Micro optical resonator, and light frequency comb generation device

Abstract

To solve the problem that, in starting a light frequency comb, the resonance wavelength and the refractive index change due to thermal deformation of a resonator, so that it has been difficult to shift the wavelength of excitation light to a region to form soliton accurately, 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, and moreover the problem that the continuous generation of soliton light comb has been unstable due to ambient temperature change.SOLUTION: Excitation light is guided to a linear waveguide without sweeping the wavelength of the excitation light, and a plurality of first to n-th ring waveguides with diameters slightly different from each other are provided. In the first to n-th ring waveguides, thermal deformation actuators are provided and the thermal deformation actuators heat and cool sequentially with heaters to start a soliton light comb. A micro-optical resonator is formed of a material with high thermal conductivity and is provided on a substrate with a structure whose thermal conductivity to the external space is low.SELECTED DRAWING: Figure 4

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 evenly spaced components (modes) along the frequency axis. To activate an optical frequency comb, simply creating an anomalous dispersion state within the cavity is insufficient; a special procedure is required. In 2014, a method for continuously sweeping the wavelength of a continuous-wave (CW) laser light source introduced into a circular optical waveguide around its resonant frequency was discovered, and many techniques based on this method have been studied. However, activation using this method is extremely complex, as it requires consideration of two factors that cause 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.

[0003] A conventional optical frequency comb generator will now be described. This micro-optical resonator is shown in Figure 18. Figure 18(a) is a plan view of the waveguide, and Figure 18(b) shows a cross-sectional view along the x-axis. 33a is a first cladding layer made of silicon dioxide (SiO2) deposited on a substrate 32. A portion of this first cladding layer 33a is etched away to embed a resonator 35, which is a circular optical resonator, and a waveguide 34, on top of which a second cladding layer 33b is formed. The resonator 35 is ring-shaped, and the waveguide 34 is linear. The resonator 35 and the waveguide 34 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 35 and the waveguide 34 are made of silicon nitride.

[0004] The operation of the conventional optical frequency comb generator configured as above will now be described. An input unit 36 ​​that receives input of excitation light 37 irradiated from a continuous wave (CW) laser light source device (not shown) is connected to the waveguide 34, and the excitation light 37, which is a CW laser light, is incident from the CW laser light source device into the waveguide 34. The incident excitation light 37 is a continuous wave having a wavelength λ1.

[0005] Figure 19(a) shows the wavelength spectrum of the incident pump light 37. When the resonant wavelength λ1 of the pump light 37 overlaps with the resonant wavelength of the resonator 35, the pump light 37 seeps out of the waveguide 34 and is captured into the resonator 35 as a so-called evanescent wave. When the modes with different wavelengths in the resonator are aligned, the peaks overlap and constructively interfere, generating a pulse 40. This state is called mode locking. This pulse is also called an optical comb.

[0006] The concept of modes is shown in Figure 20. The mode spacing is expressed by the repetition frequency frep, frep =c / (2nL) 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 FIG. 19(b).

[0007] Next, the wavelength shift of the micro optical resonator will be described with reference to FIG. The optical comb is generated by wavelength-shifting the pump light 37 from the short wavelength side to the long wavelength side with respect to the resonant wavelength λ2 of the resonant waveform 39 of the micro-optical resonator. Here, the distance between the wavelength λ1 of the pump light 37 and the resonant wavelength λ2 of the micro-optical comb is referred to as the detuning amount. The position of the resonant wavelength λ2 is called zero detune. The wavelength side shorter than zero detune is called blue detune, and the wavelength side longer than zero detune is called red detune. As the detuning amount is reduced from the blue detune side, the pump light 37 resonates as an evanescent wave from the waveguide 34, and the coupling power is guided into the resonator 35.

[0008] This gradually increases the optical power inside the resonator 35. When the coupling power of the pump light 37 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. Comb-like spectra generated using a micro-optical resonator can be broadly divided into chaotic optical combs and soliton optical combs. Of these, 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.

[0009] To generate a soliton optical comb, it is necessary to capture the soliton region 41, where the wavelength λ1 of the pump light 37 transitions to the red detuned state, exceeding the zero-tuned wavelength λ2. Figure 22 shows the process by which the pump light 37 reaches the red detuned soliton region. In Figure 22(1), the pump light 37 shifts its wavelength λ1 toward longer wavelengths. In Figure 22(2), when the pump light 37 is applied to the resonant waveform, optical energy gradually flows into the resonator 35. This optical energy heats the resonator 35. This heat causes the ring diameter of the resonator 35 to expand, shifting the resonant wavelength λ2 toward longer wavelengths. Furthermore, the refractive index also increases. Furthermore, if the optical intensity within the ring increases, the Kerr effect will occur. To avoid this, the pump light 37 must pass through the resonant peak at high speed. Then, in Figure 22(3), the pump light 37 passes the resonant peak, reaches the soliton region, and stops. To generate a soliton optical comb in this way, the pump light 37 must pass through the peak of the resonant waveform 39, inject high optical intensity into the resonator 35, and then reach the soliton region and stop. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Patent No. 10,268,100 [Non-patent literature]

[0011] [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]

[0012] The region 41 where the optical comb solitonizes is very narrow, and this region 41 moves due to heat. Therefore, it is very difficult to position the wavelength λ1 of the pump light in the region 41 where the solitonization occurs. Furthermore, in order to sweep the wavelength of the pump light at high speed, a function for sweeping the wavelength at high speed is required. In addition, in the film formation process of the cladding and waveguide, the substrate is cooled to room temperature after sputtering, but residual stress occurs in the waveguide due to thermal contraction due to the difference in the thermal expansion coefficients of the substrate and the waveguide.

[0013] This residual stress causes a difference between the stress in the substrate surface direction and the stress perpendicular to the substrate, resulting in variations in the refractive index distribution.Furthermore, changes in the external environmental temperature cause changes in the refractive index of the waveguide, making it difficult to obtain a stable soliton optical comb. The present invention is intended to solve the above-mentioned conventional problems, and has as its object to provide a micro optical resonator that can reliably convert an optical comb into a soliton and obtain a stable output. [Means for solving the problem]

[0014] To achieve this objective, the optical frequency comb generator of the present invention fixes the excitation light to a constant wavelength without sweeping the wavelength of the excitation light. Furthermore, the micro optical resonator of the present invention has a plurality of first to n-th ring waveguides with slightly different diameters relative to a linear waveguide. First to n-th thermal deformation actuators are provided within the plurality of first to n-th ring waveguides, respectively, and the first to n-th thermal deformation actuators are heated sequentially by first to n-th heaters.

[0015] Next, the first to nth thermal deformation actuators are cooled independently at a predetermined timing. This reduces the ring diameter 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, it is difficult to tune the wavelength of the pump light to the narrow soliton region of a specific ring waveguide. However, it is sufficient to tune the wavelength of the pump light of any one of the first to nth ring waveguides to the soliton region of the ring waveguide. In other words, providing multiple ring waveguides increases the probability of solitonization.

[0016] Furthermore, a substrate made of a material with high thermal conductivity and low heat transfer coefficient toward the external space is provided. This allows the micro optical resonator to be controlled and maintained at the substrate temperature during sputtering when forming the waveguide. This suppresses changes in the refractive index of the waveguide due to external environmental temperature, and furthermore, the substrate temperature can be set to the substrate temperature during the sputtering process to eliminate variations in the refractive index. [Effects of the Invention]

[0017] 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 23 to sweep the wavelength during the launch process, eliminating the need for a laser light source control device for wavelength sweeping, resulting in significant cost reductions and device miniaturization. Furthermore, multiple (n) ring waveguides are provided to position the wavelength of the pump light 23 within an extremely narrow soliton region. Furthermore, if the soliton optical comb cannot be launched at the set substrate reference temperature (T2), the reference temperature (T2) can be slightly shifted and readjusted. This ensures the launch of a soliton optical comb. Furthermore, by constantly controlling T2 to a constant value, robustness against changes in the external environmental temperature is improved, enabling stable continuous generation of a soliton optical comb. [Brief explanation of the drawings]

[0018] [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] 2 is a configuration diagram of a substrate of the micro optical comb according to the first embodiment of the present invention. FIG. [Figure 6] FIG. 2 is a plan view of a first heater of the micro optical resonator according to the first embodiment of the present invention. [Figure 7] FIG. 2 is a block diagram showing a process for controlling the temperature of a thermal deformation actuator of a micro optical resonator according to the first embodiment of the present invention. [Figure 8] 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 9]9(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. 9(a). [Figure 10] 10 is a diagram showing a movement process of a resonance waveform with a fixed excitation light 23 for explaining the operation of the micro optical resonator according to the first embodiment of the present invention. FIG. [Figure 11] 10A and 10B are diagrams showing the movement process of a plurality of resonance waveforms with the excitation light 23 fixed, for explaining the operation of the micro optical resonator according to the first embodiment of the present invention. [Figure 12] FIG. 2 is a schematic plan view for explaining a micro optical comb generated by the micro optical resonator according to the first embodiment of the present invention. [Figure 13] 10 is a graph showing a change in the resonant wavelength of each ring waveguide caused by a reference temperature (T2) fine adjustment unit of the micro optical comb according to the first embodiment of the present invention. [Figure 14] 3 shows a manufacturing process of a substrate of the micro optical comb according to the first embodiment of the present invention. [Figure 15] 3A to 3C are diagrams illustrating a manufacturing process of the micro optical comb according to the first embodiment of the present invention. [Figure 16] 3 shows material characteristics of the micro optical resonator according to the first embodiment of the present invention. [Figure 17] FIG. 10 is a layout diagram of a ring waveguide of a micro optical resonator according to a second embodiment of the present invention. [Figure 18] 1A and 1B are plan and cross-sectional views of a conventional micro-optical resonator. [Figure 19] FIG. 1 is an explanatory diagram illustrating the concept of mode locking of a micro optical resonator. [Figure 20] FIG. 1 is an explanatory diagram of a comb-shaped spectrum generated by a micro optical comb. [Figure 21] 10 is an explanatory diagram showing the relationship between the pump light 23 and the resonant waveform of the resonator when a soliton optical comb is generated by a micro optical comb. FIG. [Figure 22] 1A and 1B are diagrams showing the shift process of the pump light 23 for explaining the operation of a conventional micro optical resonator. DETAILED DESCRIPTION OF THE INVENTION

[0019] The invention described in claim 1 of the present invention comprises an input section that receives an input of excitation light having a predetermined frequency irradiated from a laser light source device, a substrate made of a material with high thermal conductivity and low heat transfer characteristics to an external space, and first to nth ring waveguides that are made of a material with a third-order nonlinear optical effect and have slightly different diameters, and a ring waveguide that receives the excitation light from the input section and is positioned to be optically coupled to the first to nth ring waveguides, and inputs evanescent light of the excitation light at least partly into a specific ring waveguide among the first to nth ring waveguides, and outputs a plurality of comb-like spectra with equal frequency intervals from the ring waveguide. The present invention includes a straight waveguide from which an optical frequency comb having a soliton frequency comb is extracted, a cladding surrounding the straight waveguide and the plurality of ring waveguides and formed on the substrate with a refractive index lower than that of the straight waveguide and the first to nth ring waveguides, first to nth cylindrical thermal deformation actuators disposed inside the first to nth ring waveguides and coaxially aligned with the center of the ring waveguide formed on the substrate, first to nth heaters for heating the first to nth thermal deformation actuators, and first to nth temperature sensors for detecting the temperatures of the first to nth thermal deformation actuators, thereby improving the start-up probability of a soliton optical comb. Furthermore, the substrate temperature can be controlled to the temperature during waveguide deposition, thereby stabilizing the refractive index.

[0020] The invention described in claim 2 of the present invention is a micro optical resonator described in claim 1, which has the substrate having multiple vacuum cavities inside, and reduces the heat transfer coefficient from the substrate surface to the external space.

[0021] The invention described in claim 3 of the present invention is the micro optical resonator described in claim 2, characterized in that the frame portion supporting the upper and lower surfaces of the multiple vacuum cavities provided in the substrate is made of a material with a lower thermal conductivity than the substrate, and further reduces the coefficient of heat transfer from the substrate surface to the external space. The invention described in claim 4 of the present invention is an optical fiber comprising: an input section for receiving an input of excitation light having a predetermined frequency irradiated from a laser light source; a substrate made of a material with high thermal conductivity and having a vacuum cavity therein; and first to nth ring waveguides each having a slightly different diameter, the excitation light from the input section being input and being 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 comb-like spectral regions having equal frequency intervals from the linear waveguide. an optical frequency comb generator including a straight waveguide from which an optical frequency comb having a RAM is extracted; a cladding surrounding the straight waveguide and the plurality of ring waveguides and formed on the substrate with a refractive index lower than that of the straight waveguide and the first to nth waveguides; first to nth cylindrical thermal deformation actuators arranged inside the first to nth ring waveguides and coaxially with the center of the ring waveguide formed on the substrate; and first to nth temperature control devices for controlling the temperatures of the first to nth thermal deformation actuators, whereby the substrate temperature can be controlled to the temperature during waveguide deposition, thereby stabilizing the refractive index.

[0022] The invention described in claim 5 of the present invention is an optical frequency comb generator as described in claim 5, which has the substrate having multiple vacuum cavities inside, thereby reducing the heat transfer rate from the substrate surface to the external space.

[0023] The invention described in claim 6 of the present invention is an optical frequency comb generator described in claim 6, characterized in that the frame portion supporting the upper and lower surfaces of the multiple vacuum cavities provided in the substrate is made of a material with lower thermal conductivity than the substrate, further reducing the heat transfer rate from the substrate surface to the external space.

[0024] 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. These straight waveguide and multiple ring waveguides are covered with a cladding 16 (shown in FIG. 4) whose refractive index is lower than that of the material of the waveguides. The multiple ring waveguides 12 consist of the first ring waveguide 12-1 to the nth ring waveguide 12-n, each with a slightly different diameter, with the diameter increasing from the closest to the input section. The diameters are set so that the center wavelength increases by the half-width of the resonant waveform. The increase in diameter is slight. However, because the difference in diameter between each ring waveguide is determined by a mask, it is easy to ensure the relative diameter accuracy of the multiple ring waveguides.

[0025] Each ring waveguide is arranged on the same plane with a small gap d between it and the straight waveguide 11. Although the ring waveguide 12 is arranged on one side of the straight waveguide 11 in FIG. 1, it may be arranged on both sides as shown in FIG. 17. An input unit 20 is provided at one end of the straight waveguide 11, which receives pump 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, which outputs the generated soliton optical comb 22. The radii r of the multiple ring waveguides 12 are arranged in a direction in which the radii increase in order, starting with ring waveguide 12-1 closest to the input unit 20.

[0026] The repetition frequency of the mode period of the ring waveguide 12 is 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 of the resonator increases in proportion to the radius r of the ring waveguide.

[0027] Figure 2 shows the resonant waveforms 24-1, 24-2, 24-3, ..., 24-n of the multiple ring waveguides 12. The horizontal axis represents wavelength. In this example, the wavelength at the maximum amplitude of the resonant wavelength 24-3 of the ring waveguide 12-3 is assumed to be 1550 nm. The wavelengths of the waveforms differ by the half-width.

[0028] FIG. 3 is an enlarged view of the third ring waveguide 12-3 of the micro-optical resonator 10. FIG. 4 is a cross-sectional view of the micro-optical resonator 10 of the present invention, showing the x-axis cross section in FIG. 3. Inside each ring waveguide, a cylindrical thermally deformable actuator 13 is arranged coaxially with the ring waveguide. The outer diameter of the thermally deformable actuator 13 is separated from the inner diameter of the ring waveguide by a distance g. The thermally deformable actuator 13 is made of a material with a high thermal expansion coefficient. A cladding portion 16x fills the gap with width g, and width g is set as small as possible without affecting the light in the waveguide. The cladding portion 16x is made of the same material as the cladding 16. The first to nth ring waveguides and the straight waveguide 11, each covered with the cladding 16, are arranged on the upper surface 15b of the substrate 15.

[0029] A temperature sensor 19 that detects the temperature of the heater 14 and the actuator is provided on the top surface 13b of the thermally deformable actuator 13. The bottom surface 13a of the thermally deformable actuator 13 is provided in contact with the top surface 15b of a silicon substrate 15 that has excellent thermal conductivity. A heat insulating layer 31 is provided on top of the cladding 16.

[0030] Figure 6 shows a plan view of the heater 14 and electrode pads 14a and 14b. The heater 14 is made of a platinum thin film and has a curved path that increases electrical resistance, with 14a and 14b being electrodes for external wiring. The temperature sensor 19 is provided with electrodes 19a and 19b. 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 16. The substrate 15 has a vacuum cavity 17 formed therein to ensure the heat insulating properties of the rear surface 15a.

[0031] Figure 5 shows the structure of substrate 15, which is a silicon wafer. The vacuum cavity has a depth of several microns and a frame structure with a planar size of several tens of microns on each side. The outer frame 18 of this vacuum cavity 17 is made of silicon oxide (SiO2), which has low thermal conductivity.

[0032] 7 is a block diagram showing the process of starting a soliton optical comb and maintaining a stable soliton optical comb by controlling the substrate temperature (T2) to a reference temperature. Each of the temperature control devices 25, 25-1 to 12-n, is provided for each of the thermal deformation actuators 13. Each of the temperature control devices 25 is equipped with a temperature sensor 19, a controller 29, and a heater 14. Furthermore, a soliton optical comb detector 28 is provided to detect whether the light output from the output unit 21 is a soliton optical comb. Also, a reference temperature (T2) fine adjustment unit 27 is provided to adjust the reference temperature (T2) if a soliton optical comb is not detected after the start-up operation of the soliton optical comb in the plurality of ring waveguides.

[0033] 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 a ring waveguide using a thermally deformable actuator 13, a feature of the present application, will be described with reference to FIG. 8. In FIG. 8(8a), the thermally deformable actuator 13 is heated by a 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 ring waveguide 12 of the resonator. On the other hand, when the heating by the heater 14 is stopped and the thermally deformable actuator 13 cools, the thermally deformable actuator contracts, thereby decreasing the circumferential length L of the ring waveguide 12. In this way, heating or cooling the thermally deformable actuator 13 elastically changes the circumferential length of the ring waveguide 12, adjusting its resonant wavelength.

[0034] Next, the operation of the optical frequency comb generator will be explained using the soliton optical comb startup block diagram of Figure 7. When starting up the soliton optical comb, the soliton optical comb startup control unit 26 applies voltage to each heater 14 to heat the thermal deformation actuator 13, thereby controlling the startup of the soliton optical comb. The soliton optical comb detector 28 detects whether a soliton optical comb has been generated in the lightwaves output from the output unit 21. If a soliton optical comb cannot be detected, the reference temperature (T2) fine adjustment unit 27 fine-tunes the reference temperature T2, and the result is fed back to the controller 29.

[0035] FIG. 9(a) is a timing chart of the frequency soliton optical comb startup by the soliton optical comb startup controller 26. In FIG. First, each temperature sensor 19 detects the temperature of the corresponding thermal deformation actuator 13. The difference between the detected temperature T3 detected by the temperature sensor 19 and the reference temperature T2 is used by each controller 29 to control the heater 14 and maintain the thermal deformation actuator 13 at the reference temperature (T2). When a command to start the soliton optical comb is received, first, all heaters 1 to n are turned on, and each thermal deformation actuator 13 is heated until it reaches a reference temperature (T2). This reference temperature T2 corresponds to the substrate temperature when the ring waveguide 12 is formed by sputtering. This reference temperature T2 is precisely controlled and managed for each thermal deformation actuator 13. At this point, preparations for the soliton optical comb start-up operation are complete. The substrate temperature is approximately 20% to 30% of the melting point of the waveguide material.

[0036] Next, the pump light 23 is activated by the soliton optical comb activation control unit 26. After that, the first to n-th heaters 14 are sequentially heated to an activation temperature (T1) at predetermined intervals. After that, when the temperature sensor 19 detects that the temperature of each thermal deformation actuator has reached T1, the heating is stopped and the temperature is cooled to and maintained at a reference temperature (T2).

[0037] 9(b) is a diagram showing the temperature of the thermally deformable actuator 13 when each thermally deformable actuator 13 is heated in sequence in FIG. 9(a). This temperature change changes the diameter of the thermally deformable actuator 13, which in turn changes the diameter of the ring waveguide 12, and the resonant wavelength of the ring waveguide 12.

[0038] 10 is a diagram showing the correlation between the wavelength (λ2) of the ring waveguide 12 and the wavelength λ1 of each excitation light 23 due to changes in the diameter of a specific thermally deformable actuator 13. The explanation will be divided into steps 10(a) to 10(e) according to the temperature change of the thermally deformable actuator 13. When the start command of the frequency soliton optical comb is turned on, first the pump light 23 is started. The wavelength λ1 of this pump light 23 is fixed to a constant value. In step Figure 10(a), the heater 14 is heated from T2 to T1. This causes the thermally deformable actuator 13 to thermally expand, and the diameter of the ring waveguide 12 increases due to elastic deformation, shifting the resonant wavelength λ2 of the ring waveguide 12 toward the longer wavelength side by Δλ2. Note that the resonant waveform at the reference temperature (T2) is shown by a dashed line, and the position of the resonant waveform 24 at T1 after heating is shown by a solid line. In step Figure 10(b), when the temperature sensor 19 detects a predetermined temperature T1, the heater 14 stops and is allowed to cool naturally, or the heater current is reduced to cool. This causes the resonant waveform 24 to shift toward the shorter wavelength side (toward the left in the figure).

[0039] 10(c) is the position where the pump light 23 overlaps the skirt of the resonance waveform 24 during the cooling process. From this point on, optical intensity is injected into the ring waveguide 12.

[0040] Step (d) of FIG. 10 is at a position where the pump light 23 overlaps the peak (zero tune) of the resonant waveform 24. At this time, a large optical intensity is injected into the ring waveguide 12, exciting four-wave mixing.

[0041] In step FIG. 10( e ), the pump light 23 reaches the solitonization region 41 in the red tune of the resonant waveform 24 .

[0042] In order to generate a soliton optical comb, the pump light 23 must remain in the soliton region immediately after it is injected into the ring waveguide in FIG. 10(d) and the optical intensity becomes high. The temperatures corresponding to steps 10(a) to 10(e) in FIG. 10 are shown in FIG. 9(b).

[0043] Figure 11 shows the soliton optical comb setup for multiple ring waveguides with different diameters. Figure 11(x) shows 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 at the reference temperature (T2), with the horizontal axis representing wavelength. 11(a) to 11(e) are diagrams showing how the wavelengths of the first to n-th resonant waveforms 24 with different wavelengths change due to heating and cooling by the heater 14. In FIG.

[0044] In Figure 11(a), at the reference temperature (T2), the waveform is at a position shorter than the wavelength λ1 of the pump light 23, and moves to the position indicated by the dashed line due to heating by the heater 14-1. It is then cooled and returns to its original position. During this process, the pump light 23 moves relatively from the short wavelength side to the long wavelength side of the resonant waveform 24 and stops. At this time, the pump light 23 is not within the soliton region 41, so no soliton optical comb is generated. The case of Figure 11(b) is the same as Figure 11(a). In Figure 11(c), when the resonant waveform 24-3 moves from the long wavelength side to the short wavelength side and stops, the pump light 23 is located within the soliton region. This generates a soliton optical comb.

[0045] Once the soliton optical comb is established in Fig. 11(c), the optical intensity of the pump light 23 in the straight waveguide 11 is used to generate the soliton optical comb in Fig. 11(c), and after Fig. 11(c), the optical intensity of the pump light 23 decreases, and thereafter, no soliton optical comb is generated in Fig. 11(d) and (e). Therefore, the soliton optical comb generated in the ring waveguide (24-3) in Fig. 11(c) is maintained.

[0046] If the soliton optical comb detector 28 fails to detect the soliton optical comb in the above series of operations, the reference temperature (T2) is slightly increased by the reference temperature (T2) fine adjustment unit 27. This causes the diameter of each thermal deformation actuator 13 to increase relatively. Figure 13 shows the increase in the resonant wavelength of each ring waveguide when the reference temperature (T2) adjustment unit 27 slightly increases the set temperature T2 of the substrate.

[0047] Thereafter, the soliton optical comb activation control unit 26 again performs the soliton optical comb activation operation for the first to n-th plurality of ring waveguides, and this activation operation is continuously repeated until the soliton optical comb is activated. Therefore, by finely adjusting the reference temperature (T2), it is possible to reliably generate a soliton optical comb. Note that Fig. 12 shows the state in which a soliton optical comb is generated in the ring waveguide 12-3.

[0048] Next, the function of the vacuum cavity 17 will be described. In this application, the micro-optical resonator 10 is operated at a high temperature. This temperature corresponds to the substrate temperature when the waveguide is deposited by sputtering. The substrate temperature corresponds to 20% to 30% of the melting point of the material of the waveguide 12, and is assumed to be between 200°C and 400°C. This results in the same temperature conditions as when the ring waveguide 12 is deposited. Therefore, residual stress within the ring waveguide 12 can be minimized and the refractive index distribution can be made uniform. This results in excellent optical properties. However, at this time, it is necessary to suppress heat dissipation to the outside and reduce the load on the heater, necessitating a strong thermal insulation structure. For this reason, a vacuum cavity 17 is provided on the back surface 15a of the substrate 15, and a frame 18 made of silicon oxide (SiO2), which has low thermal conductivity, is further provided around the vacuum cavity 17. As shown in the material properties table in Figure 16, the thermal conductivity of silicon is approximately 1 / 32 of that of silicon oxide, so nearly perfect thermal insulation can be expected. In this way, the structure is completely insulated and is not affected by external temperature changes.

[0049] On the other hand, when starting up the soliton optical comb, the thermal deformation actuator 13 needs to be cooled quickly. Therefore, the substrate 15 needs to have high thermal conductivity, and heat needs to be diffused quickly within the substrate 15 to cool it from the activation temperature T1 to the reference temperature T2. Therefore, silicon, which has high thermal conductivity, is used as the material for the substrate 15. Therefore, in Figure 4, heat insulation is ensured in the direction of arrow A by the vacuum cavity 17 and frame 18, and heat diffusion into the substrate 15 is possible in the direction of arrow B by the silicon material with high thermal conductivity. Note that a heat insulating layer 31 is provided on the top surface of the cladding 16 to prevent heat diffusion in the direction of arrow C.

[0050] FIG. 14 is a diagram showing the process of machining a vacuum cavity in a substrate 15. As shown in FIG. In step (1), a groove 18a for providing a frame 18 is formed by etching on one surface 15a of a substrate 15, which is a silicon wafer. In step (2), a film of the frame material (SiO2) is formed in the groove 18a. In step (3), excess SiO2 is removed by chemical mechanical polishing (CMP). In step (4), fine holes 30 with a high aspect ratio and a diameter of about 0.5 μm to 1 μm and a depth of about 3 μm to 5 μm are filled in the frame 18 in the area where the vacuum cavity 17 is to be formed. In step (5), the substrate 15-4 formed in step (4) is annealed at approximately 320°C. This causes adjacent high aspect ratio fine holes 30 to merge together, and at the same time, the high aspect ratio fine holes are integrated. Furthermore, the high aspect ratio fine holes are closed, forming a cavity 17. In step (6), the vacuum cavity 17 is completed by further annealing. The annealing process in step (5) is carried out in a vacuum or hydrogen-filled environment. As described above, by the process of FIG. 14, a vacuum cavity can be obtained in which the frame 18 is made of SiO2, which has low thermal conductivity, and a substrate with extremely high heat insulating properties can be constructed.

[0051] FIG. 15 shows the manufacturing process of the micro optical comb. In step (1), a silicon oxide film is formed on a substrate 15 having a vacuum cavity 17 to form a cladding layer 16a. In step (2), a groove 12a for forming the ring waveguide 12 and a groove 11a for forming the straight waveguide 11 are processed in the cladding layer 16a. In step (3), a silicon nitride film is formed. In step (4), excess silicon nitride is removed by chemical mechanical polishing. In step (5), a cladding layer 16b of silicon oxide is formed. In step (6), recesses 13a for providing the thermally deformable actuator 13 directly on the substrate 15 are processed in the cladding layers 16a and 16b. In step (7), a silicon film having high thermal conductivity is formed. In step (8), excess silicon is removed by chemical mechanical polishing. In step (9), a first heater 14 is formed on the thermally deformable actuator 13. In step (10), a cladding layer 16c of silicon oxide is formed. In step (11), a silicon oxide film is formed on the heater 14. In step (12), the heat insulating layer 16d is formed from SiO2.

[0052] As described above, according to this embodiment, the wavelength of the pump light 23 is fixed at a constant wavelength without wavelength shifting. The optical frequency comb generator of the present invention includes a linear waveguide 11 and a plurality of first through nth ring waveguides 12, each with slightly different diameters. Corresponding to the plurality of first through nth ring waveguides 12, first through nth thermal deformation actuators 13 and first through nth heaters 14 are provided. After the first through nth thermal deformation actuators 13 are sequentially heated by the first through nth heaters 14, the first through nth thermal deformation actuators 13-1 through 13-n are independently cooled at predetermined timings. The first through nth ring waveguides each have a slightly different resonant wavelength. Therefore, the wavelength of the pump light 23 falls within the solitonization region of the ring waveguide for one of the first through nth ring waveguides. Therefore, providing multiple ring waveguides increases the probability of solitonization. Furthermore, fine-tuning the reference temperature setting of the substrate 15 allows for more reliable soliton optical comb startup. Furthermore, by matching the temperature of the substrate 15 to the substrate temperature during waveguide deposition, residual stress within the waveguide can be reduced, suppressing variations in the refractive index. To maintain the reference temperature (T2) during normal operation at the high temperature during deposition, a vacuum cavity is provided within the substrate 15 to enhance thermal insulation, thereby reducing the power consumption of the heater 14. Furthermore, since the substrate 15 is made of a material with high thermal conductivity, heat can be diffused into the substrate when cooling the thermal deformation actuator 13 during soliton optical comb startup. While the thermal deformation actuator is made of silicon in this embodiment, it may also be made of silicon nitride, the same material as the ring waveguide. [Explanation of symbols]

[0053] 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 14 Heater 14a,b electrode 15 PCB 16, 16a, 16b, 16c clad 16x clad section 17 Vacuum cavity 18 slots 19 Temperature Sensor 19a, 19b electrode 20 Input section 21 Output section 22 Soliton optical comb 23 Excitation light 24 Resonance waveform 25 Substrate temperature control device 26 Soliton optical comb starter / controller 27 Reference temperature (T2) adjustment section 28 Soliton optical comb detector 29 Controller 30 High aspect ratio fine holes 31 Insulation layer 32 Silicon substrate 33(a) First cladding section 33(b) Second cladding section 34 Waveguide 35 Resonator 18 36 Input section 37 Excitation Light 38 Thermal changes in resonant wavelength of the resonator 39 Resonance waveform 40 pulses 41 Soliton region

Claims

1. an input section for receiving an input of excitation light having a predetermined frequency irradiated from a laser light source device; a substrate made of a material with high thermal conductivity and low heat transfer characteristics to an external space; and first to nth ring waveguides made of a material having a third-order nonlinear optical effect and having slightly different diameters; and a waveguide arrangement for receiving the excitation light from the input section and being optically coupled to the first to nth ring waveguides, the waveguide arrangement being arranged so that at least a portion of the excitation light is input as evanescent light of the excitation light into a specific one of the first to nth ring waveguides, and a waveguide arrangement for outputting a plurality of comb-like spectra having equal frequency intervals from the ring waveguides. a cladding surrounding the straight waveguide and the plurality of ring waveguides and formed on the substrate with a refractive index lower than that of the straight waveguide and first to nth waveguides; first to nth cylindrical thermal deformation actuators disposed inside the first to nth ring waveguides and provided coaxially with the center of the ring waveguide formed on the substrate; first to nth heaters for heating the first to nth thermal deformation actuators; and first to nth temperature sensors for detecting the temperatures of the first to nth thermal deformation actuators.

2. 10. The micro-optical resonator of claim 1, further comprising the substrate having a plurality of vacuum cavities therein.

3. 3. The micro optical resonator according to claim 2, wherein a frame supporting the upper and lower surfaces of the plurality of vacuum cavities provided in the substrate is made of a material having a lower thermal conductivity than the substrate.

4. an input section for receiving an input of excitation light having a predetermined frequency irradiated from a laser light source device; a substrate made of a material with high thermal conductivity and having a vacuum cavity therein; and first to nth ring waveguides each having a slightly different diameter, the excitation light from the input section being input and being arranged at a position where it can be 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 the linear waveguides are arranged at equal frequency intervals from each other. a cladding surrounding the straight waveguide and the plurality of ring waveguides and formed on the substrate with a refractive index lower than that of the straight waveguide and first to nth ring waveguides; first to nth cylindrical thermal deformation actuators disposed inside the first to nth ring waveguides and provided coaxially with the center of the ring waveguide formed on the substrate; and first to nth temperature control devices controlling the temperatures of the first to nth thermal deformation actuators.

5. 5. An optical frequency comb generator as claimed in claim 4, wherein the substrate has a plurality of vacuum cavities therein.

6. 6. An optical frequency comb generator according to claim 5, wherein a frame supporting the upper and lower surfaces of the plurality of vacuum cavities provided in the substrate is made of a material having a lower thermal conductivity than the substrate.

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