WO2013042757A1 - Optical waveguide and method for controlling characteristics of optical waveguide - Google Patents

Abstract

[Problem] To achieve an optical modulator with lower insertion loss and with minimal unnecessary optical phase modulation. [Solution] An optical waveguide is provided with a first and second electric potential fixing means that are provided on top of the optical waveguide and that generate a predetermined electric field within the optical waveguide according to the electric potential applied thereto. The amount of phase change in light that passes through the optical waveguide is controlled on the basis of the electric field generated within the optical waveguide, on the basis of the first electric potential of the first electrical potential fixing means and the second electric potential of the second electric potential fixing means.

Description

Optical waveguide and optical waveguide characteristic control method

The present invention relates to an optical waveguide and an optical waveguide characteristic control method.

With the explosive demand for broadband multimedia communication services such as the Internet and video distribution, the introduction of high-distance wavelength-division-multiplexed optical fiber communication systems with longer distances, larger capacities, and higher reliability is progressing in trunk and metro systems. . In addition, optical fiber access services are rapidly spreading in subscriber systems. In a communication system using such an optical fiber, it is important to reduce the installation cost of an optical fiber that is an optical transmission line and to increase the transmission band utilization efficiency per optical fiber. For this reason, a wavelength multiplexing technique that multiplexes and transmits a plurality of signal lights having different wavelengths is widely used.
An optical modulator used in an optical transmitter for a wavelength division multiplexing optical fiber communication system is required to be capable of high-speed optical modulation and to have small signal light wavelength dependency. Furthermore, the optical modulator includes an unnecessary optical phase modulation component (when the modulation method is a light intensity modulation method) or a light intensity modulation component (the modulation method is an optical phase modulation method) that causes deterioration of the received light waveform during long-distance signal transmission. In the case of ()) is required to be suppressed as much as possible. For such applications, an MZ light intensity modulator in which an optical waveguide type optical phase modulator is incorporated in an optical waveguide type Mach-Zehnder (MZ) interferometer is practical.
The MZ light intensity modulator is manufactured using an electro-optic crystal whose refractive index changes in proportion to the applied electric field strength. The MZ light intensity modulator currently in practical use is based on a so-called planar optical waveguide circuit in which titanium is diffused in the substrate surface made of lithium niobate (LiNbO3, LN), which is a typical electro-optic crystal. As a planar optical waveguide circuit, an optical phase modulator and an optical waveguide type optical multiplexer / demultiplexer region are monolithically integrated on the same LN substrate to form an MZ interferometer, and an electric field is applied to the optical waveguide type optical phase modulator. A structure provided with an electrode for application is common.
In addition, an optical waveguide type semiconductor optical phase modulator or semiconductor MZ optical modulator using a III-V group compound semiconductor such as gallium arsenide (GaAs) or indium phosphide (InP) useful for integrating light source elements. There are many attempts to develop it. An optical waveguide type semiconductor optical phase modulator or a semiconductor MZ optical modulator is a medium in which the (complex) refractive index for signal light varies with the electric field strength (multi-element mixed crystal of III-V compound semiconductor or based on it) The laminated structure is an undoped core layer. In the optical waveguide type semiconductor optical phase modulator and the semiconductor MZ optical modulator, the core layer is sandwiched from above and below by the p-type / n-type conductive clad layers, so-called pin type diodes. A single mode optical waveguide having a structure is formed. A configuration in which a reverse bias voltage is applied to the thus configured optical waveguide is widely used.
Patent Documents 1 to 4 disclose techniques related to the present invention. Patent Document 1 describes a Mach-Zehnder type semiconductor device that adjusts the phase of light propagating through the device by sequentially applying negative, positive, and negative bias voltages to bias electrodes provided on the modulator. Patent Document 2 describes an optical modulator in which two arms each have one modulation electrode and one DC bias application electrode. Further, Patent Document 3 describes a semiconductor optical modulator provided with two modulation electrodes and two DC bias application electrodes. In addition, Patent Document 4 discloses an optical modulator including an arm provided with one electrode to which a modulation voltage is supplied and one electrode to which a CW voltage is supplied.

JP2007-248850 JP 2008-158133 A JP2009-163186A Special table 2007-531022

In order to realize an ideal optical modulation operation with an optical modulator, the total phase change (total phase shift amount) and loss that the modulated optical signal undergoes while propagating through each optical phase modulator area is as designed. It needs to be. However, structural parameters such as the dimensions and effective refractive index of the optical waveguide that constitutes the propagation path of the modulated optical signal may be slightly uneven in the substrate plane during manufacturing. As a result, in the manufactured optical waveguide, there is a possibility that the total phase change deviates from the design value. When such manufacturing variations occur in the manufacturing process of the MZ optical modulator, the modulated optical signal is transmitted through the respective paths via the pair of waveguide type optical phase modulators incorporated in the MZ optical modulator. A shift (optical phase offset) occurs in the total phase change received. Such an optical phase offset causes an undesirable phenomenon in optical fiber transmission, such as deterioration of extinction characteristics of an optical modulation signal and unnecessary optical phase modulation (chirping).
The influence of such an optical phase offset on transmission characteristics can be compensated, for example, by providing a circuit that shifts the level of the bias voltage of the drive electrical signal included in the modulated electrical signal so as to cancel the optical phase offset. However, the refractive index change with respect to the modulated signal light in the core layer of the waveguide type optical phase modulator also depends on the band gap energy of the core layer and the signal light wavelength. The influence of the refractive index change on the modulated signal light is generally not linear with respect to the electric field strength. For this reason, when different bias voltages are applied to a pair of waveguide type optical phase modulators, the optical modulation characteristics of the two waveguide type optical phase modulators (optical phase change with respect to the amplitude of the drive electric signal) The balance may be lost, and unnecessary optical phase modulation may be superimposed on the optical modulation output.
As another means for compensating for the influence of the optical phase offset, a configuration in which an optical phase offset compensator capable of electrically controlling the amount of phase shift is provided in each propagation path can be used. In order to realize such an optical phase offset compensator in a semiconductor optical modulator, carriers are injected into the core layer of a predetermined region of the optical waveguide or an electric field is applied, whereby the effective refractive index with respect to the modulated optical signal. May be changed electrically. In this case, the structure of the optical phase offset compensator may be the same as that of the optical phase modulator. Therefore, when forming an optical phase offset compensator monolithically integrated in a semiconductor MZ type optical modulator, it is only necessary to partially change the mask pattern used in the electrode forming process, and an additional manufacturing process is necessary. Not. In the configuration in which a region for injecting carriers or applying an electric field is provided in the optical modulator, the element length of the optical modulator is increased by the optical phase offset compensator. However, this configuration basically does not break the balance of the light modulation characteristics of the paired optical phase modulators. For this reason, the configuration in which the optical phase offset compensator is provided is more preferable from the viewpoint of optical fiber transmission characteristics than the method of changing the bias voltage of the modulated electric signal.
However, although it is static, when an electric field is applied to the core layer of the optical waveguide or carriers are injected, ohmic resistance between the electrode and the upper part of the cladding layer (upper cladding layer) is a practical problem. It needs to be kept low to the extent that there is no. For this purpose, a III-V compound semiconductor multi-element mixed crystal such as InGaAs, which has a smaller band gap energy than that of the upper cladding layer and is doped with a high concentration of impurities, is epitaxially grown as a contact layer between the electrode and the upper cladding layer. Need to be. However, the contact layer has a small band gap energy and is doped with a high concentration of impurities. For this reason, the contact layer has an absorption coefficient that is orders of magnitude greater than that of the cladding layer, which causes a large propagation loss in the fundamental propagation mode of the signal light propagating in the optical waveguide. In order to suppress an increase in propagation loss, it is desirable to make the upper cladding layer as thick as possible so that the electromagnetic field distribution of this fundamental propagation mode is not applied to the contact layer as much as possible by appropriately selecting the structural parameters. However, there is a limit to the reduction of propagation loss by increasing the thickness of the cladding layer for manufacturing reasons. For example, in the case of an InP-based optical waveguide used as a semiconductor optical device for 1550 nm band optical fiber communication, the thickness of the upper clad layer is 1. so that the effect of large light absorption in the contact layer can be suppressed to a practical extent. Generally, it is set to about 5 to 3 μm.
The increase in propagation loss due to the provision of the optical phase offset compensator also leads to the need to prepare a light source with high output and high power consumption. Therefore, from the viewpoint of reducing the power of the optical transmission system and the reliability of the light source It is not preferable. Further, from the viewpoint of suppressing the amplitude of the driving electric signal, it is advantageous that the element of the optical modulator is as long as possible. However, the lengthening of the element causes an increase in insertion loss. Therefore, a configuration in which an optical phase offset compensator is provided in a general optical modulator has a problem that it is difficult to achieve both reduction in drive voltage amplitude and reduction in insertion loss.
In the techniques described in Patent Documents 1 to 4, the phase of the optical signal is controlled by generating an electric field only in the optical waveguide below the electrode by applying a voltage to the electrode. However, this configuration requires an electrode having a shape along the region that generates the electric field in the optical waveguide. For this reason, an increase in propagation loss due to the contact layer provided under the electrode is unavoidable, and there is still a problem that it is difficult to achieve both a reduction in drive voltage amplitude and a reduction in insertion loss.
As described above, a practical optical modulator that is advantageous for reducing the size and power consumption of the next-generation optical fiber communication system and suppressing unnecessary optical phase modulation with a low insertion loss has not yet been developed.
An object of the present invention is to provide a technique for realizing an optical modulator that has a small insertion loss and can suppress unnecessary optical phase modulation.

An optical waveguide according to the present invention includes first and second potential fixing means that are provided on the optical waveguide and generate a predetermined electric field in the optical waveguide by an applied potential, and the first potential fixing means has The phase change amount of light passing through the optical waveguide is controlled based on the electric field generated in the optical waveguide by the first potential and the second potential of the second potential fixing means.
The method for controlling characteristics of an optical waveguide according to the present invention comprises: a first potential of first potential fixing means provided on the optical waveguide and generating a predetermined electric field in the optical waveguide by an applied potential; The amount of phase change of light passing through the optical waveguide is controlled based on the electric field generated in the optical waveguide by the second electric potential of the second electric potential fixing means provided on the road.

The present invention has an effect of realizing an optical waveguide with easy phase control and low insertion loss.

It is a figure which shows the structure of the optical modulator of 1st Embodiment. It is a figure explaining potential distribution along a signal light propagation axis in a 1st embodiment. It is a figure which shows the structure of the optical modulator of 2nd Embodiment. It is a figure explaining potential distribution along a signal light propagation axis in a 3rd embodiment. It is a figure which shows the structure of the optical modulator of 4th Embodiment.

　この関係が、ｚ_２~ｚ_３間、ｚ_３~ｚ_４間においても同様に成り立つことは明らかである。ここで、光導波路の構造が均一であれば、単位長さ当りの導電率（あるいは抵抗率）は一定なので、上記の電位勾配は線形であり、一次関数で表される。従って、光導波路の持つ位相オフセットを相殺するような移相量を生じる電位勾配を上式から求め、求めた電位勾配を生ずる電圧を電位固定手段に印加することで、光導波路１０６ａと光導波路１０６ｂとに発生する位相オフセットを補償することができる。
　このように、光変調器１００では、電圧を印加した電位固定手段の間で光導波路内部に生じた電位勾配を用いて移相量が制御される。そして、光変調器１００では、電位固定手段に印加する電圧を制御することで、２本の光伝搬経路１０６ａ及び１０６ｂで生ずる位相差が少なくなるように制御されることを可能としている。
　光変調器１００においては、被変調光信号の波長において吸収の大きなコンタクト層を光導波路に沿って設ける必要がない。従って、光変調器１００は、特許文献１~４に記載された構成と比較して、位相オフセットを補償する手段を設けることによる被変調光信号の損失増加を最小限に抑えることができ、その結果、低駆動電力で実用性に優れる半導体光変調器を提供可能である。
　電位勾配は、２つの電位固定手段１１１ａと電位固定手段１１１ｂとの間や電位固定手段１１４ａ、１１４ｂ（位置ｚ_４）と光位相変調器１０４ａ、１０４ｂ（位置ｚ_５）との間にも同様に発生する。すなわち、光分波器１０２、光合波器１０３、各電位固定手段及び光位相変調器１０４ａ、１０４ｂとの間にも電位分布が生じる。そして、２本の光伝搬経路の位相差を最適化するためには、これらの電位分布をも考慮されて、それぞれの電位固定手段の電位が厳密に設定されなければならない。しかし、電位を設定するためのパラメータの数が増えるほど、最適な電位の条件を短時間で求めるための手順は複雑になる。これは、光変調器の設計コスト及び運用コストの増加に繋がる。
　そこで、各電位固定手段に設定する電位を求める際のパラメータを減らすために、電位固定手段１１１ａと電位固定手段１１４ａとを短絡してもよい。また、電位固定手段１１１ｂと電位固定手段１１４ｂとを短絡してもよい。これにより、位相オフセットを相殺するための電位を電位固定手段１１２ａ、１１２ｂ、１１３ａ、１１３ｂにどのように与えても、ｚ１~ｚ４間の電位は固定される。その結果、光変調器１００は、電位固定手段１１２ａ、１１２ｂ、１１３ａ、１１３ｂに印加される電位のみで形成された電位勾配に基づいて位相補償を行うことが可能となる。その結果、位相オフセットを補償するための位相差がより精密に設定される。
　なお、電位固定手段１１１ａ及び１１４ａは、短絡ではなくそれぞれが固定された異なる電位に接続されてもよい。同様に、電位固定手段１１１ｂ及び１１４ｂも、それぞれが異なる電位となるように接続されてもよい。
　ところで、第１の実施形態の構成では、電位固定手段１１４ａ、１１４ｂ（位置ｚ_４）と光位相変調器１０４ａ、１０４ｂ（位置ｚ_５）との間の電位分布は、厳密には固定されているわけではない。すなわち、ｚ４とｚ５との間の電位分布は、光位相変調器１０４ａ、１０４ｂへ印加される駆動電気信号Ｖ_ｍａ（ｔ）、Ｖ_ｍｂ（ｔ）に応じて動的に変化する。そして、電位固定手段１１４ａ、１１４ｂと光位相変調器１０４ａ、１０４ｂとの間の電位分布が変化するので、電位固定手段１１４ａ、１１４ｂと光位相変調器１０４ａ、１０４ｂとの間を伝搬する間に信号光に与えられる移相量も動的に変化する。その結果、光変調出力にわずかではあるものの不要な光位相変調が重畳されて光伝送特性へ影響を及ぼす恐れがある。また、互いに異なる電位に接続された領域間には、その領域間の抵抗に反比例する電流が光導波路上を流れる。従って、この電流による光導波路の信頼性への影響も考慮されなければならない。
　まず、電位固定手段と光位相変調器との間を伝搬する光の移相量については、この領域間で受ける移相量は長さにも依存することを利用して、できるだけｚ_４とｚ_５との距離を短くするように構成してもよい。その結果、ｚ_４とｚ_５との間の電位分布による位相変化が光伝送特性へ与える影響を、実用上支障のない程度に抑圧可能である。この場合、ｚ_４とｚ_５との距離に反比例してこの間の抵抗は減少する。従って、この領域の導電率を、光伝送特性への影響が実用上支障のない程度に予め低く抑えておく必要がある。この領域の導電率を低くすることにより、この領域を流れる電流による信頼性への影響を低減することが可能である。導電率を低くするためには、半導体層（この場合は上部クラッド層）に対して例えばイオン注入を用いた高抵抗化等を実施すればよい。
　なお、短絡された電位固定手段１１１ａと１１４ａ、及び電位固定手段１１１ｂと１１４ｂの４つの電極の電位は、例えば０Ｖに固定されてもよいし、または光導波路の積層構造で決まる内蔵電位を打ち消す程度に順方向バイアス電圧が印加されてもよい。
　また、これら４つの電位固定手段の短絡手段としては、光変調器素子上の電極配線で電位固定手段が予め短絡されるように電極パターンを設計してもよいし、またはそれぞれの電位固定手段を形成する電極が独立に引き出され、１つの定電圧源へ接続されるようにしてもよい。
　また、電位固定手段１１２ａと１１３ａと、あるいは電位固定手段１１２ｂと１１３ｂとがそれぞれ短絡されてもよい。これにより、位相差補償に要する定電圧源をそれぞれ１つ減らすことが可能になり、制御も容易になる。なお、その場合の短絡手段についても上述の場合と同じである。
　また、光変調器１００は、電位固定手段１１１ａ~１１４ａ及び１１１ｂ~１１４ｂが、光分波器１０２と光位相変調器１０４ａ、１０４ｂとの間にあるものとして説明した。しかし、電位固定手段１１１ａ~１１４ａ及び１１１ｂ~１１４ｂは、光位相変調器１０４ａ、１０４ｂと、光合波器１０３との間に配置されても、光変調器１００は同様に光導波路１０６ａと光導波路１０６ｂとに発生する位相オフセットを相殺できる。
　さらに、一般に、半導体光導波路においては、電位固定手段に印加する電圧を変化させることで、損失特性を変化させることができる。また、電位固定手段に逆バイアス電圧を印加することにより、電位固定手段はその近傍の光導波路を透過する光を吸収し、光吸収電流を発生させる。光吸収電流の大きさは透過している光の光強度に依存するので、光吸収電流の大きさから、光導波路内を透過する光の強度をモニタすることができる。このような機能を実現するために、光変調器１００は、電位固定手段１１１ａ~１１４ａ、１１１ｂ~１１４ｂの少なくとも１つに印加する電圧を制御する制御手段１２０を備えていてもよい。
　制御手段１２０は、それぞれの電位固定手段に印加する電圧を制御するとともに、光吸収電流をモニタする機能を備えていてもよい。制御手段１２０は、モニタした結果から求めた光変調出力信号の強度や消光比が所定の値を満たすように、各電位固定手段に印加する電圧を変化させて光導波路の損失を制御してもよい。このように、光変調器１００は、制御手段１２０を用いて電位固定手段に印加する電圧を制御し、光吸収電流をモニタすることで、伝搬する信号光の強度を制御することも可能である。
　また、第１の実施形態においては、光導波路１０６ａ及び１０６ｂがそれぞれ電位固定手段１１１ａ~１１４ａ及び１１１ｂ~１１４ｂを備えるものとして説明した。しかし、所望の位相調整量が得られるのであれば、電位固定手段は光導波路１０６ａまたは１０６ｂのいずれかのみが備えていてもよい。
　なお、第１の実施形態の最小構成は、以下のように記載される。すなわち、光導波路１０６ａは、電位固定手段１１２ａ、１１３ａを備えている。そして、電位固定手段１１２ａの電位Ｖ２ａと、電位固定手段１１３ａの電位Ｖ３ａとによって光導波路１０６ａ内の電位固定手段１１２ａと電位固定手段１１３ａとの間に生じた電場に基づいて光導波路１０６ａを通過する光の位相変化量が制御される。従って、この構成においても、位相制御が容易で挿入損失が小さい光導波路が実現される。そして、このような光導波路を用いることで、挿入損失が小さく、不要な光位相変調を抑圧可能な光変調器が実現される。 (First embodiment)
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a configuration of an optical modulator 100 according to the first embodiment of the present invention. As shown in FIG. 1, the optical modulator 100 includes an optical demultiplexer 102, an optical multiplexer 103, a pair of optical phase modulators 104a and 104b, and optical waveguides 106a and 106b that connect them. Four potential fixing means 111a to 114a are arranged in series on the optical waveguide 106a. Similarly, four potential fixing means 111b to 114b are arranged in series on the optical waveguide 106b. The potential fixing means 111a to 114a and 111b to 114b include electrodes, and generate a potential gradient in the optical waveguides 106a and 106b by applying a predetermined potential to the electrodes.
In FIG. 1, z ₁ , z ₂ , z ₃ , z ₄ and z ₅ are positions in the light propagation direction in the optical waveguides 106a and 106b. Potential fixing means 111a and 111b are both in position _{z 1.} Potential fixing means 112a and 112b are both in position _{z 2.} Similarly, the potential fixing means 113a and 113b, 114a and 114b are at the positions z ₃ and z ₄ , respectively. Incidentally, _{z 5,} each optical phase modulators 104a and 104b indicating the position of the end portion of the potential fixing means 114a and 114b side. Further, when distinguishing the bias voltages applied to the potential fixing means 111a and 111b at z1, the bias voltages are described as V _1a and V _1b , respectively. V ₂ , V ₃ and V ₄ are similarly described using subscripts as necessary.
FIG. 2 is a diagram for explaining a potential distribution along the signal light propagation axis in the first embodiment. The potential distribution V (z) is V (z ₁ ) = V _1a and V _1b in the immediate vicinity of the first potential fixing means 111a and 111b, respectively. Further, V (z ₂ ) = V _2a and V _2b are in the immediate vicinity of the second potential fixing means 112a and 112b, respectively. The same applies to the potential fixing means 113a and 113b, 114a and 114b.
Here, as shown in FIG. 2 as an example, V _1a <V _2a <V _3a <V _4a , V _1b > V _2b > V _3b > V _4b . In this case, a potential gradient of V _1a ≦ V (z) ≦ V _2a occurs at the coordinate z (z ₁ ≦ z ≦ z ₂ ) on the optical waveguide 106a between the potential fixing means 111a and 112a. Similarly, at a coordinate z (z ₁ ≦ z ≦ z ₂ ) on the optical waveguide 106b between the potential fixing means 111b and 112b, a potential gradient of V _1b ≧ V (z) ≧ V _2b occurs. This relationship is basically the same with potential fixing means at other positions z ₂ , z ₃ , and z ₄ . In this way, by providing two independent potential fixing means having a short length in the light propagation direction on the optical waveguide, a potential gradient is generated between these potential fixing means.
Here, if the optical waveguide is manufactured with the same composition and the same laminated structure as the optical phase modulator, the potential distribution V (z) in the light propagation direction created by these potential fixing means is formed on the optical waveguide. A refractive index difference n (V (z)) is generated. Therefore, for example, the phase shift amount Δφ received while the signal light propagates through z ₁ to z ₂ is expressed by the following equation (1).

It is clear that this relationship holds similarly between z ₂ to z ₃ and z ₃ to z ₄ . Here, if the structure of the optical waveguide is uniform, the electrical conductivity (or resistivity) per unit length is constant, and thus the potential gradient is linear and is expressed by a linear function. Accordingly, a potential gradient that generates a phase shift amount that cancels the phase offset of the optical waveguide is obtained from the above equation, and a voltage that generates the obtained potential gradient is applied to the potential fixing means, whereby the optical waveguide 106a and the optical waveguide 106b. Can compensate for the phase offset generated in the above.
Thus, in the optical modulator 100, the amount of phase shift is controlled using the potential gradient generated inside the optical waveguide between the potential fixing means to which a voltage is applied. In the optical modulator 100, the voltage applied to the potential fixing means is controlled so that the phase difference generated in the two light propagation paths 106a and 106b can be controlled to be small.
In the optical modulator 100, it is not necessary to provide a contact layer having a large absorption along the optical waveguide at the wavelength of the modulated optical signal. Therefore, the optical modulator 100 can minimize the increase in the loss of the modulated optical signal due to the provision of the means for compensating for the phase offset, as compared with the configurations described in Patent Documents 1 to 4. As a result, it is possible to provide a semiconductor optical modulator with low driving power and excellent practicality.
Similarly, the potential gradient is also between the two potential fixing means 111a and the potential fixing means 111b or between the potential fixing means 114a and 114b (position z ₄ ) and the optical phase modulators 104a and 104b (position z ₅ ). appear. That is, a potential distribution is also generated between the optical demultiplexer 102, the optical multiplexer 103, each potential fixing unit, and the optical phase modulators 104a and 104b. In order to optimize the phase difference between the two light propagation paths, the potential of each potential fixing means must be set strictly in consideration of these potential distributions. However, as the number of parameters for setting the potential increases, the procedure for obtaining the optimum potential condition in a shorter time becomes more complicated. This leads to an increase in design cost and operation cost of the optical modulator.
Therefore, the potential fixing unit 111a and the potential fixing unit 114a may be short-circuited in order to reduce the parameters for obtaining the potential set for each potential fixing unit. Further, the potential fixing unit 111b and the potential fixing unit 114b may be short-circuited. Thus, the potential between z1 and z4 is fixed no matter how the potential fixing means 112a, 112b, 113a, 113b is supplied with a potential for canceling the phase offset. As a result, the optical modulator 100 can perform phase compensation based on a potential gradient formed only by the potential applied to the potential fixing means 112a, 112b, 113a, 113b. As a result, the phase difference for compensating for the phase offset is set more precisely.
It should be noted that the potential fixing means 111a and 114a may be connected to different potentials that are fixed to each other instead of being short-circuited. Similarly, the potential fixing means 111b and 114b may be connected so as to have different potentials.
By the way, in the configuration of the first embodiment, the potential distribution between the potential fixing means 114a and 114b (position z ₄ ) and the optical phase modulators 104a and 104b (position z ₅ ) is strictly fixed. Do not mean. That is, the potential distribution between z4 and z5 dynamically changes according to the drive electric signals V _ma (t) and V _mb (t) applied to the optical phase modulators 104a and 104b. Since the potential distribution between the potential fixing means 114a and 114b and the optical phase modulators 104a and 104b changes, the signal is transmitted while propagating between the potential fixing means 114a and 114b and the optical phase modulators 104a and 104b. The amount of phase shift given to light also changes dynamically. As a result, a slight but unnecessary optical phase modulation may be superimposed on the optical modulation output, which may affect the optical transmission characteristics. In addition, between regions connected to different potentials, a current that is inversely proportional to the resistance between the regions flows on the optical waveguide. Therefore, the influence of the current on the reliability of the optical waveguide must also be considered.
First, as for the amount of phase shift of light propagating between the potential fixing means and the optical phase modulator, z ₄ and z as much as possible using the fact that the amount of phase shift received between these regions also depends on the length. You may comprise so that the distance with ₅ may be shortened. As a result, the influence of the phase change due to the potential distribution between z ₄ and z ₅ on the optical transmission characteristics can be suppressed to an extent that there is no practical problem. In this case, the resistance between them decreases in inverse proportion to the distance between z ₄ and z ₅ . Therefore, it is necessary to keep the conductivity in this region low in advance so that the influence on the optical transmission characteristics is not practically hindered. By reducing the conductivity of this region, it is possible to reduce the influence on the reliability due to the current flowing through this region. In order to reduce the electrical conductivity, the semiconductor layer (in this case, the upper cladding layer) may be increased in resistance using ion implantation, for example.
It should be noted that the potentials of the four electrodes of the short-circuited potential fixing means 111a and 114a and the potential fixing means 111b and 114b may be fixed at, for example, 0V, or the internal potential determined by the laminated structure of the optical waveguide is canceled. A forward bias voltage may be applied.
Further, as the short-circuiting means of these four potential fixing means, the electrode pattern may be designed so that the potential fixing means is short-circuited in advance by the electrode wiring on the optical modulator element, or each potential fixing means is The electrodes to be formed may be drawn independently and connected to one constant voltage source.
Further, the potential fixing means 112a and 113a or the potential fixing means 112b and 113b may be short-circuited, respectively. This makes it possible to reduce the number of constant voltage sources required for phase difference compensation by one and facilitate control. In this case, the short-circuit means is the same as that described above.
Further, the optical modulator 100 has been described assuming that the potential fixing means 111a to 114a and 111b to 114b are between the optical demultiplexer 102 and the optical phase modulators 104a and 104b. However, even if the potential fixing means 111a to 114a and 111b to 114b are arranged between the optical phase modulators 104a and 104b and the optical multiplexer 103, the optical modulator 100 similarly uses the optical waveguide 106a and the optical waveguide 106b. The phase offset generated in the above can be canceled out.
Further, in general, in the semiconductor optical waveguide, the loss characteristic can be changed by changing the voltage applied to the potential fixing means. Further, by applying a reverse bias voltage to the potential fixing means, the potential fixing means absorbs light transmitted through the optical waveguide in the vicinity thereof and generates a light absorption current. Since the magnitude of the light absorption current depends on the light intensity of the transmitted light, the intensity of the light transmitted through the optical waveguide can be monitored from the magnitude of the light absorption current. In order to realize such a function, the optical modulator 100 may include a control unit 120 that controls a voltage applied to at least one of the potential fixing units 111a to 114a and 111b to 114b.
The control means 120 may have a function of monitoring the light absorption current while controlling the voltage applied to each potential fixing means. The control unit 120 may control the loss of the optical waveguide by changing the voltage applied to each potential fixing unit so that the intensity and extinction ratio of the light modulation output signal obtained from the monitored result satisfy a predetermined value. Good. As described above, the optical modulator 100 can control the intensity of the propagated signal light by controlling the voltage applied to the potential fixing unit using the control unit 120 and monitoring the light absorption current. .
In the first embodiment, the optical waveguides 106a and 106b have been described as including the potential fixing means 111a to 114a and 111b to 114b, respectively. However, as long as a desired phase adjustment amount can be obtained, only the optical waveguide 106a or 106b may be provided with the potential fixing means.
Note that the minimum configuration of the first embodiment is described as follows. That is, the optical waveguide 106a includes potential fixing means 112a and 113a. Then, it passes through the optical waveguide 106a based on the electric field generated between the potential fixing means 112a and the potential fixing means 113a in the optical waveguide 106a by the potential V2a of the potential fixing means 112a and the potential V3a of the potential fixing means 113a. The amount of phase change of light is controlled. Therefore, even in this configuration, an optical waveguide with easy phase control and low insertion loss is realized. By using such an optical waveguide, an optical modulator with small insertion loss and capable of suppressing unnecessary optical phase modulation is realized.

The first embodiment is an operation verification example for the semiconductor optical modulator 100. In the first embodiment, a pair of optical phase modulators 104a and 104b (hereinafter collectively referred to as “optical phase modulator 104”) is formed on an n-InP substrate (not shown). Yes. On the n-InP substrate, a core layer and a cladding layer sandwiching the core layer from above and below are formed. The core layer is composed of an undoped AlGaInAs multiple quantum well layer and undoped InGaAsP optical confinement layers formed above and below the undoped AlGaInAs multiple quantum well layer (both not shown). That is, the optical phase modulator 104 includes a so-called separate confinement heterostructure. The structure of the undoped AlGaInAs multiple quantum well layer has 12 well layers, a well layer thickness of 10 nm, a barrier layer thickness of 6 nm, and a transition wavelength of 1400 nm. The undoped InGaAsP optical confinement layer has a wavelength composition of 1300 nm and a thickness of 20 nm. The upper and lower cladding layers are each made of p-type and n-type InP, and the optical phase modulator 104 has a p-i-n diode structure. The optical phase modulator 104 has a so-called high mesa ridge structure, and has a width of 1.8 μm and a length of 750 μm. In the optical phase modulator 104, an electric field is applied to the undoped AlGaInAs multiple quantum well layer by applying a reverse bias voltage to the pin diode structure. When an electric field is applied, the optical phase modulator 104 changes the complex refractive index for the modulated optical signal in the 1550 nm band propagating therethrough by the quantum confined Stark effect.
The optical demultiplexer 102 and the optical multiplexer 103 are two-input, two-output multimode interference (MMI) multiplexers / demultiplexers having a stacked structure similar to that of the optical phase modulator 104. The optical phase modulator 104 is formed with an electrode to which a modulated electric signal is applied. In addition, a total of eight potential fixing means 111a to 114a and 111b are provided between the optical demultiplexer 102 and the optical phase modulator 104 and between the optical phase modulator 104 and the optical multiplexer 103, respectively. ~ 114b is formed. Each potential fixing means includes an electrode. Each electrode length is 10 μm, and the distance between adjacent electrodes is 150 μm between the electrode 2 and the electrode 3, and the other distance is 10 μm. The insulation resistance between adjacent electrodes is 1 MΩ or more.
The characteristics of the paired optical phase modulators 104a and 104b were as follows: the series resistance was 5Ω, the element capacitance was 0.4 pF or less, and the frequency response band was about 14 GHz. These optical phase modulators 104 are push-pull driven with a modulated electric signal having an amplitude of 0 V to -3 V, thereby changing the phase of a modulated optical signal having a wavelength of 1550 nm incident in a TE (Transverse Electric) basic mode by π / 2. I was able to.
Further, the voltages V1a and V1b applied to the potential fixing means 111a and 111b were respectively connected to a constant voltage source of 0V. Further, the voltages V4a and V4b applied to the potential fixing means 114a and 114b were respectively connected to a constant voltage source of −1.5V. In this way, the potentials of the potential fixing means 111a, 111b, 114a, 114b were fixed. In addition, voltages V _2a , V _2b , V _3a , and V _3b applied to the potential fixing means 112a, 112b, 113a, and 113b were connected to constant voltage sources, respectively. These voltages were appropriately set so that the intensity and extinction ratio of the light modulation output signal, the photocurrent flowing through the electrode provided on the light propagation path, and the like satisfy a certain value. As a result, the optical phase offset between the optical phase modulators 104a and 104b, which is caused by the non-uniformity of the processing dimension generated at the time of manufacture, is cancelled.
In addition, a practical insertion loss of about 4 dB was obtained under the above conditions. With this configuration, a practical 10 Gb / s-NRZ (Non Return to Zero) light modulation characteristic having a good eye opening was realized. Further, when this modulated optical signal was transmitted through a single-mode optical fiber having a length of 80 km, practical characteristics were confirmed with a reception sensitivity deterioration of 0.5 dB or less.
(Second Embodiment)
An optical modulator 200 according to the second embodiment of the present invention will be described. FIG. 3 is a diagram illustrating a configuration of an optical modulator 200 according to the second embodiment of the present invention. As shown in FIG. 3, in the optical modulator 200, as compared with the optical modulator 100 of the first embodiment, potential fixing means for compensating for the optical phase offset are arranged at both the input and output of the optical phase modulator, respectively. Has been.
The optical modulator 200 includes an optical demultiplexer 202, an optical multiplexer 203, a pair of optical phase modulators 204a and 204b, and optical waveguides 206a and 206b connecting them. On the optical waveguide 206a, four potential fixing means 211a to 214a and potential fixing means 215a to 218a are arranged in series on both sides of the optical phase modulator 204a. Similarly, four potential fixing means 211b to 214b and potential fixing means 215b to 218b are arranged in series on each side of the optical waveguide 206b with the optical phase modulator 204b interposed therebetween. The basic operation of the optical modulator 200 and the control method of each potential fixing means are the same as those of the optical modulator 100 of the first embodiment. The eight potential fixing means 211a to 214a and 211b to 214b suppress the phase offset of the optical waveguides 206a and 206b by the same control as the potential fixing means 111a to 114a and 111b to 114b described in FIG. it can. Further, the phase offsets of the optical waveguides 206a and 206b can be similarly compensated by using the eight potential fixing means 215a to 218a and 215b to 218b.
The configuration of the optical modulator 200 is rotationally symmetric about the optical phase modulators 204a and 204b. By providing such a configuration, the optical modulator 200 can freely adjust the direction of the optical modulator when the optical modulator is modularized, in addition to the effects exhibited by the optical modulator 100 described in the first embodiment. There is an effect that the degree is improved. Further, by making the optical modulator a rotationally symmetric shape, the optical modulator 200 is also expected to improve the yield during manufacturing.
In the second embodiment, the optical waveguides 206a and 206b have been described as including the potential fixing means 211a to 218a and 211b to 218b, respectively. However, if a desired phase adjustment amount can be obtained, for example, the optical waveguide 206a may include only the potential fixing means 211a to 214a, and the optical waveguide 206b may include only the potential fixing means 215b to 218b. Even in such a configuration, the planar structure of the optical modulator 200 is configured to be rotationally symmetric about the optical phase modulators 204a and 204b.
Here, as in the optical modulator 100 described in the first embodiment, the optical modulator 200 controls the voltage applied to at least one of the potential fixing units 211a to 218a and 211b to 218b. May be provided. Then, the control means 220 monitors the light absorption current flowing through the electrodes of the potential fixing means, so that the voltage applied to each potential fixing means so that the intensity and extinction ratio of the light modulation output signal satisfy a predetermined value. Control may be performed. Thus, the optical modulator 200 can also control the intensity of the propagated signal light by controlling the voltage applied to the potential fixing means using the control means 220.
(Third embodiment)
Next, the control of the optical modulator in the third embodiment of the present invention will be described. FIG. 4 is a diagram for explaining a potential distribution along the signal light propagation axis in the third embodiment. In the third embodiment, the method of applying a voltage to the potential fixing means in the optical modulator 100 described in FIG. 1 is changed. As shown in FIG. 4, in the third embodiment, in the optical modulator 100 described in FIG. 1, respectively applied certain potential fixing means 114a, to 114b to potential fixing means 111a, 111b and _{z 4} in _{z 1} The voltage V _FIX was fixed at 0V. In the third embodiment, furthermore, a potential fixing means 113a in the potential fixing means 112a and z3 in the _{z 2} are short-circuited, is connected to a constant voltage source of the voltage _V 2a _{(= V 3a).} Then, the potential fixing means 113b in the potential fixing means 112b and the position _{z 3} at position z2 is connected to a constant voltage source of voltage _V 2b _{(= V 3b).} Such an arrangement, an optical phase offset is given by the voltage _{V 2a} is the waveguide 106a, the optical phase offset is given by the voltage _{V 2b} is the optical waveguide 106b. Also in this case, the optical modulator 100 can control the phase offset of each optical waveguide by the voltages V _2a and V _2b applied to the potential fixing means.
In the third embodiment, the control in the optical modulator 100 described in FIG. 1 has been described. However, the same control may be applied to the control of the optical modulator 200 described with reference to FIG. For example, the above-described control of the potential fixing units 111a to 114a and 111b to 114b of the optical modulator 100 may be replaced with the potential fixing units 211a to 214a and 211b to 214b of the optical modulator 200. Alternatively, the control of the potential fixing means 111a to 114a and 111b to 114b of the optical modulator 100 may be replaced with the potential fixing means 215a to 218a and 215b to 218b of the optical modulator 200.
(Fourth embodiment)
Next, an optical modulator according to a fourth embodiment of the present invention will be described. FIG. 5 is a diagram showing a configuration of an optical modulator according to the fourth embodiment of the present invention.
An optical modulator 400 shown in FIG. 5 includes an optical demultiplexer 401, an optical multiplexer 406, and MZ optical modulators 407 and 408. The configuration of the MZ optical modulators 407 and 408 is the same as that of the optical modulator 100. The input signal light (Input) is branched into two by the optical demultiplexer 401 and further branched by the optical demultiplexers 402 and 403 inside the MZ optical modulators 407 and 408. The four-branched input signal light is modulated by the optical modulators 409a, 409b, 410a and 410b, multiplexed by the optical multiplexers 404 to 406, and output as output signal light (Output).
The MZ optical modulator 407 includes an optical demultiplexer 402, potential fixing means 421a to 424a, 421b to 424b, and an optical multiplexer 404. The MZ optical modulator 408 includes an optical demultiplexer 403, potential fixing means 431a to 434a, 431b to 434b, and an optical multiplexer 405. Further, potential fixing means 441a to 444a and 441b to 444b are provided between the optical multiplexers 404 and 405 and the optical multiplexer 406. The operation and effect of these eight potential fixing means are the same as those of the potential fixing means of the optical modulator described in the first to third embodiments. Therefore, also in the optical modulator 400, it is possible to cancel the optical phase offset generated in the optical waveguide provided with the potential fixing means.
As described above, in the optical modulator 400 of the fourth embodiment shown in FIG. 5, the potential fixing means 441a to 444a and 441b to 444b are arranged between the MZ optical modulators 407 and 408 and the optical multiplexer 406. Yes. However, except for this point, the optical modulator 400 has a configuration in which the two optical phase modulators included in the optical modulator 100 of the first embodiment are replaced with optical intensity modulators. As this light intensity modulator, an MZ light intensity modulator having the same configuration as that of the light modulator 100 of the first embodiment is used. The optical modulator 400 is controlled so that the phase difference of the modulated optical signals passing through the MZ light intensity modulators 407 and 408 becomes π / 2. Thereby, the optical modulator 400 can perform optical quadrature modulation (optical I / Q modulation, I: In-phase, Q: Quadrature-phase). The phase offset compensation principle and the control method are the same as those described in the optical modulator 100 of the first embodiment, and thus detailed description thereof is omitted.
Note that the optical modulator 400 may include a control unit 420 that controls the voltage applied to the potential fixing unit, similarly to the optical modulators 100 and 200 described in the first and second embodiments.
The control means 420 monitors the light absorption current flowing in the potential fixing means provided on the light propagation path, and adjusts the voltage applied to each potential fixing means so that the intensity and extinction ratio of the light modulation output signal satisfy predetermined values. Control may be performed. As described above, the optical modulator 400 can also control the intensity of the propagated signal light by controlling the voltage applied to the potential fixing means using the control means 420.
While the present invention has been described with reference to the embodiments, the present invention is not limited to the above embodiments. Various changes that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention. This application is Japanese Patent Application No. 2011-208257 filed on September 23, 2011. Claiming the priority based on, the entire disclosure of which is hereby incorporated.
In addition, although embodiment of this invention can be described also as the following additional remarks, it is not limited to these.
(Appendix 1)
Provided with first and second potential fixing means provided on the optical waveguide and generating a predetermined electric field in the optical waveguide by an applied potential;
The phase of light passing through the optical waveguide based on the electric field generated in the optical waveguide by the first electric potential of the first electric potential fixing means and the second electric potential of the second electric potential fixing means. An optical waveguide whose amount of change is controlled.
(Appendix 2)
Further comprising third and fourth potential fixing means for generating a predetermined electric field in the optical waveguide by the applied potential;
The third potential fixing means, the first potential fixing means, the second potential fixing means, and the fourth potential fixing means are arranged in series on the optical waveguide in this order, and further, the third potential fixing means The phase change amount of the light passing through the optical waveguide is controlled based on the electric field generated in the optical waveguide by the third potential possessed by the potential fixing means and the fourth potential possessed by the fourth potential fixing means. The optical waveguide according to Appendix 1.
(Appendix 3)
The optical waveguide according to appendix 2, wherein the first and second potentials are applied so that a predetermined potential difference is generated between the first and second potential fixing means.
(Appendix 4)
The optical waveguide according to appendix 2 or 3, wherein the third and fourth potentials are applied so that a predetermined potential difference is generated between the third and fourth potential fixing means.
(Appendix 5)
A first optical demultiplexer that bifurcates input light, first and second optical modulators that respectively modulate the bifurcated light, and the first and second optical modulators. A first optical multiplexer for combining outputs, a first optical waveguide connecting the first optical demultiplexer and the first optical modulator, the first optical demultiplexer, and the first optical demultiplexer. A second optical waveguide connecting the two optical modulators, and at least one of the first and second optical waveguides is provided with the optical waveguide described in any one of appendixes 1 to 4 , Light modulator.
(Appendix 6)
A first optical demultiplexer that bifurcates input light, first and second optical modulators that respectively modulate the bifurcated light, and the first and second optical modulators. A first optical multiplexer for combining outputs, a third optical waveguide connecting the first optical modulator and the first optical multiplexer, the second optical modulator, and the first optical modulator. And a fourth optical waveguide connecting to the optical multiplexer, wherein the optical waveguide according to any one of appendixes 1 to 4 is disposed on at least one of the first and second optical waveguides. vessel.
(Appendix 7)
A first optical demultiplexer that bifurcates input light, first and second optical modulators that respectively modulate the bifurcated light, and the first and second optical modulators. A first optical multiplexer for combining outputs, a first optical waveguide connecting the first optical demultiplexer and the first optical modulator, the first optical demultiplexer, and the first optical demultiplexer. A second optical waveguide connecting the second optical modulator, a third optical waveguide connecting the first optical modulator and the first optical multiplexer, and the second optical modulator; A fourth optical waveguide connecting the first optical multiplexer,
In the first to fourth optical waveguides, the optical waveguide described in any one of appendixes 1 to 4 is disposed at a position that is rotationally symmetric about the positions of the first and second optical modulators. , Light modulator.
(Appendix 8)
Any one of appendixes 5 to 7, further comprising first control means for controlling the voltage applied to the first to fourth potential fixing means so that the output power of the optical modulator becomes a predetermined value. Light modulator.
(Appendix 9)
The optical modulator according to any one of appendices 5 to 8, further comprising a monitoring unit that monitors a photocurrent output from at least one of the first to fourth potential fixing units.
(Appendix 10)
The optical modulator according to appendix 9, further comprising second control means for controlling an applied voltage of the first and second potential fixing means so that the photocurrent becomes a predetermined value.
(Appendix 11)
The optical modulator according to any one of appendices 5 to 10, wherein the first and second optical modulators are optical phase modulators.
(Appendix 12)
The optical modulator according to any one of appendices 5 to 10, wherein the first and second optical modulators are optical intensity modulators.
(Appendix 13)
A second optical demultiplexer that divides the input light into two, a third and a fourth optical modulator that respectively modulate the bifurcated light, and a third and a fourth optical modulator. A second optical multiplexer for merging the outputs,
The third and fourth optical modulators are the optical modulators described in Appendix 11, respectively, and the third and fourth optical modulators further add a phase difference of π / 2 to the bifurcated light. Give a light modulator.
(Appendix 14)
Generated in the optical waveguide by the first potential of the first potential fixing means provided on the optical waveguide and the second potential of the second potential fixing means provided on the optical waveguide A method for controlling the characteristics of an optical waveguide, comprising: controlling a phase change amount of light passing through the optical waveguide based on an electric field.

100, 200, 400 Optical modulator 102, 202, 401 to 403 Optical demultiplexer 103, 203, 404 to 406 Optical multiplexer 104a, 104b, 204a, 204b Optical phase modulator 106a, 106b, 206a, 206b Optical waveguide 120 220, 420 Control means 111a to 114a, 111b to 114b Potential fixing means 211a to 218a, 211b to 218b Potential fixing means 407, 408 MZ optical modulator

Claims (10)

Provided with first and second potential fixing means provided on the optical waveguide and generating a predetermined electric field in the optical waveguide by an applied potential;
The phase of light passing through the optical waveguide based on the electric field generated in the optical waveguide by the first electric potential of the first electric potential fixing means and the second electric potential of the second electric potential fixing means. An optical waveguide whose amount of change is controlled. Further comprising third and fourth potential fixing means for generating a predetermined electric field in the optical waveguide by the applied potential;
The third potential fixing means, the first potential fixing means, the second potential fixing means, and the fourth potential fixing means are arranged in series on the optical waveguide in this order, and further, the third potential fixing means The phase change amount of the light passing through the optical waveguide is controlled based on the electric field generated in the optical waveguide by the third potential possessed by the potential fixing means and the fourth potential possessed by the fourth potential fixing means. The optical waveguide according to claim 1. 3. The optical waveguide according to claim 2, wherein the first and second potentials are applied so that a predetermined potential difference is generated between the first and second potential fixing means. 4. The optical waveguide according to claim 2, wherein the third and fourth potentials are applied so that a predetermined potential difference is generated between the third and fourth potential fixing means. A first optical demultiplexer that splits the input light into two;
First and second optical modulators that respectively modulate the bifurcated light;
A first optical multiplexer for combining the outputs of the first and second optical modulators;
A first optical waveguide connecting the first optical demultiplexer and the first optical modulator;
A second optical waveguide connecting the first optical demultiplexer and the second optical modulator,
An optical modulator, wherein the optical waveguide according to claim 1 is arranged on at least one of the first and second optical waveguides. A first optical demultiplexer that splits the input light into two;
First and second optical modulators that respectively modulate the bifurcated light;
A first optical multiplexer for combining the outputs of the first and second optical modulators;
A first optical waveguide connecting the first optical demultiplexer and the first optical modulator;
A second optical waveguide connecting the first optical demultiplexer and the second optical modulator;
A third optical waveguide connecting the first optical modulator and the first optical multiplexer;
A fourth optical waveguide connecting the second optical modulator and the first optical multiplexer, and
A position at which at least two of the first to fourth optical waveguides are rotationally symmetric about the positions of the first and second optical modulators, the optical waveguide according to any one of claims 1 to 4 An optical modulator arranged in The first control means for controlling the applied voltage to the first to fourth potential fixing means so that the output power of the optical modulator becomes a predetermined value. Light modulator. 8. The optical modulator according to claim 5, further comprising monitoring means for monitoring a photocurrent output from at least one of the first to fourth potential fixing means. 9. The optical modulator according to claim 8, further comprising second control means for controlling an applied voltage of the first and second potential fixing means so that the photocurrent becomes a predetermined value. A first potential of first potential fixing means provided on the optical waveguide and generating a predetermined electric field in the optical waveguide by the applied potential, and a second potential fixed provided on the optical waveguide A method for controlling characteristics of an optical waveguide, comprising: controlling a phase change amount of light passing through the optical waveguide based on an electric field generated in the optical waveguide by a second electric potential of the means.

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