Method for controlling whispering gallery mode microcavity to generate high-efficiency soliton crystals

Abstract

The invention discloses a control method for generating a high-efficiency soliton crystal by an echo wall mode microcavity, and the method comprises the steps: building an active locking circuit of a soliton crystal optical frequency comb, obtaining a second error signal through a feedback control module in the active locking circuit, and actively adjusting the output wavelength of a laser; on the basis of main peak offset locking of the soliton crystal of the microcavity, the soliton crystal at a high-power step is locked, and the soliton crystal state is simply locked, so that the soliton crystal state is stabilized for a long time, higher conversion efficiency of a passive locking mode is realized, and relying on microcavity parameter pre-calibration or an external compensation device is not needed; not only is the requirement for the wavelength stability of the laser reduced, but also the influence of low-frequency noise on phase adjustment is reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical frequency combs, and particularly relates to a method for controlling the generation of high-efficiency soliton crystals in a whispering gallery mode microcavity. Background Art

[0002] An optical frequency microcomb (optical frequency comb) is an optical pulse signal in the time domain and a spectrum composed of a series of frequency components with equal uniform intervals and coherent stable phase relationships in the frequency domain. Thanks to the ultrashort optical pulse characteristics and broadband coherent spectrum of the optical frequency comb, it has broad application prospects in the fields of coherent optical communication, precision spectroscopy, ultrafast ranging measurement, and frequency synthesis. Traditional optical frequency comb platforms, such as sapphire pulsed lasers and electro-optic frequency combs, have the disadvantages of large volume and high power consumption. The optical frequency comb platform based on the whispering gallery mode microcavity has advantages such as small volume and low power consumption, and has become a research hotspot in recent years.

[0003] However, since a large part of the energy of the pump laser of the optical frequency comb is not converted into effective comb tooth power, the efficiency of single-soliton microcombs is generally less than 5%. That is, the pump optical power is large, but the output effective comb tooth power is low. This is an important reason why it is difficult to apply the optical frequency comb. The simplest way to improve the efficiency is to increase the number of solitons in the microcavity. The soliton crystal optical frequency comb has multiple soliton numbers, providing a new solution to improve the efficiency of the optical frequency comb. However, the existence range of the soliton crystal optical frequency comb (soliton crystal) is small, making its stable existence a new challenge. For this reason, in 2023, Adnan only obtained soliton crystals under high pump power conditions in a low-Q on-chip microcavity by the method of laser wavelength scanning. In the same year, Cui Wenwen passively stabilized the soliton crystal optical frequency comb using an auxiliary laser in a high-Q crystal microcavity under low pump power conditions and obtained stable soliton crystals. Currently, the existing methods for obtaining stable soliton crystals are either limited to the on-chip microcavity platform or use additional lasers. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for controlling the generation of high-efficiency soliton crystals in a whispering gallery mode microcavity in view of the above problems existing in the prior art.

[0005] The above object of the present invention is achieved by the following technical means:

[0006] A method for controlling the generation of high-efficiency soliton crystals in a whispering gallery mode microcavity includes the following steps:

[0007] Step 1, build an active locking circuit for the soliton crystal optical frequency comb;

[0008] Step 2, perform phase modulation, power adjustment, and polarization adjustment on the pump optical signal in the active locking circuit in sequence to obtain an intracavity optical signal;

[0009] Step 3: The input cavity optical signal enters the whispering gallery mode microcavity, and the wavelength of the pump optical signal is scanned so that the whispering gallery mode microcavity enters the soliton crystal optical frequency comb mode;

[0010] Step 4: Collect the intracavity optical power signal of the soliton crystal, and perform homodyne demodulation on the intracavity optical power signal of the soliton crystal to obtain the first error signal;

[0011] Step 5: Subtract the first error signal from the set reference indication voltage signal to obtain the second error signal, and then adjust the wavelength of the pump optical signal according to the second error signal to lock the wavelength of the pump optical signal at the zero point of the second error signal, and stably capture the soliton crystal optical frequency comb.

[0012] The reference indication voltage signal in Step 5 as described above is set to the voltage value indicating that the whispering gallery mode microcavity enters the soliton crystal state.

[0013] The voltage value indicating that the whispering gallery mode microcavity enters the soliton crystal state as described above is the intermediate value of the soliton crystal step.

[0014] The active locking circuit of the soliton crystal optical frequency comb as described above includes a laser, an electro-optic modulator, an optical amplifier, a polarization controller, a whispering gallery mode microcavity, a photodetector, a lock-in amplifier, and a feedback circuit connected in sequence. The laser outputs the pump optical signal, and the output end of the feedback circuit is connected to the laser again. It also includes a signal generator, the output end of the signal generator is connected to the electro-optic modulator, and the output end of the signal generator is also connected to the lock-in amplifier.

[0015] Step 2 as described above specifically includes the following steps:

[0016] Step 2.1: The laser outputs the pump optical signal to the electro-optic modulator, the signal generator outputs the modulation radio frequency signal to the electro-optic modulator, and the electro-optic modulator performs phase modulation on the pump optical signal output by the laser according to the modulation radio frequency signal, and outputs the phase-modulated pump optical signal to the optical amplifier;

[0017] Step 2.2: The optical amplifier reduces the power of the phase-modulated pump optical signal, and the power of the reduced pump optical signal needs to be greater than the minimum threshold power of the optical frequency comb; then adjust the coupling loss between the optical fiber and the microcavity; finally output the pump optical signal with reduced power to the polarization controller;

[0018] Step 2.3: The polarization controller adjusts the polarization of the pump optical signal with reduced power so that the polarization of the pump optical signal with reduced power is consistent with the intracavity optical mode polarization of the whispering gallery mode microcavity to obtain the input cavity optical signal.

[0019] Step 3 described above is specifically as follows: Turn on the feedback circuit and scan the wavelength of the pump optical signal output by the laser, so that the wavelength of the intracavity optical signal enters the resonance peak of the whispering gallery mode microcavity from the blue detuning side of the resonance peak, and the whispering gallery mode microcavity successively enters the figure-eight optical frequency comb mode and the soliton crystal optical frequency comb mode.

[0020] Step 4 described above is specifically as follows: After the whispering gallery mode microcavity enters the soliton crystal optical frequency comb mode, turn on the photodetector to collect the intracavity optical power signal of the soliton crystal. The photodetector converts the intracavity optical power signal of the soliton crystal into an electrical signal and outputs it to the lock-in amplifier. The signal generator also outputs a modulated radio frequency signal to the lock-in amplifier. The lock-in amplifier performs homodyne demodulation on the electrical signal converted from the intracavity optical power signal of the soliton crystal with the modulated radio frequency signal as the reference signal to obtain the first error signal, and outputs the first error signal to the feedback circuit.

[0021] The present invention has the following beneficial effects compared with the prior art:

[0022] (1) By using the feedback circuit to actively adjust the output wavelength of the laser, based on the locking of the main peak shift of the soliton crystal of the whispering gallery mode microcavity, the soliton crystal at the high-power step is locked, simply locking the soliton crystal state, achieving a higher conversion efficiency than the passive locking method. The active locking scheme has higher robustness, significantly prolongs the existence time of the soliton crystal optical frequency comb, and requires a low pump laser power.

[0023] (2) By using the feedback circuit to obtain the second error signal and actively adjust the output wavelength of the laser, the soliton crystal state is stabilized for a long time without relying on microcavity parameter pre-calibration or external compensation devices; it not only reduces the requirement for the wavelength stability of the laser, but also reduces the influence of low-frequency noise on phase adjustment. Description of the Drawings

[0024] Figure 1 It is a schematic structural diagram of the active locking circuit of the present invention;

[0025] Figure 2 It is an evolution diagram of the intracavity optical power and the first error signal during the generation process of the soliton crystal of the present invention;

[0026] Figure 3(a) is a spectrogram of the figure-eight optical frequency comb mode under the experimental conditions of Embodiment 1 of the present invention;

[0027] Figure 3(b) is a spectrogram of the soliton crystal optical frequency comb mode under the experimental conditions of Embodiment 1 of the present invention;

[0028] Reference numerals and corresponding component names:

[0029] 1 - Laser; 2 - Electro - optic modulator; 3 - Optical amplifier; 4 - Polarization controller; 5 - Whispering - gallery - mode microcavity; 6 - Photo - detector; 7 - Signal generator; 8 - Lock - in amplifier; 9 - Feedback circuit. Detailed implementation mode

[0030] To facilitate the understanding and implementation of the present invention by those of ordinary skill in the art, the present invention will be further described in detail below in conjunction with embodiments. The embodiments described herein are only used to illustrate and explain the present invention, and are not intended to limit the present invention.

[0031] Embodiment 1:

[0032] A method for controlling high - efficiency soliton crystals generated by a whispering - gallery - mode microcavity, comprising the following steps:

[0033] Step 1: Build an active locking circuit for a soliton crystal optical frequency comb. The active locking circuit includes a laser 1, an electro - optic modulator 2, an optical amplifier 3, a polarization controller 4, a whispering - gallery - mode microcavity 5, a photo - detector 6, a lock - in amplifier 8, and a feedback circuit 9 connected in sequence. The laser 1, the electro - optic modulator 2, the optical amplifier 3, and the polarization controller 4 constitute an optical circuit module for generating an intracavity optical signal entering the whispering - gallery - mode microcavity 5. The whispering - gallery - mode microcavity 5 is used to generate an optical frequency comb. The photo - detector 6, the lock - in amplifier 8, and the feedback circuit 9 constitute a feedback control module. The output end of the feedback circuit 9 is connected to the laser 1 again. The laser 1 outputs a pump optical signal, and the wavelength of the pump optical signal output by the laser 1 is modulated by the output voltage of the feedback circuit 9. It also includes a signal generator 7. The output end of the signal generator 7 is connected to the electro - optic modulator 2, and the output end of the signal generator 7 is also connected to the lock - in amplifier 8. The laser 1, the electro - optic modulator 2, the optical amplifier 3, the polarization controller 4, and the whispering - gallery - mode microcavity 5 are connected in sequence through optical fibers;

[0034] In this embodiment, the laser 1 uses a narrow - line - width laser with adjustable wavelength, the tuning range is 1 nm, the line width is 10 KHz, and the output wavelength is in the range of 1560 nm band; the electro - optic modulator 2 uses a high - bandwidth lithium niobate electro - optic phase modulator, and its maximum modulation bandwidth can reach 10 GHz; the optical amplifier 3 uses a high - power erbium - doped fiber amplifier, and its maximum output power can reach 2 W, and the working wavelength is the same as the output wavelength of the laser 1, and the amplified laser still has low noise characteristics; the polarization controller 4 is a three - ring fiber polarization controller 4, which can withstand more than 500 mW of optical power; the whispering - gallery - mode microcavity 5 is a whispering - gallery - mode microcavity 5 made of a low - loss MgF 2 material, the free spectral range is 7.7 GHz, and it has a high quality factor of 4×10 8; The photodetector 6 is a high-bandwidth balanced detector with a response bandwidth of 75 MHz, meeting the requirement of detecting the modulated signal; the signal generator 7 has a multi-channel output function; the lock-in amplifier 8 is a multi-channel low-phase-noise lock-in amplifier 8 with a low-pass filter inside, which can filter out the high-frequency modulated signal; the feedback circuit 9 is an FPGA circuit with high bandwidth.

[0035] Step 2: Initialize the optical circuit module, including phase modulating the pump light signal output by the laser 1 through the electro-optic modulator 2, adjusting the power through the optical amplifier 3, and adjusting the polarization through the polarization controller 4 to obtain the intracavity light signal. Specifically:

[0036] Step 2.1: The laser 1 outputs a pump light signal to the electro-optic modulator 2, and the signal generator 7 outputs a modulated radio frequency signal to the electro-optic modulator 2. The electro-optic modulator 2 phase-modulates the pump light signal output by the laser 1 according to the modulated radio frequency signal.

[0037] Step 2.2: According to the formula where η is the conversion efficiency of the soliton crystal optical frequency comb, ∝ is the proportional symbol, P in is the power of the pump light signal. The optical amplifier 3 reduces the power of the phase-modulated pump light signal, and the reduced power of the pump light signal needs to be greater than the minimum threshold power P th (i.e., the minimum power required to generate a stable optical frequency comb);

[0038] Then according to the formula where κ ex is the external coupling loss between the optical fiber and the whispering gallery mode microcavity 5, and κ 0 is the internal inherent loss of the whispering gallery mode microcavity 5. The smaller the two losses, the higher the quality factor (Q value) of the whispering gallery mode microcavity 5. Adjust the external coupling loss between the optical fiber and the microcavity to obtain the highest optical frequency comb conversion efficiency;

[0039] Finally, the optical amplifier 3 outputs the pump light signal with reduced power to the polarization controller 4;

[0040] Step 2.3: The polarization controller 4 adjusts the polarization of the pump light signal with reduced power output by the optical amplifier 3 so that the polarization of the pump light signal with reduced power is consistent with the polarization of the optical mode in the whispering gallery mode microcavity 5, obtaining a suitable intracavity light signal, and the intracavity light signal enters the whispering gallery mode microcavity 5;

[0041] In this embodiment, the output power of the laser 1 is 2 mW, and the output wavelength is controlled by the voltage signal of the feedback circuit 9. The laser 1 outputs a pump optical signal with a central wavelength of 1560.4 nm and a range of 200 MHz; the modulation frequency of the modulation radio frequency signal output by the signal generator 7 is 2 MHz, and the power of the modulation radio frequency signal is 2 dBm; the power of the amplified pump optical signal output by the optical amplifier 3 is 50 mW.

[0042] Step 3: Turn on the feedback circuit 9 and scan the wavelength of the pump optical signal output by the laser 1 so that the wavelength of the incident cavity optical signal enters the resonance peak of the whispering gallery mode microcavity 5 from the blue detuning side of the resonance peak. As the wavelength of the incident cavity optical signal approaches the frequency resonance wavelength, the optical power in the whispering gallery mode microcavity 5 increases. Figure 2 It shows the intracavity optical power signal and the first error signal of the optical power in the whispering gallery mode microcavity 5. The state of the whispering gallery mode microcavity 5 is judged by monitoring the intracavity optical power signal. The whispering gallery mode microcavity 5 includes the figure-light optical frequency comb in the I stage and the soliton crystal optical frequency comb in the II stage. In the I stage, due to the thermo-optic effect of the whispering gallery mode microcavity 5, the whispering gallery mode microcavity 5 is in a thermal self-steady state, and the figure-light optical frequency comb mode as shown in Fig. 3(a) can be observed; in the II stage, the whispering gallery mode microcavity 5 enters the soliton crystal state, and the soliton crystal optical frequency comb mode as shown in Fig. 3(b) can be seen.

[0043] Step 4: In order to obtain a stable soliton crystal optical frequency comb for a long time, after the whispering gallery mode microcavity 5 enters the soliton crystal optical frequency comb mode, turn on the photodetector 6 to collect the intracavity optical power signal of the soliton crystal, and then demodulate the intracavity optical power signal of the soliton crystal by the same frequency through the lock-in amplifier 8. Specifically: turn on the photodetector 6 to collect the intracavity optical power signal of the soliton crystal. The photodetector 6 converts the intracavity optical power signal of the soliton crystal into an electrical signal and outputs it to the lock-in amplifier 8. The signal generator 7 also outputs the modulation radio frequency signal to the lock-in amplifier 8 as a reference signal. The lock-in amplifier 8 demodulates the electrical signal converted from the intracavity optical power signal of the soliton crystal with the modulation radio frequency signal as the reference signal to obtain the first error signal, and outputs the first error signal to the feedback circuit 9.

[0044] Step 5: The feedback circuit 9 subtracts the first error signal from the set reference indication voltage signal to obtain the second error signal. The feedback circuit 9 adjusts the wavelength of the pump optical signal output by the laser 1 according to the second error signal, so that the wavelength of the pump optical signal output by the laser 1 is locked at the zero point of the second error signal. The wavelength of the pump optical signal output by the laser 1 is fixed at the red detuning side of the resonance peak of the whispering gallery mode microcavity 5, thereby stably capturing the soliton crystal optical frequency comb.

[0045] Among them, the reference indication voltage signal is set to the voltage value indicating that the whispering gallery mode microcavity 5 enters the soliton crystal state (i.e., the voltage value indicating the soliton crystal signal in the second stage), and the voltage value indicating the soliton crystal signal in the second stage is preferably the intermediate value of the soliton crystal step.

[0046] In this embodiment, by comparing η = P 51 / P 41 , the conversion efficiency of the highest soliton crystal optical frequency comb is achieved, η = 60%, where P 51 is the intracavity optical power of the soliton crystal optical frequency comb mode, that is, the intracavity power of the soliton crystal, and P 41 is the power of the incident optical signal into the cavity.

[0047] The method for generating a high-efficiency soliton crystal for a whispering gallery mode microcavity proposed by the present invention (based on active circuit control) realizes a higher conversion efficiency in the passive locking mode and requires a low pump laser power by actively adjusting the output wavelength of the laser 1 through the feedback circuit 9 and locking the soliton crystal at the high-power step based on the main peak shift locking of the soliton crystal of the microcavity, simply locking the soliton crystal state.

[0048] The present invention stabilizes the soliton crystal state for a long time by using the feedback circuit 9 to obtain the second error signal and actively adjusting the output wavelength of the laser 1, without relying on microcavity parameter pre-calibration or external compensation devices; it not only reduces the requirement for the wavelength stability of the laser 1, but also reduces the influence of low-frequency noise on phase adjustment. And the cross-platform applicability of the present invention is verified on the soliton crystal optical frequency comb platform, providing a feasible and reliable solution for the large-scale application of crystal microcavity optical frequency combs.

[0049] It should be noted that the embodiments described in the present invention are only illustrative of the spirit of the present invention. Those skilled in the art of the present invention can make various modifications or supplements to the described embodiments or use similar ways to replace them, but will not deviate from the spirit of the present invention or exceed the scope defined by the appended claims.

Claims

1. A method for controlling high-efficiency soliton crystals generated by a whispering gallery mode microcavity, characterized in that: The following steps are involved: Step 1: Build an active locking circuit for the soliton crystal optical frequency comb; Step 2: performing phase modulation, power adjustment, and polarization adjustment on the pump light signal in the active locking circuit to obtain an input cavity light signal; Step 3, the incoming optical signal enters the whispering gallery mode microcavity (5), and the wavelength of the pump optical signal is scanned, so that the whispering gallery mode microcavity (5) enters the soliton crystal optical frequency comb mode; Step 4: collecting the intracavity optical power signal of the soliton crystal, and performing co-frequency demodulation on the intracavity optical power signal of the soliton crystal to obtain a first error signal; Step 5: Subtract the first error signal from the set reference indication voltage signal to obtain a second error signal, and then adjust the wavelength of the pump light signal according to the second error signal so that the wavelength of the pump light signal is locked at the zero point of the second error signal to stably capture the soliton crystal optical frequency comb.

2. According to claim 1, a method for controlling high-efficiency soliton crystals generated by a whispering gallery mode microcavity, characterized in that: The reference indicating voltage signal in step 5 is set to a voltage value indicating that the whispering gallery mode microcavity (5) enters a soliton crystal state.

3. A method for controlling high-efficiency soliton crystals generated by a whispering gallery mode microcavity according to claim 2, characterized in that: The voltage value indicating that the whispering gallery mode microcavity (5) has entered the soliton crystal state is the middle value of the soliton crystal step.

4. A method for controlling high-efficiency soliton crystals generated by a whispering gallery mode microcavity according to claim 3, characterized in that: The active locking circuit of the soliton crystal optical frequency comb comprises a laser (1), an electro-optic modulator (2), an optical amplifier (3), a polarization controller (4), a whispering gallery mode microcavity (5), a photodetector (6), a phase-locked amplifier (8), and a feedback circuit (9) which are connected in sequence. The laser (1) outputs a pump light signal, and the output end of the feedback circuit (9) is connected to the laser (1). The active locking circuit also comprises a signal generator (7), the output end of the signal generator (7) is connected to the electro-optic modulator (2), and the output end of the signal generator (7) is also connected to the phase-locked amplifier (8).

5. A method for controlling high-efficiency soliton crystals generated by a whispering gallery mode microcavity according to claim 4, characterized in that: The step 2 specifically includes the following steps: Step 2.1, the laser (1) outputs a pump light signal to the electro-optic modulator (2), the signal generator (7) outputs a modulated radio frequency signal to the electro-optic modulator (2), the electro-optic modulator (2) performs phase modulation on the pump light signal output by the laser (1) according to the modulated radio frequency signal, and outputs the phase-modulated pump light signal to the optical amplifier (3); Step 2.2, the optical amplifier (3) reduces the power of the phase-modulated pump light signal, and the power of the reduced pump light signal needs to be greater than the minimum threshold power of the optical frequency comb; then adjusts the coupling loss between the optical fiber and the microcavity; and finally outputs the reduced power pump light signal to the polarization controller (4); Step 2.3, the polarization controller (4) adjusts the polarization of the pump light signal after the power is reduced so that the polarization of the pump light signal after the power is reduced is consistent with the polarization of the optical mode in the whispering gallery mode microcavity (5), thereby obtaining an input light signal.

6. A method for controlling high-efficiency soliton crystals generated by a whispering gallery mode microcavity according to claim 5, characterized in that: The step 3 specifically comprises: starting the feedback circuit (9) and scanning the wavelength of the pump light signal output by the laser (1), so that the wavelength of the cavity-entering light signal enters the resonance peak of the whispering gallery mode microcavity (5) from the blue detuned side of the resonance peak, and the whispering gallery mode microcavity (5) enters the Turing optical frequency comb mode and the soliton crystal optical frequency comb mode in turn.

7. A method for controlling high-efficiency soliton crystals generated by a whispering gallery mode microcavity according to claim 6, characterized in that: The step 4 specifically comprises: after the whispering gallery mode microcavity (5) enters the soliton crystal optical frequency comb mode, the photodetector (6) is turned on to collect the intracavity optical power signal of the soliton crystal, the photodetector (6) converts the intracavity optical power signal of the soliton crystal into an electrical signal and outputs it to the phase-locked amplifier (8), the signal generator (7) also outputs the modulated radio frequency signal to the phase-locked amplifier (8), the phase-locked amplifier (8) uses the modulated radio frequency signal as a reference signal to perform co-frequency demodulation on the electrical signal converted from the intracavity optical power signal of the soliton crystal to obtain a first error signal, and outputs the first error signal to the feedback circuit (9).

Images