System and method for automatically locking repetition frequency of optical frequency comb

Abstract

The invention relates to the technical field of frequency control, in particular to an automatic locking system and method for the repetition frequency of an optical frequency comb, which can realize large-range high-precision locking of the repetition frequency of the optical frequency comb. Optical frequency comb frequency locking is realized based on repetition frequency drift direction detection, the system is an integrated optical frequency comb repetition automatic locking and tracking locking system, all data processing of the system is carried out in a lower computer, and a repetition frequency locking result is converted into direct detection of a direct current voltage value, so that a judgment mode has high reliability, and the reliability of the system is improved. Therefore, the problems of automatic locking and tracking locking of the repetition frequency of the optical frequency comb are solved, and the performance of the optical frequency comb is improved.

Description

Technical Field

[0001] This invention relates to the field of frequency control technology, specifically to an automatic locking system and method for the repetition frequency of an optical frequency comb. Background Technology

[0002] Optical frequency combs, serving as a bridge between optical and microwave frequency standards, have seen rapid development in recent years due to their simple structure and low cost. The advent of optical frequency combs first solved the problem of the difficulty in measuring optical frequencies, enabling rapid conversion between optical and microwave frequencies through a single system. They are the most effective tool for measuring absolute optical frequencies to date. Furthermore, due to the interrelationships between parameters such as frequency, time, and spatial scales, optical frequency combs provide an ideal research tool for time-frequency transfer, absolute distance, and absolute angle measurements, and have broad application prospects in fields such as manned spaceflight, deep space exploration, satellite timing, and modern manufacturing.

[0003] For femtosecond optical frequency combs to achieve excellent performance, the locking of the repetition frequency is crucial. Its stability is closely related to the cavity length of the laser, and temperature variations affect the cavity length. Currently, in laboratory environments, locking the repetition frequency of optical frequency combs often involves building an analog circuit phase-locked loop to control the piezoelectric ceramic (PZT) and thus lock the laser cavity length. However, the analog circuit often requires complex adjustments by the operator to achieve locking, increasing the complexity of the optical frequency comb system. Furthermore, a simple analog circuit cannot re-lock after initial locking, still requiring complex operations by the operator. While there are reports of automatic repetition frequency locking for optical frequency combs, this still relies on communication between a host computer and a slave computer. Setting and changing the reference frequency must be done through the host computer, making independent operation of the slave computer difficult. The coordinated communication between the host and slave computers further reduces the bandwidth of the phase-locked loop, thus decreasing locking accuracy and affecting the measurement accuracy of the optical frequency comb. Communication errors can cause the optical comb to disengage, and in severe cases, even damage the optical comb system. Summary of the Invention

[0004] In view of this, the present invention provides an automatic locking system and method for the repetition frequency of an optical frequency comb, which can achieve high-precision automatic locking of the repetition frequency of the optical frequency comb.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: An automatic frequency locking system for an optical frequency comb includes: an optical frequency comb optical system, a TEC temperature control system, a photodetector, a bandpass amplifier, a frequency drift detector, a rubidium atomic clock, a frequency synthesizer, a microcontroller, a distributor, an analog-to-digital converter, a peak-to-peak detection module, a zero-crossing comparator, a PID controller 1, a PID controller 2, a digital-to-analog converter, a signal amplifier, and a PZT driver. The optical frequency comb optical system controls its temperature through the TEC temperature control system. The output of the optical frequency comb optical system is connected to the first input of the frequency drift detector via the photodetector and the bandpass amplifier. The rubidium atomic clock is connected to the frequency synthesizer, and the output of the frequency synthesizer is connected to the second input of the frequency drift detector. The output of the frequency drift detector is split into three signals by the distributor. The output of the microcontroller is connected to the PID controller 1 and the PID controller 2. The output of the PID controller 1 is connected to the frequency synthesizer. The output of the PID controller 2 is connected to the PZT driver via the digital-to-analog converter and the signal amplifier. The PZT driver is connected to the optical frequency comb optical system.

[0006] The frequency drift detector comprises a high-speed comparator, a differential high-speed phase detector, and a differential amplifier. It is used to perform phase detection between the optical frequency comb repetition frequency signal and the reference frequency signal, and output an error signal. Simultaneously, it determines the magnitude relationship between the repetition frequency and the reference frequency based on the amplitude direction characteristics of the output error signal; and determines the magnitude relationship of the error signal frequency value based on the amplitude characteristics of the output error signal. The output of the frequency drift detector is split into three signals by a distributor. The first signal is input to a zero-crossing comparator, and the output of the zero-crossing comparator is connected to an analog-to-digital converter (ADC). The second signal is input to a peak-to-peak value detection module, and the output of the peak-to-peak value detection module is connected to the ADC. The third signal is input to the ADC.

[0007] The zero-crossing comparator is powered by a dual power supply, with a reference voltage of 0V. It is used to compare the error signal with 0V and output a DC voltage signal that represents the relationship between the repetition frequency and the reference frequency.

[0008] The peak-to-peak value detection module is used to detect the peak-to-peak value of the error signal and convert it into a DC voltage signal. This DC voltage signal is inversely proportional to the deviation between the optical frequency comb repetition frequency and the reference frequency.

[0009] The microcontroller is used to acquire the DC voltage signal output by the zero-crossing comparator and the peak-to-peak detection module, and use it as the input of PID controller 1. The output of PID controller 1 is used to control the output frequency of the frequency synthesizer. When the microcontroller detects a positive-to-negative jump in the output voltage of the zero-crossing comparator, it stops the control of PID controller 1. The microcontroller acquires the third signal output by the error signal through the distributor via the analog-to-digital converter, and uses it as the input of PID controller 2. The output of PID controller 2 is amplified by the PZT driver after passing through the analog-to-digital converter and the signal amplifier.

[0010] The microcontroller is also used to monitor the output voltage of the digital-to-analog converter in real time. When the voltage exceeds the set threshold, the PID controller 1 is activated to adjust the output frequency of the frequency synthesizer so that the output voltage of the digital-to-analog converter returns to the vicinity of the center value.

[0011] The set threshold values ​​are 0.1V and 4.9V, with a center value of 2.5V.

[0012] When changes in the external environment cause the lock to be lost, the microcontroller immediately shuts down the output of the PID controller 2 based on the output of the zero-crossing comparator and restarts the automatic locking process.

[0013] The present invention also provides an automatic locking method for the repetition frequency of an optical frequency comb based on the system described in the present invention, comprising the following steps: Step 1: Stabilize the temperature of the optical comb system at the set value using the TEC temperature control system; Step 2: The photodetector converts the optical signal output from the optical frequency comb system into an electrical signal, which is then filtered and amplified by a bandpass amplifier to obtain the fundamental frequency signal of the optical frequency comb repetition frequency. Step 3: The frequency drift detector performs phase detection between the optical frequency comb repetition frequency signal and the reference frequency signal output by the frequency synthesizer, and outputs an error signal. Step 4: The error signal is divided into three paths: the first path is converted into a DC voltage representing the frequency relationship between the optical frequency comb repetition frequency and the reference frequency through a zero-crossing comparator; the second path is converted into a DC voltage representing the frequency deviation through a peak-to-peak detection module; and the third path is directly acquired. Step 5: The microcontroller acquires the DC voltages converted from the first and second channels and uses them as input to the PID controller 1 to adjust the output frequency of the frequency synthesizer so that the repetition frequency of the optical frequency comb gradually approaches the reference frequency; when a jump in the output voltage of the zero-crossing comparator is detected, the adjustment of the frequency synthesizer is stopped. Step 6: The microcontroller acquires the third error signal as the input of the PID controller 2. Its output is used to adjust the PZT through the digital-to-analog converter, signal amplifier and PZT driver, thereby adjusting the cavity length of the optical comb optical system and achieving precise locking of the repetition frequency. Step 7: After locking, the microcontroller monitors the output voltage of the digital-to-analog converter in real time. When the voltage exceeds the set threshold, the output frequency of the frequency synthesizer is adjusted by the PID controller 1 so that the output voltage of the digital-to-analog converter returns to near the center value, thereby achieving long-term frequency locking. Step 8: When a loss of lock is detected, the microcontroller shuts down the output of PID controller 2 and re-executes the locking process from step 5 to step 6.

[0014] In step 7, the direction of the optical frequency comb repetition frequency drift is determined based on the direction in which the output voltage of the digital-to-analog converter exceeds the threshold, and the output frequency of the frequency synthesizer is adjusted accordingly.

[0015] Beneficial effects: 1. The system of the present invention realizes optical frequency comb frequency locking based on repetition frequency drift direction detection. It is an integrated optical frequency comb repetition automatic locking and tracking locking system. All data processing of the system is performed on the lower computer. The locking result of repetition frequency is converted into direct detection of DC voltage value, thereby making the judgment mode highly reliable. This solves the problem of optical frequency comb repetition frequency automatic locking and tracking locking and improves the performance of optical frequency comb.

[0016] 2. In this invention, a frequency drift detector is employed. This detector, in addition to the functions of a conventional phase detector, also compares the optical frequency comb repetition frequency with the reference frequency, and calculates the error signal frequency (accuracy increases as the error signal decreases, achieving better than 5Hz accuracy). It can also accurately determine the optical frequency comb repetition frequency through pre-fitting (within 5Hz error). The error signal obtained by a conventional phase detector is a sinusoidal signal that is essentially symmetrical to 0V. Since the sinusoidal signal is essentially symmetrical to 0V, it lacks directional characteristics. That is, if the optical frequency comb repetition frequency is greater than or less than the target reference frequency (with a difference of 1kHz), the signal seen on the oscilloscope will be a uniform 1kHz sinusoidal signal. Therefore, it is impossible to directly determine the relationship between the optical frequency comb repetition frequency and the target reference frequency based on this signal.

[0017] 3. In this invention, all signals from the repetition frequency locking process are fully utilized, such as the signals output by the frequency drift detector, the control voltage signals output by the digital-to-analog converter, the signals from the zero-crossing voltage comparator, and the signals from the peak-to-peak detection module. Furthermore, all signals involved are DC voltage signals (except for directly acquired error signals), ensuring high acquisition accuracy and eliminating the risk of misjudgment. Simultaneously, all signals are integrated into the optical frequency comb repetition frequency locking, thereby achieving long-term, wide-range, and precise locking of the optical frequency comb repetition frequency.

[0018] 4. The method of the present invention is based on the system of the present invention, which ingeniously integrates the control of PZT and frequency synthesizer in the optical frequency comb repetition frequency locking system, and uses two PID controllers to adjust the output frequency of the frequency synthesizer and the optical frequency comb repetition frequency to realize the automatic locking of the optical frequency comb system.

[0019] 5. In the method of the present invention, when realizing the repetition frequency locking of the optical frequency comb, the third error signal sampled by the analog-to-digital converter is not the original error signal, but a high signal-to-noise ratio and high quality error signal after effective adjustment. The frequency value of the error signal is lower than 5Hz. The low frequency and high amplitude error signal will reduce the bandwidth requirement of the phase-locked loop and improve the signal-to-noise ratio of the data sampled by the analog-to-digital converter, thereby achieving higher precision frequency locking.

[0020] 6. The method of the present invention can detect the loss of lock-up state immediately after the optical frequency comb repetition frequency is lost due to external environment (such as vibration or drastic environmental changes), and shut down the closed-loop control of PZT, thereby protecting the optical frequency comb optical system from damage. At the same time, it can complete the relocking within better than 200ms to achieve tracking and locking of the optical frequency comb repetition frequency. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of an automatic locking system for the repetition frequency of an optical frequency comb according to an embodiment of the present invention. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0023] An embodiment of the present invention provides an automatic frequency locking system for optical frequency comb repetition, such as... Figure 1 As shown, it includes an optical frequency comb optical system, a TEC temperature control system, a photodetector, a bandpass amplifier, a frequency drift detector, a rubidium atomic clock, a frequency synthesizer, a microcontroller, a distributor, an analog-to-digital converter, a peak-to-peak detection module, a zero-crossing comparator, PID controller 1, PID controller 2, a digital-to-analog converter, a signal amplifier, and a PZT driver.

[0024] Specifically, the optical frequency comb optical system is temperature-controlled by the TEC temperature control system. The TEC temperature control system controls the TEC to stabilize the temperature of the optical system inside the oscillator, and the temperature is set at around 25°C to reduce the frequency drift of the optical frequency comb repetition frequency, laying the foundation for long-term precise locking of the optical frequency comb. The laser inside the optical frequency comb optical system completes mode-locking after optical oscillation and outputs mode-locked pulse laser, which is input to the photodetector and converted into an electrical signal. The electrical signal passes through a bandpass amplifier, and the fundamental frequency signal of the optical frequency comb repetition frequency is filtered out by a bandpass filter. The fundamental frequency signal strength is then increased by the amplifier to improve the signal-to-noise ratio.

[0025] The optical frequency comb repetition frequency signal output by the bandpass amplifier and the initial reference frequency signal output by the frequency synthesizer traced back to the rubidium atomic clock are input together to the frequency drift detector. The initial reference frequency output by the frequency synthesizer is controlled by the microcontroller, and its output initial frequency value matches the optical frequency comb repetition frequency value corresponding to the actual cavity length of the femtosecond laser oscillator (in reality, there will be a certain deviation due to factors such as temperature environment).

[0026] The frequency drift detector consists of a high-speed comparator, a differential high-speed phase detector, and a differential amplifier.

[0027] Its unique circuit structure has three functions: First, it can phase-detect the optical frequency comb repetition frequency and the reference frequency output by the frequency synthesizer, outputting a phase error signal (a function achieved by conventional phase detectors). Second, the output signal has typical amplitude direction characteristics (the output signal has positive and negative signal characteristics), that is, when the repetition frequency is greater than the reference frequency, the output error signal is always greater than 0, and vice versa. Based on this characteristic of the error signal, the relative frequencies of the optical frequency comb repetition frequency and the target reference frequency can be determined. Third, the output signal has obvious amplitude characteristics, that is, as the error signal frequency changes, the error signal amplitude changes significantly; the smaller the error signal, the larger the amplitude. This allows for the detection of the error signal frequency value. Simultaneously detecting the second and third characteristics, the current repetition frequency value (deviation less than 5Hz) can be basically obtained through simple calculation.

[0028] After the frequency drift detector completes signal phase detection, it splits the output error signal into three paths via a distributor. One path is input to a dual-powered zero-crossing comparator, which detects the relationship between the current optical frequency comb repetition frequency and the reference frequency, representing this relationship with a DC voltage. The reference voltage for the zero-crossing comparator is 0V. The error signal is compared to 0V; if the output voltage is a constant negative voltage, the current repetition frequency is less than the reference frequency, and vice versa. This DC voltage value is acquired by an analog-to-digital converter and recorded by a microcontroller. The second path passes through a peak-to-peak value detection module to estimate the frequency value of the error signal. The deviation between the optical frequency comb repetition frequency and the reference frequency is closely related to the peak-to-peak value of the error signal output by the frequency drift detector. Specifically, the larger the deviation, the larger the error signal and the smaller the peak-to-peak value, exhibiting an inverse relationship. To improve the accuracy of detecting the optical frequency comb repetition frequency, this inverse relationship can be pre-fitted to generate a curve, thus achieving high-precision repetition frequency detection. The peak-to-peak value detection module converts the peak-to-peak characteristic into a DC voltage signal, which is then acquired by an analog-to-digital converter controlled by a microcontroller. The analog-to-digital converter acquires the DC voltage values ​​output from the peak-to-peak value detection module and the zero-crossing comparator detection module, using these as input signals to the PID controller 1. After PID processing, the output signal is used to modulate the signal source, gradually approaching the repetition frequency. When the microcontroller detects a sudden change in the voltage of the zero-crossing comparator (i.e., the zero-crossing comparator output voltage changes from positive to negative or vice versa), it indicates that the optical frequency comb repetition frequency is very close to the reference frequency (below 5Hz). At this point, the PID controller 1 stops controlling the frequency, maintaining the current output frequency value of the frequency synthesizer.

[0029] The microcontroller acquires the error signal output from the third distributor. Because this error signal has a small frequency and a high signal-to-noise ratio, the voltage obtained by the analog-to-digital converter (ADC) is highly accurate. This third error signal is used as the input signal for PID controller 2. After PID calculation, it is converted into an analog voltage (output voltage range 0~5V) by the digital-to-analog converter (DAC) and then amplified (2x) before being input to the PZT driver (15x) to modulate the intracavity PZT and achieve optical frequency comb repetition frequency locking. Since the optical frequency comb repetition frequency and the reference frequency are very close, the entire locking system only requires a small locking bandwidth to achieve precise optical frequency comb locking. After locking is complete, the microcontroller measures the output voltage value of the DAC in real time and uses the set threshold of the output voltage value as a switching trigger signal. When the voltage reaches the set threshold, PID 1 is triggered and uses the output voltage value as an input signal to adjust the reference frequency value output by the frequency synthesizer. Specifically, when the microcontroller detects that the DAC has triggered the set threshold condition, it starts PID controller 1 to adjust the reference frequency output by the frequency synthesizer. Since the selected PZT model of the optical frequency comb is determined, i.e., the elongation (corresponding to the change in the repetition frequency of the optical frequency comb) is determined, the magnitude of the drift frequency value generated by the repetition frequency is also completely determined. At this time, according to the trigger threshold voltage, the microcontroller controls the output of PID controller 1 to slowly change the target reference frequency output of the frequency synthesizer, so that the voltage output of the digital-to-analog converter gradually approaches 2.5V. When the output voltage reaches 2.5V±0.1V, the adjustment of the frequency synthesizer output frequency value is stopped.

[0030] This method enables ultra-long-term frequency locking of the optical frequency comb repetition frequency with extremely high locking accuracy. The specific adjustment direction of the frequency synthesizer's output frequency is as follows (the digital-to-analog converter's set thresholds are 0.1V and 4.9V): Assuming the 0.1V threshold is triggered, it indicates a decrease in the PZT drive voltage and an elongation of the PZT. This increases the oscillator cavity length (a longer cavity length reduces the repetition frequency) to counteract the increasing trend of the optical frequency comb repetition frequency. In reality, without PZT control, the optical frequency comb repetition frequency drifts towards increasing. Therefore, the frequency synthesizer needs to increase the output reference frequency to achieve a digital-to-analog converter voltage of 2.5V ± 0.1V. Conversely, triggering the 4.9V threshold requires decreasing the output reference frequency.

[0031] When the optical frequency comb repetition frequency loses lock due to external environmental factors (such as vibration or drastic environmental changes), the zero-crossing comparator will detect the lock-up change immediately. The microcontroller will then immediately disable the frequency lock of the PID controller 2 based on the change in the zero-crossing comparator's output voltage value to prevent damage to the optical frequency comb system. Simultaneously, the microcontroller will initiate the automatic locking process of the optical frequency comb to relock the repetition frequency. Testing has verified that this repetition frequency relocking process is completed within less than 200ms.

[0032] Based on the automatic optical frequency comb repetition frequency locking system of the present invention, the present invention also provides an automatic optical frequency comb repetition frequency locking method, the specific steps of which include: First, the optical system of the optical frequency comb is temperature-controlled by the TEC. The TEC temperature control system controls the TEC to stabilize the temperature of the optical system inside the oscillator, and the temperature is set at around 25°C to reduce the frequency drift of the optical frequency comb repetition frequency, laying the foundation for long-term precise locking of the optical frequency comb.

[0033] Based on temperature control, the laser within the optical frequency comb system undergoes optical oscillation to achieve mode-locking and outputs a mode-locked pulse laser. This pulse is then input to a photodetector and converted into an electrical signal. The electrical signal passes through a bandpass amplifier to filter out the fundamental frequency signal of the frequency comb repetition frequency and enhance its strength. The optical frequency comb repetition frequency signal output from the bandpass amplifier, along with the initial reference frequency signal from the frequency synthesizer traced back to the rubidium atomic clock, is input to a frequency drift detector. The initial reference frequency output from the frequency synthesizer is controlled by a microcontroller, and its output frequency value matches the optical frequency comb repetition frequency value corresponding to the actual cavity length of the femtosecond laser oscillator (i.e., the theoretical repetition frequency value of the optical frequency comb, which varies in practice due to environmental factors such as temperature). The frequency drift detector consists of a high-speed comparator, a differential high-speed phase detector, and a differential amplifier. Its special circuit structure, in addition to realizing phase detection of the optical frequency comb repetition frequency and the reference frequency output by the frequency synthesizer (i.e., outputting the error signal), can also determine the relationship between the repetition frequency and the reference frequency based on the amplitude characteristics of the error signal. For example, when the error signal is below 0V, it indicates that the optical frequency comb repetition frequency is less than the reference frequency, and vice versa. At the same time, the amplitude characteristics of the error signal can also be used as the basis for calculating the frequency value of the error signal, thereby obtaining the actual magnitude of the optical frequency comb repetition frequency.

[0034] After the frequency drift detector completes signal phase detection, it splits the output error signal into three paths via a distributor. One path is input to a dual-powered zero-crossing comparator, which detects the relationship between the current optical frequency comb repetition frequency and the reference frequency. The reference voltage is 0V. The error signal is compared to 0V; if the output voltage is a constant negative voltage, the current repetition frequency is less than the reference frequency, and vice versa. This DC voltage value is acquired by an analog-to-digital converter and recorded by a microcontroller. The second path passes through a peak-to-peak value detection module to estimate the frequency of the detected error signal (which can be used for subsequent adjustments without precise calculations). The deviation between the optical frequency comb repetition frequency and the reference frequency is closely related to the peak-to-peak value of the error signal output by the frequency drift detector; specifically, the larger the deviation, the larger the error signal and the smaller the peak-to-peak value, exhibiting an inverse relationship. The peak-to-peak value detection module converts this peak-to-peak characteristic into a slowly changing DC voltage signal, which is then acquired by an analog-to-digital converter controlled by the microcontroller. The analog-to-digital converter (ADC) acquires the DC voltage values ​​output from the peak-to-peak detection module and the zero-crossing comparator detection module, which are used as the input signal for PID controller 1. After PID calculation, the output signal is used to modulate the signal source, making it gradually approach the repetition frequency. When the microcontroller detects a sudden change in the voltage of the zero-crossing comparator through the ADC, i.e., the output voltage of the zero-crossing comparator changes from positive to negative or from negative to positive, it indicates that the optical frequency comb repetition frequency and the reference frequency are very close (below 5Hz). At this time, the control of PID controller 1 is stopped, and the output frequency value of the frequency synthesizer remains unchanged.

[0035] After adjusting the frequency synthesizer, the microcontroller acquires the error signal output from the third distributor. Because the error signal has a small frequency and a high signal-to-noise ratio, the voltage obtained by the analog-to-digital converter (ADC) is highly accurate. The third error signal acquired by the ADC is used as the input signal for the PID controller 2. After PID calculation, it is converted into an analog voltage (output voltage range 0~5V) by the digital-to-analog converter (DAC) and then amplified (2x) by a signal amplifier before being input to the PZT driver (15x amplification). This signal is used to modulate the intracavity PZT to achieve optical frequency comb repetition frequency locking. Since the optical frequency comb repetition frequency is very close to the reference frequency, the entire locking system only requires a small locking bandwidth to achieve precise optical frequency comb locking.

[0036] After locking is completed, the microcontroller monitors the output voltage of the digital-to-analog converter (DAC) and uses the set threshold value of the output voltage (i.e., output less than 0.1V and approximately 4.9V) as the switching trigger signal and continuous input signal, which are input to PID controller 1 to adjust the output voltage value of the frequency synthesizer. Specifically, when the microcontroller detects that the DAC has triggered the set threshold condition, it starts adjusting the output reference frequency of the frequency synthesizer. Since the selected PZT model (elongation) of the optical frequency comb is determined, the initial voltage value of the microcontroller (2.5V) is also determined. Therefore, the magnitude of the repetition frequency drift is also relatively determined at this time. Based on the characteristics of the trigger threshold voltage, the microcontroller slowly controls the microcontroller to change the reference frequency through the output of PID controller 1, so that the output voltage of the DAC gradually approaches 2.5V. When the output voltage reaches 2.5V ± 0.1V, the adjustment of the output frequency value of the frequency synthesizer stops. This method can achieve ultra-long-term frequency locking of the optical frequency comb repetition frequency with extremely high locking accuracy. The specific adjustment direction of the frequency synthesizer output frequency is as follows: Assuming that the 0.1V threshold is triggered at this time, it indicates that the PZT driving voltage decreases and the PZT elongates. At this time, the oscillator cavity length increases (the repetition frequency will decrease as the cavity length increases) to resist the trend of increasing optical frequency comb repetition frequency. In reality, when the PZT is not controlled, the optical frequency comb repetition frequency drifts in the direction of increasing. At this time, the frequency synthesizer needs to increase the output reference frequency to achieve a digital-to-analog converter voltage of 2.5V±0.1V.

[0037] When the optical frequency comb repetition frequency loses lock due to external environmental factors (such as vibration or drastic environmental changes), the zero-crossing comparator will detect the lock-up change immediately. The microcontroller will then immediately disable the frequency lock of the PID controller 2 based on the output voltage value of the zero-crossing comparator to prevent damage to the optical frequency comb system. Simultaneously, the microcontroller will immediately initiate the automatic locking process of the optical frequency comb to relock the repetition frequency. Testing has verified that this repetition frequency relocking process is completed within less than 200ms.

[0038] In summary, the above are merely preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An automatic repetition frequency locking system for an optical frequency comb, characterized in that, include: The system comprises an optical frequency comb, a TEC temperature control system, a photodetector, a bandpass amplifier, a frequency drift detector, a rubidium atomic clock, a frequency synthesizer, a microcontroller, a distributor, an analog-to-digital converter, a peak-to-peak detection module, a zero-crossing comparator, PID controller 1, PID controller 2, a digital-to-analog converter, a signal amplifier, and a PZT driver. The optical frequency comb is temperature-controlled by the TEC temperature control system. The output of the optical frequency comb is connected sequentially to the first input of the frequency drift detector via the photodetector and the bandpass amplifier. The rubidium atomic clock is connected to the frequency synthesizer, and the output of the frequency synthesizer is connected to the second input of the frequency drift detector. The output of the frequency drift detector is split into three signals by the distributor. The output of the microcontroller is connected to PID controller 1 and PID controller 2. The output of PID controller 1 is connected to the frequency synthesizer. The output of PID controller 2 is connected sequentially to the PZT driver via the digital-to-analog converter and the signal amplifier. The PZT driver is connected to the optical frequency comb.

2. The automatic repetition frequency locking system for an optical frequency comb as described in claim 1, characterized in that, The frequency drift detector consists of a high-speed comparator, a differential high-speed phase detector, and a differential amplifier. It is used to perform phase detection between the optical frequency comb repetition frequency signal and the reference frequency signal, and output an error signal. At the same time, it determines the magnitude relationship between the repetition frequency and the reference frequency based on the amplitude direction characteristics of the output error signal, and determines the magnitude relationship of the error signal frequency value based on the amplitude characteristics of the output error signal. The output of the frequency drift detector is divided into three signals by a distributor. The first signal is input to a zero-crossing comparator, and the output of the zero-crossing comparator is connected to an analog-to-digital converter. The second signal is input to the peak-to-peak value detection module, and the output of the peak-to-peak value detection module is connected to the analog-to-digital converter; the third signal is input to the analog-to-digital converter.

3. The automatic repetition frequency locking system for an optical frequency comb as described in claim 1, characterized in that, The zero-crossing comparator is powered by a dual power supply, with a reference voltage of 0V. It is used to compare the error signal with 0V and output a DC voltage signal that represents the relationship between the repetition frequency and the reference frequency.

4. The automatic repetition frequency locking system for an optical frequency comb as described in claim 3, characterized in that, The peak-to-peak value detection module is used to detect the peak-to-peak value of the error signal and convert it into a DC voltage signal, which is inversely proportional to the deviation between the optical frequency comb repetition frequency and the reference frequency.

5. The automatic repetition frequency locking system for an optical frequency comb as described in any one of claims 1-4, characterized in that, The microcontroller is used to acquire the DC voltage signal output by the zero-crossing comparator and the peak-to-peak detection module, and use it as the input of PID controller 1. The output of PID controller 1 is used to control the output frequency of the frequency synthesizer. When the microcontroller detects a positive-to-negative jump in the output voltage of the zero-crossing comparator, it stops the control of PID controller 1. The microcontroller acquires the third signal output by the error signal through the distributor via the analog-to-digital converter, and uses it as the input of PID controller 2. The output of PID controller 2 is amplified by the PZT driver after passing through the analog-to-digital converter and the signal amplifier.

6. The automatic repetition frequency locking system for an optical frequency comb as described in claim 5, characterized in that, The microcontroller is also used to monitor the output voltage of the digital-to-analog converter in real time. When the voltage exceeds the set threshold, the PID controller 1 is activated to adjust the output frequency of the frequency synthesizer so that the output voltage of the digital-to-analog converter returns to the vicinity of the center value.

7. The automatic repetition frequency locking system for an optical frequency comb as described in claim 6, characterized in that, The set threshold values ​​are 0.1V and 4.9V, with a center value of 2.5V.

8. The automatic repetition frequency locking system for an optical frequency comb as described in any one of claims 1-4, characterized in that, When changes in the external environment cause the lock to be lost, the microcontroller immediately shuts down the output of PID controller 2 based on the output of the zero-crossing comparator and restarts the automatic locking process.

9. A method for automatically locking the repetition frequency of an optical frequency comb based on the system described in any one of claims 1-8, characterized in that, Includes the following steps: Step 1: Stabilize the temperature of the optical comb system at the set value using the TEC temperature control system; Step 2: The photodetector converts the optical signal output from the optical frequency comb system into an electrical signal, which is then filtered and amplified by a bandpass amplifier to obtain the fundamental frequency signal of the optical frequency comb repetition frequency. Step 3: The frequency drift detector performs phase detection between the optical frequency comb repetition frequency signal and the reference frequency signal output by the frequency synthesizer, and outputs an error signal. Step 4: The error signal is divided into three paths: the first path is converted into a DC voltage representing the frequency relationship between the optical frequency comb repetition frequency and the reference frequency through a zero-crossing comparator; the second path is converted into a DC voltage representing the frequency deviation through a peak-to-peak detection module; and the third path is directly acquired. Step 5: The microcontroller acquires the DC voltage after conversion from the first and second channels and uses it as the input of PID controller 1 to adjust the output frequency of the frequency synthesizer so that the repetition frequency of the optical frequency comb gradually approaches the reference frequency. When a jump in the zero-crossing comparator output voltage is detected, the frequency synthesizer adjustment is stopped. Step 6: The microcontroller acquires the third error signal as the input of the PID controller 2. Its output is used to adjust the PZT through the digital-to-analog converter, signal amplifier and PZT driver, thereby adjusting the cavity length of the optical comb optical system and achieving precise locking of the repetition frequency. Step 7: After locking, the microcontroller monitors the output voltage of the digital-to-analog converter in real time. When the voltage exceeds the set threshold, the output frequency of the frequency synthesizer is adjusted by the PID controller 1 so that the output voltage of the digital-to-analog converter returns to near the center value, thereby achieving long-term frequency locking. Step 8: When a loss of lock is detected, the microcontroller shuts down the output of PID controller 2 and re-executes the locking process from step 5 to step 6.

10. The method as described in claim 9, characterized in that, In step 7, the drift direction of the optical frequency comb repetition frequency is determined based on the direction in which the output voltage of the digital-to-analog converter exceeds the threshold, and the output frequency of the frequency synthesizer is adjusted accordingly.

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