Laser digital frequency locking master module based on PDH technology and digital frequency locking system

Abstract

The application is based on a laser digital frequency locking master module and a digital frequency locking system of a PDH technology, which comprises a DDS, a frequency mixer, a low-pass filter, a PID filter unit, an image processing unit, a frequency locking state discriminator and a triangular wave generator realized by an FPGA of the master module; the DDS outputs two signals, one of which is directly output, and the other of which is mixed with a first PD signal, and after the error signal is obtained through the low-pass filter, it enters the PID loop filter unit; the input error signal is filtered through multiple filters and outputs three control signals, which correspond to different actuators respectively; the master controls the spot mode according to the gray distribution of the spot image and the circumscribed rectangle of the spot, calculates the different transverse mode ratios, estimates the transmitted light power, the cavity rear spot image signal and a second PD signal jointly enter the frequency locking state discriminator to judge the frequency locking state and the frequency locking quality, and the triangular wave generator controls the laser frequency sweep according to the triangular wave signal, which improves the integration and the frequency locking success rate of the system and reduces the operation complexity.

Description

Technical Field

[0001] This invention relates to the field of automatic laser frequency locking technology, and in particular to a laser digital frequency locking master control module and digital frequency locking system based on PDH technology. Background Technology

[0002] With the continuous development of laser technology, lasers are widely used in various research fields such as quantum optics, quantum information, and cold atom physics. Factors such as ambient temperature, external vibration, and sound can cause changes in laser frequency. In order to reduce the frequency drift of the laser, laser frequency stabilization technology is needed to lock the laser frequency at a certain standard frequency, such as the resonant transition frequency of atoms, molecules, or FP cavities.

[0003] Frequency-locked lasers are ubiquitous in physics experiments involving atoms, molecules, and optics, and are a core component of various high-precision measuring instruments. Laser frequency noise is a direct source of interference with the accuracy and sensitivity of testing and measuring instruments; suppressing laser frequency noise can effectively improve instrument sensitivity.

[0004] The output frequency of a freely operating laser broadens due to factors such as ambient temperature, vibration, and sound. Semiconductor lasers, in particular, have an output frequency linewidth on the order of MHz. Even with preliminary linewidth narrowing using external cavities, the linewidth remains only in the hundreds of kHz range, far from meeting experimental requirements. Passive measures such as vibration isolation, sound insulation, and precise temperature control can suppress frequency noise to some extent. However, these passive measures struggle to guarantee long-term stability and reproducibility of the laser frequency. To achieve superior frequency noise performance, active feedback suppression techniques are typically employed to lock the laser frequency at a reference frequency with extremely low noise and higher frequency stability. Active suppression techniques are categorized into two types based on frequency reference standards: one uses the center frequency of atomic or molecular transition spectral lines as the reference standard, including Lamb dip stabilization, atomic spectroscopy Zeeman effect stabilization, and atomic or molecular saturated absorption stabilization; the other uses the resonant frequency of an ultrastable optical reference cavity as the frequency reference standard. One of the most effective and widely used laser frequency stabilization methods is the Pound-Drever-Hall (PDH) laser frequency stabilization technique based on an ultrastable optical reference cavity. PDH laser frequency stabilization technology features fast servo response and low noise, making it one of the most widely used and effective frequency stabilization techniques currently available.

[0005] Currently, lasers based on PDH frequency stabilization technology used in laboratories typically employ discrete analog devices to obtain PDH error signals and solve these signals using analog PID circuits. Different actuators require multiple levels of hardware circuits with varying proportional, integral, and derivative components. However, discrete analog devices and analog PID circuits are expensive, complex, and large-scale, applicable only to single frequency-locking circuits, and once designed, difficult to modify, requiring cumbersome operations such as soldering for debugging and modification. Therefore, for frequency stabilization systems with different wavelengths and actuators, analog frequency stabilization PID circuits are difficult to adapt to and adjust different combinations of proportional, integral, and derivative components. Furthermore, the discrete devices in current analog frequency stabilization technology still require significant time for researchers to debug during experiments, and researchers still need to manually judge the frequency-locking quality and whether the mode matching with the cavity meets design requirements based on the beam intensity and mode of the back cavity spot. Summary of the Invention

[0006] To address the aforementioned problems, the first objective of this invention is to provide a laser digital frequency locking master control module and a digital frequency locking system based on PDH technology, thereby solving the problem of digital automatic frequency locking in lasers.

[0007] Therefore, the above-mentioned objectives of the present invention are achieved through the following technical solutions:

[0008] A laser digital frequency locking master control module based on PDH technology is used to control the digital frequency locking of a laser. The module comprises a DDS (Digital Controller System) implemented using an FPGA, a mixer, a low-pass filter, a PID filter unit, an image processing unit, a frequency locking state discriminator, and a triangular wave generator. The DDS outputs two signals: one signal is directly output, and the other signal is mixed with a first PD (Digital Frequency Control) signal by the mixer, then passes through a low-pass filter to obtain an error signal, which is then fed into the PID filter unit. The PID filter unit calculates the error signal, and the input error signal undergoes multi-stage filtering. The device outputs three control signals, which correspond to different actuators to control the frequency of different frequency stabilization systems. After initial frequency control, the back cavity spot image is processed by the image algorithm processing unit for filtering, noise reduction, grayscale transformation, and edge detection. The main control module determines the spot mode, calculates different transverse mode ratios, and estimates the transmitted light power based on the grayscale distribution of the back cavity spot image and the outer rectangle of the spot. The back cavity spot image signal and the second PD signal are collected by the ADC and then enter the frequency locking state discriminator to determine the frequency locking state and frequency locking quality. The triangular wave generator generates a triangular wave signal based on the frequency locking state to control the laser frequency sweep.

[0009] While adopting the above technical solutions, the present invention may also adopt or combine the following technical solutions:

[0010] As a preferred embodiment of the present invention: the PID filter unit is composed of multiple consecutive first-order IIR filters cascaded together. The input error signal is processed by a multi-stage filter algorithm of PI, PI, PD, I, I to output different control signals to the corresponding execution units. After the error signal enters the PID loop filter unit, it is first proportionally adjusted to adapt to the input conditions of the first-stage integral of the loop filter. For systems with flicker frequency noise, random walk frequency noise, or even higher frequency noise, at least two stages of integral circuits are added for suppression. The three control signals correspond at least to the system with PZT as the main actuator, the system with AOM as the main actuator, and the system with temperature control as the main actuator, respectively. For systems where PZT is the main actuator, an additional integral stage is added to the existing proportional and integral outputs to enhance the piezoelectric ceramic's suppression of low-frequency noise with large amplitude. For systems where AOM is the main actuator, a derivative stage is added to increase the feedback bandwidth. For systems where temperature control is the main actuator, an integral stage is added to suppress long-term and slow temperature drift of the device. The main control module rapidly combines multiple stages of proportional, integral, and derivative stages through different actuators of different frequency stabilization systems. Based on the type of noise to be suppressed, the number of integral stages added to the PID filter unit is determined; based on the type and characteristics of the actuator, the control stage and number of stages added to the PID filter unit are determined.

[0011] As a preferred technical solution of the present invention: the control signal of the system with temperature control as the main actuator is used as the final integral output of all components of the PID filter unit.

[0012] As a preferred technical solution of the present invention: the image processing unit analyzes the back cavity spot image signal and the pattern composition of the spot, calculates the different proportions of the fundamental mode and higher-order modes in the transmitted light, and estimates the power of the transmitted light, and performs image algorithm processing on the spot by filtering and noise reduction, grayscale transformation and edge detection.

[0013] As a preferred technical solution of the present invention: the back cavity spot image signal is analyzed into a single spot through grayscale distribution analysis. First, the minimum bounding rectangle of the spot is calculated. Different modes are classified based on the aspect ratio of the minimum bounding rectangle. The spot mode is determined according to the aspect ratio of the minimum bounding rectangle, the spot shape distribution, and the grayscale value distribution. When the aspect ratio of the spot is greater than 0.8, the spot fits to a roughly circular shape, and the grayscale value gradually decreases from the center to the periphery, it is determined to be a fundamental mode spot. When the aspect ratio of the spot is approximately 0.6, the spot fits to two ellipses, and the grayscale value gradually decreases from the center to the periphery of the two circles, it is determined to be a TEM01 or TEM10 mode spot. When analyzing a multi-mode superimposed light spot, the intensity center and secondary intensity center of the light spot are analyzed by gray value distribution. Starting from the fundamental mode light spot, higher-order mode light spots are analyzed layer by layer to calculate the proportion of different transverse modes. For light spots with a fundamental mode ratio of more than 90%, the TEM02 mode is calculated. After adjusting the back cavity light spot to the fundamental transverse mode state, higher-order transverse modes are excited in the horizontal direction when the cavity mirror is adjusted in the horizontal direction, and higher-order transverse modes are excited in the vertical direction when the cavity mirror is adjusted in the vertical direction. The different mode ratios analyzed are used to improve the optical path design, assist in optical path debugging, analyze the mode composition of the light spot, calculate the different proportions of the fundamental mode and higher-order modes in the transmitted light, and estimate the power of the transmitted light.

[0014] As a preferred technical solution of the present invention: the digital frequency locking master control module outputs an EOM drive signal, an AOM drive signal, a PZT drive signal, and a temperature control drive signal. The EOM drive signal is used to drive the EOM to perform phase modulation of the laser. The AOM drive signal is used to drive the AOM to perform laser power stabilization control. The PZT drive signal is used to drive the PZT to change the crystal length inside the laser to adjust the laser output frequency and laser frequency sweep after unlocking. The temperature control drive signal is used for precise temperature control of each key unit inside the laser.

[0015] As a preferred technical solution of the present invention, it includes a signal driving unit, which is used at the output end of the digital frequency-locked main control module to adjust the amplitude and bias of the output signal after the main control is DAC.

[0016] The second objective of this invention is to provide a laser digital frequency locking system based on PDH technology.

[0017] Therefore, the above-mentioned objectives of the present invention are achieved through the following technical solutions:

[0018] It includes a laser, AOM, EOM, first beam splitter, FP cavity, second beam splitter, and camera connected in sequence;

[0019] A portion of the incident light from the first beam splitter enters the FP cavity, while the other portion is reflected by the front cavity mirror of the FP cavity and then output to the first PD after passing through the first beam splitter.

[0020] After the incident light is split by the second beam splitter, part of it enters the camera and the other part enters the second PD.

[0021] While adopting the above technical solutions, the present invention may also adopt or combine the following technical solutions:

[0022] As a preferred technical solution of the present invention: the first PD converts the received optical signal into an electrical signal, inputs it into the digital frequency locking main control module, and obtains an error signal after processing by the mixer low-pass filter algorithm;

[0023] The second PD converts the received optical signal into an electrical signal, which is then input into the digital frequency locking master control module and processed by an algorithm to serve as a frequency locking status discrimination signal.

[0024] As a preferred technical solution of the present invention: the camera inputs the received image signal into the digital frequency locking master control module, and after image algorithm processing, it is used as another frequency locking state discrimination signal.

[0025] As a preferred technical solution of the present invention: the frequency locking state discriminator comprehensively judges the frequency locking state and frequency locking quality based on the second PD and the camera image signal, and controls the laser to sweep frequency by generating a triangular wave signal by the triangular wave generator according to the frequency locking state.

[0026] As a preferred technical solution of the present invention: the signal directly output by the DDS is used as the EOM driving signal, the signal after passing through the PI, PI, PD filter stage is used as the AOM driving signal, the signal after passing through the PI, PI, PD, I filter stage is used as the PZT driving signal, and the signal after passing through the PI, PI, PD, I, I filter stage is used as the temperature control driving signal.

[0027] This invention relates to a laser digital frequency locking master control module and system based on PDH technology. Through FPGA software algorithms, the control of the laser, AOM, EOM, and PZT, as well as error signal calculation, are integrated into the digital frequency locking master control module. The master control FPGA obtains the error signal by passing the first PD signal through a mixer and a low-pass filter. The error signal is then calculated by a PID filter unit and processed through a multi-stage filter algorithm (PI, PI, PD, I, I) to output different control signals to the corresponding execution units. By processing the second PD signal and camera image algorithms, the master control achieves intelligent and automated comprehensive judgment of the frequency locking status and quality, and realizes automatic locking and automatic relocking after loss of lock. This invention eliminates the need for separate and expensive instruments and equipment such as signal generators, AOM drivers, EOM drivers, and PZT drivers, as well as complex and single-function analog PID control circuits, required in commonly used analog frequency-stabilized laser systems. Instead, it integrates all drivers and related peripheral circuits into the digital frequency locking master control module. This invention relates to a laser digital frequency locking master control module and system based on PDH technology. The module replaces analog PID control circuits, mixers, filters, signal generators, and other equipment with a master control FPGA digital algorithm. Based on different actuators, the software algorithm rapidly assembles multi-stage filter combinations with different proportional, integral, and derivative elements. An image algorithm is added to determine the frequency locking status and quality based on the light spot image, achieving automatic locking and relocking functions, and estimating the transmitted light power and light spot pattern composition. This invention, based on PDH technology, significantly improves system integration and frequency locking success rate while reducing operational complexity, all while maintaining functional integrity. Attached Figure Description

[0028] Figure 1 This is a diagram showing the relationship between the main control digital algorithm structure and the digital frequency locking main control module and the input / output signals of the present invention;

[0029] Figure 2 This is a schematic diagram of the structure of the laser digital frequency locking system based on PDH technology of the present invention;

[0030] In the attached diagram, the components are: first PD1, second PD2, ADC3, LP4, DAC5, EOM driver 6, EOM7, AOM driver 8, AOM9, DDS10, PID filter unit 11, PZT driver 12, PZT13, temperature control driver 14, temperature control 15, laser 16, first beam splitter 17, second beam splitter 18, camera 19, and triangular wave generator 20. Detailed Implementation

[0031] The present invention will be further described below with reference to embodiments and accompanying drawings:

[0032] This invention provides a laser digital frequency locking master control module based on PDH technology, used to implement a laser digital frequency locking algorithm. The digital frequency locking master control module includes a DDS10 implemented by FPGA, a mixer, a low-pass filter, a PID filter unit, an image processing unit, a frequency locking state discriminator, and a triangular wave generator20. Specifically, the DDS outputs two signals: one is directly output, and the other is mixed with the first PD1 signal, then filtered by a low-pass filter to obtain an error signal before entering the PID filter unit. The PID filter unit11 consists of multiple consecutive first-order IIR filters cascaded together. The input signal, after passing through multiple filters, outputs three control signals. The camera signal, after being processed by the master control image algorithm processing unit, enters the frequency locking state discriminator along with the second PD signal to comprehensively judge the frequency locking state and quality. The triangular wave generator controls the laser's PZT frequency sweep according to the frequency locking state.

[0033] like Figure 1 As shown, the DDS10 IP core inside the FPGA generates two signals of the same frequency. One signal is directly output as the EOM drive signal 6; the other output, together with the first PD signal 1, is mixed with a mixer and a low-pass filter to obtain an error signal. The error signal is then processed by internal PI, PI, and PD filters to output the AOM drive signal; after passing through an I filter, the output signal is the PZT drive signal; and after passing through another I filter, the output signal is the temperature control drive signal.

[0034] Preferably, the digital frequency locking master control module includes an ADC3 and a DAC5. The ADC is located at the input terminal of the digital frequency locking master control module; the DAC is located at the output terminal of the digital frequency locking master control module.

[0035] The digital frequency-locking main control module also outputs EOM drive signals, AOM drive signals, PZT drive signals, and temperature control drive signals. The EOM drive signal drives EOM7 to perform phase modulation of the laser. The AOM drive signal drives AOM9 to perform laser power stabilization control. The PZT drive signal drives PZT13 to change the crystal length within the laser to adjust the laser output frequency and laser frequency sweep after unlocking. The temperature control drive signal is used for precise temperature control of key units within the laser.

[0036] The digital frequency locking master control module further includes a signal driving unit. The signal driving unit is located at the output terminal of the digital frequency locking master control module. The signal driving unit is used to adjust the amplitude and bias of the signal output by the master control module after passing through the DAC.

[0037] As the preferred embodiment of this application, a high-speed digital frequency-locked main control module is fabricated, which comprises three parts: a signal conditioning and driving circuit, an ADC and DAC circuit, and a main control FPGA.

[0038] The signal conditioning and driving circuit scales, applies bias, and smooths the DAC output signal to ensure it meets the signal input requirements of each peripheral. This circuit consists of four parts: EOM driver 6, AOM driver 8, PZT driver 12, and temperature control driver 14. The EOM driver signal is used to drive the EOM to perform phase modulation on the laser. The AOM driver signal power is directly related to the efficiency of the diffracted light, and changes in the driver signal power do not alter the direction of the diffracted light. Therefore, by changing the power of the driver signal, laser power stability based on the AOM is achieved, suppressing periodically changing laser power jitter. The first-order diffracted light from the AOM is detected by the first PD and fed back to the AOM driver via the main control, achieving closed-loop control of the laser power. The PZT driver signal is used to drive the PZT to change the crystal length within the laser to adjust the laser output frequency, achieving the purpose of stabilizing the laser frequency and narrowing the laser linewidth. Based on the frequency-locking state and quality determined by the frequency-locking state discriminator, if a loss of lock is detected, a triangular wave generator generates a triangular wave signal to drive the PZT to control the laser frequency sweep. Temperature control drive signals are used for precise temperature control of key units within the laser, actively suppressing slow temperature drift of the laser frequency and offsetting the thermal expansion of materials in each unit and the influence of the external environment.

[0039] The ADC and DAC circuits include analog-to-digital conversion circuits that transmit the first PD and second PD signals to the main control circuit, and digital-to-analog conversion circuits that transmit the main control signals to the EOM driver, AOM driver, PZT driver, and temperature control driver.

[0040] The main control FPGA serves as the control core of the entire system. It is responsible for processing the input signals from the first PD1 and second PD2 via the ADC, as well as the image signal acquired by the camera; it also outputs the EOM drive signal, AOM drive signal, PZT drive signal, and temperature control drive signal. The first PD receives the reflected light from the front-end cavity mirror of the FP cavity. Its AC signal is phase-shifted with the DDS10 signal inside the main control FPGA, and then processed by an internal algorithm mixer and low-pass filter to demodulate the frequency detuning information in the reflected light, i.e., the error signal. After processing by the PID filter algorithm of the main control FPGA, the error signal is fed back to the PZT, AOM, and other frequency response devices via a DAC and signal conditioning drive circuit for frequency compensation. The second PD receives the transmitted light from the FP cavity, and the camera captures the image of the light spot from behind the cavity. The second PD signal, after passing through the ADC, is input to the main control FPGA along with the image signal from the camera, and together they enter the frequency locking state discriminator to comprehensively judge the system's frequency locking state and quality. When the DC signal of the second PD is higher than the set threshold, and the back cavity spot pattern and quality meet the design requirements, the main control system is locked. When neither of these conditions is met, the main control system is unlocked. The triangular wave signal generator generates a gradually increasing triangular wave signal, which is driven by the DAC and PZT to start frequency sweeping until the system is relocked.

[0041] Furthermore, during the PZT relocking and frequency sweeping phase, if the incident light resonates with the FP cavity, and the main controller detects a pulse in the transmitted light signal received by the second PD that exceeds a set threshold, and the back-cavity spot pattern and quality captured by the camera meet the design requirements, then fixing the PZT voltage and fine-tuning the AOM, temperature control 15, and other units will complete the locking. If the main controller does not detect a signal from the second PD that exceeds the set threshold, and the camera spot pattern and quality do not meet the design requirements, then the main controller resets the output triangular wave range.

[0042] The master controller can quickly combine multiple stages of proportional, integral, and derivative components according to different actuators in different frequency stabilization systems. After the error signal enters the loop filter, it undergoes proportional adjustment to adapt to the input conditions of the first-stage integral stage of the loop filter. For systems with flicker frequency noise, random walk frequency noise, or even higher-order noise, at least two stages of integral components are needed for suppression. For systems with PZT as the main actuator, since its control bandwidth is generally on the order of kHz, which is relatively slow, an additional integral stage needs to be added to the existing proportional and integral outputs of the system to further enhance the suppression of low-frequency noise, such as temperature and vibration, which are relatively slow but have large amplitudes. For systems with AOM as the main actuator, a derivative stage is required. Since this bandwidth is limited by the propagation time of sound waves in the AOM, a derivative gain is needed to increase the feedback bandwidth. Due to its long-term drift and slow change but large tuning range, temperature is generally used as the final integral output of all stages.

[0043] Furthermore, the back-cavity light spot image signal captured by the camera is analyzed to determine the pattern composition of the light spot, calculate the different proportions of the fundamental mode and higher-order modes in the transmitted light, and estimate the power of the transmitted light. This analysis further refines the optical path design and assists in optical path debugging. After the main control performs image algorithm processing on the light spot, including filtering and noise reduction, grayscale transformation, and edge detection, if the grayscale distribution analysis indicates a single light spot, the minimum bounding rectangle of the light spot is first calculated. Different modes can be classified based on the aspect ratio of the rectangle. The light spot mode can be determined based on the aspect ratio, shape distribution, and resolution distribution. For example, when the aspect ratio of the light spot is greater than 0.8, the light spot is roughly circular, and the grayscale value gradually decreases from the center to the edges, it is identified as a fundamental mode light spot; when the aspect ratio of the light spot is approximately 0.6, the light spot is roughly two ellipses, and the grayscale value gradually decreases from the center to the edges of the two circles, it is identified as a TEM01 or TEM10 mode light spot. When analyzing a multi-mode superimposed light spot using grayscale distribution analysis, the intensity center and secondary center of the light spot are analyzed based on the grayscale value distribution. Starting with the fundamental mode, higher-order modes are removed layer by layer before further analysis to calculate the proportion of different transverse modes. For light spots where the fundamental mode accounts for more than 90%, only the TEM02 mode needs to be calculated; the proportion of higher-order modes is very small and can be ignored. After a single calibration using a power meter, the light intensity can be calculated based on the grayscale values, thereby estimating the power of the transmitted light. Researchers can then adjust the optical path based on the current transmitted light power and frequency locking quality to further refine the optical path design.

[0044] Figure 2 This is a schematic diagram of the structure of a digital frequency locking system using the PDH-based laser digital frequency locking master control module described in this invention.

[0045] like Figure 2As shown, a digital frequency locking system based on PDH technology for laser digital frequency locking master module of the present invention includes a laser 16, AOM, EOM, a first beam splitter, an FP cavity, a second beam splitter, a camera, a first PD, a second PD and a digital frequency locking master module connected in sequence.

[0046] The laser digital frequency locking system based on PDH technology uses the first PD, the second PD, and the camera signal as input signals, and EOM, AOM, PZT, and temperature control as feedback actuators. It uses the main control FPGA digital algorithm as the core to keep the output laser frequency consistent with the resonant frequency of the reference cavity, thereby achieving the purpose of stabilizing the laser frequency and narrowing the laser linewidth.

[0047] To achieve the above objectives, the present invention employs the following method:

[0048] When the laser digital frequency locking system based on PDH technology is working, the light emitted by the freely rotating laser is stabilized by the AOM, and then the phase of the laser is modulated by the EOM before entering the first beam splitter 17. Part of the incident light from the first beam splitter enters the FP cavity, and the other part is reflected by the front cavity mirror of the FP cavity and then output to the first PD after passing through the first beam splitter. The incident light from the second beam splitter 18 is split and part enters the camera 19, and the other part enters the second PD.

[0049] The first PD detects the laser signal carrying reference cavity information. The ADC outputs an electrical signal to the main control unit, and the internal DDS RF signal is demodulated through a mixer and low-pass filter using internal software algorithms to produce an error signal for system locking. This error signal is then processed through a multi-stage filter algorithm (PI, PI, PD, I, I), and fed back by a DAC to output different control signals to the corresponding execution units. This adjusts the laser output frequency, ensuring it matches the reference cavity resonant frequency. The second PD and camera feedback signals are combined to comprehensively determine the frequency locking status and quality. The composition of the laser spot and the different proportions of the fundamental and higher-order modes in the transmitted light are analyzed, along with the estimated power of the transmitted light. This further refines the optical path design and assists in optical path debugging. Using the main control FPGA digital algorithm as the core, the output laser frequency is kept consistent with the reference cavity resonant frequency, achieving the goal of stabilizing the laser frequency and narrowing the laser linewidth.

[0050] This invention addresses the problem in existing frequency stabilization systems where analog PID circuits struggle to adapt to and adjust different combinations of proportional, integral, and derivative components for varying wavelengths and actuators. It proposes a laser digital frequency locking master control module and system based on PDH technology. This module replaces numerous discrete analog components with a master control algorithm, or highly integrates them into the master control module, saving significant initial debugging time. The traditional analog PID frequency stabilization circuit is replaced with a digital PID algorithm. The master control can quickly combine multiple levels of proportional, integral, and derivative components according to the different actuators in different frequency stabilization systems. The master control determines the frequency locking status and quality based on the beam intensity and mode of the back cavity beam, assisting researchers in further analyzing the optical path and determining mode compatibility with the cavity. This invention solves the problem of automatic digital frequency locking for lasers, making the frequency stabilization system more universal, digital, intelligent, and simplified.

[0051] The above specific embodiments are used to explain and illustrate the present invention, and are only preferred embodiments of the present invention, not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A laser digital frequency-locked master module based on PDH technology, used for controlling laser digital frequency-locked, characterized in that: The laser digital frequency-locked master module based on the PDH technology comprises a DDS, a frequency mixer, a low-pass filter, a PID filter unit, an image processing unit, a frequency-locked state discriminator and a triangular wave generator realized by using an FPGA, the DDS outputs two signals, one of which is directly output, and the other of which is mixed with a first PD signal by the frequency mixer, and then enters the PID filter unit after the error signal is obtained by the low-pass filter, the error signal is solved by the PID filter unit, and three control signals are output after the error signal is filtered by multiple filters, the three control signals correspond to different actuators to adapt to the frequency control of different frequency stabilization systems; after the preliminary frequency control, the cavity rear spot image is processed by the image algorithm processing unit to filter and reduce noise, perform gray scale conversion and edge detection, and then the master module estimates the transmitted light power according to the gray scale distribution and the circumscribed rectangle of the cavity rear spot image; at the same time, the cavity rear spot image signal and a second PD signal enter the frequency-locked state discriminator together to determine the frequency-locked state and the frequency-locked quality, and the triangular wave generator generates a triangular wave signal to control the laser frequency sweep according to the frequency-locked state; the PID filter unit is composed of multiple continuous first-order IIR filters in cascade, the input error signal is filtered by PI, PI, PD, I and I multiple filter algorithms to output different control signals to corresponding execution units, and the error signal enters the PID loop filter unit to be proportionally adjusted to adapt to the input condition of the first integral of the loop filter, at least two integral links are added to suppress the flicker frequency noise, random walk frequency noise and even higher frequency noise, and the three control signals at least correspond to a system with a PZT as the main actuator, a system with an AOM as the main actuator and a system with temperature control as the main actuator, for the system with the PZT as the main actuator, an additional integral is added to the original proportional and integral output to increase the noise suppression of the piezoelectric ceramic to low frequency and large amplitude, for the system with the AOM as the main actuator, a differential link is added to increase the feedback bandwidth, and for the system with temperature control as the main actuator, an integral link is added to suppress the long-term and slow temperature drift of the device, the master module quickly combines multiple proportional, integral and differential links according to different actuators of different frequency stabilization systems; the integral link series added to the PID filter unit is determined according to the type of the suppressed noise; the control link and series added to the PID filter unit are determined according to the type and characteristics of the actuator. ​ The image processing unit calculates the different proportions of the fundamental mode and high-order modes in the transmitted light and estimates the power of the transmitted light by analyzing the cavity rear spot image signal and the mode composition of the spot, and performs image algorithm processing of filtering and reducing noise, gray scale conversion and edge detection on the spot.

2. The PDH technology based laser digital frequency locked master module as claimed in claim 1, wherein: The control signal of the system with temperature control as the main actuator is the last integral output of all links of the PID filter unit.

3. The PDH technology based laser digital frequency locked master module as claimed in claim 1, wherein: The cavity rear spot image signal is analyzed by gray scale distribution to be a single spot, the minimum circumscribed rectangle of the spot is calculated first, different mode classification is performed through the aspect ratio of the minimum circumscribed rectangle, the spot mode is judged according to the aspect ratio of the minimum circumscribed rectangle, the spot shape distribution and the gray scale value distribution, the base mode spot is judged when the spot aspect ratio is greater than 0.8, the spot is fitted as a circular shape and the gray scale value gradually decreases from the middle to the periphery, the TEM01 or TEM10 mode spot is judged when the spot aspect ratio is 0.6, the spot is fitted as two elliptical shapes and the gray scale value gradually decreases from the middle to the periphery of the two circular shapes, the higher order mode spot is analyzed after the base mode spot is removed layer by layer through the gray scale value distribution analysis when the spot is a multi-mode superimposed spot, the different transverse mode proportions are calculated, for the spot with the base mode proportion of more than 90%, the TEM02 mode is calculated, the cavity rear spot is adjusted to the base transverse mode state, the high order transverse mode is excited in the horizontal direction when the horizontal direction polarization cavity mirror is adjusted, the high order transverse mode is excited in the vertical direction when the vertical direction polarization cavity mirror is adjusted, the different mode proportions analyzed are used for perfecting the optical path design, assisting the optical path debugging, analyzing the mode composition of the spot, calculating the different proportions of the base mode and the high order mode in the transmitted light and estimating the power of the transmitted light.

4. The PDH technology based laser digital frequency locked master module as claimed in claim 1, wherein: An ADC and a DAC are provided, the ADC is located at the input end of the digital frequency locking master module, the DAC is located at the output end of the digital frequency locking master module, the digital frequency locking master module outputs an EOM drive signal, an AOM drive signal, a PZT drive signal and a temperature control drive signal, the EOM drive signal drives the EOM to perform phase modulation on the laser, the AOM drive signal drives the AOM to perform laser power stability control, the PZT drive signal drives the PZT to change the crystal length in the laser to adjust the laser output frequency and the laser sweep after losing lock, and the temperature control drive signal is used for precise control of the temperature of each key unit in the laser.

5. The PDH technology based laser digital frequency locked master module as claimed in claim 4, wherein: A signal drive unit is arranged at the output end of the digital frequency locking master module, and the signal drive unit adjusts the amplitude and bias of the output signal after the DAC.

6. The laser digital frequency locking system based on PDH technology using the digital frequency locking master module according to any one of claims 1-5, characterized in that: The system comprises a laser, an AOM, an EOM, a first light splitting prism, an F-P cavity, a second light splitting prism and a camera connected in sequence. Part of the incident light of the first light splitting prism enters the F-P cavity, and the other part is reflected by the front cavity mirror of the F-P cavity and then output to the first PD through the first light splitting prism. The second PD converts the received optical signal into an electrical signal and inputs the electrical signal into the digital frequency locking master module for algorithm processing to obtain a frequency locking state discrimination signal. The second PD converts the received optical signal into an electrical signal and inputs the electrical signal into the digital frequency locking master module for algorithm processing to obtain a frequency locking state discrimination signal. The camera inputs the received image signal into the digital frequency locking master module for image algorithm processing to obtain another frequency locking state discrimination signal.

7. A laser digital frequency locked system based on PDH technique as claimed in claim 6 wherein, The signal directly output by the DDS is used as the EOM driving signal, the signal after the PI, PI and PD filter links is used as the AOM driving signal, the signal after the PI, PI, PD and I filter links is used as the PZT driving signal, and the signal after the PI, PI, PD, I and I filter links is used as the temperature control driving signal.

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