JP2015106133A - Mz optical modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a MZ optical modulator capable of realizing stable extinction characteristics.SOLUTION: A MZ optical modulator 1 includes: an optical input waveguide 11; a demultiplexer 12 demultiplexing a light ray input from the optical input waveguide 11 to two light rays; a pair of optical waveguides 13 for giving a phase difference between the two light rays obtained by demultiplexing by the demultiplexer 12; a multiplexer 14 multiplexing the two light rays having the phase difference given therebetween by the paired optical waveguides 13 by causing the two light rays to interfere with each other; and an optical output waveguide 15 outputting the light ray obtained by multiplexing by the multiplexer 14 from one of two output ports. The paired optical waveguides 13 each include a semiconductor multilayer structure, and each include a modulation signal electrode 16 to which a modulation signal voltage is applied; a phase adjustment electrode 17 to which a phase adjustment voltage is applied; and a SOA electrode 18 to which an injection current is applied for optical amplification. The injection currents applied to the respective SOA electrodes 18 are controlled independently, thereby exerting a control such that intensities of the light rays input from the optical waveguides 13 to the multiplexer 14 are equal to each other.

Description

  The present invention relates to a Mach-Zehnder (MZ) optical modulator, and more particularly to an MZ optical modulator having a semiconductor layer structure.

  Semiconductor optical waveguide devices are integrated devices that have built-in optical signal processing functions from single-function devices represented by conventional semiconductor laser diodes, due to recent progress in crystal growth technology and high-precision processing technology. Development is progressing.

  A device in which semiconductor functional elements are integrated is formed by coupling electrically and optically separated functional elements with an input / output optical waveguide. Such an optical waveguide has functions such as signal light distribution and phase state interference by multiplexing and demultiplexing of guided light.

  As a typical optical modulator to which these functions are applied, there is a Mach-Zehnder (MZ) optical modulator shown in FIG. The outline of this operation principle is as follows. The MZ optical modulator 10 divides the light propagated from one optical input waveguide 11 into two interference waveguides 13 by a duplexer 12, and the refractive index of each waveguide 13 is controlled by a phase controller 21. By controlling the phase, the phase state of the light propagating through each waveguide 13 is controlled. In order to control the phase, an electric field is applied to the phase controller 21. Thereafter, the two lights whose phase states are controlled are interfered by the multiplexer 14 and the output light is extracted from the optical output waveguide 15.

  The MZ optical modulator 10 shown in FIG. 1 can operate as an optical modulator by controlling the output light intensity from the multiplexer. That is, when the two lights strengthen each other, the level is on, and when the two lights are weak, the level is off. The difference between the on level and the off level is the extinction characteristic, and the difference is considered to be better.

  Ideally, it is desirable that both the branching ratio of the duplexer and the multiplexing ratio of the multiplexer are 1: 1. Ideally, when the electric field is applied to the phase control unit, it is desirable that only the phase changes and the light intensity does not change at all. Further, when light is input from the two interference waveguides to the multiplexer, When the intensity of the two lights is equal, the extinction characteristic is improved.

  As the multiplexer, for example, a multi-mode interference type (multi-mode interferometer: MMI) is used.

C. Rolland, R.A. S. Moore, F.M. Shepherd, G.M. Hillier, “10 Gbit / s, 1.56 μm MULTI QUANTUM WELL InP / InGaAsP MACH-ZEHNDER OPTICAL MODULATOR”, ELECTRONICS LETTERS, March 4, 1993, Vol. 29, no. 5, p. 471-472

  However, in an actual device, the branching ratio of the demultiplexer that branches and propagates the light to the interference system is deviated from 1: 1 depending on the wavelength of the light and the device temperature, or depends on the fabrication accuracy of the original optical waveguide. There may be variations in propagation loss. In particular, in the case of a semiconductor optical modulator, fluctuations in optical absorption loss due to an applied electric field during phase control and fluctuations in optical absorption loss due to signal light wavelength may occur when determining the operating conditions of the optical modulator. This may also change the intensity of light propagated from the interference optical waveguide to the multiplexer.

  FIG. 2 shows an example of the relationship between the applied electric field strength and the light absorption loss. According to FIG. 2, for example, when an electric field strength of 6 V / μm is required at the time of phase control, an optical absorption loss of 0.3 dB occurs even when the input wavelength is 1530 nm, 2 dB, 1540 nm is 1 dB, and 1550 nm. As a result, the extinction characteristics deteriorate.

  In other words, if the intensity of light propagating to the multiplexer is different, even if the interference state is controlled by the phase condition, phase state control is performed according to the light intensity of the weaker intensity. Leakage light causes an increase in off-level intensity and a decrease in on-level intensity, resulting in deterioration of extinction characteristics.

  Therefore, in order to obtain excellent extinction characteristics, it is important to equalize the light intensities input from the two interference optical waveguides to the MMI multiplexer. In an MZ optical modulator using an LN (lithium niobate: LiNbO3) material that has been generally used so far, a multiplexer such as a branching ratio of an input-side branching filter or a subsequent propagation loss of an interference waveguide is used. Since the characteristics that affect the input light intensity are almost determined at the time of device fabrication, fabrication accuracy is important.

  On the other hand, also in the semiconductor optical waveguide MZ modulator, the branching ratio of the branching filter and the subsequent waveguide propagation loss are greatly affected by the fabrication accuracy (similar to the LN material). Furthermore, since an electric field for changing the refractive index is applied during operation as a modulator, a change in the optical absorption loss of the semiconductor due to this also affects the waveguide propagation loss. In particular, when adjusting the phase of light between the interference waveguides, the electric fields applied to the two waveguides have different values. Therefore, the light absorption loss due to this electric field application varies between the interference waveguides. It seems to become. Therefore, conventional MZ optical modulators composed of semiconductor optical waveguides are designed to include the effects on the extinction characteristics for the electric field applied to the interference waveguide as well as the accuracy of manufacturing the duplexer and multiplexer. In some cases, the operating conditions are limited.

  As described above, in the semiconductor MZ optical modulator, the branching ratio of the demultiplexer for branching the input signal light to the interference waveguide is deviated from 1: 1, the light in the interference waveguide is A difference in light intensity from the interference optical waveguide to the multiplexer caused by a difference in propagation loss and absorption loss due to an applied electric field deteriorates the extinction characteristic at the time of intensity modulation.

  The present invention has been made in view of the above-described conventional problems, and an object of the present invention is to have a function of improving the extinction characteristic when intensity modulation is obtained by controlling phase conditions, and this is controlled during continuous operation. An object of the present invention is to provide an MZ optical modulator capable of realizing a stable extinction characteristic over a long operation period. Furthermore, another object of the present invention is to provide an MZ modulator that has a function of eliminating and amplifying the propagation loss that is a problem in the optical modulator.

  In order to solve the above-described problem, an invention described in an embodiment includes an optical input waveguide, a duplexer that splits light input from the optical input waveguide into two, and the duplexer. A pair of optical waveguides that give a phase difference to the two lights branched at, and a multiplexer that multiplexes the two lights given a phase difference by the pair of optical waveguides by interfering with each other, An MZ optical modulator comprising an optical output waveguide that outputs light combined in the multiplexer from one of two output ports, wherein the pair of optical waveguides is a slab type semiconductor Each of the pair of optical waveguides has a laminated structure, and each of the pair of optical waveguides has a modulation signal electrode that applies a modulation signal voltage that modulates light propagating through the optical waveguide, and a waveguide length of the pair of optical waveguides. A phase adjusting electrode for applying a phase adjusting voltage for compensating the difference, and the optical waveguide. And an SOA electrode for applying an injection current for amplifying the light to be carried, and by independently controlling the injection current applied to each of the SOA electrodes, the light intensity of light input from the optical waveguide to the multiplexer Are controlled so as to be equal to each other.

It is a schematic diagram of a conventional semiconductor MZ light modulator. It is a figure which shows the wavelength dependence of the light absorption loss in a semiconductor waveguide structure. It is a figure which shows the structural example of the semiconductor MZ optical modulator of 1st Embodiment. It is a figure explaining the slab type semiconductor laminated structure which comprises an interference type | system | group optical waveguide. It is a figure which shows the structural example of the semiconductor MZ optical modulator of 2nd Embodiment.

  Hereinafter, embodiments of the present invention will be described in detail.

  The semiconductor MZ optical modulator according to the present embodiment is provided for an optical input waveguide, a duplexer that branches light input from the optical input waveguide into two, and two lights that are branched by the duplexer. A pair of optical waveguides that give a phase difference, a multiplexer that multiplexes two light beams that are given a phase difference by a pair of optical waveguides, and two lights that are combined in the multiplexer An MZ optical modulator having an optical output waveguide that outputs from any one of output ports, wherein independent optical intensity control regions are formed in the two optical waveguides, and interference is caused by the optical intensity control region. A configuration is provided in which the light intensity input from the system waveguide to the multiplexer is controlled to be equal. By controlling and multiplexing the phases of the signal lights having the same light intensity, it is possible to reduce components that are output without interference of the phase state. As a result, it is possible to prevent deterioration of the extinction characteristic during intensity modulation. Furthermore, in the semiconductor MZ optical modulator of the present embodiment, the propagation loss in the semiconductor MZ optical modulator can be eliminated and optical amplification can be performed by increasing the amplification factor in both of the two independent light intensity control regions. .

  As a method of controlling the intensity of light propagating from the interference semiconductor waveguide to the output-side multiplexer, a gain is obtained with respect to the wavelength of the signal light separately from the conventional electrode for applying an electric field for controlling the refractive index. Forming a semiconductor optical amplifier (SOA) having a pn junction structure and a carrier is generated by injecting a current into an electrode of the SOA, thereby amplifying the signal light propagating through the semiconductor waveguide. It becomes possible.

  Here, the light propagating through each interference waveguide is independently amplified, and the signal light controlled to have the same light intensity is combined to suppress the leakage light component from the phase-controlled light. Therefore, the extinction characteristic can be improved by reducing the off-level light output intensity and increasing the on-level light intensity. Also, if the injection current of each SOA is always controlled so that the extinction characteristic is maximized while monitoring the combined output, the high extinction characteristic is deteriorated due to fluctuation over a long period of continuous operation. It becomes possible to hold without.

  Further, the signal light intensity amplified by the SOA can be directly detected by forming a branch waveguide and a photodiode for monitoring the SOA amplified signal intensity from each interference waveguide. By controlling the injection current into the SOA so that the detected light intensities coincide with each other, a high extinction characteristic can be realized more easily.

  Also, by always controlling the propagation light intensity, even if the propagation light intensity of both interference waveguides fluctuates due to some factor during actual operation, the input to the multiplexer can be controlled by controlling the amplification factor of the SOA. Since the light intensity can always be kept constant, it is also effective for stabilizing the characteristics during long-term continuous operation of the element.

  It is also preferable to control the injection current of both SOAs by observing the output light intensity from the optical output waveguide. Even when the coupling between the semiconductor MZ modulator and the external input / output waveguide such as an optical fiber is shifted and the optical loss as the semiconductor MZ modulator is increased, compensation is performed by increasing the amplification factors of both SOAs. Can do.

(First embodiment)
First, a semiconductor MZ optical modulator having an SOA will be described. FIG. 3 is a diagram showing a schematic configuration of the semiconductor MZ optical modulator of the present embodiment. In the optical modulator 1 of this embodiment, an optical input waveguide 11, a duplexer 12, a pair of interference optical waveguides 13, a multiplexer 14, and an optical output waveguide 15 are connected in cascade. It is configured. The interference optical waveguide 13 has a modulation signal electrode 16, a phase modulation electrode 17, and an SOA electrode 18.

  The optical input waveguide 11, the duplexer 12, the pair of interference optical waveguides 13, the multiplexer 14, and the optical output waveguide 15 are a core layer that guides light on the substrate layer. And a slab-type semiconductor laminated structure in which a clad layer for confining light above and below the core layer is laminated. In this semiconductor laminated structure, the light input from the optical input waveguide 11 is branched into two by the branching filter 12 and a phase difference is given to the light branched into two by the pair of interference optical waveguides 13. The two lights given the phase difference are combined by being interfered by the multiplexer 14, and output from one of the two ports of the optical output waveguide 15.

  FIG. 4 is a diagram for explaining the layer structure of the interference optical waveguide. 4A shows an interference optical waveguide in a region where no electrode is provided, FIG. 4B shows an interference optical waveguide in a region having a modulation signal electrode or a phase modulation electrode, and FIG. 4C forms an SOA. An interference optical waveguide is shown.

  As shown in FIG. 4A, the interference optical waveguide 13 includes an n-InGaAsP contact layer 22, an n-InP clad layer 23, an i-MQW core layer 24, and a p-type layer on an n-InP substrate 21. The InP clad layer 25 is laminated.

  As shown in FIG. 4B, the modulation signal electrode 16 includes an n-side electrode 28 provided on the n-InGaAsP contact layer 22 and a p-InGaAsP contact layer stacked on the p-InP clad layer 25. 26 is a P-side electrode 27 provided on 26, and is configured as a traveling wave electrode provided along the interference optical waveguide 13. The modulation signal electrode 16 changes the speed of light propagating through the interference optical waveguide 13 by changing the refractive index of the interference optical waveguide 13 by an electric signal flowing along the interference optical waveguide 13. Thus, since the propagation speed of light changes due to the change of the electric signal, the electric signal input to the modulation signal electrode 16 is converted into an optical signal propagating through the interference optical waveguide 13. By applying different modulation signal voltages at the modulation signal electrodes 16 provided in each of the pair of interference optical waveguides 13, a phase difference is given to the two optical signals propagating through the pair of interference optical waveguides 13. Yes.

  As shown in FIG. 4B, the phase modulation electrode 17 includes an n-side electrode provided on the n-InGaAsP contact layer 22, a p-InGaAsP contact layer 26 provided on the p-InP cladding layer 25, and A P-side electrode 27 is provided. The phase modulation electrode 17 is an electrode for applying a phase adjustment voltage for compensating for a difference in waveguide length between the pair of interference optical waveguides 13. By applying a voltage for adjustment to one of the two phase modulation electrodes 17, the manufacturing error of the length of the pair of interference optical waveguides 13 can be finely adjusted by phase adjustment.

  As shown in FIG. 4C, the SOA electrode 18 is an optical amplifier in which the i-MQW core layer 24 in the semiconductor laminated structure of the interference optical waveguide 13 shown in FIG. 4A functions as the i-MQW active layer 29. Electrodes 27 and 28 for applying a voltage to the (SOA) region. The p-InP cladding layer 25 is provided with an electrode 27 via a p-InGaAsP contact layer 26. By using the SOA electrode 18 to cause a forward current to flow through the interference optical waveguide 13 having a pn junction structure, the interference optical waveguide 13 functions as an optical amplifier, and the intensity of light propagating through the i-MQW active layer 29 is increased. Can be amplified.

  In the semiconductor MZ optical modulator 1 shown in FIG. 3, light input from the input-side optical waveguide 11 to the demultiplexer 12 is branched into two interference optical waveguides 13 and propagates. Thereafter, an electric field is applied by applying a bias voltage to the electrodes 16 and 17 formed in each interference waveguide 13, and the effective optical path length is changed by changing the refractive index. Thereafter, when the signal light passes through the portion of the interference optical waveguide 13 sandwiched between the SOA electrodes 18 formed on the multiplexer 14 side of the interference waveguide 13, the signal light is amplified at an amplification factor according to the injection current. The After passing through the interference waveguide 13, the light is multiplexed by the multiplexer 14 based on the phase condition at this time.

  Here, in order to multiplex optical signals propagated from a plurality of optical waveguides, an MMI (Multi-Mode-Interferometer) multiplexer 14 is generally used. The principle of multiplexing by MMI is the position (distance and direction) where light incident from a plurality of input-side optical waveguides 11 interferes with each other in the multimode waveguide structure part, and the light intensity increases depending on the phase condition of these lights. Therefore, by creating the light output waveguide 15 at this position, it becomes possible to take out the combined output light of the light.

  Therefore, in the case of a commonly used 2-input 2-output MMI, one of the optical output waveguides 15 composed of two ports can function as an output port, and the other can function as a monitor port. It is designed so that light having an opposite phase to the output optical port is output to the monitor port. Using the output light intensity to the monitor port having a phase opposite to that of the output light, the setting of the on / off operation point according to the phase condition, the extinction characteristic at the off point, and the adjustment procedure of the light output intensity are described below.

  First, the phase difference between the two propagation lights is controlled by the reverse bias voltage applied to the interference waveguide 13 at the modulation signal electrode 16 and the phase adjustment electrode 17, and the optical output to the monitor port is maximized (the optical output to the output port). Is adjusted to the minimum). Next, the injection current to the SOA formed in the interference waveguide 13 on one side is controlled, and the injection current to the SOA electrode 18 is adjusted so that the output of the monitor port is maximized. At this point, a state is obtained in which the signal light intensities from the interference waveguide 13 to the MMI multiplexer 14 are uniform. Further, when the light output intensity from the optical modulator 1 is increased, it is possible to control the injection current to the SOA in both the interference waveguides 13 to adjust the condition to obtain a desired light output. .

  According to the semiconductor MZ optical modulator 1 of this embodiment, the SOA is independently formed in each of the two interference waveguides, thereby improving the extinction characteristic due to the phase condition and further having a gain. It is also possible to operate as In addition, by having an SOA terminal independent of the phase modulation function, compensation control can be performed for propagation loss and absorption loss of light in the interference waveguide, and branching ratio fluctuation of the duplexer, etc. It can also contribute to the long-term operational stability of the optical modulator.

(Second Embodiment)
Next, in addition to the configuration of the semiconductor MZ optical modulator of the first embodiment, a semiconductor MZ optical modulator further provided with means for monitoring the output of the SOA will be described. FIG. 5 is a diagram showing a schematic configuration of the semiconductor MZ optical modulator of the present embodiment. As shown in FIG. 5, the optical modulator 2 of the present embodiment is provided with an optical coupler waveguide 19 and a monitor PD 20 in each region of the pair of interference optical waveguides 13 before the multiplexer 14. It is a configuration.

  The configuration of the optical modulator 2 of the present embodiment from the input side optical waveguide until the signal light is amplified when passing through the SOA in the interference waveguide is the semiconductor MZ optical modulator 1 of the first embodiment. It can be configured in the same way.

  In the semiconductor MZ optical modulator of this embodiment, the optical output intensity of the interference waveguide is individually measured by having means for monitoring the signal light intensity amplified by the SOA formed in each interference waveguide. Making it possible to control.

  This is because an optical coupler 19 made of a semiconductor waveguide is formed in a portion from the amplification in the region of the interference optical waveguide 13 provided with the SOA electrode 18 to the MMI multiplexer 14, and the branched light intensity. Can be detected by the photodiode 20. Since such an optical coupler 19 can be formed with the same structure as the waveguide portion 13 of the semiconductor MZ optical modulator 2, it can be processed into a photolithographic mask pattern during actual fabrication, It is possible to form an optical coupler without the procedure of adding a new different device.

  The photodiode 20 for monitoring the branched light intensity is also formed as the same semiconductor layer structure as the region of the interference optical waveguide 13 provided with the SOA electrode 18, and sufficient light absorption is achieved by applying a reverse bias. A coefficient can be obtained. Therefore, the photodiode 20 can also be formed by mask pattern processing of photolithography without a procedure for newly adding a different apparatus. The semiconductor layer structure of the monitoring photodiode 20 can employ the same layer structure as that shown in FIG. In this semiconductor structure, the absorption coefficient can be controlled by the reverse bias voltage, and the monitor current can be adjusted.

  The photodiode 20 can also be formed with the same semiconductor structure as the SOA. In this case, the monitor current can be detected even at zero bias (without using a bias voltage application circuit).

  In a normal optical modulator, branching the monitor light intensity for control in this way causes a reduction in output light intensity. However, in this embodiment, optical amplification by SOA is possible. Since branching loss can be compensated, it does not matter. Furthermore, compared to the semiconductor MZ optical modulator 1 of the first embodiment, it becomes possible to directly monitor the intensity of the amplified light by each SOA, so that the output light intensity from each interference waveguide can be measured directly and easily. It becomes possible to control.

  According to the semiconductor MZ optical modulator of this embodiment, in addition to the effects of the semiconductor MZ optical modulator of the first embodiment, the output of the semiconductor MZ optical modulator can be further increased by having the monitor terminal capable of observing the amplified propagation light intensity. While operating so that the input light intensity to the side multiplexer becomes equal, the output light intensity itself can be controlled.

  In the above embodiment, the modulation signal electrode 16, the phase adjustment electrode 17, and the SOA electrode 18 are provided in order from the input side of the interference optical waveguide 13. However, the order is not limited to this order and is arbitrary. For example, in the configuration of FIG. 3, a modulation signal electrode 16, an SOA electrode 18, and a phase adjustment electrode 17 are provided in order from the input side, and a voltage for phase adjustment is applied to the phase adjustment electrode 17 and a current (= light receiving current). ) Can be measured, the phase adjustment electrode 17 can monitor the light intensity of the light input from the pair of interference optical waveguides to the multiplexer.

  Further, in the configuration of FIG. 3 or FIG. 5, by providing the SOA electrode 18 on the input side with respect to the modulation signal electrode 16, light that is not modulated is input to the SOA. This is effective because it can prevent the pattern effect of SOA (the effect of changing the amplification factor depending on the signal waveform).

  The branching / multiplexing unit has been described based on the MMI, but a Y-branch, a directional coupler, and an MZ coupler may be used.

DESCRIPTION OF SYMBOLS 1, 2 Optical modulator 11 Optical input waveguide 12 Demultiplexer 13 Interference system optical waveguide 14 Multiplexer 15 Optical output waveguide 16 Modulation signal electrode 17 Phase modulation electrode 18 SOA electrode 21 n-InP substrate 22 n-InGaAsP contact Layer 23 n-InP cladding layer 24 i-MQW core layer 25 p-InP cladding layer 26 p-InGaAsP contact layer 27 P-side Au electrode 28 n-side AU electrode 29 i-MQW active layer

Claims (5)

An optical input waveguide;
A duplexer that splits the light input from the optical input waveguide into two;
A pair of optical waveguides that give a phase difference to the two lights branched by the duplexer;
A multiplexer that multiplexes the light by interfering two lights having a phase difference in the pair of optical waveguides;
An MZ optical modulator comprising an optical output waveguide that outputs the light combined in the multiplexer from one of two output ports;
The pair of optical waveguides has a slab type semiconductor laminated structure, and each of the pair of optical waveguides includes a modulation signal electrode that applies a modulation signal voltage that modulates light propagating through the optical waveguide; A phase adjusting electrode for applying a phase adjusting voltage for compensating for a difference in waveguide length between the pair of optical waveguides, and an SOA electrode for applying an injection current for amplifying light propagating through the optical waveguide. The MZ optical modulator is characterized in that the light intensity of the light input from the optical waveguide to the multiplexer is controlled to be equal by independently controlling the injection current applied to each of the SOA electrodes.   2. The MZ optical modulator according to claim 1, further comprising means for monitoring light intensity of light input to the multiplexer from the pair of optical waveguides.   3. The MZ light according to claim 1, wherein the light intensity of light input to the multiplexer from the pair of optical waveguides is monitored by measuring a light receiving current at the phase adjustment electrode. 4. Modulator.   The injection current applied to the SOA electrode provided in one of the pair of optical waveguides is adjusted based on the result of monitoring the light intensity of the light input from the pair of optical waveguides to the multiplexer. 4. The MZ optical modulator according to claim 2, wherein the optical intensity of the two lights input to the multiplexer is kept constant.   And means for monitoring the light intensity of the light output from the light output waveguide and adjusting the injection current applied to both of the SOA electrodes provided in the pair of optical waveguides based on the monitoring result. The MZ optical modulator according to claim 1, wherein

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