる変調装置に関する。
【０００２】
【従来の技術】
図１４は、従来の周波数変調装置の構成を示すブロック図である。この周波数変調装置は、例えば文献（K.Kikushima, et al, "Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems", IOOC'95 Technical Digest, Vol.5 PD2-7, pp.33-34）に、その動作などが詳しく説明されている。図１４に示したこの周波数変調装置は、信号源７００と、光変調器７０２と、局発光源７０３と、第１の光導波路７０６と、第２の光導波路７０８と、光受信器７１４とを備えている。
【０００３】
上記の周波数変調装置において、信号源７００は、ＦＭ変調すべき元信号となる電気信号Ｓｉを出力する。光変調器７０２は、例えば半導体レーザで構成される。一般に、半導体レーザは、注入電流一定の条件では一定の光周波数ｆ１の光を出力し、その注入電流を振幅変調すると、光周波数も変調を受けて、光周波数ｆ１を中心とした光周波数変調信号を出力する。光変調器７０２は、この性質により、信号源７００から入力された電気信号Ｓｉを光周波数変調信号Ｌ１に変換し、出力する。局発光源７０３は、一定の光周波数ｆ２の無変調光Ｌ２を出力する。光変調器７０２および局発光源７０３から出力される各光Ｌ１，Ｌ２は、それぞれ第１の光導波路７０６および第２の光導波路７０８を介して、光受信器７１４に入力される。光受信器７１４は、自乗検波特性を有するフォトダイオードなどで構成され、入力される２つの光Ｌ１，Ｌ２の光周波数差に相当する周波数ｆｓ＝｜ｆ１−ｆ２｜において当該２つの光のビート信号を出力する（この動作は「光ヘテロダイン検波」と呼ばれる）。このようにして得られたビート信号は、信号源７００からの電気信号Ｓｉを元信号とした周波数変調信号Ｓfmとなる。
【０００４】
以上のようにして、図１４に示した従来の周波数変調装置では、半導体レーザの高い周波数変調効率（一般の電気回路方式の場合における周波数変調効率の１０倍以上）を利用することによって、一般の電気回路では作成困難な非常に高周波かつ広帯域な（周波数偏移量の大きい）ＦＭ変調信号を容易に生成することができる。
【０００５】
【発明が解決しようとする課題】
しかしながら、半導体レーザ等の光源は一般に電気発振器と比較して位相雑音が大きいため、上記従来の周波数変調装置は、それによって生成されたＦＭ変調信号の復調の際に白色雑音成分が増大するという特有の問題を有している。すなわち、光変調器７０２からの光周波数変調信号Ｌ１および局発光源７０３からの無変調光Ｌ２が図１５（Ａ）に示すような周波数スペクトルを有する場合には、光受信器７１４から出力されるＦＭ変調信号Ｓfmの周波数スペクトルは図１５（Ｂ）に示すようなものとなる。図１５（Ａ）および（Ｂ）に示されるように、ＦＭ変調信号Ｓfmに含まれる位相雑音は、光周波数変調信号Ｌ１および無変調光Ｌ２のそれぞれに含まれる位相雑音の総和となるため、このＦＭ変調信号Ｓfmを復調する際に白色雑音成分が増大する。
【０００６】
それ故に、本発明の目的は、半導体レーザによる光周波数変調と光ヘテロダイン検波とを組み合わせた構成により、高周波かつ広帯域な周波数変調を実現しながら、生成される周波数変調信号に含まれる位相雑音を抑圧し、雑音特性に優れた周波数変調装置を提供することである。
【０００７】
【課題を解決するための手段および発明の効果】
第１の発明は、光周波数の互いに異なる第１および第２の光をそれぞれ出力する第１および第２の光源を用いた光周波数変調および光ヘテロダイン検波により、入力電気信号を周波数変調信号に変換し、ＦＭ変調信号として出力する周波数変調装置であって、
前記入力電気信号で周波数変調された前記第１の光を第１の光信号として出力する第１の光変調器と、
前記第１および第２の光から自乗検波特性に基づく光検波によって得られるビート信号の搬送波成分に相当する無変調ビート信号を生成するビート信号生成部と、
前記無変調ビート信号の周波数を変換することにより周波数変換信号を生成する周波数変換器と、
前記第１の光信号と前記第２の光とのうち一方を前記周波数変換信号で光振幅変調または光強度変調することにより第２の光信号を生成する第２の光変調器と、
前記第１の光信号と前記第２の光とのうちの他方および前記第２の光信号を受け取って、自乗検波特性に基づく光検波により前記ＦＭ変調信号を生成する光受信器と、
を備える。
【０００８】
上記第１の発明によれば、２つの光源（第１および第２の光源）と２つの自乗検波器（ビート信号生成部および光受信器）とにより２つの光ヘテロダイン系が実現され、第１の光源に対応する第１の光変調器と局発光源に相当する第２の光源とビート信号生成部とで構成される第１の光ヘテロダイン系において生成されるビート信号の搬送波成分（中心周波数成分）を周波数変換することにより得られる周波数変換信号で、第１の光変調器と第２の光源（局発光源）と光受信器とで構成される第２の光ヘテロダイン系における２つの光のうちいずれか一方が振幅変調または強度変調される。これにより、第２の光ヘテロダイン系において生成されるビート信号であるＦＭ変調信号の位相雑音を抑圧し、雑音特性に優れた周波数変調を実現できる。
【０００９】
第２の発明は、第１の発明において、
前記第１の光変調器は、直接変調によって前記第１の光信号を生成し、
前記ビート信号生成部は、
前記第１の光信号および前記第２の光を受け取って、自乗検波特性に基づく光検波により、前記第１の光信号および前記第２の光の光周波数の差に相当する中心周波数を有する変調された電気信号である変調ビート信号を生成する受光素子と、
前記変調ビート信号からその搬送波成分を抽出し、当該搬送波成分を前記無変調ビート信号として出力するフィルタとを含むことを特徴とする。
【００１０】
上記第２の発明によれば、直接変調によって第１の光信号が生成され、この第１の光信号と第２の光とから生成される変調ビート信号から搬送波成分である無変調ビート信号がフィルタによって抽出され、その無変調ビート信号を周波数変換することにより得られる周波数変換信号で、第２の光ヘテロダイン系における２つの光のうちいずれか一方が振幅変調または強度変調される。これにより、光受信器により生成されるＦＭ変調信号の位相雑音を抑圧し、雑音特性に優れた周波数変調を実現できる。
【００１１】
第３の発明は、第１の発明において、
前記第１の光変調器は、
無変調光である前記第１の光を出力する前記第１の光源と、
前記第１の光源から出力された前記第１の光を前記入力電気信号で変調することにより前記第１の光信号を生成する外部光変調器とを含み、
前記第２の光源は、無変調光を前記第２の光として出力し、
前記ビート信号生成部は、無変調光である前記第１および第２の光を受け取って、自乗検波特性に基づく光検波により前記無変調ビート信号を生成する受光素子を含むことを特徴とする。
【００１２】
上記第３の発明によれば、外部変調によって第１の光から変調光である第１の光信号に変換され、ビート信号生成部では、無変調光である第１及び第２の光から無変調ビート信号が生成される。このため、変調ビート信号からその搬送波成分を抽出するためのフィルタが不要となる。
【００１３】
第４の発明は、第１の発明において、
前記第１の光変調器は、前記入力電気信号の振幅変化を前記第１の光の光周波数の変化に一意に変換することにより、所定の中心周波数ｆ１の前記第１の光信号を生成し、
前記第２の光源は、所定の周波数ｆ２の光を前記第２の光として出力し、
前記ビート信号生成部は、
前記第１の光信号と前記第２の光とを受け取って、自乗検波特性に基づく光検波により、前記第１の光信号および前記第２の光の光周波数の差に相当する周波数ｆｓ＝｜ｆ１−ｆ２｜において、変調された電気信号である変調ビート信号を生成する受光素子と、
前記変調ビート信号からその搬送波成分を抽出し、当該搬送波成分を前記無変調ビート信号として出力するフィルタとを含み、
前記周波数変換器は、前記無変調ビート信号を所定の周波数ｆx の信号に変換し、変換後の当該信号を前記周波数変換信号として出力し、
前記第２の光変調器は、前記第１の光信号と前記第２の光とのうち一方を周波数ｆx の前記周波数変換信号で光振幅変調または光強度変調することにより第２の光信号を生成し、
前記光受信器は、前記第１の光信号と前記第２の光とのうちの他方および前記第２の光信号を受け取って、自乗検波特性に基づく光検波により、周波数ｆL ＝｜ｆｓ−ｆx ｜において前記ＦＭ変調信号を生成することを特徴とする。
【００１４】
上記第４の発明によれば、第１の光変調器により入力電気信号が光周波数変調信号に変換され、第１の光変調器と局発光源に相当する第２の光源とビート信号生成部とで構成される第１の光ヘテロダイン系において、ビート信号生成部から出力される無変調ビート信号である搬送波成分が周波数変換され、第１の光変調器と第２の光源（局発光源）と光受信器とで構成される第２の光ヘテロダイン系において、第１の光変調器または第２の光源から出力される光のいずれか一方が、上記の周波数変換された搬送波成分で振幅変調または強度変調される。これにより、光受信器から出力されるビート信号であるＦＭ変調信号の位相雑音を抑圧し、雑音特性に優れた周波数変調を実現できる。
【００１５】
第５の発明は、第１の発明において、
前記入力電気信号を互いに逆相の第１および第２の電気信号に分岐させる分波器と、
前記第２の電気信号を光強度変調によって第３の光信号に変換する第３の光変調器と、
を更に備え、
前記第１の光変調器は、直接変調によって前記第１の電気信号の振幅変化を前記第１の光の光周波数の変化に一意に変換することにより、所定の中心周波数ｆ１の前記第１の光信号を生成し、
前記第２の光源は、所定の周波数ｆ２の光を前記第２の光として出力し、
前記ビート信号生成部は、
前記第１の光信号と前記第２の光とを受け取って、自乗検波特性に基づく光検波により、前記第１の光信号および前記第２の光の光周波数の差に相当する周波数ｆｓ＝｜ｆ１−ｆ２｜において、変調された電気信号である変調ビート信号を生成する受光素子と、
前記変調ビート信号からその搬送波成分を抽出し、当該搬送波成分を前記無変調ビート信号として出力するフィルタとを含み、
前記周波数変換器は、前記無変調ビート信号を所定の周波数ｆx の信号に変換し、変換後の当該信号を前記周波数変換信号として出力し、
前記第２の光変調器は、前記第１の光信号と前記第２の光とのうち一方を周波数ｆx の前記周波数変換信号で光振幅変調または光強度変調することにより第２の光信号を生成し、
前記光受信器は、前記第１の光信号と前記第２の光とのうちの他方および前記第２の光信号を受け取って、自乗検波特性に基づく光検波により、周波数ｆL ＝｜ｆｓ−ｆx｜において前記ＦＭ変調信号を生成するとともに、前記第３の光信号を受け取って、前記光検波により、前記第３の光信号に含まれる光強度変調成分に相当する電気信号を生成することを特徴とする。
【００１６】
上記第５の発明によれば、周波数変調の元信号である入力電気信号が互いに逆相の第１および第２の電気信号に分岐し、第１の光変調器により第１の電気信号が光周波数変調信号である第１の光信号に変換され、第１の光変調器と局発光源に相当する第２の光源とビート信号生成部とで構成される第１の光ヘテロダイン系において、ビート信号生成部から出力される無変調ビート信号である搬送波成分が周波数変換され、第１の光変調器と第２の光源（局発光源）と光受信器とで構成される第２の光ヘテロダイン系において、第１の光変調器または第２の光源から出力される光のいずれか一方が、上記の周波数変換された搬送波成分で振幅変調または強度変調される。さらに、第３の光変調器により第２の電気信号が光強度変調信号である第３の光信号に変換されて光受信器に入力され、そこで自乗検波により強度変調−直接検波成分（ＩＭ−ＤＤ成分）が生成される。このＩＭ−ＤＤ成分により、第１の光信号に含まれる光強度変調成分（これは直接変調に起因して発生する）に相当するＩＭ−ＤＤ成分が光受信器において相殺される。これにより、光受信器から出力されるビート信号であるＦＭ変調信号の位相雑音を抑圧し、かつ不要成分を低減した品質の良好な周波数変調を実現できる。
【００１７】
第６の発明は、第５の発明において、
前記光受信器の前記自乗検波特性に基づく光検波によって生成される強度変調−直接検波成分のうち前記第１の光信号に含まれる光強度変調成分に相当する第１のＩＭ−ＤＤ成分と、当該生成される強度変調−直接検波成分のうち前記第３の光信号に含まれる光強度変調成分に相当する第２のＩＭ−ＤＤ成分とを、互いに逆相かつ同一振幅とする位相・振幅調整手段を更に備えることを特徴とする。
【００１８】
上記第６の発明によれば、第１の光信号に含まれる光強度変調成分に相当する第１のＩＭ−ＤＤ成分と、第３の光信号に含まれる光強度変調成分に相当する第２のＩＭ−ＤＤ成分とが、互いに逆相かつ同一振幅となるように、その位相および振幅が調整される。このため、光受信器から出力されるＩＭ−ＤＤ成分がより確実に抑圧され、より高品質な高周波・広帯域ＦＭ変調信号を生成することができる。
【００１９】
第７の発明は、第１の発明において、
前記入力電気信号を互いに逆相の第１および第２の電気信号に分岐する分波器と、
所定の中心周波数ｆ２の変調光である前記第２の光を第４の光信号として出力する第４の光変調器と、
を更に備え、
前記第１の光変調器は、直接変調によって前記第１の電気信号の振幅変化を前記第１の光の光周波数の変化に一意に変換することにより、所定の中心周波数ｆ１の前記第１の光信号を生成し、
前記第４の光変調器は、直接変調によって前記第２の電気信号の振幅変化を前記第２の光の光周波数の変化に一意に変換することにより、所定の中心周波数ｆ２の前記第４の光信号を生成し、
前記ビート信号生成部は、
前記第１および第４の光信号を受け取って、自乗検波特性に基づく光検波により、前記第１および第４の光信号の光周波数の差に相当する周波数ｆｓ＝｜ｆ１−ｆ２｜において、変調された電気信号である変調ビート信号を生成する受光素子と、
前記変調ビート信号からその搬送波成分を抽出し、当該搬送波成分を前記無変調ビート信号として出力するフィルタとを含み、
前記周波数変換器は、前記無変調ビート信号を所定の周波数ｆx の信号に変換し、変換後の当該信号を前記周波数変換信号として出力し、
前記第２の光変調器は、前記第１の光信号と前記第４の光信号のうち一方を周波数ｆx の前記周波数変換信号で光振幅変調または光強度変調することにより第２の光信号を生成し、
前記光受信器は、前記第１の光信号と前記第４の光信号のうちの他方および前記第２の光信号を受け取って、自乗検波特性に基づく光検波により、周波数ｆL ＝｜ｆｓ−ｆx ｜において前記ＦＭ変調信号を生成することを特徴とする。
【００２０】
上記第７の発明によれば、周波数変調の元信号である入力電気信号が互いに逆相の第１および第２の電気信号に分岐し、第１の光変調器での直接変調により第１の電気信号が光周波数変調信号である第１の光信号に変換され、第４の光変調器での直接変調により第２の電気信号が光周波数変調信号である第４の光信号に変換される。そして、第１の光変調器と第４の光変調器とビート信号生成部とで構成される第１の光ヘテロダイン系において、ビート信号生成部から出力される無変調ビート信号である搬送波成分が周波数変換され、第１の光変調器と第４の光変調器と光受信器とで構成される第２の光ヘテロダイン系において、第１の光変調器または第４の光変調器から出力される光のいずれか一方が、上記の周波数変換された搬送波成分で振幅変調または強度変調される。これにより、光受信器から出力されるビート信号であるＦＭ変調信号の位相雑音を抑圧し、雑音特性に優れた周波数変調を実現できる。また、上記のように、第１の光変調器と第４の光変調器とがプッシュプル動作を行うことによって周波数変調信号が生成されるため、直接変調において生じる光強度変調成分に起因する不要成分であるＩＭ−ＤＤ成分が相殺される。これにより、不要成分を低減した品質の良好な周波数変調を実現できる。
【００２１】
第８の発明は、第７の発明において、
前記光受信器の前記自乗検波特性に基づく光検波によって生成される強度変調−直接検波成分のうち前記第１の光信号に含まれる光強度変調成分に相当する第１のＩＭ−ＤＤ成分と、当該生成される強度変調−直接検波成分のうち前記第４の光信号に含まれる光強度変調成分に相当する第３のＩＭ−ＤＤ成分とを、互いに逆相かつ同一振幅とする位相・振幅調整手段を更に備えることを特徴とする。
【００２２】
上記第８の発明によれば、第１の光信号に含まれる光強度変調成分に相当する第１のＩＭ−ＤＤ成分と、第４の光信号に含まれる光強度変調成分に相当する第３のＩＭ−ＤＤ成分とが、互いに逆相かつ同一振幅となるように、その位相および振幅が調整される。このため、光受信器から出力されるＩＭ−ＤＤ成分がより確実に抑圧され、より高品質な高周波・広帯域ＦＭ変調信号を生成することができる。
【００２３】
第９の発明は、第１の発明において、
前記第１の光変調器と前記ビート信号生成部との間に挿入され、前記第１の光信号からその光搬送波成分を抽出する光フィルタを更に備え、
前記第２の光源は、無変調光を前記第２の光として出力し、
前記ビート信号生成部は、前記光フィルタによって抽出された光搬送波成分と前記第２の光とを受け取って、自乗検波特性に基づく光検波により前記無変調ビート信号を生成する受光素子を含むことを特徴とする。
【００２４】
上記第９の発明によれば、第１の光変調器により入力電気信号が光周波数変調信号である第１の光信号に変換され、第１の光変調器と局発光源に相当する第２の光源とビート信号生成部とで構成される第１の光ヘテロダイン系において、第１の光変調器とビート信号生成部との間に挿入された光フィルタによって、第１の光信号からその光搬送波成分が抽出され、ビート信号生成部では、この光搬送波成分と第２の光とからフィルタを用いることなく無変調ビート信号が生成され、この無変調ビート信号すなわち搬送波成分を周波数変換することにより周波数変換信号が生成される。そして、第１の光変調器と第２の光源と光受信器とで構成される第２の光ヘテロダイン系において、第１の光変調器または第２の光源から出力される光のいずれか一方が、上記の周波数変換信号（周波数変換された搬送波成分）によって振幅変調または強度変調される。これにより、光受信器から出力されるビート信号であるＦＭ変調信号の位相雑音を抑圧し、雑音特性に優れた周波数変調を実現できる。
【００２５】
第１０の発明は、第１の発明において、
前記入力電気信号を互いに逆相の第１および第２の電気信号に分岐する分波器と、
所定の中心周波数ｆ２の変調光である前記第２の光を第４の光信号として出力する第４の光変調器と、
前記第１の光変調器と前記ビート信号生成部との間に挿入された第１の光フィルタと、
前記第４の光変調器と前記ビート信号生成部との間に挿入された第２の光フィルタと、
を更に備え、
前記第１の光変調器は、直接変調によって前記第１の電気信号の振幅変化を前記第１の光の光周波数の変化に一意に変換することにより、所定の中心周波数ｆ１の前記第１の光信号を生成し、
前記第４の光変調器は、直接変調によって前記第２の電気信号の振幅変化を前記第２の光の光周波数の変化に一意に変換することにより、所定の中心周波数ｆ２の前記第４の光信号を生成し、
前記第１の光フィルタは、前記第１の光信号からその光搬送波成分を抽出し、前記第２の光フィルタは、前記第４の光信号からその光搬送波成分を抽出し、前記ビート信号生成部は、前記第１および第２の光フィルタによって抽出された両光搬送波成分を受け取って、自乗検波特性に基づく光検波により、当該両光搬送波成分の光周波数の差に相当する周波数ｆｓ＝｜ｆ１−ｆ２｜において前記無変調ビート信号を生成する受光素子を含み、
前記周波数変換器は、前記無変調ビート信号を所定の周波数ｆx の信号に変換し、変換後の当該信号を前記周波数変換信号として出力し、
前記第２の光変調器は、前記第１の光信号と前記第４の光信号のうち一方を周波数ｆx の前記周波数変換信号で光振幅変調または光強度変調することにより第２の光信号を生成し、
前記光受信器は、前記第１の光信号と前記第４の光信号のうちの他方および前記第２の光信号を受け取って、自乗検波特性に基づく光検波により、周波数ｆL ＝｜ｆｓ−ｆx ｜において前記ＦＭ変調信号を生成することを特徴とする。
【００２６】
上記第１０の発明によれば、周波数変調の元信号である入力電気信号が互いに逆相の第１および第２の電気信号に分岐し、第１の光変調器での直接変調により第１の電気信号が光周波数変調信号である第１の光信号に変換され、第４の光変調器での直接変調により第２の電気信号が光周波数変調信号である第４の光信号に変換される。そして、第１の光変調器と第４の光変調器とビート信号生成部とで構成される第１の光ヘテロダイン系において、第１の光フィルタによって第１の光信号からその光搬送波成分が抽出され、第２の光フィルタによって第４の光信号からその光搬送波成分が抽出され、ビート信号生成部ではこれらの光搬送波成分からフィルタを用いることなく無変調ビート信号が生成される。この無変調ビート信号すなわち搬送波成分を周波数変換することにより周波数変換信号が生成され、第１の光変調器と第４の光変調器と光受信器とで構成される第２の光ヘテロダイン系において、第１の光変調器または第４の光変調器から出力される光のいずれか一方が、上記の周波数変換信号（周波数変換された搬送波成分）によって振幅変調または強度変調される。これにより、光受信器から出力されるビート信号であるＦＭ変調信号の位相雑音を抑圧し、かつ不要成分を低減した品質の良好な周波数変調を実現できる。
【００２７】
第１１の発明は、第１０の発明において、
前記光受信器の前記自乗検波特性に基づく光検波によって生成される強度変調−直接検波成分のうち前記第１の光信号に含まれる光強度変調成分に相当する第１のＩＭ−ＤＤ成分と、当該生成される強度変調−直接検波成分のうち前記第４の光信号に含まれる光強度変調成分に相当する第３のＩＭ−ＤＤ成分とを、互いに逆相かつ同一振幅とする位相・振幅調整手段を更に備えることを特徴とする。
【００２８】
上記第１１の発明によれば、上記第８の発明と同様、第１の光信号に含まれる光強度変調成分に相当する第１のＩＭ−ＤＤ成分と、第４の光信号に含まれる光強度変調成分に相当する第３のＩＭ−ＤＤ成分とが、互いに逆相かつ同一振幅となるように、その位相および振幅が調整される。このため、光受信器から出力されるＩＭ−ＤＤ成分がより確実に抑圧され、より高品質な高周波・広帯域ＦＭ変調信号を生成することができる。
【００２９】
第１２の発明は、第１の発明において、
前記第１の光源から前記ビート信号生成部を介して前記光受信器に至る経路の光および電気伝搬時間と、前記第１の光源から直接に前記光受信器に至る経路の光伝搬時間とを互いに等しくする第１の伝搬時間調整手段と、
前記第２の光源から前記ビート信号生成部を介して前記光受信器に至る経路の光および電気伝搬時間と、前記第２の光源から直接に前記光受信器に至る経路の光伝搬時間とを互いに等しくする第２の伝搬時間調整手段とを更に備えることを特徴とする。
【００３０】
上記第１２の発明によれば、第１の光源から出力されて２つに分岐した光信号のそれぞれが、各構成部を通過し、あるいは光電気変換や電気光変換等の処理を経て、光受信器に到達するまでの伝搬時間が互いに等しくなるように設定され、かつ、第２の光源から出力されて２つに分岐した光のそれぞれが、各構成部を通過し、あるいは光電気変換や電気光変換等の処理を経て、光受信器に到達するまでの伝搬時間を互いに等しくなるように設定される。これにより、光受信器から出力されるビート信号であるＦＭ変調信号の位相雑音をより適切に抑圧し、さらに雑音特性に優れた周波数変調を実現できる。
【００３１】
【発明の実施の形態】
以下、添付図面を参照しつつ本発明の各実施形態について説明する。
【００３２】
（第１の実施形態）
図１および図２（Ａ）〜（Ｃ）を参照しつつ本発明の第１の実施形態に係る周波数変調装置について以下に説明する。
【００３３】
図１は、第１の実施形態に係る周波数変調装置の構成を示すブロック図である。この周波数変調装置は、信号源１００と、第１の光変調器１０２と、局発光源１０３と、第１の光分岐回路１０４と、第２の光分岐回路１０５と、第１の光導波路１０６と、第２の光導波路１０７と、第３の光導波路１０８と、第４の光導波路１０９と、第１の光受信器１１０およびフィルタ１１１を含むビート信号生成部２００と、周波数変換器１１２と、第２の光変調器１１３と、第２の光受信器１１４とを備えている。この周波数変調装置は、光周波数の互いに異なる２つの光（第１の光Ｌ１および第２の光Ｌ２）をそれぞれ出力する２つの光源（第１および第２の光源）を用いた光周波数変調および光ヘテロダイン検波により、信号源１００から出力される電気信号Ｓｉを周波数変調信号に変換し、これをＦＭ変調信号Ｓfmとして出力する。
【００３４】
次に、図１に示す周波数変調装置の動作を詳細に説明する。第１の光変調器１０２は、直接変調方式の光変調器であって、通常、光周波数変調効果を有する第１の光源である半導体レーザで構成され、信号源１００から出力される電気信号Ｓｉによってその半導体レーザへの注入電流を振幅変調することにより、中心光周波数ｆ１の光周波数変調信号Ｌ１を第１の光として出力する。第１の光変調器１０２から出力された光信号Ｌ１は、第１の光分岐回路１０４で２つの光信号に分岐し、これらのうち一方の光信号は第２の光導波路１０７を介してビート信号生成部２００における第１の光受信器１１０に、他方の光信号は第１の光導波路１０６を介して第２の光受信器１１４に、それぞれ入力される。局発光源１０３は、第２の光源に相当し、光周波数ｆ２の無変調光Ｌ２を第２の光として出力する。局発光源１０３から出力された光Ｌ２は、第２の光分岐回路１０５で２つの光に分岐し、これらのうち一方の光は第４の光導波路１０９を介してビート信号生成部２００における第１の光受信器１１０に、他方の光は第３の光導波路１０８を介して第２の光変調器１１３に、それぞれ入力される。
【００３５】
ビート信号生成部２００は、第１の光分岐回路１０４で分岐した第１の光である光信号Ｌ１と第２の光分岐回路１０５で分岐した第２の光Ｌ２とを受け取り、下記の動作により、第１および第２の光Ｌ１，Ｌ２の光周波数の差に相当する周波数ｆｓ＝｜ｆ１−ｆ２｜を有する無変調の電気信号（以下「無変調ビート信号」という）Ｓｂを生成する。すなわち、第１の光受信器１１０は、自乗検波特性を有するフォトダイオード等の受光素子で構成され、第１の光変調器１０２からの光信号Ｌ１と局発光源１０３からの光Ｌ２とを受け取って、その自乗検波特性に基づく光ヘテロダイン検波により、それらの光周波数差ｆｓ（＝｜ｆ１−ｆ２｜）を中心周波数とするビート信号（以下「変調ビート信号」という）Ｓｂfmを生成する。この変調ビート信号Ｓｂfmは、第１の光変調器１０２から出力される光周波数変調信号Ｌ１をダウンコンバートしたＦＭ変調信号である。図２（Ａ）は、この変調ビート信号Ｓｂfmの周波数スペクトルを、信号源１００から出力される電気信号Ｓｉが周波数ｆｐを中心とする信号と想定した場合について示したものである。この変調ビート信号Ｓｂfmはフィルタ１１１に入力される。このフィルタ１１１は、帯域通過フィルタであって、ＦＭ変調信号である上記変調ビート信号Ｓｂfmの搬送波成分（中心周波数ｆｓの成分）のみを抽出し、無変調ビート信号Ｓｂとして出力する。
【００３６】
周波数変換器１１２は、例えば周波数ｆL の局部発振信号ＳL により、フィルタ１１１から出力される搬送波成分である無変調ビート信号Ｓｂを周波数変換（例えば、ダウンコンバート）し、周波数ｆx （＝｜ｆｓ−ｆL ｜）の信号を周波数変換信号Ｓｘとして出力する。図２（Ｂ）は、このような周波数変換器１１２の入力信号である無変調ビート信号Ｓｂおよび出力信号である周波数変換信号Ｓｘの周波数スペクトルを、局部発振信号ＳL の周波数スペクトルと共に示している。
【００３７】
第２の光変調器１１３は、第３の光導波路１０８の途上に挿入、配置され、局発光源１０３から出力される無変調光Ｌ２を、周波数変換器１１２から出力される周波数変換信号Ｓｘによって光学的に変調する。すなわち、第２の光変調器１１３は、光振幅変調または光強度変調によって無変調光Ｌ２を光変調信号Ｌ2xに変換し、これを出力する。
【００３８】
第２の光受信器１１４は、自乗検波特性を有するフォトダイオード等の受光素子で構成され、第１の光変調器１０２からの光信号Ｌ１と第２の光変調器１１３からの光信号Ｌ2xとを受け取って、その自乗検波特性に基づく光ヘテロダイン検波により、周波数ｆL （＝｜ｆｓ−ｆx ｜）を中心周波数とするビート信号としてＦＭ変調信号Ｓfmを生成する。このようにして生成されたＦＭ変調信号Ｓfmは、第１の光変調器１０２から出力される光周波数変調信号Ｌ１をダウンコンバートした信号であって、図２（Ｃ）に示すように位相雑音が抑圧されている。
【００３９】
本実施形態におけるＦＭ変調信号の位相雑音抑圧効果について、以下に数式を用いて説明する。
第１の光変調器１０２から出力される光周波数変調信号Ｌ１の電界成分は、
E₁(t)=A₁[1+m₁cos(2πf_pt)]^1/2cos[2πf₁t+ΔFsin(2πf_pt)+φ₁] …（１）
局発光源１０３から出力される無変調光Ｌ２の電界成分は、
E₂(t)=A₂cos(2πf₂t+φ₂) …（２）
で、それぞれ表される。ここで、ｍ１は、第１の光変調器１０２における光（強度）変調度を表し、ｆｐは、信号源１００から出力される電気信号Ｓｉ（正弦波を想定）の周波数を表し、ｆ１およびｆ２は、第１の光変調器１０２と局発光源１０３から出力される光の光周波数をそれぞれ表し、φ１、φ２は、第１の光変調器１０２と局発光源１０３から出力される光が有する位相雑音をそれぞれ表し、△Ｆは、信号源１００から出力される電気信号Ｓｉに対応して第１の光変調器１０２において与えられる周波数偏移量を表す。
【００４０】
第１の光受信器１１０がこれら２つの光Ｌ１およびＬ２の合波光を受け取ると、第１の光受信器１１０で用いられる受光素子の自乗検波特性に基づき、次式で表される光電流がその受光素子に流れる。
I₁(t)=R₁|E₁(t)+E₂(t)|²
=R₁A₁ ²[1+m₁cos(2πf_pt)]cos²[2πf₁t+ΔFsin(2πf_pt)+φ₁]
+R₁A₂ ²cos²(2πf₂t+φ₂)
+2R₁A₁A₂[1+m₁cos(2πf_pt)]^1/2cos[2πf₁t+ΔFsin(2πf_pt)+φ₁]
×cos(2πf₂+φ₂) …（３）
ここで、Ｒ１は、第１の光受信器１１０で用いられる受光素子の変換効率を表す。式（３）における第１項および第２項に相当する信号成分は、第１の光受信器１１０で用いられる受光素子の周波数応答制限によって検波されない。なお、第１項に相当する信号成分には、第１の光変調器１０２からの光信号Ｌ１における光強度変調成分が直接検波されることによって生じるＩＭ−ＤＤ（強度変調−直接検波）成分も含まれるが、このＩＭ−ＤＤ成分は後述のフィルタ１１１で除去されるので無視してもよい。
【００４１】
式（３）における第３項をさらに展開すると、
I_c1(t)=R₂A₁A₂[1+m₁cos(2πf_pt)]^1/2
×[cos[2π(f₁+f₂)t+ΔFsin(2πf_pt)+φ₁+φ₂]
＋cos[2π(f₁-f₂)t+ΔFsin(2πf_pt)+φ₁-φ₂]] …（４）
となる。式（４）における大括弧“［］”内の第１項に相当する信号成分は、第１の光受信器１１０に用いられる受光素子の周波数応答限界によって検波されず、大括弧“［］”内の第２項に相当する信号成分のみが出力される。この第２項は、信号源１００から出力される電気信号Ｓｉを元信号とし、搬送波周波数をｆｓ＝｜ｆ１−ｆ２｜、周波数偏移量を△ＦとするＦＭ変調信号を表している。この第２項のＦＭ変調信号は、第１の光変調器１０２から出力される光信号Ｌ１が有する位相雑音φ１と局発光源１０３から出力される光Ｌ２の位相雑音φ２とで決まる位相雑音を有する。
【００４２】
フィルタ１１１は、式（４）で表されるＦＭ変調信号における搬送波成分（中心周波数ｆｓの成分）として
I_s(t)=cos[2π(f₁-f₂)t+φ₁-φ₂] …（５）
で表される信号成分である無変調ビート信号Ｓｂのみを通過させる。
【００４３】
周波数変換器１１２は、周波数ｆL の局部発振信号ＳL を用いて、式（５）で表される無変調ビート信号Ｓｂを周波数変換する。例えば、無変調ビート信号Ｓｂをダウンコンバートすることによって、周波数ｆx ＝｜ｆ１−ｆ２−ｆL ｜の信号として、
I_x(t)=cos[2π(f₁-f₂-f_L)t+φ₁-φ₂]
=cos(2πf_xt+φ₁-φ₂) …（６）
で表される信号（周波数変換信号Ｓx ）に変換する。
【００４４】
第２の光変調器１１３は、式（６）で表される周波数変換信号Ｓｘにより、局発光源１０３から出力される無変調光Ｌ２（式（２）参照）を例えば光振幅変調し、電界成分が
E_2x(t)=A₂cos(2πf_xt+φ₁-φ₂)cos(2πf₂t+φ₂) …（７）
で表される光変調信号Ｌ2xを出力する。
【００４５】
第２の光受信器１１４は、第１の光変調器１０２からの光変調信号Ｌ１（式（１）参照）と、第２の光変調器１１３からの光変調信号Ｌ2x（式（７）参照）とを合波し、その自乗検波特性に基づき、
I₂(t)=R₂|E₁(t)+E_2x(t)|²
=R₂A₁ ²[1+m₁cos(2πf_pt)]cos²[2πf₁t+ΔFsin(2πf_pt)+φ₁]
+R₂A₂ ²cos²(2πf_xt+φ₁-φ₂)cos²(2πf₂t+φ₂)
+2R₂A₁A₂[1+m₁cos(2πf_pt)]^1/2cos[2πf₁t+ΔFsin(2πf_pt)+φ₁]
×cos(2πf_xt+φ₁-φ₂)cos(2πf₂+φ₂) …（８）
で表される光電流を出力する。ここで、Ｒ２は、第２の光受信器１１４で用いられる受光素子の変換効率を表す。式（８）における第１項および第２項に相当する信号成分は、第２の光受信器１１４に用いられる受光素子の周波数応答制限によって検波されない。なお、第１項に相当する信号成分には、第１の光変調器１０２からの光信号Ｌ１における光強度変調成分が直接検波されることによって生じるＩＭ−ＤＤ成分も含まれるが、このＩＭ−ＤＤ成分の周波数帯域がＦＭ変調信号Ｓfmの周波数帯域から離れている場合にはこの成分を無視してもよい。このＩＭ−ＤＤ成分の周波数帯域がＦＭ変調信号Ｓfmの周波数帯域と重なる場合には、後述の他の実施形態の構成によりこの成分を抑圧することができる。
【００４６】
式（８）における第３項をさらに展開すると、
I_c2(t)=R₂A₁A₂[1+m₁cos(2πf_pt)]^1/2
×[cos(2πf_xt+φ₁-φ₂)cos[2π(f₁+f₂)t+ΔFsin(2πf_pt)+φ₁+φ₂]
＋cos(2πf_xt+φ₁-φ₂)cos[2π(f₁-f₂)t+ΔFsin(2πf_pt)+φ₁-φ₂]]
=(R₂A₁A₂/2)[1+m₁cos(2πf_pt)]^1/2
×[cos[2π(f₁+f₂+f_x)t+ΔFsin(2πf_pt)+2φ₁]
＋cos[2π(f₁+f₂-f_x)t+ΔFsin(2πf_pt)+2φ₂]
＋cos[2π(f₁-f₂+f_x)t+ΔFsin(2πf_pt)+2(φ₁-φ₂)]
＋cos[2π(f₁-f₂-f_x)t+ΔFsin(2πf_pt)]] …（９）
となる。式（９）における大括弧“［］”内の第１項および第２項に相当する信号成分は、第２の光受信器１１４に用いられる受光素子の周波数応答限界によって検波されず、大括弧“［］”内の第３項および第４項に相当する信号成分のみがＦＭ変調信号Ｓfmとして出力される。このうち第４項は、信号源１００から出力される電気信号Ｓｉを元信号とし、搬送波周波数をｆL ＝｜ｆ１−ｆ２−ｆx ｜、周波数偏移量を△ＦとするＦＭ変調信号を表している。この第４項のＦＭ変調信号は、第１の光変調器１０２および局発光源１０３からそれぞれ出力される光Ｌ１，Ｌ２の位相雑音φ１およびφ２に依存せず、位相雑音のない信号である。
【００４７】
なお、第２の光変調器１１３が、式（６）で表される周波数変換信号Ｓx により、局発光源１０３から出力される無変調光Ｌ２（式（２）参照）を光強度変調する場合、第２の光変調器１１３から出力される光変調信号の電界成分は
E_2y(t)=A₂[1+m₂cos(2πf_xt+φ₁-φ₂)]^1/2cos(2πf₂t+φ₂)
で表される。上記式は次式のように近似することができる。
E_2y(t)=A₂[1+(m₂/2)cos(2πf_xt+φ₁-φ₂)]cos(2πf₂t+φ₂) …（１０）
ここで、ｍ2は、第２の光変調器１１３における光（強度）変調度である。式（１０）における中括弧“｛｝”内の第１項は無変調光成分を表しているので、以下では、その中括弧“｛｝”内については第２項のみを考える。
【００４８】
第２の光受信器１１４は、第１の光変調器１０２からの光変調信号Ｌ１（式（１）参照）と、第２の光変調器１１３からの光変調信号Ｌ2x（式（１０）における中括弧“｛｝”内の第２項参照）を合波し、その自乗検波特性に基づき、
I₂(t)=R₂|E₁(t)+E_2y(t)|²
=R₂A₁ ²[1+m₁cos(2πf_pt)]cos²[2πf₁t+ΔFsin(2πf_pt)+φ₁]
+R₂A₂ ²(m₂ ²/4)cos²(2πf_xt+φ₁-φ₂)cos²(2πf₂t+φ₂)
+2R₂A₁A₂[1+m₁cos(2πf_pt)]^1/2cos[2πf₁t+ΔFsin(2πf_pt)+φ₁]
×(m₂/2)cos(2πf_xt+φ₁-φ₂)cos(2πf₂+φ₂) …（１１）
で表される光電流を出力する。式（１１）における第１項および第２項に相当する信号成分は、第２の光受信器１１４に用いられる受光素子の周波数応答制限によって検波されない。なお、第１項に相当する信号成分には、第１の光変調器１０２からの光信号Ｌ１における光強度変調成分が直接検波されることによって生じるＩＭ−ＤＤ成分も含まれるが、このＩＭ−ＤＤ成分の周波数帯域がＦＭ変調信号Ｓfmの周波数帯域から離れている場合にはこの成分を無視してもよい。このＩＭ−ＤＤ成分の周波数帯域がＦＭ変調信号Ｓfmの周波数帯域と重なる場合には、後述の他の実施形態の構成によりこの成分を抑圧することができる。
【００４９】
式（１１）における第３項をさらに展開すると、
I_c2(t)=(R₂A₁A₂m₂/2)[1+m₁cos(2πf_pt)]^1/2
×[cos(2πf_xt+φ₁-φ₂)cos[2π(f₁+f₂)t+ΔFsin(2πf_pt)+φ₁+φ₂]
＋cos(2πf_xt+φ₁-φ₂)cos[2π(f₁-f₂)t+ΔFsin(2πf_pt)+φ₁-φ₂]]
=(R₂A₁A₂m₂/4)[1+m₁cos(2πf_pt)]^1/2
×[cos[2π(f₁+f₂+f_x)t+ΔFsin(2πf_pt)+2φ₁]
＋cos[2π(f₁+f₂-f_x)t+ΔFsin(2πf_pt)+2φ₂]
＋cos[2π(f₁-f₂+f_x)t+ΔFsin(2πf_pt)+2(φ₁-φ₂)]
＋cos[2π(f₁-f₂-f_x)t+ΔFsin(2πf_pt)]] …（１２）
となる。式（１２）における大括弧“［］”内の第４項は、式（９）と同様に、信号源１００から出力される電気信号Ｓｉを元信号とし、搬送波周波数をｆL ＝｜ｆ１−ｆ２−ｆx ｜、周波数偏移量を△ＦとするＦＭ変調信号を表している。この第４項のＦＭ変調信号は、第１の光変調器１０２および局発光源１０３からそれぞれ出力される光Ｌ１，Ｌ２の位相雑音φ１およびφ２に依存せず、位相雑音のない信号である。
【００５０】
なお、式（９）と式（１２）の比較により明らかなように、第２の光受信器１１４から出力されるＦＭ変調信号Ｌ2xの大きさ（振幅）に関しては、第２の光変調器１１３において光振幅変調を適用した方が２倍大きく、このＦＭ変調信号を伝送する場合等に、ＣＮＲ（搬送波対雑音電力比）において有利である。従って、以下では、第２の光変調器１１３において光振幅変調を適用する場合についてのみ説明する。
【００５１】
以上説明したように、第１の実施形態によれば、２つの光源を用いた光ヘテロダイン構成において、光源の位相雑音に依存することなく、位相雑音のない高周波・広帯域ＦＭ変調信号を生成することができる。
【００５２】
（第２の実施形態）
図３および図４（Ａ）〜（Ｃ）を参照しつつ本発明の第２の実施形態に係る周波数変調装置について以下に説明する。
【００５３】
図３は、第２の実施形態に係る周波数変調装置の構成を示すブロック図である。この周波数変調装置は、図１に示した第１の実施形態の構成に加えて、分波器３０１と第３の光変調器３１５と第５の光導波路３１７とを備える。なお、図３において、第１の実施形態における構成要素と同じ構成要素には同一の参照符号が付されている。
【００５４】
次に、図３に示す本実施形態の動作を説明する。ただし、本実施形態の構成は、前述の第１の実施形態に準ずるため、相違点のみを以下に説明する。
本実施形態の周波数変調装置では、分波器３０１は、信号源１００から出力された電気信号Ｓｉを、互いに逆相の関係になるように第１および第２の電気信号Ｓi1，Ｓi2に分岐させて出力する。第１の光変調器１０２は、分波器３０１から出力された第１の電気信号Ｓi1を、第１の実施形態と同様に、光周波数変調信号Ｌ１に変換し、出力する。第３の光変調器３１５は、通常、半導体レーザで構成され、分波器３０１から出力された第２の電気信号Ｓi2を、光強度変調信号Ｌ３に変換し、出力する。この光信号Ｌ３は、第５の光導波路３１７を介して第２の光受信器１１４に入力される。
【００５５】
第２の光受信器１１４は、第１の光変調器１０２からの光信号Ｌ１と第２の光変調器１１３からの光信号Ｌ2xとを受け取って、その自乗検波特性に基づく光ヘテロダイン検波より、それらの光信号Ｌ１およびＬ2xの光周波数差を中心周波数とするビート信号としてＦＭ変調信号Ｓfmを生成し、かつ、第３の光変調器３１５からの光強度変調信号Ｌ３を受け取って、その自乗検波特性に基づく直接検波により、その光強度変調信号Ｌ３からＩＭ−ＤＤ（強度変調−直接検波）成分を生成する。第１の光変調器１０２において、第１の実施形態で説明したように半導体レーザによる直接変調方式を採用した場合、第１の光変調器１０２からの光信号Ｌ１は、光周波数変調成分と共に光強度変調成分をも含む。そのため、第２の光受信器１１４は、図４（Ａ）に示すように、第１の光変調器１０２からの光信号Ｌ１と第２の光変調器１１３からの光信号Ｌ2xとを受け取って、光ヘテロダイン検波により、それら２つの光信号のビート信号であるＦＭ変調信号を生成すると同時に、直接検波により、第１の光変調器１０２からの光信号に含まれる光強度変調成分に相当する第１のＩＭ−ＤＤ（強度変調−直接検波）成分Ｓimdd1 を生成する。この第１のＩＭ−ＤＤ成分Ｓimdd1 は、本来必要となるＦＭ変調信号Ｓfmに対する不要成分であり、特に、図４（Ａ）に示すように、第１のＩＭ−ＤＤ成分Ｓimdd1 の占有周波数帯域がＦＭ変調信号Ｓfmの周波数帯域と重なる場合には、ＦＭ変調信号Ｓfmの品質を大きく劣化させる。第２の光受信器１１４は、第３の光変調器３１５からの光強度変調信号Ｌ３をも受け取って、直接検波により、図４（Ｂ）に示すように、その光強度変調信号Ｌ３に相当するＩＭ−ＤＤ成分、すなわち前記第１のＩＭ−ＤＤ成分Ｓimdd1 と逆相である第２のＩＭ−ＤＤ成分Ｓimdd2 を生成する。この第２のＩＭ−ＤＤ成分Ｓimdd2 で第１のＩＭ−ＤＤ成分Ｓimdd1 が相殺されることにより、図４（Ｃ）に示すように、前記第１のＩＭ−ＤＤ成分Ｓimdd1 が抑圧される。
【００５６】
以上説明したように、第２の実施形態によれば、光信号Ｌ１に含まれる光強度変調成分に起因した不要成分であるＩＭ−ＤＤ成分の発生を抑え、位相雑音のない高品質な高周波・広帯域ＦＭ変調信号Ｓfmを生成することができる。
【００５７】
（第３の実施形態）
図５を参照しつつ本発明の第３の実施形態に係る周波数変調装置について以下に説明する。
【００５８】
図５は、第３の実施形態に係る周波数変調装置の構成を示すブロック図である。この周波数変調装置は、図１に示した第１の実施形態の構成において、局発光源１０３に代えて、分波器３０１と第４の光変調器５１６とを備える。なお、図５において、第１の実施形態における構成要素と同じ構成要素には同一の参照符号が付されている。
【００５９】
次に、図５に示す本実施形態の動作を説明する。
本実施形態の周波数変調装置では、分波器３０１は、信号源１００から出力された電気信号Ｓｉを、互いに逆相の関係になるように第１および第２の電気信号Ｓi1，Ｓi2に分岐させて出力する。第１の光変調器１０２は、分波器３０１から出力された第１の電気信号Ｓi1を、第１の実施形態と同様に、光周波数変調信号Ｌ１に変換し、出力する。第１の光変調器１０２から出力された光信号Ｌ１は、第１の光分岐回路１０４で２つの光信号に分岐し、これらのうち一方の光信号は第２の光導波路１０７を介してビート信号生成部２００における第１の光受信器１１０に、他方の光信号は第１の光導波路１０６を介して第２の光受信器１１４に、それぞれ入力される。
【００６０】
第４の光変調器５１６は、直接変調方式の光変調器であって、通常、半導体レーザ等で構成され、分波器３０１から出力される第２の電気信号Ｓi2を光周波数変調信号Ｌ２に変換し、出力する。第４の光変調器５１６において直接変調方式が採用されているため、第１の光変調器１０２と同様、出力される光信号Ｌ２は、光周波数変調成分と共に光強度変調成分をも含む。第４の光変調器５１６から出力された光信号Ｌ２は、第２の光分岐回路１０５で２つの光信号に分岐し、これらのうち一方の光信号は第４の光導波路１０９を介してビート信号生成部２００における第１の光受信器１１０に、他方の光信号は第３の光導波路１０８を介して第２の光変調器１１３に、それぞれ入力される。
【００６１】
ビート信号生成部２００は、第１の光分岐回路１０４で分岐した光信号Ｌ１と第２の光分岐回路１０５で分岐した光信号Ｌ２とを受け取り、下記の動作により、第１および第２の光信号Ｌ１，Ｌ２の光周波数の差に相当する周波数ｆｓ＝｜ｆ１−ｆ２｜を有する無変調の電気信号Ｓｂを生成する。すなわち、第１の光受信器１１０は、自乗検波特性を有するフォトダイオード等の受光素子で構成され、第１の光変調器１０２からの光信号Ｌ１と第４の光変調器５１６からの光信号Ｌ２とを受け取って、その自乗検波特性に基づく光ヘテロダイン検波により、それらの光周波数差ｆｓ（＝｜ｆ１−ｆ２｜）を中心周波数とする変調ビート信号Ｓｂfmを生成する。この変調ビート信号Ｓｂfmは、第１の光変調器１０２から出力される光周波数変調信号Ｌ１をダウンコンバートしたＦＭ変調信号である。フィルタ１１１は、この変調ビート信号Ｓｂfmの搬送波成分（中心周波数ｆｓの成分）のみを抽出し、無変調ビート信号Ｓｂとして出力する。
【００６２】
周波数変換器１１２は、この無変調ビート信号Ｓｂを周波数変換（例えば、ダウンコンバート）し、周波数ｆx （＝｜ｆｓ−ｆL ｜）の信号である周波数変換信号Ｓｘを出力する。
【００６３】
第２の光変調器１１３は、第３の光導波路１０８の途上に挿入、配置され、第４の光変調器５１６から出力される光信号Ｌ２を、周波数変換器１１２から出力される周波数変換信号Ｓｘによって光学的に変調する。すなわち、第２の光変調器１１３は、光振幅変調または光強度変調によって光信号Ｌ２を光変調信号Ｌ2xに変換し、これを出力する。
【００６４】
第２の光受信器１１４は、第１の光変調器１０２からの光信号Ｌ１と第２の光変調器１１３からの光信号Ｌ2xとを受け取って、その自乗検波特性に基づく光ヘテロダイン検波により、周波数ｆL （＝｜ｆｓ−ｆx ｜）を中心周波数とするビート信号として、位相雑音の抑圧されたＦＭ変調信号Ｓfmを生成する（図２（Ｃ）参照）。また、第２の光受信器１１４は、その自乗検波特性に基づく直接検波により、第１の光変調器１０２からの光信号Ｌ１に含まれる光強度変調成分に相当する第１のＩＭ−ＤＤ成分を生成すると共に、第４の光変調器５１６からの光信号Ｌ２に含まれる光強度変調成分をも受け取って、その自乗検波特性に基づく直接検波により、前記第１のＩＭ−ＤＤ成分と逆相である第３のＩＭ−ＤＤ成分を生成する。この第３のＩＭ−ＤＤ成分によって第１のＩＭ−ＤＤ成分が相殺され、これによりＩＭ−ＤＤ成分の抑圧されたＦＭ変調信号Ｓfmが得られる。
【００６５】
以上説明したように、第３の実施形態によれば、光信号Ｌ１に含まれる光強度変調成分に起因した不要成分であるＩＭ−ＤＤ成分の発生を抑え、位相雑音のない高品質な高周波・広帯域ＦＭ変調信号Ｓfmを生成することができる。
【００６６】
（第４の実施形態）
図６を参照しつつ本発明の第４の実施形態に係る周波数変調装置について以下に説明する。
【００６７】
図６は、第４の実施形態に係る周波数変調装置の構成を示すブロック図である。この周波数変調装置は、図１に示した第１の実施形態の構成において、電気的なフィルタ１１１に代えて光フィルタ６１７を備える。なお、図６において、第１の実施形態における構成要素と同じ構成要素には同一の参照符号が付されている。
【００６８】
次に、図６に示す本実施形態の動作を説明する。ただし、本実施形態の構成は、前述の第１の実施形態に準ずるため、相違点のみを以下に説明する。
本実施形態の周波数変調装置では、光フィルタ６１７は、第１の光変調器１０２からの光信号Ｌ１のうち第１の光分岐回路１０４で分岐して第１の光受信器１１０へ向かう光信号Ｌ１の光搬送波成分Ｌ1cのみを通過させる。第１の光受信器１１０は、その光フィルタ６１７を通過した光搬送波成分Ｌ1cと局発光源１０３からの光Ｌ２とを受け取って、その自乗検波特性に基づく光ヘテロダイン検波により、それらの光周波数差ｆｓに相当する周波数を有する無変調ビート信号Ｓｂを生成する。周波数変換器１１２は、この無変調ビート信号Ｓｂを周波数変換し、周波数ｆx の周波数変換信号Ｓｘを出力する。第２の光変調器１１３は、局発光源１０３から出力される光Ｌ２に対し、前記周波数変換器１１２から出力される周波数変換信号Ｓｘを用いて光変調（光振幅変調または光強度変調）を施し、光変調信号Ｌ2xを出力する。第２の光受信器１１４は、第１の光変調器１０２からの光信号Ｌ１と第２の光変調器１１３からの光信号Ｌ2xとを受け取って、その自乗検波特性に基づく光ヘテロダイン検波により、周波数ｆL （＝｜ｆｓ−ｆx ｜）を中心周波数とするビート信号として、位相雑音の抑圧されたＦＭ変調信号Ｓfmを生成する。
【００６９】
以上説明したように、第４の実施形態によれば、２つの光源を用いた光ヘテロダイン構成において、光源の位相雑音に依存することなく、位相雑音のない高周波・広帯域ＦＭ変調信号を生成することができる。
【００７０】
なお、上述の第３の実施形態のように局発光源１０３に代えて第４の光変調器５１６を備える構成（図５参照）においては、図７に示すように、上記第４の実施形態と同様に第２の光導波路１０７の途上に第１の光フィルタ６１７を備えると共に、第４の光導波路１０９の途上にも第２の光フィルタ６２７を備え、第４の光変調器５１６から出力されて第２の光分岐回路１０５で分岐し第１の光受信器１１０に向かう光信号Ｌ２のうち、第２の光フィルタ６２７がその光搬送波成分のみを通過させるようにしてもよい。このような構成によれば、第３の実施形態と同様、第１の光受信器１１０からは無変調ビート信号Ｓｂが出力されるため、電気的なフィルタ１１１を用いることなく、位相雑音のない高周波・広帯域ＦＭ変調信号を生成することができる。
【００７１】
（第５の実施形態）
図８を参照しつつ本発明の第５の実施形態に係る周波数変調装置について以下に説明する。
【００７２】
図８は、第５の実施形態に係る周波数変調装置の構成を示すブロック図である。この周波数変調装置は、図１に示した第１の実施形態の構成において、直接変調方式の第１の光変調器１０２に代えて、第１の光Ｌ１として光周波数ｆ１の無変調光を出力する光源１０１と外部変調方式により周波数変調を行う光変調器（以下「外部光変調器」という）１０２ｅとを備え、フィルタ１１１が削除されている。図８に示すように、光源１０１から出力された無変調光Ｌ１は第１の光分岐回路１０４に入力される。また、外部光変調器１０２ｅは、第１の光導波路１０６の途上に挿入、配置され、信号源１００から元信号としての電気信号Ｓｉを受け取る。なお、図８において、第１の実施形態における構成要素と同じ構成要素には同一の参照符号が付されている。
【００７３】
次に、図８に示す本実施形態の動作を説明する。
本実施形態の周波数変調装置では、光源１０１から出力された無変調光Ｌ１は第１の光分岐回路１０４で２つの光に分岐し、それらの２つの光のうち一方の光は外部光変調器１０２ｅに入力され、他方の光は第１の光受信器１１０に入力さされる。
【００７４】
外部光変調器１０２ｅは、光源１０１からの無変調光Ｌ１を信号源１００からの電気信号Ｓｉによって変調することにより、中心光周波数ｆ１の光周波数変調信号Ｌ1mを生成する。この光信号Ｌ1mは第２の光受信器１１４に入力される。第１の光受信器１１０は、光源１０１からの無変調光Ｌ１と局発光源１０３からの無変調光Ｌ２とを受け取って、その自乗検波特性に基づく光ヘテロダイン検波により、それらの光周波数差ｆｓ＝｜ｆ１−ｆ２｜に相当する周波数を有する無変調ビート信号Ｓｂを生成する。周波数変換器１１２は、この無変調ビート信号Ｓｂを周波数変換し、周波数ｆx の周波数変換信号Ｓｘを出力する。第２の光変調器１１３は、局発光源１０３からの光Ｌ２に対し、前記周波数変換器１１２から出力される周波数変換信号Ｓｘを用いて光変調（光振幅変調または光強度変調）を施し、光変調信号Ｌ2xを出力する。この光変調信号Ｌ2xも第２の光受信器１１４に入力される。第２の光受信器１１４は、第１の光変調器１０２からの光信号Ｌ１と第２の光変調器１１３からの光信号Ｌ2xとを受け取って、その自乗検波特性に基づく光ヘテロダイン検波により、周波数ｆL （＝｜ｆｓ−ｆx ｜）を中心周波数とするビート信号として、位相雑音の抑圧されたＦＭ変調信号Ｓfmを生成する。
【００７５】
以上のように、第５の実施形態によれば、第４の実施形態と同様、第１の光受信器１１０からは無変調ビート信号Ｓｂが出力されるため、電気的なフィルタ１１１を用いることなく、位相雑音のない高周波・広帯域ＦＭ変調信号を生成することができる。しかも、第５の実施形態では、光源１０１からの無変調光Ｌ１が第１の光受信器１１０に入力されるため、第４の実施形態とは異なり、光フィルタも不要である。
【００７６】
（第１の変形例）
上記の各実施形態において、第２の光変調器１１３は、第３の光導波路１０８の途上に挿入され、局発光源１０３から出力される無変調光Ｌ２または第４の光変調器５１６から出力される光信号Ｌ２を周波数変換信号Ｓｘによって光学的に変調しているが、これに代えて、第２の光変調器１１３を第１の光導波路１０６の途上に挿入することにより、第１の光変調器１０２から出力される光信号Ｌ１を周波数変換信号Ｓｘによって光学的に変調するようにしてもよい。図９は、このような周波数変調装置の構成の一例を示すブロック図である。この周波数変調装置は、図１に示した第１の実施形態の変形例（以下「第１の変形例」という）であって、第１の実施形態において第３の光導波路１０８の途上に挿入されていた第２の光変調器１１３を、第１の光導波路１０６の途上に備えている。
【００７７】
第１の実施形態等では、第２の光変調器１１３は、周波数ｆx （＝｜ｆｓ−ｆL ｜）の周波数変換信号Ｓｘで光周波数ｆ２の無変調光Ｌ２を光振幅変調または光強度変調することにより、図１０（Ａ）に示すような光周波数スペクトルを有する光変調信号Ｌ2xを生成する（ただしｆ１＞ｆ２とする）。
【００７８】
これに対し、第１の変形例の周波数変調装置では、第２の光変調器１１３は、周波数ｆx （＝｜ｆｓ−ｆL ｜）の周波数変換信号Ｓｘで中心周波数ｆ１の光信号Ｌ１を光振幅変調または光強度変調することにより、図１０（Ｂ）に示すような光周波数スペクトルを有する光変調信号Ｌ1xを生成する（同様にｆ１＞ｆ２とする）。第１の変形例における第２の光受信器１１４は、この光変調信号Ｌ1xと局発光源１０３から出力される周波数ｆ２の無変調光Ｌ２とを受け取って、その自乗検波特性に基づく光ヘテロダイン検波により、周波数ｆL （＝｜ｆｓ−ｆx ｜）を中心周波数とするビート信号としてＦＭ変調信号Ｓfmを生成する。すなわち、第１の実施形態において得られるＦＭ変調信号Ｓfmと実質的に同一のＦＭ変調信号が生成される。したがって、第１の変形例の構成によっても、第１の実施形態の場合等と同様、光源の位相雑音に依存することなく、位相雑音のない高周波・広帯域ＦＭ変調信号を生成することができる。
【００７９】
（第２の変形例）
以上説明した各実施形態において、より望ましくは、第１の光変調器１０２（正確にはこの光変調器１０２を構成する半導体レーザ等の光源）から出力された第１の光Ｌ１が第１の光導波路１０６を介して直接に第２の光受信器１１４に到達するまでの伝搬時間と、その光Ｌ１が第２の光導波路１０７、第１の光受信器１１０および第２の光変調器１１３等を介して第２の光受信器１１４に到達するまでの伝搬時間とが互いに等しく、かつ、局発光源１０３または第４の光変調器５１６（正確にはこの光変調器５１６を構成する光源）から出力された第２の光Ｌ２が第３の光導波路１０８および第２の光変調器１１３を介して第２の光受信器１１４に到達するまでの伝搬時間と、その光Ｌ２が第４の光導波路１０９、第１の光受信器１１０および第２の光変調器１１３等を介して第２の光受信器１１４に到達するまでの伝搬時間とが、互いに等しくなるように設定する。
【００８０】
例えば、上記第１および第２の光Ｌ１，Ｌ２の伝搬経路を構成する光導波路等の光学的経路または電気的経路に１つまたは複数の光学的または電気的な遅延回路を伝搬時間の調整手段として挿入することにより、このような設定を実現することができる。この伝搬時間の調整手段としては、コストなどの面で、光学的な調整手段よりも電気的な調整手段の方が好ましい。図１１は、図１に示した第１の実施形態の変形例（以下「第２の変形例」という）の構成を示すブロック図であって、電気的な遅延回路で構成される遅延調整器７１０が上記調整手段として周波数変換器１１２と第２の光変調器１１３との間に挿入された周波数変調装置の構成を示している。なお、遅延調整器７１０の挿入位置は、上記位置に限定されるものではなく、周波数変換器１１２と第２の光変調器１１３との間に代えて、例えば、フィルタ１１１と周波数変換器１１２との間、または、第１の光受信器１１とフィルタ１１１との間に、遅延調整器７１０を挿入してもよい。
【００８１】
このような伝搬時間の調整手段によって上記の設定を実現することにより、第２の光受信器１１４から出力されるＦＭ変調信号Ｓfmの位相雑音をより確実に抑圧し、さらに高品質な高周波・広帯域ＦＭ変調信号を生成することができる。
【００８２】
（第３および第４の変形例）
上記の第２の実施形態および第３の実施形態において、より望ましくは、第２の光受信器１１４において生成される第１のＩＭ−ＤＤ成分と第２のＩＭ−ＤＤ成分とが、あるいは第１のＩＭ−ＤＤ成分と第３のＩＭ−ＤＤ成分とが、互いに逆相でかつ振幅が等しくなるように設定する。より具体的には、それらのＩＭ−ＤＤ成分の位相を調整すべく、分波器３０１で分岐した一方の信号である第１の電気信号Ｓi1が、第１の光変調器１０２により光信号Ｌ１に変換され、第１の光導波路１０６を介して第２の光受信器１１４に到達するまでの伝搬時間と、分波器３０１で分岐した他方の信号である第２の電気信号Ｓi2が、第３の光変調器３１５または第４の光変調器５１６によって光信号Ｌ３またはＬ２に変換され、第５の光導波路３１７または第３の光導波路１０８を介して第２の光受信器１１４に到達するまでの伝搬時間とが、互いに等しくなるように設定する。また、第２の光受信器１１４が受け取る光信号において第１のＩＭ−ＤＤ成分と第２のＩＭ−ＤＤ成分にそれぞれ対応する光強度変調成分の振幅が互いに等しく、または、第１のＩＭ−ＤＤ成分と第３のＩＭ−ＤＤ成分にそれぞれ対応する光強度変調成分の振幅が互いに等しくなるように設定する。
【００８３】
例えば、上記の信号の伝搬経路を構成する電気的経路または光学的経路（光導波路）に、１つまたは複数の電気的または光学的な遅延回路および可変利得増幅器（または減衰器）などを信号の位相および振幅の調整手段として挿入することにより、このような設定を実現することができる。このような位相および振幅の調整手段としては、コストなどの面で、光学的な調整手段よりも電気的な調整手段の方が好ましい。図１２は、図３に示した第２の実施形態の変形例（以下「第３の変形例」という）の構成を示すブロック図であって、電気的な遅延回路と可変利得増幅器で構成される遅延・振幅調整器７２０が上記調整手段として分波器３０１と第３の光変調器３１５との間に挿入された周波数変調装置の構成を示している。また、図１３は、図５に示した第３の実施形態の変形例（以下「第４の変形例」という）の構成を示すブロック図であって、上記の調整手段として同様の構成の遅延・振幅調整器７３０が分波器３０１と第４の光変調器５１６との間に挿入された周波数変調装置の構成を示している。なお、遅延・振幅調整器７２０、７３０の挿入位置は、上記位置に限定されるものではなく、例えば、分波器３０１と第１の光変調器１０２との間に挿入してもよい。
【００８４】
このような位相および振幅の調整手段によって上記の設定を実現することにより、第２の光受信器１１４から出力されるＩＭ−ＤＤ成分がより確実に抑圧され、さらに高品質な高周波・広帯域ＦＭ変調信号を生成することができる。
【図面の簡単な説明】
【図１】本発明の第１の実施形態に係る周波数変調装置の構成を示すブロック図。
【図２】第１の実施形態に係る周波数変調装置における第１の光受信器からの出力信号の周波数スペクトルを示す模式図（Ａ）、第１の実施形態に係る周波数変調装置における周波数変換器の入出力信号の周波数スペクトルを示す模式図（Ｂ）、および、第１の実施形態に係る周波数変調装置における第２の光受信器からの出力信号の周波数スペクトルを示す模式図（Ｃ）。
【図３】本発明の第２の実施形態に係る周波数変調装置の構成を示すブロック図。
【図４】第２の実施形態に係る周波数変調装置において第１および第２の光変調器から出力され第２の光受信器に入力される光信号に対応する検波後の信号の周波数スペクトルを示す模式図（Ａ）、第２の実施形態に係る周波数変調装置において第３の光変調器から出力され第２の光受信器に入力される光信号に対応する検波後の信号の周波数スペクトルを示す模式図（Ｂ）、および、第２の実施形態に係る周波数変調装置における第２の光受信器からの出力信号の周波数スペクトルを示す模式図（Ｃ）。
【図５】本発明の第３の実施形態に係る周波数変調装置の構成を示すブロック図。
【図６】本発明の第４の実施形態に係る周波数変調装置の構成を示すブロック図。
【図７】第４の実施形態に係る周波数変調装置の他の構成を示すブロック図。
【図８】本発明の第５の実施形態に係る周波数変調装置の構成を示すブロック図。
【図９】第１の実施形態の変形例である第１の変形例に係る周波数変調装置の構成を示すブロック図。
【図１０】第１、第２、第４および第５の各実施形態に係る周波数変調装置において第２の光受信器に入力される光信号の光周波数スペクトルを示す模式図（Ａ）、および、第１の変形例に係る周波数変調装置において第２の光受信器に入力される光信号の光周波数スペクトルを示す模式図（Ｂ）。
【図１１】第１の実施形態の他の変形例である第２の変形例に係る周波数変調装置の構成を示すブロック図。
【図１２】第２の実施形態の変形例である第３の変形例に係る周波数変調装置の構成を示すブロック図。
【図１３】第３の実施形態の変形例である第４の変形例に係る周波数変調装置の構成を示すブロック図。
【図１４】従来の周波数変調装置の構成を示すブロック図。
【図１５】従来の周波数変調装置における光受信器に入力される光信号の光周波数スペクトルを示す模式図（Ａ）、および、従来の周波数変調装置における光受信器からの出力信号の周波数スペクトルを示す模式図（Ｂ）。
【符号の説明】
１００ …信号源
１０２ …第１の光変調器
１０２ｅ…外部光変調器
１０３ …局発光源
１０４ …第１の光分岐回路
１０５ …第２の光分岐回路
１０６ …第１の光導波路
１０７ …第２の光導波路
１０８ …第３の光導波路
１０９ …第４の光導波路
１１０ …第１の光受信器
１１１ …フィルタ
１１２ …周波数変換器
１１３ …第２の光変調器
１１４ …第２の光受信器
２００ …ビート信号生成部
３０１ …分波器
３１５ …第３の光変調器
５１６ …第４の光変調器
６１７，６２７…光フィルタ
７１０ …遅延調整器
７２０，７３０…遅延・振幅調整器
Ｓｉ …（入力）電気信号
Ｓｂfm …変調ビート信号
Ｓｂ …無変調ビート信号
Ｓｘ …周波数変換信号
Ｓfm …ＦＭ変調信号（周波数変調信号）[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a modulation device that generates a wideband frequency modulation signal (hereinafter referred to as “FM modulation signal”) using optical frequency modulation and optical heterodyne detection by a semiconductor laser.
[0002]
[Prior art]
FIG. 14 is a block diagram showing a configuration of a conventional frequency modulation device. This frequency modulation apparatus is described in, for example, literature (K. Kikushima, et al, “Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems”, IOOC'95 Technical Digest, Vol. 5 PD2-7. , pp.33-34) explains the operation in detail. This frequency modulation apparatus shown in FIG. 14 includes a signal source 700, an optical modulator 702, a local light source 703, a first optical waveguide 706, a second optical waveguide 708, and an optical receiver 714. I have.
[0003]
In the above frequency modulation apparatus, the signal source 700 outputs an electric signal Si that is an original signal to be FM modulated. The optical modulator 702 is composed of, for example, a semiconductor laser. In general, a semiconductor laser outputs light having a constant optical frequency f1 under the condition of a constant injection current. When the injection current is amplitude-modulated, the optical frequency is also modulated, and an optical frequency modulation signal centered on the optical frequency f1. Is output. Due to this property, the optical modulator 702 converts the electric signal Si input from the signal source 700 into an optical frequency modulation signal L1 and outputs it. The local light source 703 outputs unmodulated light L2 having a constant optical frequency f2. The lights L1 and L2 output from the optical modulator 702 and the local light source 703 are input to the optical receiver 714 via the first optical waveguide 706 and the second optical waveguide 708, respectively. The optical receiver 714 includes a photodiode having a square detection characteristic, and the beat signal of the two lights at a frequency fs = | f1-f2 | corresponding to the optical frequency difference between the two lights L1 and L2 inputted. (This operation is called “optical heterodyne detection”). The beat signal thus obtained becomes a frequency modulation signal Sfm using the electric signal Si from the signal source 700 as an original signal.
[0004]
As described above, in the conventional frequency modulation apparatus shown in FIG. 14, by using the high frequency modulation efficiency of the semiconductor laser (more than 10 times the frequency modulation efficiency in the case of a general electric circuit system), It is possible to easily generate an FM modulation signal having a very high frequency and a wide band (a large amount of frequency deviation) that is difficult to create with an electric circuit.
[0005]
[Problems to be solved by the invention]
However, since a light source such as a semiconductor laser generally has a larger phase noise than an electric oscillator, the above-described conventional frequency modulation apparatus has a characteristic that a white noise component increases when demodulating an FM modulation signal generated thereby. Have problems. That is, when the optical frequency modulation signal L1 from the optical modulator 702 and the unmodulated light L2 from the local light source 703 have a frequency spectrum as shown in FIG. 15A, they are output from the optical receiver 714. The frequency spectrum of the FM modulation signal Sfm is as shown in FIG. As shown in FIGS. 15A and 15B, the phase noise included in the FM modulated signal Sfm is the sum of the phase noises included in the optical frequency modulated signal L1 and the unmodulated light L2, respectively. When demodulating the FM modulation signal Sfm, the white noise component increases.
[0006]
Therefore, the object of the present invention is to suppress the phase noise contained in the generated frequency modulation signal while realizing high frequency and wideband frequency modulation by combining the optical frequency modulation by the semiconductor laser and the optical heterodyne detection. And providing a frequency modulation device having excellent noise characteristics.
[0007]
[Means for Solving the Problems and Effects of the Invention]
The first invention converts an input electric signal into a frequency modulation signal by optical frequency modulation and optical heterodyne detection using first and second light sources that output first and second light beams having different optical frequencies, respectively. A frequency modulation device that outputs an FM modulated signal,
A first optical modulator that outputs the first light frequency-modulated with the input electrical signal as a first optical signal;
A beat signal generating unit that generates an unmodulated beat signal corresponding to a carrier component of a beat signal obtained by optical detection based on square detection characteristics from the first and second lights;
A frequency converter that generates a frequency converted signal by converting the frequency of the unmodulated beat signal;
A second optical modulator that generates a second optical signal by performing optical amplitude modulation or optical intensity modulation of one of the first optical signal and the second light with the frequency conversion signal;
An optical receiver that receives the other of the first optical signal and the second light and the second optical signal, and generates the FM modulated signal by optical detection based on square detection characteristics;
Is provided.
[0008]
According to the first aspect of the present invention, two optical heterodyne systems are realized by two light sources (first and second light sources) and two square detectors (beat signal generator and optical receiver). Carrier component (center frequency) of the beat signal generated in the first optical heterodyne system composed of the first optical modulator corresponding to the light source, the second light source corresponding to the local light source, and the beat signal generator. Component) is a frequency conversion signal obtained by frequency conversion, and the two lights in the second optical heterodyne system composed of the first optical modulator, the second light source (local light source), and the optical receiver. One of them is amplitude-modulated or intensity-modulated. Thereby, the phase noise of the FM modulation signal which is a beat signal generated in the second optical heterodyne system can be suppressed, and frequency modulation with excellent noise characteristics can be realized.
[0009]
According to a second invention, in the first invention,
The first optical modulator generates the first optical signal by direct modulation;
The beat signal generator
Modulation having a center frequency corresponding to the difference between the optical frequencies of the first optical signal and the second light by receiving the first optical signal and the second light and performing optical detection based on square detection characteristics A light receiving element that generates a modulated beat signal that is an electrical signal,
A filter that extracts the carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal.
[0010]
According to the second invention, a first optical signal is generated by direct modulation, and an unmodulated beat signal, which is a carrier component, is generated from the modulated beat signal generated from the first optical signal and the second light. One of the two lights in the second optical heterodyne system is amplitude-modulated or intensity-modulated by a frequency-converted signal extracted by a filter and obtained by frequency-converting the unmodulated beat signal. Thereby, it is possible to suppress the phase noise of the FM modulated signal generated by the optical receiver and realize frequency modulation with excellent noise characteristics.
[0011]
According to a third invention, in the first invention,
The first optical modulator includes:
The first light source for outputting the first light which is unmodulated light;
An external optical modulator that generates the first optical signal by modulating the first light output from the first light source with the input electrical signal;
The second light source outputs unmodulated light as the second light,
The beat signal generation unit includes a light receiving element that receives the first and second lights, which are unmodulated light, and generates the unmodulated beat signal by optical detection based on a square detection characteristic.
[0012]
According to the third aspect of the invention, the first light is converted from the first light to the modulated light by the external modulation, and the beat signal generation unit generates the non-modulated light from the first and second lights. A modulated beat signal is generated. This eliminates the need for a filter for extracting the carrier component from the modulated beat signal.
[0013]
According to a fourth invention, in the first invention,
The first optical modulator generates the first optical signal having a predetermined center frequency f1 by uniquely converting an amplitude change of the input electric signal into a change of the optical frequency of the first light. ,
The second light source outputs light having a predetermined frequency f2 as the second light,
The beat signal generator
By receiving the first optical signal and the second light and performing optical detection based on square detection characteristics, a frequency fs = | corresponding to the difference between the optical frequencies of the first optical signal and the second light. a light receiving element that generates a modulated beat signal that is a modulated electric signal at f1-f2 |
A filter that extracts the carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal;
The frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency converted signal.
The second optical modulator performs optical amplitude modulation or optical intensity modulation on one of the first optical signal and the second light with the frequency conversion signal having a frequency fx, thereby converting the second optical signal. Generate and
The optical receiver receives the other of the first optical signal and the second optical signal and the second optical signal, and performs optical detection based on a square detection characteristic, thereby performing frequency fL = | fs−fx. The FM modulation signal is generated at |.
[0014]
According to the fourth aspect of the invention, the input electrical signal is converted into the optical frequency modulation signal by the first optical modulator, the second light source corresponding to the first optical modulator, the local light source, and the beat signal generation unit. In the first optical heterodyne system composed of: a carrier component that is an unmodulated beat signal output from the beat signal generation unit is frequency-converted, and the first optical modulator and the second light source (local light source) In the second optical heterodyne system composed of the optical receiver and the optical receiver, either of the light output from the first optical modulator or the second light source is amplitude-modulated with the above-mentioned frequency-converted carrier component Or intensity modulated. As a result, it is possible to suppress the phase noise of the FM modulation signal that is the beat signal output from the optical receiver, and to realize frequency modulation with excellent noise characteristics.
[0015]
According to a fifth invention, in the first invention,
A duplexer for branching the input electrical signal into first and second electrical signals of opposite phases;
A third optical modulator that converts the second electrical signal into a third optical signal by optical intensity modulation;
Further comprising
The first optical modulator uniquely converts an amplitude change of the first electric signal into a change in the optical frequency of the first light by direct modulation, whereby the first optical modulator having a predetermined center frequency f1 is converted. Generate optical signals,
The second light source outputs light having a predetermined frequency f2 as the second light,
The beat signal generator
By receiving the first optical signal and the second light and performing optical detection based on square detection characteristics, a frequency fs = | corresponding to the difference between the optical frequencies of the first optical signal and the second light. a light receiving element that generates a modulated beat signal that is a modulated electric signal at f1-f2 |
A filter that extracts the carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal;
The frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency converted signal.
The second optical modulator performs optical amplitude modulation or optical intensity modulation on one of the first optical signal and the second light with the frequency conversion signal having a frequency fx, thereby converting the second optical signal. Generate and
The optical receiver receives the other of the first optical signal and the second optical signal and the second optical signal, and performs optical detection based on a square detection characteristic, thereby performing frequency fL = | fs−fx. The FM modulation signal is generated at |, the third optical signal is received, and an electrical signal corresponding to a light intensity modulation component included in the third optical signal is generated by the optical detection. And
[0016]
According to the fifth aspect of the invention, the input electric signal that is the original signal of the frequency modulation branches into the first and second electric signals having opposite phases, and the first optical signal is optically transmitted by the first optical modulator. In the first optical heterodyne system, which is converted into a first optical signal that is a frequency modulation signal, and includes a first light modulator, a second light source corresponding to a local light source, and a beat signal generation unit. A second optical heterodyne composed of a first optical modulator, a second light source (local light source), and an optical receiver, with a carrier wave component, which is an unmodulated beat signal output from the signal generator, frequency-converted. In the system, one of the light output from the first light modulator or the second light source is amplitude-modulated or intensity-modulated with the above-mentioned frequency-converted carrier wave component. Further, the second electric signal is converted into a third optical signal which is an optical intensity modulation signal by the third optical modulator and is input to the optical receiver, where the intensity modulation-direct detection component (IM-) is detected by square detection. DD component) is generated. With this IM-DD component, the IM-DD component corresponding to the light intensity modulation component (which occurs due to direct modulation) included in the first optical signal is canceled in the optical receiver. As a result, it is possible to suppress the phase noise of the FM modulation signal, which is the beat signal output from the optical receiver, and to realize a high-quality frequency modulation with reduced unnecessary components.
[0017]
According to a sixth invention, in the fifth invention,
A first IM-DD component corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver; Phase / amplitude adjustment in which the second IM-DD component corresponding to the light intensity modulation component included in the third optical signal among the generated intensity modulation-direct detection components is opposite in phase and has the same amplitude. The apparatus further comprises means.
[0018]
According to the sixth aspect of the invention, the first IM-DD component corresponding to the light intensity modulation component included in the first optical signal and the second IM corresponding to the light intensity modulation component included in the third optical signal. The phase and the amplitude of the IM-DD component are adjusted so that the IM-DD components have the same phase and the same amplitude. For this reason, the IM-DD component output from the optical receiver is more reliably suppressed, and a higher-quality high-frequency / wideband FM modulated signal can be generated.
[0019]
According to a seventh invention, in the first invention,
A duplexer for branching the input electrical signal into first and second electrical signals having opposite phases;
A fourth optical modulator that outputs the second light, which is modulated light having a predetermined center frequency f2, as a fourth optical signal;
Further comprising
The first optical modulator uniquely converts an amplitude change of the first electric signal into a change in the optical frequency of the first light by direct modulation, whereby the first optical modulator having a predetermined center frequency f1 is converted. Generate optical signals,
The fourth optical modulator uniquely converts an amplitude change of the second electric signal into a change in the optical frequency of the second light by direct modulation, whereby the fourth optical modulator having a predetermined center frequency f2 is converted. Generate optical signals,
The beat signal generator
The first and fourth optical signals are received and modulated at a frequency fs = | f1-f2 | corresponding to a difference in optical frequency between the first and fourth optical signals by optical detection based on square detection characteristics. A light receiving element that generates a modulated beat signal that is an electrical signal,
A filter that extracts the carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal;
The frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency converted signal.
The second optical modulator performs optical amplitude modulation or optical intensity modulation of one of the first optical signal and the fourth optical signal with the frequency conversion signal having a frequency fx, thereby converting the second optical signal. Generate and
The optical receiver receives the other one of the first optical signal and the fourth optical signal and the second optical signal, and performs optical detection based on a square detection characteristic to generate a frequency fL = | fs−fx. The FM modulation signal is generated at |.
[0020]
According to the seventh aspect of the invention, the input electric signal that is the original signal of the frequency modulation is branched into the first and second electric signals having opposite phases, and the first signal is directly modulated by the first optical modulator. The electric signal is converted into a first optical signal that is an optical frequency modulation signal, and the second electric signal is converted into a fourth optical signal that is an optical frequency modulation signal by direct modulation in the fourth optical modulator. . Then, in the first optical heterodyne system composed of the first optical modulator, the fourth optical modulator, and the beat signal generation unit, a carrier wave component that is an unmodulated beat signal output from the beat signal generation unit is Frequency-converted and output from the first optical modulator or the fourth optical modulator in the second optical heterodyne system including the first optical modulator, the fourth optical modulator, and the optical receiver. Any one of the light beams to be modulated is amplitude-modulated or intensity-modulated by the above-mentioned frequency-converted carrier component. As a result, it is possible to suppress the phase noise of the FM modulation signal that is the beat signal output from the optical receiver, and to realize frequency modulation with excellent noise characteristics. In addition, as described above, since the frequency modulation signal is generated by the push-pull operation of the first optical modulator and the fourth optical modulator, it is unnecessary due to the light intensity modulation component generated in the direct modulation. The component IM-DD component is canceled out. Thereby, it is possible to realize frequency modulation with good quality with reduced unnecessary components.
[0021]
In an eighth aspect based on the seventh aspect,
A first IM-DD component corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver; Phase / amplitude adjustment in which the third IM-DD component corresponding to the light intensity modulation component included in the fourth optical signal among the generated intensity modulation-direct detection components is opposite in phase and has the same amplitude. The apparatus further comprises means.
[0022]
According to the eighth aspect of the invention, the first IM-DD component corresponding to the light intensity modulation component included in the first optical signal and the third light intensity modulation component included in the fourth optical signal. The phase and the amplitude of the IM-DD component are adjusted so that the IM-DD components have the same phase and the same amplitude. For this reason, the IM-DD component output from the optical receiver is more reliably suppressed, and a higher-quality high-frequency / wideband FM modulated signal can be generated.
[0023]
According to a ninth invention, in the first invention,
An optical filter that is inserted between the first optical modulator and the beat signal generation unit and extracts the optical carrier component from the first optical signal;
The second light source outputs unmodulated light as the second light,
The beat signal generation unit includes a light receiving element that receives the optical carrier component extracted by the optical filter and the second light, and generates the unmodulated beat signal by optical detection based on a square detection characteristic. Features.
[0024]
According to the ninth aspect, the first optical modulator converts the input electrical signal into the first optical signal that is an optical frequency modulation signal, and the second optical modulator and the second light source corresponding to the local light source. In the first optical heterodyne system composed of the light source and the beat signal generation unit, the light is converted from the first optical signal by the optical filter inserted between the first optical modulator and the beat signal generation unit. A carrier wave component is extracted, and the beat signal generation unit generates an unmodulated beat signal from the optical carrier wave component and the second light without using a filter, and frequency-converts the unmodulated beat signal, that is, the carrier wave component. A frequency converted signal is generated. In the second optical heterodyne system including the first optical modulator, the second light source, and the optical receiver, one of the lights output from the first optical modulator or the second light source. Is amplitude-modulated or intensity-modulated by the above-described frequency conversion signal (frequency-converted carrier wave component). As a result, it is possible to suppress the phase noise of the FM modulation signal that is the beat signal output from the optical receiver, and to realize frequency modulation with excellent noise characteristics.
[0025]
In a tenth aspect based on the first aspect,
A duplexer for branching the input electrical signal into first and second electrical signals having opposite phases;
A fourth optical modulator that outputs the second light, which is modulated light having a predetermined center frequency f2, as a fourth optical signal;
A first optical filter inserted between the first optical modulator and the beat signal generator;
A second optical filter inserted between the fourth optical modulator and the beat signal generator;
Further comprising
The first optical modulator uniquely converts an amplitude change of the first electric signal into a change in the optical frequency of the first light by direct modulation, whereby the first optical modulator having a predetermined center frequency f1 is converted. Generate optical signals,
The fourth optical modulator uniquely converts an amplitude change of the second electric signal into a change in the optical frequency of the second light by direct modulation, whereby the fourth optical modulator having a predetermined center frequency f2 is converted. Generate optical signals,
The first optical filter extracts the optical carrier component from the first optical signal, the second optical filter extracts the optical carrier component from the fourth optical signal, and generates the beat signal. The unit receives both the optical carrier components extracted by the first and second optical filters, and performs optical detection based on the square detection characteristic to obtain a frequency fs = | corresponding to the difference between the optical frequencies of the two optical carrier components. a light receiving element for generating the unmodulated beat signal at f1-f2 |
The frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency converted signal.
The second optical modulator performs optical amplitude modulation or optical intensity modulation of one of the first optical signal and the fourth optical signal with the frequency conversion signal having a frequency fx, thereby converting the second optical signal. Generate and
The optical receiver receives the other one of the first optical signal and the fourth optical signal and the second optical signal, and performs optical detection based on a square detection characteristic to generate a frequency fL = | fs−fx. The FM modulation signal is generated at |.
[0026]
According to the tenth aspect of the invention, the input electric signal that is the original signal of the frequency modulation is branched into the first and second electric signals having opposite phases, and the first signal is directly modulated by the first optical modulator. The electric signal is converted into a first optical signal that is an optical frequency modulation signal, and the second electric signal is converted into a fourth optical signal that is an optical frequency modulation signal by direct modulation in the fourth optical modulator. . In the first optical heterodyne system including the first optical modulator, the fourth optical modulator, and the beat signal generation unit, the optical carrier component is generated from the first optical signal by the first optical filter. The extracted optical carrier component is extracted from the fourth optical signal by the second optical filter, and the beat signal generation unit generates an unmodulated beat signal from these optical carrier components without using a filter. A frequency conversion signal is generated by frequency-converting this unmodulated beat signal, that is, a carrier wave component, and in the second optical heterodyne system including the first optical modulator, the fourth optical modulator, and the optical receiver. One of the light output from the first optical modulator or the fourth optical modulator is amplitude-modulated or intensity-modulated by the frequency conversion signal (carrier component subjected to frequency conversion). As a result, it is possible to suppress the phase noise of the FM modulation signal, which is the beat signal output from the optical receiver, and to realize a high-quality frequency modulation with reduced unnecessary components.
[0027]
In an eleventh aspect based on the tenth aspect,
A first IM-DD component corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver; Phase / amplitude adjustment in which the third IM-DD component corresponding to the light intensity modulation component included in the fourth optical signal among the generated intensity modulation-direct detection components is opposite in phase and has the same amplitude. The apparatus further comprises means.
[0028]
According to the eleventh aspect, similar to the eighth aspect, the first IM-DD component corresponding to the light intensity modulation component included in the first optical signal and the light included in the fourth optical signal. The phase and amplitude are adjusted such that the third IM-DD component corresponding to the intensity modulation component has the same phase and opposite phase. For this reason, the IM-DD component output from the optical receiver is more reliably suppressed, and a higher-quality high-frequency / wideband FM modulated signal can be generated.
[0029]
In a twelfth aspect based on the first aspect,
Light and electrical propagation time of a path from the first light source to the optical receiver via the beat signal generation unit, and light propagation time of a path from the first light source directly to the optical receiver First propagation time adjustment means equal to each other;
A light and electrical propagation time of a path from the second light source through the beat signal generation unit to the optical receiver, and an optical propagation time of a path from the second light source directly to the optical receiver. It is further characterized by further comprising second propagation time adjusting means for equalizing each other.
[0030]
According to the twelfth aspect of the invention, each of the optical signals output from the first light source and branched into two passes through each constituent part, or undergoes processing such as photoelectric conversion or electro-optical conversion, and the light The propagation time until reaching the receiver is set to be equal to each other, and each of the light branched from the light output from the second light source passes through each component, or photoelectric conversion, Propagation times until reaching the optical receiver after processing such as electro-optical conversion are set to be equal to each other. As a result, it is possible to more appropriately suppress the phase noise of the FM modulation signal, which is a beat signal output from the optical receiver, and to realize frequency modulation with excellent noise characteristics.
[0031]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
[0032]
(First embodiment)
A frequency modulation apparatus according to a first embodiment of the present invention will be described below with reference to FIGS. 1 and 2A to 2C.
[0033]
FIG. 1 is a block diagram showing the configuration of the frequency modulation apparatus according to the first embodiment. This frequency modulation device includes a signal source 100, a first optical modulator 102, a local light source 103, a first optical branch circuit 104, a second optical branch circuit 105, and a first optical waveguide 106. A second optical waveguide 107, a third optical waveguide 108, a fourth optical waveguide 109, a beat signal generation unit 200 including a first optical receiver 110 and a filter 111, and a frequency converter 112. The second optical modulator 113 and the second optical receiver 114 are provided. This frequency modulation apparatus uses optical frequency modulation using two light sources (first and second light sources) that respectively output two lights having different optical frequencies (first light L1 and second light L2). By optical heterodyne detection, the electric signal Si output from the signal source 100 is converted into a frequency modulation signal, which is output as an FM modulation signal Sfm.
[0034]
Next, the operation of the frequency modulation apparatus shown in FIG. 1 will be described in detail. The first optical modulator 102 is a direct modulation type optical modulator, and is usually composed of a semiconductor laser that is a first light source having an optical frequency modulation effect, and an electric signal Si output from the signal source 100. Thus, the amplitude of the injection current into the semiconductor laser is modulated to output the optical frequency modulation signal L1 having the center optical frequency f1 as the first light. The optical signal L1 output from the first optical modulator 102 is branched into two optical signals by the first optical branching circuit 104, and one of these optical signals beats via the second optical waveguide 107. The other optical signal is input to the first optical receiver 110 in the signal generator 200 and the second optical receiver 114 via the first optical waveguide 106. The local light source 103 corresponds to a second light source, and outputs unmodulated light L2 having an optical frequency f2 as second light. The light L 2 output from the local light source 103 is branched into two lights by the second optical branch circuit 105, and one of these lights is passed through the fourth optical waveguide 109 to the second signal in the beat signal generation unit 200. The other light is input to the first optical receiver 110 and the second optical modulator 113 via the third optical waveguide 108.
[0035]
The beat signal generation unit 200 receives the optical signal L1 that is the first light branched by the first optical branch circuit 104 and the second light L2 that is branched by the second optical branch circuit 105, and performs the following operation. An unmodulated electric signal (hereinafter referred to as “unmodulated beat signal”) Sb having a frequency fs = | f1−f2 | corresponding to the difference between the optical frequencies of the first and second lights L1 and L2 is generated. That is, the first optical receiver 110 is composed of a light receiving element such as a photodiode having square detection characteristics, and receives the optical signal L1 from the first optical modulator 102 and the light L2 from the local light source 103. Thus, a beat signal (hereinafter referred to as “modulated beat signal”) Sbfm having the optical frequency difference fs (= | f1−f2 |) as a center frequency is generated by optical heterodyne detection based on the square detection characteristic. The modulation beat signal Sbfm is an FM modulation signal obtained by down-converting the optical frequency modulation signal L1 output from the first optical modulator 102. FIG. 2A shows the frequency spectrum of the modulated beat signal Sbfm when the electric signal Si output from the signal source 100 is assumed to be a signal centered on the frequency fp. This modulated beat signal Sbfm is input to the filter 111. This filter 111 is a band-pass filter, extracts only the carrier component (the component of the center frequency fs) of the modulated beat signal Sbfm, which is an FM modulated signal, and outputs it as an unmodulated beat signal Sb.
[0036]
The frequency converter 112 frequency-converts (for example, down-converts) the unmodulated beat signal Sb, which is a carrier wave component output from the filter 111, using, for example, the local oscillation signal SL of the frequency fL, and the frequency fx (= | fs−fL). |) Is output as the frequency conversion signal Sx. FIG. 2B shows the frequency spectrum of the unmodulated beat signal Sb that is the input signal of the frequency converter 112 and the frequency converted signal Sx that is the output signal together with the frequency spectrum of the local oscillation signal SL.
[0037]
  The second optical modulator 113 is3The unmodulated light L2 inserted and arranged in the middle of the optical waveguide 108 and output from the local light source 103 is optically modulated by the frequency conversion signal Sx output from the frequency converter 112. That is, the second optical modulator 113 converts the unmodulated light L2 into an optical modulation signal L2x by optical amplitude modulation or optical intensity modulation, and outputs this.
[0038]
The second optical receiver 114 is composed of a light receiving element such as a photodiode having square detection characteristics, and an optical signal L1 from the first optical modulator 102 and an optical signal L2x from the second optical modulator 113. And FM modulated signal Sfm is generated as a beat signal having a frequency fL (= | fs-fx |) as a center frequency by optical heterodyne detection based on the square detection characteristic. The FM modulation signal Sfm generated in this way is a signal obtained by down-converting the optical frequency modulation signal L1 output from the first optical modulator 102, and has phase noise as shown in FIG. It is suppressed.
[0039]
The phase noise suppression effect of the FM modulation signal in this embodiment will be described below using mathematical expressions.
The electric field component of the optical frequency modulation signal L1 output from the first optical modulator 102 is
E₁(t) = A₁[1 + m₁cos (2πf_pt)]^1/2cos [2πf₁t + ΔFsin (2πf_pt) + φ₁] (1)
The electric field component of the unmodulated light L2 output from the local light source 103 is
E₂(t) = A₂cos (2πf₂t + φ₂(2)
Respectively. Here, m1 represents the light (intensity) modulation degree in the first optical modulator 102, fp represents the frequency of the electric signal Si (assuming a sine wave) output from the signal source 100, and f1 and f2 Represents the optical frequencies of the light output from the first optical modulator 102 and the local light source 103, respectively, and φ1 and φ2 are the light output from the first optical modulator 102 and the local light source 103, respectively. Each represents phase noise, and ΔF represents a frequency shift amount given in the first optical modulator 102 corresponding to the electric signal Si output from the signal source 100.
[0040]
When the first optical receiver 110 receives the combined light of these two lights L1 and L2, based on the square detection characteristics of the light receiving element used in the first optical receiver 110, a photocurrent represented by the following equation is obtained. It flows to the light receiving element.
I₁(t) = R₁| E₁(t) + E₂(t) |²
= R₁A₁ ²[1 + m₁cos (2πf_pt)] cos²[2πf₁t + ΔFsin (2πf_pt) + φ₁]
+ R₁A₂ ²cos²(2πf₂t + φ₂)
+ 2R₁A₁A₂[1 + m₁cos (2πf_pt)]^1/2cos [2πf₁t + ΔFsin (2πf_pt) + φ₁]
× cos (2πf₂+ φ₂(3)
Here, R1 represents the conversion efficiency of the light receiving element used in the first optical receiver 110. The signal components corresponding to the first term and the second term in Expression (3) are not detected due to the frequency response limitation of the light receiving element used in the first optical receiver 110. The signal component corresponding to the first term also includes an IM-DD (intensity modulation-direct detection) component that is generated by directly detecting the light intensity modulation component in the optical signal L1 from the first optical modulator 102. Although included, this IM-DD component may be ignored because it is removed by the filter 111 described later.
[0041]
Further expanding the third term in equation (3),
I_c1(t) = R₂A₁A₂[1 + m₁cos (2πf_pt)]^1/2
× [cos [2π (f₁+ f₂) t + ΔFsin (2πf_pt) + φ₁+ φ₂]
+ Cos [2π (f₁-f₂) t + ΔFsin (2πf_pt) + φ₁-φ₂]] (4)
It becomes. The signal component corresponding to the first term in square brackets “[]” in the equation (4) is not detected by the frequency response limit of the light receiving element used in the first optical receiver 110, and the square brackets “[]”. Only the signal component corresponding to the second term is output. The second term represents an FM modulation signal in which the electric signal Si output from the signal source 100 is an original signal, the carrier frequency is fs = | f1-f2 |, and the frequency shift amount is ΔF. The FM modulation signal of the second term has a phase noise determined by the phase noise φ1 of the optical signal L1 output from the first optical modulator 102 and the phase noise φ2 of the light L2 output from the local light source 103. Have.
[0042]
The filter 111 is a carrier component (a component of the center frequency fs) in the FM modulation signal represented by the equation (4).
I_s(t) = cos [2π (f₁-f₂) t + φ₁-φ₂] (5)
Only the unmodulated beat signal Sb which is a signal component represented by
[0043]
The frequency converter 112 frequency-converts the unmodulated beat signal Sb expressed by the equation (5) using the local oscillation signal SL having the frequency fL. For example, by down-converting the unmodulated beat signal Sb, a signal having a frequency fx = | f1-f2-fL |
I_x(t) = cos [2π (f₁-f₂-f_L) t + φ₁-φ₂]
= cos (2πf_xt + φ₁-φ₂(6)
Is converted into a signal (frequency conversion signal Sx) expressed by
[0044]
The second optical modulator 113 performs, for example, optical amplitude modulation of the unmodulated light L2 (see Expression (2)) output from the local light source 103 by the frequency conversion signal Sx expressed by Expression (6), and generates an electric field. Ingredients
E_2x(t) = A₂cos (2πf_xt + φ₁-φ₂) cos (2πf₂t + φ₂(7)
An optical modulation signal L2x represented by
[0045]
The second optical receiver 114 includes an optical modulation signal L1 from the first optical modulator 102 (see equation (1)) and an optical modulation signal L2x from the second optical modulator 113 (see equation (7)). ) And based on its square detection characteristics,
I₂(t) = R₂| E₁(t) + E_2x(t) |²
= R₂A₁ ²[1 + m₁cos (2πf_pt)] cos²[2πf₁t + ΔFsin (2πf_pt) + φ₁]
+ R₂A₂ ²cos²(2πf_xt + φ₁-φ₂) cos²(2πf₂t + φ₂)
+ 2R₂A₁A₂[1 + m₁cos (2πf_pt)]^1/2cos [2πf₁t + ΔFsin (2πf_pt) + φ₁]
× cos (2πf_xt + φ₁-φ₂) cos (2πf₂+ φ₂(8)
The photocurrent represented by is output. Here, R2 represents the conversion efficiency of the light receiving element used in the second optical receiver 114. The signal components corresponding to the first term and the second term in Expression (8) are not detected due to the frequency response limitation of the light receiving element used in the second optical receiver 114. The signal component corresponding to the first term also includes an IM-DD component that is generated by directly detecting the light intensity modulation component in the optical signal L1 from the first optical modulator 102. If the frequency band of the DD component is far from the frequency band of the FM modulation signal Sfm, this component may be ignored. When the frequency band of this IM-DD component overlaps the frequency band of the FM modulation signal Sfm, this component can be suppressed by the configuration of another embodiment described later.
[0046]
Further expanding the third term in equation (8),
I_c2(t) = R₂A₁A₂[1 + m₁cos (2πf_pt)]^1/2
× [cos (2πf_xt + φ₁-φ₂) cos [2π (f₁+ f₂) t + ΔFsin (2πf_pt) + φ₁+ φ₂]
+ Cos (2πf_xt + φ₁-φ₂) cos [2π (f₁-f₂) t + ΔFsin (2πf_pt) + φ₁-φ₂]]
= (R₂A₁A₂/ 2) [1 + m₁cos (2πf_pt)]^1/2
× [cos [2π (f₁+ f₂+ f_x) t + ΔFsin (2πf_pt) + 2φ₁]
+ Cos [2π (f₁+ f₂-f_x) t + ΔFsin (2πf_pt) + 2φ₂]
+ Cos [2π (f₁-f₂+ f_x) t + ΔFsin (2πf_pt) +2 (φ₁-φ₂)]
+ Cos [2π (f₁-f₂-f_x) t + ΔFsin (2πf_pt)]]… (9)
It becomes. The signal components corresponding to the first and second terms in square brackets “[]” in equation (9) are not detected due to the frequency response limit of the light receiving element used in the second optical receiver 114, and the square brackets are used. Only signal components corresponding to the third and fourth terms in "[]" are output as the FM modulation signal Sfm. Of these, the fourth term represents an FM modulation signal in which the electric signal Si output from the signal source 100 is an original signal, the carrier frequency is fL = | f1-f2-fx |, and the frequency deviation is ΔF. Yes. The FM modulated signal of the fourth term is a signal having no phase noise without depending on the phase noises φ1 and φ2 of the lights L1 and L2 output from the first optical modulator 102 and the local light source 103, respectively.
[0047]
When the second optical modulator 113 modulates the light intensity of the unmodulated light L2 (see Expression (2)) output from the local light source 103 by the frequency conversion signal Sx expressed by Expression (6). The electric field component of the optical modulation signal output from the second optical modulator 113 is
E_2y(t) = A₂[1 + m₂cos (2πf_xt + φ₁-φ₂)]^1/2cos (2πf₂t + φ₂)
It is represented by The above equation can be approximated as:
E_2y(t) = A₂[1+ (m₂/ 2) cos (2πf_xt + φ₁-φ₂)] cos (2πf₂t + φ₂) ... (10)
Here, m 2 is a light (intensity) modulation degree in the second optical modulator 113. Since the first term in the braces “{}” in the expression (10) represents an unmodulated light component, only the second term is considered in the braces “{}”.
[0048]
The second optical receiver 114 includes the optical modulation signal L1 from the first optical modulator 102 (see Expression (1)) and the optical modulation signal L2x from the second optical modulator 113 (in Expression (10)). Based on the square detection characteristics of the curly braces “{}” (see item 2)
I₂(t) = R₂| E₁(t) + E_2y(t) |²
= R₂A₁ ²[1 + m₁cos (2πf_pt)] cos²[2πf₁t + ΔFsin (2πf_pt) + φ₁]
+ R₂A₂ ²(m₂ ²/ 4) cos²(2πf_xt + φ₁-φ₂) cos²(2πf₂t + φ₂)
+ 2R₂A₁A₂[1 + m₁cos (2πf_pt)]^1/2cos [2πf₁t + ΔFsin (2πf_pt) + φ₁]
× (m₂/ 2) cos (2πf_xt + φ₁-φ₂) cos (2πf₂+ φ₂(11)
The photocurrent represented by is output. Signal components corresponding to the first term and the second term in Expression (11) are not detected due to the frequency response limitation of the light receiving element used in the second optical receiver 114. The signal component corresponding to the first term also includes an IM-DD component that is generated by directly detecting the light intensity modulation component in the optical signal L1 from the first optical modulator 102. If the frequency band of the DD component is far from the frequency band of the FM modulation signal Sfm, this component may be ignored. When the frequency band of this IM-DD component overlaps the frequency band of the FM modulation signal Sfm, this component can be suppressed by the configuration of another embodiment described later.
[0049]
Further expanding the third term in equation (11),
I_c2(t) = (R₂A₁A₂m₂/ 2) [1 + m₁cos (2πf_pt)]^1/2
× [cos (2πf_xt + φ₁-φ₂) cos [2π (f₁+ f₂) t + ΔFsin (2πf_pt) + φ₁+ φ₂]
+ Cos (2πf_xt + φ₁-φ₂) cos [2π (f₁-f₂) t + ΔFsin (2πf_pt) + φ₁-φ₂]]
= (R₂A₁A₂m₂/ 4) [1 + m₁cos (2πf_pt)]^1/2
× [cos [2π (f₁+ f₂+ f_x) t + ΔFsin (2πf_pt) + 2φ₁]
+ Cos [2π (f₁+ f₂-f_x) t + ΔFsin (2πf_pt) + 2φ₂]
+ Cos [2π (f₁-f₂+ f_x) t + ΔFsin (2πf_pt) +2 (φ₁-φ₂)]
+ Cos [2π (f₁-f₂-f_x) t + ΔFsin (2πf_pt)]] ... (12)
It becomes. The fourth term in square brackets “[]” in Expression (12) is the electric signal Si output from the signal source 100 as an original signal, and the carrier frequency is fL = | f1-f2 as in Expression (9). −fx |, which represents an FM modulated signal with a frequency deviation amount ΔF. The FM modulated signal of the fourth term is a signal having no phase noise without depending on the phase noises φ1 and φ2 of the lights L1 and L2 output from the first optical modulator 102 and the local light source 103, respectively.
[0050]
As is clear from the comparison between Expression (9) and Expression (12), regarding the magnitude (amplitude) of the FM modulation signal L2x output from the second optical receiver 114, the second optical modulator 113 is used. The optical amplitude modulation is applied twice in FIG. 2 and is advantageous in CNR (carrier-to-noise power ratio) when transmitting this FM modulated signal. Therefore, hereinafter, only the case where optical amplitude modulation is applied in the second optical modulator 113 will be described.
[0051]
As described above, according to the first embodiment, in an optical heterodyne configuration using two light sources, a high-frequency / wideband FM modulated signal without phase noise is generated without depending on the phase noise of the light source. Can do.
[0052]
(Second Embodiment)
A frequency modulation apparatus according to the second embodiment of the present invention will be described below with reference to FIGS. 3 and 4A to 4C.
[0053]
FIG. 3 is a block diagram showing the configuration of the frequency modulation apparatus according to the second embodiment. This frequency modulation apparatus includes a duplexer 301, a third optical modulator 315, and a fifth optical waveguide 317 in addition to the configuration of the first embodiment shown in FIG. In FIG. 3, the same components as those in the first embodiment are denoted by the same reference numerals.
[0054]
Next, the operation of this embodiment shown in FIG. 3 will be described. However, since the configuration of the present embodiment conforms to the first embodiment described above, only the differences will be described below.
In the frequency modulation apparatus according to the present embodiment, the duplexer 301 branches the electric signal Si output from the signal source 100 into the first and second electric signals Si1 and Si2 so as to have an opposite phase relationship to each other. Output. The first optical modulator 102 converts the first electrical signal Si1 output from the duplexer 301 into an optical frequency modulation signal L1 and outputs the same as in the first embodiment. The third optical modulator 315 is usually composed of a semiconductor laser, converts the second electric signal Si2 output from the demultiplexer 301 into a light intensity modulation signal L3, and outputs it. The optical signal L3 is input to the second optical receiver 114 via the fifth optical waveguide 317.
[0055]
The second optical receiver 114 receives the optical signal L1 from the first optical modulator 102 and the optical signal L2x from the second optical modulator 113, and from optical heterodyne detection based on the square detection characteristics, An FM modulation signal Sfm is generated as a beat signal having the optical frequency difference between the optical signals L1 and L2x as a center frequency, and the light intensity modulation signal L3 from the third optical modulator 315 is received and square detection is performed. By direct detection based on characteristics, an IM-DD (intensity modulation-direct detection) component is generated from the light intensity modulation signal L3. When the first optical modulator 102 employs a direct modulation method using a semiconductor laser as described in the first embodiment, the optical signal L1 from the first optical modulator 102 is an optical frequency modulation component together with an optical frequency modulation component. An intensity modulation component is also included. Therefore, the second optical receiver 114 receives the optical signal L1 from the first optical modulator 102 and the optical signal L2x from the second optical modulator 113 as shown in FIG. In addition, an FM modulated signal that is a beat signal of the two optical signals is generated by optical heterodyne detection, and at the same time, a first optical signal corresponding to a light intensity modulation component included in the optical signal from the first optical modulator 102 is generated by direct detection. 1 IM-DD (intensity modulation-direct detection) component Simdd1 is generated. The first IM-DD component Simdd1 is an unnecessary component with respect to the FM modulation signal Sfm that is originally required. In particular, as shown in FIG. 4A, the occupied frequency band of the first IM-DD component Simdd1 is When it overlaps with the frequency band of the FM modulation signal Sfm, the quality of the FM modulation signal Sfm is greatly deteriorated. The second optical receiver 114 also receives the optical intensity modulation signal L3 from the third optical modulator 315 and corresponds to the optical intensity modulation signal L3 by direct detection as shown in FIG. 4B. The second IM-DD component Simdd2 having a phase opposite to that of the first IM-DD component Simdd1 is generated. By canceling the first IM-DD component Simdd1 with the second IM-DD component Simdd2, the first IM-DD component Simdd1 is suppressed as shown in FIG.
[0056]
As described above, according to the second embodiment, generation of an IM-DD component that is an unnecessary component due to the light intensity modulation component included in the optical signal L1 is suppressed, and a high-quality high-frequency signal without phase noise is obtained. A broadband FM modulated signal Sfm can be generated.
[0057]
(Third embodiment)
A frequency modulation apparatus according to the third embodiment of the present invention will be described below with reference to FIG.
[0058]
FIG. 5 is a block diagram showing the configuration of the frequency modulation apparatus according to the third embodiment. This frequency modulation apparatus includes a duplexer 301 and a fourth optical modulator 516 in place of the local light source 103 in the configuration of the first embodiment shown in FIG. In FIG. 5, the same components as those in the first embodiment are denoted by the same reference numerals.
[0059]
Next, the operation of this embodiment shown in FIG. 5 will be described.
In the frequency modulation apparatus according to the present embodiment, the duplexer 301 branches the electric signal Si output from the signal source 100 into the first and second electric signals Si1 and Si2 so as to have an opposite phase relationship to each other. Output. The first optical modulator 102 converts the first electrical signal Si1 output from the duplexer 301 into an optical frequency modulation signal L1 and outputs the same as in the first embodiment. The optical signal L1 output from the first optical modulator 102 is branched into two optical signals by the first optical branching circuit 104, and one of these optical signals beats via the second optical waveguide 107. The other optical signal is input to the first optical receiver 110 in the signal generator 200 and the second optical receiver 114 via the first optical waveguide 106.
[0060]
The fourth optical modulator 516 is an optical modulator of a direct modulation type, and is usually composed of a semiconductor laser or the like, and converts the second electric signal Si2 output from the demultiplexer 301 into an optical frequency modulation signal L2. Convert and output. Since the fourth optical modulator 516 employs a direct modulation method, the output optical signal L2 includes an optical intensity modulation component as well as an optical frequency modulation component, as in the first optical modulator 102. The optical signal L 2 output from the fourth optical modulator 516 is branched into two optical signals by the second optical branch circuit 105, and one of these optical signals beats via the fourth optical waveguide 109. The other optical signal is input to the first optical receiver 110 in the signal generator 200 and the second optical modulator 113 via the third optical waveguide 108.
[0061]
The beat signal generation unit 200 receives the optical signal L1 branched by the first optical branch circuit 104 and the optical signal L2 branched by the second optical branch circuit 105, and performs the first and second light by the following operation. An unmodulated electric signal Sb having a frequency fs = | f1-f2 | corresponding to the difference between the optical frequencies of the signals L1 and L2 is generated. That is, the first optical receiver 110 is composed of a light receiving element such as a photodiode having square detection characteristics, and the optical signal L1 from the first optical modulator 102 and the optical signal from the fourth optical modulator 516. L2 is received, and a modulated beat signal Sbfm having the optical frequency difference fs (= | f1-f2 |) as a center frequency is generated by optical heterodyne detection based on the square detection characteristic. The modulation beat signal Sbfm is an FM modulation signal obtained by down-converting the optical frequency modulation signal L1 output from the first optical modulator 102. The filter 111 extracts only the carrier wave component (the component of the center frequency fs) of the modulated beat signal Sbfm and outputs it as an unmodulated beat signal Sb.
[0062]
The frequency converter 112 performs frequency conversion (for example, down-conversion) on the unmodulated beat signal Sb, and outputs a frequency conversion signal Sx that is a signal having a frequency fx (= | fs−fL |).
[0063]
  The second optical modulator 113 is3The optical signal L2 inserted and arranged in the middle of the optical waveguide 108 and output from the fourth optical modulator 516 is optically modulated by the frequency conversion signal Sx output from the frequency converter 112. That is, the second optical modulator 113 converts the optical signal L2 into an optical modulation signal L2x by optical amplitude modulation or optical intensity modulation, and outputs this.
[0064]
The second optical receiver 114 receives the optical signal L1 from the first optical modulator 102 and the optical signal L2x from the second optical modulator 113, and performs optical heterodyne detection based on the square detection characteristics thereof. An FM modulated signal Sfm in which phase noise is suppressed is generated as a beat signal having a frequency fL (= | fs−fx |) as a center frequency (see FIG. 2C). In addition, the second optical receiver 114 performs first detection based on the square detection characteristic, so that the first IM-DD component corresponding to the light intensity modulation component included in the optical signal L1 from the first optical modulator 102 is obtained. And the optical intensity modulation component included in the optical signal L2 from the fourth optical modulator 516 is also received, and the first IM-DD component and the opposite phase are obtained by direct detection based on the square detection characteristic thereof. A third IM-DD component is generated. The first IM-DD component is canceled by the third IM-DD component, thereby obtaining the FM modulated signal Sfm in which the IM-DD component is suppressed.
[0065]
As described above, according to the third embodiment, generation of an IM-DD component, which is an unnecessary component due to the light intensity modulation component included in the optical signal L1, is suppressed, and high-quality high-frequency, phase noise is prevented. A broadband FM modulated signal Sfm can be generated.
[0066]
(Fourth embodiment)
A frequency modulation apparatus according to the fourth embodiment of the present invention will be described below with reference to FIG.
[0067]
FIG. 6 is a block diagram showing the configuration of the frequency modulation apparatus according to the fourth embodiment. This frequency modulation device includes an optical filter 617 instead of the electrical filter 111 in the configuration of the first embodiment shown in FIG. In FIG. 6, the same components as those in the first embodiment are denoted by the same reference numerals.
[0068]
  Next, the operation of this embodiment shown in FIG. 6 will be described. However, since the configuration of the present embodiment conforms to the first embodiment described above, only the differences will be described below.
  In the frequency modulation device of the present embodiment, the optical filter 617 is branched by the first optical branching circuit 104 out of the optical signal L1 from the first optical modulator 102 and the first optical receiver 1.10Only the optical carrier component L1c of the optical signal L1 going toward is passed. The first optical receiver 110 receives the optical carrier component L1c that has passed through the optical filter 617 and the light L2 from the local light source 103, and performs optical heterodyne detection based on the square detection characteristics thereof to determine the optical frequency difference between them. An unmodulated beat signal Sb having a frequency corresponding to fs is generated. The frequency converter 112 converts the frequency of the unmodulated beat signal Sb and outputs a frequency converted signal Sx having a frequency fx. The second light modulator 113 performs light modulation (light amplitude modulation or light intensity modulation) on the light L2 output from the local light source 103 using the frequency conversion signal Sx output from the frequency converter 112. And outputs an optical modulation signal L2x. The second optical receiver 114 receives the optical signal L1 from the first optical modulator 102 and the optical signal L2x from the second optical modulator 113, and performs optical heterodyne detection based on the square detection characteristics thereof. An FM modulated signal Sfm in which phase noise is suppressed is generated as a beat signal having a frequency fL (= | fs−fx |) as a center frequency.
[0069]
As described above, according to the fourth embodiment, in an optical heterodyne configuration using two light sources, a high-frequency / wideband FM modulated signal without phase noise is generated without depending on the phase noise of the light source. Can do.
[0070]
In the configuration including the fourth optical modulator 516 instead of the local light source 103 as in the third embodiment (see FIG. 5), the fourth embodiment is used as shown in FIG. Similarly, the first optical filter 617 is provided in the middle of the second optical waveguide 107, and the second optical filter 627 is provided in the middle of the fourth optical waveguide 109, and output from the fourth optical modulator 516. Then, the second optical filter 627 may pass only the optical carrier component of the optical signal L2 that is branched by the second optical branch circuit 105 and directed to the first optical receiver 110. According to such a configuration, as in the third embodiment, since the unmodulated beat signal Sb is output from the first optical receiver 110, there is no phase noise without using the electrical filter 111. A high frequency / wideband FM modulated signal can be generated.
[0071]
(Fifth embodiment)
A frequency modulation apparatus according to the fifth embodiment of the present invention will be described below with reference to FIG.
[0072]
FIG. 8 is a block diagram showing the configuration of the frequency modulation apparatus according to the fifth embodiment. In the configuration of the first embodiment shown in FIG. 1, this frequency modulation device outputs unmodulated light having an optical frequency f1 as the first light L1 instead of the first optical modulator 102 of the direct modulation system. And an optical modulator (hereinafter referred to as “external optical modulator”) 102e that performs frequency modulation using an external modulation method, and the filter 111 is omitted. As shown in FIG. 8, the unmodulated light L <b> 1 output from the light source 101 is input to the first optical branch circuit 104. The external optical modulator 102e is inserted and arranged in the middle of the first optical waveguide 106, and receives an electrical signal Si as an original signal from the signal source 100. In FIG. 8, the same components as those in the first embodiment are denoted by the same reference numerals.
[0073]
Next, the operation of this embodiment shown in FIG. 8 will be described.
In the frequency modulation device of this embodiment, the unmodulated light L1 output from the light source 101 is branched into two lights by the first light branching circuit 104, and one of the two lights is an external light modulator. The other light is input to the first optical receiver 110.
[0074]
The external optical modulator 102e modulates the unmodulated light L1 from the light source 101 with the electric signal Si from the signal source 100, thereby generating an optical frequency modulation signal L1m having the center optical frequency f1. The optical signal L1m is input to the second optical receiver 114. The first optical receiver 110 receives the unmodulated light L1 from the light source 101 and the unmodulated light L2 from the local light source 103, and performs optical heterodyne detection based on the square detection characteristics thereof to detect the optical frequency difference fs. = Unmodulated beat signal Sb having a frequency corresponding to | f1-f2 | is generated. The frequency converter 112 converts the frequency of the unmodulated beat signal Sb and outputs a frequency converted signal Sx having a frequency fx. The second light modulator 113 performs light modulation (light amplitude modulation or light intensity modulation) on the light L2 from the local light source 103 using the frequency conversion signal Sx output from the frequency converter 112, Outputs an optical modulation signal L2x. This optical modulation signal L2x is also input to the second optical receiver 114. The second optical receiver 114 receives the optical signal L1 from the first optical modulator 102 and the optical signal L2x from the second optical modulator 113, and performs optical heterodyne detection based on the square detection characteristics thereof. An FM modulated signal Sfm in which phase noise is suppressed is generated as a beat signal having a frequency fL (= | fs−fx |) as a center frequency.
[0075]
As described above, according to the fifth embodiment, since the unmodulated beat signal Sb is output from the first optical receiver 110 as in the fourth embodiment, the electrical filter 111 is used. Therefore, it is possible to generate a high frequency / wideband FM modulated signal without phase noise. In addition, in the fifth embodiment, since the unmodulated light L1 from the light source 101 is input to the first optical receiver 110, an optical filter is not required unlike the fourth embodiment.
[0076]
  (First modification)
  In each of the above embodiments, the second optical modulator 113 includes the first3The unmodulated light L2 output from the local light source 103 or the optical signal L2 output from the fourth optical modulator 516 is optically modulated by the frequency conversion signal Sx. However, instead of this, by inserting the second optical modulator 113 in the middle of the first optical waveguide 106, the first optical modulator 106lightThe optical signal L1 output from the modulator 102 may be optically modulated by the frequency conversion signal Sx. FIG. 9 is a block diagram showing an example of the configuration of such a frequency modulation device. This frequency modulation device is a modification of the first embodiment shown in FIG. 1 (hereinafter referred to as “first modification”).3The second optical modulator 113 inserted in the middle of the first optical waveguide 108 is provided in the middle of the first optical waveguide 106.
[0077]
In the first embodiment or the like, the second optical modulator 113 performs optical amplitude modulation or optical intensity modulation of the unmodulated light L2 having the optical frequency f2 with the frequency conversion signal Sx having the frequency fx (= | fs−fL |). Thus, an optical modulation signal L2x having an optical frequency spectrum as shown in FIG. 10A is generated (where f1> f2).
[0078]
On the other hand, in the frequency modulation device of the first modification, the second optical modulator 113 converts the optical signal L1 having the center frequency f1 into the optical amplitude with the frequency conversion signal Sx having the frequency fx (= | fs−fL |). By modulation or light intensity modulation, an optical modulation signal L1x having an optical frequency spectrum as shown in FIG. 10B is generated (similarly, f1> f2). The second optical receiver 114 in the first modification receives the optical modulation signal L1x and the unmodulated light L2 having the frequency f2 output from the local light source 103, and optical heterodyne detection based on the square detection characteristics. Thus, the FM modulation signal Sfm is generated as a beat signal having the frequency fL (= | fs−fx |) as the center frequency. That is, an FM modulation signal substantially the same as the FM modulation signal Sfm obtained in the first embodiment is generated. Therefore, also with the configuration of the first modification, as in the case of the first embodiment, it is possible to generate a high-frequency / wideband FM modulated signal without phase noise without depending on the phase noise of the light source.
[0079]
(Second modification)
In each of the embodiments described above, more preferably, the first light L1 output from the first light modulator 102 (more precisely, a light source such as a semiconductor laser constituting the light modulator 102) is the first light L1. The propagation time until the light directly reaches the second optical receiver 114 via the optical waveguide 106, and the light L1 thereof is the second optical waveguide 107, the first optical receiver 110, and the second optical modulator 113. The propagation times until reaching the second optical receiver 114 via the same are mutually equal, and the local light source 103 or the fourth optical modulator 516 (to be precise, the light source constituting the optical modulator 516) ) And the propagation time until the second light L2 output from the second optical receiver 114 reaches the second optical receiver 114 via the third optical waveguide 108 and the second optical modulator 113, and the light L2 is the fourth. Optical waveguide 109 and first optical receiver 110. Beauty propagation time to reach the second of the second optical receiver 114 through the optical modulator 113 and the like and is set to be equal to each other.
[0080]
For example, one or a plurality of optical or electrical delay circuits are provided in an optical path or an electrical path such as an optical waveguide constituting the propagation paths of the first and second lights L1 and L2, and a propagation time adjusting means Such a setting can be realized. The propagation time adjusting means is preferably an electrical adjusting means rather than an optical adjusting means in terms of cost and the like. FIG. 11 is a block diagram showing a configuration of a modified example (hereinafter referred to as “second modified example”) of the first embodiment shown in FIG. 1, and is a delay adjuster configured by an electrical delay circuit. Reference numeral 710 denotes a configuration of a frequency modulation device inserted between the frequency converter 112 and the second optical modulator 113 as the adjusting means. The insertion position of the delay adjuster 710 is not limited to the above position. For example, instead of the frequency converter 112 and the second optical modulator 113, the filter 111, the frequency converter 112, Or a delay adjuster 710 may be inserted between the first optical receiver 11 and the filter 111.
[0081]
By realizing the above setting by such a propagation time adjusting means, the phase noise of the FM modulated signal Sfm output from the second optical receiver 114 can be more reliably suppressed, and a high-quality high-frequency / wideband can be further suppressed. An FM modulated signal can be generated.
[0082]
(Third and fourth modifications)
In the second embodiment and the third embodiment described above, more desirably, the first IM-DD component and the second IM-DD component generated in the second optical receiver 114 or the first IM-DD component, The first IM-DD component and the third IM-DD component are set so as to have opposite phases and the same amplitude. More specifically, in order to adjust the phase of these IM-DD components, the first electric signal Si1 which is one of the signals branched by the duplexer 301 is converted into an optical signal L1 by the first optical modulator 102. And the second electrical signal Si2, which is the other signal branched by the demultiplexer 301, until the second optical receiver 114 reaches the second optical receiver 114 via the first optical waveguide 106. 3 is converted into an optical signal L3 or L2 by the third optical modulator 315 or the fourth optical modulator 516, and reaches the second optical receiver 114 via the fifth optical waveguide 317 or the third optical waveguide 108. Are set so that their propagation times are equal to each other. In addition, in the optical signal received by the second optical receiver 114, the amplitudes of the light intensity modulation components corresponding to the first IM-DD component and the second IM-DD component are equal to each other, or the first IM-DD The amplitudes of the light intensity modulation components corresponding to the DD component and the third IM-DD component are set to be equal to each other.
[0083]
For example, one or a plurality of electrical or optical delay circuits and variable gain amplifiers (or attenuators) are connected to the electrical path or optical path (optical waveguide) constituting the above-described signal propagation path. Such a setting can be realized by inserting as means for adjusting the phase and amplitude. As such a phase and amplitude adjusting means, an electrical adjusting means is preferable to an optical adjusting means in terms of cost and the like. FIG. 12 is a block diagram showing a configuration of a modified example (hereinafter referred to as “third modified example”) of the second embodiment shown in FIG. 3, and is configured by an electrical delay circuit and a variable gain amplifier. The delay / amplitude adjuster 720 is a configuration of a frequency modulation device in which the demultiplexer 301 and the third optical modulator 315 are inserted as the adjusting means. FIG. 13 is a block diagram showing a configuration of a modified example (hereinafter referred to as “fourth modified example”) of the third embodiment shown in FIG. A configuration of a frequency modulation device in which an amplitude adjuster 730 is inserted between the duplexer 301 and the fourth optical modulator 516 is shown. The insertion positions of the delay / amplitude adjusters 720 and 730 are not limited to the above positions, and may be inserted between the duplexer 301 and the first optical modulator 102, for example.
[0084]
By realizing the above-described setting by such a phase and amplitude adjusting means, the IM-DD component output from the second optical receiver 114 is more reliably suppressed, and further high-quality high-frequency / wideband FM modulation is performed. A signal can be generated.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of a frequency modulation apparatus according to a first embodiment of the present invention.
FIG. 2 is a schematic diagram (A) showing a frequency spectrum of an output signal from a first optical receiver in the frequency modulation device according to the first embodiment, and a frequency converter in the frequency modulation device according to the first embodiment. The schematic diagram (B) which shows the frequency spectrum of the input-output signal, and the schematic diagram (C) which shows the frequency spectrum of the output signal from the 2nd optical receiver in the frequency modulation apparatus which concerns on 1st Embodiment.
FIG. 3 is a block diagram showing a configuration of a frequency modulation apparatus according to a second embodiment of the present invention.
FIG. 4 shows a frequency spectrum of a signal after detection corresponding to an optical signal output from the first and second optical modulators and input to the second optical receiver in the frequency modulation device according to the second embodiment. Schematic diagram (A), the frequency spectrum of the signal after detection corresponding to the optical signal output from the third optical modulator and input to the second optical receiver in the frequency modulation device according to the second embodiment. The schematic diagram (B) which shows, and the schematic diagram (C) which shows the frequency spectrum of the output signal from the 2nd optical receiver in the frequency modulation apparatus which concerns on 2nd Embodiment.
FIG. 5 is a block diagram showing a configuration of a frequency modulation apparatus according to a third embodiment of the present invention.
FIG. 6 is a block diagram showing a configuration of a frequency modulation apparatus according to a fourth embodiment of the present invention.
FIG. 7 is a block diagram showing another configuration of the frequency modulation apparatus according to the fourth embodiment.
FIG. 8 is a block diagram showing a configuration of a frequency modulation apparatus according to a fifth embodiment of the present invention.
FIG. 9 is a block diagram showing a configuration of a frequency modulation apparatus according to a first modification which is a modification of the first embodiment.
FIG. 10 is a schematic diagram (A) illustrating an optical frequency spectrum of an optical signal input to a second optical receiver in the frequency modulation device according to each of the first, second, fourth, and fifth embodiments; The schematic diagram (B) which shows the optical frequency spectrum of the optical signal input into a 2nd optical receiver in the frequency modulation apparatus which concerns on a 1st modification.
FIG. 11 is a block diagram showing a configuration of a frequency modulation apparatus according to a second modification which is another modification of the first embodiment.
FIG. 12 is a block diagram showing a configuration of a frequency modulation apparatus according to a third modification which is a modification of the second embodiment.
FIG. 13 is a block diagram showing a configuration of a frequency modulation apparatus according to a fourth modification, which is a modification of the third embodiment.
FIG. 14 is a block diagram showing a configuration of a conventional frequency modulation device.
FIG. 15 is a schematic diagram (A) showing an optical frequency spectrum of an optical signal input to an optical receiver in a conventional frequency modulation device, and a frequency spectrum of an output signal from the optical receiver in the conventional frequency modulation device. Schematic diagram (B) shown.
[Explanation of symbols]
100: Signal source
102. First optical modulator
102e ... External light modulator
103 ... Local light source
104... First optical branching circuit
105 ... Second optical branching circuit
106: First optical waveguide
107… second optical waveguide
108. Third optical waveguide
109: Fourth optical waveguide
110 ... 1st optical receiver
111 ... Filter
112 ... Frequency converter
113… Second optical modulator
114 ... Second optical receiver
200 ... beat signal generator
301 ... duplexer
315 ... Third optical modulator
516 ... Fourth optical modulator
617, 627 ... Optical filter
710 ... Delay adjuster
720, 730 ... delay / amplitude adjuster
Si ... (Input) Electrical signal
Sbfm: Modulation beat signal
Sb: Unmodulated beat signal
Sx Frequency conversion signal
Sfm FM modulation signal (frequency modulation signal)

Claims

An optical signal is converted into a frequency modulation signal by optical frequency modulation and optical heterodyne detection using first and second light sources that output first and second light beams having different optical frequencies, respectively, and is used as an FM modulation signal. An output frequency modulation device comprising:
A first optical modulator that outputs the first light frequency-modulated with the input electrical signal as a first optical signal;
A beat signal generating unit that generates an unmodulated beat signal corresponding to a carrier component of a beat signal obtained by optical detection based on square detection characteristics from the first and second lights;
A frequency converter that generates a frequency converted signal by converting the frequency of the unmodulated beat signal;
A second optical modulator that generates a second optical signal by performing optical amplitude modulation or optical intensity modulation of one of the first optical signal and the second light with the frequency conversion signal;
An optical receiver that receives the other of the first optical signal and the second light and the second optical signal, and generates the FM modulated signal by optical detection based on square detection characteristics;
A frequency modulation device comprising:

The first optical modulator generates the first optical signal by direct modulation;
The beat signal generator
Modulation having a center frequency corresponding to the difference between the optical frequencies of the first optical signal and the second light by receiving the first optical signal and the second light and performing optical detection based on square detection characteristics A light receiving element that generates a modulated beat signal that is an electrical signal,
The frequency modulation device according to claim 1, further comprising: a filter that extracts a carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal.

The first optical modulator includes:
The first light source for outputting the first light which is unmodulated light;
An external optical modulator that generates the first optical signal by modulating the first light output from the first light source with the input electrical signal;
The second light source outputs unmodulated light as the second light,
The beat signal generation unit includes a light receiving element that receives the first and second lights, which are unmodulated light, and generates the unmodulated beat signal by optical detection based on square detection characteristics. Frequency modulation device.

The first optical modulator generates the first optical signal having a predetermined center frequency f1 by uniquely converting an amplitude change of the input electric signal into a change of the optical frequency of the first light. ,
The second light source outputs light having a predetermined frequency f2 as the second light,
The beat signal generator
By receiving the first optical signal and the second light and performing optical detection based on square detection characteristics, a frequency fs = | corresponding to the difference between the optical frequencies of the first optical signal and the second light. a light receiving element that generates a modulated beat signal that is a modulated electric signal at f1-f2 |
A filter that extracts the carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal;
The frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency converted signal.
The second optical modulator performs optical amplitude modulation or optical intensity modulation on one of the first optical signal and the second light with the frequency conversion signal having a frequency fx, thereby converting the second optical signal. Generate and
The optical receiver receives the other of the first optical signal and the second optical signal and the second optical signal, and performs optical detection based on a square detection characteristic, thereby performing frequency fL = | fs−fx. The frequency modulation apparatus according to claim 1, wherein the FM modulation signal is generated at |

A duplexer for branching the input electrical signal into first and second electrical signals of opposite phases;
A third optical modulator that converts the second electrical signal into a third optical signal by optical intensity modulation;
Further comprising
The first optical modulator uniquely converts an amplitude change of the first electric signal into a change in the optical frequency of the first light by direct modulation, whereby the first optical modulator having a predetermined center frequency f1 is converted. Generate optical signals,
The second light source outputs light having a predetermined frequency f2 as the second light,
The beat signal generator
By receiving the first optical signal and the second light and performing optical detection based on square detection characteristics, a frequency fs = | corresponding to the difference between the optical frequencies of the first optical signal and the second light. a light receiving element that generates a modulated beat signal that is a modulated electric signal at f1-f2 |
A filter that extracts the carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal;
The frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency converted signal.
The second optical modulator performs optical amplitude modulation or optical intensity modulation on one of the first optical signal and the second light with the frequency conversion signal having a frequency fx, thereby converting the second optical signal. Generate and
The optical receiver receives the other of the first optical signal and the second optical signal and the second optical signal, and performs optical detection based on a square detection characteristic, thereby performing frequency fL = | fs−fx. The FM modulation signal is generated in |, the third optical signal is received, and an electrical signal corresponding to a light intensity modulation component included in the third optical signal is generated by the optical detection. The frequency modulation device according to 1.

A first IM-DD component corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver; Phase / amplitude adjustment in which the second IM-DD component corresponding to the light intensity modulation component included in the third optical signal among the generated intensity modulation-direct detection components is opposite in phase and has the same amplitude. 6. The frequency modulation device according to claim 5, further comprising means.

A duplexer for branching the input electrical signal into first and second electrical signals having opposite phases;
A fourth optical modulator that outputs the second light, which is modulated light having a predetermined center frequency f2, as a fourth optical signal;
Further comprising
The first optical modulator uniquely converts an amplitude change of the first electric signal into a change in the optical frequency of the first light by direct modulation, whereby the first optical modulator having a predetermined center frequency f1 is converted. Generate optical signals,
The fourth optical modulator uniquely converts an amplitude change of the second electric signal into a change in the optical frequency of the second light by direct modulation, whereby the fourth optical modulator having a predetermined center frequency f2 is converted. Generate optical signals,
The beat signal generator
The first and fourth optical signals are received and modulated at a frequency fs = | f1-f2 | corresponding to a difference in optical frequency between the first and fourth optical signals by optical detection based on square detection characteristics. A light receiving element that generates a modulated beat signal that is an electrical signal,
A filter that extracts the carrier component from the modulated beat signal and outputs the carrier component as the unmodulated beat signal;
The frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency converted signal.
The second optical modulator performs optical amplitude modulation or optical intensity modulation of one of the first optical signal and the fourth optical signal with the frequency conversion signal having a frequency fx, thereby converting the second optical signal. Generate
The optical receiver receives the other one of the first optical signal and the fourth optical signal and the second optical signal, and performs optical detection based on a square detection characteristic to generate a frequency fL = | fs−fx. The frequency modulation device according to claim 1, wherein the FM modulation signal is generated at |.

A first IM-DD component corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver; Phase / amplitude adjustment in which the third IM-DD component corresponding to the light intensity modulation component included in the fourth optical signal among the generated intensity modulation-direct detection components is opposite in phase and has the same amplitude. The frequency modulation device according to claim 7, further comprising means.

An optical filter that is inserted between the first optical modulator and the beat signal generation unit and extracts the optical carrier component from the first optical signal;
The second light source outputs unmodulated light as the second light,
The beat signal generation unit includes a light receiving element that receives the optical carrier component extracted by the optical filter and the second light, and generates the unmodulated beat signal by optical detection based on square detection characteristics. Item 2. The frequency modulation device according to Item 1.

A duplexer for branching the input electrical signal into first and second electrical signals having opposite phases;
A fourth optical modulator that outputs the second light, which is modulated light having a predetermined center frequency f2, as a fourth optical signal;
A first optical filter inserted between the first optical modulator and the beat signal generator;
A second optical filter inserted between the fourth optical modulator and the beat signal generator;
Further comprising
The first optical modulator uniquely converts an amplitude change of the first electric signal into a change in the optical frequency of the first light by direct modulation, whereby the first optical modulator having a predetermined center frequency f1 is converted. Generate optical signals,
The fourth optical modulator uniquely converts an amplitude change of the second electric signal into a change in the optical frequency of the second light by direct modulation, whereby the fourth optical modulator having a predetermined center frequency f2 is converted. Generate optical signals,
The first optical filter extracts the optical carrier component from the first optical signal, the second optical filter extracts the optical carrier component from the fourth optical signal, and generates the beat signal. The unit receives both the optical carrier components extracted by the first and second optical filters, and performs optical detection based on the square detection characteristic to obtain a frequency fs = | corresponding to the difference between the optical frequencies of the two optical carrier components. a light receiving element for generating the unmodulated beat signal at f1-f2 |
The frequency converter converts the unmodulated beat signal into a signal having a predetermined frequency fx, and outputs the converted signal as the frequency converted signal.
The second optical modulator performs optical amplitude modulation or optical intensity modulation of one of the first optical signal and the fourth optical signal with the frequency conversion signal having a frequency fx, thereby converting the second optical signal. Generate
The optical receiver receives the other one of the first optical signal and the fourth optical signal and the second optical signal, and performs optical detection based on a square detection characteristic to generate a frequency fL = | fs−fx. The frequency modulation device according to claim 1, wherein the FM modulation signal is generated at |.

A first IM-DD component corresponding to a light intensity modulation component included in the first optical signal among intensity modulation-direct detection components generated by optical detection based on the square detection characteristics of the optical receiver; Phase / amplitude adjustment in which the third IM-DD component corresponding to the light intensity modulation component included in the fourth optical signal among the generated intensity modulation-direct detection components is opposite in phase and has the same amplitude. The frequency modulation device according to claim 10, further comprising means.

Light and electrical propagation time of a path from the first light source to the optical receiver via the beat signal generation unit, and light propagation time of a path from the first light source directly to the optical receiver First propagation time adjustment means equal to each other;
A light and electrical propagation time of a path from the second light source through the beat signal generation unit to the optical receiver, and an optical propagation time of a path from the second light source directly to the optical receiver. Second propagation time adjusting means for equalizing each other;
The frequency modulation device according to claim 1, further comprising: