A laser frequency stabilization system and method based on optical multi-frequency point cooperative locking

Abstract

The application provides a laser frequency stabilization system and method based on optical multi-frequency point cooperative locking, and relates to the technical fields of laser frequency stabilization and precision measurement. The system comprises a tunable laser unit, a multi-sideband modulation unit, an error signal generation unit and a feedback control unit. The multi-sideband modulation unit uses a controllable first radio frequency signal to modulate the phase of the laser to generate multiple sidebands; the error signal generation unit obtains a time domain electrical signal through frequency discrimination modulation. The feedback control unit calculates a total frequency discrimination error signal after the coherent superposition of multiple error signals, generates a feedback control signal according to the total frequency discrimination error signal and outputs the feedback control signal to the tunable laser unit to realize the locking of the center frequency of the laser. The application significantly improves the signal-to-noise ratio of the frequency discrimination signal through the cooperative action of multiple frequency points, can output an electrical absolute frequency signal anchored to the optical frequency reference interval while outputting high-stability frequency laser, and realizes the direct transfer of the optical domain frequency standard to the electrical domain.

Description

Technical Field

[0001] This invention relates to the field of laser frequency stabilization and precision measurement technology, specifically to a laser frequency stabilization system and method based on optical multi-frequency point cooperative locking, and an electrical signal output method with absolute frequency reference. Background Technology

[0002] Lasers, with their superior properties such as high directionality, high brightness, and high monochromaticity, have been widely used in many cutting-edge fields, including precision spectroscopy, optical metrology, optical communication, cold atom physics, and fundamental physics research. However, the output frequency of lasers is highly susceptible to external factors (such as changes in ambient temperature, mechanical vibration, acoustic noise, and air disturbances), causing frequency drift over time and with varying environmental conditions. In many high-precision applications, this frequency drift is unacceptable.

[0003] Therefore, to ensure high-precision stability of laser frequency, laser frequency stabilization technology has emerged as a key means to improve the performance of laser systems. The core idea of ​​laser frequency stabilization technology is to lock the output frequency of the laser to a highly stable reference frequency. This reference frequency is usually derived from specific transition lines of atoms or molecules (such as the hyperfine lines of iodine molecules and rubidium atoms), or the stable resonance mode of a high-fineness Fabry-Perot (FP) optical resonator.

[0004] Currently, commonly used laser frequency stabilization technologies include atomic two-way dispersion frequency stabilization (DAVLL), polarization spectral frequency stabilization, Pound-Drever-Hall (PDH) frequency stabilization, saturated absorption spectral frequency stabilization, frequency modulation spectral frequency stabilization (FMS), and modulation transfer spectral frequency stabilization (MTS).

[0005] Among the aforementioned laser frequency stabilization technologies, PDH and FMS technologies have become ideal choices for achieving the highest precision laser frequency stabilization due to their extremely high detection sensitivity, fast response bandwidth, and strong suppression of background noise. These technologies typically apply high-frequency phase or frequency modulation to the laser through external modulation devices such as electro-optic modulators (EOMs). Without directly interfering with the laser itself, they extract the frequency discrimination error signal, which sensitively reflects the detuning between the laser frequency and the reference frequency, using a photodetector combined with mixing and demodulation techniques.

[0006] Existing mainstream frequency stabilization schemes typically employ a "single-frequency-point locking" mode, which utilizes the interaction between a laser carrier and a single optical reference peak to generate a frequency discrimination error signal. However, those skilled in the art have found in practice that these mainstream technical solutions still face the following two key technical bottlenecks when pursuing higher frequency stabilization accuracy and wider application range: 1. The Signal-to-Noise Ratio (SNR) Bottleneck of the Error Signal and Compromises to Electronic Noise: In traditional PDH or FMS schemes, the frequency discrimination error signal is generated by the interaction of a single frequency component of the laser (e.g., the carrier wave) with a single resonance peak (or absorption peak) of the optical reference. The amplitude of the error signal (i.e., the discrimination slope) generated by this "single-peak locking" mode is fixed. Therefore, the signal-to-noise ratio (SNR) of this error signal is directly limited by the noise of the subsequent electronic links. To achieve high-precision locking, the photodetector, RF mixer, low-noise amplifier, and servo controller in the servo feedback loop must have extremely low noise floor (e.g., dark current noise, thermal noise, and 1 / f noise). This not only significantly increases the cost and complexity of the system, but the level of these electronic noises ultimately becomes one of the key bottlenecks limiting the ultimate accuracy of laser frequency stabilization.

[0007] 2. Lack of Optical-Electrical Frequency Transmission: Traditional frequency stabilization systems only stabilize the optical frequency and cannot directly provide an electrical frequency standard associated with that optical frequency reference. In practical applications, it is often necessary to obtain both stable laser output and a high-precision radio frequency reference signal simultaneously, but existing technologies usually require additional expensive equipment (such as optical frequency combs) to establish the connection between the optical and electrical domains.

[0008] Therefore, there is an urgent need for a new frequency stabilization architecture that can break through the single-point locked signal-to-noise ratio bottleneck and provide a high-precision electrical frequency reference while outputting frequency-stabilized laser. Summary of the Invention

[0009] The purpose of this invention is to provide a laser frequency stabilization system, method, and electrical signal output method with absolute frequency reference based on optical multi-frequency point cooperative locking. This invention significantly improves the signal-to-noise ratio (SNR) of the frequency discrimination error signal without relying on expensive low-noise electronics through multi-sideband modulation and multi-frequency point cooperative locking technology. Simultaneously, through a dual-loop locking mechanism, the physical interval of the optical frequency reference is converted in real time into an electrical radio frequency signal output with absolute frequency reference significance. This addresses the problem of limited SNR of error signals in existing technologies, significantly improving the SNR and amplitude of the frequency discrimination error signal through a novel signal enhancement mechanism without relying on extremely expensive low-noise electronics.

[0010] To achieve the above objectives, in a first aspect, the present invention provides a laser frequency stabilization system, comprising a tunable laser unit, a multi-sideband modulation unit, an error signal generation unit connected sequentially along an optical axis, and a feedback control unit electrically connected to the tunable laser unit, the multi-sideband modulation unit, and the error signal generation unit. The tunable laser unit generates a continuous tunable laser beam; the multi-sideband modulation unit modulates the phase of the tunable laser to generate multiple sidebands with different frequency intervals, forming multi-sideband light; the error signal generation unit includes an optical frequency reference with multiple discrete frequency reference peaks. Multiple sideband components in the multi-sideband light are configured to simultaneously interact with multiple adjacent frequency reference peaks of the optical frequency reference. The error signal generation unit detects the interacting optical signal and converts it into a time-domain electrical signal; the feedback control unit demodulates the time-domain electrical signal to obtain a total frequency discrimination error signal, which is a coherent superposition of multiple independent error signals generated by the interaction of multiple sideband components with their corresponding frequency reference peaks; the feedback control unit also generates a feedback control signal based on the total frequency discrimination error signal and outputs it to the tunable laser unit to achieve high-performance locking of the laser center frequency.

[0011] According to a laser frequency stabilization system provided by the present invention, the multi-sideband modulation unit includes a first modulator and a first radio frequency source. The first radio frequency source is used to generate a first radio frequency signal and drive the first modulator to generate multi-sideband light with frequency intervals determined by the frequency of the first radio frequency signal.

[0012] According to the laser frequency stabilization system provided by the present invention, the error signal generation unit further includes a second modulator, a second radio frequency source, and a photodetector; the second radio frequency source is used to generate a second radio frequency signal, and the second radio frequency signal drives the second modulator to superimpose a modulation signal for phase-sensitive detection onto the multi-sideband light; the second modulator is used to superimpose a frequency discrimination modulation signal onto the multi-sideband light; the photodetector is used to detect the transmitted or reflected light signal after interaction, and convert the light signal carrying frequency detuning information into a time-domain electrical signal.

[0013] According to the laser frequency stabilization system provided by the present invention, the feedback control unit includes a calculation module and an electrical control system; the calculation module is used to perform frequency mixing processing on the time-domain electrical signal and the second radio frequency signal to calculate the total frequency discrimination error signal; the electrical control system is used to generate a feedback control signal based on the total frequency discrimination error signal and output it to the tunable laser unit to realize the locking of the laser center frequency.

[0014] According to a laser frequency stabilization system provided by the present invention, the first radio frequency source is configured to have an external frequency control port and a signal output port. The external frequency control port is used to receive feedback control signals from the feedback control unit. When the system is in a dual-loop locked state, the electrical control system controls and locks the output frequency of the first radio frequency source according to the total frequency discrimination error signal, so that the output frequency of the first radio frequency source is strictly anchored to the spacing between adjacent frequency reference peaks of the optical frequency reference. The signal output port is used to output an electrical frequency signal with absolute frequency reference characteristics.

[0015] According to the laser frequency stabilization system provided by the present invention, the preferred range of tuning speed of the tunable laser unit is not less than 20 kHz, the preferred range of mode-hopping-free tuning is greater than 0.5 nm, and the preferred range of spectral linewidth of the output laser is less than 10 kHz.

[0016] According to the laser frequency stabilization system provided by the present invention, the first modulator is an electro-optic phase modulator, the frequency range of the first radio frequency signal is preferably not less than 1 GHz, the optical frequency reference is selected from any one of a high-precision Fabry-Perot optical resonator or a saturable absorption gas cell, and the frequency range of the second radio frequency signal is preferably not more than 100 MHz. The calculation module uses a phase-sensitive detection method to mix and low-pass filter the time-domain electrical signal output by the photodetector with the first radio frequency signal, and demodulate to obtain the total frequency discrimination error signal.

[0017] According to a laser frequency stabilization system provided by the present invention, the error signal generation unit includes a second radio frequency source and a photodetector; the second radio frequency source is used to generate a second radio frequency signal; the photodetector is used to detect the transmitted or reflected light signal after interaction, and convert the light intensity modulation signal carrying frequency detuning information into a time-domain electrical signal. The system also includes a broadband radio frequency combiner. The first radio frequency source and the second radio frequency source are both electrically connected to the broadband radio frequency combiner. The broadband radio frequency combiner is used to superimpose the first radio frequency signal and the second radio frequency signal to generate a composite signal to drive the first modulator, so that the tunable laser generates multi-sideband light through the first modulator while superimposing the frequency discrimination modulation signal.

[0018] In a second aspect, the present invention provides a laser frequency stabilization method, employing the laser frequency stabilization system of the first aspect, the laser frequency stabilization method comprising: The frequency of the first radio frequency signal is set to be numerically equal to the spacing between adjacent frequency reference peaks of the optical frequency reference. Adjust the center frequency of the tunable laser unit so that the laser carrier is aligned with a certain frequency reference peak of the optical frequency reference. At this time, multiple sidebands in the multi-sideband light fall into the frequency discrimination dynamic range of the corresponding frequency reference peak. A photodetector is used to detect the transmitted or reflected light signal after multiple sideband components interact with multiple frequency reference peaks, and the light signal is converted into a time-domain electrical signal. The time-domain electrical signal is then mixed and demodulated with a second radio frequency signal by a calculation module to obtain the total frequency discrimination error signal. The electrical control system outputs a feedback control signal to the tunable laser unit based on the total frequency discrimination error signal, so that the total frequency discrimination error signal approaches zero, thereby locking the laser center frequency.

[0019] Thirdly, the present invention provides an electrical signal output method with an absolute frequency reference, employing the laser frequency stabilization system of the first aspect. The electrical signal output method with an absolute frequency reference includes: The second laser frequency stabilization method locks the carrier center frequency of the tunable laser unit onto a certain frequency reference peak of the optical frequency reference. At this point, the laser frequency is stabilized at the [missing information - likely a specific peak or peak]. One frequency reference peak; A low-frequency jitter signal is generated inside the electrical control system and superimposed on the first radio frequency source, causing a slight frequency modulation of the output frequency of the first radio frequency source. The demodulation module demodulates the low-frequency jitter signal from the time-domain electrical signal output by the photodetector. The amplitude and phase of this electrical signal reflect the relationship between the first upper sideband and the second lower sideband. The detuning of each frequency reference peak; The demodulated electrical signal is fed back to the first radio frequency source, and the output frequency of the first radio frequency source is automatically adjusted by the electrical control system until the first upper sideband is precisely aligned with the second... One frequency reference peak; An electrical frequency signal is output through the first radio frequency source. The frequency of this electrical frequency signal is physically locked to the inherent frequency interval of the optical frequency reference, thereby outputting an electrical signal with absolute frequency reference characteristics.

[0020] This invention has at least the following beneficial effects: (1) This invention employs multi-sideband modulation and multi-frequency point collaborative locking technology, which significantly improves the signal-to-noise ratio and amplitude of the feedback control signal. By enabling multiple sidebands to interact simultaneously with multiple frequency reference peaks (including absorption peaks or resonance peaks) of the optical frequency reference, and by achieving coherent superposition of multiple error signals in the solution module, the amplitude of the total frequency discrimination error signal is multiplied. This design effectively reduces the stringent requirements of the system on the noise floor of the photodetector and subsequent electrical control system, and significantly improves the ultimate accuracy of laser frequency stabilization and the robustness of the system without adding expensive low-noise devices.

[0021] (2) This invention cleverly transforms the physical quantity of "optical frequency interval" into an "electrical absolute frequency signal" output by introducing a precisely controllable and traceable first radio frequency source. Users can directly obtain a high-precision radio frequency standard signal strictly anchored to the optical reference from this system without using an optical frequency comb. This design enables the system to have the dual functions of a "high-performance frequency-stabilized laser" and an "optical-to-electrical frequency conversion standard source", greatly expanding the application scope and value of the system. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0023] In the attached diagram: Figure 1 This is a schematic diagram of the laser frequency stabilization system based on optical multi-frequency point cooperative locking in Embodiment 2 of the present invention; Figure 2 This is a schematic diagram of the optical path of the laser frequency stabilization system based on optical multi-frequency point cooperative locking in Embodiment 1 of the present invention; Figure 3 This is a schematic diagram of the optical path of the laser frequency stabilization system based on optical multi-frequency point cooperative locking in Embodiment 2 of the present invention; Figure 4 This is a schematic diagram of the optical path of the laser frequency stabilization system based on optical multi-frequency point collaborative locking in Embodiment 3 of the present invention.

[0024] Reference numerals: 10 tunable laser unit, 20 multi-sideband modulation unit, 30 error signal generation unit, 40 feedback control unit; 11 tunable laser, 21 first modulator, 22 first radio frequency source, 23 frequency counter, 24 broadband radio frequency combiner; 31 second modulator, 32 second radio frequency source, 33 optical frequency reference, 34 photodetector; 41 calculation module, 42 electrical control system. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0026] The following detailed description of some embodiments of the present invention will be provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0027] Example 1 This embodiment aims to address the problems of small error signal amplitude and limited signal-to-noise ratio in traditional single-frequency locking schemes. This embodiment achieves high-precision stabilization of the laser frequency by constructing a multi-sideband modulation system that simultaneously locks multiple frequency reference peaks using multiple sidebands. Please refer to [link to relevant documentation]. Figure 1 and Figure 2This invention provides a laser frequency stabilization system based on optical multi-frequency point cooperative locking (hereinafter referred to as a laser frequency stabilization system). The system architecture and configuration are as follows: Figure 1 As shown, the system includes a tunable laser unit 10, a multi-sideband modulation unit 20, an error signal generation unit 30 connected in sequence along the optical axis, and a feedback control unit 40 electrically connected to the tunable laser unit 10, the multi-sideband modulation unit 20, and the error signal generation unit 30.

[0028] The tunable laser unit 10 is used to generate a continuous tunable laser; the multi-sideband modulation unit 20 is used to phase modulate the tunable laser to generate multiple sidebands with different frequency intervals, forming multi-sideband light; the error signal generation unit 30 includes an optical frequency reference with multiple discrete frequency reference peaks, and the multiple sideband components in the multi-sideband light are configured to interact simultaneously with multiple adjacent frequency reference peaks of the optical frequency reference, respectively. The error signal generation unit 30 is used to detect the optical signal after the interaction and convert it into a time-domain electrical signal; the feedback control unit 40 is used to demodulate the time-domain electrical signal to obtain a total frequency discrimination error signal, which is a coherent superposition of multiple independent error signals generated by the interaction of multiple sideband components with their corresponding frequency reference peaks; the feedback control unit 40 is also used to generate a feedback control signal based on the total frequency discrimination error signal and output it to the tunable laser unit 10 to achieve high-performance locking of the laser center frequency.

[0029] It should be noted that the laser frequency stabilization system provided by this invention includes a tunable laser unit, a multi-sideband modulation unit, an error signal generation unit, and a feedback control unit connected sequentially along the optical axis. The multi-sideband modulation unit uses a controllable first radio frequency signal to phase-modulate the laser, generating multiple sidebands; the error signal generation unit obtains a time-domain electrical signal through frequency discrimination modulation. The feedback control unit calculates the total frequency discrimination error signal after coherent superposition of multiple error signals, generates a feedback control signal based on the total frequency discrimination error signal, and outputs it to the tunable laser unit to lock the laser center frequency. This invention not only significantly improves the signal-to-noise ratio and amplitude of the total frequency discrimination error signal through multi-peak cooperative locking, but also realizes the function of converting optical frequency intervals into high-precision electrical frequency readings by locking the frequency of the first radio frequency signal to the free spectral range (FSR) or spectral line spacing of the optical reference. This invention has a compact structure, high precision, and can achieve precise measurement without the need for an expensive optical frequency comb.

[0030] For ease of description, the connections between components within each unit and between units below refer to coaxial connections (optical paths) or cable connections (circuits).

[0031] Tunable laser unit 10: Used to generate continuous, single-mode, narrow-linewidth tunable laser output. The tunable laser 11 used in this unit is selected from any one of semiconductor lasers, solid-state lasers, or fiber lasers. In this embodiment, a narrow-linewidth laser source based on a fiber laser is selected. The center wavelength can be selected from 532nm, 780nm, 1064nm, 1550nm, etc., depending on the application requirements. The selection of the operating wavelength depends on the specific application scenario requirements (such as the operating wavelength corresponding to acetylene or rubidium atomic absorption lines or FP cavities). Its key performance indicators are configured as follows: tuning speed not less than 20kHz to ensure that the feedback control unit 40 can compensate for frequency disturbances caused by the environment in a timely manner; mode-hopping tuning range greater than 0.5nm to cover multiple frequency reference peaks (i.e., reference frequency points) of the optical frequency reference; and spectral linewidth of the output laser less than 10kHz to ensure that the total frequency discrimination error signal obtained by subsequent calculation is not affected by the linewidth of the laser itself. The laser can achieve rapid fine-tuning of the output frequency through the voltage applied by the internally integrated lead zirconate titanate (PZT) piezoelectric ceramic.

[0032] Multi-sideband modulation unit 20: Its optical input is connected to the output of tunable laser unit 10, and it is used to phase modulate the tunable laser to generate multiple sidebands with different frequency intervals, forming multi-sideband light. This unit includes a first modulator 21 and a first radio frequency source 22. The first modulator 21 is preferably a broadband lithium niobate (LiNbO3) waveguide electro-optic phase modulator (EOM), which has a low radio frequency half-wave voltage and a radio frequency bandwidth of not less than 10 GHz to support the generation of large-spacing sidebands. The first radio frequency source 22 is connected to the radio frequency input of the first modulator 21, and it is used to generate a highly stable first radio frequency signal (high-frequency radio frequency signal) and drive the first modulator 21 to generate multi-sideband light with frequency intervals determined by the frequency of the first radio frequency signal. The radio frequency frequency of the first radio frequency signal is generally not less than 1 GHz, and it is used to generate large-amplitude, large-spacing modulation sidebands.

[0033] Error signal generation unit 30: Its optical path input is connected to the output of the multi-sideband modulation unit 20. This unit includes, coaxially arranged: a second modulator 31, an optical frequency reference 33, a photodetector 34, and a second radio frequency source 32 electrically connected to the second modulator 31. Second modulator 31: Employs a low-frequency electro-optic phase modulator, connected in series in the optical path, used to superimpose a frequency discrimination modulation signal onto the multi-sideband light. Second radio frequency source 32: Generates a second radio frequency signal (i.e., a reference signal), which drives the second modulator 31 to superimpose a modulation signal for phase-sensitive detection onto the multi-sideband light. Optical frequency reference 33 has multiple frequency reference peaks; multiple sideband components in the multi-sideband light can simultaneously interact with multiple frequency reference peaks. Therefore, as a frequency scale, this embodiment can employ a high-precision Fabry-Perot (FP) optical resonator or a saturable absorber filled with saturable absorber gas, utilizing the free spectral range (SFR) or hyperfine spectral lines as multi-frequency references. The photodetector 34 is used to detect transmitted or reflected light signals after interaction with multiple frequency reference peaks, and converts the light signals carrying frequency detuning information into time-domain electrical signals. Its response bandwidth is not less than 10MHz to ensure that instantaneous intensity and phase changes can be captured in a short time, providing accurate input signals for the calculation module 41. The photodetector 34 can be any one of a photodiode, avalanche photodiode, or photomultiplier tube.

[0034] Specifically, the second radio frequency signal is generally no higher than 100MHz, significantly lower than the first radio frequency signal, to avoid signal interference between the two. The first radio frequency source 22 is a high-frequency radio frequency source that can output one or more high-frequency radio frequency signals of different frequencies, which have reciprocal frequency characteristics and random phase relationship with the second radio frequency signal.

[0035] Furthermore, the first and second modulators are phase modulators, and the phase modulation bandwidth is not less than the maximum output RF frequency of the corresponding RF source, that is, the modulation bandwidth of the first modulator 21 is not less than 10GHz, and the modulation bandwidth of the second modulator 31 is not less than 100MHz.

[0036] Feedback control unit 40 includes a calculation module 41 and an electrical control system 42. The calculation module 41 includes a dual-balanced detector and a low-pass filter, used to mix the time-domain electrical signal output from the photodetector 34 with a second radio frequency signal to obtain a total frequency discrimination error signal. This total frequency discrimination error signal is the superposition of error signals generated by the interaction of multiple sideband components with their corresponding frequency reference peaks. The electrical control system 42 is used to generate a feedback control signal based on the total frequency discrimination error signal and output it to the tunable laser unit 10 to achieve laser center frequency locking. The electrical control system 42 is preferably a PID (proportional-integral-derivative) servo controller based on FPGA or analog circuits. It receives the total frequency discrimination error signal and outputs a feedback control signal, such as a feedback voltage signal, to the wavelength modulation port of the tunable laser 11 to achieve closed-loop locking of the laser center frequency. It should be noted that the voltage amplitude and polarity of the feedback voltage signal reflect the detuning of the laser frequency relative to the reference cavity resonant frequency in real time.

[0037] Specifically, the laser frequency output by the adjustable laser 11 is: After passing through the first modulator 21 in the multi-frequency modulation unit 20 (modulation frequency is...), modulation depth is The second modulator 31 in the error signal generation unit 30 (modulation frequency is...) modulation depth is ,For example After that, the light field intensity It can be represented as the superposition of multiple frequency components:

[0038] In the formula, Both represent Bezier functions of the first kind, where n and k are the orders of the corresponding Bezier functions of the first kind, respectively; , respectively, represent the modulation depths of the two-level modulation; E(t) is the light field intensity at time t, and E0 is the initial light field intensity; i represents the imaginary unit.

[0039] When adjusting When its value is precisely equal to the distance between adjacent reference frequency points of optical frequency reference 33 (such as in FSR), the laser carrier ( Align with the m-th reference frequency, and simultaneously the first upper sideband (n+1) and the first lower sideband (n-1) will automatically align with the m-th reference frequency. and A reference frequency point. At this time, the time-domain electrical signal output by the photodetector 34, i.e., the photocurrent. It contains beat frequency components generated by the interaction of all sidebands and their corresponding cavity modes. After demodulation by the demodulation module 41, the total frequency discrimination error signal... The coherent superposition of independent error signals at each sideband:

[0040] In the formula, To effectively cover the set of comb order within the optical frequency reference bandwidth, The light field intensity weight of the nth order comb tooth is given by [the value of the comb tooth]. The term characterizes the frequency discrimination error curve generated by the interaction between a single comb tooth and its corresponding reference frequency. The complex transfer function is a reference to the optical frequency, where the magnitude represents the amplitude transfer characteristic, the argument represents the phase dispersion characteristic, and the superscript... Represents the complex conjugate operation; The interference beat frequency process of each sideband component and its discrimination sideband after passing through the optical frequency reference is described.

[0041] Because the phase dispersion slopes experienced by each sideband as it passes through the reference cavity are of the same sign and phase synchronized, the amplitudes of the demodulated error signals are directly linearly superimposed (e.g., three peaks locked, resulting in an amplitude increase of approximately three times). In the electronic link, the main noise sources (such as detector thermal noise and shot noise) do not increase proportionally with the signal amplitude. Therefore, the system's signal-to-noise ratio (SNR) is significantly improved, thereby greatly reducing the noise floor of the frequency stabilization system and improving locking accuracy.

[0042] Based on the same inventive concept, this embodiment also provides a laser frequency stabilization method, which employs the aforementioned laser frequency stabilization system. The laser frequency stabilization method includes the following steps: S1. Set the frequency of the first radio frequency signal to be numerically equal to the spacing between adjacent frequency reference peaks of the optical frequency reference 33; S2. Adjust the center frequency of the tunable laser unit 10 to make the output laser frequency... Aligned with a certain frequency reference peak of optical frequency reference 33, at this time multiple sidebands in the multi-sideband light simultaneously fall into the frequency discrimination dynamic range of the corresponding frequency reference peak. It should be noted that the frequency discrimination dynamic range refers to the frequency range in which the error signal can maintain a monotonic change and an effective slope near the center frequency of the frequency reference peak (such as the transmission peak of an FP cavity or the atomic absorption peak), i.e., the "effective range that can be locked in". Only when the laser frequency is within this range can it be determined whether the frequency is too high or too low, and thus correction can be performed. If it exceeds this range, the error signal will be distorted or return to zero, resulting in the inability to lock in.

[0043] S3. The photodetector 34 is used to detect the transmitted or reflected light signal after multiple sideband components interact with multiple frequency reference peaks, and the light signal is converted into a time-domain electrical signal. The time-domain electrical signal is mixed and demodulated with the second radio frequency signal by the calculation module 41 to obtain the total frequency discrimination error signal. S4. The electrical control system 42 outputs a feedback control signal to the tunable laser unit 10 based on the total frequency discrimination error signal, so that the total frequency discrimination error signal approaches zero and the laser center frequency is locked.

[0044] Example 2 As a second embodiment, this embodiment, based on the first embodiment, provides a frequency-stabilized laser while outputting an electrical signal with an absolute frequency reference. The hardware of this embodiment is basically the same as that of the first embodiment, but specific improvements have been made to the configuration of the multi-sideband modulation unit 20 and the feedback control unit 40 to support precision measurement functions. The specific improvements are as follows: Multi-sideband modulation unit 20: The first radio frequency source 22 must be a microwave source with an external frequency control port, which is used to receive feedback control signals from the feedback control unit 40 to fine-tune its modulation frequency. .like Figure 3 As shown, a new frequency counter 23 is added, the input of which is connected to the signal monitoring output of the first radio frequency source 22, for real-time and precise reading of the locked signal. The frequency counter 23 uses a rubidium clock or a GPS disciplined clock as its time base, achieving a measurement accuracy better than 1 Hz. The first radio frequency source 22 also has a signal output port. When the system is in a dual-loop locked state, the electrical control system 42 controls and locks the output frequency of the first radio frequency source 22 according to the total frequency discrimination error signal, ensuring that the output frequency of the first radio frequency source 22 is strictly anchored to the spacing between adjacent frequency reference peaks of the optical frequency reference 33. The signal output port is used to output an electrical frequency signal with absolute frequency reference characteristics.

[0045] Feedback control unit 40: The calculation module 41 is configured to extract the total frequency discrimination error signal for the detuning of the first RF source 22. Specifically, this module internally adds a lock-in amplifier circuit for demodulating specific low-frequency jitter signals.

[0046] The electrical control system 42 is configured with dual PID outputs. The first control output is connected to the tunable laser 11 (for locking the laser frequency), and the second control output is connected to the external frequency control port of the first radio frequency source 22 (for locking the radio frequency).

[0047] Specifically, this embodiment establishes a dual feedback loop and uses an "optical frequency scale" to provide a reference for the frequency of electrical signals.

[0048] Main loop (frequency stabilization loop): Sets the laser frequency... Locked at optical frequency reference 33 Each frequency reference peak Above, that is: .

[0049] Sub-loop (measurement loop): This measures the first upper sideband frequency of the laser. Locked in Each frequency reference peak Above, that is: .

[0050] Combining the above two equations, we can obtain:

[0051] In the formula, This is the optical frequency interval to be measured (such as FSR). At this time, the modulation frequency of the first RF source 22... This is strictly equal to the optical frequency interval. At this point, the first RF source 22 is no longer a free-running oscillator, but a frequency standard regulated by an "optical ruler." Users can directly obtain this high-precision RF signal through the signal output port of the first RF source 22. The value of this signal lies in the fact that it provides an electrical frequency reference with absolute physical meaning (tracing back to the FP cavity length or atomic energy level difference). This design changes the traditional frequency stabilization system's singular output of only optical signals, enabling the system of this invention to be used as a hybrid optoelectronic frequency reference source, particularly suitable for precision measurement scenarios requiring optoelectronic frequency synchronization.

[0052] Based on the foregoing analysis, this embodiment provides an electrical signal output method with an absolute frequency reference, employing the aforementioned laser frequency stabilization system. This electrical signal output method with an absolute frequency reference includes the following steps: S1: Using the aforementioned laser frequency stabilization method, but only closing the main loop, the carrier center frequency of the tunable laser unit 10 is locked at a certain frequency reference peak of the optical frequency reference 33. At this time, the laser frequency is stabilized at the [missing information - likely a specific peak or peak]. Each frequency reference peak .

[0053] S2: A tiny low-frequency jitter signal (e.g., 1kHz) is generated inside the electrical control system 42 and superimposed on the external frequency control port of the first radio frequency source 22, so that the output frequency of the first radio frequency source 21 is slightly modulated.

[0054] S3: The decomposition module 41 uses lock-in amplification technology to demodulate the low-frequency jitter signal from the time-domain electrical signal of the photodetector 34. The amplitude and phase of this electrical signal reflect the first upper sideband and the second lower sideband. The detuning of a frequency reference peak.

[0055] S4: Close the secondary loop and feed the demodulated electrical signal back to the external frequency control port of the first RF source 22. The electrical control system 42 will automatically adjust the output frequency of the first RF source 21 until the first upper sideband is precisely aligned with the second... One frequency reference peak.

[0056] S5: When the system is in a dual-locked state, an electrical frequency signal with an absolute frequency reference is obtained directly through the signal output port of the first RF source 22. The frequency of this electrical frequency signal is physically locked to the inherent frequency interval of the optical frequency reference 33, thus serving as an electrical signal output with absolute frequency reference characteristics. The value of the frequency counter 23 is read, and this reading is the electrical frequency signal with an absolute frequency reference.

[0057] Example 3 As a third embodiment, this embodiment further proposes a highly integrated hardware implementation scheme based on embodiments one and two. This embodiment utilizes signal multiplexing technology, enabling the simultaneous generation of multi-sideband (MSB) light and the loading of the frequency discrimination modulation signal using only a single electro-optic phase modulator, thereby significantly reducing the system's size, cost, and energy consumption. This embodiment merges and reconstructs the MSB modulation unit 20 and the error signal generation unit 30, with the specific configuration as follows: like Figure 4 As shown, the system no longer uses a second modulator 31, but instead introduces a broadband RF combiner 24. The inputs of this module are connected to the first RF source 22 and the second RF source 32, respectively. This module is used to superimpose the first RF signal and the second RF signal, which have significantly different frequency bands, to generate a composite signal that drives the first modulator 21. This allows the tunable laser to generate multi-sideband light through the first modulator 21 while simultaneously superimposing a frequency discrimination modulation signal. Therefore, this embodiment simplifies the hardware while fully retaining the "error signal-to-noise ratio enhancement" function of Embodiment 1 and the "electrical signal output with absolute frequency reference" function of Embodiment 2.

[0058] In summary, this invention introduces a tunable first radio frequency source 22 into the multi-sideband modulation unit 20 to generate multi-sideband light that matches the frequency reference peak spacing of the optical frequency reference 33. Combined with the frequency discrimination modulation of the error signal generation unit 30, a unique multi-frequency point collaborative locking mechanism is constructed. On one hand, this mechanism utilizes the coherent superposition of multiple error signals to significantly improve the amplitude and signal-to-noise ratio of the frequency discrimination error signal without increasing the electronic noise floor, achieving high-precision frequency stabilization that surpasses traditional single-peak locking. On the other hand, by constructing a dual-loop feedback control architecture, the modulation frequency of the first radio frequency source 22 is... Precisely locked to the optical frequency interval, directly obtaining a high-precision radio frequency standard signal that is strictly anchored to the optical reference.

[0059] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the embodiments disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. It should be understood that the invention is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A laser frequency stabilization system, characterized in that, It includes a tunable laser unit, a multi-sideband modulation unit, an error signal generation unit, and a feedback control unit that are all electrically connected to the tunable laser unit, the multi-sideband modulation unit, and the error signal generation unit, which are connected in sequence along the optical axis. The tunable laser unit is used to generate continuous tunable laser light; the multi-sideband modulation unit is used to phase modulate the tunable laser light to generate multiple sidebands with different frequency intervals, forming multi-sideband light; the error signal generation unit includes an optical frequency reference with multiple discrete frequency reference peaks, the multiple sideband components in the multi-sideband light are configured to interact simultaneously with multiple adjacent frequency reference peaks of the optical frequency reference, and the error signal generation unit is used to detect the optical signal after the interaction and convert it into a time-domain electrical signal; The feedback control unit is used to demodulate the time-domain electrical signal to obtain the total frequency discrimination error signal. The total frequency discrimination error signal is a coherent superposition of multiple independent error signals generated by the interaction of multiple sideband components with their corresponding frequency reference peaks. The feedback control unit is also used to generate a feedback control signal based on the total frequency discrimination error signal and output it to the tunable laser unit to achieve high-performance locking of the laser center frequency.

2. The laser frequency stabilization system according to claim 1, characterized in that, The multi-sideband modulation unit includes a first modulator and a first radio frequency source. The first radio frequency source is used to generate a first radio frequency signal and drive the first modulator to generate multi-sideband light with frequency intervals determined by the frequency of the first radio frequency signal.

3. The laser frequency stabilization system according to claim 2, characterized in that, The error signal generation unit further includes a second modulator, a second radio frequency source, and a photodetector; the second radio frequency source is used to generate a second radio frequency signal, and the second radio frequency signal drives the second modulator to superimpose a modulation signal for phase-sensitive detection onto the multi-sideband light; the second modulator is used to superimpose a frequency discrimination modulation signal onto the multi-sideband light; the photodetector is used to detect the transmitted or reflected light signal after interaction, and convert the light signal carrying frequency detuning information into a time-domain electrical signal.

4. The laser frequency stabilization system according to claim 3, characterized in that, The feedback control unit includes a calculation module and an electrical control system; the calculation module is used to perform frequency mixing processing on the time-domain electrical signal and the second radio frequency signal to calculate the total frequency discrimination error signal; the electrical control system is used to generate a feedback control signal based on the total frequency discrimination error signal and output it to the tunable laser unit to lock the laser center frequency.

5. The laser frequency stabilization system according to claim 4, characterized in that, The first radio frequency source is configured to have an external frequency control port and a signal output port. The external frequency control port is used to receive feedback control signals from the feedback control unit. When the system is in a dual-loop locked state, the electrical control system controls and locks the output frequency of the first radio frequency source according to the total frequency discrimination error signal, so that the output frequency of the first radio frequency source is strictly anchored to the spacing between adjacent frequency reference peaks of the optical frequency reference. The signal output port is used to output an electrical frequency signal with absolute frequency reference characteristics.

6. The laser frequency stabilization system according to claim 5, characterized in that, The tuning speed range of the tunable laser unit is not less than 20 kHz, the mode-hopping tuning range is greater than 0.5 nm, and the spectral linewidth range of the output laser is less than 10 kHz.

7. The laser frequency stabilization system according to claim 5, characterized in that, The first modulator is an electro-optic phase modulator, and the frequency range of the first radio frequency signal is not less than 1 GHz; the optical frequency reference is selected from either a high-precision Fabry-Perot optical resonator or a saturable absorption gas cell; the frequency range of the second radio frequency signal is not greater than 100 MHz. The calculation module uses a phase-sensitive detection method to mix and low-pass filter the time-domain electrical signal output by the photodetector with the second radio frequency signal, and demodulate to obtain the total frequency discrimination error signal.

8. The laser frequency stabilization system according to claim 1, characterized in that, The error signal generation unit further includes a second radio frequency source and a photodetector; the second radio frequency source is used to generate a second radio frequency signal; the photodetector is used to detect the transmitted or reflected light signal after interaction, and convert the light intensity modulation signal carrying frequency detuning information into a time-domain electrical signal. The system also includes a broadband radio frequency combiner. The first radio frequency source and the second radio frequency source are both electrically connected to the broadband radio frequency combiner. The broadband radio frequency combiner is used to superimpose the first radio frequency signal and the second radio frequency signal to generate a composite signal to drive the first modulator, so that the tunable laser generates multi-sideband light while superimposing a frequency discrimination modulation signal.

9. A laser frequency stabilization method, characterized in that, The laser frequency stabilization system as described in any one of claims 3-8, wherein the laser frequency stabilization method comprises: The frequency of the first radio frequency signal is set to be numerically equal to the spacing between adjacent frequency reference peaks of the optical frequency reference; Adjust the center frequency of the tunable laser unit so that the laser carrier is aligned with a certain frequency reference peak of the optical frequency reference. At this time, multiple sidebands in the multi-sideband light fall into the frequency discrimination dynamic range of the corresponding frequency reference peak. A photodetector is used to detect the transmitted or reflected light signal after multiple sideband components interact with the multiple frequency reference peaks, and the light signal is converted into a time-domain electrical signal; the time-domain electrical signal is mixed and demodulated with a second radio frequency signal by a calculation module to obtain the total frequency discrimination error signal. The electrical control system outputs a feedback control signal to the tunable laser unit based on the total frequency discrimination error signal, so that the total frequency discrimination error signal approaches zero, thereby locking the laser center frequency.

10. A method for outputting an electrical signal with an absolute frequency reference, characterized in that, Employing the laser frequency stabilization system as described in any one of claims 3-8, the method for outputting the electrical signal with an absolute frequency reference includes: Using the laser frequency stabilization method as described in claim 9, the carrier center frequency of the tunable laser unit is locked at a certain frequency reference peak of the optical frequency reference, at which point the laser frequency is stabilized at the [missing information]. One frequency reference peak; A low-frequency jitter signal is generated inside the electrical control system and superimposed on the first radio frequency source, causing a slight frequency modulation of the output frequency of the first radio frequency source. The demodulation module demodulates the low-frequency jitter signal from the time-domain electrical signal output by the photodetector, and the amplitude and phase of the electrical signal reflect the first upper sideband and the second lower sideband. The detuning of each frequency reference peak; The demodulated electrical signal is fed back to the first radio frequency source, and the output frequency of the first radio frequency source is automatically adjusted by the electrical control system until the first upper sideband is precisely aligned with the first lower sideband. One frequency reference peak; An electrical frequency signal is output through the first radio frequency source. The frequency of this electrical frequency signal is physically locked to the inherent frequency interval of the optical frequency reference, thereby outputting an electrical signal with absolute frequency reference characteristics.

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