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 because of temperature rise, so that it has been difficult to shift a wavelength λ1 of excitation light 34 to a region to form soliton accurately; thus it is necessary to consider to suppress the thermal wavelength shift due to thermal expansion of a waveguide.SOLUTION: Like a first fixing plate 13 provided inside a first waveguide 12 for restricting the thermal deformation of the first waveguide, a second fixing plate with a donut shape is provided outside the first waveguide. The thermal deformation generated due to light intensity is restricted when the comb of the first waveguide is started. Thus, the certainty of the soliton optical comb starting operation can be secured and the optical comb device with a simpler and smaller structure can be achieved.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 for 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 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 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 7. Figure 7(a) shows a plan view of the waveguide, and Figure 7(b) shows a cross-sectional view of the same. Note that Figure 7(b) is a cross-sectional view taken along the x-axis of Figure 7(a). Reference numeral 30a denotes a first cladding layer formed of silicon dioxide (SiO2) 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. Note that 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] Although not shown, an input unit 33 that receives input of excitation light emitted from a CW laser light source device is connected to the waveguide 31, and excitation light 34, which is a CW laser, is incident on the waveguide 31 from the CW laser light source device.

[0008] The incident pumping light 34 is assumed to be a continuous wave having a wavelength λ1. Figure 8(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 waveform of the resonator 32, the pumping light 34 seeps 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 constructive interference occurs, generating a pulse 37. This state is called mode locking. This pulse is also called an optical comb. Figure 9 shows the concept of modes.

[0010] The mode spacing is expressed as 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, an optical comb has a spectrum in which modes are arranged at equal intervals like a comb, as shown in Figure 8(b).

[0011] 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 waveform 36 of the micro-optical resonator.

[0012] Here, wavelength sweeping of a micro optical resonator will be explained with reference to FIG. 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 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 34 resonates as an evanescent wave from the waveguide 31, and coupling power is guided into the resonator 32. Therefore, the optical power in the resonator 32 gradually increases. When the coupling power further exceeds the nonlinear optical effect, a comb mode is generated by four-wave mixing (FWM).

[0013] Comb-like spectra generated using micro-optical resonators 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.

[0014] In order to generate a soliton optical comb, it is necessary to capture a soliton region 38 in which the wavelength λ1 of the pump light 34 transitions to red detune, exceeding the zero-tuned wavelength λ2.

[0015] Figure 11 shows the process by which pump light 34 reaches the red-detuned soliton region. In Figure 11(1), the wavelength λ1 of pump light 34 shifts toward the longer wavelength side. In Figure 11(2), when the pump light 34 is applied to the resonant waveform, optical energy gradually flows into the resonator 32.

[0016] This optical energy heats the resonator 32. This heat causes the ring diameter of the resonator 32 to expand, shifting the resonant wavelength λ2 to the longer wavelength side. Furthermore, if the optical intensity inside the ring increases, the Kerr effect will appear. To avoid this, it is necessary to pass through the resonance peak at a high speed. Then, as shown in Figure 11(3), the pump light 34 passes the resonance peak and reaches the soliton region, where it stops. These operations result in a soliton optical comb. [Prior art documents] [Patent documents]

[0017] [Patent Document 1] U.S. Patent No. 10,268,100 [Non-patent literature]

[0018] [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]

[0019] 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 a comb mode must be generated by four-wave mixing (FWM).

[0020] However, as the wavelength of the pump light approaches that of the resonator, the resonant wavelength of the resonator becomes longer due to expansion caused by heat generated by the optical energy, and the region where the optical comb solitonizes shifts to the longer wavelength side. Therefore, the influence of heat has been a major problem in accurately shifting the wavelength λ1 of the pump light 34 into the region where solitonization occurs.

[0021] Furthermore, the small gap d between the resonator 32 and the waveguide 31 is usually processed by dry etching. The sides of the gap are not smooth and tend to scatter light, and there are processing limitations on how much smaller the gap d can be made.

[0022] The present invention is intended to solve the above-mentioned conventional problems, and has as its object to provide a micro optical resonator that suppresses the influence of changes in the resonant wavelength of the resonator due to heat. [Means for solving the problem]

[0023] To achieve this objective, the optical frequency comb generator of the present invention includes a first fixing section formed inside the first waveguide via a doughnut-shaped first cladding section, and a second fixing section formed outside the first waveguide via a doughnut-shaped second cladding section, to prevent deformation of the resonator due to heat generated by guiding light. Here, the fixing section has a higher Young's modulus than the cladding section. This configuration provides a micro optical resonator that suppresses changes in the resonant wavelength of the first waveguide due to heat from optical energy. [Effects of the Invention]

[0024] As described above, the present invention has the excellent effect of minimizing the change in resonant frequency due to thermal expansion caused by the optical intensity within the ring resonator when starting up a micro optical comb, and of more accurately guiding the wavelength of the pump light to the wavelength range in which the soliton optical comb is started up.

[0025] In addition, when coupling the second waveguide and the first waveguide, the waveguide surface can be polished to a mirror finish by chemical mechanical polishing, minimizing the effects of light scattering. Furthermore, in the processing step (12) in Figure 5, the ring side surface of the first waveguide 12 can be polished to a mirror finish, preventing scattering of light circulating within the first waveguide and improving resonance characteristics.

[0026] Furthermore, it is possible to suppress the change in the refractive index of the waveguide due to thermal deformation caused by the vibration of the light waves, thereby suppressing the phase noise of the soliton optical comb. [Brief explanation of the drawings]

[0027] [Figure 1] 1 is a plan view of a micro optical resonator according to a first embodiment of the present invention. [Figure 2] 1 is a cross-sectional view of a micro optical resonator according to a first embodiment of the present invention. [Figure 3] 3 shows material characteristics of the micro optical resonator according to the first embodiment of the present invention. [Figure 4] 10 is a diagram showing the sweep process of excitation light for explaining the operation of the micro optical resonator according to the first embodiment. FIG. [Figure 5] 3A to 3C are diagrams illustrating a manufacturing process of the micro optical comb according to the first embodiment of the present invention. [Figure 6] 10 is a diagram showing the effect of increasing the rigidity of the first waveguide against thermal deformation by providing a fixing plate. FIG. [Figure 7] 1A and 1B are plan and cross-sectional views of a conventional micro-optical resonator. [Figure 8] FIG. 1 is an explanatory diagram of a comb-shaped spectrum generated by a micro optical comb. [Figure 9] FIG. 1 is an explanatory diagram illustrating the concept of mode locking of micro-optical resonance. [Figure 10] FIG. 2 is a diagram illustrating a resonant waveform of a resonator. [Figure 11] 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

[0028] The invention described in claim 1 of the present invention comprises an input unit that receives an input of excitation light having a predetermined frequency irradiated from a laser device; a circular first waveguide made of a material having a third-order nonlinear optical effect; a second waveguide that is formed on a plane parallel to the plane on which the first waveguide is formed and is optically coupled to the first waveguide to input at least a portion of the excitation light into the first waveguide and extract an optical frequency comb having a comb-like spectrum with equal frequency intervals from the first waveguide; a clad layer formed between a surface on which the first waveguide is formed and a surface on which the second waveguide is formed; and a doughnut-shaped first clad portion provided inside the first waveguide. The waveguide comprises a first fixing portion provided inside the first cladding portion and having a higher Young's modulus than the material of the first cladding portion, a donut-shaped second cladding portion provided outside the first waveguide, and a second fixing portion provided along the outer periphery of the second cladding portion and having a higher Young's modulus than the material of the second cladding portion. This configuration prevents heat generation due to light introduced from the second waveguide and thermal deformation of the ring shape, thereby suppressing changes in the resonance wavelength of the first waveguide.

[0029] The invention according to claim 2 of the present invention is characterized in that the material of the first fixed portion and the second fixed portion is the same as that of the first waveguide, thereby simplifying the manufacturing process.

[0030] The invention described in claim 3 of the present invention is characterized in that a first interface between the first waveguide and the cladding layer and a second interface between the second waveguide and the first interface between the second waveguide and the cladding layer have mirror surfaces obtained by precision polishing, which can improve the optical coupling efficiency between the second waveguide and the first waveguide.

[0031] The invention described in claim 4 of the present invention includes an input unit that receives an input of excitation light having a predetermined frequency irradiated from a laser device; a circular first waveguide made of a material having a third-order nonlinear optical effect; a second waveguide that is formed on a plane parallel to the plane on which the first waveguide is formed and is optically coupled to the first waveguide to input at least a portion of the excitation light into the first waveguide and extract an optical frequency comb having a plurality of comb-like spectra with equal frequency intervals from the first waveguide; and the first fixing portion is formed inside the first waveguide, has a doughnut-shaped first cladding portion interposed therebetween, and has a higher Young's modulus than the material of the first cladding portion. This configuration prevents the ring shape of the first waveguide from being thermally deformed due to heat generated by light introduced from the second waveguide, and suppresses changes in the resonant wavelength of the first waveguide.

[0032] In the invention as set forth in claim 5, the material of the fixing portion is the same as that of the first waveguide, thereby simplifying the manufacturing process.

[0033] The invention described in claim 6 of the present invention is characterized in that a first interface between the first waveguide and the cladding layer and a second interface on the opposite side to the first interface between the second waveguide and the cladding layer have mirror surfaces obtained by precision polishing, and it is possible to improve the optical coupling efficiency between the second waveguide and the first waveguide.

[0034] Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

[0035] (Embodiment 1) 1 is a plan view of the optical frequency comb generator of the present invention, and FIG. 2 is a cross-sectional view thereof. A laser light source 20 is placed at the end of the second waveguide 11.

[0036] The second waveguide 11 is located below the circular first waveguide 12 in the z direction, and is provided parallel to the height direction with a small gap d (see FIG. 2) provided therebetween. In addition, the second waveguide 11 is arranged so as to overlap the outer periphery of the first waveguide 12 on the xy plane.

[0037] A circular first fixing plate 13 is disposed inside the first waveguide 12 at a uniform distance g via a first clad portion 21. A second fixing plate 14 is disposed outside the first waveguide 12 at a uniform distance g from the outer side surface of the first waveguide 12. A doughnut-shaped first clad portion 21 is formed between the first waveguide 12 and the first fixing plate 13 to prevent light circulating within the first waveguide from leaking to the outside. Similarly, a doughnut-shaped second clad portion 22 is formed between the first waveguide 12 and the second fixing plate 14. The width g of the first clad portion and the second clad portion is set small enough not to affect the light passing through the first waveguide.

[0038] In FIG. 2, clad layers 16, 17, 18, and 19 are formed in a laminated structure on a substrate 15. A second waveguide 11 is formed in the clad layer 16 by etching a groove of a predetermined depth and then filling the groove with silicon nitride (SiO2). A clad layer 17 is formed on the clad layer 16 with a predetermined thickness d, which determines the distance (in the Z direction) between the second waveguide 11 and the first waveguide 12. The first waveguide 12, the first fixing plate 13, and the second fixing plate 14 are formed in the clad layer 18, and a clad layer 19 is formed thereon to form a micro-optical resonator. Here, a first interface 26 between the clad layer 17 and the clad layer 18 and a second interface 27 between the clad layer 17 and the clad layer 16 are mirror-finished by precision chemical mechanical polishing.

[0039] The radius r of the first waveguide is the repetition frequency of the mode period, Frep=c / (2nL) L=2πr where n is the effective refractive index of the first waveguide. The second waveguide 11 and the first waveguide 12 are made of silicon nitride (Si3N4) which has a third-order nonlinear optical effect. The material of the cladding layer and cladding portion is an optical material having a smaller refractive index than the material of the first waveguide, such as silicon oxide (SiO2). The material properties in this example are shown in FIG.

[0040] Furthermore, it is desirable for the material of the fixing plate to have a larger modulus of longitudinal elasticity, but when considering the manufacturing process, it is more reasonable to use silicon nitride, the same material as the first waveguide. Silicon nitride has a modulus of longitudinal elasticity about four times that of silicon oxide, ensuring sufficient performance.

[0041] Figure 6 shows a comparison of the ability of fixing plates to suppress thermal deformation of the ring waveguide. (a) shows a model without the first fixing plate 13 and second fixing plate 14. (b) shows the case where only the first fixing plate is provided. (c) shows the case where the first fixing plate 13 and second fixing plate 14 are provided. The fixing plates are made of silicon nitride, the same material as the ring waveguide, and the cladding 18 is made of silicon oxide.

[0042] The graph shows the thermal deformation suppression force when the second waveguide is thermally deformed due to the injection of optical energy. Model (a) is set to 1 as the reference, and the rigidity of models (b) and (c) at suppressing deformation is compared. Model (c) is more than three times more rigid than when no fixing plate is used.

[0043] (Operation of the first embodiment) The operation of the micro optical resonator configured as above will be explained with reference to Fig. 4. Fig. 4 is a diagram showing how the pump light 23 approaches from the long wavelength side toward zero tune.

[0044] First, in Fig. 4(1), the pumping light 23 is swept (shifted) in the → direction. Then, in Fig. 4(2), when the pumping light 23 reaches the base of the resonant waveform 28, optical power is gradually injected. After that, it moves beyond the peak of the resonant waveform 28. At this time, the pumping light 23 irradiated from the laser light source 20 acts as an evanescent wave, and coupling power is injected from the second waveguide 11 into the first waveguide 12. Then, the first waveguide 12 is heated by the optical power of the pumping light 23.

[0045] However, here, the first fixing plate 13 and the second fixing plate 14 are not heated. The reason for this is that, first, the first fixing plate 13 and the second waveguide 11 are spaced far enough apart that light is not coupled, and therefore light does not reach the first fixing plate, causing no deformation due to heating. The second fixing plate 14 and the second waveguide 11 overlap, separated by a distance d, the thickness of the cladding layer 16. However, because the average radius is significantly different from the radius of the first waveguide 12, the resonant frequency does not match the wavelength of the excitation light 23. Therefore, no resonance occurs and no heat is generated. Therefore, like the first fixing plate 13, there is no deformation due to heating.

[0046] Therefore, when excitation light 23 is input at the resonant wavelength of first waveguide 12, the resonant wavelength of first waveguide 12 tends to increase due to thermal expansion, but the deformation of first waveguide 12 is restricted by first fixing plate 13 and second fixing plate 14, which are made of materials with large longitudinal elastic modulus. Therefore, there is no change in the ring diameter, and there is little movement of the resonant waveform.

[0047] Furthermore, since the first waveguide 31 is located at a small distance g from the fixed plates 13 and 14, the heat generated in the first waveguide 31 moves to the fixed plates. This is expected to have the effect of suppressing the temperature rise of the first waveguide, thereby reducing thermal deformation. Therefore, the resonant wavelength is less likely to change, making it easier to more reliably match the pump light 23 with the soliton region.

[0048] FIG. 5 shows the manufacturing process of the micro optical comb. In step (1), a silicon oxide film is formed on a substrate 15 to form a cladding layer 16 . In step (2), a groove 25 for forming a second waveguide is processed. In step (3), a silicon nitride film is formed and buried in the groove 25 for forming the second waveguide. In step (4), the silicon nitride film formed in step (3) is removed by chemical mechanical polishing (CMP) to remove excess silicon nitride and polish the top surface of the second waveguide to a mirror finish. This allows the top surface 11a of the second waveguide facing the first waveguide to be polished to a mirror finish. The top surface 11a is flush with the second interface 27. In step (5), a silicon oxide cladding layer 17 is formed. In step (6), the cladding layer 17 is polished by CMP to a predetermined thickness. The thickness d of the cladding layer 17 requires precision in order to transmit the evanescent light from the second waveguide 11 to the first waveguide 12. In addition, the first interface 26 has a mirror surface. Step (7) is a step of applying a resist 24 for forming a first waveguide. In step (8), the resist is patterned into the shape of the first waveguide. In step (9), silicon nitride films are formed for the first waveguide and the fixing plate. At this time, the lower surface 12a of the first waveguide is formed on the cladding layer 17 polished by CMP, and therefore has a mirror-like surface. In step (10), the silicon nitride is polished by CMP. In step (11), the resist is removed. In step (12), the side wall 12b of the ring is smoothed by wet etching to give it a mirror finish, thereby minimizing scattering of light circulating within the first waveguide. In step (13), the cladding layer 18 is formed. In step (14), the upper surface is polished by CMP to form the first waveguide 12. In step (15), a cladding layer 19 is formed on the upper surface to complete the process.

[0049] As described above, according to this embodiment, there is provided an input section that receives input of excitation light having a predetermined frequency irradiated from a laser device; a circular first waveguide made of a material having a third-order nonlinear optical effect; a second waveguide that is formed on a plane parallel to the plane on which the first waveguide is formed and that is optically coupled to the first waveguide to input at least a portion of the excitation light into the first waveguide and extract an optical frequency comb having a plurality of comb-like spectra with equal frequency intervals from the first waveguide; a cladding layer that is formed between the plane on which the first waveguide is formed and the plane on which the second waveguide is formed; and a doughnut-shaped first cladding section that is provided inside the first waveguide. A configuration including a first fixing part provided inside the first cladding part and having a higher modulus of elasticity than the material of the first cladding part, a donut-shaped second cladding part provided outside the first waveguide, and a second fixing part provided along the outer periphery of the second cladding part and having a higher modulus of elasticity than the material of the second cladding part makes it possible to more reliably launch a soliton optical comb.

[0050] In this embodiment, even if only the first fixing portion is provided without the second fixing portion, or even if only the second fixing portion is provided without the first fixing portion, the effect of suppressing thermal deformation of the first waveguide is not lost. However, when both the first and second fixing portions described in this embodiment are provided, the effect of suppressing thermal deformation of the first waveguide is naturally greater. [Explanation of symbols]

[0051] 11 Second waveguide 11a: top surface of second waveguide 12 First waveguide 12a Lower surface of first waveguide 12b Sidewall surface of the first waveguide 13 First fixing plate 14 Second Fixation 15 PCB 16 Cladding layer 17 Cladding layer 18 Cladding layer 19 Cladding layer 20 Laser light source 21 First clad section 22 First clad section 23 Excitation light 24 Resist 25 groove 26 First Interface 27 Second Interface 28 Resonance waveform 29 Silicon substrate 30a First cladding layer 30b Second cladding layer 31 Waveguide 32 resonator 33 Input section 34 Excitation light 35 Thermal change in resonant wavelength of the resonator (δλ) 36 Resonance waveform 37 Pulse 38 Soliton Region

Claims

1. an input section that receives an input of excitation light having a predetermined frequency irradiated from a laser device; a circular first waveguide made of a material having a third-order nonlinear optical effect; a second waveguide that is formed on a plane parallel to the plane on which the first waveguide is formed and that is optically coupled to the first waveguide to input at least a portion of the excitation light into the first waveguide and extract an optical frequency comb having a comb-like spectrum with equal frequency intervals from the first waveguide; a cladding layer formed between a surface on which the first waveguide is formed and a surface on which the second waveguide is formed; a doughnut-shaped first cladding portion provided inside the first waveguide; a first fixing portion provided inside the first cladding portion and having a higher Young's modulus than a material of the first cladding portion; a doughnut-shaped second cladding portion provided outside the first waveguide; and a second fixing portion provided along an outer periphery of the second cladding portion and having a higher Young's modulus than a material of the second cladding portion.

2. 2. The micro optical resonator according to claim 1, wherein the material of the first fixed portion and the second fixed portion is the same as that of the first waveguide.

3. 3. The micro-optical resonator according to claim 1, wherein a first interface between the first waveguide and the cladding layer and a second interface between the second waveguide and the cladding layer have mirror surfaces.

4. a first circular waveguide formed on a plane parallel to the plane on which the first waveguide is formed and optically coupled to the first waveguide so as to input at least a portion of the excitation light into the first waveguide and extract an optical frequency comb having a plurality of comb-like spectra with equal frequency intervals from the first waveguide; and a first fixing part formed inside the first waveguide and provided via the doughnut-shaped first cladding part, the first fixing part having a higher longitudinal elastic modulus than the material of the first cladding part.

5. 5. The micro optical resonator according to claim 4, wherein the material of the first fixed portion is the same as that of the first waveguide.

6. 6. The micro-optical resonator according to claim 4, wherein the cladding layer formed between the surface on which the first waveguide is formed and the surface on which the second waveguide is formed, a first interface between the first waveguide and the cladding layer, and a second interface between the second waveguide and the first interface between the second waveguide and the cladding layer have mirror surfaces.

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