Optical frequency comb generation device and method based on lithium niobate film waveguide series double micro-rings

Abstract

The invention discloses an optical frequency comb generation device and method based on lithium niobate film waveguide series connection double micro-rings. The device comprises a first bus waveguide, a first annular waveguide, a second annular waveguide and a second bus waveguide which are etched on a lithium niobate film. Wherein one end of the first bus waveguide is a pump light input end, the other end of the first bus waveguide is an auxiliary light input end, the first bus waveguide and the first annular waveguide are mutually coupled, the first annular waveguide and the second annular waveguide are mutually coupled, the second annular waveguide is a soliton cavity, and a temperature regulation and control device is arranged below the soliton cavity. The temperature of the soliton cavity is regulated and controlled to enable the harmonic peak of the soliton cavity to drift, so that the pump light enters a soliton state, the auxiliary light performs power compensation on the soliton state of the pump light, and a soliton optical frequency comb is generated. In combination with auxiliary photo-thermal modulation, the micro-cavity thermal effect is compensated, the soliton stability is improved, and finally a comb-shaped structure spectrum with stable performance, high coherence and high flatness is output from a soliton cavity port.

Description

Technical Field

[0001] The present invention relates to the fields of optical integrated optoelectronics and nonlinear optics, and in particular to an optical frequency comb generating device and method based on lithium niobate thin film waveguide series-connected double microrings. Background Art

[0002] An optical frequency comb (OFC) is a coherent spectrum composed of a series of discrete, equally spaced frequency components. It has unprecedented accuracy in frequency and timing and is widely used in important fields such as precision measurement, microwave photonics, quantum optics, optical communication, and optical computing. Traditional OFCs are usually implemented based on solid-state or fiber mode-locked lasers. They use optical fibers to form a loop structure similar to a resonant cavity. However, optical fibers are easily affected by external factors such as temperature and humidity, so that they cannot produce a stable frequency comb. In addition, the loop needs to contain a gain medium, which results in a large system size and high cost. In contrast, the Kerr optical frequency comb generated by a microring resonator provides a more compact, highly integrated solution with a larger repetition frequency range. It can be integrated on a single chip and has the advantages of low cost, low loss, and low power consumption. Compared with the Fabry-Perot (FP) cavity, which uses the superposition of light fields with opposite transmission directions in the cavity to generate standing waves to form resonance, the microring resonator can achieve stronger confinement of light, achieve an extremely small mode volume, and obtain an extremely high quality factor.

[0003] Lithium niobate on insulator (LNOI) has attracted extensive attention due to its strong linear electro-optic effect and large second-order and third-order nonlinear effects. In recent years, the emergence of thin film lithium niobate (TFLN) has pushed the research on lithium niobate to a peak. By localizing the light field in a very small thin film lithium niobate waveguide, the electro-optic effect and nonlinear effect are greatly enhanced on the basis of lithium niobate. At the same time, due to the strong nonlinearity of the ultra-high quality factor LNOI microring resonator, it has become an excellent platform for generating optical frequency combs. At present, how to reduce optical loss and improve the efficiency of converting pump light to optical solitons through the microcavity while improving the stability and duration of solitons is the key to achieving high coherence and high flatness microcavity optical frequency combs. Summary of the invention

[0004] In order to solve or partially solve the problems mentioned in the background technology, the present invention provides an optical frequency comb generation device and method based on a lithium niobate thin film waveguide with dual microrings in series. The specific technical solution is as follows:

[0005] In one aspect of the present invention, there is provided an optical frequency comb generating device based on a lithium niobate film waveguide connected in series with a double micro-ring, comprising a first bus waveguide, a first ring waveguide, a second ring waveguide and a second bus waveguide etched on a lithium niobate film;

[0006] One end of the first bus waveguide is a pump light input end, and the other end is an auxiliary light input end. The first bus waveguide is coupled with the first ring waveguide. The pump light and the auxiliary light input through the first bus waveguide are coupled into the first ring waveguide respectively. The first ring waveguide is a pump cavity. The pump light and the auxiliary light resonate in the pump cavity.

[0007] The first annular waveguide and the second annular waveguide are coupled to each other, and the pump light and the auxiliary light in the first annular waveguide are coupled into the second annular waveguide respectively; the second annular waveguide is a soliton cavity, and a temperature control device is arranged below the soliton cavity, and the temperature of the soliton cavity is controlled by the temperature control device to make the resonance peak of the soliton cavity drift, so that the pump light enters the soliton state, and the auxiliary light performs power compensation on the soliton state of the pump light, so as to generate a soliton optical frequency comb;

[0008] The second ring waveguide is coupled to the second bus waveguide so that the soliton optical frequency comb generated on the second ring waveguide is coupled into the second bus waveguide and coupled out through one end of the second bus waveguide.

[0009] Further, the first bus waveguide has straight waveguides on both sides and is curved in the middle to form a curved coupling in the coupling area with the first ring waveguide, and / or the second bus waveguide has straight waveguides on both sides and is curved in the middle to form a curved coupling in the coupling area with the second ring waveguide.

[0010] Further, the first bus waveguide and the second bus waveguide are symmetrically arranged with respect to a coupling region of the first ring waveguide and the second ring waveguide.

[0011] Furthermore, the first bus waveguide and the second bus waveguide are single-slot ridge waveguide structures, and the first ring waveguide and the second ring waveguide are strip waveguide structures.

[0012] Further, the first bus waveguide and the second bus waveguide satisfy a single-mode condition, and the dispersion of the first ring waveguide and the second ring waveguide is anomalous dispersion.

[0013] Furthermore, the total height of the lithium niobate film is 600nm, and the etching depth is 500nm; the width of the first ring waveguide and the second ring waveguide is 1800nm, and the cavity length is 188.496μm; the width of the first bus waveguide and the second bus waveguide is 750nm, and the slit width is 40nm.

[0014] Another aspect of the present invention provides a method for generating an optical frequency comb based on a lithium niobate thin film waveguide and a double microring in series based on the above-mentioned device, the method comprising:

[0015] S1. Inputting pump light and auxiliary light at both ends of the first bus waveguide respectively, the pump light is close to the blue detuning position of the first resonance peak in the soliton cavity, the auxiliary light is close to the blue detuning position of the second resonance peak in the soliton cavity, and the absolute value of the detuning amount of the pump light is smaller than the absolute value of the detuning amount of the auxiliary light;

[0016] S2. Adjust the temperature of the soliton cavity to blue-shift the resonance peak of the soliton cavity, and reduce the detuning of the pump light and the auxiliary light to trigger modulation instability, so that the optical frequency comb generated by the pump light is in a chaotic state;

[0017] S3. Continue to adjust the temperature of the soliton cavity to make the resonance peak of the soliton cavity continue to blue-shift, so that the pump light enters the red detuning region of the first resonance peak of the soliton cavity, and the optical frequency comb generated by the pump light is transformed from a chaotic state to a soliton state. At the same time, the auxiliary light is further close to the second resonance peak of the soliton cavity and is in a blue detuning position to compensate for the reduced optical power of the pump light to generate stable optical solitons and obtain a soliton optical frequency comb.

[0018] Furthermore, in step S3, the method further includes:

[0019] Continuously adjusting the temperature of the soliton cavity until the shift of the first resonance peak of the soliton cavity passes through the central wavelength of the pump light;

[0020] The central wavelength position of the auxiliary light is adjusted multiple times to adapt to the temperature control of the soliton cavity until a stable optical soliton is achieved in the soliton cavity.

[0021] Furthermore, when executing step S2 and step S3, the method further includes: when adjusting the temperature of the soliton cavity, keeping the central wavelengths of the pump light and the auxiliary light unchanged.

[0022] The optical frequency comb generation device and method based on lithium niobate thin film waveguide and double microrings in series of the present invention have the following advantages:

[0023] 1. With the help of the double microcavity superposition method, the conversion efficiency limit of the traditional single microcavity is overcome. By refining the coupling spacing between the microring and the bus waveguide, the coupling distance between the double microrings and the detuning amount, the coupling and phase matching of the two microrings can be precisely controlled, and the resonant power of light in the cavity can be enhanced, so that photons interact with each other for a long time in the cavity, thereby obtaining higher coupling efficiency and lower loss.

[0024] 2. Two beams of light with opposite frequencies are input from the microcavity into the pump cavity and the soliton cavity respectively. The two cavities respectively provide pump energy and generate and output solitons. Combined with auxiliary optical thermal modulation, the thermal effect of the microcavity is compensated and the stability of solitons is improved. Finally, a comb structure spectrum with stable performance, high coherence and high flatness is output from the soliton cavity port.

[0025] 3. By using the method of double microcavity superposition bending coupling, the coupling efficiency is greatly improved, and the conversion efficiency limit of the traditional single microcavity can be overcome, achieving a breakthrough of nearly 100% pump-to-optical soliton energy conversion rate.

[0026] 4. The ridge slot waveguide combines the strong confinement of the light field by the strip waveguide and the simplicity of the ridge waveguide device process, which strongly confines most of the mode field to the slit, that is, the high refractive index core area, thereby significantly reducing the effective mode area and improving the nonlinear performance of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 Schematic diagram of a lithium niobate thin film waveguide series double micro-ring structure according to an embodiment of the present invention.

[0028] Figure 2 It is a series double-coupled microcavity structure.

[0029] Figure 3 Schematic diagram of the cross-section of the lithium niobate thin film optical waveguide structure.

[0030] Figure 4 Schematic diagram of simulation results of waveguide modes of lithium niobate thin films with different structures according to an embodiment of the present invention.

[0031] Figure 5 This is a partial cross-sectional structural diagram of a lithium niobate thin film waveguide series double micro-ring structure according to an embodiment of the present invention.

[0032] Figure 6 This is a diagram of experimental simulation data of a lithium niobate thin film waveguide according to an embodiment of the present invention;

[0033] Figure 6 (a) is a comparison curve of the effective refractive index data of lithium niobate film waveguides with different cross-sectional widths;

[0034] Figure 6 (b) is a comparison curve of dispersion data of lithium niobate film waveguides with different cross-sectional widths;

[0035] Figure 7 This is an experimental system diagram of an optical frequency comb generating device based on a lithium niobate thin film waveguide connected in series with double microrings according to an embodiment of the present invention.

[0036] Figure 8 It is the principle diagram of degenerate four-wave mixing and non-degenerate four-wave mixing;

[0037] Fig. 9 This is a schematic diagram of a method for generating an optical frequency comb based on a lithium niobate thin film waveguide connected in series with double microrings according to an embodiment of the present invention. DETAILED DESCRIPTION

[0038] In order to better understand the purpose, structure and function of the present invention, the optical frequency comb generating device based on lithium niobate thin film slit waveguide and double microrings in series and the preparation method thereof are further described in detail below in conjunction with the accompanying drawings.

[0039] The microcavity optical frequency comb generated by the traditional frequency tuning scheme has the problems of low pump conversion efficiency, large thermal effect, poor soliton stability, etc. Based on the above problems, an embodiment of the present invention provides a Kerr optical frequency comb device based on a thin film lithium niobate material optical waveguide, which adopts a series double microring coupling structure.

[0040] A specific embodiment of the present invention provides an optical frequency comb generating device based on a lithium niobate thin film waveguide and a double microring in series. Figure 1 As shown, it specifically includes a first bus waveguide 1, a first ring waveguide 3, a second ring waveguide 4 and a second bus waveguide 2 etched on a lithium niobate film.

[0041] The present invention uses a dual-pump thermal tuning method to generate a stable Kerr optical frequency comb. Specifically, one end of the first bus waveguide 1 is a pump light input end (M end), and the other end is an auxiliary light input end (T end). The first bus waveguide 1 is coupled with the first ring waveguide 3. The pump light and the auxiliary light input through the first bus waveguide 1 are coupled into the first ring waveguide 3 respectively. The first ring waveguide 3 is a pump cavity. The pump light and the auxiliary light resonate in the pump cavity.

[0042] Furthermore, the first ring waveguide 3 and the second ring waveguide 4 are coupled to each other, and the pump light and the auxiliary light in the first ring waveguide 2 are coupled into the second ring waveguide 3 respectively; the second ring waveguide 3 is a soliton cavity, and a temperature control device 5 is arranged below the soliton cavity, and the temperature of the soliton cavity is controlled by the temperature control device 5 to make the resonance peak of the soliton cavity drift, so that the pump light enters the soliton state, and the auxiliary light performs power compensation on the soliton state of the pump light, so as to generate a soliton optical frequency comb;

[0043] The second ring waveguide 4 is coupled to the second bus waveguide 2 so that the soliton optical frequency comb generated on the second ring waveguide 4 is coupled into the second bus waveguide 2 and coupled out through one end (D end) of the second bus waveguide.

[0044] In the embodiment of the present invention, the pump cavity is used to transfer the energy of the pump light to the soliton cavity, and the soliton cavity can accommodate Kerr solitons. The pump light is input from the pump light input end (M end), coupled into the pump cavity through the first bus waveguide 1, resonates in the pump cavity, and the light intensity in the cavity increases, which is equivalent to amplifying the power of the pump light. The light is coupled through the two cavities and finally output through the D port of the soliton cavity, thereby obtaining a soliton optical frequency comb. The pump cavity is used to provide pump energy for the soliton cavity, and the soliton cavity is responsible for the generation and output of solitons.

[0045] In addition, the microcavity optical frequency comb in the embodiment of the present invention is regulated by dual-pump thermal tuning, that is, thermal control is added at the soliton cavity to compensate for the soliton instability caused by the sudden drop in intracavity power when the pump light jumps from the blue detuning zone to the red detuning zone in the traditional frequency tuning scheme.

[0046] Further, in an optional embodiment of the present invention, the series double micro-ring coupling structure is as follows Figure 2 As shown, Figure 2 (a) is a straight waveguide coupling form, that is, the first bus waveguide 1 and the second bus waveguide 2 are both straight waveguides. The preferred embodiment of the present invention is Figure 2 (b) shows a curved waveguide coupling form, in which the first bus waveguide 1 has straight waveguides on both sides and is curved in the middle to form a curved coupling in the coupling region with the first ring waveguide 3, and / or the second bus waveguide 2 has straight waveguides on both sides and is curved in the middle to form a curved coupling in the coupling region with the second ring waveguide 4. The bus waveguide and the ring waveguide adopt a curved coupling form in the coupling region, which can further increase the area and time of light coupling into the ring, thereby greatly enhancing the coupling efficiency, and can overcome the conversion efficiency limit of a traditional single microcavity, achieving a breakthrough of nearly 100% pump-to-optical soliton energy conversion rate.

[0047] Furthermore, in a preferred embodiment of the present invention, the first bus waveguide 1 and the second bus waveguide 2 are symmetrically arranged relative to the coupling region of the first ring waveguide 3 and the second ring waveguide 3 to ensure that the optical signal achieves the same coupling effect and power conversion in the coupling action of the two bus waveguides and the ring waveguide, and has the same coupling coefficient on different paths, avoiding output signal distortion caused by phase difference and causing additional nonlinear effects. In addition, by optimizing the coupling spacing parameter value of the two microcavities and controlling the coupling strength, flexible control of pump-optical soliton conversion is achieved to meet different application scenarios.

[0048] Furthermore, in a preferred embodiment of the present invention, the first bus waveguide 1 and the second bus waveguide 2 are etched into a single slit ridge waveguide (strip-loaded slot-waveguide) structure consisting of two parallel waveguides and a gap region in the middle. This is because the electric field will have a sudden change at the interface between the high refractive index material and the low refractive index material, and the slit region will produce extremely strong restriction on the light.

[0049] Furthermore, the present invention further verifies the advantages of the single-slit ridge waveguide through simulation experiments. Figure 3 The schematic diagram of the cross section of the lithium niobate thin film optical waveguide structure is shown. cladding 、n TFLN 、n box are the refractive indices of the cladding, lithium niobate film waveguide and buried oxide layer, Figure 4 The simulation results of the modes of lithium niobate film waveguides with different structures are shown. When the total thickness of the lithium niobate film and the waveguide width are limited to 600nm and 750nm respectively, and the slit width is set to 40nm, the effective mode areas of the three structures are 0.6607μm 2 、0.6872μm 2 、0.5394μm 2 Therefore, from the analysis of experimental data, it can be seen that the ridge slot waveguide combines the strong confinement of the light field by the strip waveguide and the simplicity of the ridge waveguide device process, which strongly confines most of the mode field to the slit, that is, the high refractive index core area, thereby significantly reducing the effective mode area and improving the nonlinear performance of the device.

[0050] Furthermore, in order to achieve a balance between dispersion and nonlinearity, intracavity loss and gain, in an embodiment of the present invention, the bus waveguide width is set to satisfy the single-mode condition, and the ring waveguide width is set to satisfy the anomalous dispersion state to generate an optical frequency comb.

[0051] Furthermore, in order to meet the anomalous dispersion condition and better stimulate the nonlinear effect, the ring waveguide is usually set to be wider to adapt to the transmission of light in the cavity to generate an optical frequency comb; and in order to meet the single-mode transmission, the bus waveguide width is set to be relatively narrow, which enhances the constraint on the light field distribution, reduces the loss caused by mode coupling, and improves the transmission characteristics of the device. Therefore, the cross-sectional structure of the optical frequency comb generation device based on lithium niobate thin film waveguide series double microring provided in the embodiment of the present invention is as follows Figure 4 shown.

[0052] Furthermore, the present invention designs the structural parameters of the lithium niobate thin film waveguide in series with double microrings based on simulation technology, and based on the simulation results, the effective refractive index of the lithium niobate thin film strip optical waveguide with different widths at a thickness of 600nm is as follows: Figure 6As shown in (a), when the width is about 750nm, the effective refractive index of the core waveguide is higher than the refractive index of the silica cladding 1.44. At this time, the waveguide only supports one TE fundamental mode and one TM fundamental mode, achieving the condition of total reflection. Therefore, in the specific embodiment of the present invention, the width W2 of the first bus waveguide 1 and the second bus waveguide 2 is 750nm, the total height H of lithium niobate is 600nm, and the etching depth D is 500nm. Figure 6 (a) The dotted line is the refractive index of the silica cladding

[0053] Furthermore, in order to meet the phase matching condition for triggering the four-wave mixing effect in the process of generating the microcavity optical frequency comb, the dispersion curves of waveguides with different widths at a thickness of 600 nm are simulated so that the dispersion value is greater than 0, such as Figure 6 (b), taking into account the size and dispersion value, the present invention selects a lithium niobate thin film waveguide with a width of 1800nm, so that the microcavity presents an anomalous dispersion state near the C band. Therefore, in a specific embodiment of the present invention, the width W1 of the first ring waveguide 3 and the second ring waveguide 4 is 1800nm, the total height H of lithium niobate is 600nm, the etching depth D is 500nm, and the cavity length of the pump cavity and the soliton cavity is 188.496μm, and the free spectral range FSR is about 958.75GHz.

[0054] Furthermore, in a specific embodiment of the present invention, the coupling distance between the first ring waveguide 3 and the second ring waveguide 4 is 0.25 μm. To achieve critical coupling, that is, the optical field resonance component in the bus waveguide can be coupled into the ring waveguide to the maximum extent, the coupling spacing between the ring waveguide and the bus waveguide is set to 0.7 μm, that is, the coupling spacing between the first bus waveguide 1 and the first ring waveguide 3 is 0.7 μm, and the coupling spacing between the second bus waveguide 2 and the second ring waveguide 4 is also 0.7 μm, so that it has a higher quality factor while achieving critical coupling.

[0055] Furthermore, the process of preparing the lithium niobate thin film waveguide in the embodiment of the present invention is as follows: using bonding technology, spin-coating BCB (phenylcyclobutene) on the surface of the SOI substrate, and then attaching the commercial lithium niobate substrate to the SOI substrate, performing high-temperature annealing and curing, and removing the silicon substrate and silicon dioxide on the commercial lithium niobate substrate in combination with an etching process to expose the lithium niobate film. Subsequently, the substrate with the lithium niobate film is subjected to process steps such as coating, electron beam exposure, development, fixing, and ICP etching to prepare a waveguide structure pattern, and a silicon dioxide isolation layer is grown on the waveguide layer by atomic layer deposition as an upper cladding layer, such as Figure 5 As shown, the device can remain stable at high temperatures and protect the device from external factors such as moisture.

[0056] Furthermore, in order to verify the effect of the optical frequency comb generating device based on lithium niobate thin film slit waveguide and double microrings in series provided by the embodiment of the present invention, a Figure 7 The experimental system shown in the figure includes a pump light source input device and an auxiliary light source input device, wherein the pump light output by the pump light source (Continuous-wave Pump, CWPump) in the pump light source input device is power amplified by the first erbium-doped fiber amplifier (Erbium-doped FiberAmplifier 1, EDFA1), directionally transmitted by the first circulator (Circulator1), polarized by the first polarization controller (FiberPolarization Controller 1, FPC1) and coupled into the pump light input end (M end) of one end of the first bus waveguide 1; the auxiliary light output by the auxiliary light source (Continuous-wave Auxiliary, CW Aux.) in the auxiliary light source input device is power amplified by the second erbium-doped fiber amplifier (Erbium-doped Fiber Amplifier 2, EDFA2), directionally transmitted by the second circulator (Circulator2), polarized by the second polarization controller (FiberPolarization Controller 2, FPC2) adjusts the polarization state and couples it into the auxiliary light input end (T end) at the other end of the first bus waveguide 1.

[0057] Furthermore, a coupler is connected to the output end (D end) of the soliton optical frequency comb, and the output end of the coupler is respectively connected to a first optical spectrum analyzer (Optical Spectrum Analyzer 1, OSA1) for observing and analyzing the output spectrum; and a bandpass filter (BPF), a photodetector (PD) and an oscilloscope (Oscilloscope, Osc.). A second spectrum analyzer (Optical Spectrum Analyzer 2, OSA2) is also connected to the other end (N end) of the second total waveguide 2 for further observing the spectrum on the second total waveguide.

[0058] In addition, the present invention also provides a method for generating an optical frequency comb based on a lithium niobate thin film slot waveguide and a double microring in series based on the above device. Before introducing the optical frequency comb generating method in the present invention, a brief introduction to its working principle is given first.

[0059] First, when light of a certain wavelength enters the microcavity (ring waveguide), self-phase modulation and cross-phase modulation will occur. The self-phase modulation (SPM) effect means that the refractive index of the microcavity medium changes due to the change of the input light field, thereby achieving the modulation of the phase of the incident light field itself. The phase shift caused by SPM is shown in formula (1). The phase shift is proportional to the size of the light field and the transmission distance.

[0060]

[0061] Where n2 is the nonlinear refractive index, L K is the effective propagation length, and k0 represents the wave number corresponding to the center frequency.

[0062] Cross-phase modulation (XPM) means that the input light field and the light field of different frequencies exist in the optical waveguide at the same time, affecting the refractive index of each other's light field, and nonlinear phase modulation is performed between the light fields. The phase shift caused by XPM is shown in (2).

[0063]

[0064] Both SPM and XPM are manifested as nonlinear phase shifts continuously introduced into the microresonator during the transmission of the light field. Usually, the phase shift caused by XPM is about twice that caused by SPM. At the same time, SPM and XPM describe the mutual influence of the phase between light fields and will change the instantaneous frequency of the time domain signal.

[0065] Furthermore, the microcavity will also produce four-wave mixing (FWM) effects, which involve the annihilation of two photons and the generation of two new frequency component photons. Depending on whether the frequencies of the two annihilated photons are the same, it can be divided into two cases: degenerate and non-degenerate four-wave mixing, as shown in Figure 2. Figure 8 As shown in (a) and (b), degenerate four-wave mixing involves the interaction of three photons of different frequencies, i.e., 2ω1=2ω2=ω3+ω4; while non-degenerate four-wave mixing involves the interaction of four photons of different frequencies, i.e., ω1+ω2=ω3+ω4.

[0066] When the external input power reaches the nonlinear power threshold of the microcavity, the pump light can generate two symmetrical new frequency sidebands under the action of degenerate four-wave mixing, and then further realize efficient frequency conversion through non-degenerate four-wave mixing effect, generate new frequencies and broaden the optical comb spectrum.

[0067] Furthermore, the process in which new frequency components are generated around the pump light due to the four-wave mixing effect in the cavity is called modulation instability. Specifically, when the continuous light power in the cavity rises to a certain value, the power and energy will fluctuate slightly, resulting in fluctuations such as noise, which will introduce parametric amplification. When the nonlinear gain is greater than the total loss of the optical microcavity, a pair of frequency-symmetrical comb teeth will be generated on both sides of the pump light frequency.

[0068] The optical frequency comb generation method based on lithium niobate thin film slit waveguide and double microrings in series provided by the embodiment of the present invention is implemented on the basis of the above principle. The present invention compensates for the thermal effect in the cavity by inputting auxiliary light that is transmitted in the opposite direction to the pump light. The principle is as follows: Fig. 9 As shown, the blue vertical line and the red vertical line are the central wavelengths of the auxiliary light source (Pump) and the pump light source (Aux.), respectively. The peak position of the red curve is the first resonance peak of the central wavelength of the pump light in the soliton cavity, and the peak position of the blue curve is the second resonance peak of the central wavelength of the auxiliary light in the soliton cavity, that is, the pump light and the auxiliary light are both close to the resonance peak positions of the soliton cavity and the pump cavity. The input frequencies of the pump light injected from the M end (pump light input end) and the auxiliary light injected from the T port (auxiliary light input end) are kept unchanged. It should be noted that the structural parameters of the pump cavity and the soliton cavity in the embodiment of the present invention are the same. The pump light and the auxiliary light resonate after being coupled into the pump cavity, so that the optical power of both is amplified. The present invention uses thermal regulation to change the temperature of the soliton cavity, causing the resonance peak of the microcavity to drift, thereby achieving a change in the detuning amount. Specifically, the following steps are included:

[0069] S1, input pump light and auxiliary light at both ends of the first bus waveguide 1, respectively, the pump light is close to the blue detuning position of the first resonance peak in the soliton cavity, the auxiliary light is close to the blue detuning position of the second resonance peak in the soliton cavity, and the absolute value of the detuning amount of the pump light is smaller than the absolute value of the detuning amount of the auxiliary light; that is ... auxiliary light is close to the blue detuning position of the second resonance peak in the soliton cavity, and the auxiliary light is close to the blue detuning position of the second resonance peak in the soliton cavity, and the auxiliary light is close to the blue detuning position of the second resonance peak Fig. 9 State 1 in

[0070] In the embodiment of the present invention, the first resonance peak is the resonance peak of the pump light in the soliton cavity, and the second resonance peak is the resonance peak of the auxiliary light in the soliton cavity. The central wavelength of the auxiliary light can be smaller than the central wavelength of the pump light or larger than the central wavelength of the pump light. It is only necessary to ensure that the absolute value of the detuning amount of the auxiliary light is smaller than the detuning amount of the pump light, and both are within the 3dB bandwidth range of the resonance peak.

[0071] S2. Adjust the temperature of the soliton cavity to blue-shift the resonance peak of the soliton cavity, and reduce the detuning of the pump light and the auxiliary light to trigger modulation instability, so that the optical frequency comb generated by the pump light is in a chaotic state;

[0072] In the embodiment of the present invention, the temperature of the soliton cavity is reduced by thermal regulation, and the blue shift of the resonance peak of the soliton cavity specifically refers to that the first resonance peak of the pump light in the soliton cavity moves to the left (blue shift), so that the central wavelength of the pump light is further close to the first resonance peak, and the second resonance peak of the auxiliary light in the soliton cavity moves to the left (blue shift), so that the central wavelength of the auxiliary light is further close to the second resonance peak, and the corresponding central wavelength of the pump light is closer to the second resonance peak and the central wavelength of the auxiliary light is closer to the second resonance peak, thereby triggering modulation instability, increasing the power in the cavity, and raising the temperature. The optical comb generated by the pump light is in a chaotic state, that is, state 2, and at this time, both the pump light and the auxiliary light are in a blue detuned position.

[0073] S3. Continue to adjust the temperature of the soliton cavity to make the resonance peak of the soliton cavity continue to blue-shift, so that the pump light enters the red detuning region of the first resonance peak of the soliton cavity, and the optical frequency comb generated by the pump light is transformed from a chaotic state to a soliton state. At the same time, the auxiliary light is further close to the second resonance peak of the soliton cavity and is in a blue detuning position to compensate for the reduced optical power of the pump light to generate stable optical solitons and obtain a soliton optical frequency comb.

[0074] In the embodiment of the present invention, the temperature of the microcavity is further reduced, and the resonance peak of the soliton cavity continues to blueshift. At this time, when the pump light enters the red detuning region of the first resonance peak of the soliton cavity, the optical comb state changes from the chaotic state to the soliton state, the operating power in the cavity drops sharply, and the pump light coupled into the cavity decreases. At the same time, since the absolute value of the detuning amount of the pump light is smaller than the absolute value of the detuning amount of the auxiliary light, the auxiliary light will be closer to the second resonance peak of the soliton cavity, and the power in the cavity will increase, thereby compensating for the optical power reduced by the pump light, reducing the thermal sensitivity of the microcavity resonance peak drift, and generating stable optical solitons, that is, state 3.

[0075] Specifically, in order to ensure that the soliton optical frequency comb is in the state in step 3, the operation process of step 3 is specifically as follows: continuously adjust the temperature of the soliton cavity until the offset of the first resonance peak of the soliton cavity passes through the central wavelength of the pump light; after the offset of the first resonance peak passes through the central wavelength of the pump light, adjust the central wavelength position of the auxiliary light multiple times to adapt to the temperature control of the soliton cavity until a stable optical soliton is achieved in the soliton cavity. In the process of adjusting the temperature, the output state of the D-port optical comb and the light transmittance of the N-port can be observed by a spectrometer respectively until the offset of the resonance peak passes through the pump light, to verify whether the auxiliary light can compensate for the power drop of the microcavity optical frequency comb from the modulation unstable state to the soliton state. In addition, it should be noted that when adjusting the temperature of the soliton cavity, the central wavelengths of the pump light and the auxiliary light are kept unchanged, so as to simplify the adjustment process by controlling the variables.

[0076] In a specific embodiment of the present invention, the central wavelengths of the pump light source and the auxiliary light source are set on the basis of confirming the resonance wavelength of the microcavity. In the preferred embodiment of the present invention, the central wavelength of the auxiliary light should be smaller than the pump light to prevent the occurrence of mode hopping and other phenomena, so that the auxiliary light fails to compensate for the thermal effect introduced by the pump light. After the reverse light sources at both ends are power amplified by an erbium-doped fiber amplifier (EDFA), directional transmission by a circulator, and polarization state adjustment by a polarization controller (FPC), they enter the microcavity through the M and T ports respectively. After the light resonates through the pump cavity, the light intensity increases, and the two beams of light with amplified power enter the soliton cavity respectively. The pump light is combined with the auxiliary light for thermal control, thereby generating an optical frequency comb.

[0077] An optical frequency comb generation device and method based on a lithium niobate film waveguide and a double microring in series provided by an embodiment of the present invention adopts a curved coupled slit waveguide and a double coupling structure, and enhances the resonance of light in the cavity by precisely controlling the coupling and phase matching of the two microrings, so that the photons interact for a long time in the cavity, thereby obtaining higher coupling efficiency and lower loss. At the same time, two beams of light with opposite frequencies are input from the microcavity into the pump cavity and the soliton cavity respectively. The two cavities respectively complete the provision of pump energy and the generation and output of solitons. Combined with auxiliary optical thermal tuning, the thermal effect of the microcavity is compensated, and the stability of the soliton is improved. Through the thermo-optical effect and Kerr nonlinear effect of the microcavity, a comb structure spectrum with stable performance, high coherence and high flatness is finally output from the soliton cavity port. The results show that based on the lithium niobate film series double coupled microring resonant cavity, under the auxiliary optical thermal tuning scheme, not only the conversion efficiency between pump light and soliton and the coupling efficiency between the two cavities are improved, but also the optical loss is reduced, and optical solitons with high stability are obtained.

[0078] The present invention is further described above with the help of specific embodiments, but it should be understood that the specific description here should not be understood as limiting the essence and scope of the present invention. Various modifications made to the above embodiments by ordinary technicians in this field after reading this specification belong to the scope of protection of the present invention. The various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. In order to avoid unnecessary repetition, the embodiments of the present invention will not be described separately for various possible combinations.

[0079] If the embodiments of the present invention involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative position relationship, movement status, etc. between the components under a certain specific posture (as shown in the accompanying drawings). If the specific posture changes, the directional indication will also change accordingly.

Claims

1. An optical frequency comb generator based on lithium niobate thin film waveguide and double microrings in series, characterized in that: It includes a first bus waveguide, a first ring waveguide, a second ring waveguide and a second bus waveguide etched on a lithium niobate film; One end of the first bus waveguide is a pump light input end, and the other end is an auxiliary light input end. The first bus waveguide is coupled with the first ring waveguide. The pump light and the auxiliary light input through the first bus waveguide are coupled into the first ring waveguide respectively. The first ring waveguide is a pump cavity. The pump light and the auxiliary light resonate in the pump cavity. The first annular waveguide and the second annular waveguide are coupled to each other, and the pump light and the auxiliary light in the first annular waveguide are coupled into the second annular waveguide respectively; the second annular waveguide is a soliton cavity, and a temperature control device is arranged below the soliton cavity, and the temperature of the soliton cavity is controlled by the temperature control device to make the resonance peak of the soliton cavity drift, so that the pump light enters the soliton state, and the auxiliary light performs power compensation on the soliton state of the pump light, so as to generate a soliton optical frequency comb; The second ring waveguide is coupled to the second bus waveguide so that the soliton optical frequency comb generated on the second ring waveguide is coupled into the second bus waveguide and coupled out through one end of the second bus waveguide.

2. The device according to claim 1, characterized in that The first bus waveguide has straight waveguides on both sides and is curved in the middle to form a curved coupling in a coupling region with the first ring waveguide, and / or the second bus waveguide has straight waveguides on both sides and is curved in the middle to form a curved coupling in a coupling region with the second ring waveguide.

3. The device according to claim 1 or 2, characterized in that: The first bus waveguide and the second bus waveguide are symmetrically arranged with respect to a coupling region of the first ring waveguide and the second ring waveguide.

4. The device according to claim 1 or 2, characterized in that: The first bus waveguide and the second bus waveguide are single-slot ridge waveguide structures, and the first ring waveguide and the second ring waveguide are strip waveguide structures.

5. The device according to claim 4, characterized in that The first bus waveguide and the second bus waveguide meet the single-mode condition, and the dispersion of the first ring waveguide and the second ring waveguide is anomalous dispersion.

6. The device according to claim 5, characterized in that The total height of the lithium niobate film is 600nm, and the etching depth is 500nm; the width of the first ring waveguide and the second ring waveguide is 1800nm, and the cavity length is 188.496μm; the width of the first bus waveguide and the second bus waveguide is 750nm, and the slit width is 40nm.

7. A method for generating an optical frequency comb based on a lithium niobate thin film waveguide connected in series with double microrings based on the device according to any one of claims 1 to 6, characterized in that: The method comprises: S1. Inputting pump light and auxiliary light at both ends of the first bus waveguide respectively, the pump light is close to the blue detuning position of the first resonance peak in the soliton cavity, the auxiliary light is close to the blue detuning position of the second resonance peak in the soliton cavity, and the absolute value of the detuning amount of the pump light is smaller than the absolute value of the detuning amount of the auxiliary light; S2. Adjust the temperature of the soliton cavity to blue-shift the resonance peak of the soliton cavity, and reduce the detuning of the pump light and the auxiliary light to trigger modulation instability, so that the optical frequency comb generated by the pump light is in a chaotic state; S3. Continue to adjust the temperature of the soliton cavity to make the resonance peak of the soliton cavity continue to blue-shift, so that the pump light enters the red detuning region of the first resonance peak of the soliton cavity, and the optical frequency comb generated by the pump light is transformed from a chaotic state to a soliton state. At the same time, the auxiliary light is further close to the second resonance peak of the soliton cavity and is in a blue detuning position to compensate for the reduced optical power of the pump light to generate stable optical solitons and obtain a soliton optical frequency comb.

8. The method according to claim 7, characterized in that In step S3, the method further includes: Continuously adjusting the temperature of the soliton cavity until the shift of the first resonance peak of the soliton cavity passes through the central wavelength of the pump light; The central wavelength position of the auxiliary light is adjusted multiple times to adapt to the temperature control of the soliton cavity until a stable optical soliton is achieved in the soliton cavity.

9. The method according to claim 7 or 8, characterized in that: When executing step S2 and step S3, the method further includes: keeping the central wavelengths of the pump light and the auxiliary light unchanged when adjusting the temperature of the soliton cavity.

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