Optical device

Abstract

An optical device is provided with: an undercladding layer (102) formed on a substrate (101); a disk-type resonator (103) formed on the undercladding layer (102); and an optical waveguide (104) formed on the undercladding layer (102). In addition, the device is provided with a heat dissipation via (106) that is composed of a metal, and that penetrates the central part of the disk-type resonator (103), further penetrates the undercladding layer (102), and reaches the substrate (101).

Description

Optical Devices

[0001] The present invention relates to optical devices.

[0002] In recent years, optical frequency Kerr comb technology utilizing third-order nonlinear effects in micro-optical resonators has been advancing. In particular, compound semiconductor-on-insulator (CSOI) optical waveguides can utilize the high optical nonlinearity of compound semiconductors and can achieve strong optical confinement due to the large refractive index difference between compound semiconductors and SiO2, thereby enabling highly efficient generation of frequency comb light with extremely low pump light intensity.

[0003] Furthermore, by appropriately selecting the band gap of the compound semiconductor material, it is possible to suppress optical nonlinear absorption under high light intensity. Up to now, CSOI optical waveguides have been proposed using AlGaAs, InP, InGaP, GaP as III-V compound semiconductors, SiC or SiGe as IV compound semiconductor materials, and GaN or AlN as II-IV materials. In particular, CSOI optical waveguides using AlGaAs have been proposed in the past. 6 A ring optical resonator with a high Q factor exceeding 1 THz has been realized, and it has been reported that this high Q ring optical resonator can be used to generate frequency comb light with an FSR of 1 THz at an extremely low threshold power of 36 μW (Non-Patent Document 1).

[0004] Among frequency comb lights, the frequency comb light realized in Non-Patent Document 1 generates optical intensity periodically in the frequency (wavelength) domain, and coherence between the individual comb lights is not maintained. On the other hand, a state in which coherence between comb lights is maintained is called a soliton comb. This is achieved by creating a state in which soliton pulse light circulates within an optical resonator. Achieving a soliton comb state is important for applications such as spectroscopy and frequency standards that handle the phase information of each comb line.

[0005] In recent years, it has been shown that this soliton comb state can be achieved by continuously sweeping the pump light wavelength from the short wavelength side (negative pump light wavelength detuning) to the long wavelength side (positive wavelength detuning) of the resonant wavelength of the optical resonator (Non-Patent Document 2).

[0006] The relationship between the pump wavelength detuning and the generated comb intensity when generating a soliton comb state is shown by the solid line (a) in Figure 4. As the wavelength sweep proceeds from the negative wavelength detuning state to the long wavelength side, wavelengths begin to enter the cavity, and the optical intensity exceeds the threshold, wavelength conversion occurs due to four-wave mixing (FWM), a third-order nonlinear optical effect, and the comb begins to be generated.

[0007] Because the Kerr effect, which is also a third-order nonlinear optical effect, also shifts the resonant wavelength of the optical resonator toward longer wavelengths depending on the input light intensity (in the case of general nonlinear materials), in order to obtain an effective zero-detuning state, the wavelength sweep is continued toward longer wavelengths. As the zero-detuning is approached, stronger light intensity exists inside the optical resonator, and the power of the FWM wavelength-converted light also increases. As FWM occurs in a cascade manner, multiple comb lines are generated, and the overall comb light intensity increases.

[0008] On the other hand, when the wavelength detuning exceeds the resonant wavelength, the comb light intensity drops sharply and maintains a constant comb light intensity relative to the wavelength detuning, as shown in region (b) (soliton step). This state, known as the soliton comb state, can be achieved by appropriately setting the wavelength sweep speed, etc. [Non-Patent Document 2]. Further wavelength sweeping leads to a detuning point where the optical power supplied to the optical resonator decreases and soliton pulses can no longer be maintained, at which point the soliton comb state disappears and the comb light intensity becomes zero.

[0009] The solid line (a) is drawn taking into account the Kerr effect. However, in reality, there is also a resonant wavelength shift due to thermal effects (such as the thermo-optic effect, thermal resistance, and heat dissipation rate). The presence of thermal effects makes it even more difficult to generate a soliton comb state, as described in Non-Patent Document 2. That is, as wavelength sweeping continues and the effective zero detuning state is exceeded, the optical intensity in the optical resonator decreases, causing the optical resonator temperature to decrease, resulting in a blue shift of the resonant wavelength due to the thermo-optic effect. Therefore, the actual comb intensity reaches a soliton step through the process shown by the dashed line (c) in Figure 7. In other words, on the low-wavelength side of the pump wavelength detuning that forms the soliton step, an unrealizable soliton comb state, shown in region (d), exists.

[0010] Therefore, in order to realize the soliton comb state, it is important to obtain as large a realizable soliton step as possible, as shown in region (e), and the slope (-K eff ) as small as possible, i.e., K eff As described in Non-Patent Document 2, it is important to make K eff is expressed by the following formula:

[0011]

[0012] In formula (1), n g is the group refractive index, K c is the thermal conductance of the optical resonator system (W / K), κ a is the linear absorption loss rate (rad / s) inside the optical resonator, κ is the loss rate (rad / s) of the entire optical resonator, ∂n / ∂T is the effective thermo-optic coefficient (1 / K) of the optical waveguide that constitutes the optical resonator, and ω p is the angular momentum frequency of the pump light (rad / s), t R is the "round-trip time" (s) of the optical resonator. From equation (1), K eff To increase c Increase κ a / κ is reduced, ∂n / ∂T, which shows the relationship between the refractive index n and the temperature T, is reduced, t R This provides guidelines such as reducing the

[0013] L. Chang et al., "Ultra-efficient frequency comb generation in AlGaAs-on-insulator microresonators", Nature Communications, vol. 11, no. 1, Article number:1331, 2020.Q. LI et al., "Stably accessing octave-spanning microresonator frequency combs in the soliton regime", Optica, vol. 4, no. 2, pp. 193-203, 2017.Gregory Moille et al., "Dissipative Kerr Solitons in a III-V Microresonator", Laser & Photonics Reviews, vol. 14, issue 8, 2000022, 2020.

[0014] There have been many reports on soliton comb generation, and in the CSOI optical waveguide, ∂n / ∂T is about two orders of magnitude larger than that of the SiN optical waveguide used in Non-Patent Document 2. eff becomes small, so that the broken line in FIG. 7 does not intersect with the soliton step, making it extremely difficult to generate a soliton comb.

[0015] To solve this problem, Non-Patent Document 3 describes how an optical resonator made using an AlGaAs optical waveguide is cooled to an environment of 20 K or less in a refrigerator and pump light is injected into it, thereby reducing ∂n / ∂T by about two orders of magnitude compared to room temperature and realizing soliton comb generation. However, the need for equipment such as a refrigerator to obtain an extremely low temperature environment of 20 K or less poses a major problem.

[0016] Optical resonators using CSOI optical waveguides are expected to generate soliton combs with submilliwatt pump light intensities, and it is anticipated that a single-chip soliton comb light source will be realized and put to practical use using a semiconductor laser light source integrated on the same chip as the pump light source. However, laser driving is difficult in cryogenic environments. Even in such future developments, technology for generating soliton combs at room temperature, rather than operating in cryogenic environments, is required.

[0017] The present invention has been made to solve the above problems, and has as its object to enable soliton comb generation in a room temperature environment in an optical resonator using a CSOI optical waveguide.

[0018] The optical device according to the present invention comprises a cladding layer formed on a substrate, a disk-shaped resonator formed on the cladding layer and made of a compound semiconductor and having a circular shape in a plan view, an optical waveguide formed on the cladding layer and made of a core made of a compound semiconductor and arranged at a distance that allows optical coupling with the disk-shaped resonator, and a heat dissipation via made of metal that penetrates the center of the disk-shaped resonator and further penetrates the cladding layer to reach the substrate.

[0019] As described above, according to the present invention, a heat dissipation via made of metal is provided that penetrates the center of the disk-type resonator and further penetrates the undercladding layer to reach the substrate, so that soliton comb generation is possible in a room temperature environment in an optical resonator using a CSOI optical waveguide.

[0020] Fig. 1 is a cross-sectional view showing the configuration of an optical device according to an embodiment of the present invention. Fig. 2 is a cross-sectional view showing the configuration of another optical device according to an embodiment of the present invention. Fig. 3A is a characteristic diagram showing chromatic dispersion of an optical device according to an embodiment of the present invention. Fig. 3B is a distribution diagram showing a part of the propagation modes of an optical device according to an embodiment of the present invention. Fig. 4 is an explanatory diagram for explaining generation of a soliton comb state.

[0021] An optical device according to an embodiment of the present invention will be described below with reference to Fig. 1. This optical device includes an undercladding layer 102 formed on a substrate 101, a disk-shaped resonator 103 formed on the undercladding layer 102, and an optical waveguide 104 formed on the undercladding layer 102.

[0022] The substrate 101 is made of, for example, Si. The undercladding layer 102 can be made of, for example, silicon oxide. For example, the undercladding layer 102 can be formed by thermally oxidizing the surface of the substrate 101 made of Si. The undercladding layer 102 can have a thickness of, for example, 2 μm.

[0023] The disk-type resonator 103 is made of a compound semiconductor such as AlGaAs (Al composition 20%) and is circular in plan view. In this example, the disk-type resonator 103 is formed so that the central portion 103a is thinner than the annular peripheral portion 103b. For example, the disk-type resonator 103 is circular with a diameter of 20.3 μm in plan view. The peripheral portion 103b is 400 nm thick, and the central portion 103a is 100 nm thick. The central portion 103a is circular with a diameter of 19.7 μm in plan view. The disk-type resonator 103 is a so-called CSOI disk-type resonator.

[0024] The optical waveguide 104 is composed of a core 105 made of a compound semiconductor such as AlGaAs (Al composition 20%). The optical waveguide 104 is arranged at a distance that allows optical coupling with the disk-shaped resonator 103. The optical waveguide 104 is a so-called CSOI optical waveguide. An overclad layer 107 is formed on the underclad layer 102. The overclad layer 107 is formed to cover the core 105 and the disk portion of the disk-shaped resonator 103. The overclad layer 107 can be made of, for example, silicon oxide.

[0025] In addition to the above-described configuration, the optical device according to the embodiment includes a heat dissipation via 106 made of metal that penetrates the center of the disk-shaped resonator 103 (central portion 103a), further penetrates the undercladding layer 102, and reaches the substrate 101. The heat dissipation via 106 is, for example, cylindrical. In this example, the upper portion of the heat dissipation via 106 is formed to protrude from the upper surface of the disk-shaped resonator 103. The heat dissipation via 106 can be made of, for example, Au.

[0026] 2, a via cap 108 may be further provided. The via cap 108 is formed in contact with the upper end of the heat dissipation via 106 and in contact with the surface of the disk-type resonator 103 around the heat dissipation via 106. The via cap 108 is made of a metal such as Au. The via cap 108 may be circular in plan view, for example.

[0027] As described above, by using a CSOI disk-type resonator and further providing a structure (heat dissipation via) at the center of the disk-type resonator to enhance heat dissipation, K eff As mentioned above, in order to realize the soliton comb state, K eff It is important to make K as large as possible. c By increasing eff can be made larger.

[0028] In the embodiment, first, by using a disk-type resonator, the peripheral part of the disk-type resonator, which is responsible for optical confinement within the resonator and at the same time serves as a heat source, and the central part of the disk-type resonator, which is responsible for heat dissipation, are connected (integrally formed) with a compound semiconductor material having high thermal conductivity. Furthermore, a structure (heat dissipation via) that enhances heat dissipation provided in the central part of the disk-type resonator makes it possible to efficiently dissipate heat generated in the peripheral part of the disk-type resonator. As a result, K c By increasing the , it is possible to generate soliton combs at room temperature.

[0029] In the disk-type resonator 103, the thickness of the central portion 103a is made thinner than that of the peripheral portion 103b in order to control the wavelength dispersion of the optical propagation mode within the disk. By appropriately designing the difference in thickness between the central portion 103a and the peripheral portion 103b, and the difference in diameter in a plan view of the thin central portion 103a and the diameter of the disk-type resonator 103 itself, it is possible to obtain the desired dispersion while also ensuring thermal conductivity between the central portion 103a and the peripheral portion 103b.

[0030] According to the embodiment, it is possible to form a heat conduction path in which heat generated in the disk-type resonator 103 is efficiently dissipated to the substrate 101 having high thermal conductivity via the heat dissipation vias 106. At the same time, it is possible to obtain an excellent effect of appropriately designing the optical propagation mode and wavelength dispersion of the disk-type resonator 103 while suppressing the optical influence of the heat dissipation vias 106 at the center of the disk-type resonator 103.

[0031] A method for manufacturing an optical device according to an embodiment of the present invention will now be described. A sacrificial layer is formed on a GaAs substrate, and an AlGaAs layer is formed on the sacrificial layer to form a growth substrate by crystal growth using a general MOCVD apparatus. A Si substrate is also prepared, and the surface of this Si substrate is thermally oxidized to form a silicon oxide layer on the surface of the Si substrate.

[0032] The AlGaAs layer of the growth substrate and the silicon oxide layer of the Si substrate are bonded together using a common surface hydrophilic bonding technique. Next, the GaAs substrate and sacrificial layer of the growth substrate are removed by wet etching. This process results in a CSOI substrate in which an AlGaAs layer is formed on the Si substrate via a cladding layer made of silicon oxide.

[0033] Next, SiO2 is deposited on the AlGaAs layer using a typical plasma CVD or sputtering method to form a hard mask layer. Next, a first resist pattern is formed on the hard mask layer using electron beam lithography or ultraviolet photolithography. The first resist pattern is a pattern for forming the overall shape of the disk-shaped resonator and the core. Next, using the first resist pattern as a mask, the hard mask layer is dry-etched until the AlGaAs layer is exposed, forming a hard mask pattern using the hard mask layer. Next, using the hard mask pattern as a mask, the AlGaAs layer is dug to a depth of approximately 400 nm using a dry etching technique to form the disk portion and core of the disk-shaped resonator.

[0034] Next, a second resist pattern with an opening in the area where the thickness of the central portion of the disk-shaped resonator is to be thinned is formed on the hard mask pattern using electron beam lithography or ultraviolet photolithography. Next, the hard mask pattern is dry-etched using the second resist pattern as a mask to expose the disk portion (AlGaAs layer) in the area to be thinned. Next, the center of the disk portion is thinned by 300 nm using dry etching with the hard mask pattern as a mask. This results in a disk portion whose center is thinner than the annular periphery.

[0035] Next, the heat dissipation vias are fabricated by the following process. First, a third resist pattern having openings at the locations where the heat dissipation vias will be formed in the center of the disk-shaped resonator is formed by electron beam lithography or ultraviolet photolithography. The third resist pattern is first formed to a thickness sufficient to form deep through-holes for providing the heat dissipation vias that penetrate the thin central portion of the disk portion and the cladding layer. Furthermore, the third resist pattern is formed to the above-mentioned thickness plus a thickness that includes a portion that serves as a lift-off mask for forming a seed layer for forming the heat dissipation vias by plating.

[0036] After forming the third resist pattern, the disk portion (AlGaAs layer) and the 2 μm thick cladding layer are etched using the third resist pattern as a mask to form a through hole that penetrates through them and reaches the substrate. During this etching, the substrate can be over-etched to some extent to ensure reliable thermal conduction between the substrate and the heat dissipation vias.

[0037] After the through hole is formed by the above process, the remaining portion of the third resist pattern is used as a lift-off mask, and Au is deposited on top of this by sputtering or vapor deposition to form a seed layer on the bottom surface of the formed through hole (upper surface of the substrate). The lift-off mask is then removed to remove the seed layer in areas other than the bottom surface of the through hole. After this, Au is plated and grown inside the through hole by electroplating, forming a heat dissipation via inside the through hole. Note that to ensure better thermal contact between the formed heat dissipation via and the disk portion, a via cap made of Au can be formed by lift-off using a lift-off mask formed by known lithography techniques. Finally, an overcladding layer is formed by depositing silicon oxide using a conventional plasma CVD method.

[0038] The materials and structures described above are merely examples, and the materials and structures can be appropriately designed based on the concepts described in the Summary of the Invention, taking into consideration the realization of anomalous dispersion, the optical confinement factor, the comb generation threshold, etc.

[0039] Next, the characteristics of the optical device according to the embodiment (chromatic dispersion of the resonator) will be described. Figure 3A shows the chromatic dispersion of the disk-shaped resonator 103. Here, the disk-shaped resonator 103 has a diameter of 20.3 mm in plan view, and the thickness of the peripheral portion 103b is 400 nm. The diameter of the thinner central portion 103a in plan view is 19.7 mm. The chromatic dispersion was calculated by varying the thickness of the central portion 103a to 100 nm, 150 nm, and 200 nm. In Figure 3A, (a) is 100 nm, (b) is 150 nm, and (c) is 200 nm. As shown in Figure 3A, it can be seen that chromatic dispersion can also be controlled by controlling the thickness of the central portion 103a. Controlling the thickness of the central portion 103a makes it possible to achieve the anomalous dispersion state commonly used for soliton comb generation. Figure 3B shows the intracavity propagation mode when the thickness of the central portion 103a is 100 nm.

[0040] As described above, according to the present invention, a heat dissipation via made of metal is provided that penetrates the center of the disk-type resonator and then penetrates the undercladding layer to reach the substrate, thereby enabling soliton comb generation in a room temperature environment in an optical resonator using a CSOI optical waveguide.

[0041] Some or all of the above-described embodiments may also be described as, but are not limited to, the following supplementary notes.

[0042] [Supplementary Note 1] An optical device comprising: a cladding layer formed on a substrate; a disk-shaped resonator formed on the cladding layer and made of a compound semiconductor and having a circular shape in a plan view; an optical waveguide formed on the cladding layer and made of a core made of a compound semiconductor and arranged at an interval allowing optical coupling with the disk-shaped resonator; and a heat dissipation via made of metal that penetrates a center of the disk-shaped resonator and further penetrates the cladding layer to reach the substrate.

[0043] [Supplementary Note 2] In the optical device according to Supplementary Note 1, the disk-shaped resonator is formed so that the central portion is thinner than the peripheral portion.

[0044] [Supplementary Note 3] The optical device according to Supplementary Note 1 or 2, further comprising a via cap made of metal in contact with an upper end of the heat dissipation via and formed in contact with the surface of the disk-shaped resonator around the heat dissipation via.

[0045] [Supplementary Note 4] The optical device according to Supplementary Note 3, wherein the via cap is made of Au.

[0046] [Supplementary Note 5] In the optical device according to any one of Supplementary Notes 1 to 4, the heat dissipation vias are made of Au.

[0047] It should be noted that the present invention is not limited to the embodiments described above, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0048] 101...substrate, 102...undercladding layer, 103...disk-shaped resonator, 103a...center portion, 103b...periphery portion, 104...optical waveguide, 105...core, 106...heat dissipation via, 107...overcladding layer, 108...via cap.

Claims

1. An optical device comprising: a cladding layer formed on a substrate; a disk-shaped resonator formed on the cladding layer and made of a compound semiconductor and having a circular shape in a plan view; an optical waveguide formed on the cladding layer and made of a core made of a compound semiconductor and arranged at a distance that allows optical coupling with the disk-shaped resonator; and a heat dissipation via made of metal that penetrates the center of the disk-shaped resonator and further penetrates the cladding layer to reach the substrate.

2. An optical device according to claim 1, wherein the disk-type resonator is formed so that the central portion is thinner than the peripheral portion.

3. An optical device according to claim 1 or 2, further comprising a via cap made of metal in contact with the upper end of said heat dissipation via and formed in contact with the surface of said disk-type resonator around said heat dissipation via.

4. An optical device according to claim 3, wherein the via cap is made of Au.

5. An optical device according to claim 1 or 2, wherein the heat dissipation vias are made of Au.

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