A method for echo wall mode microcavity to generate high-efficiency soliton crystal control

Abstract

This invention discloses a method for controlling the generation of high-efficiency soliton crystals in a microcavity in whispering-gallery mode. By constructing an active locking circuit for the soliton crystal optical frequency comb, and then using the feedback control module in the active locking circuit to obtain a second error signal and actively adjust the laser output wavelength, the method achieves soliton crystal main peak offset locking based on the microcavity, locking the soliton crystal at the high-power step, and simply locking the soliton crystal state, thereby stabilizing the soliton crystal state for a long time. This achieves higher conversion efficiency than the passive locking method, without relying on microcavity parameter pre-calibration or external compensation devices. It not only reduces the requirements for laser wavelength stability but also reduces the impact of low-frequency noise on phase adjustment.

Description

Technical Field

[0001] This invention belongs to the field of optical frequency comb technology, specifically relating to a method for controlling the generation of high-efficiency soliton crystals in whispering-gallery mode microcavities. Background Technology

[0002] Optical frequency combs (FFCs) are optical pulse signals in the time domain and consist of a spectrum of uniformly spaced frequency components with coherent and stable phase relationships in the frequency domain. Benefiting from their ultrashort pulse characteristics and broadband coherent spectrum, FCCs have broad application prospects in coherent optical communication, precision spectroscopy, ultrafast ranging, and frequency synthesis. Traditional optical frequency comb platforms, such as sapphire pulsed lasers and electro-optic frequency combs, suffer from drawbacks such as large size and high power consumption. Optical frequency comb platforms based on whispering-gallery mode microcavities offer advantages such as small size and low power consumption, making them a research hotspot in recent years.

[0003] However, because a large portion of the pump laser energy of an optical frequency comb is not converted into effective comb power, the efficiency of single soliton microcombs is generally less than 5%. That is, the pump power is high, but the effective comb power output is low. This is a major reason why optical frequency combs are difficult to apply. The simplest way to improve efficiency is to increase the number of solitons within the microcavity. Soliton crystal optical frequency combs have multiples of solitons, providing a new solution for improving their efficiency. However, the limited range of soliton crystal optical frequency combs (soliton crystals) makes their stable existence a new challenge. To address this, in 2023, Adnan obtained soliton crystals in a low-Q on-chip microcavity under high pump power conditions using only laser wavelength scanning. In the same year, Cui Wenwen passively stabilized a soliton crystal optical frequency comb in a high-Q crystal microcavity under low pump power conditions using an auxiliary laser, obtaining a stable soliton crystal. Currently, existing methods for obtaining stable soliton crystals are either limited to on-chip microcavity platforms or use additional lasers. Summary of the Invention

[0004] The purpose of this invention is to address the aforementioned problems in the prior art by providing a method for controlling the generation of high-efficiency soliton crystals in whispering-gallery mode microcavities.

[0005] The above-mentioned objectives of the present invention are achieved by the following technical means:

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

[0007] Step 1: Construct the active locking circuit for the soliton crystal optical frequency comb;

[0008] Step 2: In the active locking circuit, the pump light signal is sequentially subjected to phase modulation, power adjustment, and polarization adjustment to obtain the cavity light signal;

[0009] Step 3: The inlet optical signal enters the whispering-gallery mode microcavity and scans the wavelength of the pump optical signal, causing the whispering-gallery mode microcavity to enter the soliton crystal optical frequency comb mode;

[0010] Step 4: Collect the intracavity optical power signal of the soliton crystal and demodulate the intracavity optical power signal of the soliton crystal at the same frequency 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. 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, and stably capture the soliton crystal optical frequency comb.

[0012] As described above, the reference indicator voltage signal in step 5 is set to the voltage value that indicates the whispering-gallery mode microcavity entering the soliton crystal state.

[0013] As mentioned above, the voltage value at which the whispering-gallery mode microcavity enters the soliton crystal state is the midpoint of the soliton crystal step.

[0014] As described above, the active locking circuit of the soliton crystal optical frequency comb 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 a pump light signal, and the output of the feedback circuit is then connected to the laser. The circuit also includes a signal generator, the output of which is connected to the electro-optic modulator and the output of which is also connected to the lock-in amplifier.

[0015] As described above, step 2 specifically includes the following steps:

[0016] Step 2.1: The laser outputs a pump light signal to the electro-optic modulator, the signal generator outputs a modulated radio frequency signal to the electro-optic modulator, the electro-optic modulator modulates the phase of the pump light signal output by the laser according to the modulated radio frequency signal, and outputs the phase-modulated pump light signal to the optical amplifier.

[0017] Step 2.2: The optical amplifier 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 adjust the coupling loss between the optical fiber and the microcavity; finally, output the pump light signal with reduced power to the polarization controller.

[0018] Step 2.3: The polarization controller 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, and the cavity light signal is obtained.

[0019] As described above, step 3 specifically involves: turning on the feedback circuit and scanning the wavelength of the pump light signal output from the laser, so that the wavelength of the input light signal enters the resonant peak of the whispering-gallery mode microcavity from the blue detuned side of the resonant peak, and the whispering-gallery mode microcavity sequentially enters the Turing optical frequency comb mode and the soliton crystal optical frequency comb mode.

[0020] As described above, step 4 specifically involves: after the whispering-gallery mode microcavity enters the soliton crystal optical frequency comb mode, the photodetector is activated 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 the modulated radio frequency signal to the lock-in amplifier. The lock-in amplifier uses the modulated radio frequency signal as a reference signal to demodulate the electrical signal converted from the intracavity optical power signal of the soliton crystal at the same frequency to obtain the first error signal, and outputs the first error signal to the feedback circuit.

[0021] Compared with the prior art, the present invention has the following advantages:

[0022] (1) The present invention uses a feedback circuit to actively adjust the output wavelength of the laser, and locks the soliton crystal based on the main peak shift of the soliton crystal in the whispering-gallery mode microcavity. It locks the soliton crystal at the high power step, simply locks the soliton crystal state, achieves higher conversion efficiency of the passive locking method, and the active locking scheme has higher robustness, significantly extends the existence time of the soliton crystal optical frequency comb, and requires low pump laser power.

[0023] (2) The present invention obtains the second error signal by using a feedback circuit and actively adjusts the output wavelength of the laser to stabilize the soliton crystal state for a long time without relying on microcavity parameter pre-calibration or external compensation devices; it not only reduces the requirements for the wavelength stability of the laser, but also reduces the impact of low-frequency noise on phase adjustment. Attached Figure Description

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

[0025] Figure 2 This is a diagram showing the evolution of intracavity optical power and the first error signal during the soliton crystal generation process of the present invention.

[0026] Figure 3(a) is a spectrum of the Turing optical frequency comb mode under the experimental conditions of Example 1 of the present invention;

[0027] Figure 3(b) shows the spectrum of the soliton crystal optical frequency comb mode under the experimental conditions of Example 1 of the present invention;

[0028] Figure 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-Photodetector; 7-Signal generator; 8-Lock-in amplifier; 9-Feedback circuit. Detailed Implementation

[0030] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention will be further described in detail below with reference to embodiments. The embodiments described herein are for illustration and explanation only and are not intended to limit the present invention.

[0031] Example 1:

[0032] A method for controlling the generation of high-efficiency soliton crystals in whispering-gallery mode microcavities includes the following steps:

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

[0034] In this embodiment, laser 1 is a wavelength-tunable narrow-linewidth laser with a tuning range of 1 nm, a linewidth of 10 kHz, and an output wavelength in the 1560 nm band. Electro-optic modulator 2 is a high-bandwidth lithium niobate electro-optic phase modulator with a maximum modulation bandwidth of 10 GHz. Optical amplifier 3 is a high-power erbium-doped fiber amplifier with a maximum output power of 2 W. Its operating wavelength is the same as the output wavelength of laser 1, and the amplified laser still has low noise characteristics. Polarization controller 4 is a three-ring fiber polarization controller that can withstand optical power of over 500 mW. Whispering-gallery mode microcavity 5 is a low-loss MgF2 material whispering-gallery mode microcavity with a free spectral range of 7.7 GHz and a high quality factor of 4 × 10⁻⁶. 8The photodetector 6 is a high-bandwidth balanced detector with a response bandwidth of 75MHz, which meets the requirements for detecting modulated signals; 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 with an internal low-pass filter that can filter out high-frequency modulated signals; the feedback circuit 9 is an FPGA circuit with high bandwidth.

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

[0036] Step 2.1: Laser 1 outputs a pump light signal to electro-optic modulator 2, and signal generator 7 outputs a modulated radio frequency signal to electro-optic modulator 2. Electro-optic modulator 2 performs phase modulation on the pump light signal output by 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 proportionality sign, and P in To reduce the power of the pump optical signal, optical amplifier 3 reduces the power of the phase-modulated pump optical signal, and the reduced power of the pump optical signal must be greater than the minimum threshold power P of the optical frequency comb. th (i.e., the minimum power required to generate a stable optical frequency comb);

[0038] Then according to the formula Among them, κ ex κ0 represents the external coupling loss between the optical fiber and the whispering-gallery mode microcavity 5, and κ0 represents the internal inherent loss of the whispering-gallery mode microcavity 5. The smaller the loss of both, the higher the quality factor (Q value) of the whispering-gallery mode microcavity 5. By adjusting the external coupling loss between the optical fiber and the microcavity, the highest optical frequency comb conversion efficiency can be obtained.

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

[0040] Step 2.3: The polarization controller 4 adjusts the polarization of the pump light signal output by the optical amplifier 3 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, and a suitable in-cavity light signal is obtained. The in-cavity light signal enters the whispering-gallery mode microcavity 5.

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

[0042] Step 3: Activate feedback circuit 9 to scan the wavelength of the pump light signal output from laser 1, causing the wavelength of the cavity light signal to enter the resonant peak of the whispering-gallery mode microcavity 5 from the blue detuned side of the resonant peak. As the wavelength of the cavity light signal approaches the frequency resonant wavelength, the optical power inside the whispering-gallery mode microcavity 5 increases. Figure 2 The image shows the intracavity optical power signal and the first error signal within the whispering-gallery mode microcavity 5. The state of the whispering-gallery mode microcavity 5 is determined by monitoring the intracavity optical power signal. The whispering-gallery mode microcavity 5 includes a Turing optical frequency comb in stage I and a soliton crystal optical frequency comb in stage II. In stage I, due to the thermo-optical effect of the whispering-gallery mode microcavity 5, the whispering-gallery mode microcavity 5 is in a thermally self-stable state, and the Turing optical frequency comb mode shown in Figure 3(a) can be observed. In stage II, the whispering-gallery mode microcavity 5 enters the soliton crystal state, and the soliton crystal optical frequency comb mode shown in Figure 3(b) can be observed.

[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, the photodetector 6 is turned on to collect the intracavity optical power signal of the soliton crystal, and then the intracavity optical power signal of the soliton crystal is demodulated at the same frequency by the lock-in amplifier 8. Specifically, 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 lock-in amplifier 8, the signal generator 7 also outputs the modulated radio frequency signal to the lock-in amplifier 8 as a reference signal, and the lock-in amplifier 8 uses the modulated radio frequency signal as a reference signal to demodulate the electrical signal converted from the intracavity optical power signal of the soliton crystal at the same frequency 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 light signal output by the laser 1 according to the second error signal, so that the wavelength of the pump light signal output by the laser 1 is locked at the zero point of the second error signal. The wavelength of the pump light signal output by the laser 1 is fixed at the red detuning of the resonant peak of the whispering-gallery mode microcavity 5, thereby stabilizing the capture of the soliton crystal optical frequency comb.

[0045] The reference indicator voltage signal is set to the voltage value that indicates the soliton crystal state of the whispering-gallery mode microcavity 5 (i.e., the voltage value that indicates the soliton crystal signal in stage II). The voltage value that indicates the soliton crystal signal in stage II is preferably the middle value of the soliton crystal step.

[0046] In this embodiment, by comparing η=P 51 / P 41 The highest conversion efficiency η = 60% was achieved in the soliton crystal optical frequency comb, P 51 P represents the intracavity optical power in the soliton crystal's optical frequency comb mode, i.e., the intracavity power of the soliton crystal. 41 This represents the power of the optical signal entering the cavity.

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

[0048] This invention achieves long-term stable soliton crystal states by using feedback circuit 9 to acquire the second error signal and actively adjust the output wavelength of laser 1, without relying on microcavity parameter pre-calibration or external compensation devices. This not only reduces the requirements for the wavelength stability of laser 1 but also reduces the impact of low-frequency noise on phase adjustment. The cross-platform applicability of this invention was verified on a 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 this invention are merely illustrative of the spirit of the invention. Those skilled in the art to which this invention pertains can make various modifications or additions to the described embodiments or use similar methods to substitute them, without departing from the spirit of the invention or exceeding the scope defined by the appended claims.

Claims

1. A method for controlling the generation of high-efficiency soliton crystals in whispering-gallery mode microcavities, characterized in that, Includes the following steps: Step 1: Construct the active locking circuit for the soliton crystal optical frequency comb; Step 2: In the active locking circuit, the pump light signal is sequentially subjected to phase modulation, power adjustment, and polarization adjustment to obtain the cavity light signal; Step 3: The inlet optical signal enters the whispering-gallery mode microcavity (5) and scans the wavelength of the pump optical signal, so that the whispering-gallery mode microcavity (5) enters the soliton crystal optical frequency comb mode; Step 4: Collect the intracavity optical power signal of the soliton crystal and demodulate the intracavity optical power signal of the soliton crystal at the same frequency to obtain the first error signal; Step 5: Subtract the first error signal from the set reference indication voltage signal to obtain the second error signal. 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, and stably capture the soliton crystal optical frequency comb.

2. The method for controlling the generation of high-efficiency soliton crystals in a microcavity in whispering-gallery mode according to claim 1, characterized in that, The reference indicator voltage signal in step 5 is set to the voltage value that indicates the whispering-gallery mode microcavity (5) entering the soliton crystal state.

3. The method for controlling the generation of high-efficiency soliton crystals in a microcavity in whispering-gallery mode according to claim 2, characterized in that, The voltage value at which the indicated whispering-gallery mode microcavity (5) enters the soliton crystal state is the midpoint of the soliton crystal step.

4. The method for controlling the generation of high-efficiency soliton crystals in a microcavity in whispering-gallery mode according to claim 3, characterized in that, The active locking circuit of the soliton crystal optical frequency comb includes 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 lock-in amplifier (8), and a feedback circuit (9) connected in sequence. The laser (1) outputs a pump light signal, and the output of the feedback circuit (9) is connected to the laser (1). The circuit also includes a signal generator (7), the output of which is connected to the electro-optic modulator (2) and the output of which is also connected to the lock-in amplifier (8).

5. The method for controlling the generation of high-efficiency soliton crystals in a microcavity in whispering-gallery mode according to claim 4, characterized in that, Step 2 specifically includes the following steps: 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) modulates 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 adjust the coupling loss between the optical fiber and the microcavity; finally output the pump light signal with reduced power 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 light mode in the whispering-gallery mode microcavity (5), and obtains the cavity light signal.

6. The method for controlling the generation of high-efficiency soliton crystals in a microcavity in whispering-gallery mode according to claim 5, characterized in that, Step 3 specifically involves: turning on the feedback circuit (9), scanning the wavelength of the pump light signal output by the laser (1), so that the wavelength of the cavity light signal enters the resonant peak of the whispering-gallery mode microcavity (5) from the blue detuned side of the resonant peak, and the whispering-gallery mode microcavity (5) sequentially enters the Turing optical frequency comb mode and the soliton crystal optical frequency comb mode.

7. The method for controlling the generation of high-efficiency soliton crystals in a microcavity in whispering-gallery mode according to claim 6, characterized in that, Step 4 is as follows: After the whispering-gallery mode microcavity (5) enters the soliton crystal optical frequency comb mode, the photodetector (6) is turned on to collect the cavity optical power signal of the soliton crystal. The photodetector (6) converts the cavity 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 modulated radio frequency signal to the lock-in amplifier (8). The lock-in amplifier (8) uses the modulated radio frequency signal as a reference signal to demodulate the electrical signal converted from the cavity optical power signal of the soliton crystal at the same frequency to obtain the first error signal, and outputs the first error signal to the feedback circuit (9).

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