Micro optical resonator, and light frequency comb generation device

Abstract

To solve the problem that, in starting a light frequency comb, the change in resonance wavelength and the refractive index due to thermal deformation of a resonator results in instability, and the serious problem in terms of size reduction and cost because the function of sweeping the excitation light to a soliton region of the resonator requires to change the wavelength of the excitation light.SOLUTION: A plurality of first to n-th ring waveguides with diameters slightly different from each other are provided with respect to a linear waveguide. In each ring waveguide, a thermal deformation actuator is provided, and the thermal deformation actuator heats and cools sequentially with an actuator heater to start a soliton light comb. In the first to n-th ring waveguides, the resonance wavelengths are different from each other slightly. With this structure, it is only necessary that any one of the plurality of ring waveguides matches the wavelength of the excitation light with the soliton region of the ring waveguide; thus, the soliton light comb can be generated more certainly.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an apparatus and method for controlling the frequency of an optical frequency comb, and more particularly to an apparatus and method for controlling the frequency of an optical frequency comb that reliably starts a soliton optical comb and continuously generates and maintains a soliton optical comb. [Background technology]

[0002] An optical frequency comb is an optical signal with a comb-shaped spectrum consisting of components (modes) arranged at equal intervals on the frequency axis.

[0003] To activate an optical frequency comb, it is not enough to simply create an anomalous dispersion state within the cavity; a special procedure is required. In 2014, a method was discovered in which the wavelength of a continuous-wave (CW) laser light source introduced into a circular optical waveguide is continuously swept around its resonant frequency, and many techniques based on this method have been researched.

[0004] However, activation using the above method is extremely complicated because it is necessary to consider two factors in the resonant frequency shift in the circular optical waveguide: the optical shift due to the Kerr nonlinear effect, and the thermal shift due to the change in the refractive index of the material itself due to temperature rise and thermal expansion.

[0005] A conventional optical frequency comb generator will now be described. This micro-optical resonator is shown in Figure 15. Figure 15(a) is a plan view of the waveguide, and Figure 15(b) shows a cross-sectional view along the x-axis. 30a is a first cladding layer made of silicon dioxide (SiO2) deposited on a silicon substrate 29. A portion of this first cladding layer 30a is etched away to embed a resonator 32, which is a circular optical resonator, and a waveguide 31, on top of which a second cladding layer 30b is formed. The resonator 32 is ring-shaped, and the waveguide 31 is linear. The resonator 32 and the waveguide 31 are arranged with a small gap d between them. The z-direction side of the gap d is formed by a surface processed by dry etching. The resonator 32 and the waveguide 31 are made of silicon nitride.

[0006] The operation of the conventional optical frequency comb generator configured as above will now be described.

[0007] An input unit 33 is connected to the waveguide 31, which receives input of excitation light irradiated from a continuous wave (CW) laser light source device not shown, and excitation light 34, which is CW laser light, is incident on the waveguide 31 from the CW laser light source device.

[0008] The incident pumping light 34 is a continuous wave having a wavelength λ1. Figure 16(a) shows the wavelength spectrum of the incident pumping light 34. When the resonant wavelength λ1 of the pumping light 34 overlaps with the resonant wavelength of the resonator 32, the pumping light 34 leaks out of the waveguide 31 and is taken into the resonator 32 as a so-called evanescent wave.

[0009] When the modes with different wavelengths inside the resonator are aligned, their peaks overlap and constructively interfere with each other, generating a pulse 37. This state is called mode locking. This pulse is also called an optical comb. Figure 17 shows the concept of mode locking.

[0010] The mode spacing is expressed as the repetition frequency frep, frep =mc / (2nL) m=1, 2, 3,... where n is the effective refractive index of the waveguide material, L is the perimeter of the resonator, and c is the speed of light. When viewed on the frequency axis, the optical comb has a spectrum in which modes are arranged at equal intervals like a comb, as shown in Figure 16(b).

[0011] Here, wavelength sweeping of a micro optical resonator will be explained with reference to FIG. The optical comb is generated by sweeping the wavelength of the pump light 34 from the short wavelength side to the long wavelength side with respect to the resonant wavelength λ2 of the resonant waveform 36 of the micro-optical resonator.

[0012] Here, the distance between the wavelength λ1 of the pump light 34 and the resonant wavelength λ2 of the micro optical comb is defined as the detuning amount. The position of the resonant wavelength λ2 is called zero detune. The wavelengths shorter than zero detune are called blue detune, and the wavelengths longer than zero detune are called red detune. As the detuning amount is reduced from the blue detune side, the pump light 34 resonates as an evanescent wave from the waveguide 31, and the coupling power is guided into the resonator 32. This gradually increases the optical power within the resonator 32. When the coupling power of the pump light 34 exceeds the threshold of the nonlinear optical effect, a comb mode is generated by four-wave mixing (FWM). What is generated at this time is a chaotic optical comb.

[0013] Comb-like spectra generated using a micro-optical resonator can be broadly divided into chaotic optical combs and soliton optical combs. An optical comb that is not mode-locked is called a chaotic optical comb. On the other hand, an optical comb that is mode-locked and is the most stable in terms of phase, frequency, pulse waveform, etc. is called a soliton optical comb. To generate a soliton optical comb, it is necessary to capture the soliton region 38, where the wavelength λ1 of the pump light 34 transitions to red detune, exceeding the zero-tune wavelength λ2.

[0014] Figure 19 shows the process by which pump light 34 reaches the red-detuned soliton region. In Figure 19(1), the pump light 34 shifts its wavelength λ1 toward the longer wavelength side. In Figure 19(2), when the pump light 34 is applied to the resonant waveform, optical energy gradually flows into the resonator 32. This optical energy causes the resonator 32 to generate heat. This heat causes the ring diameter of the resonator 32 to thermally expand, shifting the resonant wavelength λ2 toward the longer wavelength side. 35 represents the change in the resonant wavelength of the resonator due to heat.

[0015] Furthermore, the refractive index also increases. Furthermore, if the optical intensity inside the ring becomes high, the Kerr effect will appear. To avoid this, it is necessary to pass through the resonance peak at a high speed. After that, as shown in Figure 19(3), the pump light 34 passes the resonance peak and reaches the soliton region in the red tune, where it stops. In this way, the pump light 34 must pass through the peak of the resonance waveform 36, inject high optical intensity into the resonator 32, and then reach the soliton region and stop. These operations result in a soliton optical comb. [Prior art documents] [Patent documents]

[0016] [Patent Document 1] U.S. Patent No. 10,268,100 [Non-patent literature]

[0017] [Non-Patent Document 1] Jacob S.Levy,et al.,Nature Photonics,4,pp.37-40(2010) [Non-patent document 2] T. Herr,et al.,Nature Photonics,8,pp.145-152(2014) Summary of the Invention [Problem to be solved by the invention]

[0018] In the above-described conventional configuration, when generating an optical frequency comb, the wavelength of the pump light must be shifted from a wavelength smaller than the resonant wavelength of the resonator to a wavelength larger than the resonant wavelength of the resonator, and an optical comb mode must be generated by four-wave mixing.

[0019] However, as the wavelength of the pump light approaches that of the resonator, thermal expansion due to optical energy lengthens the resonator's resonant wavelength, shifting the region where the optical comb solitonizes toward longer wavelengths. Furthermore, the refractive index increases due to heat, shortening the wavelength of the pump light. To compensate for this shortfall, it becomes necessary to increase the wavelength being swept. Originally, the region 38 where the optical comb solitonizes is itself a very narrow region, and the target region is displaced by heat, making it extremely difficult to accurately position the wavelength λ1 of the pump light 34 within the region where solitonization occurs.

[0020] Furthermore, devices for shifting the wavelength of pump light are complex and expensive. The present invention aims to solve the above-mentioned problems of the conventional technology by providing a micro-optical resonator that utilizes the change in the resonant wavelength of the resonator due to heat and reliably converts an optical comb into a soliton with a simple configuration. [Means for solving the problem]

[0021] To achieve this objective, the optical frequency comb generator of the present invention fixes the excitation light to a constant wavelength without sweeping its wavelength. Furthermore, the micro-optical resonator of the present invention has a plurality of first to nth ring waveguides with slightly different diameters relative to a linear waveguide. First to nth thermally deformable actuators are provided within the plurality of first to nth ring waveguides, respectively, and the first to nth thermally deformable actuators are heated sequentially by first to nth actuator heaters. Next, the first to nth thermally deformable actuators are independently cooled at predetermined timings. This reduces the ring diameters of the first to nth ring waveguides, shifting their resonant wavelengths toward shorter wavelengths. The first to nth ring waveguides each have a slightly different resonant wavelength. Therefore, although it is difficult to tune the wavelength of the pump light to the narrow soliton region of a specific ring waveguide, it is sufficient to tune the wavelength of the pump light to the soliton region of any one of the multiple ring waveguides (1st to nth). In other words, by providing multiple ring waveguides, the probability of solitonization can be increased. [Effects of the Invention]

[0022] As described above, the present invention enables the launch of a soliton optical comb by fixing the wavelength of the pump light 23 during the launch process of a micro optical comb. This eliminates the need to control the pump light and sweep the wavelength during the launch process. A laser light source control device for wavelength sweeping is not required, allowing for significant cost reductions and device miniaturization. Furthermore, multiple ring waveguides are provided to position the wavelength of the pump light within an extremely narrow soliton region. This ensures reliable launch of a soliton optical comb. Furthermore, robustness against changes in the external environmental temperature is improved, enabling stable continuous generation of a soliton optical comb. [Brief explanation of the drawings]

[0023] [Figure 1] 1 is a plan view of a micro optical resonator according to a first embodiment of the present invention. [Figure 2] FIG. 3 is a diagram showing wavelengths of a plurality of ring waveguides according to the first embodiment of the present invention. [Figure 3] 1 is a plan view showing the configuration of a straight waveguide and a ring waveguide according to a first embodiment of the present invention. [Figure 4] 1 is a cross-sectional view of a micro optical resonator according to a first embodiment of the present invention. [Figure 5] FIG. 2 is a plan view of a first ring waveguide heater of the micro optical resonator according to the first embodiment of the present invention. [Figure 6] 4 is an operation diagram of the thermal deformation actuator of the micro optical resonator according to the first embodiment of the present invention. FIG. [Figure 7] 7(a) is a timing chart illustrating the operation of the micro optical resonator according to the first embodiment of the present invention, and (b) is a graph showing the change over time in the resonant wavelength of each ring waveguide corresponding to FIG. [Figure 8] 10A and 10B are diagrams showing the movement process of a resonance waveform with a fixed excitation light for explaining the operation of the micro optical resonator according to the first embodiment of the present invention. [Figure 9] 10A and 10B are diagrams showing the movement process of a plurality of resonance waveforms with fixed excitation light for explaining the operation of the micro optical resonator in the first embodiment of the present invention. [Figure 10] 3 is a schematic plan view illustrating the state of a micro optical comb generated by the micro optical resonator according to the first embodiment of the present invention. FIG. [Figure 11] FIG. 3 is a block diagram illustrating a process for controlling the substrate temperature of the micro optical resonator according to the first embodiment of the present invention. [Figure 12] 4 is a graph showing changes in the resonant wavelength of each ring waveguide caused by a substrate temperature setting unit of the micro optical comb according to the first embodiment of the present invention. [Figure 13] 3A to 3C are diagrams illustrating a manufacturing process of the micro optical comb according to the first embodiment of the present invention. [Figure 14] 3 shows material characteristics of the micro optical resonator according to the first embodiment of the present invention. [Figure 15] 1A and 1B are plan and cross-sectional views of a conventional micro-optical resonator. [Figure 16] FIG. 1 is an explanatory diagram illustrating the concept of mode locking of a micro optical resonator. [Figure 17] This is an illustration of the comb-shaped spectrum generated by a micro optical comb. [Figure 18] 1 is an explanatory diagram showing the relationship between pump light and the resonant waveform of a resonator when a soliton optical comb is generated using a micro optical comb. FIG. [Figure 19] 1A and 1B are diagrams illustrating the sweep process of excitation light for explaining the operation of a conventional micro optical resonator. DETAILED DESCRIPTION OF THE INVENTION

[0024] The invention described in claim 1 of the present invention comprises an input section that receives an input of excitation light having a predetermined wavelength irradiated from a laser light source device, first to nth ring waveguides that are made of a material having a third-order nonlinear optical effect and have slightly different diameters, and the excitation light from the input section is input and is arranged at a position where it is optically coupled to the first to nth ring waveguides, and at least a part of the excitation light is input as evanescent light of the excitation light into a specific ring waveguide among the first to nth ring waveguides, and a plurality of wavelengths having equal frequency intervals are output from the first waveguide. The optical frequency comb includes a straight waveguide that extracts an optical frequency comb having a comb-like spectrum, a cladding that surrounds the straight waveguide and the plurality of ring waveguides and has a refractive index lower than that of the first waveguide and the second waveguide, first to n-th thermal deformation actuators that are formed inside the first to n-th ring waveguides, respectively, and a substrate that cools the thermal deformation actuators, wherein the first to n-th thermal deformation actuators increase the diameters of the first to n-th waveguides by heating, thereby shifting the resonant wavelengths of the first to n-th ring waveguides to longer wavelengths. Thereafter, the heaters of the first to n-th actuators are stopped and the first to n-th thermal deformation actuators are cooled by the substrate, thereby shifting the resonant wavelengths of the first to n-th ring waveguides to shorter wavelengths.

[0025] This configuration eliminates the need for the conventional wavelength sweeping operation of continuously changing the wavelength of the pump light when starting a soliton optical comb. Furthermore, the presence of multiple ring waveguides increases the probability of matching the pump light with the soliton region. This makes it possible to more reliably perform the soliton optical comb startup process and eliminates the need for the conventional system of sweeping the pump light.

[0026] In the invention described in claim 2, the diameter of each ring waveguide is set to be different so that the resonant wavelengths of the first to nth ring waveguides are successively increased by a fixed amount from the resonant wavelength of the first ring waveguide closest to the input section. With this configuration, a soliton optical comb can be systematically and efficiently initiated from the ring waveguide with the smallest resonant wavelength.

[0027] In the invention described in claim 3, the increment of the resonant wavelength of each of the first to nth ring waveguides is set to be equal to the half-width of the resonant waveform. With this configuration, it is possible to start up all soliton optical combs.

[0028] The invention described in claim 4 of the present invention includes first to n-th actuator heaters that heat the first to n-th thermal deformation actuators, first to n-th actuator temperature sensors that detect the temperatures of the first to n-th thermal deformation actuators, an actuator temperature control device that controls the temperatures of the first to n-th actuator heaters, a substrate heater that heats the reference temperature of the substrate to a predetermined temperature, a substrate temperature sensor that detects the substrate temperature, and a substrate temperature control device that controls the reference temperature of the substrate to a value higher than the ambient temperature. This makes it possible to achieve optimal soliton optical comb activation and stable generation and maintenance of a soliton optical comb that is not affected by changes in the ambient temperature.

[0029] In the fifth aspect of the present invention, the first to nth thermal deformation actuators are in contact with the first to nth actuator heaters. This improves the response speed of the first to nth thermal actuators, thereby enabling the soliton optical comb startup process to be carried out more reliably.

[0030] In the sixth aspect of the present invention, the first to nth thermal deformation actuators are in contact with the substrate. This allows the temperature of the thermal deformation actuators to be cooled quickly, improving the response speed of the thermal deformation actuators. This allows the soliton optical comb startup process to be carried out more reliably.

[0031] The seventh aspect of the present invention includes a soliton optical comb detector that detects the soliton optical comb output from the linear waveguide, a substrate temperature regulator that adjusts the substrate temperature, and a soliton optical comb startup controller that controls the startup of the soliton optical comb, wherein the soliton optical comb detector detects whether a soliton optical comb was generated during the initial soliton optical comb startup operation, and if not, raises the set temperature of the substrate by a predetermined amount and commands another soliton optical comb startup operation. Furthermore, by repeating this loop until a soliton optical comb is generated, the startup of the soliton optical comb is further ensured.

[0032] Hereinafter, an embodiment of the present invention will be described with reference to FIGS. (Embodiment 1) FIG. 1 is a plan view of a micro-optical resonator 10 of the present invention. The micro-optical resonator 10 is provided with a straight waveguide 11 and multiple ring waveguides 12. The straight waveguide and multiple ring waveguides are surrounded by a cladding 16 whose refractive index is lower than that of the material of the waveguides.

[0033] The multiple ring waveguides 12, consisting of a first ring waveguide 12-1 to an n-th ring waveguide 12-n, are provided on the same plane with a small gap d between them and the straight waveguide 11. An input unit 20 is provided at one end of the straight waveguide 11 to receive pumping light 23, which is a CW laser beam from a laser light source. An output unit 21 is provided at the other end of the straight waveguide 11 to output the generated soliton optical comb 22.

[0034] The radii r of the multiple ring waveguides are arranged in a direction in which the radii increase in order from the ring waveguide 12-1 closest to the input unit 20. The repetition frequency of the mode period of the ring waveguide 12 is given by fref=mc / (2·n·L) (m=1, 2, 3,...) L=2πr where n is the effective refractive index of the waveguide, r is the radius of the ring, L is the perimeter of the resonator, and c is the speed of light. The resonant wavelength is λ=2πr·n / m (m=1, 2, 3,...) Therefore, the resonant wavelength λ changes in proportion to the radius r.

[0035] 2 shows the resonant waveforms 24-1, 24-2, 24-3, ..., 24-n of the multiple ring waveguides 12. The horizontal axis represents wavelength. Starting with the ring waveguide 12-1 closest to the input section 20, the waveforms are arranged so that they increase by the half-width of the resonant waveform 24-1.

[0036] 3 is an enlarged view of the third ring waveguide 12-3 of the micro-optical resonator 10. Inside each ring waveguide, a cylindrical thermal deformation actuator 13 is placed at a distance g. The gap with width g is filled with a cladding portion 16x, and width g is set as small as possible without affecting the light inside the waveguide.

[0037] Figure 4 is a cross-sectional view of the micro optical resonator 10 of the present invention, showing the x-axis cross section in Figure 3. An actuator heater 14 and an actuator temperature sensor 19 that detects the temperature of the actuator are provided on the top surface 13b of the cylindrical thermally deformable actuator 13. This actuator temperature sensor 19 is a thermocouple and uses the temperature of the substrate 15 as its reference. The thermally deformable actuator 13 is also provided on the bottom surface 13a of the substrate 15, which is made of silicon and has excellent thermal conductivity.

[0038] Figure 5 shows a plan view of the actuator heater 14 and electrode pads 14a and 14b. The actuator heater 14 is made of a thin platinum film and has a curved shape that increases electrical resistance, and electrode pads 14a and 14b are partially exposed so that they can be wired externally. Actuator temperature sensor 19 is provided with pads 14a and 14b, and is partially exposed so that it can be wired externally. In this embodiment, the straight waveguide 11 and the ring waveguide 12 are made of silicon nitride (Si3N4). Silicon oxide (SiO2) is used as the cladding material. The material characteristics are shown in Figure 14. The refractive index and diameter of the waveguide change sensitively with temperature changes, making temperature control necessary. Therefore, a substrate heater 17 and a substrate temperature sensor 18 for detecting the temperature of the substrate 15 are provided on the underside of the substrate 15.

[0039] The soliton optical comb startup operation of the micro optical resonator configured as above will now be described. First, the function of adjusting the resonant wavelength of the ring waveguide using the thermally deformable actuator 13, a feature of the present application, will be described with reference to FIG. 6. In FIG. 6(a), the thermally deformable actuator 13 is heated by the actuator heater 14 and displaced in the direction (arrow direction) in which the radius of its cylindrical surface increases. This thermal deformation mechanically deforms the inner surface of the ring waveguide 12 in the circumferential direction, increasing the circumferential length L of the resonator. On the other hand, when the heating by the actuator heater 14 is stopped and the thermally deformable actuator 13 is cooled, the thermally deformable actuator contracts, thereby decreasing the circumferential length L. In this way, heating or cooling the thermally deformable actuator 13 changes the circumferential length of the first waveguide 12 and adjusts its resonant wavelength.

[0040] FIG. 7(a) is a timing chart showing the activation operation of multiple first to nth ring waveguides when activating a frequency soliton optical comb. FIG. 11 is a diagram showing a system for activating a frequency soliton optical comb. When activation of the soliton optical comb begins, the soliton optical comb activation control unit 26 first activates the pump light 23. Then, multiple first to nth actuator heaters 14 are heated sequentially at predetermined time intervals. When the actuator temperature sensor detects that the temperature of each thermal deformation actuator has reached a predetermined temperature rise, heating is stopped.

[0041] Figure 7(b) shows the change Δλ2 in the resonant wavelength λ of each ring waveguide when the actuator heater 14 is activated. When the thermally deformable actuator 13 is heated by the actuator heater 14, the diameter of the ring waveguide 12 increases due to thermal expansion, increasing the resonant wavelength Δλ2. When the actuator temperature sensor 19 detects the temperature T1 at which the ring waveguide 12 has reached a predetermined diameter, the actuator heater 14 stops, and the ring waveguide 12 then cools naturally.

[0042] Because the thermally deformable actuator 13 and substrate 15 are made of silicon and have high thermal conductivity, the temperature T1 is rapidly cooled and returns to the reference temperature T2. The reference temperature T2 is kept constant by the substrate heater and substrate temperature sensor 18 and is set to a temperature higher than the maximum ambient temperature in which the micro-optical resonator is used. As the actuator heaters are turned on and off sequentially from the first ring waveguide to the n-th ring waveguide, the time-varying resonant wavelength of each ring waveguide shifts as shown in Figure 7(b).

[0043] Figure 8 shows the correlation between the wavelength λ1 of the pump light 23 and the zero tune λ2 of the resonant waveform 24 at each position of the ring waveguide 14-3. Each position will be explained as step (1) to step (5). When the start command for the frequency soliton optical comb is turned on, the pump light 23 starts first. The wavelength λ1 of the pump light is fixed to a constant value.

[0044] In (1), the actuator heater 14-3 is turned on and heated by an external power source. This causes the thermally deformable actuator 13-3 to thermally expand and reach a predetermined diameter. This deformation increases the diameter of the ring waveguide 12-3, shifting the resonant wavelength λ2 to the longer wavelength side by Δλ2. The resonant waveform before heating is shown by the dashed line, and the resonant wavelength after heating is shown by the solid line. In (2), when the actuator temperature sensor 19-3 detects a predetermined temperature T1, the actuator heater 14-3 is turned off. When the actuator heater 14-3 is turned off, the thermal deformation actuator 13-3 is cooled to the reference temperature T2 of the substrate 15, and the resonant waveform shifts toward the short wavelength side (to the left in the figure). (3) is the position where the pump light 23 overlaps the base of the resonant waveform 24. From this point on, the optical intensity is injected into the first waveguide 12. (4) is the position where the pump light 23 overlaps the peak (zero tune) of the resonance waveform 24. At this time, a large optical intensity is injected into the first waveguide 12, and four-wave mixing is excited. In (5), the pump light 23 reaches the soliton region in the red tune of the resonant waveform 24. It is important that the soliton optical comb is generated by staying in the soliton region immediately after the pump light 23 is injected into the ring waveguide at (4) and the optical intensity becomes high.

[0045] Next, using FIG. 9, we will show how to launch a soliton optical comb from multiple ring waveguides. The upper diagram in Figure 9 shows a comparison of the wavelength positions of the resonant waveforms 24-1, 24-2, . . . 24-n of the first to nth ring waveguides 12-1, 12-2, . . . 12-n, with wavelength on the horizontal axis. Also, (a) to (e) in Figure 9 show the wavelength change of the waveguide 12 caused by the actuator heater 14 at each wavelength position. The diagram shows how each ring waveguide 12 launches a soliton optical comb using the process described in Figure 8.

[0046] (a) and (b) show cases where no soliton optical comb is generated. Heating by the actuator heater 14 moves it to the position indicated by the dashed line, and when the actuator heater is turned off, cooling begins, causing it to exceed the wavelength λ1 of the pump light 23 and return to its original wavelength λ2. At this time, the pump light 23 is not in the soliton region of the resonant waveform, so no soliton optical comb is generated. Note that when heated, the resonant wavelength 24-1 crosses the wavelength λ1 of the pump light 23. This does not affect the startup operation. In (c), the pump light 23 enters the soliton region, generating a soliton optical comb. When the soliton optical comb is generated at (c), the optical intensity of the pump light in the straight waveguide is consumed in generating the soliton optical comb at (c), and no pump light intensity is supplied to (d) to (e) downstream of (c). Therefore, once the soliton optical comb is generated at (c), the soliton optical comb at (c) is maintained thereafter.

[0047] FIG. 10 shows a case where the Lithium-ion comb shown in FIG. 9 is generated in the ring waveguide 12-3.

[0048] 11 is a block diagram showing the process of controlling the substrate temperature. Normally, the range of the operating environment temperature T3 in which the micro optical resonator 10 is used is set in advance in the product specifications. For example, if the product specifications allow the use of the micro optical resonator 10 up to a maximum operating temperature T3 = 85°C, the reference temperature T2 of the substrate is T2>T3 In this case, the substrate 15 is maintained at the reference temperature T2, that is, power proportional to (T2 - T3) is continuously supplied to the second heater 17 while controlling the temperature. This stabilizes the substrate temperature within the operating temperature range.

[0049] 11, temperature control device 25 feeds back the difference between temperature T3 detected by substrate temperature sensor 18 and reference temperature T2 to second substrate heater 17. This makes it possible to always maintain the substrate temperature at a constant value T2. For this reason, substrate temperature adjustment unit 27 is provided, which has the function of adjusting the substrate reference temperature T2.

[0050] 7 to 10, the soliton optical comb startup process has been explained. If the soliton optical comb detector in this process does not detect a soliton optical comb, the soliton optical comb startup controller 26 is configured to increase the reference temperature T2 by several degrees Celsius and repeat the soliton optical comb startup operation. As a result, as shown in Fig. 12, the resonant wavelengths of the entire first to nth ring waveguides are slightly shifted by δλ, and the soliton optical comb startup operation is repeated.

[0051] FIG. 13 shows the manufacturing process of the micro optical comb. In step (1), a silicon oxide film is formed on a silicon wafer 15 to form a cladding layer 16a. In step (2), a groove 12a for forming the first waveguide 12 and a groove 11a for forming the second waveguide 11 are processed in the cladding layer 16a. In step (3), a recess 13x for providing the thermally deformable actuator 13 directly on the substrate 15 is processed in the cladding layer 16a. In step (4), a silicon nitride film is formed. In step (5), excess silicon nitride is removed by chemical mechanical polishing. In step (6), a cladding layer 16b of silicon oxide is formed. In step (7), a recess 13y for the thermal deformation actuator 13 is formed. In step (8), a silicon nitride film is formed. In step (9), excess silicon nitride is removed by chemical mechanical polishing. In step (10), a first actuator heater 14 is formed on the thermally deformable actuator 13. In step (11), a silicon oxide film is formed on the first actuator heater 14. An actuator temperature sensor 19 is formed on the first actuator heater 14. 16c is a cladding that covers the first actuator heater 14 and the temperature sensor 19.

[0052] As described above, according to this embodiment, the wavelength of the pump light is fixed at a constant wavelength without wavelength sweeping. The optical frequency comb generator of the present invention has a linear waveguide and a plurality of first to nth ring waveguides with slightly different diameters. Corresponding to the plurality of first to nth ring waveguides, first to nth thermal deformation actuators and first to nth ring waveguide heaters are provided. After the first to nth thermal deformation actuators are heated sequentially by the first to nth ring waveguide heaters, the first to nth thermal deformation actuators are cooled independently at predetermined timings. The first to nth ring waveguides each have a slightly different resonant wavelength.

[0053] Therefore, the wavelength of the pump light will be located in the soliton region of one of the first to nth ring waveguides. Therefore, by providing multiple ring waveguides, the probability of solitonization can be increased. Furthermore, by fine-tuning the temperature setting of the substrate 15, the soliton optical comb can be generated more reliably. In this embodiment, the material of the thermal deformation actuator is silicon nitride, but it may be made of a silicon wafer. [Explanation of symbols]

[0054] 10 Micro-optical resonators 11 Straight waveguide 12 Ring waveguide 13 Thermal deformation actuator 13a Bottom of the thermal deformation actuator 13b Top surface of the thermal deformation actuator 13x, 13y recess 14 Actuator heater 14a, b Electrode pads 15 PCB 16 Clad 16a Clad 16b clad 16c clad 16x clad section 17 Substrate heater 18 Board temperature sensor 19 Actuator temperature sensor 19a, 19b Electrode pads for actuator temperature sensor 20 Input section 21 Output section 22 Soliton optical comb 23 Excitation light 24 Resonant waveform 25 Substrate temperature control device 26 Soliton optical comb starter / controller 27 Board temperature adjustment section 28 Soliton optical comb detector 29 Silicon substrate 30(a) First cladding section 30(b) Second cladding section 31 Waveguide 32 resonator 33 Input section 34 Excitation light 35 Thermal changes in resonant wavelength of the resonator 36 Resonance waveform 37 Pulse 38 Soliton Region

Claims

1. a linear waveguide that receives the excitation light from the input section and is positioned so as to be optically coupled to each of the first to nth ring waveguides, and that inputs evanescent light of at least a portion of the excitation light into a specific one of the first to nth ring waveguides, and extracts from the first waveguide an optical frequency comb having a plurality of comb-like spectra with equal frequency intervals; a cladding that surrounds the linear waveguide and the multiple ring waveguides and has a refractive index lower than that of the first waveguide and the first to nth waveguides; first to nth thermal deformation actuators that are formed inside the first to nth ring waveguides, respectively; and a substrate that cools the thermal deformation actuators.

2. 2. The micro optical resonator according to claim 1, wherein the diameters of the first to n-th ring waveguides are set to be different so that the resonant wavelengths of the first to n-th ring waveguides are successively increased by a fixed amount from the resonant wavelength of the first ring waveguide closest to the input section.

3. 3. The micro-optical resonator according to claim 1, wherein the increment of the resonant wavelength of each of the first to nth ring waveguides is set to increase by an amount equal to the half-width of the resonant waveform.

4. The micro-optical resonator according to claims 1 to 2, characterized in that it comprises first to nth actuator heaters that heat the first to nth thermal deformation actuators, first to nth actuator temperature sensors that detect the temperatures of the first to nth thermal deformation actuators, an actuator temperature control device that controls the temperatures of the first to nth actuator heaters, a substrate heater that heats the reference temperature of the substrate to a predetermined temperature, a substrate temperature sensor that detects the substrate temperature, and a substrate temperature control device that controls the reference temperature of the substrate to a value higher than the ambient temperature.

5. 3. The micro optical resonator according to claim 1, wherein the first to n-th thermal deformation actuators and the first to n-th actuator heaters are in contact with each other.

6. 3. The micro optical resonator according to claim 1, wherein the first to n-th thermal deformation actuators are in contact with the substrate.

7. A micro-optical resonator according to claims 1 to 2, comprising a soliton optical comb detection unit that detects the soliton optical comb output from the linear waveguide, a substrate temperature adjustment unit that finely adjusts the substrate temperature, and a soliton optical comb startup control unit that controls the startup of the soliton optical comb.

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