Dual-frequency modulation transfer spectroscopy laser frequency stabilization device and method

Abstract

The application discloses a kind of dual-frequency modulation transfer spectroscopy laser frequency stabilization device and method, construct the dual-frequency modulation signal including first frequency component and second frequency component, according to dual-frequency modulation signal driving fiber electro-optic phase modulator to laser is carried out dual-frequency electro-optic phase modulation;Photoelectric detector carries out detection to the polarized beam splitter output light and obtains saturated absorption signal, to saturated absorption signal in turn carries out quadrature demodulation and low-pass filter processing and obtains first error signal and second error signal, first error signal and second error signal are superimposed to obtain dual-frequency error signal;Further, tunable laser is carried out closed-loop feedback control, realizes laser frequency stabilization.The application obtains two different frequency components error signal using dual-frequency modulation transfer spectroscopy detection technology, improves slope and intensity by adding operation, improves frequency discrimination sensitivity and anti-interference ability, is conducive to peak search determination and realizes the precision locking of laser frequency to rubidium absorption peak, with good practicability.

Description

Technical Field

[0001] This invention belongs to the field of laser frequency stabilization technology, and in particular relates to a dual-frequency modulation transfer spectrum laser frequency stabilization device and method. Background Technology

[0002] Laser interferometry has important applications in ultra-precision machining and instrument testing and calibration. The stability and traceability of the laser wavelength directly affect measurement accuracy. Free-flowing lasers are affected by temperature and current noise, and their wavelengths often drift or fluctuate, making them unsuitable for precision measurements. Locking the laser to the absorption peaks of gases such as rubidium and acetylene is an important frequency stabilization method for obtaining highly stable lasers. The key lies in obtaining the error signal corresponding to the frequency difference between the laser frequency and the absorption peak. Traditional saturated absorption detection methods are susceptible to DC drift and noise. To address this issue, modulation-transfer spectroscopy converts the saturated absorption signal into an AC signal, improving anti-interference capabilities. Further optimization of the error signal is currently a research hotspot.

[0003] Currently, modulation-transfer spectroscopy technology mainly employs single-frequency phase modulation of the pump laser. After mixing and filtering the saturated absorption signal, an error signal is obtained. The slope and intensity of the error signal play a decisive role in the laser frequency locking performance. The slope at the zero-crossing point of the error signal determines the frequency discrimination sensitivity, and the error signal intensity determines the anti-interference capability of the lock. In engineering applications, the frequency discrimination sensitivity determines the accuracy of laser frequency locking, and the magnitude of the error signal is crucial to the locking control process. During absorption peak identification, it is often necessary to determine whether the laser frequency has scanned to the absorption peak based on the zero-crossing point or peak value of the error signal. If the error signal intensity is too low, it can easily lead to identification errors. In terms of lock status monitoring, it is often necessary to monitor whether the error signal deviates from zero or whether there are abrupt changes to determine whether the lock has been lost. If the error signal intensity is too low, it can easily lead to identification errors.

[0004] The slope and intensity of the error signal are mainly affected by three factors: gas concentration and temperature, the power and polarization state of the pump and probe light, and the power and frequency of the modulation signal. In practical applications, these factors in the modulation-transfer-detection device are adjusted based on debugging experience to optimize the error signal. However, the error signal profile has certain peculiarities; adjusting the modulation frequency to increase the error signal intensity does not necessarily improve the slope at the zero-crossing point. For example, when the ratio of the modulation frequency to the natural linewidth of the absorption spectrum is approximately 1 and 1.5, respectively, the slope and intensity of the error signal can reach their maximum values. That is, it is difficult to simultaneously achieve optimal slope and intensity at a single modulation frequency.

[0005] Therefore, how to simultaneously improve the slope and intensity of the error signal is a key technical problem that needs to be solved in modulation transfer spectroscopy. Summary of the Invention

[0006] To address the problems existing in the background art, this invention discloses a dual-frequency modulation transfer spectroscopy laser frequency stabilization method and device. It uses dual-frequency modulation transfer detection technology to obtain error signals with different frequency components, and improves the slope and intensity through addition operations, thereby enhancing the frequency discrimination sensitivity and anti-interference capability. This solves the problems of difficulty in enhancing the error signal intensity and improving the sensitivity in the background art.

[0007] The technical solution adopted in this invention is: The device in this embodiment includes a tunable laser, an optical fiber beam splitter, a first collimator, a polarizer, an optical fiber electro-optic phase modulator, a gas cell, a polarizing beam splitter, a second collimator, a photodetector, an analog-to-digital converter, and a laser frequency stabilization module. The output of the tunable laser is connected to the input of the fiber optic beam splitter. The two outputs of the fiber optic beam splitter are connected to the inputs of the fiber electro-optic phase modulator and the first collimator, respectively. The output of the fiber electro-optic phase modulator is connected to the input of the second collimator. The output of the second collimator outputs spatial light, which is incident on the gas cell from right to left through the polarizing beam splitter. The output of the first collimator outputs spatial light, which is incident on the gas cell from left to right through the polarizer. After exiting the gas cell, the light is reflected by the polarizing beam splitter to the photodetector. The photodetector is electrically connected to the analog-to-digital converter (ADC). The ADC, the tunable laser, and the fiber electro-optic phase modulator are all electrically connected to the laser frequency stabilization module. The photodetector collects the light signal reflected by the polarizing beam splitter and, after conversion by the ADC, sends it to the laser frequency stabilization module. The laser frequency stabilization module is used to control the tunable laser and the fiber electro-optic phase modulator to achieve laser frequency stabilization.

[0008] The output laser from the tunable laser is split into two laser beams with a power ratio of 1:1 by an optical fiber beam splitter. One laser beam is input into an optical fiber electro-optic phase modulator for dual-frequency electro-optic phase modulation. The output light of the optical fiber electro-optic phase modulator is used as pump light. After passing through a second collimator and a polarizing beam splitter in sequence, it is incident on the gas cell from right to left. The other laser beam is used as probe light. After passing through a first collimator and a polarizer in sequence, it is incident on the gas cell from left to right. After exiting the gas cell, it is reflected by the polarizing beam splitter to the photodetector.

[0009] The laser frequency stabilization module includes a first multiplier, a first local oscillator, a first filter, a second multiplier, a second local oscillator, a second filter, a first adder, a laser scanning and locking module, a first digital-to-analog converter, a first signal source, a third multiplier, a second signal source, a fourth multiplier, a second adder, and a second digital-to-analog converter; The output of the analog-to-digital converter is connected to the input of the first multiplier and the second multiplier, respectively. The outputs of the first local oscillator and the second local oscillator are also connected to the input of the first multiplier and the second multiplier, respectively. The outputs of the first multiplier and the second multiplier are connected to the input of the first adder after passing through the first filter and the second filter, respectively. The output of the first adder is connected to the input of the laser scanning and locking module. The output of the laser scanning and locking module is connected to the input of the first digital-to-analog converter. The output of the first digital-to-analog converter is connected to the tunable laser. The first signal source and the second signal source are respectively connected to the inputs of the third multiplier and the fourth multiplier. The outputs of the third multiplier and the fourth multiplier are connected to the inputs of the second adder. The output of the second adder is connected to the input of the second digital-to-analog converter. The output of the second digital-to-analog converter is connected to the fiber optic electro-optic phase modulator.

[0010] The dual-frequency modulation transfer spectroscopy laser frequency stabilization method includes the following steps: S1. Construct a dual-frequency modulation signal containing a first frequency component and a second frequency component, and drive an optical fiber electro-optic phase modulator to perform dual-frequency electro-optic phase modulation on the laser according to the dual-frequency modulation signal. S2. The photodetector detects the output light of the polarization beam splitter to obtain the saturated absorption signal, and inputs the saturated absorption signal into the FPGA development board after analog-to-digital conversion. S3. In the FPGA development board, the saturated absorption signal is synchronously demodulated with the first frequency local oscillator signal and the second frequency local oscillator signal, respectively, and then low-pass filtered to obtain the first error signal and the second error signal. The first error signal and the second error signal are superimposed to obtain the dual-frequency error signal. S4. Closed-loop feedback control of the tunable laser is performed based on the dual-frequency error signal to lock the laser frequency to the target absorption peak position, thereby achieving laser frequency stabilization.

[0011] The dual-frequency modulation signal is specifically set according to the following formula: in, z ( t This indicates a dual-frequency modulated signal. f 1 and f 2 represents the first frequency and the second frequency, respectively. K 1 and K 2 represents the corresponding amplitude of the first frequency and the corresponding amplitude of the second frequency, respectively.

[0012] The saturation absorption signal is specifically set according to the following formula: Where S(t) represents the saturation absorption signal, These are the subscript numbers of the two modulation frequencies. , These represent the signal components with absorption and dispersion line shapes, respectively, and Δ represents the frequency difference between the laser beam and the specific absorption peak of rubidium atoms. Indicates phase, This represents the amplitude coefficients of two frequency components in a saturated absorption signal. Indicates the first modulation frequency. Indicates the second modulation frequency. For constant terms, F represents the natural linewidth constant of the gas absorption spectral line, which is a gain coefficient related to the modulation depth.

[0013] Specifically, S3 is: S3.1 The saturated absorption signal is multiplied by the first multiplier and the local oscillator signal with the first frequency output by the first local oscillator source. Then, it is low-pass filtered by the first filter to obtain the first error signal. S3.2 The saturated absorption signal is multiplied by the second multiplier and the local oscillator signal with the second frequency output from the second local oscillator source. Then, it is low-pass filtered by the second filter to obtain the second error signal. S3.3. Superimpose the first error signal and the second error signal to obtain a dual-frequency error signal.

[0014] The dual-frequency error signal is specifically set according to the following formula: in, This indicates a dual-frequency error signal. and These represent the amplitude coefficients of the first frequency and the first frequency component in the saturated absorption signal, respectively. and These represent the first and second frequency signal components of the dispersive line shape, respectively, and Δ represents the frequency difference between the laser and the specific absorption peak of the rubidium atom.

[0015] The beneficial effects of this invention are: This invention uses dual-frequency modulation transfer spectroscopy detection technology to obtain error signals of two different frequency components. By adding the components, the slope and intensity are improved, thus enhancing frequency discrimination sensitivity and anti-interference ability. This is beneficial for peak finding and judgment and for achieving precise locking of the laser frequency to the rubidium absorption peak, and has good practicality. Attached Figure Description

[0016] Figure 1 This is a block diagram illustrating the principle of a dual-frequency modulation transfer spectroscopy laser frequency stabilization method and device.

[0017] Figure 2The figures show the simulation results of the relationship between the peak-to-peak value, zero-crossing slope and modulation depth of the error signals of two single-frequency modulation transfer spectroscopy techniques, namely 6MHz (a) and 9MHz (b).

[0018] Figure 3 The waveform simulation results are shown for the voltage and frequency difference when the peak-to-peak value and zero-crossing slope of the error signal are at their maximum for both 6MHz and 9MHz single-frequency modulation transfer spectrum techniques.

[0019] Figure 4 These are grayscale images of the peak-to-peak values ​​and the zero-crossing slope of the dual-frequency error signal under different modulation depth combinations (a) and (b).

[0020] Figure 5 This is a comparison chart of the simulation results of the dual-frequency error signal and the error signal waveforms of the traditional method with two modulation frequencies of 6MHz and 9MHz.

[0021] In the diagram: 1. Tunable laser, 2. Fiber beam splitter, 3. First collimator, 4. Polarizer, 5. Fiber electro-optic phase modulator, 6. Gas cell, 7. Polarizing beam splitter, 8. Second collimator, 9. Photodetector, 10. Analog-to-digital converter, 11. First multiplier, 12. First local oscillator, 13. First filter, 14. Second multiplier, 15. Second local oscillator, 16. Second filter, 17. First adder, 18. Laser scanning and locking module, 19. First digital-to-analog converter, 20. First signal source, 21. Third multiplier, 22. Second signal source, 23. Fourth multiplier, 24. Second adder, 25. Second digital-to-analog converter. Detailed Implementation

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

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the scope of protection of this invention.

[0024] like Figure 1 As shown, the device in this embodiment includes a tunable laser 1, an optical fiber beam splitter 2, a first collimator 3, a polarizer 4, an optical fiber electro-optic phase modulator 5, a gas cell 6, a polarizing beam splitter 7, a second collimator 8, a photodetector 9, an analog-to-digital converter 10, and a laser frequency stabilization module. The output of the tunable laser 1 is connected to the input of the fiber beam splitter 2. The two outputs of the fiber beam splitter 2 are connected to the inputs of the fiber electro-optic phase modulator 5 and the first collimator 3, respectively. The output of the fiber electro-optic phase modulator 5 is connected to the input of the second collimator 8. The output of the second collimator 8 outputs spatial light, which is incident from right to left into the gas cell 6 through the polarizing beam splitter 7. The output of the first collimator 3 outputs spatial light, which is incident from left to right into the gas cell 6 through the polarizer 4. After passing through the gas cell 6, the light is reflected by the polarizing beam splitter 7 to the photodetector 9. The photodetector 9 is electrically connected to the analog-to-digital converter 10. The analog-to-digital converter 10, the tunable laser 1, and the fiber electro-optic phase modulator 5 are all electrically connected to the laser frequency stabilization module. The photodetector 9 collects the light signal reflected by the polarizing beam splitter 7, converts it through the analog-to-digital converter 10, and sends it to the laser frequency stabilization module. The laser frequency stabilization module is used to control the tunable laser 1 and the fiber electro-optic phase modulator 5 to achieve laser frequency stabilization.

[0025] In the embodiments of the present invention, the tunable laser output wavelength is 780.24 nm, and the laser wavelength can be adjusted by a voltage-controlled adjustment terminal; the gas cell is filled with rubidium gas and the rubidium absorption spectrum is used as the laser frequency reference, and the natural linewidth constant of the rubidium atomic absorption spectrum is F = 6 MHz; the fiber electro-optic phase modulator adopts a 100 MHz high-bandwidth lithium niobate electro-optic phase modulator; the modulation frequency is set to... f 1=F=6 MHz f 2 = 1.5F = 9 MHz. The sampling frequency of the analog-to-digital converter and the update rate of the digital-to-analog converter are both 125 MHz; the FPGA chip on the FPGA development board is ZYNQ7010.

[0026] Tunable laser 1 outputs continuous laser light, which is split into two laser beams with a power ratio of 1:1 by fiber beam splitter 2. One of the laser beams is input into fiber electro-optic phase modulator 5 for frequency modulation. f 1 and f The dual-frequency electro-optic phase modulation of 2 uses the output light of the fiber electro-optic phase modulator 5 as the pump light, which passes through the second collimator 8 and the polarizing beam splitter 7 in sequence and is then incident from right to left into the gas cell 6. Another laser beam is used as the probe light, which passes through the first collimator 3 and the polarizer 4 in sequence and is then incident from left to right into the gas cell 6. After passing through the gas cell 6, it is reflected by the polarizing beam splitter 7 to the photodetector 9.

[0027] The laser frequency stabilization module includes a first multiplier 11, a first local oscillator 12, a first filter 13, a second multiplier 14, a second local oscillator 15, a second filter 16, a first adder 17, a laser scanning and locking module 18, a first digital-to-analog converter 19, a first signal source 20, a third multiplier 21, a second signal source 22, a fourth multiplier 23, a second adder 24, and a second digital-to-analog converter 25. The photodetector 9 collects the light signal reflected by the polarizing beam splitter 7, and the output of the analog-to-digital converter 10 is connected to the first multiplier 11 and the second multiplier 14 respectively. The outputs of the first local oscillator 12 and the second local oscillator 15 are also connected to the first multiplier 11 and the second multiplier 14 respectively. The outputs of the first multiplier 11 and the second multiplier 14 are connected to the first adder 17 after passing through the first filter 13 and the second filter 16 respectively. The output of the first adder 17 is connected to the input of the laser scanning and locking module 18. The output of the laser scanning and locking module 18 is connected to the first digital-to-analog converter 19. The output of the first digital-to-analog converter 19 is connected to the tunable laser 1. The first signal source 20 and the second signal source 22 are respectively connected to the third multiplier 21 and the fourth multiplier 23. The output terminals of the third multiplier 21 and the fourth multiplier 23 are connected to the second adder 24. The output terminal of the second adder 24 is connected to the second digital-to-analog converter 25. The output terminal of the second digital-to-analog converter 25 is connected to the fiber optic electro-optic phase modulator 5.

[0028] The output center frequency of tunable laser 1 is A continuous laser beam is split into two beams with a power ratio of 1:1 by fiber beam splitter 2. One beam is then modulated by fiber electro-optic phase modulator 5 to a frequency of... f 1 and f The dual-frequency electro-optic phase modulation of the fiber beam splitter 2 results in a pump beam that is converted into a p-polarized spatial beam by the second collimator 8 and transmitted through the polarizing beam splitter 7 in a right-to-left direction through the gas chamber 6. The other laser beam output from the fiber beam splitter 2 serves as the probe beam, converted into an s-polarized spatial beam by the first collimator 3 and transmitted through the polarizer 4 in a left-to-right direction through the gas chamber 6. It is then reflected by the polarizing beam splitter 7 to the photodetector 9. The photodetector 9 outputs a dual-frequency modulated saturated absorption signal. S ( t The tunable laser 1 performs a triangular frequency sweep under the control of the laser scanning and locking module 18. The laser scanning and locking module 18 outputs a triangular wave to the first digital-to-analog converter 19, which then transmits it to the voltage-controlled adjustment terminal of the tunable laser 1 for triangular frequency sweep. The first signal source 20 and the second signal source 22 generate frequencies of... f 1 and f The sinusoidal signal 2 is multiplied by amplitude coefficients K1 and K2 by the third multiplier 21 and the fourth multiplier 23, respectively. The two multiplied sinusoidal signals are then added by the second adder 24, and finally converted and superimposed by the second digital-to-analog converter 25. f 1 and f 2. The dual-frequency modulation signal with two frequency components is used as the modulation signal of the fiber electro-optic phase modulator 5.

[0029] Dual-frequency modulation transfer saturated absorption signal S ( t The signal is sampled by the analog-to-digital converter 10 and transmitted to the FPGA for further processing. Within the FPGA, the saturated absorption signal is transferred via dual-frequency modulation. S ( t The frequency output by the first multiplier 11 and the first local oscillator 12 is: f The local oscillator signal is multiplied and then low-pass filtered by the first filter 13 to obtain the error signal. e 1( t ), e 1( t )= A 1 Q 1(Δ) / 2; Saturated absorption signal of dual-frequency modulation transfer S ( t The frequency output by the second multiplier 14 and the second local oscillator 15 is: f The two local oscillator signals are multiplied, and then low-pass filtered by the second filter 16 to obtain the error signal. e 2( t ), e2( t ) = A 2 Q 2(Δ) / 2. Error signal e 1( t ) and error signal e 2( t The signals are added together by the first adder 17 to obtain a dual-frequency error signal with a greater slope and intensity. e s ( t )= e 1( t )+ e 2( t The cutoff frequencies of the first filter 13 and the second filter 16 are 1 MHz; the output frequency of the first local oscillator 12 is... f The local oscillator signal of 1 has been pre-tuned to saturate absorption signal. S ( t The frequency in ) is f The frequency components of 1 are in phase, and the output frequency of the second local oscillator 15 is... f The local oscillator signal of 2 has been pre-tuned to saturate absorption signal. S ( t The frequency in ) is f The frequency components of 2 are in phase.

[0030] Dual-frequency error signal e s ( tCompared to single-frequency error signals, dual-frequency error signals have a steeper zero-crossing slope, resulting in higher frequency discrimination sensitivity. They also exhibit greater overall strength and stronger resistance to noise interference, which is beneficial for precise laser frequency locking. e s ( t The signal is transmitted to the laser scanning and locking module 18 for closed-loop locking. The laser scanning and locking module 18 uses the dual-frequency error signal... e s ( t The intensity is used to find the peak. When the laser scans to the absorption peak, the laser scanning and locking module 18 will automatically pause the triangular scanning and start closed-loop control, and finally lock the tunable laser to a specific rubidium absorption peak.

[0031] The dual-frequency modulation transfer spectroscopy laser frequency stabilization method includes the following steps: S1. Construct a dual-frequency modulation signal containing a first frequency component and a second frequency component. Drive the fiber electro-optic phase modulator 5 to perform dual-frequency electro-optic phase modulation on the laser according to the dual-frequency modulation signal to obtain dual-frequency modulated pump light. Then, make the dual-frequency modulated pump light and the unmodulated probe light undergo modulation transfer in the gas chamber. Specifically, the frequency is f 1 and f The two sinusoidal signals 2 are multiplied by amplitude coefficients K1 and K2 respectively, and the sum is used as the modulation signal of the electro-optic phase modulator to simultaneously modulate the laser at a frequency of . f 1 and f The electric field of the dual-frequency electro-optic phase modulation of 2, where the modulated laser is used as the pump light for the dual-frequency modulation transfer spectrum, can be expressed as: in For the pump light amplitude, Indicates the center frequency of the laser. and These represent the modulation depths corresponding to the two modulation frequencies. It is a time variable.

[0032] After the above dual-frequency electro-optic phase modulation, the pump light generates a frequency domain located at... , Four laser sidebands are located at the position. Another unmodulated laser beam is used as the probe beam, and it enters the gas cell from the left and right directions, respectively, along with the pump beam. Inside the gas cell, under the action of four-wave mixing, the modulation effect of the pump beam is transferred to the unmodulated probe beam, which will generate four laser sidebands with the same distribution.

[0033] S2. The photodetector 9 detects the output light of the polarizing beam splitter 7 to obtain the saturated absorption signal, and inputs the saturated absorption signal into the FPGA development board after analog-to-digital conversion. The saturated absorption signal of dual-frequency modulation transfer contains f 1 and f Two frequency components can be used to demodulate the error signals corresponding to the two modulation frequencies.

[0034] S3. In the FPGA development board, the saturated absorption signal is sequentially subjected to quadrature demodulation and low-pass filtering to obtain the first error signal and the second error signal. The first error signal and the second error signal are superimposed to obtain the dual-frequency error signal. Specifically, the saturated absorption signal of dual-frequency modulation transfer S ( t The signal is sampled by an analog-to-digital converter and then transmitted to the FPGA. Within the FPGA, the saturated absorption signal is transferred via dual-frequency modulation. S ( t ) respectively with frequencies of f 1 and f The error signal is obtained by multiplying the two local oscillator signals and then passing them through a low-pass filter. e 1( t )= A 1 Q 1(Δ) / 2 and e 2( t ) = A 2 Q 2(Δ) / 2; Error signal e 1( t )and e 2( t The dual-frequency error signal is obtained through summation. e s ( t The formula is as follows: Among them, the frequency is f 1 and f The local oscillator signals of the two have been pre-adjusted to saturation absorption signals. S ( t The frequency in ) is f 1 and f The dispersion linear components of 2 are in phase. Therefore, the above error signal e 1( t ), e 2( t )and e s ( t All of them contain only dispersive line types.

[0035] Dual-frequency error signal e s ( tCompared to single-frequency error signals generated by traditional methods, the slope at the zero-crossing point is greater, resulting in higher frequency discrimination sensitivity. At the same time, the overall strength is greater and the anti-noise interference capability is stronger.

[0036] S4. Based on the dual-frequency error signal, the tunable laser 1 is subjected to closed-loop feedback control to lock the laser frequency to the target absorption peak position, thereby achieving laser frequency stabilization.

[0037] Specifically, during the laser frequency scanning process, based on the dual-frequency error signal... e s ( t Peak finding is achieved using intensity, when the dual-frequency error signal... e s ( t When the peak-to-peak value exceeds a certain threshold, the laser frequency is considered to be aligned with the absorption peak, and the center value of the scanning voltage corresponding to the peak-to-peak value is recorded. At this point, the laser frequency sweep is paused, and the scanning voltage is finely adjusted to the corresponding center value. Then, closed-loop control is immediately initiated, using a PID algorithm (proportional-integral-derivative) to analyze the dual-frequency error signal. e s ( t The feedback signal is processed to perform closed-loop adjustment of the laser frequency, ultimately locking the tunable laser to a specific absorption peak. During peak finding, the tunable laser 1 undergoes triangular frequency sweeping under the control of the laser scanning and locking module 18. The laser scanning and locking module 18 outputs a triangular wave to the first digital-to-analog converter 19, which then transmits it to the voltage-controlled adjustment terminal of the tunable laser 1 for triangular frequency sweeping.

[0038] In practical implementation, frequency f 1=F、 f 2 = 1.5F. Adjust amplitude coefficients K1 and K2 to achieve modulation depths of 0.968 and 0.414 for the sinusoidal phase modulation frequencies of the laser within the dual-frequency modulation transfer detector unit. During laser frequency sweeping, the triangular scanning voltage needs to be pre-adjusted to ensure the laser sweep range covers the absorption peak to be locked. The dual-frequency error signal when the laser sweeps past the absorption peak is then used as the basis for this adjustment. e s ( t The peak-to-peak value is 0.5 times the peak value as the threshold for peak detection.

[0039] Compared with the traditional single-frequency error signal, the dual-frequency error signal obtained by the present invention through the above steps has a larger zero-crossing slope, corresponding to higher frequency discrimination sensitivity. At the same time, it has a larger overall strength and stronger anti-noise interference capability, which is beneficial for peak finding and precise locking of laser frequency.

[0040] In practical implementation, frequency f 1=F、 f2 = 1.5F. Adjust amplitude coefficients K1 and K2 to achieve modulation depths of 0.968 and 0.414 for the sinusoidal phase modulation frequencies of the laser within the dual-frequency modulation transfer detector unit. During laser frequency sweeping, the triangular scanning voltage needs to be pre-adjusted to ensure the laser sweep range covers the absorption peak to be locked. The dual-frequency error signal when the laser sweeps past the absorption peak is then used as the basis for this adjustment. e s ( t The peak-to-peak value is 0.5 times the peak value as the threshold for peak detection.

[0041] Compared with the traditional single-frequency error signal, the dual-frequency error signal obtained by the present invention through the above steps has a larger zero-crossing slope, corresponding to higher frequency discrimination sensitivity. At the same time, it has a larger overall strength and stronger anti-noise interference capability, which is beneficial for peak finding and precise locking of laser frequency.

[0042] The dual-frequency modulation signal is specifically set according to the following formula: in, z ( t This indicates a dual-frequency modulated signal. f 1 and f 2 represents the first modulation frequency and the second modulation frequency, respectively. K 1 and K 2 represents the corresponding amplitude of the first modulation frequency and the corresponding amplitude of the second modulation frequency, respectively.

[0043] The saturation absorption signal S(t) is specifically set according to the following formula: Where S(t) represents the saturation absorption signal, These are the subscript numbers of the two modulation frequencies. , These represent the signal components with absorption and dispersion line shapes, respectively, and Δ represents the frequency difference between the laser beam and the specific absorption peak of rubidium atoms. Indicates phase, This represents the amplitude coefficient of the i-th frequency component in the saturated absorption signal. Indicates the first frequency. Indicates the second frequency. For constant terms, To be compatible with modulation depth and The relevant gain coefficient, F, represents the natural linewidth constant of the gas absorption spectral line.

[0044] Specifically, S3 is: S3.1 The saturated absorption signal is multiplied by the first multiplier 11 and the local oscillator signal with the first frequency output by the first local oscillator source 12, and then low-pass filtered by the first filter 13 to obtain the first error signal. S3.2 The saturated absorption signal is multiplied by the second multiplier 14 and the local oscillator signal with the second frequency output by the second local oscillator source 15. Then, it is low-pass filtered by the second filter 16 to obtain the second error signal. S3.3. Superimpose the first error signal and the second error signal to obtain a dual-frequency error signal.

[0045] The dual-frequency error signal is specifically set according to the following formula: in, This indicates a dual-frequency error signal. and These represent the amplitude coefficients of the first frequency and the first frequency component in the saturated absorption signal, respectively. and These represent the first and second frequency signal components of the dispersive line shape, respectively, and Δ represents the frequency difference between the laser and the specific absorption peak of the rubidium atom.

[0046] In modulation-transfer spectral detection, the pump light is subjected to dual-frequency sinusoidal phase modulation to obtain a saturated absorption signal for dual-frequency modulation transfer. The saturated absorption signal is multiplied by the dual-frequency local oscillator signals and low-pass filtered to obtain error signals corresponding to the two frequencies. These error signals are then added together to obtain the dual-frequency error signal. The advantages are a larger slope and intensity at the zero-crossing point, resulting in higher frequency discrimination sensitivity and stronger noise immunity. The laser scanning and locking module uses the intensity of the dual-frequency error signal to find the peak. When the laser scan reaches the absorption peak, the laser frequency scan is paused and closed-loop control is initiated, ultimately locking the laser to a specific absorption peak. This invention solves the problem of simultaneously improving the sensitivity and intensity of the error signal in single-frequency modulation transfer technology. High frequency discrimination sensitivity improves the laser frequency locking accuracy, and good noise immunity improves the accuracy of peak finding.

[0047] like Figure 2The simulation results show the relationship between peak-to-peak value, zero-crossing slope, and modulation depth of the error signal for two single-frequency modulation transfer spectroscopy techniques: 6MHz (a) and 9MHz (b). It can be seen that for single-frequency modulation transfer spectroscopy, the peak-to-peak value and zero-crossing slope of the error signal are maximized when the modulation depth is 1.08. However, when the modulation depth exceeds 1.08, increasing the modulation depth actually decreases the peak-to-peak value and zero-crossing slope of the error signal. Comparing the modulation frequencies of 6MHz and 9MHz, the 6MHz frequency corresponds to a larger zero-crossing slope (0.031547) and a peak-to-peak value of 0.210780, which is 90.9% of the peak-to-peak value of the 9MHz error signal. The 9MHz frequency corresponds to a larger peak-to-peak value (0.231873), but the corresponding maximum zero-crossing slope is reduced by about half (0.016404). This indicates that for single-frequency modulation transfer spectroscopy, it is difficult to simultaneously achieve optimal peak-to-peak value and zero-crossing slope when optimizing the error signal by adjusting the modulation frequency. To further explain this phenomenon... Figure 3 The simulation results show the waveforms at their maximum peak-to-peak value and zero-crossing slope for error signals from two single-frequency modulation transfer spectroscopy techniques at 6MHz and 9MHz. It can be seen that 9MHz corresponds to the largest peak-to-peak value, but the error curve exhibits some distortion near the zero-crossing point, causing it to no longer have a linear shape.

[0048] In the embodiments, the modulation frequencies are respectively set to f 1=F=6 MHz f 2 = 1.5F = 9 MHz. The modulation corresponding to the two modulation frequencies was controlled to vary from 0 to 2 in steps of 0.001. The peak-to-peak value and the maximum value of the zero-crossing slope of the dual-frequency error signal under each combination were analyzed. Relevant simulation results are shown below. Figure 4 .like Figure 4 As shown, the horizontal and vertical axes represent... f 1 and f 2. The modulation frequency corresponds to the modulation depth of the electro-optic phase modulation. The gray value of each data point in the figure is the peak-to-peak value / zero-crossing slope under the corresponding modulation depth combination. Figure 4In (a) and (b), the gray values ​​of the contour lines are equal to the maximum peak-to-peak value and the maximum zero-crossing slope of the traditional single-frequency modulation transfer method, respectively. The modulation depth combinations within the contour lines are all greater than the contour line values, meaning that all combinations within the contour line region can increase the peak-to-peak value or the maximum zero-crossing slope of the error signal. Simulation results show that when the modulation depths are 0.968 and 0.414, the zero-crossing slope of the dual-frequency error signal reaches its maximum value of 0.034256, corresponding to a peak-to-peak value of 0.262045; when the modulation depths are 0.675 and 0.785, the peak-to-peak value of the dual-frequency error signal reaches its maximum value of 0.276279, corresponding to a zero-crossing slope of 0.031741. Under these two modulation depth combinations, both the slope and peak-to-peak value at the zero-crossing position exceed the maximum zero-crossing slope of 0.031547 and the maximum peak-to-peak value of 0.231873 of the single-frequency modulation transfer spectroscopy technique. Using a combination of dual-frequency error signals with larger slopes (m1=0.968, m2=0.414) for laser frequency stabilization, the peak-to-peak value was increased by 24.3% and the slope by 8.6% compared to the traditional method with a maximum slope of 6MHz; compared to the traditional method with a maximum peak-to-peak value of 9MHz, the peak-to-peak value was increased by 13.0% and the slope by 108.8%. Figure 5 The figure shows the waveform simulation results of the dual-frequency error signals m1=0.968, m2=0.414 and the error signals of the traditional method at 6MHz and 9MHz, with a modulation depth of 1.08. Clearly, the dual-frequency error signal has a larger zero-crossing slope and a larger peak-to-peak value. The above simulation analysis results indicate that the method described in this invention can simultaneously improve both the zero-crossing slope and the peak-to-peak value of the error signal.

[0049] In summary, this invention uses dual-frequency modulation transfer detection technology to obtain error signals with two different frequency components. By adding the signals, the slope and intensity are improved, thus enhancing frequency discrimination sensitivity and anti-interference capability. This is beneficial for peak finding and judgment and for achieving precise locking of the laser frequency to the rubidium absorption peak, demonstrating good practicality.

[0050] The above detailed embodiments illustrate the technical solution and beneficial effects of the present invention. It should be understood that the above description is only the most preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A dual-frequency modulation transfer spectral laser frequency stabilization device, characterized in that: The device includes a tunable laser (1), an optical fiber beam splitter (2), a first collimator (3), a polarizer (4), an optical fiber electro-optic phase modulator (5), a gas cell (6), a polarizing beam splitter (7), a second collimator (8), a photodetector (9), an analog-to-digital converter (10), and a laser frequency stabilization module. The output end of the tunable laser (1) is connected to the input end of the fiber beam splitter (2). The two output ends of the fiber beam splitter (2) are connected to the input ends of the fiber electro-optic phase modulator (5) and the first collimator (3), respectively. The output end of the fiber electro-optic phase modulator (5) is connected to the input end of the second collimator (8). The output end of the second collimator (8) outputs spatial light, which is incident from right to left into the gas cell (6) through the polarizing beam splitter (7). The output end of the first collimator (3) outputs spatial light, which is incident from left to right into the gas cell (6) through the polarizer (4). After passing through the gas chamber (6), the light signal is reflected by the polarizing beam splitter (7) to the photodetector (9). The photodetector (9) is electrically connected to the analog-to-digital converter (10). The analog-to-digital converter (10), the tunable laser (1), and the fiber electro-optic phase modulator (5) are all electrically connected to the laser frequency stabilization module. The photodetector (9) collects the light signal reflected by the polarizing beam splitter (7), and after conversion by the analog-to-digital converter (10), it is sent to the laser frequency stabilization module. The laser frequency stabilization module is used to control the tunable laser (1) and the fiber electro-optic phase modulator (5) to achieve laser frequency stabilization.

2. The dual-frequency modulation transfer spectral laser frequency stabilization device according to claim 1, characterized in that: The output laser of the tunable laser (1) is split into two laser beams with a power ratio of 1:1 by the fiber beam splitter (2). One laser beam is input into the fiber electro-optic phase modulator (5) for dual-frequency electro-optic phase modulation. The output light of the fiber electro-optic phase modulator (5) is used as pump light. After passing through the second collimator (8) and the polarizing beam splitter (7) in sequence, it is incident from right to left into the gas cell (6). The other laser beam is used as probe light. After passing through the first collimator (3) and the polarizer (4) in sequence, it is incident from left to right into the gas cell (6). After passing through the gas cell (6), it is reflected by the polarizing beam splitter (7) to the photodetector (9).

3. The dual-frequency modulation transfer spectral laser frequency stabilization device according to claim 1, characterized in that: The laser frequency stabilization module includes a first multiplier (11), a first local oscillator (12), a first filter (13), a second multiplier (14), a second local oscillator (15), a second filter (16), a first adder (17), a laser scanning and locking module (18), a first digital-to-analog converter (19), a first signal source (20), a third multiplier (21), a second signal source (22), a fourth multiplier (23), a second adder (24), and a second digital-to-analog converter (25). The output of the analog-to-digital converter (10) is connected to the input of the first multiplier (11) and the second multiplier (14), respectively. The outputs of the first local oscillator (12) and the second local oscillator (15) are also connected to the input of the first multiplier (11) and the second multiplier (14), respectively. The outputs of the first multiplier (11) and the second multiplier (14) are connected to the input of the first adder (17) after passing through the first filter (13) and the second filter (16), respectively. The output of the first adder (17) is connected to the input of the laser scanning and locking module (18), and the output of the laser scanning and locking module (18) is connected to the input of the first digital-to-analog converter (19). The output of the first digital-to-analog converter (19) is connected to the tunable laser (1). The first signal source (20) and the second signal source (22) are respectively connected to the input of the third multiplier (21) and the fourth multiplier (23). The outputs of the third multiplier (21) and the fourth multiplier (23) are connected to the input of the second adder (24). The output of the second adder (24) is connected to the input of the second digital-to-analog converter (25). The output of the second digital-to-analog converter (25) is connected to the fiber electro-optic phase modulator (5).

4. A dual-frequency modulation transfer spectral laser frequency stabilization method applied to the apparatus described in any one of claims 1-3, characterized in that, The method includes the following steps: S1. Construct a dual-frequency modulation signal containing a first frequency component and a second frequency component, and drive the fiber electro-optic phase modulator (5) to perform dual-frequency electro-optic phase modulation on the laser according to the dual-frequency modulation signal; S2. The photodetector (9) detects the output light of the polarization beam splitter (7) to obtain the saturated absorption signal, and inputs the saturated absorption signal into the FPGA development board after analog-to-digital conversion. S3. In the FPGA development board, the saturated absorption signal is synchronously demodulated with the first frequency local oscillator signal and the second frequency local oscillator signal, respectively, and then low-pass filtered to obtain the first error signal and the second error signal. The first error signal and the second error signal are superimposed to obtain the dual-frequency error signal. S4. Based on the dual-frequency error signal, the tunable laser (1) is subjected to closed-loop feedback control to lock the laser frequency to the target absorption peak position, thereby achieving laser frequency stabilization.

5. The dual-frequency modulation transfer spectral laser frequency stabilization method according to claim 4, characterized in that: The dual-frequency modulation signal is specifically set according to the following formula: in, z ( t This indicates a dual-frequency modulated signal. f 1 and f 2 represents the first frequency and the second frequency, respectively. K 1 and K 2 represents the corresponding amplitude of the first frequency and the corresponding amplitude of the second frequency, respectively.

6. The dual-frequency modulation transfer spectral laser frequency stabilization method according to claim 4, characterized in that: The saturation absorption signal is specifically set according to the following formula: Where S(t) represents the saturation absorption signal, These are the subscript numbers of the two modulation frequencies. , These represent the signal components with absorption and dispersion line shapes, respectively, and Δ represents the frequency difference between the laser beam and the specific absorption peak of rubidium atoms. Indicates phase, This represents the amplitude coefficients of two frequency components in a saturated absorption signal. Indicates the first modulation frequency. Indicates the second modulation frequency. For constant terms, F represents the natural linewidth constant of the gas absorption spectral line, which is a gain coefficient related to the modulation depth.

7. The dual-frequency modulation transfer spectral laser frequency stabilization method according to claim 4, characterized in that: Specifically, S3 is: S3.1 The saturated absorption signal is multiplied by the first multiplier (11) and the local oscillator signal with the first frequency output by the first local oscillator source (12), and then low-pass filtered by the first filter (13) to obtain the first error signal; S3.2 The saturated absorption signal is multiplied by the second multiplier (14) and the local oscillator signal with the second frequency output by the second local oscillator source (15), and then low-pass filtered by the second filter (16) to obtain the second error signal; S3.

3. Superimpose the first error signal and the second error signal to obtain a dual-frequency error signal.

8. The dual-frequency modulation transfer spectral laser frequency stabilization method according to claim 4, characterized in that: The dual-frequency error signal is specifically set according to the following formula: in, This indicates a dual-frequency error signal. and These represent the amplitude coefficients of the first frequency and the first frequency component in the saturated absorption signal, respectively. and These represent the first and second frequency signal components of the dispersive line shape, respectively, and Δ represents the frequency difference between the laser and the specific absorption peak of the rubidium atom.

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