JP3567451B2 - Optical amplifier - Google Patents

Description

【００３８】
入力端光コネクタ１０に入力する信号光を波長１５４３ｎｍから１５５８ｎｍまでの１ｎｍ刻みの１６波ＷＤＭ信号とし、各信号光それぞれの強度を−２２ｄＢｍとし、１６波信号光の全強度を−１０ｄＢｍとした。そして、この多波長の信号光を一括増幅すべく、励起光源３２として波長０．９８μｍのレーザ光を出力する半導体レーザ光源を用い、励起光源８２として波長１．４８μｍのレーザ光を出力する半導体レーザ光源を用い、また、光バンドパスフィルタ６２の通過波長帯域の中心波長を１５６５ｎｍとし、光フィルタ７０の遮断波長も１５６５ｎｍとした。
【００３９】
以上のように構成される光ファイバ増幅器中の３点それぞれにおける光のスペクトル図を図２に示す。図２（ａ）は、入力端光コネクタ１０の入力点（図１中のＡ点）における信号光のスペクトル図であり、図２（ｂ）は、光フィルタ７０の入力点（図１中のＢ点）における光のスペクトル図であり、図２（ｃ）は、出力端光コネクタ９０の出力点（図１中のＣ点）における光のスペクトル図である。
【００４０】
前段光増幅部３０、光分波器４０、光帰還部６０および光合波器２０からなる閉ループで構成されるレーザ発振器では、光バンドパスフィルタ６２の通過波長帯域の中心波長である１５６５ｎｍでレーザ発振する。このレーザ発振に伴い前段光増幅部３０のＥＤＦ３４における反転分布が固定され、その結果、前段光増幅部３０では約２０ｄＢの安定した利得が得られた。したがって、光フィルタ７０の入力点（図１中のＢ点）には、図２（ｂ）のスペクトル図に示すように、光増幅された１６波信号光および波長１５６５ｎｍのレーザ発振光が現れる。なお、このとき、各信号光の強度は約−２ｄＢｍであり、１６波信号光の全強度は約１０ｄＢｍであった。
【００４１】
この光フィルタ７０に入力した光のうち、波長１５６５ｎｍのレーザ発振光は、光フィルタ７０により反射され、１６波信号光は、光フィルタ７０を通過し、さらに、後段光増幅部８０に入力し一括増幅される。この後段光増幅部８０における利得は、受光素子８６Ａおよび８６Ｂそれぞれにより検出された入力光および出力光それぞれの強度の比に基づいて、制御回路８７により７ｄＢに一定に制御されている。したがって、出力端光コネクタ９０の出力点（図１中のＣ点）では、図２（ｃ）のスペクトル図に示すように、さらに光増幅された１６波信号光が出力される。このとき、後段光増幅部８０にはレーザ発振光が入力することはないので、出力される１６波信号光の全強度は、後段光増幅部８０における最大出力である約１７ｄＢｍが得られた。以上より、この光ファイバ増幅器全体の利得は、２７ｄＢが得られた。
【００４２】
なお、閉ループを構成することなく前段光増幅部３０を単体で使用した場合には、Ｂ点における１６波信号光の全強度は、約１２．５ｄＢｍが得られる。しかし、本実施形態のように閉ループによりレーザ発振させた場合には、入力する１６波信号光の強度が大きくなると、レーザ発振が停止するため、反転分布が固定され得なくなり、利得を一定に保てなくなる。したがって、本実施形態では、前段光増幅部３０における利得を安定に一定に維持するためには、Ｂ点における１６波信号光の全強度の最大値は、約１０．０ｄＢｍとなり、単体で使用した場合に比較して約２．５ｄＢｍ低くなる。
【００４３】
図３は、入力する全信号光の強度に対する利得の依存性を示す図である。この図で、実線は本実施形態の場合を示し、破線は従来技術の場合を示す。従来の光ファイバ増幅器のように、Ａ点およびＣ点それぞれにおける光の強度（すなわち、光ファイバ増幅器全体に対する入力光および出力光の強度）をモニタし、両者に比に基づいて利得を一定に制御する場合には、出力光にＡＳＥ光が含まれることから、図３中の破線に示すように、入力する信号光の全強度が小さくなると、全体の利得は小さくなる。
【００４４】
これに対して、本実施形態に係る光ファイバ増幅器では、前段光増幅部３０および後段光増幅部８０それぞれにおいてＡＳＥ光が発生するが、前段光増幅部３０において発生するＡＳＥ光が元となったＡＳＥ光成分に比べて、後段光増幅部８０において発生するＡＳＥ光は、無視できるほど小さい。これは、前段光増幅部３０に入力する信号光の強度よりも、後段光増幅部８０に入力する信号光の強度の方が大きいので、後段光増幅部８０のＥＤＦ８４Ａおよび８４Ｂにおける反転分布は、殆どが信号光の増幅に費やされ、ＡＳＥ光の発生に費やされるのが僅かになるからである。したがって、本実施形態に係る光ファイバ増幅器では、図３中の実線に示すように、入力端光コネクタ１０に入力する信号光の全強度が変動しても、全体の利得は約２７ｄＢに略一定に保たれる。
【００４５】
また、レーザ発振光は、光フィルタ７０により反射され、光分波器４０を経て、受光素子５０に入力し受光される。この受光素子５０により受光されたレーザ発振光の強度をモニタすることにより、前段光増幅部３０における利得が一定に制御されているか否かを判定することができる。すなわち、既述したように、入力端光コネクタ１０に入力する信号光の波数が増えたりした場合のように、入力信号光の全強度が所定値を越えると、レーザ発振が停止し、反転分布が固定され得なくなり、利得を一定に保てなくなるからである。したがって、受光素子５０により受光されたレーザ発振光の強度が所定値より低くなったときには、利得一定制御が阻害されたとみなして、その旨の警報等を発することができ、また、この警報等に基づいて、光ファイバ増幅器やこれを含む光伝送システムのメインテナンス等を行うことができる。
【００４６】
本発明は、上記実施形態に限定されるものではなく種々の変形が可能である。上記実施形態の説明では、前段光増幅部および後段光増幅部それぞれにおいて光増幅作用をするものとして、Ｅｒ元素が添加された光ファイバ（ＥＤＦ）について説明したが、これに限られるものではない。例えば、光増幅作用を有する物質としてＥｒ元素以外の希土類元素（例えば、Ｎｄ（ネオジウム）元素、Ｐｒ（プラセオジウム）元素、等）が添加された光ファイバを用いたものであってもよいし、光ファイバではなく平面型光導波路を用いたものであってもよい。
【００４７】
また、レーザ発振光をモニタする手段としては、前段光増幅部３０から後段光増幅部８０へ向かう光のうちからレーザ発振光を取り出す光分波手段を設け、その取り出されたレーザ発振光を受光し検出してもよい。また、この場合には、この光分波手段と光フィルタ７０とを兼用することもできる。例えば、光ファイバの光軸に対して斜めに形成されたグレーティングを有する光ファイバグレーティングであって、レーザ発振光をブラッグ反射させて当該光ファイバの側方に出射させ、一方、信号光をそのまま通過させるものが用いられ得る。
【００４８】
【発明の効果】
以上、詳細に説明したとおり、本発明によれば、前段光増幅部、光分波器、光帰還部および光合波器からなる閉ループは、進行波型のレーザ発振器を構成しており、光帰還部における通過波長（信号光波長帯域以外の波長）でレーザ発振する。このレーザ発振に伴い、前段光増幅部における反転分布は固定され、前段光増幅部における信号光の利得は一定に維持される。前段光増幅部から出力された信号光（信号光波長帯域の光）は、光フィルタを通過し、後段光増幅部に入力し、一定利得で更に光増幅され出力する。
【００４９】
このような構成としたことにより、入力信号光の強度が小さいときでも、信号光は前段光増幅部において一定利得で光増幅され、また、その前段光増幅部により光増幅された信号光が後段光増幅部に入力するので、後段光増幅部においても一定利得で光増幅される。したがって、入力信号光の強度が小さいときであっても、光増幅器の利得は、充分に大きな値で且つ安定して一定に維持される。
【００５０】
前段光増幅部は、(1) 励起光が供給されているときに入力した所定波長帯域の光を光増幅して出力する光導波路と、(2) 励起光を出力し光導波路に供給する励起手段と、を備えるものが好適である。また、後段光増幅部は。(1) 励起光が供給されているときに入力した所定波長帯域の光を光増幅して出力する光導波路と、(2) 励起光を出力し光導波路に供給する励起手段と、(3) 光導波路に入力する光の強度を検出する入力光検出手段と、(4) 光導波路から出力される光の強度を検出する出力光検出手段と、(5) 入力光検出手段および出力光検出手段それぞれにより検出された光の強度の比に基づいて、励起手段から出力される励起光を制御する制御手段と、を備えるものが好適である。そして、前段光増幅部および後段光増幅部の光導波路は光ファイバである場合には、光ファイバ線路との接続損が少ない。また、前段光増幅部および後段光増幅部の光導波路は、光増幅作用を奏する物質としてＥｒ元素が添加されている場合には、光通信において最も多用されている波長１．５５μｍ帯の信号光が増幅される。
【００５１】
また、光フィルタは、光軸に沿って周期的な屈折率変化を有するブラッグ・グレーティングが形成された光ファイバ・グレーティングである場合には、光ファイバとの接続損が少ない。
【００５２】
また、信号光波長帯域以外の光の強度をモニタしてレーザ発振を監視する監視手段を更に備える場合には、前段光増幅部を含む閉ループにおいて発生したレーザ発振光の強度が監視手段によりモニタされ、前段光増幅部における光増幅動作の状態が判断され得る。また、この監視手段による監視結果に基づいて警報等を発することができる。
【図面の簡単な説明】
【図１】本実施形態に係る光ファイバ増幅器の構成図である。
【図２】本実施形態に係る光ファイバ増幅器中の３点それぞれにおける光のスペクトル図である。
【図３】入力する全信号光の強度に対する利得の依存性を示す図である。
【図４】従来の光増幅器の構成図である。
【符号の説明】
１０…入力端光コネクタ、２０…光合波器、３０…前段光増幅部、３１Ａ，３１Ｂ…光アイソレータ、３２…励起光源、３３…ＷＤＭカプラ、３４…ＥＤＦ、４０…光分波器、５０…受光素子、６０…光帰還部、６１…可変アッテネータ、６２…光バンドパスフィルタ、７０…光フィルタ、８０…後段光増幅部、８１Ａ，８１Ｂ…光アイソレータ、８２…励起光源、８３…ＷＤＭカプラ、８４Ａ，８４Ｂ…ＥＤＦ、８５Ａ，８５Ｂ…光分波器、８６Ａ，８６Ｂ…受光素子、８７…制御回路、９０…出力端光コネクタ。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical amplifier for collectively amplifying multi-wavelength signal light in a WDM optical transmission system.
[0002]
[Prior art]
With the increase in the capacity and speed of optical communication, research and development on a wavelength division multiplex (WDM) transmission system have been conducted. In the WDM transmission system, one of the most important optical elements is an optical amplifier that collectively amplifies multi-wavelength signal light. As one of such optical amplifiers, an optical fiber amplifier (EDFA: Er-Doped Fiber Amplifier) using an amplifying optical fiber (EDF: Er-Doped Fiber) doped with an Er (erbium) element has been conventionally used. ing.
[0003]
Since this optical amplifier is provided in each repeater in the optical transmission system, if the gain characteristic is not stable, particularly in the case of a network-like optical transmission system in which the number of signal light wavelengths increases or decreases, In some cases, the signal light arrives at the receiving station with sufficient intensity, but at other times, the intensity of the signal light may be weak, and an error may occur in the reception of the signal light. Therefore, several techniques for controlling the gain of an optical amplifier to a constant value have been conventionally known.
[0004]
For example, a technique is known in which the intensity of input light and the output light of an optical amplifier are monitored to determine the ratio between the two, and the gain is controlled to be constant based on the value of the ratio. However, in this technique, the output light from the optical amplifier includes, in addition to the signal light input to the optical amplifier and optically amplified, ASE (Amplified Spontaneous Emission) light generated inside the optical amplifier. Therefore, when the intensity of the signal light input to the optical amplifier is low, there is a problem that the ratio of the ASE light included in the output light increases and the intensity of the output signal light decreases.
[0005]
As a technique for solving such a problem, an optical closed loop including an optical amplifier is provided, and the laser is oscillated in the closed loop, so that the state of the optical waveguide having an optical amplifying action in the optical amplifier is maintained constant, Thus, a technique for controlling the gain of an optical amplifier to be constant has been disclosed (J. Chung et al., "All-optical gain-clamped EDFAs with differing feedback waveguides for the next year's newsletter. , 1996, vol.32, no.23, pp.2159-2161).
[0006]
FIG. 4 is a configuration diagram of this conventional optical amplifier. As shown in this figure, a part of the output light of the optical amplifying unit 1 is taken out via an optical demultiplexer 2 provided on the output side of the optical amplifying unit 1, and the variable attenuator 3 and the optical bandpass filter 4 are removed. Then, the light is input to the optical amplifier 1 again via the optical multiplexer 5 provided on the input side of the optical amplifier 1. By causing the laser to oscillate at the center wavelength of the optical bandpass filter 4 in the closed loop configured as described above, the population inversion in the optical waveguide having the optical amplifying function in the optical amplifying unit 1 is maintained constant, and the optical amplifying unit 1 The gain is controlled to be constant.
[0007]
[Problems to be solved by the invention]
However, in the above-described conventional technique, the energy of the pump light supplied to the optical waveguide in the optical amplifier 1 is consumed not only for optically amplifying the signal light but also for laser oscillation. Therefore, it is impossible to sufficiently amplify the input signal light, and it is possible to achieve only an optical output of about half of the optical output when the optical amplifier 1 alone amplifies the light.
[0008]
SUMMARY OF THE INVENTION The present invention has been made to solve the above problem, and has as its object to provide an optical amplifier having a stable and large gain even when the intensity of input signal light is low.
[0009]
[Means for Solving the Problems]
An optical amplifier according to the present invention comprises: (1) a pre-stage optical amplifier that optically amplifies and outputs an input light of a predetermined wavelength band; and (2) receives a light output from the pre-stage optical amplifying unit, An optical demultiplexer that demultiplexes the light into the second light, (3) an optical feedback unit that passes light of the first light other than the signal light wavelength band, and (4) a signal light to be amplified. Combine with the light output from the optical feedback section Pre-stage optical amplifier (5) an optical filter that allows passage of light in the signal light wavelength band of the second light, and (6) a constant gain based on the ratio of the intensity of each of the input light and the output light. And a post-stage optical amplifying unit that inputs the light output from the optical filter and amplifies and outputs the light.
[0010]
According to this optical amplifier, the closed loop including the pre-stage optical amplifier, the optical demultiplexer, the optical feedback unit, and the optical multiplexer constitutes a traveling-wave laser oscillator, and the transmission wavelength (signal light) in the optical feedback unit is used. Laser oscillation at a wavelength other than the wavelength band). With this laser oscillation, the population inversion in the pre-stage optical amplifier is fixed, and the gain of the signal light in the pre-stage optical amplifier is kept constant. The signal light (light in the signal light wavelength band) output from the first-stage optical amplifier passes through the optical filter, enters the second-stage optical amplifier, is further optically amplified with a constant gain, and is output.
[0011]
Further, in the optical amplifier according to the present invention, the pre-stage optical amplifier includes: (1) an optical waveguide for optically amplifying and outputting light of a predetermined wavelength band inputted when the pumping light is supplied; A pumping means for outputting the pumping light and supplying the pumping light to the optical waveguide. In this case, when the pumping light output by the pumping unit is supplied to the optical waveguide, the light of a predetermined wavelength band input to the optical waveguide is optically amplified by the optical waveguide.
[0012]
Further, in the optical amplifier according to the present invention, the post-stage optical amplifier is provided. (1) an optical waveguide for optically amplifying and outputting light in a predetermined wavelength band input when the pumping light is supplied, (2) an excitation means for outputting the pumping light and supplying the light to the optical waveguide, (3) Input light detection means for detecting the intensity of light input to the optical waveguide, and (4) from the optical waveguide output Output light detection means for detecting the intensity of the light to be output, and (5) controlling the excitation light output from the excitation means based on the ratio of the light intensities detected by the input light detection means and the output light detection means, respectively. And control means for performing the control. In this case, when the pumping light output by the pumping unit is supplied to the optical waveguide, the light of a predetermined wavelength band input to the optical waveguide is optically amplified by the optical waveguide. The intensity of each of the input light and the output light of the optical waveguide is detected by each of the input light detection means and the output light detection means, and based on the ratio between the two, the control means controls the excitation light output from the excitation means. The intensity is controlled, and the gain of the post-stage optical amplifier is kept constant.
[0013]
Further, in the optical amplifier according to the present invention, the optical waveguides of the first-stage optical amplifier and the second-stage optical amplifier are optical fibers. In this case, the connection loss with the optical fiber line is small. Further, the optical amplifier according to the present invention is characterized in that the optical waveguides of the first-stage optical amplifier and the second-stage optical amplifier are added with an Er element as a substance having an optical amplification effect. In this case, signal light in the wavelength band of 1.55 μm, which is most frequently used in optical communication, is amplified.
[0014]
Further, in the optical amplifier according to the present invention, the optical filter is an optical fiber grating in which a Bragg grating having a periodic change in refractive index along the optical axis is formed. In this case, the connection loss with the optical fiber is small.
[0015]
Further, the optical amplifier according to the present invention further reduces the intensity of light other than the signal light wavelength band. Monitor laser oscillation It is characterized by further comprising monitoring means for monitoring. In this case, the intensity of the laser oscillation light generated in the closed loop including the pre-stage optical amplification unit is monitored by the monitoring unit, and the state of the optical amplification operation in the pre-stage optical amplification unit can be determined.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description. Hereinafter, an optical fiber amplifier which is one of the optical amplifiers will be described.
[0017]
FIG. 1 is a configuration diagram of the optical fiber amplifier according to the present embodiment. This optical fiber amplifier optically amplifies the signal light input to the input end optical connector 10 and outputs the amplified signal light from the output end optical connector 90. The optical fiber amplifier is connected between the input end optical connector 10 and the output end optical connector 90. The optical filter 20, the pre-stage optical amplifier 30, the optical demultiplexer 40, the optical filter 70, and the post-stage optical amplifier 80 are cascaded in this order. An optical feedback unit 60 is provided between the optical demultiplexer 40 and the optical multiplexer 20, and the light receiving element 50 is also connected to the optical demultiplexer 40. These components are optically connected to each other by an optical waveguide such as an optical fiber.
[0018]
The signal light input to the input end optical connector 10 first reaches the optical multiplexer 20. The optical multiplexer 20 receives the signal light from the input end optical connector 10 and the light output from the optical feedback unit 60, multiplexes the two, and outputs the multiplexed light.
[0019]
The pre-stage optical amplifier 30 for inputting the light output from the optical multiplexer 20 includes optical isolators 31A and 31B, an excitation light source 32, a WDM coupler 33, and an EDF. The optical isolator 31A passes the light input from the optical multiplexer 20 to the WDM coupler 33, but does not pass the light in the opposite direction. An excitation light source (for example, a semiconductor laser light source) 32 outputs excitation light having a constant intensity to be supplied to the EDF 34. The WDM coupler 33 receives the light arriving from the optical isolator 31A and the pumping light output from the pumping light source 32, and outputs both to the EDF. The EDF 34 optically amplifies light of a predetermined wavelength input from the optical isolator 31A when the excitation light is supplied. The optical isolator 31B allows the light output from the EDF 34 to pass, but does not allow the light to pass in the opposite direction. That is, when the pump light is supplied from the pump light source 32 to the EDF 34 via the WDM coupler 33, the pre-stage optical amplifying unit 30 uses the EDF 34 to convert the light of a predetermined wavelength input through the optical isolator 31A and the WDM coupler 33 by the EDF 34. The light is amplified and output through the optical isolator 31B.
[0020]
The optical demultiplexer 40 receives the light output from the optical amplifier 40, outputs a part of the light to the optical feedback unit 60, and outputs the rest to the optical filter 70. The optical demultiplexer 40 receives light input from the optical filter 70 and outputs a part or all of the light to the light receiving element 50. The light receiving element (for example, a photodiode) 50 receives the light arriving from the optical demultiplexer 40 and outputs an electric signal according to the intensity.
[0021]
The optical feedback unit 60 is configured such that a variable attenuator 61 and an optical bandpass filter 62 are optically cascaded. The optical feedback unit 60 receives the light output from the pre-stage optical amplifying unit 30 and arrives via the optical demultiplexer 40, applies a predetermined loss to the light by the variable attenuator 61, The filter 62 allows light of the light other than the signal light wavelength band to pass through, and outputs only light of the predetermined wavelength band. The light output from the optical feedback unit 60 is input again to the pre-stage optical amplifier 30 via the optical multiplexer 20.
[0022]
The optical filter 70 receives the light that has arrived through the optical demultiplexer 40 among the lights output from the pre-stage optical amplifier 30, and allows the light in the signal light wavelength band (ie, the signal light) to pass therethrough. As the optical filter 70, an optical fiber grating in which a Bragg grating having a periodic refractive index change along the optical axis is preferably used. In this case, the optical filter 70 and another optical fiber are Connection loss is small. At this time, the Bragg wavelength in the Bragg grating is matched with the wavelength of the light to be cut off.
[0023]
The post-stage optical amplifying unit 80 that inputs the light passing through the optical filter 70 includes optical isolators 81A and 81B, an excitation light source 82, a WDM coupler 83, EDFs 84A and 84B, optical demultiplexers 85A and 85B, light receiving elements 86A and 86B, and It comprises a control circuit 87. The optical isolator 81A allows the light input from the optical filter 70 to pass through the optical splitter 85A, but does not allow the light to pass in the opposite direction. The optical splitter 85A receives the light arriving from the optical isolator 81A, outputs a part of the light to the light receiving element 86A, and outputs the remainder to the EDFs 84A and 84B. The EDFs 84A and 84B are cascaded with each other, and optically amplify light of a predetermined wavelength input from the optical demultiplexer 85A when the pumping light output from the pumping light source 82 is supplied.
[0024]
An excitation light source (for example, a semiconductor laser light source) 82 outputs excitation light to be supplied to the EDFs 84B and 84A. The WDM coupler 83 receives the pump light output from the pump light source 82 and outputs the pump light to the EDFs 84B and 84A, and outputs the light output from the EDFs 84A and 84B to the optical isolator 81B. The optical isolator 81B allows the light reaching from the WDM coupler 83 to pass, but does not allow the light to pass in the opposite direction. The optical splitter 85B receives the light reaching from the optical isolator 81B, outputs a part of the light to the light receiving element 86B, and outputs the remaining part to the output end optical connector 90.
[0025]
Each of the light receiving elements (for example, photodiodes) 86A and 86B receives the light branched by each of the optical demultiplexers 85A and 85B, and generates an electric signal corresponding to the intensity of the input light and the output light of the post-stage optical amplifier 80. Output a signal. The control circuit 87 receives the electric signal output from each of the light receiving elements 86A and 86B, controls the intensity of the excitation light output from the excitation light source 82 based on the ratio between the two, and controls the gain of the post-amplifier 80. Is controlled to be constant.
[0026]
That is, when the pumping light is supplied from the pumping light source 82 to the EDFs 84A and 84B via the WDM coupler 83, the post-stage optical amplifying section 80 outputs the light of a predetermined wavelength input through the optical isolator 81A and the optical demultiplexer 85A. Is optically amplified by the EDFs 84A and 84B, and the optically amplified light is output through the WDM coupler 83, the optical isolator 81B, and the optical demultiplexer 85B. At this time, the gain of the post-amplifier 80 is controlled to be constant by the light receiving elements 86A and 86B and the control circuit 87.
[0027]
The optical fiber amplifier configured as described above operates as follows. In the pre-stage optical amplifier 30, the excitation light is output from the excitation light source 32 and supplied to the EDF 34 via the WDM coupler 33. Further, in the latter-stage optical amplifier 80, the pump light is output from the pump light source 82 and supplied to the EDFs 84B and 84A via the WDM coupler 83.
[0028]
As described above, when the excitation light is supplied to the EDF 34 in the pre-stage optical amplification unit 30, the Er element included in the EDF 34 is excited, and a population inversion is generated. Then, light is generated when the excited Er element transitions to a lower energy state. Of the light, light that can pass through the optical bandpass filter 62 in the optical feedback unit 60 (light other than the signal light wavelength band) passes through the optical demultiplexer 40, the optical feedback unit 60, and the optical multiplexer 20, and It is again input to the pre-stage optical amplifier 30, and stimulated emission is caused in the EDF.
[0029]
That is, the closed loop including the pre-stage optical amplifier 30, the optical demultiplexer 40, the optical feedback unit 60, and the optical multiplexer 20 forms a traveling-wave laser oscillator. The wavelength of the laser oscillation in the closed loop is a wavelength in the pass wavelength band of the optical bandpass filter 62, and the oscillation intensity is determined by the transmission loss in the variable attenuator 61. Then, when laser oscillation occurs, the population inversion in the EDF 34 of the pre-stage optical amplifier 30 is fixed.
[0030]
At this time, when a signal light is input to the input terminal optical connector 10 of the optical fiber amplifier, the signal light is input to the pre-amplifier 30 through the optical multiplexer 20 and is optically amplified and output. Since the population inversion in the EDF 34 of the section 30 is fixed, the gain of the signal light in the pre-stage optical amplification section 30 is kept constant.
[0031]
The light output from the pre-stage optical amplifier 30 includes the signal light that has been optically amplified and the laser oscillation light, and is input to the optical filter 70 via the optical demultiplexer 40. Of the light input to the optical filter 70, the signal light passes through the optical filter 70, but the laser oscillation light is cut off and reflected by the optical filter 70. The laser oscillation light reflected by the optical filter 70 is received by the light receiving element 50 through the optical demultiplexer 40, and an electric signal corresponding to the intensity is output. Then, by monitoring this electric signal, the optical amplification operation in the pre-stage optical amplifier 30 can be monitored.
[0032]
On the other hand, the signal light that has passed through the optical filter 70 is input to the subsequent-stage optical amplifying unit 80, optically amplified and output, and further output from the output terminal optical connector 90. In the latter-stage optical amplifying section 80, the intensities of the input light and the output light are detected by the light receiving elements 86A and 8B, respectively, and based on the ratio between the two, the control circuit 87 controls the excitation light supplied from the excitation light source 82 to the EDFs 84B and 84A. The light intensity is controlled, and the gain of the optical amplification of the signal light is kept constant.
[0033]
As described above, since the inversion distribution in the EDF 34 of the pre-stage optical amplifier 30 is fixed by the laser oscillation, the gain in the pre-stage optical amplifier 30 is stably maintained constant. In addition, since the signal light whose intensity is equal to or higher than a certain level by the first-stage optical amplifier 30 is input to the second-stage optical amplifier 80, the gain in the second-stage optical amplifier 80 is also stably maintained at a constant level. Therefore, the constant gain control of the entire optical fiber amplifier is also stabilized.
[0034]
Further, even if the intensity of the output signal light is not sufficient only by the gain in the first-stage optical amplifier 30, the optical amplifier is further amplified by the second-stage optical amplifier 80, so that the gain of the entire optical fiber amplifier is sufficiently large. The optical fiber amplifier outputs a signal light having a sufficiently large intensity.
[0035]
Next, more specific examples of the optical fiber amplifier according to the present embodiment and experimental results will be described.
[0036]
The concentrations, the 1.53 μm band absorption peaks, and the lengths of the respective additional elements of the EDFs 34, 84A, and 84B used herein are those described in the following table.
[0037]
[Table 1]

[0038]
The signal light input to the input end optical connector 10 was a 16-wave WDM signal with a wavelength of 1543 nm to 1558 nm in increments of 1 nm, the intensity of each signal light was -22 dBm, and the total intensity of the 16 wave signal light was -10 dBm. In order to collectively amplify the multi-wavelength signal light, a semiconductor laser light source that outputs laser light having a wavelength of 0.98 μm is used as the excitation light source 32, and a semiconductor laser that outputs laser light having a wavelength of 1.48 μm is used as the excitation light source 82. A light source was used, and the center wavelength of the pass wavelength band of the optical bandpass filter 62 was 1565 nm, and the cutoff wavelength of the optical filter 70 was 1565 nm.
[0039]
FIG. 2 shows a spectrum diagram of light at each of three points in the optical fiber amplifier configured as described above. 2A is a spectrum diagram of signal light at an input point (point A in FIG. 1) of the input end optical connector 10, and FIG. 2B is an input point of the optical filter 70 (FIG. 1). FIG. 2C is a spectrum diagram of light at an output point (point C in FIG. 1) of the output end optical connector 90. FIG.
[0040]
In a laser oscillator composed of a closed loop including the pre-stage optical amplifying unit 30, the optical demultiplexer 40, the optical feedback unit 60, and the optical multiplexer 20, the laser oscillates at 1565 nm which is the center wavelength of the pass band of the optical bandpass filter 62. I do. With this laser oscillation, the population inversion in the EDF 34 of the pre-stage optical amplifier 30 was fixed, and as a result, a stable gain of about 20 dB was obtained in the pre-stage optical amplifier 30. Therefore, at the input point (point B in FIG. 1) of the optical filter 70, as shown in the spectrum diagram of FIG. 2B, the optically amplified 16-wave signal light and the laser oscillation light having the wavelength of 1565 nm appear. At this time, the intensity of each signal light was about -2 dBm, and the total intensity of the 16-wave signal light was about 10 dBm.
[0041]
Of the light input to the optical filter 70, the laser oscillation light having a wavelength of 1565 nm is reflected by the optical filter 70, and the 16-wave signal light passes through the optical filter 70, and is further input to the post-stage optical amplifying unit 80 and collectively. Amplified. The gain in the post-amplifier 80 is controlled to be constant at 7 dB by the control circuit 87 based on the ratio of the intensity of each of the input light and the output light detected by each of the light receiving elements 86A and 86B. Therefore, at the output point (point C in FIG. 1) of the output end optical connector 90, as shown in the spectrum diagram of FIG. 2C, the further amplified 16-wave signal light is output. At this time, since no laser oscillation light is input to the rear-stage optical amplifier 80, the total intensity of the output 16-wave signal light was about 17 dBm, which is the maximum output of the rear-stage optical amplifier 80. As described above, the gain of the entire optical fiber amplifier was 27 dB.
[0042]
When the pre-stage optical amplifier 30 is used alone without forming a closed loop, the total intensity of the 16-wave signal light at the point B is about 12.5 dBm. However, when laser oscillation is performed in a closed loop as in this embodiment, if the intensity of the input 16-wave signal light increases, laser oscillation stops, so that the population inversion cannot be fixed and the gain is kept constant. Lost. Therefore, in this embodiment, the maximum value of the total intensity of the 16-wave signal light at the point B is about 10.0 dBm in order to stably maintain the gain in the pre-stage optical amplifying unit 30 and is used alone. It is about 2.5 dBm lower than the case.
[0043]
FIG. 3 is a diagram illustrating the dependence of gain on the intensity of all input signal lights. In this figure, the solid line shows the case of the present embodiment, and the broken line shows the case of the prior art. As in the conventional optical fiber amplifier, the light intensity at each of the points A and C (that is, the intensity of the input light and the output light with respect to the entire optical fiber amplifier) is monitored, and the gain is controlled to be constant based on the ratio between the two. In this case, since the output light includes the ASE light, when the total intensity of the input signal light decreases as shown by the broken line in FIG. 3, the overall gain decreases.
[0044]
On the other hand, in the optical fiber amplifier according to the present embodiment, ASE light is generated in each of the pre-stage optical amplification unit 30 and the post-stage optical amplification unit 80, but the ASE light generated in the pre-stage optical amplification unit 30 is based on the ASE light. As compared with the ASE light component, the ASE light generated in the latter-stage light amplification unit 80 is so small as to be negligible. This is because the intensity of the signal light input to the rear-stage optical amplifier 80 is higher than the intensity of the signal light input to the front-stage optical amplifier 30, and the inversion distribution in the EDFs 84A and 84B of the rear-stage optical amplifier 80 is This is because most of the time is spent on amplifying the signal light and only a little is spent on generating the ASE light. Therefore, in the optical fiber amplifier according to the present embodiment, as shown by the solid line in FIG. 3, even if the total intensity of the signal light input to the input end optical connector 10 fluctuates, the overall gain is substantially constant at about 27 dB. Is kept.
[0045]
The laser oscillation light is reflected by the optical filter 70, passes through the optical demultiplexer 40, is input to the light receiving element 50, and is received. By monitoring the intensity of the laser oscillation light received by the light receiving element 50, it is possible to determine whether or not the gain in the pre-stage optical amplifier 30 is controlled to be constant. That is, as described above, when the total intensity of the input signal light exceeds a predetermined value, such as when the wave number of the signal light input to the input end optical connector 10 increases, laser oscillation stops, and the population inversion occurs. Cannot be fixed, and the gain cannot be kept constant. Therefore, when the intensity of the laser oscillation light received by the light receiving element 50 becomes lower than a predetermined value, it is considered that the constant gain control has been disturbed, and an alarm or the like can be issued to that effect. Based on this, maintenance of an optical fiber amplifier and an optical transmission system including the same can be performed.
[0046]
The present invention is not limited to the above embodiment, and various modifications are possible. In the description of the above embodiment, the optical fiber (EDF) doped with the Er element has been described as having the optical amplification function in each of the first-stage optical amplification unit and the second-stage optical amplification unit. However, the present invention is not limited to this. For example, an optical fiber to which a rare earth element other than the Er element (for example, an Nd (neodymium) element, a Pr (praseodymium) element, or the like) is added as a substance having an optical amplification action may be used. Instead of a fiber, a planar optical waveguide may be used.
[0047]
As means for monitoring the laser oscillation light, there is provided an optical demultiplexing means for extracting the laser oscillation light from the light traveling from the pre-stage optical amplification unit 30 to the post-stage optical amplification unit 80, and receiving the extracted laser oscillation light. May be detected. Further, in this case, the optical demultiplexing means and the optical filter 70 can also be used. For example, an optical fiber grating having a grating formed obliquely with respect to the optical axis of the optical fiber, wherein the laser oscillation light is Bragg-reflected and emitted to the side of the optical fiber, while the signal light passes directly. What causes it to be used can be used.
[0048]
【The invention's effect】
As described above in detail, according to the present invention, the closed loop including the pre-stage optical amplifier, the optical demultiplexer, the optical feedback unit, and the optical multiplexer constitutes a traveling-wave laser oscillator, and the optical feedback The laser oscillates at the passing wavelength (wavelength outside the signal light wavelength band) in the section. With this laser oscillation, the population inversion in the pre-stage optical amplifier is fixed, and the gain of the signal light in the pre-stage optical amplifier is kept constant. The signal light (light in the signal light wavelength band) output from the pre-stage optical amplification unit passes through the optical filter, enters the post-stage optical amplification unit, is further optically amplified with a constant gain, and is output.
[0049]
With such a configuration, even when the intensity of the input signal light is low, the signal light is optically amplified with a constant gain in the pre-stage optical amplification unit, and the signal light that has been optically amplified by the pre-stage optical amplification unit is post-staged. Since the light is input to the optical amplifying unit, it is also optically amplified with a constant gain in the subsequent optical amplifying unit. Therefore, even when the intensity of the input signal light is low, the gain of the optical amplifier is maintained at a sufficiently large value and stably.
[0050]
The pre-stage optical amplifier includes (1) an optical waveguide that optically amplifies and outputs light in a predetermined wavelength band input when the pumping light is supplied, and (2) pumping that outputs the pumping light and supplies the light to the optical waveguide. Means are preferred. Also, the post-stage optical amplifier. (1) an optical waveguide for optically amplifying and outputting light in a predetermined wavelength band input when the pumping light is supplied, (2) an excitation means for outputting the pumping light and supplying the light to the optical waveguide, (3) Input light detection means for detecting the intensity of light input to the optical waveguide, and (4) from the optical waveguide output Output light detection means for detecting the intensity of the light to be output, and (5) controlling the excitation light output from the excitation means based on the ratio of the light intensities detected by the input light detection means and the output light detection means, respectively. And a control unit that performs the control. When the optical waveguides of the pre-stage optical amplification unit and the post-stage optical amplification unit are optical fibers, the connection loss with the optical fiber line is small. When the Er element is added as a substance having an optical amplifying action, the optical waveguides of the first-stage optical amplifier and the second-stage optical amplifier have a signal light of a wavelength of 1.55 μm which is most frequently used in optical communication. Is amplified.
[0051]
Further, when the optical filter is an optical fiber grating in which a Bragg grating having a periodic refractive index change along the optical axis is formed, the connection loss with the optical fiber is small.
[0052]
In addition, the intensity of light outside the signal light wavelength band is Monitor laser oscillation When the monitoring means for monitoring is further provided, the intensity of the laser oscillation light generated in the closed loop including the pre-stage optical amplification unit is monitored by the monitoring unit, and the state of the optical amplification operation in the pre-stage optical amplification unit can be determined. Further, an alarm or the like can be issued based on the monitoring result by the monitoring means.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an optical fiber amplifier according to an embodiment.
FIG. 2 is a spectrum diagram of light at each of three points in the optical fiber amplifier according to the embodiment.
FIG. 3 is a diagram showing the dependence of gain on the intensity of all input signal light.
FIG. 4 is a configuration diagram of a conventional optical amplifier.
[Explanation of symbols]
Reference Signs List 10: input end optical connector, 20: optical multiplexer, 30: pre-stage optical amplifier, 31A, 31B: optical isolator, 32: excitation light source, 33: WDM coupler, 34: EDF, 40: optical demultiplexer, 50: Light receiving element, 60: optical feedback unit, 61: variable attenuator, 62: optical bandpass filter, 70: optical filter, 80: post-stage optical amplifier, 81A, 81B: optical isolator, 82: excitation light source, 83: WDM coupler, 84A, 84B: EDF, 85A, 85B: optical splitter, 86A, 86B: light receiving element, 87: control circuit, 90: output end optical connector.

Claims (7)

A pre-stage optical amplifier for optically amplifying and outputting the input predetermined wavelength band light,
An optical demultiplexer that receives the light output from the pre-stage optical amplifying unit and splits the light into a first light and a second light;
An optical feedback unit that transmits light other than the signal light wavelength band of the first light;
An optical multiplexer for multiplexing the signal light to be amplified and the light output from the optical feedback unit and inputting the multiplexed light to the pre-stage optical amplification unit ;
An optical filter that allows passage of light in the signal light wavelength band of the second light;
A gain is controlled to be constant based on the ratio of the intensity of each of the input light and the output light, and a post-stage optical amplifying unit that inputs the light output from the optical filter and amplifies and outputs the light.
An optical amplifier comprising: The pre-stage optical amplifier,
An optical waveguide that optically amplifies and outputs light in the predetermined wavelength band that is input when excitation light is supplied,
An excitation unit that outputs the excitation light and supplies the excitation light to the optical waveguide;
The optical amplifier according to claim 1, further comprising: The post-stage optical amplifier,
An optical waveguide that optically amplifies and outputs light in the predetermined wavelength band that is input when excitation light is supplied,
An excitation unit that outputs the excitation light and supplies the excitation light to the optical waveguide;
Input light detection means for detecting the intensity of light input to the optical waveguide,
Output light detection means for detecting the intensity of light output from the optical waveguide,
A control unit that controls the excitation light output from the excitation unit based on a ratio of light intensities detected by the input light detection unit and the output light detection unit,
The optical amplifier according to claim 1, further comprising: The optical amplifier according to claim 2, wherein the optical waveguide is an optical fiber. 5. The optical amplifier according to claim 4, wherein said optical waveguide is doped with Er element as a substance having an optical amplification effect. The optical amplifier according to claim 1, wherein the optical filter is an optical fiber grating on which a Bragg grating having a periodic change in refractive index along an optical axis is formed. 2. The optical amplifier according to claim 1, further comprising monitoring means for monitoring laser intensity by monitoring the intensity of light other than the signal light wavelength band.

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