JP6701830B2 - Modulator and Mach-Zehnder modulator - Google Patents

Description

  The present invention relates to a modulator and a Mach-Zehnder type modulator.

  In optical communication, a modulator that modulates an optical signal is used, and for example, a multilevel modulator element that performs pulse-amplitude modulation (PAM) is used. In particular, there is known a modulator that carries out PAM modulation by mounting a plurality of phase shifters on both arms of a Mach-Zehnder interferometer and modulating each of these phase shifters with individual data.

FIG. 1 is a diagram showing a schematic configuration of a PAM-16 type silicon modulator disclosed in Patent Document 3 and Non-Patent Document 1.
In the modulator shown in FIG. 1, a Mach-Zehnder interferometer is formed by coupling two silicon waveguides 2A and 2B separated by a 2×2 MMI coupler (Multi-Mode Interference) 1X with a 2×2 MMI coupler 1Y. It is formed. The two silicon waveguides 2A and 2B are referred to as the two arms of the Mach-Zehnder interferometer. A phase shifter that changes the capacitance of the waveguides according to an external signal is loaded on the waveguides of the two arms to perform modulation.

In the modulator of FIG. 1, four phase shifters are provided, and the amount of change generated in each phase shifter is weighted so that modulation according to digital data can be performed. Specifically, as shown in FIG. 1, four phase shifters 11A, 11B; 12A, 12B; 13A, 13B; 14A, 14B are provided. The amount of phase change in the phase shifter changes according to the length of the phase shifter for the same drive voltage, so the ratio of the four phase shifter lengths is designed to be 1:2:2 ² :2 ^3. There is. With this design, each phase shifter is mounted on both arms, making it a PAM-16 type modulator that can perform push-pull drive.

  In the modulator of FIG. 1, assuming that each of the phase shifters having different lengths is driven by each driver circuit, the longer the phase shifter, the larger the driving force is required. Therefore, it is necessary to reduce the output impedance. There is. Driving with NRZ signals using driver circuits with different output impedances drives each phase shifter separately. The sum of these phase shift amounts becomes the phase difference between both arms. Therefore, a plurality of intermediate states are formed between the maximum value and the minimum value of the optical output amplitude according to the combination of the signal states of NRZ given to each phase shift, and PAM multilevel modulation is realized.

International Publication No. 2011/043079 JP, 2005-37547, A U.S. Pat. No. 8,320,720

X. Wu et. al, “A 20Gb/s NRZ/PAM-4 1V Transmitter in 40nm CMOS. Driving a Si-Photonic Modulator in 0.13μm CMOS” ISSCC2013 7.7 T. Baba et. al, “50-Gb/s ring-resonator-based silicon modulator” Optics Express 21, pp. 11869-11876 (2013).

The PAM modulator in Fig. 1 uses a phase shifter with a length ratio of 1:2:2 ² :2 ³ , so in actual driving it is necessary to prepare multiple driver circuits with different output impedances. There is a problem that it becomes a complicated drive system. Even if drivers with the same output impedance are used, PAM modulation cannot be performed because the amount of each phase change is not proportional to the length.
An object of the present invention is to realize a PAM-type multi-valued modulator that can obtain a modulation amount proportional to multi-values with a simple multi-valued modulation drive system using a plurality of driver circuits having the same design (output impedance). ..

In one aspect, the modulator includes a waveguide and N (N>1) phase shifters loaded in the waveguide and having a lumped constant electrode structure. When numbering from 1 to N in order from the shortest length to the N phase shifters, at least in some i (i is a natural number of 1 or more and N-1 or less), i-th and i+1-th the ratio of the length of the phase shifter is set to be greatly than 2. Each of the N phase shifters has a driver circuit driven by the same drive voltage with the same output impedance .

Further, in one aspect, the Mach-Zehnder modulator has two arms including a waveguide and at least two phase shifter units loaded in at least one of the two arms, and the phase shifter unit includes: The electrode structure has N (N>1) phase shifters of lumped constant type. When numbering from 1 to N in order from the shortest length to N phase shifters, at least i (i is 1 or more and N-1 or less) is i-th and i+1-th the ratio of the length of the phase shifter is set to be greatly than 2. Each of the N phase shifters has a driver circuit driven by the same drive voltage with the same output impedance .

  As one aspect, in a modulator and a Mach-Zehnder modulator, a multivalued pulse amplitude modulation drive by a plurality of driver circuits of the same design can obtain a modulation amount proportional to a multivalue.

FIG. 3 is a diagram showing a schematic configuration of a PAM-16 type silicon modulator disclosed in Patent Document 3 and Non-Patent Document 1. It is a figure which shows the equivalent circuit of the phase shifter used for calculation of the amount of phase shifts. It is a figure which shows the calculation result and calculation formula of the phase shifter length dependence of the phase shift amount in each driver impedance. It is a top view which shows the shape of the PAM-4 Mach-Zehnder modulator of 1st Embodiment, and the figure which shows the structure of the waveguide part of a phase shifter, (A) is a top view, (B) is a phase shifter guide. It is a structural diagram of a waveguide part. It is sectional drawing of AA in (A) of FIG. It is a figure which shows the drive system of the Mach-Zehnder type|mold modulator of 1st Embodiment, and the phase difference characteristic produced by drive, (A) shows a drive system and (B) shows a characteristic. It is a top view which shows the shape of the PAM-8 Mach-Zehnder type|mold modulator of 2nd Embodiment. It is a figure which shows the drive system of the Mach-Zehnder type|mold modulator of 2nd Embodiment. It is a figure which shows the phase difference characteristic produced by drive of the Mach-Zehnder type|mold modulator of 2nd Embodiment. It is a figure which shows the structure of the waveguide part of the phase shifter of the Mach-Zehnder type|mold modulator of 3rd Embodiment.

First, the reason why the PAM modulator of FIG. 1 cannot obtain the amount of phase change proportional to multiple values will be considered.
FIG. 2 is a diagram showing an equivalent circuit of the phase shifter used for calculating the phase shift amount. In FIG. 2, circuit parameters are also shown.
FIG. 3 shows the calculation result and the formula of the phase shifter length dependency of the phase shift amount at each driver impedance. (A) is a graph of the phase shifter length dependency of the phase shift amount, and (B) is the calculation formula. Show.

In the equivalent circuit of FIG. 2, the signal from the signal source 21 is applied to the equivalent circuit 23 of the diode forming the phase shifter via the driver impedance 22. The equivalent circuit 23 has a series resistance R _{s of} a diode, which constitutes the phase shifter, a capacitance (capacitance) C _F of the diode, and a resistance component R _F due to the leak path of the diode, which are connected as shown in the figure. . The power supply voltage amplitude of the signal of the signal source 21 is represented by ΔV (=1V), and the driver impedance is represented by Z _d . Furthermore, the amount of charge required for the phase shift of _π is defined as Q _π .

Further, considering that R _S and R _F are inversely proportional to each other and C _F is proportional to the length (L _act ) of the phase shifter, the power supply voltage 1 V, using the calculation formula shown in FIG. The result of calculation assuming a frequency of 12.5 GHz is shown in the graph of FIG. The phase shifter shown in FIG. 2 is assumed to be a silicon modulator using the free carrier plasma effect, and a phase change proportional to the amount of carriers accumulated in the waveguide core of the phase shifter having a length L _act propagates. It occurs against light. As shown in the graph of FIG. 3A, when the driver impedance is 0Ω, the phase shift amount changes linearly with the length. On the other hand, when the driver impedance is other than 0Ω, it exhibits nonlinearity with respect to the length.

Considering that the impedance of the driver is not actually 0Ω, the PAM modulator in Fig. 1 has a ratio of phase shifter length of 1:2:2 ² :2 ³ , so the amount of phase shift of each phase shifter. Does not reach the target ratio. It is considered that this is the reason why the phase change amount proportional to the multi-value is not obtained and the desired multi-value modulation cannot be performed.

  It is possible to solve this problem by changing the driving force (output impedance) of the driver that drives each phase shifter. However, in order to change the driving force (output impedance) of the driver that drives each phase shifter, a driver with a different output impedance is required to drive each phase shifter, so a driver with the same configuration (output impedance) is used. There is a problem that the drive system has a more complicated design and configuration than the case where it is used. Hereinafter, drivers having the same driving force (output impedance) will be referred to as drivers having the same configuration with respect to drivers having different driving forces (output impedance).

  4A and 4B are a top view showing the shape of the PAM-4 Mach-Zehnder modulator of the first embodiment and a view showing the structure of the waveguide part of the phase shifter, where FIG. 4A is a top view and FIG. It is a structural diagram of the waveguide part of the phase shifter.

MZ modulator according to the first embodiment, the modulator is formed on the two arms of the formed silicon (Si) waveguide insulating layer SiO ² formed on the substrate, SiO ² thereon The insulating layer, the ground electrode pad layer and the electrode pad layer are formed.

  As shown in FIG. 4A, the two arms separated by the 2×2 MMI coupler are coupled by another 2×2 MMI coupler. The silicon waveguides 30A and 30B correspond to two arms. The silicon waveguides 30A and 30B are provided with phase shifters 31A and 31B forming a first phase shifter and phase shifters 41A and 41B forming a second phase shifter.

The phase shifters 31A, 31B, 41A, and 41B are high-speed driving phase shifters formed of a PIN diode and an electric circuit having an equalizer function. The length of the phase shifters 31A and 31B in the waveguide direction is 90 μm, and the length of the phase shifters 31A and 31B in the waveguide direction is 270 μm, which are different lengths. Phase shifters 31A, 31B, 41A and 41B form a PAM-4 modulator. The phase shifters 31A, 31B, 41A and 41B include P-doped regions 32A, 32B, 42A and 42B provided on one side of the waveguide and N-doped regions 33A, 33B provided on the other side of the waveguide. 43A and 43B. N-doped regions 33A, 33B, 43A and 43B have a capacitance region of the equalizer and a resistance region of the equalizer and are connected to electrode pad layers 51A, 51B, 52A and 52B. A ground electrode pad layer 50 and electrode pad layers 51A, 51B, 52A and 52B shown by dotted lines are formed above the region of the Mach-Zehnder modulator shown in FIG. 4A via an insulating layer of SiO ^2. It The electrode pad layers 51A, 51B, 52A and 52B form a capacitance with the equalizer capacitance regions of the N-doped regions 33A, 33B, 43A and 43B.

Each phase shifter has the values shown in FIG. 2 for electrical parameters per unit working length such as C _F , R _F , R _S , and Q _π . The length of each phase shifter is determined based on the relational expression of FIG. 3, and the phase shifters 41A and 41B forming the second phase shifter have the lengths of the phase shifters 31A and 31B forming the first phase shifter. It is set to 270 μm, which is longer than twice the length of 90*2 μm=180 μm. In other words, the length of each phase shifter is set in consideration of the impedance of the driver.

  As shown in FIG. 4B, the waveguide portion of each phase shifter has the side grating waveguide described in Non-Patent Document 2. Side grating portions 35 and 37 and capacitance regions 46 and 48 are formed on both side surfaces of the side grating waveguide 30 formed of Si. Although not shown, the resistance regions and electrodes of the equalizer are formed ahead of the capacitance regions 46 and 48. The side grating portion 35 and the capacitance region 46 are included in the P-doped regions 32A, 32B, 42A and 42B in FIG. 4A. The side grating portion 37 and the capacitance region 48 are included in the N-doped regions 33A, 33B, 43A and 43B in FIG. 4A. The side grating portion 35 is divided into a P− doped region 45 and a P+ doped region which is the same as the capacitance region 46. The side grating portion 37 is divided into an N-doped region 47 and an N+-doped region which is the same as the capacitance region 48.

In FIG. 4B, the core width P of the side grating is 480 nm, the waveguide height Q is 220 nm, the side grating width R is 80 nm, the side grating length is 1 μm, and the period is 285 nm. The four kinds of doped regions are formed by implanting N+/N-/P-/P+ four times around the waveguide core, the N+ and P+ regions are 10 ²⁰ cm ^-3, and the N- and P- regions are 10 ¹⁸ cm ^-3. cm ^-3 . The waveguide shape other than the phase shifter uses a channel type waveguide, and the core width is 480 nm and the height is 220 nm.

FIG. 5 is a sectional view taken along line AA in FIG.
An insulating layer 68 of SiO ² is formed on the substrate 67, and a waveguide, a side grating portion, a capacitance region and the like are formed on the insulating layer 68. Further thereon, an insulating layer 68 of SiO ² , a ground electrode pad layer 50 and electrode pad layers 52A and 52B are formed. The waveguide 30 in FIG. 4B corresponds to the portions 30A and 30B in FIG. P-doped region 45, P+-doped region 46, N-doped region 47 and N+-doped region 48 in FIG. 4B are the portions 62A and 62B, the portions 61A and 61B, the portions 63A and 63B in FIG. , 64A and 64B. N-doped regions 65A and 65B are formed ahead of the N+ doped regions 64A and 64B, and N+ doped regions 66A and 66B are further formed ahead thereof. A conductor layer is formed on the N+ doped regions 66A and 66B, an insulating layer 68 of SiO ^{2 having} a thickness T (T=1 μm) is formed on the conductor layer, and an electrode pad layer 52A, 52B is formed. The N+ doped regions 66A and 66B, the conductor layer, the insulating layer 68 of SiO ² of thickness T and the electrode pad layers 52A, 52B form the equalizer capacitance. N-doped regions 65A and 65B function as a resistor of the equalizer. N+ doped regions 66A and 66B are electrically connected to electrode pad layers 52A and 52B. As a result, an electric circuit corresponding to the equivalent circuit in which the capacitance and the resistor of FIG. 2 are connected in parallel is formed.

The 90 μm long phase shifters 31A and 31B have a size of 100 μm in the horizontal direction and 60 μm in the vertical direction, and the capacitors of the 270 μm long phase shifters 41A and 41B have a horizontal size of 160 μm and a vertical direction of 60 μm. As described above, the resistor of the equalizer circuit uses a low-doped silicon thin wire, the doping concentration is the same as that of the N-region 10 ¹⁸ cm ^-3 , the equalizer resistor in each phase shifter is 220 nm in height, 1 μm in width. is there. The length of the resistors of the 90 μm long phase shifters 31A and 31B is 12 μm, the length of the resistors of the 270 μm long phase shifters 41A and 41B is 9 μm, and the resistance values are 6 kΩ and 4 kΩ, respectively.

6A and 6B are diagrams showing a drive system of the Mach-Zehnder modulator of the first embodiment and a phase difference characteristic generated by the drive. FIG. 6A shows the drive system and FIG. 6B shows the characteristic.
As shown in FIG. 6A, here, the driver impedance is set to 50 Ω, which is common to the phase shifters, and each driver drives the phase shifter. Peak-to-peak 3.0Vpp 25Gb/s NRZ electrical signal is input to each phase shifter. The drive system drives the phase shifters 31A and 31B and the phase shifters 41A and 41B with a current of 72 μA or 200 μA.

  The combination of the phase difference between both arms and the optical output level caused by driving each phase shifter by this drive system is as shown in FIG. 6(B). Since the first and second phase shifters are loaded on the arm of this modulator and driven by push-pull, four (ON,ON),(OFF,ON),(ON,OFF),(OFF,OFF) The driving conditions are as follows, and four combinations L1-L4 of the optical output levels are obtained. As shown in the figure, turning on the first phase shifter causes a phase difference of 0.12π compared to OFF, and turning on the second phase shifter causes a phase difference of 0.24π compared to OFF. The phase difference between them is 0.36π, 0.24π, 0.12π, and zero under the four driving conditions. As described above, when the first and second phase shifters are driven by the drive systems having the same configuration (same drive voltage), the phase difference generated by turning on/off the second phase shifter is 0.24π, This is twice the phase difference of 0.12π caused by turning on/off the phase shifter of. As a result, PAM-4 modulation can be performed with a drive system having the same configuration (same drive voltage).

FIG. 7 is a top view showing the shape of the PAM-8 Mach-Zehnder modulator of the second embodiment.
The Mach-Zehnder modulator of the second embodiment has a similar configuration to the Mach-Zehnder modulator of the first embodiment, but the number of loaded phase shifters is increased from 4 to 6, that is, one arm The difference is that three phase shifters are loaded. The Mach-Zehnder modulator according to the second embodiment includes phase shifters 71A and 71B that form a first phase shifter, phase shifters 72A and 72B that form a second phase shifter, and phase shifters that form a third phase shifter. And shifters 73A and 73B. The phase shifters 72A and 72B are the same as the phase shifters 31A and 31B of the first embodiment, the length in the waveguide direction is 90 μm, and the size of the capacitor is 100 μm in the horizontal direction and 60 μm in the vertical direction. The phase shifters 73A and 73B are the same as the phase shifters 41A and 41B of the first embodiment, the length in the waveguide direction is 270 μm, and the size of the capacitor is 160 μm in the horizontal direction and 60 μm in the vertical direction. The phase shifters 71A and 71B have a configuration similar to the other phase shifters, the length in the waveguide direction is 40 μm, and the size of the capacitor is 70 μm in the horizontal direction and 60 μm in the vertical direction.

The lengths of the phase shifters 71A and 71B to 73A and 73B in the waveguide direction are set to 40 μm, 90 μm, and 270 μm based on the relational expression of FIG. 3B, and 1:2:2 ² =4 The ratio is not 1:2.25:6.75. Thus, in the second embodiment, the length ratio of the shortest phase shifters 71A and 71B to the second shortest phase shifters 72A and 72B is 90/40=2.25, which is relatively close to 2. On the other hand, the ratio of the lengths of the second shortest phase shifters 72A and 72B and the third shortest (longest here) phase shifters 73A and 73B is 270/90=3, and the deviation from 2 becomes larger. More generalized, if the N phase shifters are numbered from 1 to N in order from the shortest length, at least some number i (i is 1 or more and N-1 or less. Here, 1 or 2 ), the ratio of the lengths of the i-th and i+1-th phase shifters is greater than 2.

FIG. 8 is a diagram showing a drive system of the Mach-Zehnder modulator of the second embodiment.
FIG. 9 is a diagram showing a phase difference characteristic generated by driving the Mach-Zehnder modulator of the second embodiment.

  As shown in FIG. 8, also in the drive system of the second embodiment, the driver impedance is set to 50Ω and each driver drives the phase shifter. A peak-to-peak 3.0 Vpp 25 Gb/s NRZ electrical signal is input to each phase shifter.

  The combination of the phase difference between both arms and the optical output level caused by driving each phase shifter by this drive system is as shown in FIG. Since the first to third phase shifters are loaded and push-pull driven on the arm of this modulator, (ON,ON,ON),(OFF,ON,ON),(ON,OFF,ON),( There are 8 driving conditions: (OFF, OFF, ON), (ON, ON, OFF), (OFF, ON, OFF), (ON, OFF, OF), (OFF, OFF, OFF). By driving the Mach-Zehnder modulator of the second embodiment under this driving condition, eight combinations L1-L8 of optical output levels can be obtained. As shown in the figure, turning on the first phase shifter causes a phase difference of 0.06π compared to OFF, and turning on the second phase shifter causes a phase difference of 0.12π compared to OFF. Turning on the phase shifter 3 causes a phase difference of 0.24π compared to turning it off. The phase difference between the arms is 0.32π, 0.36π, 0.3π, 0.24π, 0.18π, 0.12π, 0.06π, and zero under eight driving conditions. As described above, when the first to third phase shifters are driven by the drive system having the same configuration (same drive voltage), the phase difference generated by turning on/off the second phase shifter is 0.12π, It is twice the phase difference of 0.06π caused by turning on/off the phase shifter. Similarly, the phase difference generated by turning on/off the third phase shifter is 0.24π, which is four times the phase difference 0.06π generated by turning on/off the first phase shifter, and the second phase shifter. It is twice the phase difference of 0.12π caused by turning ON/OFF. As a result, PAM-8 modulation can be performed with a drive system having the same configuration (same drive voltage).

  As can be seen from the calculation result of FIG. 3, between the relatively short phase shifters on which the driver impedance is relatively smaller than the impedance of the phase shifter on the same arm, the lengths of the adjacent phase shifters are the same as before. The ratio of the height is 2. On the other hand, among the relatively long phase shifters, it is possible to obtain the effect of improving the linearity of the multilevel modulation also by making the ratio of the lengths of the adjacent phase shifters larger than 2 by the calculation shown in FIG. .

  The Mach-Zehnder modulator of the third embodiment to be described next has a configuration similar to that of the first embodiment, except that the waveguide part of the phase shifter is formed of PN diodes instead of PIN diodes. .

  FIG. 10 is a diagram showing the configuration of the waveguide portion of the phase shifter of the Mach-Zehnder modulator of the third embodiment, and is a diagram corresponding to FIG. 4(B).

Also in the third embodiment, the waveguide portion of the phase shifter has the same side-face grating waveguide as in the first embodiment. However, in the first embodiment, the waveguide portion 30 is formed of silicon, whereas in the third embodiment, the waveguide portion 85 is divided into two regions in the extending direction of the waveguide. Region is a P-doped region, and the other region is an N-doped region. In other words, the region on the side of the P-doped region is doped to 10 ¹⁸ cm ⁻³ in the same manner as the P-doped region 82 on the side lattice portion, and the region on the side of the N-doped region is N-doped on the side lattice portion. Like region 83, it is doped to 10 ¹⁸ cm ^-3 . As for the doping condition, N+/N-/P-/P+ is injected four times around the waveguide core as in the first embodiment, but the injection is performed to the inside of the waveguide core to form the PN diode in the waveguide. Is going. The concentration of N+-doped region and P+-doped region is 10 ²⁰ cm ^-3 .

  The drive system in the third embodiment is driven in the same manner as in the first embodiment with a reverse bias of 3V applied.

  As described above, in the embodiment, the plurality of phase shifters formed in both arms of the Mach-Zehnder modulator are driven by a simple configuration using the drivers of the same configuration, and thus the phase change amount of each phase shifter is increased. Can be PAM-modulated in order of increasing from 2 to 2.

Although the first to third embodiments have been described, it goes without saying that various modifications are possible.
For example, in a PAM modulator that drives each phase shifter with the same driver having a driver impedance other than 0Ω, the nonlinearity of the phase change amount with respect to the action length represented by the calculation formula of FIG. To Specifically, when the phase shift amounts of the phase shifters are arranged in ascending order, the phase shifter lengths are set so that the phase changes of the front and rear phase shifters have a double relationship. That is, when a plurality of phase shifters are arranged in ascending order, the ratio of the lengths of the adjacent phase shifters is set to be larger than 2.

  Further, the waveguide portion of the phase shifter formed by the PN diode of the third embodiment can be applied to the second embodiment.

  Furthermore, in the first to third embodiments, the silicon modulator using the carrier plasma effect has been described. However, the phase shifter is not limited to the one using a silicon diode, but may be an InP quantum well structure or a Stark structure. The same effect can be obtained by using the effect.

Further, although the case where the driver impedance is set to 50Ω has been described as an example, even when the driver impedance is other than 0Ω such as 25Ω, the same effect can be obtained by setting the length of the phase shifter according to the driver impedance.
Further, the example in which the phase shifters are arranged in order from the end in the ascending order has been described, but they may be arranged in any order.

Further, in the embodiment, an example of push-pull driving in which the phase shifters are provided in the silicon waveguides of the two arms has been described, but it is also possible to provide the phase shifter in only one arm.
Further, the configuration of the present invention can be applied not to the Mach-Zehnder type modulator but also to a modulator that PAM-modulates a signal propagating in one silicon waveguide.

  Although the embodiments have been described above, all the examples and conditions described herein are provided for the purpose of helping understanding of the invention and the concept of the invention applied to the technology. The specifically described examples and conditions are not intended to limit the scope of the invention, and the construction of such examples in the specification does not represent advantages or disadvantages of the invention. Although the embodiments of the invention have been described in detail, it should be understood that various changes, substitutions and modifications can be made without departing from the spirit and scope of the invention.

30A, 30B Silicon waveguide (arm)
31A, 31B Phase shifter (first phase shifter)
41A, 41B Phase shifter (second phase shifter)
32A, 32B, 42A, 42B P-doped region 33A, 33B, 43A, 43B N-doped region 50 Ground electrode pad layer 51A, 51B, 52A, 52B Electrode pad layer

Claims (8)

A waveguide,
Loaded on the waveguide, the electrode structure has a lumped constant type N (N>1) phase shifter,
If the N phase shifters are numbered from 1 to N in order from the shortest length, at least in some i (i is a natural number of 1 or more and N-1 or less), i-th and i+1 th ratio of the length of the phase shifter is set to from 2 becomes greatly,
The modulator, wherein each of the N phase shifters has a driver circuit driven by the same drive voltage with the same output impedance . The modulator according to claim 1, wherein PAM modulation is performed with a constant light intensity difference or phase shift difference between the levels indicated by the operation signal of the modulator. The modulator according to claim 1 or 2, wherein the N phase shifters are formed of PIN diodes and have an electric circuit structure having an equalizer function formed so as to be electrically connected to the PIN diodes. The modulator according to claim 1, wherein the waveguide of each of the N phase shifters is formed of a PN diode. Two arms including the waveguide,
At least two phase shifter units loaded in the waveguide of at least one of the two arms,
The phase shifter unit has N (N>1) phase shifters whose electrode structure is a lumped constant type,
If the N phase shifters are numbered from 1 to N in order from the one with the shortest length, at least in some i (i is a natural number of 1 or more and N-1 or less), i-th and i+1 th ratio of the length of the phase shifter is set to from 2 becomes greatly,
The Mach-Zehnder modulator , wherein each of the N phase shifters has a driver circuit driven by the same drive voltage with the same output impedance . The Mach-Zehnder modulator according to claim 5, wherein PAM modulation is performed with a constant light intensity difference or phase shift difference between the levels indicated by the operation signal of the Mach-Zehnder modulator. 7. The Mach-Zehnder modulator according to claim 5, wherein the N phase shifters are formed of PIN diodes and have an electric circuit structure having an equalizer function formed so as to be electrically connected to the PIN diodes. 7. The Mach-Zehnder type modulator according to claim 5 , wherein the waveguide of each phase shifter is formed of a PN diode.

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