2.05μm Solid-State Single-Frequency Laser Frequency Stabilization System Based on CO2 Gas

Abstract

The present application relates to a kind of 2.05 micrometer seed laser frequency stabilization system of integral path difference absorption radar based on carbon dioxide concentration detection.The system includes fiber-coupled LD pumped Tm,Ho:YLF reference main laser,reference main laser closed-loop feedback control system,fiber-coupled LD pumped Tm,Ho:YLF slave laser,slave laser optical phase-locked loop closed-loop feedback control system.The present application uses the R30 absorption line of carbon dioxide gas as reference standard,locks 2.05 μm solid-state single-frequency reference main laser to 2050.967 nm,while using master-slave laser frequency tracking locking scheme,obtains the frequency deviation of microwave radio frequency band based on optical phase-locked loop method,combines the frequency deviation of frequency-stable reference main laser and slave laser,locks the frequency deviation to 2GHz external microwave radio frequency reference source,realizes the frequency locking of slave laser.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and more specifically to a 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas. Background Technology

[0002] Global climate change is a major environmental issue affecting human economic and social development. Improving the accuracy of CO2 detection is of great significance for scientific research on the carbon cycle and the prediction of global climate change. Differential absorption lidar (DIAL) is considered one of the most accurate methods for CO2 concentration detection; therefore, obtaining a high-precision and high-stability laser is crucial.

[0003] There are two frequency reference standards for laser frequency stabilization. The first is to lock the laser frequency to the resonance absorption lines of atoms and molecules, including methods such as saturation absorption spectroscopy, polarization spectroscopy, modulation-to-transfer spectroscopy, and frequency modulation spectroscopy. The second is to use an ultra-stable, high-precision Fabry-Perot cavity as a frequency reference to lock the laser frequency. Because the atomic and molecular absorption spectroscopy method has advantages over the ultra-stable optical resonator method, such as specific absorption lines, better frequency stability, stronger anti-interference ability, and simpler equipment, this invention selects atomic and molecular absorption spectroscopy as the frequency reference standard.

[0004] Frequency stabilization techniques based on atomic and molecular absorption spectra mainly include: saturated absorption spectroscopy, polarization spectroscopy, tune-to-transfer spectroscopy, and frequency modulation spectroscopy. The advantages of saturated absorption spectroscopy are: simple apparatus and excellent frequency stability and reproducibility. The disadvantages are: low signal-to-noise ratio of the discrimination signal and a wide stabilization linewidth due to the Doppler broadening effect. The advantages of polarization spectroscopy are: higher frequency stability compared to saturated absorption spectroscopy. The disadvantages are: only effective for closed-loop transitions and susceptible to external stray magnetic fields. The advantages of tune-to-transfer spectroscopy are: the tuned spectrum is generated using a four-wave mixing method, the discrimination signal is free from background noise interference, the discrimination curve has a narrow linewidth, the locked frequency linewidth is narrow, and the frequency stability is high. The disadvantages are: the electro-optic phase modulator is susceptible to residual amplitude modulation effects caused by environmental temperature and other factors. Frequency modulation spectroscopy has a simple and compact structure, a high signal-to-noise ratio of the detection signal, can suppress low-frequency noise, and has a wide control bandwidth and extremely high frequency stability.

[0005] Therefore, designing a 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas remains an urgent problem to be solved. Summary of the Invention

[0006] The technical problem this invention aims to solve is: to better address global climate change, it is necessary to improve the accuracy of CO2 detection. Differential absorption lidar (DIAL) is considered one of the most accurate methods for CO2 concentration detection. Therefore, obtaining a high-precision and high-stability laser is crucial. This invention proposes a high-precision and high-stability laser to improve the accuracy of CO2 concentration detection.

[0007] To achieve the aforementioned objectives of this invention, an embodiment of the present invention provides a 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas, comprising: a Tm,Ho:YLF master laser unit, including a 2.05μm Tm,Ho:YLF master laser obtained by pumping a Tm,Ho:YLF crystal with a 792nm fiber-coupled LD through a coupling system; a master laser closed-loop feedback control unit connected to the Tm,Ho:YLF master laser unit; a Tm,Ho:YLF slave laser unit, including a 2.05μm Tm,Ho:YLF slave laser obtained by pumping a Tm,Ho:YLF crystal with a 792nm fiber-coupled LD through a coupling system; and a slave laser optical phase-locked loop closed-loop feedback control system connected to the Tm,Ho:YLF slave laser unit.

[0008] Among them, the Tm,Ho:YLF main laser and the Tm,Ho:YLF are inserted into the resonant cavity of the laser by two FP etalons for mode selection; and PZT piezoelectric ceramic plates are attached to the output mirror as drivers to change the cavity length.

[0009] Furthermore, the Tm,Ho:YLF master laser unit also includes a first optical isolator, a first half-wave plate, and a first polarizing beam splitter sequentially disposed after the Tm,Ho:YLF master laser; the laser beam split by the first polarizing beam splitter is injected into the closed-loop feedback control unit of the master laser in one path, and serves as the frequency reference standard of the Tm,Ho:YLF slave laser in the other path.

[0010] The first optical isolator ensures unidirectional laser operation and prevents laser backflow from damaging the laser.

[0011] Further, the main laser closed-loop feedback control unit includes: a photoelectric phase modulator; a quartz gas chamber for receiving the laser modulated by the photoelectric phase modulator, the quartz gas chamber being filled with pure CO2 gas at a predetermined pressure; a first photodetector for receiving the laser emitted through the quartz gas chamber and converting the laser signal into a transmitted electrical signal carrying resonant phase information; a first phase detector for receiving the electrical signal from the first photodetector and for mixing, filtering, and demodulating the electrical signal with the local oscillator frequency to obtain an error signal; a first signal generator for generating two radio frequency signals, one input to the photoelectric phase modulator and the other providing a local oscillator signal to the first phase detector; a first PID servo circuit for obtaining a feedback control signal based on the error signal from the first phase detector; and a main laser driver for receiving the feedback control signal from the first PID servo circuit and adjusting the output frequency of the Tm,Ho:YLF main laser according to the feedback control signal.

[0012] The first photodetector can be an 82MHz resonant photodetector.

[0013] The first phase detector consists of a power amplifier, a mixer, and a filter.

[0014] Furthermore, the main laser closed-loop feedback control unit also includes a lens, a second half-wave plate, a second polarizing beam splitter, and a mirror assembly arranged sequentially. The mirror assembly is located on one side of the quartz gas chamber.

[0015] Furthermore, the mirror assembly includes four 45° mirrors.

[0016] Furthermore, the Tm,Ho:YLF laser unit further includes: a second optical isolator, a third half-wave plate, and a second polarizing beam splitter sequentially disposed after the Tm,Ho:YLF laser; the laser beam split by the second polarizing beam splitter is injected into a single-frequency pulse laser through an optical switch, and the other is used for frequency locking by an optical phase-locked loop.

[0017] The second optical isolator ensures unidirectional laser operation and prevents laser backflow from damaging the laser.

[0018] Furthermore, the laser optical phase-locked loop closed-loop feedback control system includes: a second photodetector for converting the optical signal after combining two laser beams into an electrical signal and generating a beat frequency; a frequency divider for preprocessing the beat frequency signal from the second photodetector; a second phase detector for receiving the electrical signal from the frequency divider and for mixing, filtering, and demodulating the electrical signal with a local oscillator reference signal to obtain an error signal; a second signal generator for providing a reference frequency signal to the second phase detector; a second PID servo circuit for obtaining a feedback control signal based on the error signal from the second phase detector; and a laser slave driver for receiving the feedback control signal from the second PID servo circuit and adjusting the output frequency of the Tm,Ho:YLF laser slave according to the feedback control signal.

[0019] The frequency divider is used to reduce noise in the free-running signal.

[0020] Furthermore, it also includes a third polarizing beam splitter and a fourth half-wave plate disposed in front of the second photodetector.

[0021] Furthermore, the feature is that the Tm,Ho:YLF master laser and the Tm,Ho:YLF are inserted into the cavity of the laser as FP etalon.

[0022] Furthermore, it also includes a drive circuit, which is used for current driving and temperature control; after the current signal set by the main control circuit is input, it is filtered and driven by the current driving circuit, and the output is the working current of the laser. The current feedback signal is passed through a low-noise feedback circuit and serves as the feedback current of the laser, which is superimposed with the working current as the laser current output; the thermistor of the laser reflects the temperature of the laser, and a PI control network is used to control the working temperature of the laser.

[0023] This invention uses a CO2 gas absorption cell as a reference source and employs frequency modulation spectroscopy to lock the frequency of the reference master laser to the CO2 R30 absorption peak (2050.967nm). It adopts a master-slave laser frequency tracking and locking scheme and, based on the optical phase-locked loop method, obtains the frequency deviation of the microwave radio frequency band by combining the slave laser with the frequency-stable reference master laser, and locks this frequency deviation to a 2GHz external microwave radio frequency reference source. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the laser frequency stabilization system according to an embodiment of the present invention;

[0026] Figure 2 This is a schematic diagram illustrating the principle of closed-loop feedback control of the main laser in an embodiment of the present invention.

[0027] Figure 3 This is a schematic diagram illustrating the principle of closed-loop feedback control of the laser in an embodiment of the present invention.

[0028] Figure 4 This is a schematic diagram of the quartz gas chamber according to an embodiment of the present invention.

[0029] Explanation of reference numerals in the attached figures:

[0030] 1-Tm,Ho: YLF master laser, 2-First optical isolator, 3-First half-wave plate, 4-First polarizing beam splitter, 5-Photoelectric phase modulator, 6-Lens, 7-Second half-wave plate, 8-Second polarizing beam splitter, (9, 10, 11, 12)-45° reflector, 13-Quartz gas cell, 14-First photodetector, 15-First PID servo circuit, 16-First phase detector, 17-First signal generator, 18-Tm,Ho:YLF From the laser, 19-second optical isolator, 20-third half-wave plate, 21-second polarizing beam splitter, 22-fourth half-wave plate, 23-third polarizing beam splitter, 24-second photodetector, 25-second PID servo circuit, 26-second phase detector, 27-second signal generator, 28-frequency divider, 29-master laser driver, 30-slave laser driver, 31-temperature control circuit, 32-filter, 33-mixer, 34-power amplifier. Detailed Implementation

[0031] The description of the embodiments in this specification should be taken in conjunction with the accompanying drawings, which should form part of the complete specification. In the drawings, the shape or thickness of the embodiments may be exaggerated and may be indicated in a simplified or convenient manner. Furthermore, parts of the various structures in the drawings will be described separately; it is worth noting that elements not shown in the figures or not described in words are in a form known to those skilled in the art.

[0032] The descriptions of the embodiments herein, including any references to directions and orientations, are for ease of description only and should not be construed as limiting the scope of the invention. The following description of preferred embodiments involves combinations of features, which may exist independently or in combination; the invention is not particularly limited to the preferred embodiments. The scope of the invention is defined by the claims.

[0033] This invention employs frequency modulation spectral stabilization technology to achieve frequency stabilization.

[0034] CO2 gas absorbers generally come in two forms: free-space type and hollow fiber type. Compared with free-space type absorbers, hollow fiber type absorbers suffer from more severe losses and are prone to background noise. Therefore, considering noise and frequency stability, this invention selects the free-space type gas absorber.

[0035] The principle of the laser frequency stabilization system of this invention is as follows:

[0036] Step 1: The 792nm semiconductor LD is pumped through a coupling shaping system to obtain a 2.05μm reference Tm,Ho:YLF master laser 1. Two FP etalons are inserted into the laser cavity for mode selection.

[0037] Step 2: The output laser is split into two paths after passing through the first optical isolator 2, the first half-wave plate 3, and the first polarization beam splitter 4. One laser path is injected into the closed-loop feedback control system of the reference master laser, and the other laser path provides the frequency standard for Tm,Ho:YLF slave laser 18.

[0038] Step 3: The laser entering the closed-loop feedback control system of the reference master laser is first phase modulated by the photoelectric phase modulator 5 (photoelectric phase modulator, i.e., EOM).

[0039] Step 4: After photoelectric phase modulation, the laser sequentially passes through collimating lens 6, second half-wave plate 7, second polarizing beam splitter prism 8, and a 45° reflecting mirror 9 before entering a four-way quartz gas chamber 13 filled with pure carbon dioxide at a certain pressure. A pair of orthogonally placed right-angle prisms allows the laser to circulate four times within the quartz gas chamber 13 (see reference). Figure 4 (The orthogonally placed right-angled prisms are located on both sides of the quartz gas cell, which doubles the absorption length.)

[0040] Step 5: After the laser travels a distance in the quartz gas chamber 13, it passes through the 45° reflectors 10, 11, and 12 in sequence and is incident on the first photodetector 14, converting the optical signal into an electrical signal.

[0041] Step 6: The first signal generator 17 generates two radio frequency signals. One signal is amplified and used as the modulation frequency signal to drive the photoelectric phase modulator 5. The other signal is passed through a phase shifter and used to provide the demodulation local oscillator signal for the first phase detector 16.

[0042] Step 7: The transmitted electrical signal carrying the resonant phase information obtained by the first photodetector 14 enters the first phase detector 16. After mixing and filtering the electrical signal with the local oscillator frequency provided by the first signal generator 17, the error signal is obtained by demodulation.

[0043] Step 8: The error signal generated by the first phase detector 16 enters the first PID servo circuit 15. The first PID servo circuit 15 obtains a feedback control signal by acquiring and processing the error signal, and this signal is applied to the main laser driver 29 (reference). Figure 2 The feedback driving voltage is formed in a PZT driver to realize closed-loop control of the output laser frequency of the Tm,Ho:YLF master laser 1, thereby achieving frequency locking of the reference master laser.

[0044] Step 9: Tm,Ho:YLF The laser output from laser 18 passes sequentially through the second optical isolator 19, the third half-wave plate 20, and the second polarization beam splitter 21, splitting into two paths. One path is injected into the subsequent single-frequency pulse laser (not shown) through an optical switch, while the other path is used for frequency locking by an optical phase-locked loop.

[0045] Step 10: One of the laser beams used for frequency locking in the optical phase-locked loop is combined with one of the laser beams output from the reference master laser. The combined beam passes through the fourth half-wave plate 22 and the third polarizing beam splitter 23 in sequence, and then enters the second photodetector 24. The optical signal is converted into an electrical signal, and a beat frequency is generated on the second photodetector 24.

[0046] Step 10: After preprocessing by frequency divider 28, the beat frequency signal enters the second phase detector 26, and then passes through filter 32 and power amplifier 34 (see reference). Figure 3 The error signal is obtained by phase detection and filtering of the local oscillator reference signal provided by the signal generator in mixer 33.

[0047] Step 11: The second PID servo circuit 25 acquires and processes the error signal to obtain a feedback control signal, which is applied to the laser driver 30 (reference). Figure 3 This can be a PZT driver used to adjust the output frequency of the laser 18 (Tm,Ho:YLF) to form a closed loop, thereby achieving laser frequency locking.

[0048] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas, characterized in that, include: The Tm,Ho:YLF master laser unit includes a 2.05μm Tm,Ho:YLF master laser obtained by pumping a Tm,Ho:YLF crystal through a coupling system using a 792nm fiber-coupled LD (1). The main laser closed-loop feedback control unit is connected to the Tm,Ho:YLF main laser unit; The Tm,Ho:YLF laser unit comprises a 2.05μm Tm,Ho:YLF laser obtained by pumping a Tm,Ho:YLF crystal through a coupling system using a 792nm fiber-coupled LD. The laser optical phase-locked loop closed-loop feedback control system is connected to the Tm,Ho:YLF laser unit. The main laser closed-loop feedback control unit includes: Photoelectric phase modulator (5); Quartz gas chamber (13) receives laser light modulated by the photoelectric phase modulator (5), and the quartz gas chamber (13) is filled with pure CO2 gas at a predetermined pressure. The first photodetector (14) receives the laser emitted through the quartz gas chamber (13) and converts the laser signal into a transmitted electrical signal carrying resonant phase information. The first phase detector (16) receives the electrical signal from the first photodetector (14) and uses it to mix and filter the electrical signal with the local oscillator frequency and then demodulate it to obtain an error signal. The first signal generator (17) generates two radio frequency signals, one of which is input to the photoelectric phase modulator (5), and the other provides a local oscillator signal to the first phase detector (16); The first PID servo circuit (15) is used to obtain a feedback control signal based on the error signal of the first phase detector (16); The main laser driver (29) receives the feedback control signal from the first PID servo circuit (15) and adjusts the output frequency of the Tm,Ho:YLF main laser (1) according to the feedback control signal. The main laser closed-loop feedback control unit also includes a lens (6), a second half-wave plate (7), a second polarizing beam splitter (8), and a set of mirrors (9, 10, 11, 12) arranged in sequence. The reflector group (9, 10, 11, 12) is located on one side of the quartz gas chamber.

2. The 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas according to claim 1, characterized in that, The Tm,Ho:YLF main laser unit also includes a first optical isolator (2), a first half-wave plate (3) and a first polarizing beam splitter (4) sequentially disposed after the Tm,Ho:YLF main laser (1). The laser beam split by the first polarization beam splitter (4) is injected into the closed-loop feedback control unit of the main laser in one path, and serves as the frequency reference standard of the Tm,Ho:YLF slave laser (18) in the other path.

3. The 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas according to claim 1, characterized in that, The mirror group (9, 10, 11, 12) includes four 45° mirrors.

4. The 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas according to claim 1, characterized in that, The Tm,Ho:YLF laser unit also includes a second optical isolator (19), a third half-wave plate (20), and a second polarizing beam splitter (21) sequentially disposed after the Tm,Ho:YLF laser (18). The laser beam split by the second polarizing beam splitter (21) is injected into a single-frequency pulse laser through an optical switch, and the other path is used for frequency locking by an optical phase-locked loop.

5. The 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas according to claim 1, characterized in that, The laser optical phase-locked loop closed-loop feedback control system includes: The second photodetector (24) is used to convert the optical signal after the two laser beams are combined into an electrical signal and generate a beat frequency; Frequency divider (28) is used to preprocess the beat frequency signal of the second photodetector (24); The second phase detector (26) receives the electrical signal from the frequency divider (28) and uses it to mix and filter the electrical signal with the local oscillator reference signal and then demodulate it to obtain an error signal. The second signal generator (27) is used to provide a reference frequency signal for the second phase detector (26); The second PID servo circuit (25) is used to obtain a feedback control signal based on the error signal of the second phase detector (26); The laser driver (30) receives feedback control signals from the second PID servo circuit (25) and adjusts the output frequency of the laser (18) according to the feedback control signals.

6. The 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas according to claim 5, characterized in that, It also includes a third polarizing beam splitter (23) and a fourth half-wave plate (22) disposed in front of the second photodetector (24).

7. The 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas according to any one of claims 1-6, characterized in that, The Tm,Ho:YLF master laser (1) and the Tm,Ho:YLF are inserted into the cavity of the laser (18) using the FP etalon.

8. The 2.05μm solid-state single-frequency laser frequency stabilization system based on CO2 gas according to any one of claims 1-6, characterized in that, It also includes a drive circuit, which includes a current drive circuit and a temperature control circuit. The current signal set by the main control circuit is input, filtered and driven by the current driving circuit, and output as the laser's operating current. The current feedback signal is fed through a low-noise feedback circuit and used as the laser's feedback current. It is then superimposed with the operating current to become the laser's current output. The laser's thermistor reflects its temperature, and a PI control network is used to control the laser's operating temperature.

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