WO2023248490A1

WO2023248490A1 - Optical modulator

Info

Publication number: WO2023248490A1
Application number: PCT/JP2022/037012
Authority: WO
Inventors: 百合子川村; 雅之高橋; 祥吾山中; 健太郎本田; 陽介雛倉
Original assignee: 日本電信電話株式会社
Priority date: 2022-06-21
Filing date: 2022-10-03
Publication date: 2023-12-28
Also published as: WO2023248352A1

Abstract

The present invention provides an optical modulator having high modulation efficiency without increasing power consumption. Provided is an optical modulator that is a Mach-Zehnder interferometer-type optical modulator (200) in which optical waveguide cores of first and second waveguides (102, 103) comprise semiconductor layers having a p-n junction, and includes first and second high frequency electrodes (108, 109) for applying a modulated electrical signal and a bias voltage to the semiconductor layers, the optical modulator comprising: a first phase modulation region (110) which is formed in the first waveguide and has a first conductivity type semiconductor layer (105a) connected to the first high frequency electrode (108) and a second conductivity type semiconductor layer (106a) connected to the second high frequency electrode (109); and a second phase modulation region (111) which is formed in the second waveguide (103) and has a second conductivity type semiconductor layer (106b) connected to the first high frequency electrode and a first conductivity type semiconductor layer (105b) connected to the second high frequency electrode.

Description

light modulator

The present invention relates to a Mach-Zehnder interferometer type optical modulator used in optical communications.

A Mach-Zehnder interferometer type optical modulator has small wavelength dependence, has no wavelength chirp component in principle, and can operate at high speed. For this reason, it is widely used in medium and long-distance optical communication systems using a coherent method, and IMDD (direct detection direct modulation) optical communication systems for short distances of several hundred meters.

A Mach-Zehnder interferometer type optical modulator splits the light incident on the input optical waveguide into two optical waveguides (arm optical waveguides) with an intensity of approximately 1:1, and propagates the split light over a fixed length. It has a structure in which the signals are combined again and then output. By changing the phase of the two lights using the phase modulation region provided in the optical waveguide that is split into two, the interference conditions of the lights when they are combined are changed, and the intensity and phase of the output light are modulated. be able to.

As materials constituting the optical waveguide in the phase modulation region, dielectrics such as LiNbO ₃ and semiconductors such as InP, GaAs, and Si (silicon) are used. By inputting a modulated electrical signal to an electrode placed near the optical waveguide in the phase modulation region and applying a voltage to the optical waveguide, the phase of light propagating through the optical waveguide is changed. In recent years, optical modulators using Si as an optical waveguide material have been actively researched and developed because optical circuits can be integrated in a small size.

A high-speed optical modulator is required for high-capacity optical communication. As a broadband Mach-Zehnder interferometer type optical modulator, there is a traveling wave electrode type Mach-Zehnder optical modulator. The traveling wave electrode is an electrode that matches the velocity of the modulated electrical signal and the light propagating through the optical waveguide (phase velocity matching), and interacts with the light while propagating the electrical signal. For example, Non-Patent Document 1 discloses a Si Mach-Zehnder optical modulator with a modulator length of about several millimeters.

(Travelling wave electrode type Mach-Zehnder optical modulator)
FIG. 1 shows the structure of a conventional traveling wave electrode type Mach-Zehnder optical modulator. FIG. 1 is a perspective view of the structure of an optical waveguide and electrodes of a single-electrode Mach-Zehnder optical modulator, viewed from above. FIG. 2 shows a cross section taken along II-II' in FIG. The Si optical modulator 100 includes an input optical coupler (or Y-shaped splitter) 101 connected to an input optical waveguide, and one-to-two parallel first waveguides 102 that guide split input light into two. It includes a second waveguide 103 and an output optical coupler 104 that combines output light from the two waveguides and outputs it to an output optical waveguide.

The optical waveguide of the Si optical modulator 100 is composed of a Si layer 2 sandwiched between upper and lower SiO ₂ cladding layers 1 and 3. The Si waveguide for confining light has a structure called a rib waveguide with different thicknesses. That is, as shown in FIG. 2, the rib waveguide is composed of

thick Si layers

151a and 151b at the center and thin slab regions 152a to 152c on both sides thereof. The central

thick Si layers

151a and 151b of the Si layer 2 are used as the cores of the first waveguide 102 and the second waveguide 103, respectively, and the difference in refractive index with the surrounding SiO ₂ cladding layers 1 and 3 is utilized. , constitutes an optical waveguide that confines light propagating in a direction perpendicular to the plane of the paper.

The optical waveguide core of the Si layer 2 each has a pn junction in which the p-

type semiconductor layers

105a, 105b and the n-type semiconductor layer 106 are joined at the center of the core. A metal bias electrode 107 is provided on the n-type semiconductor layer 106 sandwiched between the two optical waveguides, a first high-frequency electrode 108 is provided on the p-type semiconductor layer 105a, and a first high-frequency electrode 108 is provided on the p-type semiconductor layer 105b. A second high frequency electrode 109 is provided. Modulated electrical signals and bias voltages are applied via these electrodes.

The phase modulation region formed in the first waveguide 102 is referred to as a first modulation region 110, and the phase modulation region formed in the second waveguide 103 is referred to as a second modulation region 111. Since these modulation regions are pn junctions, they can be regarded as capacitive pn diodes. That is, as shown in the equivalent circuit of FIG. 3, the first modulation region 110 and the second modulation region 111 are connected in series as a pn diode, and then the first high-frequency electrode 108 and the second high-frequency electrode It is loaded between 109 and 109.

Note that in the structure of the Si optical modulator 100, the n-type semiconductor layer and the p-type semiconductor layer may be formed in reverse. Furthermore, the vicinity of the central

thick Si layers

151a and 151b may be a low concentration region, and the side of the slab regions 152a to 152c connected to the electrodes may be a high concentration region.

In the Si optical modulator 100, unless the pn junction is in a reverse bias state, the modulation speed will deteriorate. Therefore, a voltage is applied to the metal bias electrode 107 such that the pn junction is in a reverse bias state, no matter what bias state each of the first modulation region 110 and the second modulation region 111 is in. . A differential modulated electrical signal is input to the first modulation region 110 and the second modulation region 111 from a driver amplifier connected to the first high-frequency electrode 108 and the second high-frequency electrode 109, respectively. This modulates the light.

As shown in the equivalent circuit of FIG. 3, the Si optical modulator 100 has a first modulation region 110 and a second modulation region 111 connected in series, and a first high-frequency electrode 108 and a second high-frequency electrode It is loaded between the electrode 109. Therefore, the differential modulated electrical signals applied to the first high-frequency electrode 108 and the second high-frequency electrode 109 are voltage-divided. In other words, if the amplitude voltage value of each differential modulator signal is X, then only a voltage of X/2 is applied to each of the first modulation region 110 and the second modulation region 111, which reduces the modulation efficiency. will deteriorate. On the other hand, when the amplitude voltage value is increased in order to improve modulation efficiency, there is a problem in that power consumption increases.

An object of the present invention is to provide an optical modulator with high modulation efficiency without increasing power consumption.

In order to achieve such an object, one embodiment of the present invention includes an input optical coupler, and a pair of first and second optical fibers that guide the input light branched into two by the input optical coupler. A Mach-Zehnder interferometer type optical modulator including a waveguide and an output optical coupler that combines output lights of the first and second waveguides, the optical waveguides of the first and second waveguides The core is made of a semiconductor layer having a pn junction and is connected to the first high frequency electrode in an optical modulator including first and second high frequency electrodes for applying a modulated electric signal and a bias voltage to the semiconductor layer. a first phase modulation region formed in the first waveguide having a first conductivity type semiconductor layer connected to the second high frequency electrode; A second waveguide formed in the second waveguide having a second conductive type semiconductor layer connected to the first high frequency electrode and a first conductive type semiconductor layer connected to the second high frequency electrode. A phase modulation region.

FIG. 1 is a plan view showing the structure of a conventional traveling wave electrode type Mach-Zehnder optical modulator. FIG. 2 is a cross-sectional view showing the structure of a conventional traveling wave electrode type Mach-Zehnder optical modulator. FIG. 3 is a diagram showing an equivalent circuit in the modulation region of a conventional traveling wave electrode type Mach-Zehnder optical modulator. FIG. 4 is a plan view showing the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to Embodiment 1 of the present invention; FIG. 5 is a cross-sectional view showing the structure of the traveling wave electrode type Mach-Zehnder optical modulator of Example 1, FIG. 6 is a cross-sectional view showing the structure of the traveling wave electrode type Mach-Zehnder optical modulator of Example 1, FIG. 7 is a diagram showing an equivalent circuit in the modulation region of the traveling wave electrode type Mach-Zehnder optical modulator of Example 1, FIG. 8 is a plan view showing the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to a second embodiment of the present invention; FIG. 9 is a cross-sectional view showing the structure of the traveling wave electrode type Mach-Zehnder optical modulator of Example 2, FIG. 10 is a diagram showing the relationship between the maximum modulation frequency bandwidth and wavelength of a modulated electrical signal; FIG. 11 is a plan view showing the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to Embodiment 3 of the present invention; FIG. 12 is a diagram showing the loss when a rib waveguide made of a silicon waveguide is bent by 90 degrees, FIG. 13 is a diagram showing the structure of a rib waveguide according to Example 3, FIG. 14 is a plan view showing the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to Example 4 of the present invention; FIG. 15 is a plan view showing the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to Example 5 of the present invention; FIG. 16 is a cross-sectional view showing the structure of the traveling wave electrode type Mach-Zehnder optical modulator of Example 5, FIG. 17 is a diagram showing an equivalent circuit in the modulation region of the traveling wave electrode type Mach-Zehnder optical modulator of Example 5, FIG. 18 is a diagram showing an example of application of capacitance in the traveling wave electrode type Mach-Zehnder optical modulator of Example 5, FIG. 19 is a diagram showing another application example of the capacitance in the traveling wave electrode type Mach-Zehnder optical modulator of Example 5, FIG. 20 is a diagram showing simulation results for the traveling wave electrode type Mach-Zehnder optical modulator of Example 5.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, parts having the same functions are given the same numbers for clarity of explanation. However, it is obvious to those skilled in the art that the present invention is not limited to the contents described in the embodiments shown below, and that the form and details can be changed in various ways without departing from the spirit of the invention disclosed in this specification etc. be. Furthermore, configurations according to different embodiments can be implemented in combination as appropriate.

FIG. 4 shows the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to Example 1 of the present invention. FIG. 4 is a perspective view of the structure of the optical waveguide and electrodes of the single-electrode Mach-Zehnder optical modulator 200, viewed from above. FIG. 5 shows a cross section taken along line V-V' in FIG. FIG. 6 shows a cross section taken along VI-VI' in FIG. The Si optical modulator 200 includes an input optical coupler (or Y-shaped splitter) 101 connected to an input optical waveguide, and two parallel first waveguides 102 that guide split input light into two. It includes a second waveguide 103 and an output optical coupler 104 that combines output light from the two waveguides and outputs it to an output optical waveguide.

The optical waveguide of the Si optical modulator 200 is composed of a Si layer 2 sandwiched between upper and lower SiO ₂ cladding layers 1 and 3. The Si waveguide for confining light has a structure called a rib waveguide with different thicknesses. That is, as shown in FIGS. 5 and 6, the rib waveguide is composed of

thick Si layers

151a and 151b in the center and thin slab regions 152a to 152d on both sides thereof. The central

thick Si layers

151a and 151b of the Si layer 2 are used as the cores of the first waveguide 102 and the second waveguide 103, respectively, and the difference in refractive index with the surrounding SiO ₂ cladding layers 1 and 3 is utilized. , constitutes an optical waveguide that confines light propagating in a direction perpendicular to the plane of the paper. The difference from the conventional traveling wave electrode type Mach-Zehnder optical modulator is that the slab regions 202b and 202c between the cores of the first waveguide 102 and the second waveguide 103 are electrically separated by an insulating layer 121. This is the point. The insulating layer 121 is a region in which the Si layer 2 is not doped with anything.

The optical waveguide core of the Si layer 2 has a pn junction in which p-

type semiconductor layers

105a, 105b and n-

type semiconductor layers

106a, 106b are joined at the center of the core. A first high-frequency electrode 108 is provided on the p-type semiconductor layer 105a, and a second high-frequency electrode 109 is provided on the p-type semiconductor layer 105b. Further, the n-type semiconductor layer 106a is connected to the second high-frequency electrode 109, and the n-type semiconductor layer 106b is connected to the first high-frequency electrode 108. Modulated electrical signals and bias voltages are applied via these electrodes. The n-

type semiconductor layers

106a, 106b and the high-frequency electrodes are connected by extending T-shaped electrodes from the first high-frequency electrode 108 and the second high-frequency electrode 109, as shown in FIG. However, the connection is not limited to this, and may be connected in any shape.

The phase modulation region formed in the first waveguide 102 is referred to as a first modulation region 110, and the phase modulation region formed in the second waveguide 103 is referred to as a second modulation region 111. Since these modulation regions are pn junctions, they can be regarded as capacitive pn diodes. That is, as shown in the equivalent circuit of FIG. 7, the first modulation region 110 is loaded between the first high-frequency electrode 108 and the second high-frequency electrode 109, and the second modulation region 111 is loaded between the It is loaded between the high frequency electrode 109 and the first high frequency electrode 108.

In the Si optical modulator 200, as shown in the equivalent circuit of FIG. are loaded independently between them. Therefore, the differential modulated electrical signal applied to the first high-frequency electrode 108 and the second high-frequency electrode 109 is not voltage-divided, and the amplitude voltage value X of the differential modulated electrical signal is different from the first modulated region 110. It is applied to each of the second modulation regions 111. Therefore, compared to a conventional traveling wave electrode type Mach-Zehnder optical modulator, the optical modulator can be driven with twice the modulation efficiency.

Note that the structure of the Si optical modulator 200 may be such that the n-type semiconductor layer and the p-type semiconductor layer are formed in reverse. Furthermore, the vicinity of the central thick Si layers 201a and 201b may be a low concentration region, and the side of the slab regions 202a to 202c connected to the electrodes may be a high concentration region. Furthermore, with regard to Example 1, the case of a Si modulator using a _Si semiconductor has been described. It may be used as a semiconductor.

FIG. 8 shows the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to a second embodiment of the present invention. FIG. 8 is a perspective view of the structure of the optical waveguide and electrodes of the single-electrode Mach-Zehnder optical modulator 300, viewed from above. FIG. 9 shows a cross section taken along line IX-IX' in FIG. Descriptions of components common to the first embodiment will be omitted.

The difference from the conventional traveling wave electrode type Mach-Zehnder optical modulator is that the first modulation region 110 formed in the first waveguide 102 and the second modulation region 111 formed in the second waveguide 103 are different from the conventional traveling wave electrode type Mach-Zehnder optical modulator. , which are divided into a plurality of points along the light propagation direction and arranged alternately. The first modulation region 110 and the second modulation region 111 are connected to the first high-frequency electrode 108 and the second high-frequency electrode 109 by extending T-shaped electrodes, as in the first embodiment. ing. As shown in FIG. 9, the core of the second waveguide 103 adjacent to the first modulation region 110 of the first waveguide 102 is a rib waveguide in which the Si layer 2 is not doped with anything. , acts as an insulating layer.

Similar to the first embodiment, in the Si optical modulator 300, as shown in the equivalent circuit of FIG. It is independently loaded between the two high frequency electrodes 109. Therefore, the differential modulated electrical signals applied to the first high-frequency electrode 108 and the second high-frequency electrode 109 are not voltage-divided, and compared to the conventional traveling wave electrode type Mach-Zehnder optical modulator, the optical modulation efficiency is twice as high. A modulator can be driven.

A method of dividing the first modulation area 110 and the second modulation area 111 will be explained. The length of each divided modulation region in the light propagation direction is set to be 1/20 or less of the wavelength included in the modulated electrical signal. In FIG. 10, when the refractive index n of the cores of the first waveguide 102 and the second waveguide 103 is set to 3.5, a modulated electric signal propagating through the first high-frequency electrode 108 and the second high-frequency electrode 109 is shown. shows the relationship between maximum modulation frequency bandwidth and wavelength. For example, when the modulated electrical signal includes a signal of maximum 60 GHz, 70 μm is obtained from the relationship: speed of light x refractive index/maximum modulation frequency bandwidth/20.

The first high frequency electrode 108 and the second high frequency electrode 109 are traveling wave electrodes. As described above, the traveling wave electrode is an electrode that matches the velocity of the modulated electrical signal and the light propagating through the optical waveguide (phase velocity matching), and interacts with the light while propagating the electrical signal. One end of the traveling wave electrode is connected to a driver amplifier, and the other end is terminated by a terminating resistor, and the traveling wave electrode therebetween is configured as a high frequency line having characteristic impedance. When a modulated electrical signal with a frequency bandwidth propagates through a traveling wave electrode, if the length exceeds about 1/20 wavelength of the maximum wavelength of the modulated electrical signal, the characteristic impedance has a large frequency dependence. become.

Therefore, the first modulation region 110 and the second modulation region 111 are divided into 1/20 or less of the maximum wavelength of the modulated electrical signal. As a result, the length of the T-shaped electrode in the light propagation direction that applies a modulated electrical signal to the n-type semiconductor layer 106 becomes a length that is not affected by frequency dependence. The T-shaped electrode that applies a modulated electrical signal to the p-type semiconductor layer 105 also has the same configuration. Therefore, in addition to the effects of the first embodiment, the Si optical modulator 300 can suppress the quality deterioration of the modulated electrical signal applied to the semiconductor layer and improve the high frequency characteristics.

The number of first modulation regions 110 and second modulation regions 111 should be approximately the same in the light propagation direction. This is to equalize the amount of phase modulation in each of the first waveguide 102 and second waveguide 103 divided by the input optical coupler 101 in the Mach-Zehnder optical modulator. This is because modulated output light without chirp (waveform distortion) can be output.

FIG. 11 shows the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to Example 3 of the present invention. FIG. 11 is a perspective view of the structure of the optical waveguide and electrodes of the single-electrode Mach-Zehnder optical modulator 400, viewed from above. Descriptions of components common to Example 2 will be omitted.

The difference from the conventional traveling wave electrode type Mach-Zehnder optical modulator is that the first modulation region 110 formed in the first waveguide 102 and the second modulation region 111 formed in the second waveguide 103 are different from the conventional traveling wave electrode type Mach-Zehnder optical modulator. , are points divided into multiple parts. Furthermore, the first modulation region 110 and the second modulation region 111 are optically connected by

curved rib waveguides

412 and 413, respectively. The

rib waveguides

412 and 413 have a structure including four 90 degree bends, and are formed of undoped insulating silicon waveguides. The cross-sectional structures of the rib waveguides 312 and 313 are the same as those of the first waveguide 102 and the second waveguide 103, respectively.

Comparing the region 150 of the conventional Si optical modulator 100 (see FIG. 1) with the region 410 of the Si optical modulator 400 according to the third embodiment, the capacity loaded per unit length is halved. . Specifically, in the configuration of FIG. 1, two capacitances Cpn across the pn junction of the first modulation region 110 and a capacitance Cpn across the pn junction of the second modulation region 111 are connected between the

signal electrodes

108 and 109. are connected. On the other hand, in the configuration of FIG. 11, only one capacitor Cpn is connected between the

signal electrodes

108 and 109. The capacity per area is 1/(1/Cpn+1/Cpn)=Cpn/2 in the configuration of the Si optical modulator 100 in FIG. 1, but becomes Cpn in the configuration of the Si optical modulator 400 in FIG. . This means that twice the capacity is loaded per unit length.

Generally, the propagation speed of a high-frequency signal propagating through an optical modulator is inversely proportional to the square root of the interelectrode capacitance. becomes late. Although the signal propagation speed can be adjusted by changing the width and spacing of the signal electrodes, it is difficult to adjust a larger capacitance than in the conventional configuration.

Therefore, in the third embodiment,

bent rib waveguides

412 and 413 are provided to serve as delay lines for optical signals, thereby effectively slowing down the propagation of the optical signals. That is, the optical signal is also delayed in accordance with the delay of the high frequency signal due to the increase in the interelectrode capacitance, so that the propagation speeds of the optical signal and the high frequency signal are made equal. Thereby, when modulating an optical signal with a high frequency signal, it is possible to prevent band deterioration due to speed mismatch between the two.

Conventionally, such optical signal delay lines have been realized not by rib waveguides but by channel waveguides. The channel waveguide has a rectangular cross section and can be bent at a smaller radius without loss. Specifically, a delay line using a channel waveguide is created in the optical modulation region formed by the rib waveguide via a rib-channel waveguide converter, and then a delay line is created using a channel waveguide via a rib-channel waveguide converter, and then a delay line is created using a channel waveguide via a rib-channel waveguide converter. It was coupled to a light modulation region formed by a waveguide. This Ribou channel waveguide converter has an optical loss of about 0.2 dB per conversion, and when a large number of them are arranged in one optical modulator, the optical loss becomes a problem.

However, according to the configuration of the third embodiment, the first modulation region 110 and the second modulation region 111 are arranged alternately, and the

rib waveguides

412 and 413 are arranged in the region where the PN junction is not arranged. is forming. Therefore, since a sufficient length in the propagation direction of the optical signal can be secured between the modulation regions, a rib waveguide with a large bending radius for low loss can be used, and a delay line with a desired amount of delay can be formed. can be provided.

FIG. 12 shows the loss when a rib waveguide made of a silicon waveguide is bent by 90 degrees. As described above, the length of the divided modulation regions (110, 111) is set to be 1/20 or less of the wavelength included in the modulated electrical signal. When a modulated electrical signal of 40 GHz is applied, the length of 1/20 of the wavelength is about 100 μm. On the other hand, as shown in FIG. 12, in a rib waveguide made of a general silicon waveguide, bending loss increases rapidly when the bending radius becomes smaller than about 10 μm.

FIG. 13 shows the structure of the rib waveguide according to Example 3. Therefore, the

bent rib waveguides

412 and 413 have a structure including four 90 degree bends (1/4 arc) with a bending radius R=20 μm. Referring to FIG. 9, the bending loss of this rib waveguide can be made sufficiently small as about 0.025 dB for four 90 degree bends.

Since the length of the

rib waveguides

412 and 413 is 80 μm with respect to the light propagation direction of the Si optical modulator 400, it is possible to arrange them between the modulation regions (110 and 111) divided as described above. can. Furthermore, an appropriate delay amount for the delay line can be set by providing a straight line portion as shown in FIG. 13 and adjusting its length Lst.

FIG. 14 shows the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to Example 4 of the present invention. FIG. 14 is a perspective view of the structure of the optical waveguide and electrodes of the single-electrode Mach-Zehnder optical modulator 500, viewed from above. Descriptions of components common to Example 2 will be omitted.

The difference from the conventional traveling wave electrode type Mach-Zehnder optical modulator is that the first modulation region 110 formed in the first waveguide 102 and the second modulation region 111 formed in the second waveguide 103 are different from the conventional traveling wave electrode type Mach-Zehnder optical modulator. , are points divided into multiple parts. Furthermore, the difference from the Si optical modulator 300 of Example 2 is that the divided modulation regions are not arranged alternately, but are arranged collectively on one side. As shown in FIG. 14, by providing bent portions in the first waveguide 102 and the second waveguide 103, the first modulation region 110 and the second modulation region 111 can be bent along the propagation direction of light. The layout is arranged in such a way that the

The configurations of the divided first high-frequency electrode 108 and second high-frequency electrode 109 are substantially the same as the Si optical modulator 300 of the second embodiment, and in addition to the effects of the first embodiment, there are also the advantages of the second embodiment. The same effects can be achieved. Furthermore, since the divided modulation regions do not have a periodic structure, there is a high degree of freedom in manufacturing design.

FIG. 15 shows the structure of a traveling wave electrode type Mach-Zehnder optical modulator according to Example 5 of the present invention. FIG. 15 is a perspective view of the structure of the optical waveguide and electrodes of the single-electrode Mach-Zehnder optical modulator 600, viewed from above. FIG. 16 shows a cross section taken along line XVI-XVI' in FIG. 15. Descriptions of components common to Example 2 will be omitted.

The p-type semiconductor layer 105 of the first modulation region 110 is connected to the first high-frequency electrode 108, and the n-type semiconductor layer 106 is electrically connected to the electrode 109 via the capacitor 113. The p-type semiconductor layer 105 of the second modulation region 111 is connected to the second high-frequency electrode 109, and the n-type semiconductor layer 106 is electrically connected to the first high-frequency electrode 108 via the capacitor 113. The electrode structure between the n-type semiconductor layer 106 and the capacitor 113 will be referred to as a node 116 for the following explanation. The capacitor 113 has a structure in which the second high-frequency electrode 109 and the node 116 are opposed to each other by a predetermined distance in the light propagation direction with a predetermined interval.

As shown by the broken line in the cross-sectional views of FIGS. 15 and 16, a bias electrode 115 is connected to the node 116 via a resistor 114. An external bias power supply is connected to the bias electrode 115, so that a bias voltage can be applied separately from the modulated electrical signal. The bias electrode 115 is formed using a metal wiring layer below the second high frequency electrode 109.

In addition to the effects of Examples 1-4, the Si optical modulator 600 can further improve high frequency characteristics. In order to operate the Si optical modulator at high speed, it is necessary to always apply a reverse bias to the PN junction of the first modulation region 110 and the second modulation region 111. However, if the modulated electrical signal and bias voltage are applied using the first high-frequency electrode 108 and the second high-frequency electrode 109 in common, the PN junction may become forward biased depending on the amplitude of the modulated electrical signal. do. As shown in the equivalent circuit of FIG. 17, the Si optical modulator 500 has a configuration similar to a so-called bias T, consisting of a capacitor 113 and a resistor 114, so that a bias voltage can be applied separately from the modulated electrical signal. can. With this configuration, the PN junction can always be set to forward bias, so high-speed operation is possible.

Furthermore, the phase modulation efficiency with respect to the applied voltage of the Si modulator changes depending on the bias voltage of the PN junction. In the Si optical modulator 600 of Example 5, since any bias voltage can be applied to the PN junction, the modulation efficiency can be further improved by operating at the bias voltage that provides the best modulation efficiency.

(Application example of capacity 113)
A specific capacitance value of the capacitor 113 will be illustrated. In a capacitor 113 in which metal electrodes of a predetermined length are opposed to each other with a predetermined interval, the capacitance value per unit length is defined as Cbias. Further, in the PN junction between the first modulation region 110 and the second modulation region 111, the capacitance value per unit length in the light propagation direction is defined as Cpn. In the equivalent circuit of FIG. 17, when the resistor 114 is sufficiently large, the current flowing through the resistor 114 can be almost ignored. The voltage X applied to the first high frequency electrode 108 and the second high frequency electrode 109 is proportional to the reciprocal sum of the capacitance (1/Cbias+1/Cpn), and the voltage applied to Cpn involved in phase modulation is the reciprocal of the capacitance 1/Cbias. is proportional to.

Therefore, the modulation efficiency is Cpn/(Cbias+Cpn), so in order to obtain twice the modulation efficiency as compared to the conventional traveling wave electrode type Mach-Zehnder optical modulator, Cbias should be set to the same capacitance value or more with respect to Cpn. There is a need to. For example, when Cpn per unit length is about 100 fF/mm, Cbias is set to 5 pF/mm, and the value of the resistor 114 is set to approximately 1 kΩ or more.

FIG. 18 shows an application example of the capacitor 113. As is clear from FIG. 15, the length of the capacitor 113 in the light propagation direction is limited by the structure of the divided modulation regions. A desired capacitance value may not be obtained simply by making the second high-frequency electrode 109 and the node 116 face each other. Therefore, it may be possible to implement it by providing an MIM (Metal-Insulator-Metal) structure using a lower wiring layer. As shown in FIG. 18, a larger capacitance value can be obtained by arranging the second high-frequency electrode 109 and the node 116 to face each other by a predetermined area with the SiO ₂ cladding layer 3 in between.

FIG. 19 shows another application example of the capacitor 113. When manufacturing the first modulation region 110 and the second modulation region 111, a pn junction connecting a p-type semiconductor layer and an n-type semiconductor layer is manufactured separately from the modulation region. A desired capacitance value can also be obtained by using this pn junction as the capacitor 113.

FIG. 20 shows simulation results when a modulated electrical signal is applied to the Si optical modulator 500 of Example 5. The above-mentioned Cpn is set to 100 fF/mm, Cbias is set to 5 pF/mm, and the resistance value of the resistor 114 is set to 10 kΩ. A differential modulated electrical signal of 30 Gbaud and an amplitude of 2 V was input to the first high frequency electrode 108 and the second high frequency electrode 109 shown in FIG. FIG. 20(a) shows an eye pattern of a modulated electrical signal input from the driver amplifier to each high-frequency electrode. FIG. 20(b) shows an eye pattern at the termination resistor after propagating through the high-frequency electrode. FIG. 20(c) is an eye pattern of the voltage signal applied to the PN junction of the first modulation region 110.

As is clear from FIG. 20(c), a voltage signal of approximately -2V to -4V is applied, and it is found that the device operates with reverse bias. Furthermore, since a voltage signal of about -2V to -4V is applied to the phase modulation section 111, the Mach-Zehnder optical modulator operates with an amplitude of about 4V in response to a differential input signal with an amplitude of 2V. . Furthermore, FIGS. 20(b) and 20(c) show good eye openings, and it can be seen that quality deterioration of the modulated electrical signal is suppressed. Therefore, compared to the conventional traveling wave electrode type Mach-Zehnder optical modulator, it is possible to drive the optical modulator with twice the modulation efficiency, realizing an optical modulator with high modulation efficiency without increasing power consumption. It can be seen that

According to the Si optical modulator of this embodiment, the differential modulated electric signal applied to the first high-frequency electrode 108 and the second high-frequency electrode 109 is not voltage-divided, and is different from the conventional traveling wave electrode type Mach-Zehnder optical modulator. By comparison, it is possible to drive an optical modulator with twice the modulation efficiency. Therefore, an optical modulator with high modulation efficiency can be realized without increasing power consumption.

Regarding the modulation region of the Si optical modulator of this embodiment, only the polarity of the n-type semiconductor layer and the p-type semiconductor layer has been described, but the region near the thick Si layer at the center, which becomes the optical waveguide core, is a low concentration region, and the electrode The side of the slab region to be connected may be a high concentration region. Alternatively, a pin junction may be used in which an undoped i-type (intrinsic) semiconductor is sandwiched between pn junction structures.

In this embodiment, a Si modulator using a Si semiconductor has been described, but the material of the modulation region can also be made of other materials such as InP. This is because even in an InP modulator, reverse bias operation can be performed using a pn junction or a pin junction.

The present invention can be generally used in optical communication systems. In particular, it can be applied to an optical modulator in an optical transmitter of an optical communication system.

Claims

an input optical coupler, a pair of first and second waveguides that guide the input light branched into two by the input optical coupler, and combine the output lights of the first and second waveguides. A Mach-Zehnder interferometer type optical modulator including an output optical coupler, wherein the optical waveguide cores of the first and second waveguides are made of a semiconductor layer having a pn junction, and the semiconductor layer is provided with a modulated electrical signal. and an optical modulator including first and second high frequency electrodes for applying a bias voltage,
A first waveguide formed in the first waveguide having a first conductive type semiconductor layer connected to the first high frequency electrode and a second conductive type semiconductor layer connected to the second high frequency electrode. 1 phase modulation region;
a second conductive type semiconductor layer connected to the first high frequency electrode; and a first conductive type semiconductor layer connected to the second high frequency electrode. An optical modulator comprising two phase modulation regions.
The optical modulator according to claim 1, wherein the first and second phase modulation regions are divided into a plurality of regions along the light propagation direction.
The optical modulator according to claim 2, wherein the divided first phase modulation regions and the divided second phase modulation regions are arranged alternately along the light propagation direction.
4. The first divided phase modulation area and the second divided phase modulation area are connected by a bent rib waveguide. light modulator.
The rib waveguide is made of an insulating silicon optical waveguide,
5. The optical modulator according to claim 4, wherein the rib waveguide has the same cross-sectional structure as the first waveguide or the second waveguide.
The optical modulator according to claim 5, wherein the bending radius of the rib waveguide is 10 μm or more.
A divided first phase modulation region is arranged along the light propagation direction, and a divided second phase modulation region is arranged along the light propagation direction with the bent portions of the first and second optical waveguides in between. 3. The optical modulator according to claim 2, wherein the optical modulator is arranged as follows.
a first capacitor inserted between the first phase modulation region and the second high frequency electrode;
a first resistor having one end connected between the first phase modulation region and the first capacitor;
a second capacitor inserted between the second phase modulation region and the first high frequency electrode;
further comprising a second resistor having one end connected between the second phase modulation region and the second capacitor,
7. The optical modulator according to claim 1, wherein a bias voltage is applied to the other ends of the first and second resistors.
The optical modulator according to claim 8, wherein the first and second capacitors have an MIM structure using a wiring layer below the first and second high-frequency electrodes.
The optical modulator according to claim 8, wherein the first and second capacitors are made of semiconductor layers having pn junctions.