Device and method for inhibiting influence of frequency abrupt change on optical cavity frequency locking

Abstract

The invention belongs to the technical field of laser frequency control, and provides a device and method for inhibiting the influence of frequency mutation on optical cavity frequency locking, and the device is characterized in that the device comprises a laser driving assembly, an optical path assembly, a locking loop adjustment assembly, and an error signal switch assembly; the optical path assembly comprises a laser, an isolator, a first reflector, a laser frequency tuner, a first focusing lens, a second reflector, an optical resonant cavity and a second focusing lens which are connected in sequence; the locking loop adjusting assembly comprises a photoelectric detector, a frequency mixer, a low-pass filter, an electronic switch, a PID servo system and a driver of a laser frequency coordinator which are connected in sequence; the error signal switch assembly judges whether the optical resonant cavity is in a frequency locking state or a frequency unlocking state according to the received electric signal of the photoelectric detector, and then the locking loop adjusting assembly is connected or disconnected. According to the device, the stability of frequency locking and frequency correction of the optical resonant cavity during continuous laser scanning is improved.

Description

Technical Field

[0001] This invention relates to the field of laser frequency control technology, specifically to a method and apparatus for suppressing the influence of frequency abrupt changes on optical cavity frequency locking. Background Technology

[0002] Narrow-linewidth, low-frequency-noise lasers have important applications in cutting-edge fields such as optical clocks, gravitational wave detection, coherent communication, and precision spectroscopy. These fields all require relatively low linewidths down to sub-kHz and frequency drifts better than 10. -15 The center frequency of a freely operating semiconductor or solid-state laser often exhibits random drift on the order of MHz to GHz due to the coupling of ambient temperature fluctuations, mechanical vibrations, atmospheric pressure, and spontaneous emission noise. Therefore, active frequency stabilization techniques such as saturated absorption spectrum locking, Pound-Drever-Hall (PDH) locking, and optical feedback locking are required.

[0003] One limitation is that saturated absorption spectroscopy can only lock the laser to a fixed position (i.e., the center of the molecular absorption spectrum), and cannot achieve stability for different laser frequencies. This cannot meet the needs of many fields, such as spectral measurements, which require laser frequencies to cover the entire spectrum, multi-point excitation spectra, and linear fitting of absorption to ensure accurate spectral information.

[0004] Both PDH (Power-Down Detection) and optical feedback technologies rely on an external frequency reference source, namely an optical cavity. The optical cavity consists of two or three highly reflective mirrors. It utilizes the reflection of the laser between the mirrors to create interference. When the laser wavelength and cavity length satisfy a certain relationship, the laser forms constructive interference, at which point the eigenmodes of the laser and the optical cavity resonate. The eigenmodes of the optical cavity typically have very narrow linewidths, ranging from Hz to kHz. Using PDH and optical feedback, laser-to-cavity frequency locking can be achieved, thereby narrowing the laser linewidth to the Hz level. Furthermore, because the optical cavity has multiple eigenmodes (up to 10³ to 10⁴), laser frequency tuning can be achieved by locking the laser frequency to different cavity modes.

[0005] However, in the locked state, the laser frequency is required to jump from one cavity mode eigenmode to the next eigenmode. During the jump, there may be a brief loss of lock. How to ensure that the servo system does not jump out of the control range during the brief loss of lock, and how to ensure that the servo system can quickly and stably perform frequency correction during the recovery of lock, has become a technical problem that urgently needs to be solved. Summary of the Invention

[0006] The technical problem to be solved by this invention is how to improve the stability of optical resonant cavity frequency locking and frequency correction during continuous laser scanning.

[0007] The present invention solves the above-mentioned technical problems through the following technical means: The present invention provides a device for suppressing the effect of frequency abrupt changes on optical cavity frequency locking, comprising a laser driving assembly, an optical path assembly, a locking loop adjustment assembly, and an error signal switching assembly; The optical path assembly includes a laser, an isolator, a first reflector, a laser frequency tuner, a first focusing lens, a second reflector, an optical resonant cavity, and a second focusing lens connected in sequence. The locking loop adjustment assembly includes a photodetector, a mixer, a low-pass filter, an electronic switch, a PID servo system, and a laser frequency coordinator driver connected in sequence. The error signal switching component determines whether the optical resonant cavity is in a frequency-locked state or a frequency-unlocked state based on the received electrical signal from the photodetector, and then connects or disconnects the locking circuit adjustment component.

[0008] Preferably, the laser driving component includes a function generator, a radio frequency source, an adder, and a laser driver; the function generator generates a triangular wave scanning signal, the radio frequency source generates a continuous frequency sinusoidal radio frequency signal, and the two signals are added together by the adder and used as a control signal, which is then sent to the input terminal of the laser driver.

[0009] Preferably, the error signal switching assembly includes a pulse generator and an electronic switch; the input and output terminals of the pulse generator are respectively connected to the output terminal of the photodetector and the control terminal of the electronic switch; the pulse generator is used to automatically generate a pulse signal when the system experiences mode skipping or signal interruption, and send it to the electronic switch to cut off invalid error signals and freeze the PID servo control state, and automatically release and restore the output after the system recaptures a valid signal; The electronic switch is connected in series between the low-pass filter and the PID servo system; the radio frequency source is connected to the mixer.

[0010] Preferably, the error signal switching assembly further includes a voltage follower, a potentiometer threshold network, a comparator module, a sample-and-hold module, and a delay control module.

[0011] Preferably, the voltage follower is used to achieve impedance isolation and buffer amplification between the input signal and the subsequent circuit, ensuring that the amplitude of the input signal is not affected by the load and is stably transmitted to the demodulation and comparison terminals.

[0012] Preferably, the potentiometer threshold network consists of a voltage divider adjustment structure composed of potentiometers and pull-up / pull-down resistors, used to provide an adjustable and low-noise threshold voltage V for the comparator module. R It is used to determine whether the optical resonator is in a frequency-locked state.

[0013] Preferably, the comparator module is used to compare the electrical signal with the threshold voltage V in real time. R The magnitude; when the electrical signal is higher than V R When the pulse generator outputs a high-level control signal, the pulse generator outputs a high-level control signal; when the electrical signal is lower than V... R At that time, the pulse generator outputs a low-level control signal.

[0014] Preferably, the sample-and-hold module is used to detect when the electrical signal is below V. R When the signal is detected to be higher than V, it automatically enters a hold state, freezing the current PID servo system input level to prevent integral saturation or misadjustment under invalid error signals; when the detected electrical signal is higher than V... R When the sampling and holding module releases its hold function, it resumes real-time signal sampling.

[0015] Preferably, the delay control module is used to generate a fixed-delay release control signal when the frequency lock state is restored, to ensure that the PID loop resumes input only after the system stabilizes, and to avoid oscillation during the transition phase.

[0016] The present invention also provides a method for suppressing the effect of frequency abrupt changes on optical cavity frequency locking, based on the above-described apparatus, comprising the following steps: S1. The laser's emitted light enters the optical resonant cavity through the optical path, and the transmitted light output from the optical resonant cavity is converted into an electrical signal by a photodetector. S2. After the electrical signal is modulated and demodulated by the mixer and low-pass filter, a corresponding error signal is generated and input to the electronic switch; the electrical signal is also output to the error signal switching component. S3, Electrical signal and threshold voltage V R Comparison: If the electrical signal is higher than the threshold voltage V R The pulse generator outputs a high-level control signal, the electronic switch is turned on, and the PID servo system controls the laser frequency tuner based on the error signal feedback to correct the laser frequency in the current frequency-locked state; if the electrical signal is lower than the threshold voltage V... R The pulse generator outputs a low-level control signal, the electronic switch is turned off, and the sample-and-hold module holds the last frame error signal, thus realizing the PID servo system stop frequency correction. S4. When the laser scan recaptures the next longitudinal mode of the optical resonator, the laser frequency is relocked, and the electrical signal is once again above the threshold voltage V. R The pulse generator outputs a high-level control signal, the electronic switch is turned on, and the PID servo system controls the laser frequency tuner based on the error signal feedback to achieve laser frequency correction under the new frequency lock state.

[0017] The advantages of this invention are: (1) The device of the present invention adds an error signal switching component including a pulse generator and an electronic switch on the basis of traditional frequency locking. It can actively cut off the invalid error signal and freeze the PID control input at the moment of laser longitudinal mode jump, so as to avoid PID integral term saturation, output disorder and mode-locking state collapse due to signal sudden change or loss, thereby realizing continuous and stable locking between longitudinal modes without manual re-capture.

[0018] (2) The device of the present invention introduces a sample-and-hold module and a delay control module. It can restore sampling after a delay after the system recaptures the cavity mode, realize the smooth release of the frozen state, avoid signal shock and oscillation at the moment of recovery, and ensure seamless transition and fast relock of the PID servo system loop. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of a device for suppressing the effect of frequency abrupt changes on optical cavity frequency locking according to an embodiment of the present invention; Figure 2 This is a schematic diagram of a narrow-linewidth laser frequency-stable step output in a scanning range of 97.5 GHz according to an embodiment of the present invention; Reference numerals: 1. Laser; 2. Isolator; 3. First reflector; 4. Laser frequency tuner; 5. First focusing lens; 6. Second reflector; 7. Optical resonant cavity; 8. Second focusing lens; 9. Laser driver; 10. Adder; 11. Function generator; 12. Radio frequency source; 13. Mixer; 14. Pulse generator; 15. Low-pass filter; 16. Electronic switch; 17. PID servo system; 18. Driver for laser frequency tuner. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Example 1 This embodiment provides a device to suppress the impact of frequency abrupt changes on optical cavity frequency locking. To avoid overflow in the servo system during laser tuning, which could lead to unstable locking, this embodiment combines an error signal switching component with a frequency locking servo system. PDH or optical feedback is used to achieve laser-to-optical resonant cavity frequency locking. Based on this, the laser controller continuously scans the injected current to achieve wide-range longitudinal mode traversal. The error signal switching component actively cuts off invalid error signals and freezes the PID servo system state at the moment of mode jump, seamlessly releasing it after capturing the next cavity mode. Throughout the process, the output frequency interval is clamped to a single longitudinal mode interval in real time, ensuring no jumps or re-peak seeking throughout the current scan, and continuously maintaining stable single-longitudinal-mode output. The specific structure of the device is as follows: Figure 1 As shown, it includes a laser drive assembly, an optical path assembly, a locking loop adjustment assembly, and an error signal switch assembly; The optical path assembly includes a laser 1, an isolator 2, a first reflector 3, a laser frequency tuner 4, a first focusing lens 5, a second reflector 6, an optical resonant cavity 7, and a second focusing lens 8 connected in sequence. The isolator 2 is used to isolate the reflected light from the front mirror of the cavity and can be used to adjust the feedback rate in optical feedback locking. The reflector and lens matching element is disposed between the laser 1 and the optical resonant cavity 7 to shape the beam emitted by the laser 1 and improve the coupling efficiency of the laser to the optical resonant cavity 7.

[0022] The locking loop adjustment assembly includes a photodetector 19, a mixer 13, a low-pass filter 15, an electronic switch 16, a PID servo system 17, and a laser frequency tuner driver 18 connected in sequence (for PDH technology, the tuner can be an external acousto-optic modulator; for optical feedback technology, the tuner is an external feedback phase tuner). The error signal switching component determines whether the optical resonant cavity 7 is in a frequency locked state or a frequency unlocked state based on the electrical signal received from the photodetector 19, and then connects or disconnects the locking circuit adjustment component.

[0023] The laser driving assembly includes a function generator 11, an RF source 12, an adder 10, and a laser driver 9. The function generator 11 generates a triangular wave scanning signal, and the RF source 12 generates a continuous frequency sinusoidal RF signal. The two signals are added together by the adder 10 and used as a control signal, which is then sent to the input terminal of the laser driver 9.

[0024] The error signal switching assembly includes a pulse generator 14 and an electronic switch 16. The input and output terminals of the pulse generator 14 are respectively connected to the output terminal of the photodetector 19 and the control terminal of the electronic switch 16. The pulse generator 14 is used to automatically generate a pulse signal when the system experiences mode skipping or signal interruption, and send it to the electronic switch 16 to cut off invalid error signals and freeze the PID servo control state. After the system recaptures a valid signal, it automatically releases and resumes output. The electronic switch 16 is connected in series between the low-pass filter 15 and the PID servo system 17; the radio frequency source 12 is connected to the mixer 13. The mixer 13 is used to mix the electrical signal with a radio frequency signal at the same frequency as the modulation signal. The low-pass filter 15 is used to filter out the high-frequency noise after mixing to obtain the error signal corresponding to the electrical signal.

[0025] The error signal switching assembly further includes a voltage follower (not shown in the figure), a potentiometer threshold network (not shown in the figure), a comparator module (not shown in the figure), a sample-and-hold module (not shown in the figure), and a delay control module (not shown in the figure). In this embodiment, the recommended components are the MCP6001 voltage follower, the RP1 potentiometer threshold network, the LM393 comparator module, the LF398 sample-and-hold module, and the CD4538 delay control module.

[0026] The voltage follower is used to achieve impedance isolation and buffer amplification between the input signal and the subsequent circuit, ensuring that the amplitude of the input signal is not affected by the load and is stably transmitted to the demodulation and comparison terminals.

[0027] The potentiometer threshold network consists of potentiometers and pull-up / pull-down resistors forming a voltage divider adjustment structure, used to provide an adjustable and low-noise threshold voltage V for the comparator module. R This is used to determine whether the optical resonant cavity 7 is in a frequency-locked state.

[0028] The comparator module is used to compare the electrical signal with the threshold voltage V in real time. R The magnitude; when the electrical signal is higher than V R When the pulse generator 14 outputs a high-level control signal, the pulse generator 14 outputs a high-level control signal; when the electrical signal is lower than V... R At this time, pulse generator 14 outputs a low-level control signal. It also suppresses signal jitter through hysteresis feedback, achieving reliable identification of the frequency lock state.

[0029] The sample-and-hold module is used to detect an electrical signal below V. R When the signal is detected to be higher than V, it automatically enters a hold state, freezing the current PID servo system input level 17 to prevent integral saturation or misadjustment under invalid error signals; when the detected electrical signal is higher than V... RWhen the sampling and holding module releases its hold function and resumes real-time signal sampling, a smooth transition is achieved.

[0030] The delay control module is used to generate a fixed-delay release control signal when the frequency lock state is restored, to ensure that the PID loop resumes input only after the system is stable, and to avoid oscillation during the transition phase.

[0031] The PID servo system 17 is used to receive the error signal output by the electronic switch and to provide feedback control for the laser frequency tuner.

[0032] This embodiment also provides experimental results using the above-described apparatus, such as... Figure 2 As shown, the above device enables the output frequency to be stably and quickly locked from the current frequency to the next longitudinal mode frequency when the scanning range is 97.5 GHz and the interval between adjacent locked frequencies is 680 MHz under the locked state.

[0033] Example 2 It should be further explained that, based on the same inventive concept, this embodiment provides a method for suppressing the influence of frequency abrupt changes on the frequency locking of an optical cavity, based on the apparatus described in Embodiment 1, including the following steps: S1. The emitted light from laser 1 enters the optical resonant cavity 7 through the optical path, and the transmitted light output from the optical resonant cavity 7 is converted into an electrical signal by the photodetector 19. S2. After the electrical signal is modulated and demodulated by the mixer 13 and the low-pass filter 15, a corresponding error signal is generated and input to the electronic switch 16; the electrical signal is also output to the error signal switch assembly. S3, Electrical signal and threshold voltage V R Comparison: If the electrical signal is higher than the threshold voltage V R The pulse generator 14 outputs a high-level control signal, the electronic switch 16 is turned on, and the PID servo system 17 controls the laser frequency tuner 4 based on the error signal feedback to correct the laser frequency under the current frequency lock state; if the electrical signal is lower than the threshold voltage V R The pulse generator 14 outputs a low-level control signal, the electronic switch 16 is turned off, the sample and hold module holds the last frame error signal, and the PID servo system 17 stops frequency correction. S4. When the laser scan recaptures the next longitudinal mode of the optical resonator 7, the laser frequency is relocked, and the electrical signal is once again above the threshold voltage V. R The pulse generator 14 outputs a high-level control signal, the electronic switch 16 is turned on, and the PID servo system 17 controls the laser frequency tuner 4 according to the error signal feedback to realize the correction of the laser frequency in the new frequency locking state.

[0034] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A device for suppressing the effect of frequency abrupt changes on optical cavity frequency locking, characterized in that, This includes a laser drive assembly, an optical path assembly, a locking loop adjustment assembly, and an error signal switch assembly; The optical path assembly includes a laser (1), an isolator (2), a first reflector (3), a laser frequency tuner (4), a first focusing lens (5), a second reflector (6), an optical resonant cavity (7), and a second focusing lens (8) connected in sequence. The locking loop adjustment assembly includes a photodetector (19), a mixer (13), a low-pass filter (15), an electronic switch (16), a PID servo system (17), and a laser frequency coordinator driver (18) connected in sequence. The error signal switch assembly determines whether the optical resonant cavity (7) is in a frequency locked state or a frequency unlocked state based on the electrical signal received from the photodetector (19), and then connects or disconnects the locking circuit adjustment assembly.

2. The device for suppressing the influence of frequency abrupt changes on optical cavity frequency locking according to claim 1, characterized in that, The laser driving component includes a function generator (11), a radio frequency source (12), an adder (10), and a laser driver (9). The function generator (11) generates a triangular wave scanning signal, and the radio frequency source (12) generates a sinusoidal radio frequency signal with a continuous frequency. The two signals are added together by the adder (10) and used as a control signal, which is then sent to the input terminal of the laser driver (9).

3. The device for suppressing the influence of frequency abrupt changes on optical cavity frequency locking according to claim 1, characterized in that, The error signal switching assembly includes a pulse generator (14) and an electronic switch (16); the input and output terminals of the pulse generator (14) are connected to the output terminal of the photodetector (19) and the control terminal of the electronic switch (16), respectively; the pulse generator (14) is used to automatically generate a pulse signal when the system experiences mode skipping or signal interruption, and send it to the electronic switch (16) to cut off invalid error signals and freeze the PID servo control state, and automatically release and restore output after the system recaptures a valid signal; The electronic switch (16) is connected in series between the low-pass filter (15) and the PID servo system (17); the radio frequency source (12) is connected to the mixer (13).

4. The device for suppressing the effect of frequency abrupt changes on optical cavity frequency locking according to claim 1, characterized in that, The error signal switching assembly also includes a voltage follower, a potentiometer threshold network, a comparator module, a sample-and-hold module, and a delay control module.

5. The device for suppressing the effect of frequency abrupt changes on optical cavity frequency locking according to claim 4, characterized in that, The voltage follower is used to achieve impedance isolation and buffer amplification between the input signal and the subsequent circuit, ensuring that the amplitude of the input signal is not affected by the load and is stably transmitted to the demodulation and comparison terminals.

6. The device for suppressing the effect of frequency abrupt changes on optical cavity frequency locking according to claim 4, characterized in that, The potentiometer threshold network consists of potentiometers and pull-up / pull-down resistors forming a voltage divider adjustment structure, used to provide an adjustable and low-noise threshold voltage V for the comparator module. R This is used to determine whether the optical resonator (7) is in a frequency-locked state.

7. The device for suppressing the effect of frequency abrupt changes on optical cavity frequency locking according to claim 4, characterized in that, The comparator module is used to compare the electrical signal with the threshold voltage V in real time. R The magnitude; when the electrical signal is higher than V R When the pulse generator (14) outputs a high-level control signal, when the electrical signal is lower than V... R At that time, the pulse generator (14) outputs a low-level control signal.

8. The device for suppressing the effect of frequency abrupt changes on optical cavity frequency locking according to claim 4, characterized in that, The sample-and-hold module is used to detect an electrical signal below V. R When the signal is detected to be higher than V, it automatically enters the hold state, freezing the current PID servo system (17) input level to prevent integral saturation or misadjustment under the action of invalid error signals; when the electrical signal is detected to be higher than V R When the sampling and holding module releases its hold function, it resumes real-time signal sampling.

9. The device for suppressing the effect of frequency abrupt changes on optical cavity frequency locking according to claim 4, characterized in that, The delay control module is used to generate a fixed-delay release control signal when the frequency lock state is restored, to ensure that the PID loop resumes input only after the system is stable, and to avoid oscillation during the transition phase.

10. A method for suppressing the effect of frequency abrupt changes on optical cavity frequency locking, characterized in that, Based on the apparatus according to claims 1-9, the method includes the following steps: S1. The emitted light from the laser (1) enters the optical resonant cavity (7) through the optical path, and the transmitted light output from the optical resonant cavity (7) is converted into an electrical signal by the photodetector (19). S2. After the electrical signal is modulated and demodulated by the mixer (13) and the low-pass filter (15), a corresponding error signal is generated and input to the electronic switch (16); the electrical signal is simultaneously output to the error signal switch assembly. S3, Electrical signal and threshold voltage V R Comparison: If the electrical signal is higher than the threshold voltage V R The pulse generator (14) outputs a high-level control signal, the electronic switch (16) is turned on, and the PID servo system (17) controls the laser frequency tuner (4) according to the error signal feedback to realize the correction of the laser frequency under the current frequency locking state; if the electrical signal is lower than the threshold voltage V R The pulse generator (14) outputs a low-level control signal, the electronic switch (16) is turned off, the sample and hold module holds the last frame error signal, and the PID servo system (17) stops frequency correction. S4. When the laser scan recaptures the next longitudinal mode of the optical resonant cavity (7), the laser frequency is relocked, and the electrical signal is once again higher than the threshold voltage V. R The pulse generator (14) outputs a high-level control signal, the electronic switch (16) is turned on, and the PID servo system (17) controls the laser frequency tuner (4) according to the error signal feedback to realize the correction of the laser frequency under the new frequency lock state.

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