JP2013003236A - Optical phase modulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress frequency chirp generated at the time of bit transition in an optical phase modulator.SOLUTION: An optical phase modulator (100) comprises: a branching part (120) that branches an incident optical signal into two; a first modulator arm (130) that includes a first phase shifter (132); a second modulator arm (140) that includes a second phase shifter (142); a combination part (150) that combines a first optical signal transmitted through the first modulator arm with a second optical signal transmitted through the second modulator arm, and outputs the combined optical signal. A loss adjustment part (A) provided in the optical phase modulator (100) makes a transmission loss of the first modulator arm (130) independent of phase shift of the first optical signal due to the first phase shifter (132) and a transmission loss of the second modulator arm (140) independent of phase shift of the second optical signal due to the second phase shifter (142) different from each other.

Description

  The present invention relates to an optical phase modulator, and more particularly to a Mach-Zehnder interference type optical phase modulator.

In recent years, in ultra-long-distance optical communications, various modulation formats have been the subject of research and development in order to further expand transmission capacity and improve frequency utilization efficiency.
The modulation format combined with phase modulation is expected to bring about significant changes in both transmission capacity and quality by enabling multi-level and coherent detection, and has been used in the past. A shift from intensity modulation to phase modulation has already begun.

  As an example of a conventional optical phase modulator, FIG. 6 shows a configuration example of a Mach-Zehnder interference type phase modulator. The optical phase modulator 600 includes two modulator arms 630 and 640, a branching device 620 that branches light input from the input waveguide 610 and inputs the light to the two modulator arms 630 and 640, It comprises a coupler 650 that combines the light input from the modulator arms 630 and 640 and outputs it to the output waveguide 660. The first modulator arm (hereinafter sometimes referred to as “upper arm”) 630 includes a waveguide 631, a phase shifter 632, and a waveguide 633 that are connected in series. Similarly, the second modulator arm (hereinafter also referred to as “lower arm”) 640 includes a waveguide 641, a phase shifter 642, and a waveguide 643 connected in series.

In the phase modulator 600, the light incident on the input waveguide 610 is equally divided into two lights by the branching device 620 and inputted to the two modulator arms. The light input to the upper arm 630 propagates through the waveguide 631, undergoes a refractive index change by the phase shifter 632, propagates through the waveguide 633, and enters the coupler 650. Similarly, light input to the lower arm 640 propagates through the waveguide 641, undergoes a refractive index change by the phase shifter 642, propagates through the waveguide 643, and enters the coupler 650.
At this time, when a voltage is applied so that the phase shifter 632 increases the refractive index and the phase shifter 642 decreases the refractive index (this operation is called “push-pull operation”), the two modulator arms 630 and 640 are applied. Therefore, output lights having constant intensities and phases different from each other by π (180 °) can be output. The two lights whose phases change in opposite directions interfere with each other in the coupler 650 and are output from the output waveguide 660 as an optical signal.

FIG. 7 shows the coupling from each modulator arm to the coupler while the optical signal output of the Mach-Zehnder interferometric phase modulator described above transitions between the signal “0” (703) and the signal “1” (704). 6 is a bit transition diagram showing changes in intensity and phase of light incident on 650 on an IQ plane. FIG.
The change in the phase of the light in each modulator arm during the transition of the optical signal output between the signal “0” and the signal “1” indicates that the phase of the light phase-shifted in the first modulator arm 111 is the clock. The phase of the light phase-shifted in the second modulator arm 112 proceeds counterclockwise. In other words, the locus 701 of the optical signal modulated by the phase shifter 632 changes from 0 degrees to 180 degrees counterclockwise, and the locus 702 of the optical signal modulated by the phase shifter 642 changes from 0 degrees to 180 degrees clockwise. .

For example, when the electro-optic effect (Pockels effect) that is generally used in a LiNbO ₃ modulator or the like is used, the loss of two modulator arms is equal to each other, so that the signal “0” ( 703) and the signal “1” (704) are made only on the I axis. Therefore, in such a Mach-Zehnder interferometric phase modulator, frequency chirp at the time of bit transition can be suppressed.
In addition, as prior art documents regarding frequency chirp, there are Non-Patent Document 1, Non-Patent Document 2, and the like.

  While the phase modulation described above is expected to bring about significant changes in both transmission capacity and quality, at the same time, transceivers that support advanced modulation formats will increase the size and complexity of devices and systems. This may lead to higher costs, which is a major concern for the practical application of new modulation formats.

  Therefore, a large-scale monolithic integration technique for optical / electronic devices using group IV semiconductor materials typified by silicon attracts attention. Since this technology has excellent advantages such as mass production and easy miniaturization, it can be used in a wide range from ultrashort-distance optical communication represented by interchip optical interconnection to medium / long-distance optical communication. Application is expected.

For example, it is possible to reduce the size of the optical phase modulator by configuring the above-described Mach-Zehnder interference type optical phase modulator using a silicon material.
When a Mach-Zehnder interference type optical phase modulator is formed from a silicon material, the phase shifters 632 and 642 are provided in the vicinity of the silicon fine wire waveguide in which a PN junction is formed along the axial direction and the silicon fine wire waveguide. And an electrode for applying a bias to the PN junction. When a control voltage is applied to this electrode from outside to change the carrier density in the waveguide, the refractive index of the silicon wire waveguide changes due to the carrier plasma dispersion effect, and the phase of the light passing therethrough can be shifted. .

A. H. Gnauck, P. J. Winzer, Journal of Lightwave Technology, Vol.23, No.1, pp.115-130 (2005) F. Koyama, K. Iga, Journal of Lightwave Technology, Vol. 6, No. 1, pp. 87-93 (1988) David J. Lockwood, Lorenzo Pavesi (Eds.), Silicon Photonics II -Components and Integration-, Springer (2010) G. T. Reed, G. Mashanovich, F. Y. Gardes & D. J. Thomson ”Silicon optical modulators” Nature Photonics, August 2010, Volume 4, No 8, pp518-526

However, when the carrier density is modulated, the refractive index change and the absorptance change are simultaneously modulated by the carrier plasma dispersion effect.
For example, in a silicon optical modulator, the refractive index change and the absorptance change due to the carrier plasma dispersion effect at a wavelength of 1.55 μm are empirically expressed as the following formula (1 ) And formula (2), respectively (see Non-Patent Document 4).

Here, Δn is a change in refractive index, Δα is a change in absorptance, ΔN _e is a change in electron carrier density, and ΔN _h is a change in hole carrier density.

  On the other hand, the phase shift Δφ accompanying the refractive index change Δn is expressed by the following equation (3).

  Here, L is the waveguide length of the phase shifter, and λ is the wavelength.

On the other hand, the change in absorptance appears as a change in light propagation loss (hereinafter, in this specification, “light propagation loss” and “light propagation loss change” are referred to as “propagation loss” and “propagation loss change”, respectively. ).
In this specification, unless otherwise specified, “propagation loss” and “propagation loss change” mean a propagation loss per unit length and a propagation loss change per unit length, respectively.

As described above, in the conventional Mach-Zehnder interference type optical phase modulator using the electro-optic effect, the propagation loss of the two modulator arms is equal even at the time of bit transition. However, in a Mach-Zehnder interferometric optical phase modulator using the carrier plasma dispersion effect, if one phase shifter is forward biased and the other phase shifter is reverse biased to perform push-pull operation, the carrier in the two phase shifters The change in density has opposite signs and unequal values. Therefore, at the time of bit transition, as shown in the equations (1) and (2), not only the refractive index change Δn but also the propagation loss change Δα _plasma is simultaneously modulated, and the propagation of the upper arm at this time The direction of change in loss change Δα _{1, plasma} and the direction of change in propagation loss change Δα _{2, plasma in} the lower arm are opposite to each other as expressed in equations (4) and (5). Become.

Where α ₁ is the total propagation loss of the upper arm, α ₂ is the total propagation loss of the lower arm, α _{1, prop} is the change in the propagation loss of the upper arm without being affected by the carrier plasma dispersion effect, α _{2 , prop} is the propagation loss change of the lower arm without being affected by the carrier plasma dispersion effect, Δα _{1, plasma} is the propagation loss change due to the carrier plasma dispersion effect of the upper arm, and Δα _{2, plasma} is the lower arm propagation effect. Changes in propagation loss due to the carrier plasma dispersion effect are shown.

In general, since the upper arm and the lower arm are formed symmetrically, the propagation losses α _{1, prop} and α _{2, prop} in the state not affected by the carrier plasma dispersion effect are the same as those of the upper arm. It becomes equal between the lower arm and can be expressed as α _{1, prop} = α _{2, prop} = α _prop .

  The complex electric field amplitude of the output light of the Mach-Zehnder interference type optical phase modulator having two phase shifters is expressed by the following equation (6).

Where E _out is the output complex electric field amplitude, a is the input optical branching ratio, b is the output optical coupling ratio, A is the input optical electric field amplitude, and Δφ ₁ (t) is the phase shift by the upper arm (the phase by the first phase shifter) Shift), Δφ ₂ (t) is the phase shift by the lower arm (phase shift by the second phase shifter), α ₁ (t) is the propagation loss of the upper arm, and α ₂ (t) is the propagation loss of the lower arm. , T is time and L is the waveguide length.
Note that the phase shifts Δφ ₁ (t) and Δφ ₂ (t) and propagation losses α ₁ (t) and α ₂ (t) that change under the influence of the carrier plasma dispersion effect are functions of time. In this specification, for the sake of simplicity, they are expressed as Δφ ₁ , Δφ ₂ , α ₁ , and α ₂ .

Here, regarding the Mach-Zehnder interferometric phase modulator using the carrier plasma dispersion effect, FIG. 8A shows a bit transition diagram on the IQ plane showing changes in the intensity and phase of the optical signal output from each modulator arm at the time of bit transition. . Then, the phase of the optical signal modulated by the phase shifter 632 and output from the first modulator arm 630 changes from 0 degrees to 180 degrees counterclockwise as indicated by a locus 801, and is modulated by the phase shifter 642. Even if the phase of the optical signal output from the second modulator arm 640 changes clockwise from 0 degrees to 180 degrees as indicated by a locus 802, the expressions (2), (4), and (4) As can be seen from (5), in the phase shifter 632, the propagation loss α ₁ is reduced, so that the amplitude of the optical signal output from the first modulator arm 630 increases (track 801), and the phase shifter 642 propagates. Since the loss α ₂ increases, the amplitude of the optical signal output from the second modulator arm 640 decreases (trajectory 802).

Therefore, it can be seen that the transition between the signal “0” (803) and the signal “1” (804) in the phase-modulated optical signal extends over both the I axis and the Q axis. The numerical calculation result shown in FIG. 8B also shows that a Q-axis component is generated at the time of bit transition.
As described above, when optical phase modulation is performed using the carrier plasma dispersion effect, at the time of bit transition between the signal “0” (803) and the signal “1” (804) of the optical signal output of the optical phase modulator. Frequency change occurs continuously. As a result, this frequency change remains as a so-called frequency chirp, which causes deterioration of the phase modulation characteristics, particularly during long distance transmission.

  Accordingly, an object of the present invention is to suppress frequency chirp that occurs at the time of bit transition in an optical phase modulator.

  In order to achieve the above-described object, an optical phase modulator according to the present invention includes a branching unit that branches an incident optical signal into two, and a first optical signal that propagates a first optical signal branched by the branching unit. A first optical waveguide, a second optical waveguide that propagates the second optical signal branched by the branching unit, and a first optical waveguide that is provided in the first optical waveguide and that shifts the phase of the first optical signal. 1 phase modulation section, a second phase modulation section provided in the second optical waveguide and shifting the phase of the second optical signal, and the first light propagating through the first optical waveguide A coupling unit that multiplexes and emits the signal and the second optical signal propagated through the second optical waveguide, and shifts the phase of the first optical signal by the first phase modulation unit. Propagation loss of the first optical waveguide that does not depend on the second phase modulation unit and the second phase modulation unit. And the propagation loss of the second optical waveguide that is independent of the shift of the phase of the signal is different from each other.

  In the present invention, the propagation loss of the first optical waveguide that does not depend on the phase shift of the first optical signal by the first phase modulation unit and the phase shift of the second optical signal by the second phase modulation unit In making the propagation loss of the second optical waveguide independent from each other, for example, the loss adjustment for adjusting the propagation loss of the optical waveguide to at least one of the first optical waveguide and the second optical waveguide is performed. A portion may be provided, and at least one of the area and the shape of the cross section perpendicular to the propagation direction of the optical signal of the first optical waveguide and the second optical waveguide may be different from each other.

  In the case where the loss adjusting unit is provided here, the loss adjusting unit is, for example, a portion of the first optical waveguide or the second optical waveguide, and a cross section of the optical waveguide perpendicular to the propagation direction of the optical signal. At least one of the area and the shape may be a portion that changes along the propagation direction, or is formed in a part of the first optical waveguide or the second optical waveguide, and the optical waveguide has a predetermined curvature. It is good also as a bent part bent with a radius.

  In the optical phase modulator according to the present invention, each of the first optical waveguide and the second optical waveguide has a core made of a semiconductor, and the first phase modulation unit and the second phase modulation unit are Each including at least one of an n-type semiconductor and a p-type semiconductor serving as a supply source of carriers injected into the core and an electrode for controlling injection of carriers into the core, and a phase of an optical signal by a carrier plasma dispersion effect May be shifted.

According to the present invention, the propagation loss of the first optical waveguide that does not depend on the phase shift in the first phase modulation unit and the propagation loss of the second optical waveguide that does not depend on the phase shift in the second phase modulation unit. By adjusting the loss so as to be different from each other, the locus of the optical signal of each optical waveguide phase-shifted by the first and second phase modulators can be adjusted independently. Change in optical propagation loss caused by shifting the phase of one optical signal in the first direction, and light produced by shifting the phase of the second optical signal in the second direction in the second phase modulator Even if the change in propagation loss differs from each other, the frequency chirp at the time of transition between bits can be relaxed.
That is, the propagation loss that does not depend on the phase shift in the first phase modulation unit in the propagation loss of the first optical waveguide and the phase in the second phase modulation unit in the propagation loss of the second optical waveguide. By making the propagation loss not dependent on the shift different from each other, the influence of the change in the absorptance (propagation loss change) accompanying the phase modulation is canceled out, and the signal between the signal “0” and the signal “1” in the optical phase modulation is canceled. Frequency chirp generated at the time of bit transition can be suppressed.

FIG. 1A is a plan view showing a configuration of an optical phase modulator according to the first embodiment of the present invention. FIG. 1B is a cross-sectional view showing the configuration of the phase shifter of the optical phase modulator according to the first embodiment of the present invention. FIG. 1C is a plan view showing the configuration of the loss adjustment unit of the optical phase modulator according to the first embodiment of the present invention. FIG. 1D is a cross-sectional view showing the configuration of the loss adjustment unit of the optical phase modulator according to the first embodiment of the present invention. FIG. 2 is a diagram showing the dependence of propagation loss on the change of the core width of the silicon waveguide. FIG. 3A is a bit transition diagram (outline) on the IQ plane when frequency chirp is suppressed by the optical phase modulator according to the first embodiment of the present invention. FIG. 3B is a bit transition diagram (numerical calculation result) in the IQ plane when the frequency chirp is suppressed by the optical phase modulator according to the first embodiment of the present invention. FIG. 4A is a plan view showing a configuration of an optical phase modulator according to the second embodiment of the present invention. FIG. 4B is a plan view showing the configuration of the loss adjustment unit of the optical phase modulator according to the second embodiment of the present invention. FIG. 5 is a diagram showing the dependence of insertion loss on the bending radius of a silicon waveguide. FIG. 6 is a diagram illustrating a configuration example of a conventional Mach-Zehnder interference type optical phase modulator. FIG. 7 is a bit transition diagram in the IQ plane when frequency chirp is suppressed. FIG. 8A is a bit transition diagram (outline) in the IQ plane when frequency chirp is not suppressed. FIG. 8B is a bit transition diagram (numerical calculation result) on the IQ plane when frequency chirp is not suppressed by numerical calculation.

Embodiments of the present invention will be described below with reference to the drawings.
<First Embodiment>
The optical phase modulator according to the first embodiment of the present invention is a Mach-Zehnder interference optical phase modulator, and adjusts the cross-sectional structure or shape of the waveguide as means for adjusting the propagation loss of the modulator arm. Thus, asymmetry is provided between the two modulator arms.

<Basic concept>
The optical phase modulator according to the embodiment of the present invention has a propagation loss that does not depend on the phase shift of the optical signal among the propagation losses of the two modulator arms, that is, between the refractive index and the absorption rate (propagation loss). Independent of the Kramers-Kronig relationship, the propagation loss derived from the waveguide structure is adjusted to provide asymmetry between the two modulator arms.

  At this time, the propagation loss of the two modulator arms is obtained by adding a new loss adjustment term in Equations (4) and (5) in order to correct the propagation loss change due to the influence of the carrier plasma dispersion effect (7). ) And formula (8).

Where α ₁ is the total propagation loss of the upper arm, α ₂ is the total propagation loss of the lower arm, and α _prop (= α _{1, prop} = α _{2, prop} ) is not affected by the carrier plasma dispersion effect. Propagation loss of upper arm and lower arm, Δα _{1, plasma} is the propagation loss change due to carrier plasma dispersion effect of upper arm, Δα _{2, plasma} is the propagation loss change due to carrier plasma dispersion effect of lower arm, Δα _{1, adjust Represents the} change in propagation loss due to the structural adjustment of the upper arm, and Δα _{2, adjust} represents the change in propagation loss due to the structural adjustment of the lower arm.

<Configuration of optical phase modulator>
1A to 1D show the configuration of the optical phase modulator according to the present embodiment. The optical phase modulator 100 includes a branching unit (demultiplexer) 120 that branches the optical signal incident from the input waveguide 110 into two, and a first optical signal that propagates the first optical signal branched by the branching unit 120. Optical waveguide (hereinafter referred to as “first modulator arm” or “upper arm”) 130 and a second optical waveguide (hereinafter referred to as “first optical arm”) that propagates the second optical signal branched by the branching unit 120. 2 modulator arms ”or“ lower arm ”) 140, and the first optical signal propagated through the upper arm 130 and the second optical signal propagated through the lower arm 140 are combined and output. A coupling portion (multiplexer) 150 that emits to the waveguide 160 is configured. These are silicon waveguides formed on an SOI substrate. Among these, the optical path lengths of the upper arm 130 and the lower arm 140 are formed to be equal. Further, the shape and size of the cross section perpendicular to the optical signal propagation direction of the optical waveguide cores of the upper arm 130 and the lower arm 140 are also formed to be equal to each other except for a loss adjusting portion 141 ′ described later.

The upper arm 130 is provided with a first phase shifter (first phase modulator) 132 that shifts the phase of the first optical signal propagating through the upper arm 130. The phase shifter 132 is connected to the branching unit 120 and the coupling unit 150 via the optical waveguide 131 and the optical waveguide 133, respectively. Similarly, the lower arm 140 is provided with a second phase shifter (second phase modulator) 142 that shifts the phase of the second optical signal propagating through the lower arm 140. The phase modulation unit 142 is connected to the branching unit 120 and the coupling unit 150 via the optical waveguide 141 and the optical waveguide 143, respectively.
Hereinafter, the first phase shifter 132 and the second phase shifter 142 may be simply referred to as a “phase shifter”.

  FIG. 1B shows a cross-sectional view of the first phase shifter 132 taken along line II (see FIG. 1A). The first phase shifter 132 becomes a silicon waveguide core 175 formed on the SOI substrate including the silicon layer 171 and the silicon oxide layer 172, and a carrier supply source serving as a supply source of carriers injected into the core 175. Impurity regions 173a and 173b, and a first electrode 176 and a second electrode 177 electrically connected to the impurity regions 173a and 173b, respectively, are provided.

  The silicon waveguide core 175 is formed by changing the thickness of the silicon layer 173 formed on the silicon oxide layer 172 stepwise, and the silicon layer 173 including the silicon waveguide core 175 is made of silicon oxide. The over clad layer 174 is covered.

Impurity regions 173a and 173b serving as carrier supply sources are formed in the silicon layer 173 in the vicinity of the silicon waveguide core 175. In the present embodiment, an n + type region 173a is formed on one side of the silicon waveguide core 175, and a p + type region 173b is formed on the other side. A via hole formed in the cladding layer 174 is formed, and the first electrode 176 and the second electrode 177 formed in the via hole are electrically connected to the n + region 173a and the p + region 173b, respectively.
The phase shifter 142 is also configured similarly to the phase shifter 132.

  The phase shifters 132 and 142 can modulate the carrier concentration in the silicon waveguide core 175 and shift the phase of the optical signal by applying a control bias from the outside between the first and second electrodes. In this embodiment, when carriers are injected into the silicon waveguide core 175, a control voltage is applied to the phase shifter 132 and the phase shifter 142 so as to be forward biased and reverse biased respectively.

In the present embodiment, the phase shifters 132 and 142 have been described as having a so-called PIN structure. However, in the present invention, the configuration of the phase modulation unit is PIN if it can inject carriers into the silicon waveguide core 175. For example, a PN junction may be formed, and at least one of an n-type silicon region and a p-type silicon region may be formed in the vicinity of the silicon waveguide core 175. In particular, the configuration of the phase modulation unit in which chirp suppression is important is a PN junction structure capable of high-speed operation (depletion type by applying a reverse bias and pulling out a carrier), for example, P + type / P type / N type / N + type It is a structure like this.
A configuration example of the phase shifter is also disclosed in Non-Patent Document 4.

  In the Mach-Zehnder interference type optical phase modulator 100 according to the present embodiment, the lower arm 140 is provided with a loss adjustment unit 141 ′ that adjusts the propagation loss of the lower arm 140. 1C and 1D are an enlarged plan view and a cross-sectional view of the loss adjusting portion 141 ', respectively. As shown in FIG. 1C, the loss adjusting unit 141 ′ is obtained by partially widening the core width of the silicon waveguide 141 of the lower arm 140. At this time, since a rapid change in the cross-sectional area of the waveguide core causes reflection and scattering of light, as shown in FIG. 1C, the width of the waveguide core of the loss adjusting unit 141 ′ is continuous (change in refractive index of the cross section To reduce as much as possible).

Since the loss adjusting unit 141 ′ as described above is a strong confinement system with a high refractive index difference structure typified by a silicon waveguide, it is possible to adjust the propagation loss with high precision and reproducibility by adjusting the cross-sectional shape and dimensions. Is possible.
For example, by using electron beam exposure or ultraviolet exposure, the width of the waveguide core having a resolution of 10 nm or less can be adjusted, and its reproducibility has also been confirmed.

In the optical waveguide having the structure shown in FIG. 1D, the silicon layer may be formed by selectively etching the SOI layer using a known etching method. Further, an over clad may be formed by forming a silicon oxide layer on the silicon core. Further, the silicon oxide layer (BOX layer) on the SOI substrate may be an underclad.
In the present embodiment, the height of the core of the silicon waveguide 141 is constant over the length direction.

  FIG. 2 is a diagram showing the dependence of propagation loss on the core width (Core width) when the silicon waveguide core height is fixed at 200 nm, which is cited from Non-Patent Document 3. . As can be seen from FIG. 2, the propagation loss decreases as the core width increases.

  By taking advantage of this feature, it is possible to adjust the propagation loss by reducing or enlarging the waveguide core cross-sectional area of one or both of the modulator arms and to provide asymmetry to the propagation losses of the two modulator arms. The optical phase modulator according to this embodiment adjusts the propagation loss by partially expanding the cross-sectional area of the waveguide core of one arm and suppresses the generation of frequency chirp. That is, by forming the core width of the silicon waveguide 141 partially as the loss adjusting unit 141 ′, the propagation loss of the lower arm 140 becomes smaller than the propagation loss of the upper arm 130. Propagation loss asymmetry is given to the arm 140.

In the present embodiment, in addition to simplifying the structure, the waveguide core cross-sectional area is not reduced in order to prevent excessive loss, and only the core cross-sectional area is increased by adjusting the waveguide width in the lower arm 140. ing. In this case, Δα _{1, adjust} in equation (7) is set to 0, and only Δα _{2, adjust} in equation (8) is added.

<Operation of optical phase modulator>
Next, the operation of the optical phase modulator 100 according to the present embodiment will be described.
In this phase modulator 100, the light incident on the input waveguide 110 is equally divided into two lights by the splitter 120. In the upper arm 130, the light propagates through the waveguide 131 and enters the phase shifter 132, propagates through the waveguide 133, and reaches the coupling unit 150. On the other hand, in the lower arm 140, the phase shifter 142 undergoes a phase shift due to a change in refractive index through the loss adjustment region 141 ′ of the waveguide 141, and then propagates through the waveguide 143 to reach the coupling unit 150. Thus, the optical signal incident from the upper modulation arm 130 and the optical signal incident from the lower modulation arm 140 interfere with each other in the coupling unit 150, and pass through the output waveguide 160 as an optical signal output. Is output.

In such a Mach-Zehnder interferometric optical phase modulator, when transitioning between the signal “0” and the signal “1” of the optical signal output, one of the phase shifters 132 is forward biased by a push-pull operation, When the other phase shifter 142 is reverse-biased, as shown in the equations (1) and (2), the refractive index change Δn and the propagation loss change Δα _plasma are simultaneously modulated by the carrier plasma dispersion effect, and the upper arm at this time The direction of change of the propagation loss change Δα _{1, plasma} and the direction of change of the propagation loss change Δα _{2, plasma} of the lower arm are opposite to each other, as shown in equations (7) and (8). Become. However, by providing the loss adjusting portion 141 ′ in the waveguide 141, the propagation loss α ₂ of the lower arm 140 is reduced more than the propagation loss α ₁ of the upper arm 130.

Bit transition diagrams in the IQ plane at this time are shown in FIGS. 3A and 3B.
For example, when the optical signal output of the optical phase modulator 100 transitions from the signal “0” (303) to the signal “1” (304), the phase of the optical signal propagating through the upper arm 130 is modulated by the phase shifter 132. Thus, the phase of the optical signal propagating through the lower arm 140 is modulated by the phase shifter 142 and changes clockwise from 0 to 180 degrees (trajectory). 302).

At this time, since the loss adjustment unit 141 ′ is provided in the lower arm 140 and −Δα _{2, adjust} is added (see the equation (8)), Δα _{2, plasma} is offset by Δα _{2, ADJUST} , since the effects of [Delta] [alpha] _{2, plasma} relative to the total propagation loss alpha ₂ side arm becomes relatively small, the optical signal intensity and phase of the trajectory 302 propagating through the lower arm 140, lower Q-axis direction (Fig. 3A Swell in the direction). As a result, the signal “0” (303) of the optical signal output obtained by the interference between the optical signal incident on the coupling unit 150 from the upper arm 130 and the optical signal incident on the coupling unit 150 from the lower arm 140. And the signal “1” (304), the component of the optical signal output away from the I axis along the Q axis can be reduced.

For example, α ₁ = α ₂ suffices. From the equations (7) and (8), it is desirable that the change in the propagation loss of the upper arm and the change in the propagation loss by the loss adjusting unit 141 ′ of the lower arm 140 are desirably Δα _{1. , adjust} + Δα2 _{, adjust} = Δα1 _{, plasma} + Δα2 _{, plasma} , the trajectory of the optical signal output in the bit transition can be regarded as almost only on the I axis. However, depending on the carrier density and other conditions, the electric field amplitude ratio at the phase 0 and π may differ greatly, so the propagation loss change should be set in consideration of these conditions.

As a more specific example, FIGS. 3B and 8B show numerical analysis results of a chirp suppression method by propagation loss control of a silicon modulator using hole carriers. 8B corresponds to before the loss adjustment unit, and FIG. 3B corresponds to after the loss.
Here, when the input branching ratio a = (0.5) 1/2, the output branching ratio b = (0.5) 1/2, the wavelength λ = 1.550 nm, and the waveguide length L of the phase shifter L = 10 mm, By setting the propagation loss change Δα _{2, adjust} = 0.6 dB / cm due to the structure adjustment of the lower arm, the frequency chirp almost shifted on the I axis as shown in FIG. 3B. As a result, it is understood that the light intensity having the frequency chirp can be suppressed to 1/6 or less.
In addition, in a modulator that uses a change in electron carrier density instead of a hole carrier, or a modulator that uses a change in both carrier densities, a more prominent chirp suppression effect is expected due to the relationship between equations (1) and (2). Is done.

As described above, according to the optical phase modulator according to the present embodiment, only one of the modulator arms has a structure in which the waveguide cross-sectional area is increased, and the locus of each modulator arm is adjusted independently, thereby modulating the modulator. Therefore, frequency chirp can be suppressed without increasing excess loss.
In other words, one modulator arm (lower arm) 140 is provided with a loss adjusting unit 141 ′ by changing the core width of a part of the silicon waveguide, and is modulated by the influence of the carrier plasma dispersion effect. By relatively reducing the influence of the propagation loss change Δα _{2, plasma} of the modulator 142 of the arm 140, the locus on the IQ plane of the optical signal incident on the coupling unit 150 from the lower arm at the time of bit transition 3A swells downward as shown in FIG. 3A, so that light obtained by interference between an optical signal incident on the coupling portion 150 from the upper arm 130 and an optical signal incident on the coupling portion 150 from the lower arm 140 is obtained. Frequency chirp at the time of bit transition of signal output can be suppressed.

<Second Embodiment>
Next, a second embodiment of the present invention will be described with reference to FIGS. 4A, 4B, and 5. FIG. In addition, about the component which is common in 1st Embodiment mentioned above, the same name and the same referential mark are used, and the description is abbreviate | omitted.

<Configuration of optical phase modulator>
In the optical phase modulator according to the second embodiment, a bending portion is provided in the silicon waveguide of the modulator arm of the Mach-Zehnder interference type phase modulator as means for adjusting the loss. That is, the adjustment of the propagation loss change Δα _adjust is realized by adjusting the bending radius of the silicon waveguide to increase or decrease the bending loss.

FIG. 4A is a diagram showing an outline of the asymmetric Mach-Zehnder interference type silicon phase modulator according to the present embodiment. The optical phase modulator 200 includes a branching unit 120 that splits an optical signal incident from the input waveguide 110 into two parts, and a first modulator arm that propagates the first optical signal branched by the branching unit 120 ( (Or upper arm) 130, second modulator arm (or lower arm) 240 that propagates the second optical signal branched by branching unit 120, and first optical signal propagated through upper arm 130 and The coupling unit 150 is configured to multiplex the second optical signal propagating through the side arm 240 and output the multiplexed signal to the output waveguide 160. These are silicon waveguides formed on an SOI substrate.
Among these, the silicon waveguide core 241 constituting the second modulator arm 240 is provided with a bent portion B as a loss adjusting portion.

  4B is an enlarged view of the bent portion B. FIG. As shown in FIG. 4B, in the bent portion B, the waveguide core 241 is bent by approximately 90 ° with a bending radius r on the order of several μm.

  FIG. 5 is a diagram showing the dependence of insertion loss on the bending radius of a silicon waveguide, which is cited from Non-Patent Document 3. According to FIG. 5, it is understood that the loss increases as the bending radius of the silicon waveguide decreases. Utilizing this feature, the waveguide bending loss of one or both of the two modulator arms can be adjusted by reducing or expanding the bending radius, thereby suppressing the occurrence of frequency chirp.

  It is practically possible to adjust the bending radius r with a resolution smaller than 100 nm by fabricating the silicon waveguide using electron beam exposure or ultraviolet exposure. Accordingly, since the silicon waveguide is a strong confinement system having a high refractive index difference structure, it is possible to adjust the loss accurately and with high reproducibility by changing the bending radius r.

In the present embodiment, in order to simplify the structure, a bent portion is provided in the silicon waveguide core 241 in the lower arm 240 to give a bending loss by adjusting the bending radius. That is, Δα _{1, adjust} in equation (7) was set to 0, and Δα _{2, adjust} in equation (8) was set.
In this embodiment, since the bending loss used for adjusting the propagation loss is generated at the bent portion of the silicon waveguide because of its structure, it matches the above-described equations (7) and (8). Therefore, the propagation loss change Δα _{2, adjust} is a value obtained by averaging the loss change caused by adjusting the bending radius of the bent portion by the length L of the waveguide of the modulator.
The optical path lengths of the upper arm 130 and the lower arm 240 are formed to be equal. The cross-sectional shape of the silicon waveguide core is the same as that of the first embodiment described above.

<Operation of optical phase modulator>
In the phase modulator 200 according to the present embodiment, the light incident on the input waveguide 110 is equally divided into two lights by the splitter 120. In the upper arm 130, the optical signal propagates through the waveguide 131, undergoes a phase shift due to a change in refractive index by the phase shifter 132, propagates through the waveguide 133, and enters the coupling unit 150. On the other hand, in the lower arm 240, the optical signal propagates through the waveguide core 241 imparted with bending loss, undergoes a phase shift by the phase shifter 142, propagates through the waveguide 143, and enters the coupling unit 150. .
The loss α ₂ of the second modulator arm 240 includes a propagation loss change Δα _{2, adjust} caused by adjusting the bending loss of the bent portion. Accordingly, the waveguide bending loss of the bent portion is adjusted by reducing and expanding the bending radius, and the influence of the propagation loss change Δα _{2, plasma} of the modulator 142 of the lower arm 240 modulated by the influence of the carrier plasma dispersion effect is adjusted. By relatively reducing the frequency chirp, generation of frequency chirp at the time of bit transition can be suppressed.

  According to the present embodiment, the propagation loss is adjusted by controlling the bending radius of the waveguide core. Therefore, compared with the first embodiment in which the width of the waveguide core is controlled, the accuracy in the manufacturing process is improved. There is an advantage of mitigation.

<Other embodiments>
In the above-described embodiment, a Mach-Zehnder interference type phase modulator made of a silicon waveguide has been described as an example. However, in the present invention, any material can be used as long as the loss can be adjusted independently of the refractive index. And does not depend on the interference structure.

In the two embodiments described above, the propagation loss is adjusted by changing the width of the silicon waveguide core or by controlling the bending radius of the silicon waveguide core. The means for loss adjustment is not limited to these embodiments.
For example, as the loss adjusting part, an impurity introducing part doped with impurities may be formed in at least a part of at least one silicon waveguide core. Since silicon can adjust the light absorption according to the impurity concentration, the propagation loss of the modulator arm can be adjusted by providing such an impurity introduction portion. Such an impurity introducing portion can be simultaneously formed by forming a silicon waveguide core after introducing impurities into a predetermined region of the silicon layer in a process of forming a phase shifter (phase modulating portion).

  Furthermore, in the present invention, the propagation loss that does not depend on the phase shift of the optical signal in the first modulator arm is different from the propagation loss that does not depend on the phase shift of the optical signal in the second modulator arm. In other words, it is only necessary to provide an asymmetry in the propagation loss that does not depend on the phase shift between the two modulator arms. Therefore, as in the embodiment described above, a loss adjusting unit may be provided in a part of the waveguide, and the shape or size of the waveguide core (in the propagation direction of the optical signal) between the two modulator arms. There may be a difference in at least one of the area and shape of the vertical cross section.

  The present invention can be used in the fields of optical communication and optical communication devices.

  DESCRIPTION OF SYMBOLS 110 ... Input waveguide, 120 ... Branch part, 130, 140, 240 ... Modulator arm, 131, 133, 141, 143, 241 ... Optical waveguide (core), 132, 142 ... Phase shifter, 141 ', 241' ... Loss adjustment unit, 150 ... coupling unit, 160 ... output waveguide.

Claims (7)

A branching section for splitting the incident optical signal into two;
A first optical waveguide for propagating a first optical signal branched by the branch portion;
A second optical waveguide for propagating a second optical signal branched by the branch part;
A first phase modulation unit that is provided in the first optical waveguide and shifts a phase of the first optical signal;
A second phase modulation unit provided in the second optical waveguide and configured to shift a phase of the second optical signal;
A coupling portion for combining and emitting the first optical signal propagated through the first optical waveguide and the second optical signal propagated through the second optical waveguide;
The optical propagation loss of the first optical waveguide independent of the phase shift of the first optical signal by the first phase modulation unit, and the phase of the second optical signal by the second phase modulation unit An optical phase modulator characterized in that the optical propagation loss of the second optical waveguide independent of shift is different from each other. The optical phase modulator as claimed in claim 1.
At least one of the first optical waveguide and the second optical waveguide includes a loss adjusting unit that adjusts an optical propagation loss of the optical waveguide. The optical phase modulator according to claim 2, wherein
The loss adjusting unit is
A portion of the first optical waveguide or the second optical waveguide, wherein at least one of the cross-sectional area and shape perpendicular to the propagation direction of the optical signal changes along the propagation direction. An optical phase modulator comprising: The optical phase modulator according to claim 2, wherein
The loss adjusting unit is
An optical phase modulator, wherein the optical phase modulator is a bent portion formed in a part of the first optical waveguide or the second optical waveguide, wherein the optical waveguide is bent with a predetermined radius of curvature. The optical phase modulator according to claim 2, wherein
The first optical waveguide and the second optical waveguide have a core made of silicon,
The loss adjusting unit is
An optical phase modulator comprising an impurity introduction portion in which an impurity is introduced into at least a part of the core of the first optical waveguide or the second optical waveguide. The optical phase modulator as claimed in claim 1.
An optical phase modulator characterized in that at least one of an area and a shape of a cross section perpendicular to an optical signal propagation direction of the first optical waveguide and the second optical waveguide is different from each other. The optical phase modulator according to any one of claims 1 to 6,
Each of the first optical waveguide and the second optical waveguide has a core made of a semiconductor,
The first phase modulation unit and the second phase modulation unit respectively inject at least one of an n-type semiconductor and a p-type semiconductor serving as a supply source of carriers injected into the core, and inject carriers into the core. An optical phase modulator comprising: an electrode to be controlled; and a phase of an optical signal shifted by a carrier plasma dispersion effect.