WO2023105585A1

WO2023105585A1 - Optical modulator

Info

Publication number: WO2023105585A1
Application number: PCT/JP2021/044757
Authority: WO
Inventors: 泰彰橋詰; 義弘小木曽; 常祐尾崎; 光映石川
Original assignee: 日本電信電話株式会社
Priority date: 2021-12-06
Filing date: 2021-12-06
Publication date: 2023-06-15

Abstract

This optical modulator includes: an optical wave guide that is formed on a semiconductor substrate and includes an optical waveguide layer in which a lower cladding layer, a core layer, and an upper cladding layer are layered in this order; an optical demultiplexer (407XI) that demultiplexes an optical signal propagated through the optical waveguide; and a protective layer covering the optical waveguide and the optical demultiplexer. The optical waveguide includes an inside course section (409XIa) and an outside course section (409XIb) for propagating the optical signals demultiplexed by the optical demultiplexer, a delay section in which the interval between the inside course section and the outside course section varies and the length of the inside course section is longer than that of the outside course section, and a folded section in which the interval between the inside course section and the outside course section is constant and the direction of extension changes. The protective layer in the area including the delay section and the folded section comprises an MZM (401X1) having a protective layer non-formation area Rd in which the thickness is thinner than that of the protective layer above the optical demultiplexer.

Description

optical modulator

The present invention relates to an optical modulator.

With the increasing capacity of optical communication systems, there is a demand for high-speed optical modulators that support advanced optical modulation methods. In particular, multilevel optical modulators using digital coherent technology play a major role in realizing large-capacity transceivers exceeding 100 Gbps. The multilevel optical modulator incorporates in parallel and in multiple stages optical modulators (MZMs) configured by Mach-Zehnder interference type optical waveguides (MZM optical waveguides) and capable of zero-chirp driving. The MZ optical waveguide is configured to split an optical input into two arm waveguides, change the phase, combine the waves, and output interference. With such a configuration, the multilevel optical modulator can add independent signals to the amplitude and phase of light.

The IQ optical modulator is a typical polarization multiplexing optical modulator that is currently spreading to communication networks. The IQ optical modulator has a so-called nested structure in which each arm of a parent MZM is composed of child MZMs. The MZM (Quad-parallel MZM) is an optical modulator having two sub-MZMs arranged in parallel corresponding to each of the X and Y polarization channels, for a total of four sub-MZMs. Two arms of each child MZM are provided with traveling-wave electrodes to which RF-modulated electrical signals for modulating the optical signal propagating in the optical waveguide are input. In each polarization channel, one such pair of child MZMs corresponds to the I channel and the other to the Q channel. In such a polarization multiplexed IQ optical modulator, an electro-optical effect is generated by inputting an RF modulated electric signal to one end of a modulation electrode provided along an arm optical waveguide of the child MZM. It modulates the phases of two optical signals propagating in the optical waveguide. Such configurations are known, for example, from US Pat.

FIG. 1 is a top view illustrating a known polarization multiplexed IQ optical modulator 100. FIG. A polarization multiplexing IQ optical modulator 100 is formed on a semiconductor substrate 21 (FIG. 2). The polarization multiplexing IQ optical modulator 100 has an MZM 101X for the X polarization channel and an MZM 101Y for the Y polarization channel. MZM 101X has MZM 101XI for I channel and MZM 101XQ for Q channel. The MZM 101Y has an I-channel MZM 101YI and a Q-channel MZM 101YQ. An input optical waveguide 102 is formed on one side of a cleaved semiconductor substrate (hereinafter referred to as a “semiconductor chip”), passes between

MZMs

101X and 101Y, and is connected to an optical demultiplexer 103 . The output of optical demultiplexer 103 is connected to

optical demultiplexers

104X and 104Y. The output of optical demultiplexer 104X is connected to MZMs 101XI and 101XQ, and the output of optical demultiplexer 104Y is connected to MZMs 101YI and 101YQ. The MZM101X and MZM101Y have the same configuration, and the MZM101XI and MZM101XQ, and the MZM101YI and MZM101YQ have the same configuration formed line-symmetrically with each other. Therefore, only the MZM101XI will be described below.

The MZM 101XI for the X polarization channel is provided together with the phase adjuster 106XI between the optical demultiplexer 104X and the optical multiplexer 105X that multiplexes the optical signals demultiplexed by the optical demultiplexer 104X. . The MZM 101XI also includes MZ optical waveguides 109XIa, 109XIb, 111XIa, 111XIb, phase adjusters 110XIa, 110XIb, and traveling wave electrodes 112 between the optical demultiplexer 107XI and the optical multiplexer 108XI. The traveling wave electrode 112 is connected to an RF signal line (not shown) to receive an RF modulation signal.

FIG. 2 is a cross-sectional view of the MZ optical waveguide 109XIa along arrows A and A' in FIG. In FIG. 2, the z direction indicated by the x, y, and z coordinates shown in FIG. 2 is the height h direction, and the x direction is the width W direction. Then, the side with the larger z-axis coordinate value is defined as “above” or “above” the relatively smaller side. The IQ optical modulator 100 includes a semiconductor substrate 21, lower clad layers 22 and 23 formed on the upper surface of the semiconductor substrate 21, a core layer 24 formed on the upper surface of the lower clad layer 23, and formed on the upper surface of the core layer 24. It has upper

clad layers

25 and 26 . The oxide film 27 covers the side surfaces of the lower clad layers 22 , 23 , the core layer 24 , the upper

clad layers

25 , 26 and the upper surface of the upper clad layer 26 . A benzocyclobutene (BCB) layer 28 is formed on the oxide film 27 . A configuration in which the BCB layer 28 covers the optical waveguide is described in Non-Patent Document 1, for example.

In the above configuration, for example, the semiconductor substrate 21 is a SI (Semi-Insulating)-InP substrate, the lower clad layer 22 is an n-type InP lower clad layer, the lower clad layer 23 is a non-doped InP lower clad layer, and the core layer 24 is a non-doped multiple quantum A multi-quantum well (MQW) core layer and upper

clad layers

25 and 26 are non-doped InP upper clad layers. In FIG. 2, both the

upper cladding layers

25 and 26 are non-doped InP upper cladding layers. be layered. Such a structure is formed by laminating the lower clad layers 22 and 23, the core layer 24, the upper clad layer 25, and the p-type InP upper clad layer on the semiconductor substrate 21 and patterning them by exposure and etching. In patterning, the p-type InP upper cladding layer is removed leaving only the areas where phase modulation is to be applied. The removed p-type InP upper cladding layer is replaced with the non-doped InP upper cladding layer 26 by regrowing the non-doped InP cladding layer in the removed portion.

The configuration shown in FIG. 2 forms a high mesa structure above the lower clad layer 22 and realizes an optical waveguide structure that confines light in the xy plane. The high mesa structure has a width W of about 2 μm and a height of about h4 μm. The oxide film 27 serves as a protective layer for protecting the high mesa structure. The BCB layer 28 is a protective layer intended for planarization and protection of the optical waveguide. The film thickness of the BCB layer 28 is about 0.3 μm on the upper surface of the high mesa structure. An electrode for applying an electric field is formed so as to contact the p-type InP clad layer by partially removing the BCB layer and the SiO2 layer above the p-type InP clad layer.

International publication WO/2018/174083 International publication WO/2021/049004

However, in the known optical modulator, the optical waveguide has a refractive index applied to the light guided in the high mesa structure by covering the high mesa structure with a BCB layer 28 as shown in FIG. is strongly affected by the stress of This is described, for example, in Non-Patent Document 2. That is, BCB has a coefficient of thermal expansion of about 42 ppm/° C., whereas InP has a coefficient of thermal expansion of about 4.5 ppm/° C. It has a thermal expansion coefficient about one order of magnitude larger than that of InP. In the manufacturing process of the IQ optical modulator 100, inside the high mesa structure made of InP covered with the BCB layer 28, extremely large stress is generated due to the difference in thermal expansion coefficient between InP and BCB. As a result, the optical waveguide undergoes a stress-induced refractive index change via the photoelastic effect due to the stress from the BCB layer 28 .

Furthermore, BCB is known to be unstable as a material compared to InP, and to bond with the surrounding atmosphere and internal components and deteriorate over a long period of time. Due to the alteration of the BCB, the internal stress of the BCB layer 28 changes, and the equivalent refractive index in the high mesa structure changes over time via the photoelastic effect.

The present disclosure has been made in view of the above points, and relates to an optical modulator that relieves the stress applied to the optical waveguide from the protective layer and suppresses changes in the refractive index of the optical waveguide, thereby having stable characteristics.

In order to achieve the above object, an optical modulator according to one embodiment of the present disclosure includes an optical waveguide formed on a semiconductor substrate and having an optical waveguide layer in which a lower clad layer, a core layer, and an upper clad layer are laminated in this order; a first optical demultiplexer that demultiplexes an optical signal propagating through an optical waveguide; and a protective layer that covers the optical waveguide and the first optical demultiplexer, wherein the optical waveguide is connected to the first optical demultiplexer. including a first optical waveguide and a second optical waveguide for propagating an optical signal demultiplexed by an optical demultiplexer, the distance between the first optical waveguide and the second optical waveguide being different, and a delay section in which the optical path length of the second optical waveguide is longer than the optical path length of the first optical waveguide; and the protective layer in the range including the folded portion has a region thinner than the protective layer overlying the first optical demultiplexer.

According to the above configuration, it is possible to provide an optical modulator having stable characteristics by relieving the stress applied from the protective layer to the optical waveguide and suppressing changes in the refractive index of the optical waveguide.

1 is a top view illustrating a known polarization multiplexed IQ optical modulator; FIG. 2 is a cross-sectional view of the MZ optical waveguide in FIG. 1; FIG. It is a top view for explaining an optical modulator of one embodiment of the present invention. 4 is a diagram for explaining the delay section and the folding section shown in FIG. 3, and is an enlarged view of a part of the MZM; FIG. 4 is a cross-sectional view of the MZ optical waveguide in FIG. 3; FIG. FIG. 4 is a diagram showing changes in phase difference occurring in the optical modulator of the present embodiment in comparison with changes in phase difference in a known IQ optical modulator;

An embodiment of the present disclosure will be described below. FIG. 3 is a top view for explaining the optical modulator 4 of one embodiment of the invention. 4 is an enlarged view of the protective layer non-formation region Rd shown in FIG. 3, and FIG. 5 is a cross-sectional view of the incourse portion 409XIa along arrows B and B' in FIG. In this embodiment, the top and bottom are determined along the z-axis of the x, y, and z coordinates shown in FIGS. ”. The x-direction is the width direction of the optical modulator, and the y-direction is the length direction.

This embodiment will be described with an example in which the optical modulator 4 is a polarization multiplexed IQ optical modulator. However, this embodiment is not limited to such a configuration. This embodiment is a Mach-Zehnder interferometric circuit, a portion where a difference occurs in the length of two arm waveguides (hereinafter also referred to as a “waveguide length difference”) such as a folded portion, and the waveguide length It can be applied to a circuit having a section in the interferometer with a structure in which a portion of one of the two waveguides separates from the other due to the delay in order to cancel (compensate) the difference.

As shown in FIG. 3, the optical modulator 4 is formed on a semiconductor substrate 51 to constitute a semiconductor chip. As shown in FIG. 3, the optical modulator 4 includes a Mach-Zehnder interferometer (hereinafter referred to as "MZM"). The optical modulator 4 has an MZM401X for the X polarization channel and an MZM401Y for the Y polarization channel.

MZMs

401X and 401Y have I-channel MZMs 401XI and 401XQ for optical modulation using IQ quadrature modulation. MZM401Y has MZM40YI and 401YQ. A configuration in which an MZM contains further MZMs is also called a "nested structure".

As shown in FIG. 3, the MZMs 401XI, 401XQ, 401YI, and 401YQ are arranged parallel to each other in the x-axis direction indicated by the x, y, and z coordinates in the drawing. The

MZMs

401X and 401Y are arranged axisymmetrically, the MZMs 401XI and 401XQ are arranged axisymmetrically, and the MZMs 401YI and 401YQ are arranged axisymmetrically. The optical modulator 4 also has an input optical waveguide 402 and output

optical waveguides

416X and 416Y, and receives an optical signal from the input optical waveguide 402 and outputs from the output

optical waveguides

416X and 416Y. The MZM401XI is configured to process the optical signal of the I channel of the X polarization channel. The MZM401XQ is configured to process an optical signal of the Q channel of the X polarization channel. The MZM401YI is configured to process an optical signal of the I channel of the Y polarization channel. The MZM401YQ is configured to process an optical signal of the Q channel of the Y polarization channel.

As shown in FIG. 5, the MZM401XI is formed on the upper surface of a semiconductor substrate 51, and has an optical waveguide layer in which lower clad

layers

52 and 53, a core layer 54, and upper clad

layers

55 and 56 are laminated in this order; An oxide film 57 and a BCB layer 58, which are protective layers covering the optical waveguide, are included. Also, as shown in FIG. 3, the MZM401XI has an optical demultiplexer 407XI, and the optical waveguides of the MZM401XI include a first optical waveguide and a second optical waveguide for propagating the optical signal demultiplexed by the optical demultiplexer 407XI. Including wave path. The oxide film 57 and the BCB layer 58, which are protective layers, are also formed above the optical demultiplexer 407XI.

As shown in FIG. 4, the first optical waveguide includes incourse portions 409XIa, 411XIa, 412XIa, 413XIa, 414XIa, and 415XIa. The second optical waveguide includes outcourse portions 409XIb, 411XIb, 412XIb, 413XIb, 414XIb, 415XIb. A portion where the distance between the first optical waveguide and the second optical waveguide is different and the optical path length of the second optical waveguide is longer than that of the first optical waveguide is defined as a delay portion Rb. Further, a portion where the interval between the first optical waveguide and the second optical waveguide is constant and the extending direction changes is referred to as a folded portion Ra. In addition, although the folded portion shown in the present embodiment changes its extending direction by 90 degrees, this embodiment is not limited to changing the extending direction of the folded portion by 90 degrees. The extending direction may be changed by . In the MZM 401XI, the protective layer in the area including the delay portion Rb and the turn-around portion Ra has a region thinner than the protective layer overlying the optical demultiplexer 407XI. This embodiment will explain an example in which this region is a protective layer non-formation region Rd in which at least a portion of the protective layer is not formed. Such an example corresponds to an example in which at least a portion of the protective layer has a thickness of "0". The above configuration corresponds to the first optical modulation unit of this embodiment, and the optical demultiplexer 407XI corresponds to the first optical demultiplexing section of this embodiment.

In the IQ optical modulator 4, a high-speed RF modulated signal is input from the -y direction in the drawing, and a driver such as an amplifier is arranged on the -y direction side. For the input/output of optical signals, it is necessary to arrange a lens close to the end surface of the semiconductor chip, and the input/output of optical signals cannot be performed on the -y direction side of the semiconductor chip. For this reason, the IQ optical modulator 4 has a structure in which the MZM optical waveguide is folded back from the input side toward the input side, and input and output of optical signals are performed on the y-direction side of the semiconductor chip. .

Furthermore, the MZM 401XI includes a phase modulator 406XI, an optical multiplexer 408XI, phase modulators 410XIa and 410XIb, and a traveling wave electrode 422.

The MZM 401XQ includes a phase modulator 406XQ, an optical demultiplexer 407XQ, an optical multiplexer 408XQ, phase modulators 410XQa and 410XQb, and a traveling wave electrode 422. The MZM waveguide of MZM 401XQ includes a delay section including an incourse section 409XQa and an outcourse section 409XQb, and a folded section including an incourse section 411XQa and an outcourse section 411XQb.

The MZM 401YI includes a phase modulator 406YI, an optical demultiplexer 407YI, an optical multiplexer 408YI, phase modulators 410YIa and 410YIb, and a traveling wave electrode 422. The MZM waveguide of MZM 401YI includes a delay portion including an incourse portion 409YIa and an outcourse portion 409YIb, and a folded portion including an incourse portion 411YIa and an outcourse portion 411YIb.

The MZM 401YQ includes a phase modulator 406YQ, an optical demultiplexer 407YQ, an optical multiplexer 408YQ, phase modulators 410YQa and 410YQb, and a traveling wave electrode 422. The MZM waveguide of MZM 401YQ includes a delay section including an incourse section 409YQa and an outcourse section 409YQb, and a folded section including an incourse section 411YQa and an outcourse section 411YQb.

In the above configuration, MZM401XQ, MZM401YI and MZM401YQ each correspond to the first optical modulation unit of this embodiment, like MZM401XI. In addition, the MZM401X includes an optical demultiplexer 404X that demultiplexes and inputs an optical signal propagating through the input optical waveguide 402 (one optical waveguide) to each of the MZM401XI and MZM401XQ, and the MZM401XI and MZM401XQ from the MZM401XI and MZM401XQ It includes an optical multiplexer 405X that multiplexes the output optical signals, and an output optical waveguide 416X that outputs the optical signal multiplexed by the optical multiplexer 405X. Such a configuration corresponds to the second optical modulation unit of this embodiment, and the optical demultiplexer 404X corresponds to the second optical demultiplexer. Furthermore, the optical modulator 4 includes

MZMs

401X and 401Y, and an optical demultiplexer 403 that demultiplexes and inputs the optical signal before being demultiplexed by the optical demultiplexer 404X to each of the

MZMs

401X and 401Y. . The optical demultiplexer 403 corresponds to a third optical demultiplexer. The first optical modulator is a single optical modulator, the second optical modulator is an IQ optical modulator, and the optical modulator 4 is a polarization multiplexed IQ optical modulator.

Next, the operation of the optical modulator of this embodiment will be described. However, among the above configurations, those having the same reference numerals have similar functions and operate in the same manner. Therefore, in the present embodiment, the functions and operations of the MZM401XI will be described to replace redundant descriptions of other MZMs. The input optical waveguide 402 inputs an optical signal from the y direction toward the -y direction in the semiconductor chip. An optical demultiplexer 403 is provided on the input optical waveguide 402, and

optical demultiplexers

404X and 404Y are further formed at two outputs of the optical demultiplexer 403. FIG. The MZM 401XI inputs one of the optical signals demultiplexed by the optical demultiplexer 404X. Phase modulator 406XI adjusts the phase of the optical signal input to MZM401XI.

The optical demultiplexer 407XI demultiplexes the phase-adjusted optical signal. The phase modulators 410XIa and 410XIb respectively modulate the phases of the demultiplexed optical signals. In the MZM waveguide, the in-course portion 409XIa propagates the optical signal phase-modulated by the phase modulator 410XIa, and the out-course portion 409Ib propagates the optical signal phase-modulated by the phase modulator 410XIb. The in-course portion 411XIa propagates the optical signal that has passed through the in-course portion 409Ia, and the out-course portion 411XIb propagates the optical signal that has passed through the out-course portion 409Ib. The optical multiplexer 408XI multiplexes the optical signals that have passed through the in-course portion 411XIa and the out-course portion 411XIb, and outputs them toward the output optical waveguide 416X.

An optical multiplexer 405X is provided between the optical multiplexer 408XI and the output optical waveguide 416X. The optical multiplexer 405X multiplexes the optical signal output toward the output optical waveguide 416X with the optical signal output from the MZM 401XQ, and outputs the combined optical signal to the output optical waveguide 416X.

As shown in FIG. 4, the MZM optical waveguide of the MZM 401XI includes a delay portion Rb including an in-course portion 409XIa and an out-course portion 409XIb, a straight portion including an in-course portion 412XIa and an out-course portion 412XIb, an in-course portion 413XIa and an out-course portion 412XIa. A 90-degree bend including a course portion 413XIb, an in-course portion 414XIa, a straight portion including an out-course portion 414XIb, an in-course portion 415XIa, a 90-degree bend including an out-course portion 415XIb, an in-course portion 411XIa, and an out-course portion 411XIb. It includes a folded portion Ra having a straight portion including.

In the above configuration, in the delay portion Rb, both the incourse portion 409XIa and the outcourse portion 409XIb extend along the y-axis direction in FIG. The interval between is different. In the example shown in FIG. 4, the incourse portion 409XIa moves away from the outcourse portion 409XIb in the -y direction from the y direction, so the interval between the incourse portion 409XIa and the outcourse portion 409XIb is continuously widened. After passing through the peak, this interval narrows because the in course portion 409XIa approaches the out course portion 409XIb. In the folded portion Ra, the in-course portion 412XIa and the out-course portion 412XIb extend along the y-axis, the in-course portion 414XIa and the out-course portion 414XIb extend along the x-axis, and the in-course portion 411XIa and the out-course portion 411XIa extend along the x-axis. 411XIb extends along the y-axis. The extension direction of the incourse portion 413XIa, the outcourse portion 413XIb, the incourse portion 415XIa, and the outcourse portion 415XIb changes in the tangential direction of the partial circle with a central angle of 90 degrees. During this time, the in course portions 412XIa, 413XIa, 414XIa, 415XIa and 411XIa and the out course portions 412XIb, 413XIb, 414XIb, 415XIb and 411XIb are close to each other, and the intervals therebetween are constant.

This embodiment differs from a known polarization multiplexing IQ modulator in that the BCB layer in the region (protective layer non-formed region Rd) including the delay portion Rb and the folded portion Ra is eliminated. The formation of the protective layer non-formation region Rd and the effect of forming the protective layer non-formation region Rd will be described below.

As shown in FIG. 5, the IQ optical modulator 4 includes a semiconductor substrate 51, lower

clad layers

52 and 53 formed on the upper surface of the semiconductor substrate 51, a core layer 54 formed on the upper surface of the lower clad layer 53, a core layer It has upper clad

layers

55 and 56 formed on the upper surface of 54 . The oxide film 57 covers the side surfaces of the lower

clad layers

52 and 53 , the core layer 54 , the upper clad

layers

55 and 56 and the upper surface of the upper clad layer 56 . In the present embodiment, for example, the semiconductor substrate 51 is a SI (Semi-Insulating)-InP substrate, the lower clad layer 52 is an n-type InP lower clad layer, the lower clad layer 53 is a non-doped InP lower clad layer, and the core layer 54 is a non-doped multiple layer. A quantum well (MQW: Multi-quantum Well) core layer and upper clad

layers

55 and 56 are non-doped InP upper clad layers.

In this embodiment, both of the upper cladding layers 55 and 56 are non-doped InP upper cladding layers, as in the known configuration, but the RF modulated electric signal is Only the upper clad layer at the portion where the phase modulation is performed by the p-type InP upper clad layer. Such a structure is formed by laminating the lower

clad layers

52 and 53, the core layer 54, the upper clad layer 55, and the p-type InP upper clad layer on the semiconductor substrate 51 and patterning them by exposure and etching. In patterning, the p-type InP upper cladding layer is removed leaving only the areas where phase modulation is to be applied. The removed p-type InP upper cladding layer is replaced with the non-doped InP upper cladding layer 56 by regrowing the non-doped InP cladding layer in the removed portion.

In this embodiment, similarly to the known configuration shown in FIG. 2, after forming a BCB film on the upper surface of the upper clad layer, the BCB film is selectively removed from the protective layer non-formation region Rd. As a result, a protective layer non-formation region Rd is formed in which at least a portion of the protective layer is not formed. Here, as shown in FIG. 5, most of the BCB is removed, but the BCB layer 58 remains slightly on the bottom of both sides of the high mesa structure, that is, on the upper surface of the oxide film 57 covering the lower clad layer 52. . In the protective layer non-formation region Rd of the present embodiment, it is sufficient if the protective layer covering the upper clad

layers

55 and 56 and the core layer 54 is not formed (non-formation), and the oxidation that covers the lower clad layer 52 is sufficient. A case where a protective layer is formed on the upper surface of the film 57 is allowed. In this embodiment, the oxide film 57 constitutes a part of the protective layer together with the BCB layer 58, but it is sufficient that at least a part of the protective layer is not formed, and the oxide film 57 consists of the core layer 54 and the lower clad.

Layers

52, 53 and upper cladding layer 55 are allowed to be covered.

The present inventors consider the effect of stress due to BCB on MWQ, which is the core layer, as a problem, and the effect of stress relaxation is obtained when the difference ΔS between the top surface S1 of the BCB layer and the bottom surface S2 of the MQW is smaller than about 1 μm. I have found that it is sufficient. For this reason, in the present embodiment, the protective layer in the protective layer non-formation region should be thinner than the other portions such as the optical demultiplexer 407XI as long as the stress relaxation effect is exhibited. The removal of the BCB in the protective layer non-formation region Rd can be achieved by a known photolithography process and etching.

Next, the effect of removing the BCB in the protective layer non-formation region Rd will be described with reference to FIG. From the viewpoint of stabilizing the phase, the two arm waveguides of the MZM optical waveguide must have the same physical length (arm waveguide length). However, as described above, the optical modulator 4 needs to fold the MZM optical waveguide, and the folding causes a difference (ΔL) in the length of the arm waveguides. In order to solve this problem, the MZM optical waveguide of the optical modulator 4 is provided with a delay portion Rb, and the in-course portion 409XIa passing through the inside of the two MZM optical waveguides is curved in the -x direction in FIG. Therefore, the length of the in course portion 409XIa is increased. The folded portion Ra is configured by serially connecting a straight portion, a 90-degree bent portion, a straight portion, a 90-degree bent portion, and a straight portion for the purpose of changing the propagation direction of an optical signal by 180 degrees.

In the first embodiment, there is no other optical waveguide around the incourse portion 409XIa that curves away from the outcourse portion 409XIb, and the outcourse portion 409XQb of the MZM 401XQ exists relatively near the outcourse portion 409XIb. . Therefore, the in course portion 409XIa and the out course portion 409XIb receive different stresses from the BCB since the BCB layer covering the mesa structure has a different volume. That is, the problem is that the refractive index of the optical waveguide changes due to the stress change associated with the deterioration of the BCB layer. The stress from the BCB layer depends on the amount (volume) of the BCB layer. If the optical waveguides are close to each other, the volume of the BCB layer is small, and if the optical waveguides are far apart, the volume of the BCB layer is large.

Furthermore, when the BCB is formed on the upper layer of the in course portion 409XIa and the out course portion 409XIb, the stress change due to the deterioration of the BCB is also different between the in course portion 409XIa and the out course portion 409XIb. The change in stress causes a photoelastic effect, resulting in a difference in equivalent refraction change, the phase difference between the in course portion 409XIa and the out course portion 409XIb changes over time, and the optical path length obtained by dividing the phase difference by the wave number (2π/λ) The difference also changes with time.

FIG. 6 is a diagram showing changes in the phase difference generated in the optical modulator 4 of this embodiment in comparison with changes in the phase difference of the known IQ optical modulator 100 described above. The experiment whose results are shown in FIG. 6 was left for 2000 hours while the temperature of the chipped modulator was kept at 50 degrees. The horizontal axis of FIG. 6 indicates the standing time, and the vertical axis indicates the change in the phase difference. The plot ● in FIG. 6 indicates the result of the known IQ optical modulator 100, and the x indicates the result of the optical modulator 4 of this embodiment. According to FIG. 6, the phase difference between the MZ optical waveguides 109XIa and 109XIb in the IQ optical modulator 100 increases to 0.09 rad, 0.25 rad, 0.41 rad, and 0.58 rad according to the standing time. On the other hand, the optical modulator 4 of the present embodiment in which the BCB in the protective layer non-formation region Rd is removed has a phase difference of about 0.1 rad even after 2000 hours, and the phase difference is more stable than the known IQ optical modulator 100. It is clear that the performance is excellent. Such a point is the time-dependent change in the stress from the BCB applied to the pair of (two) optical waveguides arranged in the protective layer non-formation region Rd by removing the BCB in the protective layer non-formation region Rd. It is considered that this is because the difference in is reduced. In other words, in the present embodiment, the stress change caused by the BCB is reduced by removing the BCB layer in the protective layer non-formation region Rd. It is thought that the temporal variations of the equivalent refractive indices of the two arm waveguides became equal, and the change in the phase difference between the two arm waveguides was suppressed.

In addition, in the embodiment described above, since the high-frequency wiring and the like are patterned, the BCB is not removed from the entire area of the folded portion. For this reason, although it is not possible to completely eliminate the temporal change in the phase difference between the two arm waveguides, it is clear that a sufficient suppression effect can be obtained.

It should be noted that the present disclosure is not limited to the embodiments described above. For example, in the present embodiment, the delay portion and the folded portion are formed as an integrated protective layer non-formation region, but there may be a plurality of protective layer non-formed regions, and a wider range including the delay portion and the folded portion may be used. or a minimum area including the delay portion and the folding portion. The folded portion is not limited to including the straight portion and the bent portion as in the present embodiment, and may refer to only the bent portion. Furthermore, the protective layer non-formation region is not limited to being removed after the protective layer is formed on the entire surface of the semiconductor chip, and the protective layer may be formed in advance on the region excluding the protective layer non-formation region.

4 Optical modulator 51

Semiconductor substrates

52, 53 Lower clad layer 54 Core layers 55, 56 Upper clad layer 57 Oxide film 58 BCB layer 401X, 401XI, 401XQ, 401Y, 401YI, 401YQ MZM
402 Input

optical waveguides

403, 404X, 404Y, 407XI, 407XQ, 407YI, 407YQ Optical demultiplexers 405X, 408XI, 408XQ, 408YI, 408YQ Optical multiplexers 406XI, 406XQ, 406YI, 406YQ Phase modulators 409Ia, 409XIa, 4 09XQa, 409YIa , 409YQa, 411XIa, 411XQa, 411YIa, 411YQa, 412XIa, 413XIa, 414XIa, 415XIa in-course section 409Ib, 409XIb, 409XQb, 409YIb, 409YQb, 411XIb, 411XQb, 411YIb, 411YQb, 412XIb, 413XIb, 414XIb, 415XIb Out course section 410XIa , 410XIb, 410XQa, 410XQb, 410YIa, 410YIb, 410YQa,

410YQb phase modulators

416X, 416Y output optical waveguides 422 traveling wave electrodes

Claims

an optical waveguide formed on a semiconductor substrate and having an optical waveguide layer in which a lower clad layer, a core layer and an upper clad layer are laminated in this order;
a first optical demultiplexer for demultiplexing an optical signal propagating through the optical waveguide;
a protective layer covering the optical waveguide and the first optical demultiplexer;
The optical waveguide includes a first optical waveguide and a second optical waveguide that propagate an optical signal demultiplexed by the first optical demultiplexer, and the first optical waveguide and the second optical waveguide and a delay portion in which the optical path length of the second optical waveguide is longer than the optical path length of the first optical waveguide, and the interval is constant and the extension direction changes. and a folded portion,
a first optical modulation unit, wherein the protective layer in the range including the delay section and the folding section has a region thinner than the protective layer overlying the first optical demultiplexer; optical modulator.
2. The optical modulator according to claim 1, wherein the thin area of the protective layer is a protective layer non-formed area where the protective layer is not formed.
two said first optical modulation units;
a second optical demultiplexer that demultiplexes and inputs an optical signal propagating through one of the optical waveguides to each of the two first optical modulation units;
an optical multiplexer for multiplexing the optical signals output from the two first optical modulation units;
3. The optical modulator according to claim 1, comprising a second optical modulation unit including an output optical waveguide for outputting the optical signal multiplexed by said optical multiplexer.
two said second optical modulation units;
and a third optical demultiplexer for demultiplexing and inputting an optical signal before being demultiplexed by the second optical demultiplexer to each of the two second optical modulation units. Item 4. The optical modulator according to item 3.
The optical modulator according to any one of claims 1 to 4, wherein the protective layer contains Benzocyclobutene.