Micro optical resonator and optical frequency comb generator

Abstract

To solve the problem that since the resonance wavelength and refractive index of a resonator due to thermal deformation brought about by a rise of temperature at start of an optical frequency comb, there is difficulty moving the wavelength of excitation light to a solitonizing region with good accuracy, and that the function to sweep excitation light requires changing the wavelength of excitation light and, therefore, is detrimental in terms of downsizing and cost.SOLUTION: Sweeping of excitation light is not performed, and a plurality of first to n-th ring waveguides of the same nominal diameter, with diameters randomly distributed on the basis of statistical establishment. A thermal deformation actuator is provided in each of the plurality of first to n-th ring waveguides, and the thermal deformation actuators are simultaneously heated by an actuator heater and then cooled, thereby starting up a soliton optical comb. Since the wavelength of excitation light needs to match the soliton region of a ring waveguide for only one of the plurality of first to n-th ring waveguides, it is possible to generate a soliton optical comb more reliably.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 a micro-optical resonator and an optical frequency comb generator for reliably starting a soliton optical comb and continuously generating and maintaining 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 14. Figure 14(a) is a plan view of the waveguide, and Figure 14(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, and a resonator 32, which is a circular optical resonator, and a waveguide 31 are embedded therein. A second cladding layer 30b is formed on top of this. 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.

[0004] The operation of the conventional optical frequency comb generator configured as above will now be described. An input unit 33 is connected to the waveguide 31, and receives pumping light irradiated from a continuous-wave (CW) laser light source (not shown). Pumping light 34, a CW laser beam, is incident on the waveguide 31. The incident pumping light 34 is a continuous wave having a wavelength λ1. Figure 15(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 seeps out of the waveguide 31 and is captured into the resonator 32 as a so-called evanescent wave. When the modes with different wavelengths in the resonator are aligned, their peaks overlap and constructively interfere, generating a pulse 37. This state is called mode locking. This pulse is also called an optical comb.

[0005] The concept of mode locking is shown in Figure 16. The mode spacing is expressed by 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 15(b).

[0006] 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 short to long wavelengths relative to the resonant wavelength λ2 of the resonant waveform 36 of the micro-optical resonator. Here, the distance between the wavelength λ1 of the pump light 34 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 shorter than zero detune is called blue detune, and the wavelength longer than zero detune is called red detune. As the detuning amount decreases from the blue detune side, the pump light 34 resonates as an evanescent wave from the waveguide 31, and 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.

[0007] 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.

[0008] Figure 18 shows the process by which pump light 34 reaches the red-detuned soliton region. In Figure 18(1), the pump light 34 shifts its wavelength λ1 toward the longer wavelength side. In Figure 18(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 heat up. This heat causes the ring diameter of the resonator 32 to thermally expand, shifting the resonant wavelength λ2 toward the longer wavelength side. 35 is the change in the resonator's resonant wavelength (δλ2) due to heat. Furthermore, as the optical intensity within the ring increases, the refractive index also increases due to heat generation. The Kerr effect also appears.

[0009] To avoid this, it is necessary to pass through the resonance peak at high speed. Then, as shown in Figure 18(3), the pump light 34 passes the resonance peak and reaches the soliton region in the red tune, where it stops. After the pump light 34 passes through the peak of the resonance waveform 36 and injects high optical intensity into the resonator 32, it must reach the soliton region and stop. These operations result in a soliton optical comb. A high-speed wavelength shifter is used to sweep the wavelength of the pump light 34 from short to long wavelengths relative to the resonant wavelength λ2 of the resonant waveform 36 of the micro-optical resonator. An optical signal amplifier (EDFA) is also required to amplify the optical intensity attenuated by the high-speed wavelength shifter. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] U.S. Patent No. 316,829 [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] In the conventional configuration described above, 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. However, as the wavelength of the pump light is detuned less from the blue-tuned side of the resonator's wavelength, the resonant wavelength of the resonator becomes longer due to thermal expansion caused by optical energy, and the region where the optical comb becomes soliton shifts to the longer wavelength side. 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.

[0013] Originally, the region 38 where the optical comb solitonizes is very narrow, and the target region changes due to heat, making it extremely difficult to precisely position the wavelength λ1 of the pump light 34 within the soliton region. Furthermore, the high-speed wavelength shifter and optical signal amplifier (EDFA) for the pump light are complex and expensive. This also hinders efforts to miniaturize the optical frequency comb generator and reduce its chip size.

[0014] The present invention is intended to solve the above-mentioned conventional problems, and aims to provide a micro optical resonator that utilizes changes 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]

[0015] 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. The micro optical resonator of the present invention has a linear waveguide and a plurality of first to n-th ring waveguides of the same nominal diameter whose diameters are randomly distributed based on statistical probability. The first to n-th thermal deformation actuators are provided within the plurality of first to n-th ring waveguides, respectively.

[0016] When the first through nth thermally deformable actuators are heated by the first through nth actuator heaters, their diameters increase due to thermal expansion. This deformation also increases the diameter of the ring waveguide. Next, when the first through nth thermally deformable actuators are cooled simultaneously at a predetermined timing, the ring diameters of the first through nth ring waveguides shrink accordingly, and their resonant wavelengths shift toward shorter wavelengths. Although the first through nth ring waveguides have the same nominal diameter, they vary within the tolerance range, resulting in different diameters. This causes their resonant wavelengths to differ slightly.

[0017] Although it is difficult to tune the wavelength of pump light to the narrow soliton generation region of a single ring waveguide, it is sufficient to tune the wavelength of pump light to the soliton generation region of one of the first through nth ring waveguides. In other words, by providing multiple ring waveguides, the probability of generating a soliton optical comb in a single startup operation can be increased. Furthermore, if the pump light is not positioned in the soliton generation region, the cladding temperature adjustment unit raises the reference temperature of the cladding surrounding the ring waveguide by a predetermined amount, shifting the soliton generation regions of the multiple ring waveguides toward longer wavelengths. The startup operation is then performed again. In this way, if the startup of a soliton comb is not successful in a single startup attempt, the soliton generation regions of the multiple ring waveguides can be shifted slightly and the soliton optical comb startup operation repeated, enabling reliable and efficient startup of a soliton optical comb. [Effects of the Invention]

[0018] As described above, the present invention enables the launch of a soliton optical comb by fixing the wavelength of the pump light 34 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, significantly reducing costs and downsizing the device. The launch of a soliton optical comb can be performed reliably and reliably. Furthermore, robustness against changes in the external environmental temperature can be improved, enabling the stable continuous generation of a soliton optical comb. [Brief explanation of the drawings]

[0019] [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] 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 6] 4A is a timing chart illustrating the operation of the micro optical resonator according to the first embodiment of the present invention, and FIG. 4B is a graph showing the change over time in the resonant wavelength of each ring waveguide corresponding to FIG. [Figure 7] 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 8] 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 9] 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 10] 1 is a block diagram showing a process of an optical frequency generating device according to a first embodiment of the present invention. [Figure 11] 4 is a graph showing changes in the resonance wavelength of each ring waveguide caused by a cladding temperature setting unit of the frequency generating device in the first embodiment of the present invention. [Figure 12] 3A to 3C are diagrams illustrating a manufacturing process of the micro optical comb according to the first embodiment of the present invention. [Figure 13] 3 shows material characteristics of the micro optical resonator according to the first embodiment of the present invention. [Figure 14]1A and 1B are plan and cross-sectional views of a conventional micro-optical resonator. [Figure 15] FIG. 1 is an explanatory diagram illustrating the concept of mode locking of a micro optical resonator. [Figure 16] FIG. 1 is an explanatory diagram of a comb-shaped spectrum generated by a micro optical comb. [Figure 17] 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 18] 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

[0020] The invention described in claim 1 of the present invention comprises an input section that receives input of pumping light having a predetermined wavelength irradiated from a laser light source device; first to nth ring waveguides having the same nominal diameter, each of which is made of a material having a third-order nonlinear optical effect and whose diameters are randomly distributed based on statistical probability; a linear waveguide that receives the pumping light from the input section and is positioned to optically couple with the first to nth ring waveguides, and inputs evanescent light of at least a portion of the pumping light into a specific one of the first to nth ring waveguides, and extracts an optical frequency comb having a plurality of comb-like spectra with equal frequency intervals from the first waveguide; and a linear waveguide that couples the linear waveguide and the first to nth ring waveguides. The optical waveguide includes a cladding surrounding the straight waveguide and formed with a refractive index lower than that of the straight waveguide and the first to nth ring waveguides, first to nth thermal deformation actuators formed inside the first to nth ring waveguides, respectively, for increasing or decreasing the diameter of the first to nth ring waveguides, first to nth actuator heaters for heating the first to nth thermal deformation actuators, an actuator temperature sensor for detecting the temperature of the thermal deformation actuators, and a substrate for cooling the thermal deformation actuators, wherein the first to nth thermal deformation actuators increase the diameter of the first to nth ring waveguides by heating, thereby shifting the resonant wavelengths of the first to nth ring waveguides toward longer wavelengths. Thereafter, heating of the first to nth actuator heaters is stopped, and the first to nth thermal deformation actuators are cooled by the substrate. The resonant wavelengths of the first to nth ring waveguides are shifted toward shorter wavelengths.

[0021] This configuration makes it possible to eliminate the conventional wavelength sweeping operation that continuously changes the wavelength of the pump light when starting a soliton optical comb. This eliminates the need for a high-speed wavelength shifter and an EDFA (Electron-Digital Frequency Amplifier) ​​that change and sweep the wavelength of the pump light. Furthermore, the presence of multiple ring waveguides increases the probability of matching the pump light with the soliton region.

[0022] In the invention described in claim 2, the first to nth thermal deformation actuators are in contact with the first to nth actuator heaters, respectively, which improves the response speed of the first to nth thermal deformation actuators and makes it possible to more reliably perform the soliton optical comb startup process.

[0023] In the invention described in claim 3, the first to nth thermal deformation actuators are in contact with the substrate, which allows the temperature of the thermal deformation actuators to be cooled quickly, improves the response speed of the thermal deformation actuators, and more reliably performs the soliton optical comb startup process.

[0024] The invention described in claim 4 of the present invention comprises an input section that receives input of pumping light having a predetermined wavelength irradiated from a laser light source device; first to nth ring waveguides having the same nominal diameter, each of which is made of a material having a third-order nonlinear optical effect and whose diameters are randomly distributed based on statistical probability; a linear waveguide that receives the pumping light from the input section and is positioned to optically couple with the first to nth ring waveguides, and inputs evanescent light of at least a portion of the pumping light into a specific one of the first to nth ring waveguides, and extracts an optical frequency comb having a plurality of comb-like spectra with equal frequency intervals from the first waveguide; and a linear waveguide that couples the linear waveguide and the first to nth ring waveguides. The soliton optical comb includes a cladding surrounding the waveguide and formed with a refractive index lower than that of the straight waveguide and the first to nth ring waveguides, first to nth thermal deformation actuators formed inside the first to nth ring waveguides, respectively, for increasing or decreasing the diameter of the ring waveguide, first to nth actuator heaters for heating the first to nth thermal deformation actuators, actuator temperature sensors for detecting the temperatures of the thermal deformation actuators, a substrate for cooling the thermal deformation actuators, a second heater for heating the cladding to a predetermined temperature, a second temperature sensor for detecting the temperature of the cladding, and a temperature control device for controlling a reference temperature of the cladding to a value higher than the ambient temperature. After the soliton optical comb is activated, even if the ambient temperature changes, the reference temperature T2 higher than the ambient temperature is controlled by the second heater to maintain a constant value, thereby achieving stable maintenance of the soliton optical comb.

[0025] The invention described in claim 5 of the present invention includes a soliton optical comb detector that detects the soliton optical comb output from the linear waveguide, a cladding temperature adjuster that increases the reference temperature of the cladding by a fixed amount each time a soliton optical comb is started, and a soliton optical comb start-up controller that controls the start-up of the soliton optical comb. The soliton optical comb detector detects whether a soliton optical comb was generated during the initial soliton optical comb start-up operation. If not, the cladding temperature adjuster increases the set temperature by a predetermined value and commands the soliton optical comb start-up controller to start the soliton optical comb again. This start-up operation repeats this loop until a soliton optical comb is generated. This ensures the start-up of a soliton optical comb.

[0026] Hereinafter, an embodiment of the present invention will be described with reference to FIGS. 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, which has a refractive index lower than that of the material of the waveguides. The cladding 16 is formed on a silicon substrate 15. The multiple ring waveguides 12 consist of a first ring waveguide 12-1 to an n-th ring waveguide 12-n. The nominal diameter r of each ring waveguide is the same. However, the diameters are randomly distributed based on a standard deviation within the dimensional tolerance range.

[0027] Each ring waveguide is provided on the same plane with a small gap d between it and the straight waveguide 11. An input section 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 device. An output section 21 is provided at the other end of the straight waveguide 11 to output the generated soliton optical comb 22.

[0028] 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 changes in proportion to the radius r.

[0029] Figure 2 shows the resonant waveforms 24-1, 24-2, 24-3, 24-m, 24-n of multiple ring waveguides 12. The horizontal axis represents wavelength. Each resonant waveform has the same nominal radius r of the ring waveguide 12, but is randomly distributed based on its standard deviation. Figure 3 is an enlarged view of the m-th ring waveguide 12-m of the micro-optical resonator 10. The m-th ring waveguide 12-m is located at the center of the n ring waveguides. A cylindrical thermal deformation actuator 13 is placed inside each ring waveguide 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. The first to n-th actuator heaters 14 are connected in series, and a voltage is applied to terminals 14a and 14b. The actuator heater 14 is made of a platinum thin film and has a curved shape that increases electrical resistance.

[0030] Figure 4 is a cross-sectional view of the micro-optical resonator 10 of the present invention, showing the y-axis cross section of the ring waveguide 12-m in Figure 3. The top surface 13b of the cylindrical thermally deformable actuator 13 is provided with an actuator heater 14 and an actuator temperature sensor 19 for detecting the temperature of the actuator. The actuator temperature sensor 19 is installed only on the thermally deformable actuator 13-m corresponding to the ring waveguide 12-m. This actuator temperature sensor 19 is a thermocouple and uses the temperature of the cladding 16 as a reference. The ring waveguide 12-m is located at the center of the n ring waveguides, and the actuator temperature sensor 19 detects the average temperature of the n thermally deformable actuators. The thermally deformable actuator 13 is mounted on a silicon substrate 15 with excellent thermal conductivity on its bottom surface 13a, improving the cooling efficiency of the thermally deformable actuator. The thermally deformable actuator 13 is made of silicon, which has a high thermal conductivity.

[0031] Since the refractive index and resonant wavelength of the waveguide are sensitive to temperature changes, precise temperature control is required. For this reason, a second heater 17 and a second temperature sensor 18 for detecting the temperature of the cladding 16 are provided on the cladding 16 on the upper surface of the resonator.

[0032] 10 shows a system for starting a soliton optical comb. This system includes a soliton optical comb start-up controller 26 that controls the startup of the soliton optical comb, a soliton optical comb detector 28 that detects whether the output light from the output unit 21 contains a soliton optical comb, a cladding temperature setting unit 27 that resets the reference temperature of the cladding, and a temperature control device that maintains the cladding temperature at the reference temperature.

[0033] In this example, the straight waveguide 11 and the ring waveguide 12 are made of silicon nitride (Si3N4), the cladding 16 is made of silicon oxide (SiO2), and the thermal deformation actuator is made of silicon. The material properties are shown in Figure 14.

[0034] 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. 5. In FIG. 5(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, as shown in FIG. 5(b), 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.

[0035] Figure 6(a) is a timing chart when the frequency soliton optical comb is activated. When the soliton optical comb activation is turned on, the heater 14 is turned on first. After the thermal deformation actuator 13 reaches a predetermined temperature, it is turned off. At the same time that the heater 14 is turned off, the pump light 23 is turned on. Figure 6(b) shows the change in the resonance wavelength λ2 over time due to the expansion and contraction of the diameter of the ring waveguide 12. The temperature of the thermal deformation actuator 13 before heating the actuator 14 is T2, and the temperature after heating is T1. Furthermore, this change in temperature from T2 to T1 causes the resonance wavelength of the ring waveguide to change by δλ2.

[0036] 7 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 for the case of one ring waveguide among n ring waveguides in which the soliton optical comb is activated. Each position will be explained as steps (1) to (5).

[0037] In Figure 7(1), when the soliton optical comb startup command is turned on, the actuator heater 14 is turned on and heated by an external power source to a temperature T1. As a result, all of the thermally deformable actuators 13 thermally expand to reach a predetermined diameter. This deformation increases the diameter of the ring waveguide 12 located on the outer periphery of the thermally deformable actuator 13, and its resonant wavelength λ2 shifts to the longer wavelength side by Δλ2. The resonant waveform before heating is shown by a dashed line, and the resonant wavelength after heating is shown by a solid line.

[0038] In Figure 7(2), when the actuator temperature sensor 19 detects a predetermined heating temperature T1, the actuator heater 14 is turned off. At this time, excitation light 23 with a wavelength λ1 fixed at a constant value is activated. The thermally deformable actuator 13 is cooled to a reference temperature T2, and the resonant waveform moves toward the shorter wavelength side (to the left in the figure).

[0039] 7(3) shows the position where the pump light 23 overlaps the base of the resonance waveform 24 during the cooling process from temperature T1 to T2. From this point on, optical intensity is injected into the ring waveguide 12.

[0040] 7(4) shows 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, exciting four-wave mixing.

[0041] In Figure 7(5), the pump light 23 reaches the soliton region in the red tune of the resonant waveform 24. Note that for the establishment of the soliton optical comb, it is important that the pump light 23 remains in the soliton region immediately after it is injected into the ring waveguide in Figure 7(4) and the optical intensity becomes high.

[0042] The steps for establishing a soliton optical comb have been explained above. However, because the positions of the resonant wavelengths 24 are distributed, the probability of establishing a soliton optical comb in a single startup operation is low. Therefore, multiple ring waveguides are provided.

[0043] Next, using FIG. 8, we will explain soliton optical comb setup of multiple ring waveguides with the same nominal diameter and diameters randomly distributed within the tolerance. Figure 8 shows the arrangement of resonant waveforms 24-1, 24-2, ..., 24-n of the first to nth ring waveguides 12-1, 12-2, ..., 12-n, respectively, with wavelength on the horizontal axis. This corresponds to Figure 2. Also, (a) to (e) of Figure 8 show the wavelength changes of the first to nth waveguides 12 due to heating and cooling of each actuator heater 14.

[0044] 8(a), first, the resonant waveform 24-1 moves to the position indicated by the dashed line due to heating by the actuator heater 14. When the actuator heater is turned off, cooling begins, and the wavelength returns to the original wavelength λ2, exceeding the wavelength λ1 of the pump light 23. However, because the pump light 23 is not in the soliton region of the resonant waveform, no soliton optical comb is generated.

[0045] In FIG. 8(b), the pump light 23 does not enter the soliton region after the actuator heater 14 is heated and cooled, as shown in FIG. 8(b), so a soliton optical comb does not occur. Furthermore, when two or more resonance wavelengths are close to each other, the optical intensity of the pump light is dispersed between the two waveforms, and so neither soliton optical comb occurs. The probability that two or more resonance wavelengths overlap within the range of their half-widths is extremely small.

[0046] Only in Fig. 8(c) does a soliton optical comb occur when pump light 23 enters the soliton region. When a soliton optical comb is established in Fig. 8(c), the optical intensity of the pump light in the linear waveguide is consumed in generating the soliton optical comb in Fig. 8(c), and no optical intensity of the pump light is supplied downstream from Fig. 8(d) to Fig. 8(e). Therefore, once a soliton optical comb is generated in Fig. 8(c), the soliton optical comb in Fig. 8(c) is maintained thereafter.

[0047] FIG. 9 is an image diagram showing a case where the Littlon optical comb shown in FIG. 8 is generated in the ring waveguide 12-3. 8 illustrates the case where the wavelength λ1 of the pump light 23 enters the soliton region of one of the n ring waveguides, ring waveguide 12-3, i.e., a soliton optical comb is established. However, this is not usually the case, and there is a high probability that the wavelength of the pump light 23 will not enter the soliton region of the n ring waveguides. To address this, the temperature T2 of the cladding 16 is slightly increased, thereby shifting the soliton regions of all ring waveguides to the longer wavelength side, allowing the pump light 23 to enter the soliton region.

[0048] Usually, the ambient temperature T3 at which the micro optical resonator 10 is used is preset in the product specifications. A lower limit is also specified. For example, if the product specifications allow the maximum operating temperature to be 85°C, then: T3 = 85℃ Let's say. The reference temperature T2 of the cladding 16 is always set to a temperature higher than the upper limit T3 of the ambient temperature. T2>T3 Here, T2=T3+ΔT3 and ΔT3 is a temperature margin that is set in advance to take into account variations.

[0049] 10. When the soliton optical comb activation starts, the soliton optical comb activation control unit 26 first applies voltage to the terminals 14a and 14b of the actuator heaters 14, heating the first through nth actuator heaters 14 simultaneously. After that, heating continues for a predetermined time. After the actuator temperature sensor detects that the temperature has reached a predetermined temperature rise value T1, the voltage application to the terminals 14a and 14b is turned off, and heating stops. At the same time, the pump light is activated, and the first soliton optical comb is activated by cooling the thermal deformation actuator 13.

[0050] If the soliton optical comb detector 28 does not detect a soliton optical comb at this time, the cladding temperature adjustment unit 27 adds the temperature increase ΔT2 corresponding to the half-width of the resonant waveform 24 to the reference temperature T2, sets the new T2, and feeds this back to the temperature control device 25. The change in the resonant wavelength at this time is shown in Figure 11. The soliton optical comb startup operation is then performed again. Note that in this case, the change in the resonant wavelength of the ring waveguide 12 is the sum of the change due to thermal expansion and the change in the refractive index due to heat. This operation of updating and starting the reference temperature T2 is repeated until the generation of a soliton optical comb is detected. After the soliton optical comb has started up, the cladding temperature adjusting unit 27 stops and maintains the reference temperature T2 at the time of start-up.

[0051] FIG. 12 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 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 13x 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 is formed. In step (8), excess silicon is removed by chemical mechanical polishing. In step (9), a first actuator heater 14 is formed on the thermally deformable actuator 13. In step (10), an actuator temperature sensor 19 is formed on the thermally deformable actuator 13. In step (11), a silicon oxide film is formed on the first actuator heater 14 and the actuator temperature sensor 19. In step (12), a second heater 17 and a second sensor 18 are formed on the silicon oxide film.

[0052] As described above, according to this embodiment, the wavelength of the pump light is fixed at a constant value without wavelength sweeping. The probability of launching can be improved by providing the first to nth ring waveguides with the same nominal diameter, whose diameters are randomly distributed based on statistical probability, relative to the linear waveguide. Furthermore, by slightly increasing the reference temperature T2 of the cladding 16 and repeatedly launching the soliton optical comb, the soliton optical comb can be launched reliably.

[0053] In this example, the material of the thermal deformation actuator was silicon. It may also be made of silicon nitride. Even in the case of a single ring waveguide, although variations in the startup time occur, it is possible to start up a soliton optical comb. [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 Second Heater 18 Second 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 Temperature control device 26 Soliton optical comb starter / controller 27 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 Resonant waveform 37 Pulse 38 Soliton Region

Claims

1. 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 having the same nominal diameter, the diameters of which are randomly distributed based on statistical probability and made of a material having a third-order nonlinear optical effect; a linear waveguide that receives the excitation light from the input section and is positioned to be optically coupled to the first to nth ring waveguides, the linear waveguide inputting evanescent light of at least a portion of the excitation light into a specific one of the first to nth ring waveguides, and extracting an optical frequency comb having a plurality of comb-like spectra with equal frequency intervals from the first waveguide; a cladding surrounding the straight waveguide and the first to nth ring waveguides and formed with a refractive index lower than that of the straight waveguide and the first to nth ring waveguides; first to nth thermal deformation actuators formed inside the first to nth ring waveguides, respectively, for increasing or decreasing the diameter of the first to nth ring waveguides; first to nth actuator heaters for heating the first to nth thermal deformation actuators; actuator temperature sensors for detecting the temperatures of the thermal deformation actuators; and a substrate for cooling the thermal deformation actuators.

2. 3. The micro optical resonator according to claim 1, wherein the first to nth thermal deformation actuators and the first to nth actuator heaters are in contact with each other, respectively.

3. 4. The micro optical resonator according to claim 1, wherein the first to nth thermal deformation actuators are in contact with the substrate.

4. an input section that receives 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 the same nominal diameter with diameters that are randomly distributed based on statistical probability; a straight waveguide that receives the excitation light from the input section and is positioned to be optically coupled to the first to nth ring waveguides, respectively, and that inputs evanescent light of at least a portion of the excitation light into a specific ring waveguide among the first to nth ring waveguides and extracts an optical frequency comb having a plurality of comb-like spectra with equal frequency intervals from the first waveguide; and a straight waveguide that surrounds the straight waveguide and the first to nth ring waveguides and extracts an optical frequency comb having a plurality of comb-like spectra with equal frequency intervals from the first waveguide. and claddings formed with a refractive index lower than the refractive index of the first to nth ring waveguides; first to nth thermal deformation actuators formed inside the first to nth ring waveguides, respectively, for increasing or decreasing the diameter of the ring waveguides; first to nth actuator heaters for heating the first to nth thermal deformation actuators; actuator temperature sensors for detecting the temperatures of the thermal deformation actuators; a substrate for cooling the thermal deformation actuators; a second heater for heating the cladding to a predetermined temperature; a second temperature sensor for detecting the temperature of the cladding; and a temperature control device for controlling a reference temperature of the cladding to a value higher than the ambient temperature.

5. 5. The optical frequency comb generator according to claim 4, further comprising: a soliton optical comb detector for detecting the soliton optical comb output from the linear waveguide; a cladding temperature adjusting unit for increasing the reference temperature of the cladding by a fixed amount each time the soliton optical comb is started up; and a soliton optical comb start-up control unit for controlling the start-up of the soliton optical comb.

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