JP5288033B2 - Light modulator - Google Patents

Description

  The present invention relates to an optical modulator, and more particularly to an optical modulator in which a plurality of Mach-Zehnder type waveguides are combined in a nested structure.

  In the optical communication field and the optical measurement field, an optical waveguide element in which an optical waveguide is formed on an electro-optic effect substrate is frequently used. Among them, an optical modulator in which a Mach-Zehnder type waveguide is formed to perform optical modulation such as optical intensity modulation is highly utilized due to advantages such as easy integration and high efficiency of optical modulation.

  In recent years, modulation schemes for various applications such as high-speed, large-capacity communication and long-distance transmission have been proposed using optical modulators with optical waveguides that combine multiple Mach-Zehnder waveguides in a nested structure. Has been. For example, in a DQPSK modulator and an SSB modulator, two Mach-Zehnder type waveguides are combined in a nested structure. For such an optical modulator using a two-stage Mach-Zehnder type waveguide, various control techniques have been proposed for stabilizing control of the modulation operation.

  On the other hand, an optical modulator, such as quadrature amplitude modulation (QAM), which is disclosed in Non-Patent Document 1 and combines four or more Mach-Zehnder type waveguides as shown in FIG. QAM modulators have been proposed. FIG. 1 shows an optical waveguide used in a 16QAM modulator. As described above, a control method for stabilizing modulation driving such as stabilization of an output signal has not been established for an optical modulator including a large number of optical modulation units (MZM) having Mach-Zehnder type waveguides.

  The 16QAM modulator of FIG. 1 will be described. The shape of the optical waveguide is a combination of four Mach-Zehnder type waveguides (11 to 14) in a nested structure. The input waveguide 41 is divided into two at the branching portion 31 and further divided into two at the next branching portions (32, 33), and each of the input waveguides 41 is used as an input waveguide of each Mach-Zehnder type waveguide (11-14). It is connected. The output waveguides of the Mach-Zehnder type waveguides are merged at the multiplexing unit (34, 35), and further merged at the multiplexing unit 36 to form one output waveguide 42.

  In order to modulate the light wave propagating through the Mach-Zehnder type waveguide, each Mach-Zehnder type waveguide is provided with a modulation electrode (21 to 24), and an independent modulation signal (I1, Q1, I2, Q2) is provided for each. ) Is applied. The light wave 1 incident from the input waveguide 41 is modulated in each Mach-Zehnder type waveguide by these modulation signals, and the modulated light emitted from the output waveguide 42 corresponds to the constellation at the time of 16QAM modulation shown in FIG. Have information to do.

  However, in order to equalize the distance between the constellations of the output signals shown in FIG. 2 (signal position indicated by the symbol m), the output difference between the output signal P1 of the modulation unit DPMZM1 and the output signal P2 of the modulation unit DPMZM2 It is necessary to give an optimum value of 6 dB. If the output difference between P1 and P2 deviates from the optimum value, a quadrangle (a quadrangle composed of the line S) connecting the center positions n (white circles) of the four signal points m in each quadrant of the IQ coordinates is obtained. Distorted from the square, and a situation occurs in which the distance between each signal is close. This causes a signal discrimination error.

  However, it is difficult to realize an optical waveguide structure that gives an accurate signal output difference due to manufacturing variations and the like. In addition, a method of trimming one of the waveguides and tuning the propagation constant in the middle of the manufacturing process of the optical modulator is also conceivable. However, processing for each product such as manufacturing error and wavelength dependency is required, and the manufacturing cost increases. .

T. Sakamoto, A. Chiba and T. Kawanishi, "50-Gb / s 16 QAM by a quad-parallel Mach-Zehnder modulator", ECOC2007, PDP2.8, Berlin, Germany, 21 September 2007

  The present invention solves the above-described problems, and appropriately adjusts the output signal intensity to be combined with the multiplexing unit in an optical modulator in which a plurality of Mach-Zehnder waveguides are combined in a nested structure, thereby easily stabilizing the signal output. It is an object to provide an optical modulator that can be used.

  In order to solve the above-mentioned problems, an invention according to claim 1 is directed to a substrate having an electro-optic effect, a plurality of Mach-Zehnder type waveguides formed in parallel on the substrate, and an input of the Mach-Zehnder type waveguide An optical waveguide having a branching portion configured by branching a waveguide connected to the waveguide, a multiplexing portion where an output waveguide from the Mach-Zehnder type waveguide is joined, and an optical wave propagating through the optical waveguide. In an optical modulator provided with a modulation electrode for modulation, between the branch portion and an input waveguide of the Mach-Zehnder type waveguide to which a waveguide branched from the branch portion is connected, or the Mach-Zehnder type A Y-branch switch is provided at least between the output waveguide of the waveguide and the multiplexing unit, and the branching ratio of the Y-branch switch can be adjusted by applying an electric field to at least one of the branch waveguides. Be And wherein the door.

According to a second aspect of the present invention, in the optical modulator according to the first aspect, 2 ^N (N is an integer of 2 or more) Mach-Zehnder type waveguides are arranged in parallel, and adjacent Mach-Zehnder A 2 ^{N + 2} QAM modulator having a nested structure in which two input waveguides and two output waveguides are sequentially coupled, and each Mach-Zehnder waveguide is digitally phase-modulated with a driving voltage of 2Vπ. It is characterized by.

According to a third aspect of the present invention, in the optical modulator according to the second aspect, the 2 ^{N + 2} QAM modulator is a 16 QAM modulator, and the Y branch is provided at one of the output waveguides that joins the final stage multiplexing unit. A switch is provided, and the branching ratio of the Y branch switch is adjusted so that the output difference of the light wave that joins the multiplexing unit is 6 dB.

  According to the first aspect of the present invention, a substrate having an electro-optic effect and a plurality of Mach-Zehnder type waveguides formed in parallel on the substrate and connected to the input waveguide of the Mach-Zehnder type waveguide are arranged. An optical waveguide having a branching portion formed by branching the waveguide, and a multiplexing portion where the output waveguide from the Mach-Zehnder type waveguide joins, and a modulation electrode for modulating the light wave propagating through the optical waveguide. In the optical modulator provided, between the branch portion and an input waveguide of the Mach-Zehnder type waveguide to which the waveguide branched from the branch portion is connected, or an output waveguide of the Mach-Zehnder type waveguide In order to make it possible to adjust the branching ratio of the Y branch switch by applying an electric field to at least one of the branch waveguides, at least one of the Y branch switches is provided at least between the multiplexing unit and finally, Multiplexer The intensity ratio of the output signals to be merged can be adjusted to the optimum state, and even with an optical modulator that combines a plurality of Mach-Zehnder type waveguides in a nested structure, only by adjusting the branch ratio of the Y branch switch, It is possible to provide an optical modulator that can easily stabilize the signal output.

According to the second aspect of the present invention, 2 ^N (N is an integer of 2 or more) Mach-Zehnder type waveguides are arranged in parallel in the optical waveguide, and the input waveguide and the output waveguide of the adjacent Mach-Zehnder type waveguide are arranged. QAM which has a nested structure that couples two by two in sequence, and each Mach-Zehnder type waveguide constitutes a 2 ^{N + 2} QAM modulator in which digital phase modulation is performed with a driving voltage of 2Vπ, so that the signal output can be stabilized. It is possible to provide a modulator.

According to the invention of claim 3, the 2 ^{N + 2} QAM modulator is a 16QAM modulator, and a Y-branch switch is provided in one of the output waveguides that joins the final stage multiplexing unit, and the optical wave that joins the multiplexing unit Since the branch ratio of the Y-branch switch is adjusted so that the output difference is 6 dB, it is possible to provide a 16QAM modulator that can stabilize the signal output.

It is a figure explaining the optical waveguide structure etc. of 16QAM modulator. It is a figure which shows the constellation of the output signal in 16QAM modulation. It is a figure explaining a mode that the Y branch switch was provided between the output waveguide of a Mach-Zehnder type waveguide, and a multiplexing part in the optical modulator of this invention. It is a figure which shows an example of the Y branch switch used for the optical modulator of this invention. It is an Example which shows the structure which adjusts automatically the branching ratio of the Y branch switch used for the optical modulator of this invention. It is another Example which shows the structure which adjusts automatically the branching ratio of the Y branch switch used for the optical modulator of this invention. It is a figure explaining a mode that the Y branch switch was provided in the branch part in the optical modulator of this invention. In the optical modulator of the present invention, it is a diagram illustrating a state in which a Y branch switch is provided between a branch portion and an input waveguide of a Mach-Zehnder type waveguide to which a waveguide branched from the branch portion is connected. It is a figure explaining a mode that the Y branch switch was provided in the multiplexing part in the optical modulator of this invention.

The optical modulator of the present invention will be described in detail below.
FIG. 3 is a diagram illustrating an example of the optical modulator of the present invention. 3 to 9, in principle, the same reference numerals are used for parts that have the same configurations as those in FIG. 1 and the drawings.
A feature of the present invention is that a substrate 3 having an electro-optic effect and a plurality of Mach-Zehnder type waveguides (11 to 14) formed in the substrate are arranged in parallel, and an output waveguide from the Mach-Zehnder type waveguide In an optical modulator provided with an optical waveguide having a multiplexing portion 36 where the optical waveguides are combined and a modulation electrode (not shown) for modulating the optical wave propagating through the optical waveguide, the output guides of the plurality of Mach-Zehnder type waveguides A Y branch switch (51, 52) is provided between at least one of the waveguides from the Mach-Zehnder type waveguide to the multiplexing unit 36, and the branch ratio of the Y branch switch can be adjusted. To do.

  As shown in FIG. 4, the branch switch used in the present invention has a waveguide 61 (an output waveguide of a Mach-Zehnder type waveguide, or an output waveguide output from a multiplexing unit that combines a plurality of output waveguides). (Corresponding) is divided into two branch waveguides (62, 63) by the Y branch portion 51, and at least one of the branch waveguides is provided with an electrode 52 (electrodes 521 and 522 in FIG. 4). Have. By adjusting the voltage applied between the electrode 521 and the electrode 522 by the voltage applying means 523 in FIG. 4, the branching ratio at which the light wave a branches into the light waves b and c can be adjusted.

  In the embodiment of FIG. 3, not only the multiplexing unit 51 of the optical modulator but also the output waveguide 64 output from the multiplexing unit 35 of the two Mach-Zehnder waveguides (13, 14) includes a Y branch unit. Waveguides (53, 66, 67) are formed. This Y-branch waveguide is not necessarily required for the present invention, but when automatically adjusting the intensity ratio of the output signal to be merged with the multiplexing unit 36, a part of the intensity of the light wave to be merged with the multiplexing unit. It is possible to use as a means for monitoring

Next, a configuration for automatically adjusting the branching ratio of the Y branch switch, which is a reference example of the present invention, will be described.
FIG. 5 is a reference example showing a configuration for automatically adjusting the branching ratio, and the optical modulator according to the reference example of the present invention uses the light wave 4 (L1) branched by the Y branch switch (51, 52). A Y-branch waveguide portion 53 is provided in a part of the output waveguide of the first branch light monitoring means 71 to be monitored and another Mach-Zehnder type waveguide that joins the multiplexing portion. The branching ratio of the Y branch switch is controlled based on the second branch light monitor means 74 for monitoring the branched light wave 5 (L2) and the detection signals (72, 75) of the first and second branch light monitor means. Control means 73 (applied voltage output from the control means is indicated by reference numeral 76).

  In the control means 73, the detection signal 72 from the first branch light monitor means 71 and the detection signal 75 from the second branch light monitor means 74 have a predetermined relationship, for example, the optical modulator of FIG. When used as a 16QAM modulator as shown in FIG. 1, the ratio of the detection signals is set so that the output difference of the output signals joined to the multiplexing unit 36 is 6 dB.

  In FIG. 5, the two monitor means 71 and 74 are employed corresponding to the two monitor lights 4 (L1) and 5 (L2). However, the monitor means may be constituted by one light receiving element. In that case, it is also possible to irradiate two monitor lights at different positions of the light receiving element using an optical component such as an optical fiber to detect the light intensity for each monitor light.

FIG. 6 is another reference example showing a configuration for automatically adjusting the branch ratio of the Y branch switch.
In FIG. 6, the output light monitoring means 83 for monitoring a part 82 of the light wave output from the multiplexing unit 36, and the branching ratio of the branch switches (51, 52) are controlled based on the detection signal 84 of the output light monitoring means. And a control means 85 for controlling.

  As the means 81 for extracting a part of the output light from the multiplexing unit 36, an optical coupler or the like can be suitably used. Further, the light wave 4 output from the Y branch switch is detected by the monitor means 71, and the detection signal 72 is provided to the control means 84. However, in the embodiment of the present invention, the monitor means 71 is not necessarily required. Not. For example, each signal electrode (not shown) is driven in advance with a predetermined modulation pattern, and it is determined whether or not a signal corresponding to the predetermined modulation pattern is output by monitoring a part 82 of the output light. Automatic adjustment can be performed by adjusting the voltage applied to the electrode 52 of the Y-branch switch so as to obtain a modulation pattern.

  By detecting the monitor light 4, the branch state of the Y branch switch can be directly observed. With reference to these detection results, the voltage applied to the electrode 52 of the Y branch switch can be adjusted, It is possible to further stabilize the signal output.

The optical modulator of the present invention is not limited to the 16QAM modulator described above, and 2 ^N (N is an integer of 2 or more) Mach-Zehnder type waveguides are arranged in parallel as optical waveguides, and adjacent Mach-Zehnder type waveguides are arranged. 2 ^{N + 2} QAM modulator having a nested structure in which two input waveguides and two output waveguides are sequentially coupled, and each Mach-Zehnder type waveguide is digitally phase-modulated with a driving voltage of 2Vπ. Function.

  In such a case, as shown in FIG. 3, the Y branch switch (51, 52) is not only provided in a part of the output waveguide 61 immediately before the final stage multiplexing unit 36, but also the multiplexing unit. A similar Y-branch switch is also provided in the output waveguide of the Mach-Zehnder type waveguide 11 immediately before 34 so that the intensity ratio of the output signals to be combined can be adjusted for a plurality of multiplexing units. It is also possible.

  In the above embodiments and reference examples, the Y-branch switch has been described focusing on the optical modulator provided between the output waveguide of the Mach-Zehnder type waveguide and the multiplexing unit. However, the optical modulator of the present invention is described above. For example, as shown in FIG. 7, the branching unit 151 is used, and the branching unit 151 and at least one branching waveguide of the two branching waveguides (320, 330) branched from the branching unit are used. A Y-branch switch including an electrode 152 that applies an electric field to the waveguide can be configured. With this Y branch switch, it is possible to adjust the light intensity of the light wave divided by the branching unit 151 and finally adjust the light intensity (P1, P2) of the two light waves that join at the multiplexing unit 36 in advance. .

Further, as shown in FIG. 9, the Y-branch switch can be provided in the multiplexing unit. In FIG. 9, an electrode 352 for applying an electric field to the combining unit 351 and at least one of the two branching waveguides (68, 69) connected to the combining unit 351 by using the combining unit 351. A Y-branch switch consisting of The Y branch switch in FIG. 9 plays a role of adjusting the intensity ratio of the light wave that joins the multiplexing unit 351.
Note that the electrodes 152, 252 and 352 described above are not shown in the figure, but each have the structure described with reference to FIG.

  Further, as shown in FIG. 8, a Y branch switch is provided between the branching section 31 and the input waveguide of the Mach-Zehnder type waveguide (11, 12) to which the waveguide branched from the branching section is connected. Is also possible. In FIG. 8, a Y branch switch is configured by the branching portion 251 and the electrode 252 in the middle of the branching waveguide branched by the branching portion 31, and the two propagation waveguides 253 and 254 branched by the branching portion 251 are propagated. The branching ratio of the light wave is adjusted.

  In FIG. 8, a Y-branch waveguide portion 255 is provided in the middle of the other branch waveguide branched from the branch portion 31, and the light wave propagating through the branch waveguide 256 is introduced toward the Mach-Zehnder type waveguide (13, 14). The light wave propagating through the other branching waveguide 257 is led out of the substrate 3. Then, the light wave L1 emitted from the branch waveguide 253 is monitored by the first branch light monitor means, and the light wave L2 emitted from the branch waveguide 257 is monitored by the second branch light monitor means, which is described in FIG. The configuration for automatically adjusting the branching ratio of the Y branch switch can be similarly implemented in the configuration shown in FIG.

  As the method for extracting the monitor lights L1 and L2 for adjusting the Y branch switch as shown in FIG. 5, the light waves branched from the Y branch switch are always used in FIGS. The optical modulator is not limited to such an embodiment. For example, as shown in FIG. 7, a first Y-branch waveguide section 100 is provided in a part of the output waveguide of the Mach-Zehnder type waveguide (11, 12) that joins the multiplexing section 36, and the first The light wave L1 output from the branched waveguide 101 from the Y-branch waveguide unit 100 and a part of the output waveguide of the other Mach-Zehnder type waveguides (13, 14) that merge with the multiplexing unit 36 It is also possible to provide two Y-branch waveguide sections 102 and use the light waves L2 output from the branched waveguide 103 from the second Y-branch waveguide section 102 as monitor lights.

Further, as shown in FIG. 9, a first Y-branch waveguide 110 is provided in a part of the waveguide branched from the branch portion, and the light is output from the waveguide 111 branched from the first Y-branch waveguide 110. A second Y-branch waveguide portion 112 is provided in a part of another waveguide branched from the branch portion and the light wave L2 branched from the second Y-branch waveguide portion 112 is monitored. It is also possible to use it as light and input it to the first and second branched light monitoring means as shown in FIG.
That is, the Y branch switch shown in FIGS. 3, 7, 8, and 9 and the means for generating monitor light using the Y branch waveguide shown in FIGS. Any combination can be realized.

  As described above, according to the present invention, it is possible to appropriately adjust the output signal intensity to be combined with the multiplexing unit in the optical modulator in which a plurality of Mach-Zehnder type waveguides are combined in a nested structure, and to easily stabilize the signal output. It is possible to provide an optical modulator that can be realized.

DESCRIPTION OF SYMBOLS 1 Input light 2 Output light 3 Board | substrate 4,5,82, L1, L2 Monitor light 11-14 Mach-Zehnder type | mold waveguides 21-24 Signal electrodes 31-33 Branch part 34-36 Combined part 41 Input waveguide 42,61 , 64 Output waveguide 51, 151, 251, 351 Y branch switch (branch waveguide)
52,152,252,352,521,522 Y branch switch (electrode)
71, 74, 83 Detection means 73, 85 Control means

Claims (3)

A branch formed by arranging a substrate having an electro-optic effect, a plurality of Mach-Zehnder type waveguides formed in parallel on the substrate, and a waveguide connected to the input waveguide of the Mach-Zehnder type waveguide. In an optical modulator comprising an optical waveguide having a coupling portion where the output waveguide from the Mach-Zehnder-type waveguide joins, and a modulation electrode for modulating a light wave propagating through the optical waveguide,
Between the branching section and the input waveguide of the Mach-Zehnder type waveguide to which the waveguide branched from the branching section is connected, or between the output waveguide of the Mach-Zehnder type waveguide and the multiplexing section At least one of them is provided with a Y branch switch,
An optical modulator characterized in that the branching ratio of the Y-branch switch can be adjusted by applying an electric field to at least one branching waveguide. 2. The optical modulator according to claim 1, wherein the optical waveguide includes 2 ^N (N is an integer of 2 or more) Mach-Zehnder type waveguides arranged in parallel, and an input waveguide of an adjacent Mach-Zehnder type waveguide and 2. An optical modulator comprising a nested structure in which two output waveguides are sequentially coupled, and each Mach-Zehnder type waveguide constitutes a 2 ^{N + 2} QAM modulator in which digital phase modulation is performed with a driving voltage of 2Vπ. 3. The optical modulator according to claim 2, wherein the 2 ^{N + 2} QAM modulator is a 16 QAM modulator, and the Y branch switch is provided in one of the output waveguides that joins the final stage multiplexing unit. An optical modulator characterized in that the branching ratio of the Y-branch switch is adjusted so that the output difference of the light wave that joins the unit is 6 dB.

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