A method for generating low-repetition-rate soliton microcombs for aluminum nitride integrated optical microcavities

Abstract

The present invention discloses a method for generating a low-repetition-rate soliton microcomb for an aluminum nitride integrated optical microcavity. By using an aluminum nitride integrated optical microring resonator with a photonic crystal structure, the present invention translates the microcavity resonance peak near the stimulated Raman gain peak of aluminum nitride to reduce the effect of the stimulated Raman effect, rendering the competition between Raman gain and Kerr nonlinear gain ineffective, and enabling the generation of a soliton optical frequency comb in an aluminum nitride optical microcavity with a repetition rate less than 100 GHz. The present invention is a novel and effective method, which can enable the aluminum nitride microcavity to still generate a soliton-state optical microcomb at a low repetition rate less than 100 GHz, effectively solving the limitations in aspects such as waveguide design and fabrication when using an aluminum nitride optical microcavity to generate a soliton microcomb. The lower repetition rate can be compatible and adapted with existing mature electronic devices and equipment, and can be used in fields such as astronomical spectroscopy, microwave photonics, and dense integrated optical communication.

Description

Technical Field

[0001] The present invention relates to nonlinear optics, nonlinear frequency conversion, optical solitons, integrated optical microcavities, and the generation of integrated optical frequency combs. Specifically, it relates to a method for generating a low-repetition-rate soliton microcomb for an aluminum nitride integrated optical microcavity. By using an aluminum nitride integrated optical microring resonator with a photonic crystal structure, the competition between the Raman gain of the aluminum nitride material and the Kerr nonlinear gain fails, and a soliton optical frequency comb can be generated in an aluminum nitride optical microcavity with a repetition rate (free spectral range) less than 100 GHz. This method is mainly applied to the generation and application fields of integrated optical soliton microcombs. Background Art

[0002] In the current fields of integrated optics and nonlinear optics, the research on generating soliton-state optical microcombs in integrated optical microresonators based on external coherent injection of tunable laser sources has always been a research hotspot. Soliton microcombs can be generated on many integrated optical waveguide platforms, require extremely low pump energy, have a spectrum composed of frequency-comb teeth with equal frequency intervals and phase-locked, and a bandwidth that can exceed one octave. Due to the dual balance of dispersion and nonlinearity, loss and gain, they have a very stable structure and extremely low noise, making them very high-quality ultra-wideband coherent light sources on integrated chips, which can be used in many fields such as spectroscopy, metrology, astronomy, and optical communication.

[0003] Among the reported integrated optical waveguide platforms for soliton microcomb generation, aluminum nitride optical microcavities have strong competitiveness. Aluminum nitride materials are easy to integrate, have an extremely wide transparent window, can cover the deep ultraviolet to mid-infrared bands, have a relatively high refractive index, and simultaneously have relatively high second- and third-order nonlinear effects, and a very large bandgap, with almost no influence of multi-photon absorption effects, making it an ideal choice for nonlinear optical interactions. In addition, the second-order nonlinear effect of aluminum nitride is crucial for chip-based f-2f self-referenced locked carrier-envelope phase, making the fully integrated self-referenced soliton microcomb generation feasible. However, aluminum nitride materials naturally have strong stimulated Raman scattering, and its Raman gain competes with the Kerr nonlinear gain required for soliton generation, thus suppressing the excitation of soliton microcombs. Existing reported soliton microcomb generations all require aluminum nitride microcavities with a relatively high repetition rate, so that the Raman gain peak falls in the middle of the microcavity resonance peak, and the frequency overlap part is extremely small. This requires a repetition rate greater than 100 GHz or even up to several hundred GHz. Such a high repetition rate makes its detection and compatibility with existing electronic devices very difficult, and a new method is needed to solve the generation of soliton microcombs in low-repetition-rate (less than 100 GHz) aluminum nitride microcavities. Therefore, how to reduce the competition between Raman gain and Kerr gain in aluminum nitride microcavities is the goal of our design of integrated aluminum nitride waveguides.

[0004] Photonic crystal structure integrated microcavities have been reported in recent research. By periodically modulating the width (or thickness) of an aluminum nitride waveguide along the light transmission direction of the microcavity, the Bragg bandgap effect can be generated to excite the backward-propagating light field, regulate the effective refractive index of the microcavity, and then adjust the resonant peak frequency of the microcavity. According to whether the modulation period number is odd or even, the resonant peak of the microcavity can be selectively translated or split to achieve flexible control of the dispersion near the resonant peak. Importantly, for the translated or split microcavity resonant peak, its optical quality factor will not be affected and can remain basically the same as that of the original microcavity, thus not affecting the excitation conditions of the soliton microcomb. Through the photonic crystal microcavity structure, the two resonant peaks near the stimulated Raman peak of the aluminum nitride microcavity can be flexibly translated backward, making the overlapping part of the Raman gain peak and the resonant peak extremely small, thereby weakening the role of Raman gain and eliminating the competitive influence of Raman gain on Kerr gain, enabling the Kerr gain necessary for the generation of soliton microcomb to play a dominant role. Summary of the Invention

[0005] The object of the present invention is to provide a method for generating low-repetition-rate soliton microcomb for aluminum nitride integrated optical microcavities in view of the deficiencies of the prior art. This method uses an aluminum nitride integrated optical microring resonator with a photonic crystal structure to translate the microcavity resonant peak near the aluminum nitride stimulated Raman gain peak, thereby reducing the effect of the stimulated Raman effect, making the competition of Raman gain for Kerr nonlinear gain fail, and enabling the generation of soliton optical frequency combs in low-repetition-rate aluminum nitride optical microcavities with a repetition rate (free spectral range) less than 100 GHz.

[0006] The method for generating low-repetition-rate soliton microcomb provided by the present invention uses a tunable continuous laser as the pump light source. The tunable continuous laser can continuously tune the wavelength in real time in the communication band. Compared with pulsed laser sources, continuous light sources have lower prices, more mature preparation technologies, more portable laser volumes, and very low maintenance costs. By injecting the tunable continuous laser source into the optical microcavity from the outside and scanning the frequency of the laser from high frequency to low frequency to sweep through the resonant peak of the microcavity, the cascaded four-wave mixing effect is excited to generate soliton microcomb.

[0007] For the aluminum nitride integrated optical waveguide described in the present invention, single-crystal aluminum nitride is used as the waveguide core layer material. The aluminum nitride thin film grows on the sapphire material substrate along the c-plane (0001) direction, and the aluminum nitride ridge waveguide core layer structure is realized by photolithography. This ridge structure is composed of a thinner rectangular flat layer and a trapezoidal strip layer, and the aluminum nitride waveguide core layer is covered with a silica cladding.

[0008] The aluminum nitride integrated optical microcavity described in the present invention adopts a microring resonator structure. The microring resonator structure of the aluminum nitride waveguide core layer is coupled to the aluminum nitride ring cavity waveguide through a straight waveguide. By controlling the gap distance between the straight waveguide and the aluminum nitride ring cavity, the percentage of the optical field coupled into the aluminum nitride ring cavity is adjusted; by controlling the radius of the aluminum nitride ring cavity, the repetition frequency of the aluminum nitride optical microcavity is made less than 100 GHz.

[0009] The method for generating low-repetition-frequency soliton microcombs provided by the present invention adopts a novel photonic crystal structure microring resonator. By periodically modulating the width (or thickness) of the aluminum nitride waveguide along the optical transmission direction of the microcavity, the Bragg bandgap effect can be generated to excite the backward-propagating optical field, and the effective refractive index of the microcavity can be flexibly regulated, thereby adjusting the resonant peak frequency of the microcavity. According to whether the number of modulation periods is odd or even, the resonant peak of the microcavity can be selectively translated or split, realizing flexible control of the dispersion near the resonant peak. Through the photonic crystal microcavity structure, the two resonant peaks near the stimulated Raman peak of the aluminum nitride microcavity are translated backward, so that the overlapping part of the Raman gain peak and the resonant peak frequency is extremely small, thereby weakening the effect of Raman gain and eliminating the competitive influence of Raman gain on Kerr gain.

[0010] The present invention proposes to use the aluminum nitride integrated optical microcavity with a photonic crystal structure to eliminate the competitive influence of the strong stimulated Raman gain of the aluminum nitride material itself on the Kerr nonlinear gain. By externally injecting a tunable continuous laser source into the microring resonator, soliton microcombs are generated through frequency scanning. This method can generate soliton microcombs in aluminum nitride microcavities with a repetition frequency lower than 100 GHz, and has a small size and is easy to integrate, and can be realized using a single photonic crystal structure aluminum nitride microcavity. This method has universality and can be extended to other high-nonlinear integrated optical microcavities with strong stimulated Raman effects.

[0011] The beneficial effects of the present invention:

[0012] (1) The present invention uses aluminum nitride as the integrated optical waveguide material. The aluminum nitride material is inexpensive and easy to obtain, easy to integrate and hetero-integrate, has an extremely wide transparent window, can cover the deep ultraviolet to mid-infrared bands, has a relatively high refractive index, takes into account relatively high second-order and third-order nonlinear effects, and has a very large bandgap, with almost no influence of multi-photon absorption effects. In addition, the second-order nonlinear effect of aluminum nitride is crucial for chip-based f-2f self-referenced locked carrier-envelope phase, making fully integrated self-referenced soliton microcomb generation feasible;

[0013] (2) The present invention adopts a straight waveguide-coupled ring cavity structure as the resonant microcavity. The micro-ring resonator has a compact volume and is easy to fabricate. It belongs to a two-dimensional microcavity structure, and the supported modes are relatively simple, making it easy to maintain fundamental mode transmission. It is not affected by multimode nonlinear cross-coupling, and the fundamental mode transmission can be obtained by controlling the coupling direction angle of the injected pump light. The percentage of the optical field coupled into the ring cavity can be adjusted by controlling the gap distance between the straight waveguide and the ring cavity. By controlling the radius of the ring cavity, the repetition frequency of the optical microcavity is made less than 100 GHz;

[0014] (3) The present invention adopts a novel photonic crystal structure to optimize the micro-ring resonator. By reversely translating two resonant peaks near the stimulated Raman peak of the aluminum nitride microcavity, the overlapping part of the Raman gain peak and the resonant peak in frequency is made extremely small, thereby weakening the effect of Raman gain and eliminating the competitive influence of Raman gain on Kerr gain. The optical quality factor of the translated resonant peak is not affected and still meets the conditions for soliton microcomb excitation. The fabrication process of the photonic crystal structure is relatively mature and does not introduce complexity;

[0015] (4) The present invention uses an externally injected tunable continuous laser source into the micro-ring resonator to generate soliton microcombs through frequency scanning. This method can generate soliton microcombs in aluminum nitride microcavities with a repetition frequency lower than 100 GHz. The low repetition frequency is compatible with existing mature electronic devices and test equipment. Moreover, the overall size is small and easy to integrate, and it can be realized using a single photonic crystal structure aluminum nitride microcavity. This method has universality and can be extended to other high-nonlinear integrated optical microcavities with strong stimulated Raman effects. Description of the Drawings

[0016] Figure 1 It is a schematic cross-sectional view of an aluminum nitride optical waveguide.

[0017] Figure 2 It is a top view schematic of an aluminum nitride integrated optical microcavity with a photonic crystal structure.

[0018] Figure 3 It is a schematic diagram of the experimental device system for soliton microcomb generation.

[0019] Figure 4 It is an instantaneous frequency domain simulation diagram of the generated soliton microcomb.

[0020] Figure 5 It is an instantaneous time domain simulation diagram of the generated soliton microcomb. Detailed Embodiments

[0021] The present invention will be further described below in conjunction with the drawings and specific implementation examples of the method for generating low-repetition-frequency soliton microcombs for aluminum nitride integrated optical microcavities.

[0022] The present invention discloses a method for generating a low-repetition-rate soliton microcomb for an aluminum nitride integrated optical microcavity. By using an aluminum nitride integrated optical microring resonator with a photonic crystal structure, the present invention translates the microcavity resonance peaks near the stimulated Raman gain peak of aluminum nitride, thereby reducing the effect of the stimulated Raman effect, making the competition between Raman gain and Kerr nonlinear gain fail, and enabling the generation of soliton optical frequency combs in aluminum nitride optical microcavities with a repetition rate less than 100 GHz. The present invention is a novel and effective method, which can enable aluminum nitride microcavities to still generate soliton-state optical microcombs under low repetition rates less than 100 GHz, effectively solving the limitations in waveguide design, fabrication, etc. when using aluminum nitride optical microcavities to generate soliton microcombs. The lower repetition rate can be compatible with existing mature electronic devices and equipment, and can be used in fields such as astronomical spectroscopy, microwave photonics, and dense integrated optical communication.

[0023] Figure 1 It is a schematic cross-sectional view of an aluminum nitride optical waveguide. The substrate 4 is sapphire, the waveguide core layer 3 is single-crystal aluminum nitride, the first cladding layer 2 is silicon dioxide, and the second cladding layer 1 is air.

[0024] Figure 2 It is a top view schematic diagram of an aluminum nitride integrated optical microcavity with a photonic crystal structure. 5 is an aluminum nitride strip straight waveguide, and 6 is an aluminum nitride ring cavity. It can be seen that along the transmission direction of the ring cavity, the width of the aluminum nitride waveguide is periodically modulated, and the modulation period number is an odd multiple 2m - 1 of the smaller one m among the mode numbers of the two microcavity resonance peaks to be translated.

[0025] In the simulation calculation, the width of the aluminum nitride optical waveguide core layer is 1500 nm, the core layer height is 830 nm, the flat waveguide height of the core layer is 400 nm, the thickness of the first cladding layer is 4 μm, and the thickness of the substrate layer is 1000 μm, which is easy to achieve in actual processes; the nonlinear refractive index of aluminum nitride is 2.3×10 -19 m 2 / w, the stimulated Raman gain peak value is 0.45 cm / GW, the Raman frequency shift is 18.3 THz, the Raman gain linewidth is 138 GHz, the linear transmission loss of the aluminum nitride microcavity is 0.1 dB / cm, the optical quality factor > 1×10 6 , the repetition rate of the microcavity is 80.8 GHz, the mode numbers of the resonance peaks to be translated are 226 and 227, the resonance peak translation amount is 10 GHz, and such an aluminum nitride microring resonator is easy to process in actual processes; the simulation is based on the normalized coupled Ikedamap external drive damping transmission equation, the power of the continuous light pump source is 600 mW, and the frequency scanning time is 0.4 μs.

[0026] Figure 3It is a schematic diagram of the experimental device system for soliton microcomb generation. The tunable continuous laser source 7 outputs pump laser, which is amplified by the erbium-doped fiber amplifier 8 and then enters the bandpass filter 9 to filter out the ASE noise introduced by the amplifier. Then it enters the polarization controller 10 to be adjusted to the TM transmission mode. Subsequently, it passes through the lens fiber 11 and is incident on the photonic crystal aluminum nitride microcavity 12 from the end face. The output light of the microcavity is led out by the lens fiber 13, and is split into three paths by the beam splitter 14, and respectively enters the spectrometer 17 to observe the frequency-domain pattern, enters the detector 15 connected to the oscilloscope 18 to monitor the change of the microcavity output power, and enters the detector 16 connected to the radio frequency spectrum analyzer 19 to observe the low-frequency noise characteristics.

[0027] Figure 4 It is the instantaneous frequency-domain simulation diagram of the generated soliton microcomb. It can be seen that through the photonic crystal aluminum nitride integrated optical microcavity, the soliton state optical frequency comb is successfully excited. The frequency comb presents an equally spaced and discrete comb tooth structure in the frequency domain, and its frequency interval is strictly equal to 80.8 GHz, and dozens of comb teeth can be excited simultaneously. From the envelope of the frequency-domain pattern, the generated is a two-soliton state optical frequency comb, and its frequency-domain envelope is a hyperbolic secant shape modulated by a period, and the modulation period is strictly equal to the reciprocal of the time difference between the two optical solitons generated in the time domain. The strong single-frequency component at the center frequency of the generated frequency comb is the residual continuous light pump, which can be filtered out by a band-stop filter in subsequent applications.

[0028] Figure 5 It is the instantaneous time-domain simulation diagram of the generated soliton microcomb. It can be seen that the slow-varying envelope of the time-domain optical field presents the shape of two optical solitons. Its specific shape is that two hyperbolic secant optical solitons sit on the background light of a strong continuous light at the bottom. This background light is the residual continuous light pump. The interference between the optical soliton and the residual continuous light causes the pulse pedestal to present an uneven depression, but the generated optical soliton can still remain stable for a long time. The generated two solitons have the same peak power and pulse width, and are locked together and transmitted at the same speed. By controlling the pump laser frequency scanning time, the single-soliton state microcomb can be further excited, but its excitation is probabilistic affected by noise. If deterministic excitation of the single-soliton microcomb is required, subsequent methods such as pump laser phase modulation and induced mode hybridization to excite dispersion waves can be considered to introduce a time-domain potential well to ensure the deterministic generation of the low-repetition-rate soliton microcomb.

[0029] The above embodiments are used to explain and illustrate the present invention, rather than to limit the present invention. Within the spirit and scope of the protection of the claims of the present invention, any modification and change made to the present invention fall within the protection scope of the present invention.

Claims

1. A method for generating a low-repetition-rate soliton microcomb for an aluminum nitride integrated optical microcavity, characterized in that: This method realizes the selective translation of the microcavity resonance peak through an aluminum nitride integrated photonic crystal micro-ring resonator with a photonic crystal structure, thereby eliminating the competitive effect of stimulated Raman gain on Kerr nonlinear gain. Based on this aluminum nitride photonic crystal micro-ring resonator, soliton microcombs are generated when the repetition rate is lower than 100 GHz. The specific structure of the aluminum nitride photonic crystal micro-ring resonator is as follows: Aluminum nitride with a photonic crystal is used as the material of the waveguide core layer (3). The aluminum nitride thin film is grown on the sapphire substrate (4). The aluminum nitride ridge waveguide structure is realized through photolithography. This ridge waveguide structure is composed of a thin rectangular flat layer and a trapezoidal strip layer. There is a first cladding layer (2) on the aluminum nitride waveguide core layer (3), and the first cladding layer (2) is a silica cladding. The aluminum nitride waveguide core layer (3) includes an aluminum nitride strip straight waveguide (5) and an aluminum nitride ring cavity (6). The width or thickness of the aluminum nitride waveguide is modulated periodically along the optical transmission direction of the aluminum nitride ring cavity (6), thereby generating a Bragg bandgap effect to excite the backward transmission optical field, realizing the selective translation of the microcavity resonance peak near the stimulated Raman peak of the aluminum nitride waveguide, making the overlapping part of the Raman gain peak and the microcavity resonance peak smaller, thereby weakening the effect of Raman gain and preventing it from restricting the generation of soliton microcombs through competition with Kerr nonlinear gain.

2. The method for generating a low-repetition-rate soliton microcomb for an aluminum nitride integrated optical microcavity according to claim 1, characterized in that Based on the micro-ring resonator structure of the aluminum nitride waveguide core layer (3), it is coupled to the aluminum nitride ring cavity (6) waveguide through a straight waveguide. By controlling the gap distance between the straight waveguide and the aluminum nitride ring cavity (6), the percentage of the optical field coupled into the aluminum nitride ring cavity (6) is adjusted; by controlling the radius of the aluminum nitride ring cavity (6), the repetition rate of the aluminum nitride optical microcavity is made less than 100 GHz.

3. The method for generating a low-repetition-rate soliton microcomb for an aluminum nitride integrated optical microcavity according to claim 2, wherein The number of modulation periods is an odd multiple 2m - 1 of the smaller mode number m among the mode numbers of the two microcavity resonance peaks to be translated.

4. The method for generating a low-repetition-rate soliton microcomb for an aluminum nitride integrated optical microcavity according to claim 2, wherein Using the aluminum nitride photonic crystal micro-ring resonator, the generation of soliton-state optical microcombs is realized by pumping the micro-ring resonator with a tunable laser source.

5. The method for generating a low-repetition-rate soliton microcomb for an aluminum nitride integrated optical microcavity according to claim 2, characterized in that The device used in this method is as follows: The tunable continuous laser source (7) outputs pump laser, which is amplified by the erbium-doped fiber amplifier (8) and then enters the bandpass filter (9) to filter out the ASE noise introduced by the amplifier. Then it enters the polarization controller (10) to be adjusted to the TM transmission mode. Subsequently, it passes through the lens fiber (11) and is incident on the aluminum nitride photonic crystal micro-ring resonator (12) from the end face. The output light of the micro-ring resonator is led out through the lens fiber (13), and is divided into three paths by the beam splitter (14), and enters the spectrometer (17) to observe the frequency-domain pattern, enters the detector (15) connected to the oscilloscope (18) to monitor the change of the microcavity output power, and enters the detector (16) connected to the radio frequency spectrum analyzer (19) to observe the low-frequency noise characteristics; Using the aluminum nitride integrated optical microcavity with a photonic crystal structure, the competitive influence of the strong stimulated Raman gain of the aluminum nitride material itself on the Kerr nonlinear gain is eliminated. By externally injecting the micro-ring resonator with a tunable continuous laser source, soliton microcombs are generated through frequency scanning; Soliton microcombs are generated in the aluminum nitride microcavity with a repetition rate lower than 100 GHz, and they are small in size and convenient for integration.

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