Compact laser frequency automatic long-term locking system and method

Abstract

The invention is suitable for the technical field of laser measurement, and provides a compact laser frequency automatic long-term locking system and method, which can obtain an absorption spectrum through triangular wave scanning by adjusting the laser wavelength to be close to a target spectral line. Parameters such as spectral line number, amplitude and distance threshold are set, a target spectral line is automatically identified, and a corresponding voltage value is recorded. And then, through fast and slow PID control, the frequency is locked at a spectral line zero crossing point through PZT and temperature adjustment. And the system monitors the frequency discrimination signal in real time, the PID is closed if the lock is lost, the output value is kept to avoid frequency hopping, then the target spectral line is re-identified through small-range oblique wave scanning and locking is recovered, and long-term stable automatic frequency locking is realized. Therefore, the device can realize many application scenes of laser frequency locking based on the atom / molecule absorption cell, is very suitable for research of high-precision and advanced physical experiments and interference measurement technologies in which a frequency-stabilized light source participates, can fully ensure high accuracy of the laser light source, and can simplify an experiment or measurement device.

Description

Technical Field

[0001] This invention relates to the field of laser measurement technology and provides a compact automatic long-term laser frequency locking system and method. Background Technology

[0002] Lasers, with their advantages of good monochromaticity, high coherence, and good collimation, are widely used in research on precision measurement, quantum metrology, physics, and precision spectroscopy. They are an important component of the light source in various measurement systems, and their performance, in most cases, determines the upper limit of the accuracy of the measurement results.

[0003] However, due to the low frequency stability and wide linewidth of freely operating lasers, their coherence is reduced, which cannot meet the requirements of current precision measurement technology and related physical research. Therefore, laser frequency stabilization technology (which also has the effect of narrowing linewidth) is a current research hotspot and the foundation of a large number of measurement technology research and physical experiment research.

[0004] Based on their underlying principles, laser frequency stabilization methods mainly include active and passive frequency stabilization. Passive frequency stabilization relies on external conditions to ensure stable laser operation, but this type of method cannot achieve high-accuracy frequency stabilization; it can only ensure the smooth operation of the laser. Active frequency stabilization, on the other hand, compares the laser output frequency with a relatively constant frequency reference source and uses feedback control to lock the laser frequency to the reference frequency—an active control method. Typical methods for actively controlling and stabilizing laser frequencies can be divided into three categories. The first category utilizes the stability of atomic or molecular energy level transition spectral lines, such as saturated absorption and modulation transfer methods, to lock the laser frequency at the center frequency of the transition spectral line, thus achieving laser frequency stabilization. The second category utilizes the length stability of physical structures to obtain a stable frequency reference based on standing wave conditions, such as the Pound-Drever-Hall (PDH) laser frequency stabilization technique based on an optical reference cavity. This method combines phase modulation spectroscopy with optical heterodyne detection to precisely lock the laser frequency at the resonant frequency of the Fabry-Pérot (FP) cavity. The third category, the most traditional but with performance limits far inferior to the above two methods, includes Zeeman effect stabilization and Lamb's dip stabilization. These two methods are only used in scenarios where high frequency stabilization requirements are not necessary. As the requirements for frequency stabilization accuracy increase, the third category of methods is gradually being phased out; therefore, the following comparisons will only focus on the first two methods.

[0005] Active control methods are more effective, locking the laser wavelength to a fixed frequency point at the moment the light source is generated, and exhibiting a certain degree of adaptability to external vibrations and environmental conditions. Furthermore, wavelength locking methods based on atomic or molecular energy level transition lines are based on the physical properties of molecules or atoms. Compared to wavelength locking methods based on the length stability of physical structures, these methods have a significant advantage in terms of environmental adaptability in practical applications. Wavelength locking methods based on atomic or molecular energy level transition lines only require ensuring the relative stability of the atomic or molecular state, while wavelength locking methods based on the length stability of physical structures have much stricter requirements regarding external vibrations and other environmental conditions. For example, obtaining the saturated absorption spectrum of iodine molecules only requires ensuring that the temperature of the iodine molecule absorption cell is within the range of (-15±0.2)℃.

[0006] Techniques capable of locking wavelengths to molecular spectral lines primarily include molecular saturated absorption and modulation-transfer spectroscopy. Molecular saturated absorption obtains the absorption spectrum signal by internally modulating the laser, typically through adjusting the cavity length or injecting current. This results in frequency modulation in the actual output laser, meaning the laser wavelength itself exhibits non-noise fluctuations, leading to linewidth broadening. Furthermore, sufficient optical power is required for strong molecular absorption in molecular saturated absorption, necessitating higher optical power for wavelength locking, which reduces the output power of the locked laser. Therefore, modulation-transfer spectroscopy offers advantages over molecular saturated absorption. Firstly, it's an external modulation technique, tuning only the splitter path and largely preserving the stable light source's output linewidth. Secondly, it demodulates the sideband detuning of the probe light through four-wave mixing, requiring less laser power, exhibiting strong anti-interference capabilities, and eliminating Doppler background broadening.

[0007] (2) Prior art related to the present invention

[0008] Existing laser frequency locking devices are generally frequency-stabilized helium-neon lasers, which use the Zeeman effect for frequency locking, and the wavelength uncertainty after locking is approximately 10. -8 These lasers, while capable of operating on a large scale, suffer from low frequency stability and are primarily used in general laser interferometry research. While frequency-locked lasers exist for wavelengths such as 780nm and 1542nm, these devices are integrated with the light source and can only lock onto a single wavelength. Furthermore, for laser frequency locking devices, the LaserLock from TEM (Germany) and the Moku:Lab modular platform from Liquid Instruments (Australia) are among the most widely used and mature products on the market. However, these products are designed for post-processing and control of electrical signals and cannot be directly used with the relevant light source in practical applications, nor do they possess automatic locking functionality.

[0009] Therefore, based on the current research status and market products, existing laser devices or frequency-locking equipment have the following drawbacks:

[0010] 1. Laser devices can only lock onto a single wavelength and are integrated with the light source, making them unable to adapt to light sources of other wavelengths;

[0011] 2. Frequency locking devices can only process and control electrical signals, cannot be directly used for light sources, and do not have an automatic locking function;

[0012] 3. Frequency locking equipment requires the construction of a frequency stabilization optical path to achieve laser frequency locking. It cannot be well integrated with the frequency locking equipment, which will result in a large size of the frequency locking system and increase the difficulty of using the product. Summary of the Invention

[0013] To overcome the shortcomings of existing technologies, this invention provides a compact automatic long-term laser frequency locking system and method, aiming to solve the problems mentioned in the background art; the compact automatic long-term laser frequency locking system includes a laser, a laser frequency locking optical path module, and a laser frequency locking control module;

[0014] The laser frequency-locking optical path module includes an optical fiber beam splitter, at least two acousto-optic frequency shifters, and matching AOM controllers, optical fiber attenuators, at least two optical fiber collimators, a mirror, an absorption cell, a PBS polarization beam splitter, and a PD detector; the laser frequency-locking control module includes a co-source signal source, a lock-in amplifier, and at least two PID controllers.

[0015] The laser is connected to the input end of the fiber optic beam splitter via an optical fiber. One output end of the fiber optic beam splitter is connected in series with two acousto-optic frequency shifters and a first fiber optic collimator via an optical fiber. The other output end of the fiber optic beam splitter is connected in series with an optical fiber attenuator and a second fiber optic collimator via an optical fiber.

[0016] The combination of the absorption cell, the reflector, and the PBS polarization beam splitter is as follows: when the two beams of light from the first fiber collimator and the second fiber collimator are incident on the absorption cell, under the action of the reflector and the PBS polarization beam splitter, the positive and negative beams are collinear and the polarization directions are perpendicular to each other, thus achieving a doubling of the absorption range in the absorption cell for a total of 4 times in 2 round trips.

[0017] The light emitted from the second fiber collimator is reflected by the PBS polarization beam splitter and then enters the PD detector; the two control signal output ports of the same source signal source are respectively connected to two AOM controllers, and the two AOM controllers are respectively connected to the signal input terminals of two acousto-optic frequency shifters; the other signal output port is connected to the lock-in amplifier; the lock-in amplifier of the laser frequency lock control module is electrically connected to the PD detector and the same source signal source;

[0018] Two PID controllers are connected in series between the lock-in amplifier and the laser; at the same time, the second output terminal of the PID controller closer to the lock-in amplifier is electrically connected to the laser; a switch is provided between the laser and the two PID controllers.

[0019] Furthermore, the PID controller includes a first PID controller and a second PID controller;

[0020] The output of the lock-in amplifier is electrically connected in series to a low-pass filter, a first PID controller, a second switch, a third switch, and the first input terminal of the laser.

[0021] The second output terminal of the first PID controller is electrically connected to the second PID controller, and the output terminal of the second PID controller is electrically connected in series to the fourth switch, the fifth switch, and the second input terminal of the laser.

[0022] Furthermore, there are two independent bias voltages between the second switch and the third switch, and a triangular wave signal source is electrically connected to one of the bias voltages through the first switch.

[0023] An independent bias voltage is provided between the fourth switch and the fifth switch.

[0024] Furthermore, voltage limiting elements are provided between the fifth switch and the laser light source, and between the third switch and the laser light source.

[0025] Furthermore, the output terminal of the PD detector is electrically connected to a signal amplifier, and the output terminal of the signal amplifier is electrically connected to a lock-in amplifier.

[0026] A compact method for automatic long-term locking of laser frequency includes the following steps:

[0027] S1: Adjust the center wavelength Adjust the center wavelength of the laser to be near the target spectral line;

[0028] S2: Set the triangular wave scanning voltage and scan frequency Start scanning;

[0029] S3: Set feature parameters, number of spectral features , Amplitude 'a', Distance threshold ;

[0030] Using the target spectral line as a reference point, the system automatically locates the target spectral line by detecting the distance between the reference point and multiple spectral lines on both sides. The number of spectral lines on the left and right sides is set to... , , representing the number of spectral lines that need to be identified and calculated; amplitude 'a' is the minimum amplitude required to identify the spectral lines; and distance threshold... This is the main basis for judging spectral lines;

[0031] S4: Determine whether the calculated feature parameters meet the threshold and record the scan voltage value. ;

[0032] S5: Set the target spectral line voltage value and enable PID control, i.e., fast PID and slow PID control;

[0033] The scan voltage value recorded in step S4 Add it to the PZT port and simultaneously enable fast PID and slow PID to achieve automatic peak locking of the laser wavelength.

[0034] Furthermore, step S4 specifically involves defining a distance value based on the frequency difference between the target spectral line and the first spectral line on the left or right. The distances between other spectral lines are defined as follows: Multiples of this value are used to obtain the distance values ​​from multiple spectral lines to the target spectral line. When the distance values ​​on the left and right sides simultaneously meet the set threshold coefficient At that time, the position of the corresponding target spectral line can be found and the scanning voltage value at that position can be recorded. .

[0035] Furthermore, step S5 specifically involves: the input signal of the fast PID is the target spectral line signal, i.e., the frequency discrimination signal; the fast PID adjusts the laser's PZT interface, quickly responds to the frequency discrimination signal and locks it to the zero-crossing point; the slow PID uses the output value of the fast PID as input, and feeds back the output value of the fast PZT towards zero by sending a feedback signal to the laser's Temp. port.

[0036] Furthermore, the method also includes an automatic relocking step after the lock is lost:

[0037] S6: Determine if the lock is engaged;

[0038] By monitoring the peak-to-peak value of the frequency discrimination signal, it can be determined whether it is in a locked state.

[0039] When locked, the peak-to-peak value should be significantly greater than 0;

[0040] When the lock is lost, the peak-to-peak value is a smaller value that approaches 0;

[0041] S7: Disable PID control after lockout;

[0042] Upon detecting a current loss of lock, the PID control is immediately turned off, the current PID output value is memorized and set as the offset value, and it is directly loaded into the PZT port and Temp. port of the laser to ensure that the laser frequency does not change abruptly.

[0043] S8: Perform a small-range oblique wave scan;

[0044] Custom amplitude and frequency The search is conducted to find the target spectral line. Since the frequency of the laser changes gradually, only a single target spectral line exists in the vicinity.

[0045] S9: Zero-crossing point identification of absorption lines; identify the zero-crossing point of the target spectral line and record the corresponding voltage value. .

[0046] S10: After loading the voltage value recorded in step 9, start PID control;

[0047] The process of steps S6 to S10 is repeated to ensure that the laser frequency is locked for a long time.

[0048] This invention features a compact structure, automatic frequency locking, and ease of use. It is suitable for various applications of laser frequency locking based on atomic / molecular absorption cells. It is particularly well-suited for high-precision physics experiments and interferometric measurement research involving frequency-stabilized light sources. It can fully guarantee the high accuracy of the laser source and simplify experimental or measurement devices. It is of great significance for miniaturization, portability improvement, and simplified operation of the device. Attached Figure Description

[0049] Figure 1 The connection diagram of each component in the system;

[0050] Figure 2 for Figure 1 Enlarged view of part A;

[0051] Figure 3 The diagram shows multiple absorption lines of iodine molecules within the R(56) 32-0 spectral line;

[0052] Figure 4 Flowchart of a method for automatic long-term locking of laser frequency;

[0053] In the diagram: 01-Laser frequency locking optical path module; 02-Laser frequency locking control module; 03-Laser module;

[0054] 1-Fiber optic beam splitter; 11-First acousto-optic frequency shifter; 111-First AOM controller; 12-Second acousto-optic frequency shifter; 121-Second AOM controller; 131-First fiber optic collimator; 132-Second fiber optic collimator;

[0055] 2-Absorption cell; 21-First reflector; 22-Second reflector; 23-Third reflector; 24-Absorption cell temperature control;

[0056] 3-Fiber optic attenuator;

[0057] 41-PBS polarization beam splitter; 42-PD detector; 51-Same source signal source; 52-Signal amplifier; 6-Lock-in amplifier;

[0058] 701 - First switch; 702 - Second switch; 703 - Third switch; 704 - Fourth switch; 705 - Fifth switch;

[0059] 71-Gain component; 72-Low-pass filter; 73-Bias voltage; 741-First PID controller; 742-Second PID controller; 75-Triangular wave signal source; 76-Voltage limiting component;

[0060] 8-Laser source; 81-Fourth reflector; 9-Coupler. Detailed Implementation

[0061] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0062] It should be noted that, in the description of this invention, unless otherwise stated, "a plurality of" means two or more; the terms "upper," "lower," "left," "right," "inner," "outer," "front end," "rear end," "head," "tail," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this invention. Furthermore, the terms "first," "second," "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0063] Furthermore, in the description of this invention, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0064] 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. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0065] See Figure 1-4 The purpose of this invention is to provide a compact automatic long-term laser frequency locking system applicable to lasers of different wavelengths. The system includes:

[0066] Laser frequency-locked optical path module 01: includes fiber beam splitter 1, acousto-optic frequency shifter and matching AOM controller, fiber attenuator 3, fiber collimator (Probe and Pump), mirror, atomic / molecular absorption cell, absorption cell temperature controller 24, PBS polarization beam splitter 41 and PD detector 42.

[0067] The fiber optic bundle splitter is a 90:10 fiber optic bundle splitter; the fiber optic collimator includes two components: a probe and a pump, namely a first fiber optic collimator 131 and a second fiber optic collimator 132; the acousto-optic frequency shifter includes a first acousto-optic frequency shifter 11 and a second acousto-optic frequency shifter 12; the AOM controller includes a first AOM controller 111 and a second AOM controller 121; and the reflectors include a first reflector 21, a second reflector 22, and a third reflector 23.

[0068] Laser frequency locking control module 02 includes a homogeneous signal source 51, a signal amplifier 52, a lock-in amplifier 6, a low-pass filter 72, a PID controller (PZT and Temp.), several switches, a triangular wave signal source 75, and a voltage limiting element 76.

[0069] The PID controller includes a first PID controller 741 and a second PID controller 742, which are labeled "PZT" and "Temp." respectively; the switches include a first switch 701, a second switch 702, a third switch 703, a fourth switch 704, and a fifth switch 705.

[0070] It also includes laser module 03, which includes laser source 8 (laser), fourth reflector 81, and coupler 9. The laser has two control signal input terminals, namely PZT signal input terminal and Temp. signal input terminal.

[0071] The location and wiring of laser frequency-locked optical path module 01 are as follows:

[0072] The output light source of the laser source 8 is coupled into the optical fiber through the coupler 9 and connected to the input end of the optical fiber beam splitter 1. The light is split at a ratio of 90:10. 90% of the light enters the acousto-optic frequency shifters AOM1 and AOM2 in sequence (i.e., the first acousto-optic frequency shifter 11 and the second acousto-optic frequency shifter 12 in sequence), and then enters the first optical fiber collimator 131 (Pump); at the same time, 10% of the light enters the optical fiber attenuator 3 and the second optical fiber collimator 132 (Probe) in sequence.

[0073] The positions ensure that the emitted light from the first fiber collimator 131 (Pump) and the emitted light from the second fiber collimator 132 (Probe) are collinear and opposite in direction, and the two collimators are symmetrical about the diameter center along the length of the absorption cell 2. The first reflector 21 and the second reflector 22 are located at the upper end of the absorption cell 2, and the third reflector 23 is located at the lower end of the absorption cell 2 (between the first fiber collimator 131 (Pump) and the second fiber collimator 132 (Probe)). The end faces of the first reflector 21, the second reflector 22, the third reflector 23, and the absorption cell 2 are parallel to each other. Two beams of light pass through absorption cell 2 (atomic / molecular absorption cell). Under the action of the first fiber collimator 131 (Pump), the second fiber collimator 132 (Probe), and the reflectors (first reflector 21, second reflector 22, and third reflector 23), they achieve bidirectional collinearity and perpendicular polarization directions, resulting in a four-fold absorption range multiplication within absorption cell 2 (two round trips). Finally, the light emitted from the second fiber collimator 132 (Probe) is reflected by the PBS polarizing beam splitter 41 (positioned to ensure the reflected light is within the detection surface of the PD detector 42) and then enters the PD detector 42.

[0074] Preferably, the temperature of the absorption cell 2 is controlled during this process to ensure that the temperature stability is better than 0.01℃.

[0075] The connection between laser frequency locking optical path module 01 and laser frequency locking control module 02 is as follows:

[0076] Both acousto-optic frequency shifters have control signal input terminals, and each control signal input terminal is connected to an AOM controller. The input terminals of both AOM controllers are connected to a common signal source 51. Specifically, the common signal source 51 has two control signal output ports, which are respectively connected to the first AOM controller 111 and the second AOM controller 121; the output terminals of the first AOM controller 111 and the second AOM controller 121 are respectively connected to the control signal input terminals of the first acousto-optic frequency shifter 11 and the second acousto-optic frequency shifter 12.

[0077] Therefore, the output of the co-source signal source 51 in the laser frequency locking control module 02 serves as the control signal for the acousto-optic frequency shifters AOM1 and AOM2 in the laser frequency locking optical path module 01. One signal is a frequency-modulated radio frequency signal, and the other is an unmodulated co-source radio frequency signal. The former directly forward-shifts and modulates the laser signal to generate pump light; the latter negatively shifts the frequency to adjust the laser frequency back to the absorption spectrum, and the two acousto-optic frequency shifters can stabilize the laser power. The output of the PD detector 42 in the laser frequency locking optical path module 01 serves as the input of the laser frequency locking control module 02 and is directly connected to the signal amplifier 52.

[0078] Based on the above, the wiring relationship for laser frequency locking control module 02 is as follows:

[0079] The outputs of signal amplifier 52 and co-source signal source 51 are both electrically connected to lock-in amplifier 6. The output of lock-in amplifier 6 is sequentially electrically connected to gain element 71, low-pass filter 72, first PID controller 741, second switch 702, third switch 703 and the first input terminal of laser source 8.

[0080] The second output terminal of the first PID controller 741 is electrically connected to the second PID controller 742. The output terminal of the second PID controller 742 is sequentially electrically connected to the fourth switch 704, the fifth switch 705, and the second input terminal of the laser light source 8.

[0081] Specifically, a separate bias voltage 73 is provided between the low-pass filter 72 and the first PID controller 741; two independent bias voltages 73 are provided between the second switch 702 and the third switch 703, and a triangular wave signal source 75 is electrically connected to the bias voltage 73 near the third switch 703 through the first switch 701. A separate bias voltage 73 is provided between the fourth switch 704 and the fifth switch 705. More specifically, voltage limiting elements 76 are provided between the fifth switch 705 and the laser source 8, and between the third switch 703 and the laser source 8.

[0082] Therefore, the input signal amplified by the signal amplifier 52 (the output signal of the PD detector 42 is the input signal of the laser frequency locking control module 02) and the synchronization signal (referring to the frequency modulation signal) of the same source signal source 51 are both input into the lock-in amplifier 6; after the processed signal passes through the gain element 71, it passes through the low-pass filter 72 to obtain the frequency discrimination error signal used as feedback control.

[0083] The frequency discrimination error signal is first input to the first PID controller 741 (PZT). When the second switch 702 and the third switch 703 are closed, the frequency of the laser's PZT signal input terminal can be tuned and controlled. The signal after proportional control in the first PID controller 741 (PZT) is used as the input to the second PID controller 742 (Temp.). When the fourth switch 704 and the fifth switch 705 are closed, the frequency of the laser's Temp. signal input terminal is tuned and controlled. The triangular wave signal source 75 and switch 1 are used to achieve laser frequency scanning and switching and control of the non-scanning state when frequency locking is required.

[0084] In summary, this system features a compact structure, integrating a frequency-locking optical path module and a frequency-locking control module. It can use different atomic / molecular absorption cells as locking sources and, based on modulation-transfer spectroscopy technology, combines power-on self-locking and unlock-off re-locking functions to achieve long-term automatic locking, locking the laser frequency to a high-accuracy and high-stability modulation-transfer fine spectral line. The working principle of this invention is as follows:

[0085] After the laser light source 8 output enters the laser frequency-locked optical path module 01, the fiber beam splitter 1 splits the beam into a probe beam and a pump beam. The probe and pump beams undergo near-degenerate four-wave mixing in the atomic / molecular absorption cell. Under the action of the absorption cell 2, the modulation sideband applied to the pump beam is transferred to the probe beam, ultimately resulting in the corresponding signal on the PD detector 42:

[0086]

[0087] In the formula:

[0088] C is the signal amplitude constant; Spectral linewidth; The modulation index of the modulator; The modulation frequency of the modulator; For Bessel functions; It is a Lorentz line type, i.e., an absorption line type; It is a dispersive line type; This is the initial phase.

[0089] The formulas for calculating absorption line type and dispersion line type are as follows:

[0090]

[0091] In the formula:

[0092] This is the laser frequency detuning, which is the deviation value relative to the spectral line frequency of the absorption cell.

[0093] After the signal in equation (1) enters the lock-in amplifier 6, the phase is adjusted so that the amplitude signal of the corresponding absorption line can be extracted when the triangular wave signal source 75 performs frequency scanning, and used as the frequency discrimination error signal of the laser frequency locking system.

[0094] However, since multiple similar absorption lines may exist in the frequency domain, as shown in the attached figure... Figure 3 As shown, iodine molecules exhibit multiple absorption lines within the R(56) 32-0 spectrum. Furthermore, after locking the absorption lines, the system may experience a loss of lock due to perturbations. Therefore, to ensure the system remains locked for an extended period, a corresponding algorithm is needed to guarantee relocking after a loss of lock.

[0095] Therefore, this invention proposes a compact automatic long-term laser frequency locking method. The method uses the following algorithm to automatically find the corresponding absorption spectral line and lock it, while ensuring relocking after loss of lock.

[0096] The specific algorithm flow is as follows: Figure 4 As shown: It is worth noting that in the following steps, steps S2 to S5 are automatic peak finding and locking, while steps S6 to S10 are the automatic relocking process after lock loss. The steps include:

[0097] S1: Adjust the center wavelength ;

[0098] Generally speaking, the fast tuning (PZT applied voltage) range of a laser is limited. Therefore, in order to ensure that multiple curves, including the target spectral line, can be scanned during fast tuning, the center wavelength of the laser needs to be adjusted to be near the target spectral line first.

[0099] S2: Set the triangular wave scanning voltage and scan frequency Start scanning;

[0100] Based on the laser's fast tuning coefficient and response rate, the corresponding triangular wave scanning voltage can be set. and scan frequency The scanning frequency is typically set in the range of mHz to Hz. To accurately determine the target spectral line, it should be ensured that all spectral lines near the target line can be scanned.

[0101] S3: Set feature parameters, number of spectral features , Amplitude 'a', Distance threshold ;

[0102] Using the target spectral line as a reference point, the target spectral line is automatically located by detecting the distance between the reference point and multiple spectral lines on both sides. Therefore, it is necessary to set the response characteristic parameters. The number of spectral lines on the left and right sides are set to... , , representing the number of spectral lines that need to be identified and calculated; amplitude 'a' is the minimum amplitude required to identify the spectral lines; and distance threshold... This is the main basis for judging spectral lines, as explained in step 4.

[0103] S4: Determine if the calculated feature parameters meet the requirements?

[0104] The theoretical value of the spacing between multiple spectral lines is the frequency difference in the frequency domain. To quantify the distance, the frequency difference between the target spectral line and the first spectral line to its left (or right) is used as a reference and defined as one distance value. The distances between other spectral lines are defined as follows: The multiples of this can be used to obtain the distance values ​​from multiple spectral lines to the target spectral line. When the distance values ​​on the left and right sides simultaneously meet the set threshold coefficient When the condition is generally set to a positive range, such as [0.85, 0.95], that is: ∈ It can locate the position of the corresponding target spectral line and record the scanning voltage value at that position. .

[0105] S5: Set the target spectral line voltage value and enable PID control (fast PID, slow PID).

[0106] The scan voltage value recorded in step 4 Add the laser to the PZT port and simultaneously enable both fast and slow PID controllers to achieve automatic peak finding and locking of the laser wavelength. The input signal for the fast PID controller is the target spectral line signal (generally called the frequency discrimination signal), and it adjusts the laser's PZT interface to quickly respond to the frequency discrimination signal and lock it to the zero-crossing point. The slow PID controller, on the other hand, uses the output value of the fast PID controller as input and feeds back the output value of the fast PZT towards zero by sending a feedback signal to the laser's Temp. port, ensuring frequency locking even when the PZT's tuning range is limited.

[0107] S6: Lock status check, is it locked?

[0108] By monitoring the peak-to-peak value of the frequency discrimination signal, it can be determined whether the signal is in a locked state. When the signal is locked, the peak-to-peak value should be significantly greater than 0; when the signal is unlocked, the peak-to-peak value should be a smaller value close to 0.

[0109] S7: Disable PID control (fast PID, slow PID);

[0110] Upon detecting a current loss of lock, the PID control is immediately disabled, but the current PID output value is memorized and set as an offset value, which is then directly loaded into the PZT port and Temp. port of the laser to ensure that the laser frequency does not change abruptly.

[0111] S8: Small-range ramp scan (custom amplitude) and frequency );

[0112] A small-scale oblique wave scan is performed to search for the target spectral line. Since the laser frequency changes gradually, only a single target spectral line exists in the vicinity.

[0113] S9: Identification of zero-crossing points of absorption lines;

[0114] Identify the zero-crossing points of the target spectral lines and record the corresponding voltage values.

[0115] S10: Set the voltage and enable PID control (fast PID, slow PID); after loading the voltage value recorded in step 9, enable PID control to lock the laser frequency again. Then repeat steps 6 to 10 to ensure the laser frequency is locked for a long time.

[0116] In summary, the compact laser frequency automatic long-term locking system and method of this invention features a compact structure, automatic frequency locking, and ease of use. It is suitable for numerous applications involving laser frequency locking based on atomic / molecular absorption cells, and is particularly well-suited for high-precision physics experiments and interferometric measurement research involving frequency-stabilized light sources. It not only ensures high accuracy of the laser source but also simplifies experimental or measurement devices. Furthermore, it is of great significance for miniaturization, portability improvement, and simplified operation of the device. The corresponding project title is: Research on Wavelength Meter Calibration Technology Based on Iodine Molecular Absorption Spectrum Reference; the project number is: JSJL2023212B001.

[0117] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. A compact automatic long-term laser frequency locking system, characterized in that, It includes a laser, a laser frequency-locking optical path module (01), and a laser frequency-locking control module (02). The laser frequency-locking optical path module (01) includes an optical fiber beam splitter (1), at least two acousto-optic frequency shifters and matching AOM controllers, optical fiber attenuators (3), at least two optical fiber collimators, a reflector, an absorption cell (2), a PBS polarization beam splitter (41), and a PD detector (42); the laser frequency-locking control module (02) includes a co-source signal source (51), a lock-in amplifier (6), and at least two PID controllers; The laser is connected to the input end of the fiber beam splitter (1) via an optical fiber. One of the output ends of the fiber beam splitter (1) is connected in series with two acousto-optic frequency shifters and the first fiber collimator (131) via an optical fiber. The other output end of the fiber beam splitter (1) is connected in series with an optical fiber attenuator (3) and the second fiber collimator (132) via an optical fiber. The combination of the absorption cell (2), the reflector, and the PBS polarization beam splitter (41) is as follows: When the two beams of light from the first fiber collimator (131) and the second fiber collimator (132) are incident on the absorption cell (2), under the action of the reflector and the PBS polarization beam splitter (41), the positive and negative bidirectional collinearity and the polarization directions are perpendicular to each other, thereby achieving a doubling of the absorption range in the absorption cell (2) for a total of 4 round trips. The light emitted from the second fiber collimator (132) is reflected by the PBS polarization beam splitter (41) and then enters the PD detector (42); The two control signal output ports of the co-source signal source (51) are connected to two AOM controllers respectively, and the two AOM controllers are connected to the signal input terminals of two acousto-optic frequency shifters respectively; the other signal output port is connected to the lock-in amplifier (6); the lock-in amplifier (6) of the laser frequency lock control module (02) is electrically connected to the PD detector (42) and the co-source signal source (51). Two PID controllers are connected in series between the lock-in amplifier (6) and the laser; at the same time, the second output terminal of the PID controller closer to the lock-in amplifier (6) is electrically connected to the laser; a switch is provided between the laser and the two PID controllers.

2. The compact laser frequency automatic long-term locking system according to claim 1, characterized in that, The PID controller includes a first PID controller (741) and a second PID controller (742). The output terminal of the lock-in amplifier (6) is electrically connected in series with a low-pass filter (72), a first PID controller (741), a second switch (702), a third switch (703), and the first input terminal of the laser. The second output terminal of the first PID controller (741) is electrically connected to the second PID controller (742). The output terminal of the second PID controller (742) is electrically connected in series to the fourth switch (704), the fifth switch (705), and the second input terminal of the laser.

3. The compact laser frequency automatic long-term locking system according to claim 2, characterized in that, Two independent bias voltages (73) are provided between the second switch (702) and the third switch (703), and a triangular wave signal source (75) is electrically connected to one of the bias voltages (73) through the first switch (701). An independent bias voltage (73) is provided between the fourth switch (704) and the fifth switch (705).

4. The compact laser frequency automatic long-term locking system according to claim 3, characterized in that, A voltage limiting element (76) is provided between the fifth switch (705) and the laser source (8), and between the third switch (703) and the laser source (8).

5. The compact laser frequency automatic long-term locking system according to claim 1, characterized in that, The output terminal of the PD detector (42) is electrically connected to a signal amplifier (52), and the output terminal of the signal amplifier (52) is electrically connected to a lock-in amplifier (6).

6. A compact laser frequency automatic long-term locking method, based on the compact laser frequency automatic long-term locking system of claim 1, characterized in that, Includes the following steps: S1: Adjust the center wavelength Adjust the center wavelength of the laser to be near the target spectral line; S2: Set the triangular wave scanning voltage and scan frequency Start scanning; S3: Set feature parameters, number of spectral features , Amplitude 'a', Distance threshold ; Using the target spectral line as a reference point, the system automatically locates the target spectral line by detecting the distance between the reference point and multiple spectral lines on both sides. The number of spectral lines on the left and right sides is set to... , , representing the number of spectral lines that need to be identified and calculated; amplitude 'a' is the minimum amplitude required to identify the spectral lines; and distance threshold... This is the main basis for judging spectral lines; S4: Determine whether the calculated feature parameters meet the threshold and record the scan voltage value. ; S5: Set the target spectral line voltage value and enable PID control, i.e., fast PID and slow PID control; The scan voltage value recorded in step S4 Add it to the PZT port and simultaneously enable fast PID and slow PID to achieve automatic peak locking of the laser wavelength.

7. The compact laser frequency automatic long-term locking method according to claim 6, characterized in that, Step S4 specifically involves defining a distance value based on the frequency difference between the target spectral line and the first spectral line on the left or right. The distances between other spectral lines are defined as follows: Multiples of this value are used to obtain the distance values ​​from multiple spectral lines to the target spectral line. When the distance values ​​on the left and right sides simultaneously meet the set threshold coefficient At that time, the position of the corresponding target spectral line can be found and the scanning voltage value at that position can be recorded. .

8. The compact laser frequency automatic long-term locking method according to claim 6, characterized in that, Step S5 specifically involves: the input signal of the fast PID is the target spectral line signal, i.e., the frequency discrimination signal; the fast PID adjusts the laser's PZT interface, quickly responds to the frequency discrimination signal and locks it to the zero-crossing point; the slow PID uses the output value of the fast PID as input, and feeds back the output value of the fast PZT towards zero by sending a feedback signal to the laser's Temp. port.

9. The compact laser frequency automatic long-term locking method according to claim 8, characterized in that, It also includes an automatic relocking procedure after the lock is lost: S6: Determine if the lock status is locked; By monitoring the peak-to-peak value of the frequency discrimination signal, it can be determined whether it is in a locked state. When locked, the peak-to-peak value should be significantly greater than 0; When the lock is lost, the peak-to-peak value is a smaller value that approaches 0; S7: Disable PID control after lockout; Upon detecting a current loss of lock, the PID control is immediately turned off, the current PID output value is memorized and set as the offset value, and it is directly loaded into the PZT port and Temp. port of the laser to ensure that the laser frequency does not change abruptly. S8: Perform a small-range oblique wave scan; Custom amplitude and frequency The search is conducted to find the target spectral line. Since the frequency of the laser changes gradually, only a single target spectral line exists in the vicinity. S9: Zero-crossing point identification of absorption lines; identify the zero-crossing point of the target spectral line and record the corresponding voltage value. . S10: After loading the voltage value recorded in step 9, start PID control; The process of steps S6 to S10 is repeated to ensure that the laser frequency is locked for a long time.

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