WO2024029011A1

WO2024029011A1 - Optical modulator

Info

Publication number: WO2024029011A1
Application number: PCT/JP2022/029859
Authority: WO
Inventors: 侑祐齋藤; 悠太上田; 光映石川
Original assignee: 日本電信電話株式会社
Priority date: 2022-08-03
Filing date: 2022-08-03
Publication date: 2024-02-08

Abstract

Provided is an optical modulator capable of suppressing spreading of light caused by a mirror that converts an optical path and of increasing connection efficiency with an external optical component. An optical modulator (10) comprising a laminate substrate having a lower clad layer (12), a core layer (13), and upper clad layers (14, 15) stacked in the stated order on a substrate (11) is provided with: an optical waveguide that includes a core (13) formed from the core layer (13) and that has waveguide end surfaces in grooves (41, 42) formed in the laminate substrate; mirrors (43a, 44a) that are formed on slopes facing the waveguide end surfaces and that convert optical paths in the vertical direction of the substrate; and insulation films (18a, 18b) that insulate ground electrodes (62a-62c) and signal electrodes (61a, 61b) provided near the optical waveguide, and fill gaps between the waveguide end surfaces and the mirrors (43a, 44a).

Description

light modulator

The present invention relates to an optical modulator, and more particularly to an optical modulator that allows optical coupling in a direction substantially perpendicular to the main surface of a substrate.

In recent years, with the advancement of information and communication technology, network traffic has increased rapidly, and optical communication systems that can meet this demand are required to have even higher speeds and lower power consumption. An optical modulator is a key device that determines the performance of the entire optical communication system, and its central optical circuit is fabricated using materials such as InP, Si, or lithium niobate (LN: LiNbO ₃ ). . Furthermore, the optical modulator described in Non-Patent Document 1 uses benzocyclobutene (BCB), an organic material with a low dielectric constant, as the insulating material of the signal electrode, and exhibits good frequency response characteristics. There is.

On the other hand, there are many problems with the configuration in which light guided through a conventional optical circuit including an optical modulator is connected to an external optical system. Optical circuits are generally formed on a flat surface near the surface of a substrate, and require connection to the outside at an end surface formed on a side surface of the substrate. Therefore, manufacturing processes such as cleaving, polishing, and anti-reflection coating of the end face of the optical waveguide, and mounting processes such as alignment of the spatial optical system are required, which has been an issue in reducing manufacturing costs.

In order to solve this problem, methods have been considered in which the light guided through the optical circuit is emitted onto the top surface of the substrate. For example, Patent Document 1 proposes a method of providing a mirror in an optical circuit. The light emitted from the waveguide end face of the optical circuit has its optical path converted in the vertical direction of the substrate, and is emitted into the free space above the substrate. Generally, an optical waveguide made of a semiconductor has a refractive index of 3.0 or more, which is about three times that of free space using air as a medium. Therefore, when light is emitted from the end face of the waveguide, the spot spreads. Conventionally, in order to keep the required dimensions of the mirror small, the distance between the end of the optical waveguide and the mirror has been designed to be small.

However, while a structure including a mirror can be easily manufactured, there is a problem in that it is subject to design constraints in order to suppress the spread of light. Another problem was that it was difficult to efficiently connect external optical components such as optical fibers due to the spread of light.

International Publication No. 2021/255862 International Publication No. 2020/246042

An object of the present invention is to provide an optical modulator that improves connection efficiency with external optical components and has excellent high frequency characteristics in a structure in which a mirror is provided in an optical circuit.

In order to achieve such an object, the present invention provides an optical modulator including a laminated substrate in which a lower cladding layer, a core layer, and an upper cladding layer are laminated in order on a substrate, in which the an optical waveguide including a core formed therein, the optical waveguide having a waveguide end face in a groove formed in the laminated substrate; It is characterized by comprising a converting mirror, and an insulating film that insulates a signal electrode and a ground electrode provided near the optical waveguide and fills a gap between the waveguide end face and the mirror.

FIG. 1 is a diagram showing the manufacturing process of an optical modulator according to a first embodiment of the present invention, FIG. 2 is a diagram showing the manufacturing process of the optical modulator of the first embodiment, FIG. 3 is a diagram showing the manufacturing process of the optical modulator of the first embodiment, FIG. 4 is a diagram showing the manufacturing process of the optical modulator of the first embodiment, FIG. 5 is a diagram showing the manufacturing process of the optical modulator of the first embodiment, FIG. 6 is a diagram showing the manufacturing process of the optical modulator of the first embodiment, FIG. 7 is a diagram showing the manufacturing process of the optical modulator of the first embodiment, FIG. 8 is a diagram showing the manufacturing process of the optical modulator of the first embodiment, FIG. 9 is a diagram showing the manufacturing process of the optical modulator of the second embodiment, FIG. 10 is a diagram showing the structure of an optical modulator according to the third embodiment.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this embodiment, an optical modulator using an InP-based semiconductor material will be explained as an example. However, if the waveguide can be configured using a refractive index difference using Si or LN, and the optical modulator can be manufactured by etching, Any material may be used.

[First embodiment]
FIGS. 1-8 show the manufacturing process of the optical modulator according to the first embodiment of the present invention. As shown in FIG. 1(a), the optical modulator 10 has n-type doped InP (n-InP) layered as a lower cladding layer 12 on the upper surface of a semi-insulating InP (SI-InP) substrate 11. . Further, the optical modulator 10 has a multi-quantum well (MQW) structure as a core layer 13 and a p-type doped InP (p-InP) as a first upper cladding layer 14 stacked in this order on the upper surface of the lower cladding layer 12. It is formed from a laminated substrate. The MQW structure is made of elements such as In, Ga, As, P, and Al, and the optical modulator 10 functions as an electroabsorption modulator.

As shown in FIG. 1(b), the first upper cladding layer 14 is removed by photolithography and etching, leaving the light modulation region 21. Furthermore, in the

mirror formation regions

22a and 22b, the core layer 13 is also removed by photolithography and etching. FIG. 1C shows the top surface of the substrate from which the first upper cladding layer 14 and the core layer 13 have been removed.

Next, as shown in FIG. 2A, the light modulation region 21 is masked with SiO ₂ or the like, and backfilled with SI-InP as the second upper cladding layer 15 using metal organic vapor phase epitaxy. . The growth thickness from the lower cladding layer 12 to the first upper cladding layer 14 and the second upper cladding layer 15 was set to 2.0 μm, but if it is possible to confine light in the optical waveguide, the growth thickness may be, for example, 0.5 μm or more. It is possible to design up to about 10 μm.

When the SiO ₂ mask is removed using hydrofluoric acid or the like, as shown in FIG. 2(b), the light modulation region 21 is composed of the first upper cladding layer 14 made of p-InP, and the regions other than the light modulation region are It is composed of a second upper cladding layer 15 made of SI-InP. Since SI-InP functions as an insulating material, it is possible to isolate electrodes to be formed later between the light modulation region 21 and other regions. In addition, optical absorption loss due to free carriers in other regions can be reduced.

Next, an optical waveguide that constitutes the optical modulator 10 is formed. The optical modulator 10 is a Mach-Zehnder interferometer type optical modulator, as shown in FIG. Two

arm waveguides

32 and 33 are connected to the duplexer 35b. Both optical waveguides have a high mesa structure formed by photolithography and etching. The width of the optical waveguide, that is, the width of the core of the high mesa structure was set to 2.0 μm in order to satisfy the single mode condition, but it is also possible to design the width to be 1.0 to 3.0 μm, for example. Furthermore, the configuration of the optical modulator 10 is not limited to the Mach-Zehnder interferometer type, but may also be configured without an interferometer.

FIG. 3B is a cross-sectional view taken along IIIb-IIIb' in FIG. 15c. FIG. 3(c) is a cross-sectional view taken along IIIc-IIIc' in FIG. 3(a), and the optical waveguides that become the

arm waveguides

32 and 33 are composed of the first

upper cladding layers

14a and 14b made of p-InP. be done. Although the high mesa height was set to 5.0 μm, any height may be used as long as light can be confined in the core of the optical waveguide. In addition to the high mesa structure, a ridge optical waveguide or the like can also be used.

Furthermore, in this step, a groove 41 is formed so that the waveguide end surface of the input waveguide 31 is exposed, and a groove 42 is formed so that the waveguide end surface of the output waveguide 34 is exposed, each with a depth that reaches the lower cladding layer 12. Form. The

grooves

41 and 42 are formed to expose the end face of the waveguide facing a mirror, which will be described later, and furthermore, the distance between the end face of the waveguide and the reflection surface of the mirror is defined by the width of the groove. Note that as the etching method, a general semiconductor process such as reactive ion etching (RIE) and etching using inductively coupled plasma (ICP) can be used.

Next, a portion of the laminated substrate is removed at a position facing the waveguide end surfaces of the

grooves

41 and 42 to form a slope that will become a reflective surface of the mirror. As shown in FIG. 4(a), by a so-called grayscale lithography process, the resist is coated on the surface of the substrate at the positions that will become the

slopes

43 and 44 so that the exposure intensity is varied and the resist becomes a concave surface. do. By performing the dry etching process, the shape of the resist is transferred to the laminated substrate, and as shown in FIG. 4(b),

slopes

43 and 44 on which concave mirrors are formed are formed. The curvature of the concave surface is designed so that the waveguide 31 and the output waveguide 34 are provided at a predetermined distance from the waveguide end faces, respectively, and condense the input light or make the output light collimated. ing. The distance from the waveguide end face to the

slopes

43 and 44 is desirably 5 μm to 30 μm or less in order to prevent vignetting on the mirror due to the spread of light, but this is not a limitation.

Note that when the slope of the mirror is a flat surface, the slope that will become the mirror can be formed by normal lithography and wet etching, without using the above-mentioned method. In that case, it is desirable that the distance from the waveguide end face to the slope be 3 μm to 15 μm or less.

FIG. 5A shows the top surface of a substrate on which a passivation layer (SiON) 17 is formed on the optical modulator 10. 5(b) is a sectional view taken along Vb-Vb' in FIG. 5(a), and FIG. 5(c) is a sectional view taken along Vc-Vc' in FIG. 5(a). Although the film thickness of the passivation layer 17 was set to 300 nm, it can be set to a range of approximately 100 nm to 1000 nm depending on the design of the optical waveguide, mirror, and the like. The passivation layer 17 is a protective film for protecting the surface of an optical circuit such as an optical waveguide formed as a Mach-Zehnder interferometer type optical modulator. The material of the passivation layer 17 may include various materials other than SiON, such as SiO ₂ and SiN, and may be a multilayer film made of a plurality of materials. Further, as a method of film formation, any method used in standard semiconductor processes, such as plasma CVD (Chemical Vapor Deposition) and spin-on glass, may be used.

FIG. 6(a) shows a state in which the SiON of the ground electrode forming portions 51-53 of the light modulation region of the optical modulator 10 is removed by opening. FIG. 6(b) is a sectional view taken along VIb-VIb' in FIG. 6(a). The SiON in the ground electrode forming portions 51-53 in the light modulation region is removed by photolithography and etching. FIG. 7A shows a state in which ground electrode contacts 51a-53a and mirror reflection surfaces 43a and 44b are formed. 7(b) is a sectional view taken along VIIb-VIIb' in FIG. 7(a), and FIG. 7(c) is a sectional view taken along VIIc-VIIc' in FIG. 7(a). Thereafter, metal is deposited by vacuum evaporation, and ground electrode contacts 51a-53a and mirror reflection surfaces 43a and 44b are simultaneously formed by lift-off. The metal material may be any material used in general semiconductor processes, such as Au, Pt, Ti, and Cr.

Next, as shown in FIG. 8(a), benzocyclobutene (BCB) 18 is formed as an insulating film on the surface of the substrate of the optical modulator 10. BCB 18 is formed by spin coating and curing. The thickness of the BCB 18 is set to approximately the height of the high mesa of the optical waveguide by adjusting the rotation speed of spin coating. Here, it was set to 5.0 μm. FIG. 8(b) is a sectional view taken along VIIIb-VIIIb' in FIG. 8(a), and FIG. 8(c) is a sectional view taken along VIIIc-VIIIc' in FIG. 8(a). Through this step, the periphery of the optical waveguide is protected by the

BCBs

18a and 18b, and as will be described later, it is possible to isolate the signal electrode and ground electrode provided near the optical waveguide. At the same time, the gaps between the waveguide end faces exposed in the

grooves

41 and 42 and the mirror reflection surfaces 43a and 44b are also filled with the

BCBs

18a and 18b.

The refractive index of BCB is approximately 1.5, and the difference in refractive index with the waveguide material is smaller than that of air, resulting in a smaller diffraction angle, so that light enters the end of the optical waveguide or is emitted from the end of the optical waveguide. The spread of light is suppressed. Therefore, there are advantages such as miniaturization of the mirror and relaxation of the curvature conditions necessary for condensing light onto the end of the optical waveguide. Although BCB is used as the insulating film, other materials commonly used as interlayer insulating films, such as SiO ₂ and polyimide, may be used. However, in pursuit of higher speed optical modulators, materials with lower dielectric constants are desirable.

After that, a signal electrode and a ground electrode for connecting to an external control circuit for making the optical modulator 10 function are formed. The signal electrode, which becomes the traveling wave electrode, is formed by photolithography and etching on the signal electrode forming portion 61 on the first upper cladding layers 14a and 14b, where the BCB and SiON on the optical waveguide in the optical modulation region are removed. . BCB and SiON can be removed by standard RIE or hydrofluoric acid soak processes. Next, patterns for signal electrodes and ground electrodes are formed by photolithography, then metal is deposited by vacuum evaporation, and

signal electrodes

61a, 61b and ground electrodes 62a-62c are formed by lift-off.

As the electrode material, the above-mentioned metal materials can be used, and the formation method is not limited to vacuum deposition, but a plating method or the like may also be used. Further, the electrode pattern is not limited to the illustrated pattern, but may be a capacitively loaded structure as shown in Non-Patent Document 1, for example.

[Second embodiment]
In the first embodiment, the slope for forming the mirror was created by etching the laminated substrate, but in the second embodiment, the slope is created using BCB. In the following, only the differences from the first embodiment will be described, and overlapping points will be omitted.

First, the grayscale lithography process shown in FIGS. 4(a) and 4(b) is not performed, and the positions of the

slopes

43 and 44 are located at the lower part, similar to the

grooves

41 and 42 shown in FIG. 3(b). It is removed to a depth that reaches the cladding layer 12. The formation of the passivation layer 17 shown in FIG. 5 and the formation of the ground electrode contacts 51a-53a shown in FIGS. 6 and 7 are the same as in the first embodiment.

Second, as shown in FIG. 9(b), benzocyclobutene (BCB) 18 is formed as an insulating film on the surface of the substrate of the optical modulator 10. At this time, by a so-called gray scale lithography process, a portion of the BCB 18 inside the

grooves

41 and 42 forms slopes 18a and 18d that become reflective surfaces of the mirrors. A resist is applied to the surface of the substrate, and the resist is applied to the surface of the substrate so that the exposure intensity is varied so that the resist has a concave surface. By performing dry etching, the shape of the resist is transferred to the BCB, forming slopes 18a and 18d that serve as concave mirrors.

The slopes 18a and 18d are provided at a predetermined distance from the end of the optical waveguide, and are used to condense light input to the end of the optical waveguide, or to collimate light output from the end of the optical waveguide. Curvature is designed. Thereafter, metal is deposited by vacuum evaporation, and mirror reflection surfaces 43a and 44a are simultaneously formed by lift-off.

Next, as shown in FIGS. 9(a) and 9(c), similarly to the first embodiment, after removing the BCB and SiON of the signal electrode forming portion 61 on the optical waveguide in the optical modulation region, After forming patterns for signal electrodes and ground electrodes by photolithography, metal is deposited by vacuum evaporation, and signal electrodes and ground electrodes 62 are formed by lift-off.

[Third embodiment]
In the optical modulator 10 of the first and second embodiments, a fiber block is manufactured to optically couple the light reflected by the mirror with the optical fiber. In the first and second embodiments, after forming the signal electrode 61 and the ground electrode 62, a film of photosensitive epoxy resin is formed on the surface of the substrate of the optical modulator 10 by spin coating. The thickness of the photosensitive epoxy resin is approximately 100 μm.

Next, a fiber block structure as shown in FIGS. 10(a) and 10(b) is produced by lithography and etching steps. The mirrors are installed on the

BCBs

18a, 18b in the vertical direction of the substrate with respect to the mirror reflection surfaces 43a, 44b so that the optical path of the light reflected by the mirrors and the optical axis of the optical fiber are aligned. FIG. 10 shows an example of square-shaped fiber blocks 71 and 72 that correspond to receptacles. By inserting a fiber block corresponding to a prismatic plug to which a fiber core wire is fixed into this hollow part, the optical waveguide of the optical modulator 10 and the optical fiber core wire can be connected via a mirror by passive alignment. can.

The connection with the optical fiber is not limited to the shape of this embodiment, and the fiber blocks 71 and 72 may have a cylindrical hollow portion and the optical fiber core wire may be directly inserted therein. Furthermore, it is also possible to combine optical components such as lenses, filters, and isolators with fiber blocks.

[Summary of embodiments]
According to this embodiment, in the optical modulator, the space from the end of the optical waveguide to the mirror is filled with a low dielectric constant material whose refractive index is higher than that of air, thereby suppressing the spread of light and preventing interference with external optical components. Connection efficiency can be increased. Further, the material filling the space from the end of the optical waveguide to the mirror can also be used as an insulating film for a signal electrode, and an optical modulator with excellent high frequency characteristics can be obtained. Furthermore, optical coupling is possible in a direction substantially perpendicular to the main surface of the substrate, and a simple connection method using passive alignment can improve mounting efficiency and simplify the mounting process.

Claims

In an optical modulator consisting of a laminated substrate in which a lower cladding layer, a core layer, and an upper cladding layer are laminated in order on a substrate,
an optical waveguide including a core formed from the core layer, the optical waveguide having a waveguide end face in a groove formed in the laminated substrate;
a mirror formed on a slope facing the end face of the waveguide and converting the optical path in a direction perpendicular to the substrate;
An optical modulator comprising: an insulating film that insulates a signal electrode and a ground electrode provided near the optical waveguide and fills a gap between the end face of the waveguide and the mirror.
The optical modulator according to claim 1, wherein the slope is formed as a flat or concave surface by removing a part of the laminated substrate, and the mirror has a reflective surface made of metal.
The optical modulator according to claim 1, wherein the slope is formed by forming a part of the insulating film as a flat or concave surface, and the mirror has a reflective surface made of metal.
The optical modulator according to claim 1, wherein the insulating film is made of an organic material with a low dielectric constant.
The optical modulator according to any one of claims 1 to 4, further comprising a fiber block installed in a direction perpendicular to the substrate with respect to the mirror.
The optical modulator according to any one of claims 1 to 4, wherein the core layer has an optical modulation region including a multi-quantum well structure, and constitutes an electroabsorption modulator.
The optical modulator according to claim 6, wherein the optical modulation region constitutes a Mach-Zehnder interferometer using the optical waveguide.
The optical modulator according to claim 6, wherein the upper cladding layer in a region other than the optical modulation region is made of an insulating material.