A frequency discrimination sensitivity in-situ calibration method based on closed-loop dual-channel disturbance injection

Abstract

This invention proposes an in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection. Under closed-loop locked conditions, this method injects a single-frequency interference signal into the error voltage path and simultaneously measures the response amplitudes of the error voltage and the output laser frequency. Theoretical analysis shows that the ratio of the error voltage response amplitude to the output laser frequency response amplitude can directly and accurately obtain the frequency discrimination sensitivity, and this result is independent of the complex dynamic characteristics within the servo loop. The frequency discrimination sensitivity results obtained by this method are highly consistent with the theoretical model expectations, and repeated measurements demonstrate excellent stability, fully verifying the accuracy and reliability of this in-situ calibration method. This invention has advantages such as clear principle, simple operation, and no need to interrupt system operation, providing key technical support for the performance evaluation of high-precision frequency stabilization systems.

Description

Technical Field

[0001] This invention belongs to the field of laser frequency stabilization technology, and in particular relates to an in-situ accurate calibration method for a key parameter of laser frequency stabilization system—frequency discrimination sensitivity. Background Technology

[0002] The frequency stability of freely operating lasers is often insufficient for the demands of precision measurement. To improve laser frequency stability, active frequency control is necessary. Precise laser frequency control is a crucial technological foundation for cutting-edge scientific fields such as time and frequency metrology, high-precision radar detection, and deep space navigation and communication. Common frequency stabilization methods include Pound-Drever-Hall (PDH) stabilization, fiber optic interferometer stabilization, and modulation-transfer spectroscopy (MTSPS). PDH stabilization uses mode matching to resonate the laser with a highly stable optical reference cavity, thereby obtaining an excellent discrimination signal to lock the laser frequency. Fiber optic interferometer stabilization uses two beams of light to beat at the interferometer's output port, obtaining a beat signal containing discrimination information to stabilize the laser frequency. MTSPS utilizes precise transitions of atoms or molecules to obtain a discrimination signal with high signal-to-noise ratio, low drift, and no Doppler shift, which is then used to stabilize the laser frequency. In these active servo frequency stabilization systems, discrimination sensitivity is crucial. It is a key parameter related to noise suppression performance and frequency stability.

[0003] Traditional measurement methods typically require an open-loop system, introducing a known frequency change by modulating a laser, and measuring the corresponding error signal voltage change for calculation. This method is cumbersome, requires interrupting normal system operation, and the inherent instability of the laser frequency in the open-loop state can affect the accuracy of the calibration results. Therefore, there is an urgent need for a technical solution that can achieve in-situ, high-precision calibration in a closed-loop state. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and solve the problems of existing frequency discrimination sensitivity measurement methods requiring system interruption (open-loop measurement) and low measurement accuracy caused by the instability of the laser's own frequency.

[0005] An in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection is proposed. The dual channels refer to two measurement channels that simultaneously measure the error voltage response amplitude and the output laser frequency response amplitude. This method establishes a linearized model of the laser frequency stabilization system, wherein… For the complex variables of the Laplace transform, , , , , and These are the frequency setpoint, frequency error signal, error voltage, single-frequency disturbance signal, control signal, and output laser frequency, respectively. The frequency discrimination sensitivity to be calibrated for the frequency discriminator. For the transfer function of the servo controller, The transfer function for the actuator; When the laser frequency stabilization system is in closed-loop locked state, under error voltage Injecting a single-frequency perturbation signal along the path ; Synchronous measurement error voltage Power spectral density and output laser frequency The power spectral density is used to extract the error voltage. and the output laser frequency For the single-frequency perturbation signal The response amplitude; calculate the ratio of the two response amplitudes to obtain ; According to the principle of superposition of linear systems, the disturbance injection point (i.e., the single-frequency disturbance signal) is used. From the injection location to the observation point (i.e., the error voltage) and output laser frequency The transfer functions for the measurement location are as follows: (1) (2) For angular frequency single-frequency disturbance (i.e. The ratio of the amplitude of the error voltage to the amplitude of the output laser frequency is: (3) The frequency of the single-frequency disturbance signal must be within the response bandwidth of the phase detector and must avoid interference from 50 Hz mains power and its harmonics.

[0006] The error voltage power spectral density is obtained by measuring with a spectrum analyzer, and the response amplitude of the error voltage is read at the perturbation frequency.

[0007] The method for measuring the output laser frequency power spectral density is to beat the laser under test with a reference laser; use a phase noise analyzer to measure the frequency noise power spectral density of the beat signal, and read the response amplitude of the output laser frequency at the perturbation frequency.

[0008] The frequency discrimination sensitivity is measured by repeatedly measuring and averaging under multiple single-frequency disturbance signals of different frequencies to obtain the final frequency discrimination sensitivity calibration result.

[0009] The single-frequency disturbance signal can be generated by a software algorithm and directly superimposed on the control command of the digital servo controller (without the need for an external signal generator and adder circuit), or injected into the analog servo circuit through an external signal generator and adder.

[0010] The frequency of the single-frequency perturbation signal can be dynamically adjusted according to the bandwidth of the phase detector used by the system. During calibration, multiple single-frequency perturbation signals of different frequencies can be injected in sequence to obtain the consistency verification of the frequency discrimination sensitivity in different frequency bands.

[0011] The calibration method can be integrated into the automated calibration module of the laser frequency stabilization control system. The controller periodically triggers the injection of disturbance signals and automatically reads the error voltage and the response amplitude of the laser frequency at the disturbance frequency, thereby realizing online automatic in-situ monitoring of frequency discrimination sensitivity.

[0012] When measuring the error voltage response amplitude, if there is mechanical vibration or electromagnetic interference of a specific frequency in the system environment, you can choose to avoid the disturbance signal at the interference frequency, or use a narrowband filter to extract the response component at the disturbance frequency to improve the measurement signal-to-noise ratio.

[0013] Compared with the prior art, the beneficial technical effects of the present invention are as follows: 1) This invention establishes a linearized transfer function model for the laser frequency stabilization control system, rigorously proving the feasibility and rationality of the test method from a control theory perspective. Theoretical results show that the transfer function of the servo controller, the dynamic characteristics of the actuator, and the absolute amplitude of the injected disturbance signal are completely independent of the test results. This means that this method does not depend on specific hardware models or complex system parameter tuning, and has strong theoretical universality and wide applicability to equipment.

[0014] 2) Real-time calibration is achieved during normal system operation. The calibration process does not require changes to the physical structure of the system or interruption of frequency stabilization, avoiding the risks of thermal drift and frequency jump caused by system relocking. This makes it possible to perform "online performance evaluation" and "health status prediction" in precision physics experiments or long-term timekeeping systems, greatly improving the practicality of the system.

[0015] 3) Compared to the traditional open-loop calibration mode, the instantaneous frequency jitter and long drift of the laser itself are directly superimposed on the measurement signal in the open-loop mode, causing deviations in the slope fitting of the frequency discrimination curve. This method eliminates the interference of laser drift on the measurement during open-loop measurement, resulting in more accurate and reliable measurement results. Attached Figure Description

[0016] Figure 1 This is a schematic diagram illustrating the theoretical derivation of the in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to the present invention. ① Frequency setpoint 1 is a subtractor, and ② is the frequency error signal. 2 is the frequency discriminator, and ③ is the error voltage. 3 is an adder, ④ is the sum of the error voltage and the single-frequency disturbance signal, and 4 is a servo controller. ⑤ is the control signal 5 represents the actuator, and ⑥ represents the output laser frequency. ⑦ is a single-frequency disturbance signal. . Detailed Implementation

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings. Consider a typical laser frequency stabilization system; in this embodiment, it is an iodine molecule modulation transfer spectrum frequency stabilization system, which can be linearized and modeled. The frequency setpoint... For ①, frequency error signal For ②, the error voltage ③ is the sum of the error voltage and the single-frequency disturbance signal, ④ is the control signal. ⑤, Output laser frequency For ⑥, the injected single-frequency disturbance signal for ⑦, Subtractor is 1, Frequency Discriminator is 2, Adder is 3, Servo Controller is 4, Actuator is 5. For example... Figure 1 As shown, frequency setpoint ① (This example is the R(56)32-0 transition of the iodine molecule) The spectral line frequency is input to subtractor 1 and compared with the feedback output laser frequency. ⑥ Subtract to obtain the frequency error signal ②; The signal passes through frequency discriminator 2 (the frequency discrimination sensitivity to be calibrated is...). Convert to error voltage ③; Error voltage ③ With the injected single-frequency disturbance signal ⑦ (This example is provided by the signal generator) The signals are added at adder 3, which is used for single-frequency disturbance signal injection, to obtain signal ④ (i.e., the sum of the error voltage and the single-frequency disturbance signal). This sum signal is sent to the servo controller. 4 (This example uses an analog servo controller; in other embodiments, the servo controller can also be a digital servo controller); the servo controller outputs control signals. ⑤ (In this example, the frequency shifting of the piezoelectric ceramic and acousto-optic modulator of the control feedback laser) is achieved via actuator 5 (whose transfer function is...). This process generates the output laser frequency. ⑥, and feed back to subtractor 1 to form a closed loop.

[0018] The calibration steps are as follows: With the laser frequency stabilization system in closed-loop locked state, inject a single-frequency perturbation signal into the error voltage path; simultaneously measure the power spectral density of the error voltage and the output laser frequency; extract the response amplitude of the error voltage to the single-frequency perturbation signal and the response amplitude of the output laser frequency to the single-frequency perturbation signal; calculate the ratio of the response amplitude of the error voltage to the single-frequency perturbation signal to the response amplitude of the output laser frequency to the single-frequency perturbation signal to obtain the frequency discrimination sensitivity. .

[0019] The frequency of the single-frequency perturbation signal is within the response bandwidth of the phase detector and avoids interference from 50 Hz mains power and its harmonics. In this example, the error voltage power spectral density is measured using a Fast Fourier Transform spectrum analyzer SR770, and the amplitude of the response is read at the perturbation frequency. The output laser frequency power spectral density is measured by beating the laser under test with a reference laser; the frequency noise power spectral density of the beta signal is measured using a phase noise analyzer, and the amplitude spectral density value of the response is read at the perturbation frequency. In this example, the reference laser is a femtosecond optical comb locked onto a frequency-stabilized laser, and the frequency stability of this frequency-stabilized laser is an order of magnitude better than that of the laser under test.

[0020] Frequency discrimination sensitivity is measured by repeatedly measuring and averaging under multiple single-frequency disturbance signals at different frequencies to obtain the final frequency discrimination sensitivity calibration result.

[0021] As a specific example of this embodiment, four frequency points were selected: 23Hz, 48Hz, 1.27kHz, and 4.38kHz. The amplitude of the injected disturbance signal at each frequency point was set to 10 mV (peak-to-peak value), and each measurement and recording time was 1 minute. On the spectrum analyzer, the peak value of the error voltage power spectral density at each disturbance frequency was read, and the voltage response amplitude was obtained by taking the exponent. On the phase noise analyzer, the peak value of the frequency noise power spectral density at the same disturbance frequency was read, and the frequency response amplitude was obtained by taking the exponent. The ratio of the two was calculated to obtain the frequency discrimination sensitivity at each frequency point, and the arithmetic mean was taken as the final calibration result.

[0022] This embodiment successfully achieved in-situ calibration of the frequency discrimination sensitivity of the iodine molecule modulation transfer spectrum frequency stabilization system through the above steps.

Claims

1. A method for in-situ calibration of frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection, characterized in that, Includes the following steps: Establish a linearized model of the laser frequency stabilization system, in which For the complex variables of the Laplace transform, , , , , and These are the frequency setpoint, frequency error signal, error voltage, single-frequency disturbance signal, control signal, and output laser frequency, respectively. The frequency discrimination sensitivity to be calibrated for the frequency discriminator. For the transfer function of the servo controller, The transfer function for the actuator; When the laser frequency stabilization system is in closed-loop locked state, under error voltage Injecting a single-frequency perturbation signal along the path ; Synchronous measurement error voltage Power spectral density and output laser frequency The power spectral density is used to extract the error voltage. and the output laser frequency For the single-frequency perturbation signal The response amplitude; calculate the ratio of the two response amplitudes to obtain ; According to the principle of superposition of linear systems, the disturbance injection point (i.e., the single-frequency disturbance signal) is used. From the injection location to the observation point (i.e., the error voltage) and output laser frequency The transfer functions for the measurement location are as follows: （1） （2） For angular frequency single-frequency disturbance (i.e. The ratio of the amplitude of the error voltage to the amplitude of the output laser frequency is: （3）。 2. The in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to claim 1, characterized in that: The frequency of the single-frequency disturbance signal must be within the response bandwidth of the phase detector and must avoid interference from 50 Hz mains power and its harmonics.

3. The in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to claim 1, characterized in that: The error voltage power spectral density is obtained by measuring with a spectrum analyzer, and the response amplitude of the error voltage is read at the perturbation frequency.

4. The in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to claim 1, characterized in that: The method for measuring the output laser frequency power spectral density is to beat the laser under test with a reference laser; use a phase noise analyzer to measure the frequency noise power spectral density of the beat signal, and read the response amplitude of the output laser frequency at the perturbation frequency.

5. The in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to claim 1, characterized in that: The final frequency discrimination sensitivity calibration result is obtained by repeatedly measuring and averaging under multiple single-frequency disturbance signals of different frequencies.

6. The in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to claim 1, characterized in that: The servo controller is a digital servo controller, and the single-frequency disturbance signal is generated by a software algorithm and directly superimposed on the control command of the digital servo controller.

7. The in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to claim 1, characterized in that: The servo controller is an analog servo controller, and the single-frequency disturbance signal is injected into the input path of the analog servo controller through an external signal generator and an adder.

8. The in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to claim 1, characterized in that: The calibration method is integrated into the automated calibration module of the laser frequency stabilization system. The controller triggers the injection of disturbance signals at regular intervals and automatically reads the error voltage and the response amplitude of the output laser frequency at the disturbance frequency, thereby realizing online automatic in-situ monitoring of frequency discrimination sensitivity.

9. The in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to claim 5, characterized in that: The frequency of the single-frequency disturbance signal is dynamically adjusted according to the bandwidth of the phase detector to obtain consistent verification of the frequency discrimination sensitivity in different frequency bands.

10. The in-situ calibration method for frequency discrimination sensitivity based on closed-loop dual-channel perturbation injection according to claim 3, characterized in that: When reading the error voltage response amplitude, a narrowband filter is used to extract the response component at the disturbance frequency to improve the measurement signal-to-noise ratio; or disturbance frequencies with mechanical vibration or electromagnetic interference are avoided.

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