An FPGA laser automatic frequency stabilization system and method based on atomic absorption spectroscopy

Abstract

The present invention discloses an FPGA laser automatic frequency stabilization system based on atomic absorption spectrum, comprising: a laser; a beam splitter, which divides the laser output by the laser into multiple beams; a wavelength meter, which measures the frequency of the laser; a frequency discriminator, which receives the laser split by the beam splitter, processes the laser, and obtains an analog signal; a first conversion module, which receives the analog signal output by the frequency discriminator, and converts the analog signal into a digital signal; an FPGA module, which receives the measured value output by the wavelength meter and the digital signal output by the first conversion module, determines the locking state of the laser frequency, and automatically locks the laser frequency according to the locking state or automatically relocks it after losing the lock; a second conversion module, which converts the digital signal output by the FPGA module into an analog signal, and feeds it back to the laser. The present invention can realize automatic locking of the laser frequency, loss of lock detection, and automatic relocking after losing the lock, and has the characteristics of simple control system, strong robustness, and wide applicability.

Description

Technical Field

[0001] The invention relates to the field of laser frequency stabilization, and in particular to an FPGA laser automatic frequency stabilization system based on atomic absorption spectroscopy and a method thereof. Background Art

[0002] Among the many applications of lasers, laser frequency stability is an extremely important parameter. Therefore, with the development of laser applications, laser frequency stabilization technology has become an important tool for basic scientific research and a key component of cutting-edge science, playing an increasingly important role in modern science and technology.

[0003] High-resolution spectroscopy has important applications in cold atom physics, quantum optics, quantum precision measurement and sensing, etc. Stabilized lasers are crucial to high-resolution spectroscopy. To improve the stability of the laser output frequency, the laser frequency stabilization technology that locks the laser frequency to the atomic spectrum has been developed for more than 30 years, resulting in the birth of a series of technologies such as saturated absorption spectroscopy (SAS) technology without Doppler broadening, polarization spectroscopy technology, modulation transfer spectroscopy technology, and magneto-dichroism spectroscopy, which further narrow the spectral line width and improve the stability of the stabilized laser.

[0004] Although laser frequency stabilization technology has been developed for many years and has been widely used in many fields, it still has some shortcomings, mainly reflected in the fact that the laser frequency stabilization system is not stable enough and is prone to loss of lock. Since the effective locking frequency range of the frequency stabilization system is narrow, when the laser system is subjected to unavoidable interference, the laser frequency is likely to jump out of the effective locking frequency range, resulting in loss of lock. The existence of the loss of lock phenomenon greatly limits the application scope of frequency stabilization lasers. Summary of the invention

[0005] In order to overcome the deficiencies of the prior art, the purpose of the present invention is to provide an FPGA laser automatic frequency stabilization system and method based on atomic absorption spectroscopy, which can realize automatic locking of the laser frequency and relocking after loss of lock, so that the stabilized frequency laser can be used in an environment with interference.

[0006] To solve the above problems, the technical solution adopted by the present invention is as follows:

[0007] An FPGA laser automatic frequency stabilization system based on atomic absorption spectroscopy, comprising:

[0008] A laser, used for outputting laser light;

[0009] A beam splitter, which splits the laser output by the laser into multiple beams;

[0010] A wavelength meter receives the laser light split by the beam splitter and measures the frequency of the laser light;

[0011] A frequency discriminator receives the laser split by the beam splitter and processes the laser to obtain an atomic absorption spectrum signal and a frequency discrimination error signal;

[0012] A first conversion module receives the atomic absorption spectrum signal and the discrimination error signal output by the discriminator, and converts the atomic absorption spectrum signal and the discrimination error signal into digital signals;

[0013] An FPGA module receives the measurement value output by the wavelength meter and the digital signal output by the first conversion module, determines the locking state of the laser frequency, and automatically locks the laser frequency according to the locking state or automatically relocks the laser frequency after losing the lock;

[0014] The second conversion module converts the digital signal output by the FPGA module into an analog signal, and feeds the analog signal back to the laser.

[0015] Furthermore, the FPGA module includes:

[0016] A re-locking monitoring module, a digital PID module, a locking point identification module and a locking point selection and scaling module, wherein the digital PID module, a laser, a frequency discriminator, a first conversion module and a second conversion module form a laser closed-loop locking circuit, and the locking point identification module, the locking point selection and scaling module, the laser, the frequency discriminator, the wavelength meter, the first conversion module and the second conversion module form a laser self-locking circuit;

[0017] A relock monitoring module, used to determine the locking state of the laser frequency according to the mean value of the frequency discrimination error signal output by the frequency discriminator in real time, and close the laser closed-loop locking circuit or the laser self-locking circuit according to the locking state;

[0018] A digital PID module, used for receiving the frequency discrimination error signal output by the frequency discriminator, and outputting a control quantity, which is fed back to the laser through the second conversion module;

[0019] A locking point recognition module, which adjusts the laser to a self-locking state according to the laser frequency measured by the wavelength meter, wherein the number of resonance peaks of the real-time absorption spectrum of the laser output by the laser corresponds to the number of resonance peaks in the standard atomic absorption spectrum;

[0020] The locking point selection zoom module searches and locates all lockable points in the real-time atomic absorption spectrum when the laser is in a self-locking state, and adjusts the laser frequency to an effective locking frequency range according to the position of the locking point. The effective locking frequency range is that the laser contains only one identifiable locking point within the scanning range.

[0021] Furthermore, the frequency discriminator comprises:

[0022] An electro-optic modulator, used for receiving the laser light split by the beam splitter and performing modulation;

[0023] An optical fiber electro-optic modulator, used for receiving and modulating the laser light modulated by the electro-optic modulator;

[0024] An atomic gas chamber, used for absorbing the laser light modulated by the optical fiber electro-optic modulator;

[0025] The reflection module reflects the laser light passing through the atomic gas chamber into the atomic gas chamber for absorption again;

[0026] A photoelectric detector converts the reabsorbed laser light into two atomic absorption spectrum signals, and transmits one of the atomic absorption spectrum signals to the first conversion module;

[0027] The demodulation module demodulates the other atomic absorption spectrum signal into a frequency discrimination error signal, and transmits the frequency discrimination error signal to the first conversion module.

[0028] An FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy includes the following steps:

[0029] Use the relock monitoring module to monitor the working status of the frequency stabilization system in real time;

[0030] If the laser frequency is in a locked state, the laser closed-loop locking circuit remains operational, locking the laser frequency at the target frequency value;

[0031] If the laser frequency is in an unlocked state, the locking point recognition module in the FPGA module adjusts the laser to a working state that allows self-locking;

[0032] Then use the locking point selection and scaling module in the FPGA module to identify all lockable points in the atomic absorption spectrum, determine the target locking point, adjust the laser frequency to the preset effective locking frequency range according to the target locking point, reactivate the laser closed-loop locking circuit, and relock the laser frequency after it is lost.

[0033] Furthermore, the specific process of locking the laser frequency at the target frequency value is: acquiring the atomic absorption spectrum signal and the frequency discrimination error signal of the laser in the current state in real time; the digital PID module in the FPGA module calculates the control amount fed back to the laser according to the frequency error; and according to the feedback control amount, the output frequency of the laser is feedback-corrected through the DAC.

[0034] Furthermore, the specific process of the FPGA module to achieve relocking after loss of lock is: automatically adjusting the driving voltage of the laser PZT to change the scanning range of the laser so that its frequency range is sufficient to cover a certain group of atomic transition spectral lines; automatically adjusting the scanning compensation voltage of the laser PZT, and making a preliminary judgment through a wavelength meter so that the laser scanning frequency includes the target frequency value; determining whether the laser is in a working state available for self-locking through the atomic absorption spectrum; the locking point selection and scaling module searches and locates all lockable points in the real-time atomic absorption spectrum, that is, locates the absorption peak in the atomic absorption spectrum, and based on this, scales the laser frequency to the effective locking frequency range corresponding to the target locking point, until within the current scanning range, the real-time atomic absorption spectrum peak recognition can only find a unique absorption peak.

[0035] Furthermore, when the laser is adjusted to a working state capable of self-locking, the locking point recognition module adjusts the scanning compensation voltage within a constant scanning frequency range, determines the number of extreme values ​​in the spectrum, and uses the extreme value points to correspond to the resonance peaks of the real-time atomic absorption spectrum. When the number of resonance peaks in the real-time absorption spectrum corresponds to the number of resonance peaks in the standard atomic absorption spectrum, the laser is adjusted to a working state capable of self-locking characterized by the number of peaks in the standard atomic absorption spectrum.

[0036] Furthermore, the specific process of preliminarily adjusting the laser output frequency range by means of a wavelength meter is as follows: preliminarily measuring the laser output frequency range by means of a wavelength meter; if the laser output frequency range includes the target laser frequency, determining whether the laser is in a self-locking working state by means of an atomic absorption spectrum; if the laser output frequency range does not include the laser frequency to be locked, determining the direction of the PZT scanning compensation change based on the wavelength meter reading and the target frequency value; and driving the PZT scanning compensation change by means of a step change function with a preset change amplitude, changing the laser frequency scanning range with a fixed change step size until the laser output frequency range includes the target laser frequency.

[0037] Furthermore, when the locking point identification module adjusts the scanning compensation voltage, the locking point identification module needs to set appropriate extreme value limiting parameters, including extreme value spacing and extreme value height, and perform preprocessing and real-time processing.

[0038] Furthermore, the specific process of the preprocessing and real-time processing is as follows: in the preprocessing, for a specific scanning amplitude, that is, covering the frequency range corresponding to a certain group of atomic transition spectral lines, the standard atomic absorption spectrum data is derived, extreme value identification is performed, and correct extreme value limit parameters are set so that the program can correspondingly identify the number and order of atomic resonance transition absorption peaks; in the real-time processing, the atomic absorption spectrum signal is obtained in real time, input into the FPGA module through the ADC, and the number of extreme values ​​is extracted according to the same extreme value identification parameters as the preprocessing, and compared with the number of standard atomic absorption spectrum peaks in the preprocessing to determine whether the laser is working in a state that can be self-locked at this time.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] The digital signal in the present invention is processed by the FPGA module, which can greatly improve the operating efficiency. When the relock monitoring module finds that the laser frequency is in an unlocked state, the FPGA module automatically relocks the laser frequency. Therefore, the present invention can realize automatic locking of the laser frequency, lockout detection, and automatic relocking after lockout, and has the characteristics of simple control system, strong robustness, and wide applicability.

[0041] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 - is a structural diagram of an FPGA laser automatic frequency stabilization system based on atomic absorption spectroscopy according to an embodiment of the present invention;

[0043] Figure 2 - is a structural diagram of an FPGA module according to an embodiment of the present invention;

[0044] Figure 3 - is a step diagram of an FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy according to an embodiment of the present invention;

[0045] Figure 4 - is a logic block diagram of an FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy according to an embodiment of the present invention;

[0046] Figure 5 - is a block diagram of a real-time locking point identification module according to an embodiment of the present invention;

[0047] Figure 6 - is a block diagram of a lock point selection zoom module in an embodiment of the present invention.

[0048] Explanation of the accompanying drawings: 1. Laser; 2. Beam splitter; 3. Wavelength meter; 4. Frequency discriminator; 5. First conversion module; 6. FPGA module; 7. Second conversion module; 8. Re-locking monitoring module; 9. Digital PID module; 10. Locking point identification module; 11. Locking point selection and scaling module; 12. Electro-optical modulator; 13. Fiber-optic electro-optical modulator; 14. Atomic gas chamber; 15. Reflection module; 16. Photodetector; 17. Demodulation module. DETAILED DESCRIPTION

[0049] The FPGA laser automatic frequency stabilization system based on atomic absorption spectroscopy provided by the present invention is as follows: Figure 1As shown, it includes: a laser 1, a beam splitter 2, a wavelength meter 3, a frequency discriminator 4, a first conversion module 5, an FPGA module 6 and a second conversion module 7. The laser 1, further, the laser 1 has a piezoelectric ceramic (PZT), and the laser 1 is used to output laser light; the beam splitter 2, further, the number of the beam splitters 2 is 2, and the beam splitter 2 is used to split the laser light output by the laser 1 into multiple beams; the wavelength meter 3 receives the laser light split by the beam splitter 2 and measures the frequency of the laser light; the frequency discriminator 4, further, the frequency discriminator 4 is a Doppler-free dichroic locking signal (Doppler-free dichroic locking signal lock, DFDL) discriminator, used to receive the laser splittered by the beam splitter 2, and process the laser to obtain the atomic absorption spectrum signal and the discrimination error signal; the first conversion module 5, receives the atomic absorption spectrum signal and the discrimination error signal output by the discriminator 4, and converts the atomic absorption spectrum signal and the discrimination error signal into digital signals; the FPGA module 6, receives the measurement value output by the wavelength meter 3 and the digital signal output by the first conversion module 5, determines the locking state of the laser frequency, and automatically locks the laser frequency according to the locking state or automatically relocks it after losing lock; the second conversion module 7, converts the digital signal output by the FPGA module 6 into an analog signal, and feeds the analog signal back to the laser 1.

[0050] When the present invention is in use, the laser output by the laser 1 is divided into three beams by two beam splitters 2. The first two low-power laser beams are used for the wavelength meter 3 and the frequency discriminator 4 respectively, and the third high-power laser beam is used for output. The wavelength meter 3 measures the frequency of the laser and feeds it back to the FPGA module 6. After the split light enters the frequency discriminator 4, it is converted into an atomic absorption spectrum signal and a frequency discrimination error signal, and then converted into a digital signal by the first conversion module 5, and then transmitted to the FPGA module 6. The FPGA module 6 determines the locking state of the laser frequency. If the laser frequency is in the locking state, the current locking state continues to be maintained. If the laser frequency is in the unlocked state, the FPGA module 6 starts to automatically lock the laser frequency. After the laser frequency is adjusted back to the preset effective locking frequency range through a series of automatic control processes, the FPGA module 6 determines the locking state again to maintain the locking state.

[0051] Furthermore, if Figure 2 As shown, the FPGA module 6 includes: a re-locking monitoring module 8, a digital PID module 9, a locking point identification module 10, and a locking point selection and scaling module 11. The digital PID module 9, the laser 1, the frequency discriminator 4, the first conversion module 5, and the second conversion module 7 form a laser closed-loop locking circuit. The locking point identification module 10, the locking point selection and scaling module 11, the laser 1, the frequency discriminator 4, the wavelength meter 3, the first conversion module 5, and the second conversion module 7 form a laser self-locking circuit.

[0052] The relock monitoring module 8 monitors the mean value of the frequency discrimination error signal output by the frequency discriminator 4 in real time, determines the locking state of the laser frequency, and closes the laser closed-loop locking circuit or the laser self-locking circuit according to the locking state. The specific judgment process is as follows: if the mean value of the frequency discrimination signal is less than a preset minimum value, the laser 1 is in a locked state; if the mean value of the frequency discrimination signal is greater than a preset minimum value, the laser 1 is in an unlocked state.

[0053] The digital PID module 9 is used to receive the frequency discrimination error signal output by the frequency discriminator 4 and output a control quantity, which is fed back to the laser 1 through the second conversion module.

[0054] The locking point recognition module 10 adjusts the laser 1 to a self-locking state according to the laser frequency measured by the wavelength meter 3. The self-locking state is that the number of resonance peaks of the real-time absorption spectrum of the laser output by the laser 1 corresponds to the number of resonance peaks in the standard atomic absorption spectrum.

[0055] The locking point selection zoom module 11, when the laser 1 is in a self-locking state, searches and locates all lockable points in the real-time atomic absorption spectrum, and adjusts the laser frequency to an effective locking frequency range according to the position of the locking point. The effective locking frequency range is that the laser contains only one identifiable locking point within the scanning range.

[0056] If the relock monitoring module 8 determines that the laser frequency is in a locked state, the laser closed-loop locking circuit continues to be closed and the laser self-locking circuit is disconnected, so that the laser frequency is locked at the target frequency value. The specific process is as follows:

[0057] (1) The digital PID module 9 obtains the frequency discrimination error signal of the laser in the current state in real time;

[0058] (2) The digital PID module 9 calculates the control quantity fed back to the laser based on the frequency discrimination error signal;

[0059] (3) According to the feedback control amount, the piezoelectric ceramic (PZT) voltage of the laser 1 is feedback controlled through the second conversion module 7, so that the laser frequency is locked at the target frequency value.

[0060] Preferably, the frequency discriminator 4 includes: an electro-optic modulator 12 , a fiber electro-optic modulator 13 , an atomic gas chamber 14 , a reflection module 15 , a photodetector 16 and a demodulation module 17 .

[0061] The electro-optic modulator 12 is used to receive the laser beam split by the beam splitter 2 and perform modulation.

[0062] The optical fiber electro-optic modulator 13 is used to receive the laser light modulated by the electro-optic modulator 12 and perform modulation.

[0063] The atomic gas chamber 14 is used to absorb the laser light modulated by the optical fiber electro-optic modulator 13 .

[0064] The reflection module 15 reflects the laser light passing through the atomic gas chamber 14 into the atomic gas chamber 14 for absorption again.

[0065] The photodetector 16 converts the reabsorbed laser light into two atomic absorption spectrum signals, and transmits one of the atomic absorption spectrum signals to the first conversion module 7 .

[0066] The demodulation module demodulates the other atomic absorption spectrum signal into a frequency discrimination error signal, and transmits the frequency discrimination error signal to the first conversion module 7 .

[0067] The specific output process of the discriminator 4 is as follows: the split light entering the discriminator 4 is modulated by the electro-optic modulator 12 (EOM), and then modulated by the fiber electro-optic modulator 13 (F-EOM), and then incident on the atomic gas chamber 14. After being reflected by the reflection module 15, it returns to the original path and is incident on the atomic gas chamber 14 again. The light absorbed by the atomic gas chamber 14 is converted into an electrical signal by the photodetector 16 (PD), and an atomic absorption spectrum signal can be obtained. The atomic absorption spectrum signal is divided into two paths, one of which is transmitted to the FPGA module 6 after passing through the first conversion module 5; the other atomic absorption spectrum signal is demodulated by the demodulation module 17 to obtain a discrimination signal (FDS). The discrimination signal is transmitted to the FPGA module 6 after passing through the first conversion module 5.

[0068] Further, the reflection module 15 includes a quarter wave plate and a reflector. A quarter wave plate is a birefringent single crystal wave plate of a certain thickness. When light passes through the wave plate from normal incidence, the phase difference between ordinary light (o light) and extraordinary light (e light) is equal to π / 2 or an odd multiple thereof, and such a chip is called a quarter wave plate or 1 / 4 wave plate.

[0069] Furthermore, the demodulation module 17 includes a multiplier and a low-pass filter. After the reflected light signal is converted into an electrical signal by a photodetector (PD), the multiplier is used to generate sum-frequency and difference-frequency DC signals, and the low-pass filter is used to filter out the sum-frequency signal and extract the discrimination error signal.

[0070] Furthermore, the laser 1 is a semiconductor laser, which has the advantages of small size, high efficiency, simple modulation and convenient use.

[0071] Furthermore, the accuracy of the wavelength meter 3 is 1 GHz. The wavelength meter 3 is used to preliminarily measure the output frequency range of the laser 1, and to determine the direction of the piezoelectric ceramic (PZT) scanning compensation change according to the reading of the wavelength meter 3 and the target frequency value. Therefore, the use of the wavelength meter 3 with an accuracy of 1 GHz can not only meet the requirements, but also effectively control the cost of the entire frequency stabilization system.

[0072] Furthermore, the atomic gas chamber 14 is a rubidium atomic gas chamber, and the transition frequency between the rubidium ground state hyperfine energy levels is used as the reference frequency, so that the laser frequency is locked by the rubidium atomic reference frequency, thereby ensuring that the laser 1 outputs a stable laser frequency.

[0073] Furthermore, the first conversion module 5 is an analog-to-digital conversion module. After the reflected light signal is converted into an electrical signal by the photodetector 12 (PD), it is converted into a digital signal by the analog-to-digital converter (ADC).

[0074] Furthermore, the second conversion module 7 is a digital-to-analog conversion module. The demodulation error signal is controlled and processed by the digital PID module 9 to obtain a digital laser frequency tuning signal. This signal is converted into an analog signal by the digital-to-analog conversion module (DAC) and acts on the frequency tuning port of the laser 1 to achieve frequency stabilization control of the laser 1.

[0075] The FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy provided by the present invention is as follows: Figure 3 and Figure 4 As shown, the following steps are included:

[0076] S1: Use the relock monitoring module to monitor the working status of the frequency stabilization system in real time;

[0077] S2: If the laser frequency is in the locked state, the laser closed-loop locking circuit keeps working to lock the laser frequency at the target frequency value;

[0078] S3: If the laser frequency is in an unlocked state, the locking point recognition module in the FPGA module adjusts the laser to a working state that allows self-locking;

[0079] S4: Then use the lock point selection and zoom module in the FPGA module to identify all lockable points in the atomic absorption spectrum, determine the target lock point, adjust the laser frequency to the preset effective lock frequency range according to the target lock point, reactivate the laser closed-loop lock circuit, and relock the laser frequency after it is lost.

[0080] In the above step S2, the specific process of locking the laser frequency at the target frequency value is: real-time acquisition of the atomic absorption spectrum signal and the frequency discrimination error signal of the laser in the current state; the digital PID module in the FPGA module calculates the control amount fed back to the laser according to the frequency error; and feedback correction is performed on the output frequency of the laser through the DAC according to the feedback control amount.

[0081] In the above step S4, the specific process of the FPGA module to achieve relocking after loss of lock is: automatically adjust the driving voltage of the laser PZT to change the scanning range of the laser so that its frequency range is sufficient to cover a certain group of atomic transition spectral lines; automatically adjust the scanning compensation voltage of the laser PZT, and make a preliminary judgment through the wavelength meter to make the laser scanning frequency contain the target frequency value; determine whether the laser is in a working state for self-locking through the atomic absorption spectrum; the locking point selection and scaling module searches and locates all lockable points in the real-time atomic absorption spectrum, that is, locates the absorption peak in the atomic absorption spectrum, and based on this, scales the laser frequency to the effective locking frequency range corresponding to the target locking point, until the real-time atomic absorption spectrum peak recognition can only find a unique absorption peak within the current scanning range.

[0082] The specific process of preliminarily adjusting the laser output frequency range by using a wavelength meter is as follows: preliminarily measuring the laser output frequency range by using a wavelength meter; if the laser output frequency range includes the target laser frequency, determining whether the laser is in a self-locking working state by using an atomic absorption spectrum; if the laser output frequency range does not include the laser frequency to be locked, determining the direction of the PZT scanning compensation change based on the wavelength meter reading and the target frequency value; driving the PZT scanning compensation change by using a step change function with a preset change amplitude, changing the laser frequency scanning range with a fixed change step size until the laser output frequency range includes the target laser frequency.

[0083] In the above step S3, when the laser is adjusted to a working state capable of self-locking, the locking point recognition module adjusts the scanning compensation voltage within a constant scanning frequency range, determines the number of extreme values ​​in the spectrum, and uses the extreme value points to correspond to the resonance peaks of the real-time atomic absorption spectrum. When the number of resonance peaks in the real-time absorption spectrum corresponds to the number of resonance peaks in the standard atomic absorption spectrum, the laser is adjusted to a working state capable of self-locking characterized by the number of peaks in the standard atomic absorption spectrum.

[0084] When the locking point recognition module adjusts the scanning compensation voltage, the locking point recognition module shall set appropriate extreme value limiting parameters, including extreme value spacing and extreme value height, and perform preprocessing and real-time processing.

[0085] The specific process of preprocessing and real-time processing is as follows: in preprocessing, for a specific scanning amplitude, that is, the frequency range corresponding to a certain group of atomic transition spectral lines, the standard atomic absorption spectrum data is exported, extreme value identification is performed, and correct extreme value limit parameters are set so that the program can identify the number and order of atomic resonance transition absorption peaks; in real-time processing, the atomic absorption spectrum signal is obtained in real time, input into the FPGA module through the ADC, and the number of extreme values ​​is extracted according to the same extreme value identification parameters as the preprocessing, and compared with the number of standard atomic absorption spectrum peaks in the preprocessing to determine whether the laser is working in a state that can be self-locked at this time.

[0086] Furthermore, if Figure 5 and Figure 6 As shown in the figure, the relock monitoring module is a module that runs together with the digital PID module when the laser locking is successfully completed. It can monitor the system status and start the self-locking loop after the laser loses lock. When the laser is locked, the discrimination signal input from the ADC passes through the digital low-pass filter and is collected by the register. The collected discrimination signal is averaged at regular intervals. When the system is in the locked state, the variation of the discrimination signal over time should be stable at the reference value 0. At this time, the relock monitoring module and the digital PID module continue to run; on the contrary, when the laser loses lock, the accumulated discrimination signal mean is much greater than zero. At this time, the relock monitoring module and the digital PID module are turned off, and the self-locking loop is entered to start the real-time absorption spectrum lock point identification module.

[0087] When the re-lock monitoring module detects that the laser frequency is in a locked state, only the digital PID module works, and the other locking point identification modules and locking point selection scaling modules have no signal input. At this time, the frequency discrimination signal input from the ADC enters the digital PID module and stabilizes at a reference value of 0. The real-time feedback output signal is transmitted to the PZT driver through the ADC, thereby stabilizing the laser frequency for a long time.

[0088] When the relock monitoring module detects that the laser frequency is in an unlocked state, the digital PID module and the relock monitoring module are turned off, and the real-time absorption spectrum lock point identification module is turned on. First, a preset sawtooth wave signal corresponding to a scanning range of 8 GHz is directly added to the PZT driver through the amplitude DAC to control the scanning range of the laser frequency.

[0089] At the same time, the measured value of the wavelength meter enters the FPGA module through the ADC, is compared with the target frequency, and the direction of the PZT scanning compensation is given. If it is not included, the PZT scanning compensation voltage is roughly adjusted, that is, the voltage step corresponding to 1 / 10 of the relative frequency is sequentially adjusted until the target frequency is included in the laser frequency range measured in real time;

[0090] On the contrary, when included, the laser PZT scanning compensation voltage is adjusted in fine steps to put the laser frequency in an operating state that can be self-locked.

[0091] By using the preset identification parameters, the peak number of the atomic absorption spectrum signal inputted by the ADC is identified in real time and compared with the peak number of the standard atomic absorption spectrum signal. If the two are equal, it is in a self-locking working state.

[0092] When the laser frequency is in the working state where self-locking is available, turn on the locking point selection and scaling module, and input the atomic absorption spectrum signal into the locking point selection and scaling module. Move the locking point peak to the center of the scanning spectral line through the DC compensation signal, and then gradually reduce the amplitude of the sawtooth wave function until there is only one absorption peak in the entire scanning spectral line, that is, complete the DFDL judgment. Turn on the digital PID module and the relocking monitoring module, turn off the locking point identification module, the locking point selection and scaling module, and the sawtooth wave scanning signal, and enter the relocking monitoring loop. So far, the entire system realizes the cycle of laser frequency unlocking and self-locking.

[0093] The following is a detailed introduction to the process of self-locking frequency:

[0094] Step B1: In this case, the laser frequency is locked to the hyperfine level transition of the D1 line of Rb-87 atoms. First, increase the scanning amplitude of the piezoelectric ceramic (PZT) of laser 1 to a preset value to ensure that the corresponding transition spectral line of the D1 line of Rb-87 atoms is covered, and its frequency range is approximately 377104 < Freq < 377112 GHz, and the relative frequency size is 8 GHz.

[0095] Step B2: After step B1, measure the laser frequency F through the 1 GHz wavemeter 3 A , determine whether the current laser frequency range contains the target frequency F T , and give the direction of PZT scanning compensation. When the target frequency is not within the laser frequency range measured in real time, roughly adjust the scanning compensation voltage of the piezoelectric ceramic (PZT) of laser 1, that is, with a voltage step ΔV corresponding to 1 / 10 of the relative frequency size large , sequentially adjust the scanning compensation voltage T of the piezoelectric ceramic (PZT) of laser 1 c = ΔV large * i until the target frequency is included in the laser frequency range measured in real time.

[0096] Step B3: After step B2, the locking point identification module 10 needs to adjust the scanning compensation voltage T of the piezoelectric ceramic (PZT) of the laser with a fine step ΔV small to make the frequency of laser 1 in the working state where self-locking is available. An effective method to determine whether the semiconductor laser is in the working state where self-locking is available is to analyze its Doppler-free dichroic locking signal DFDL. The specific analysis process is as follows: d = ΔV small * j.

[0097] Step B3-1: First, in order to obtain the limiting parameters for finding the extreme points and realize the correspondence between the extreme points and the transition spectral lines, in this case, a standard DFDL signal is used as a template for preprocessing to exclude the influence of extreme points caused by stray backgrounds. By setting appropriate parameters such as the extreme height and extreme spacing, the number of extracted extreme points Ns Corresponding to the resonance peak of the spectrum.

[0098] Step B3-2: After obtaining appropriate extreme value constraint parameters through preprocessing, perform extreme value analysis on the real-time acquisition spectrum under the same parameter conditions to obtain the corresponding extreme value number N peak , if and only if the number of extreme values ​​N is obtained in real time peak Equal to the number of extreme values ​​of the standard DFDL spectrum line N s , the system is in a state where it can be locked automatically and the next operation can be carried out.

[0099] Step B4: After adjusting the laser 1 to a self-locking working state through the extreme value of the atomic absorption spectrum signal, the locking point selection and scaling module 11 locates the position of the locking point in the DFDL spectrum through the coordinates of the extreme value, and calculates the sweep compensation and sweep amplitude of the PZT voltage to scale the laser frequency to the set effective locking range. At this time, the DFDL only contains an absorption peak corresponding to an ultrafine energy level transition, and the DFDL judgment is completed. Subsequently, the closed-loop locking circuit is activated and the sweep is closed to achieve re-locking of the laser frequency to the target reference frequency.

[0100] Compared with the prior art, the present invention has the following beneficial effects:

[0101] The digital signal in the present invention is processed by the FPGA module, which can greatly improve the operating efficiency. When the relock monitoring module finds that the laser frequency is in an unlocked state, the FPGA module automatically relocks the laser frequency. Therefore, the present invention can realize automatic locking of the laser frequency, lockout detection, and automatic relocking after lockout, and has the characteristics of simple control system, strong robustness, and wide applicability.

[0102] The above-mentioned embodiments are only preferred embodiments of the present invention and cannot be used to limit the scope of protection of the present invention. Any non-substantial changes and substitutions made by technicians in this field on the basis of the present invention shall fall within the scope of protection required by the present invention.

Claims

1. An FPGA laser automatic frequency stabilization system based on atomic absorption spectroscopy, characterized in that: include: A laser, used for outputting laser light; A beam splitter, which splits the laser light output by the laser into multiple beams; A wavelength meter receives the laser light split by the beam splitter and measures the frequency of the laser light; A frequency discriminator receives the laser split by the beam splitter and processes the laser to obtain an atomic absorption spectrum signal and a frequency discriminator error signal; A first conversion module receives the atomic absorption spectrum signal and the discrimination error signal output by the discriminator, and converts the atomic absorption spectrum signal and the discrimination error signal into digital signals; An FPGA module receives the measurement value output by the wavelength meter and the digital signal output by the first conversion module, determines the locking state of the laser frequency, and automatically locks the laser frequency according to the locking state or automatically relocks the laser frequency after losing the lock; A second conversion module converts the digital signal output by the FPGA module into an analog signal, and feeds the analog signal back to the laser; Wherein, the FPGA module includes: A re-locking monitoring module, a digital PID module, a locking point identification module and a locking point selection and scaling module, wherein the digital PID module, a laser, a frequency discriminator, a first conversion module and a second conversion module form a laser closed-loop locking circuit, and the locking point identification module, the locking point selection and scaling module, the laser, the frequency discriminator, the wavelength meter, the first conversion module and the second conversion module form a laser self-locking circuit; A relock monitoring module, used to determine the locking state of the laser frequency according to the mean value of the frequency discrimination error signal output by the frequency discriminator in real time, and close the laser closed-loop locking circuit or the laser self-locking circuit according to the locking state; A digital PID module, used for receiving the frequency discrimination error signal output by the frequency discriminator, and outputting a control quantity, which is fed back to the laser through the second conversion module; A locking point recognition module, which adjusts the laser to a self-locking state according to the laser frequency measured by the wavelength meter, wherein the number of resonance peaks of the real-time absorption spectrum of the laser output by the laser corresponds to the number of resonance peaks in the standard atomic absorption spectrum; The locking point selection zoom module searches and locates all lockable points in the real-time atomic absorption spectrum when the laser is in a self-locking state, and adjusts the laser frequency to an effective locking frequency range according to the position of the locking point. The effective locking frequency range is that the laser contains only one identifiable locking point within the scanning range.

2. The FPGA laser automatic frequency stabilization system based on atomic absorption spectroscopy according to claim 1 is characterized in that: The frequency discriminator comprises: An electro-optic modulator, used for receiving the laser light split by the beam splitter and performing modulation; An optical fiber electro-optic modulator, used for receiving and modulating the laser light modulated by the electro-optic modulator; An atomic gas chamber, used for absorbing the laser light modulated by the optical fiber electro-optic modulator; The reflection module reflects the laser light passing through the atomic gas chamber into the atomic gas chamber for absorption again; A photoelectric detector converts the reabsorbed laser light into two atomic absorption spectrum signals, and transmits one of the atomic absorption spectrum signals to the first conversion module; The demodulation module demodulates the other atomic absorption spectrum signal into a frequency discrimination error signal, and transmits the frequency discrimination error signal to the first conversion module.

3. An FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy, characterized in that: The method uses the FPGA laser automatic frequency stabilization system based on atomic absorption spectrum as claimed in claim 1 or 2 to perform automatic frequency stabilization, and the method comprises the following steps: Use the relock monitoring module to monitor the working status of the frequency stabilization system in real time; If the laser frequency is in a locked state, the laser closed-loop locking circuit remains operational, locking the laser frequency at the target frequency value; If the laser frequency is in an unlocked state, the locking point recognition module in the FPGA module adjusts the laser to a working state that allows self-locking; Then use the locking point selection and scaling module in the FPGA module to identify all lockable points in the atomic absorption spectrum, determine the target locking point, adjust the laser frequency to the preset effective locking frequency range according to the target locking point, reactivate the laser closed-loop locking circuit, and relock the laser frequency after it is lost.

4. The FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy according to claim 3 is characterized in that: The specific process of locking the laser frequency at the target frequency value is as follows: the atomic absorption spectrum signal and the frequency discrimination error signal of the laser in the current state are obtained in real time; the digital PID module in the FPGA module calculates the control quantity fed back to the laser based on the frequency error; and the output frequency of the laser is feedback-corrected through the DAC based on the feedback control quantity.

5. The FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy according to claim 3 is characterized in that: The specific process of the FPGA module to achieve relocking after loss of lock is: automatically adjusting the driving voltage of the laser PZT to change the scanning range of the laser so that its frequency range is sufficient to cover a certain group of atomic transition spectral lines; automatically adjusting the scanning compensation voltage of the laser PZT, and making a preliminary judgment through a wavelength meter so that the laser scanning frequency contains the target frequency value; Atomic absorption spectroscopy is used to determine whether the laser is in a self-locking working state; The lock point selection and zoom module searches and locates all lockable points in the real-time atomic absorption spectrum, that is, locates the absorption peak in the atomic absorption spectrum, and based on this, scales the laser frequency to the effective lock frequency range corresponding to the target lock point until the real-time atomic absorption spectrum peak recognition can only find the unique absorption peak within the current scanning range.

6. The FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy according to claim 3 is characterized in that: When the laser is adjusted to a working state capable of self-locking, the locking point recognition module adjusts the scanning compensation voltage within a constant scanning frequency range, determines the number of extreme values ​​in the spectrum, and uses the extreme value points to correspond to the resonance peaks of the real-time atomic absorption spectrum. When the number of resonance peaks in the real-time absorption spectrum corresponds to the number of resonance peaks in the standard atomic absorption spectrum, the laser is adjusted to a working state capable of self-locking characterized by the number of peaks in the standard atomic absorption spectrum.

7. The FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy according to claim 5, characterized in that: The specific process of preliminarily adjusting the laser output frequency range by using a wavelength meter is as follows: preliminarily measuring the laser output frequency range by using a wavelength meter; if the laser output frequency range includes the target laser frequency, determining whether the laser is in a self-locking working state by using an atomic absorption spectrum; if the laser output frequency range does not include the laser frequency to be locked, determining the direction of the PZT scanning compensation change based on the wavelength meter reading and the target frequency value; driving the PZT scanning compensation change by using a step change function with a preset change amplitude, changing the laser frequency scanning range with a fixed change step size until the laser output frequency range includes the target laser frequency.

8. The FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy according to claim 6, characterized in that: When the locking point recognition module adjusts the scanning compensation voltage, the locking point recognition module needs to set appropriate extreme value limiting parameters, including extreme value spacing and extreme value height, and perform preprocessing and real-time processing.

9. The FPGA laser automatic frequency stabilization method based on atomic absorption spectroscopy according to claim 8, characterized in that: The specific process of the preprocessing and real-time processing is as follows: in the preprocessing, for a specific scanning amplitude, that is, covering the frequency range corresponding to a certain group of atomic transition spectral lines, the standard atomic absorption spectrum data is derived, extreme value identification is performed, and correct extreme value limit parameters are set so that the program can correspondingly identify the number and order of atomic resonance transition absorption peaks; in the real-time processing, the atomic absorption spectrum signal is obtained in real time, input into the FPGA module through the ADC, and the number of extreme values ​​is extracted according to the same extreme value identification parameters as the preprocessing, and compared with the number of standard atomic absorption spectrum peaks in the preprocessing to determine whether the laser is working in a state that can be self-locked at this time.

Images