Laser frequency locking device for lithium isotope separation and use method

Abstract

The invention discloses a laser frequency locking device for lithium isotope separation and a use method thereof, and relates to the technical fields of lithium isotope separation and laser frequency stabilization, comprising a laser module, an optical device, a lithium atom gas chamber, and a frequency locking circuit module; the laser module comprises a tunable semiconductor laser with a central wavelength of 671nm, an optical isolator, and a coupled shaping optical path device; the optical device is used for locking the frequency of the laser; the high-temperature and high-density lithium atom gas chamber comprises a vacuum, gas distribution, water cooling, temperature control, and air blowing subsystems, which can prevent lithium vapor from adhering to a glass window and blocking the optical path; the frequency locking circuit module is used for generating an error signal and then feeding it back negatively to the laser, modulating and locking the output light frequency of the laser; the device has the characteristics of a simple optical path and easy adjustment; the high-temperature and high-density lithium atom gas chamber not only effectively avoids the problem of lithium atoms adhering to glass windows on both sides and blocking the optical path, but also avoids lithium oxidation to obtain purer lithium atom vapor.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium isotope separation and laser frequency stabilization, and more specifically to a laser frequency locking device for lithium isotope separation and a use method thereof. Background Art

[0002] With the growth of the global population and the rapid development of the economy, the demand for energy is also increasing, among which nuclear energy is particularly prominent. 6Li and 7Li are two stable isotopes of lithium, with natural abundances of 7.52% and 92.48% respectively. In a nuclear fusion reactor, 6Li is bombarded with neutrons to produce tritium and helium, thereby achieving tritium proliferation, making 6Li a value-added material for controlled nuclear fusion. At the same time, 7Li is used as a core coolant and thermal conductor for nuclear fission reactors, and can be used as a medium for the fourth generation of thorium-based molten salt reactors. Therefore, countries around the world attach great importance to the separation of lithium isotopes.

[0003] Among the currently published methods for separation of lithium isotopes, the only method for large-scale production of lithium isotopes is the lithium amalgam method. However, the huge potential harm to human body and environment caused by the large amount of mercury used in the separation process restricts its development. The processes of the other chemical methods are cumbersome, requiring multi-stage phase separation, concentration, phase conversion and other operations, and generating a large amount of waste that needs to be processed, which is not conducive to industrial production. The physical method of separating lithium isotopes based on the selective excitation of lithium isotopes by laser beams has high selectivity, short process flow, no waste generation, and is green and environmentally friendly, and has received widespread attention. Among them, the narrow line width and frequency stability of the laser have a crucial influence on the separation of lithium isotopes.

[0004] Tunable semiconductor lasers can use the feedback oscillation of the laser in the external cavity to compress the line width of the laser to the MHz or even hundreds of kHz level. However, due to the instability of the external cavity environment and the thermal effects of components such as the laser tube, the frequency of the laser drifts a lot over a long period of time, usually reaching an offset of about 10 MHz per hour, and may even cause the laser mode to change. The frequency instability cannot meet the technical requirements of lithium isotope separation. Therefore, it is necessary to lock the laser frequency to narrow the laser line width while reducing the long-term drift of the frequency.

[0005] Saturation absorption frequency locking is the most commonly used frequency locking method, with a simple device and good frequency stability. The most important thing in this method is the overlap technology of the atomic gas chamber and the opposing light beam. For lithium metal atoms, they have strong chemical activity and need to produce high-density atomic vapor at high temperatures. When the temperature rises, the high-density lithium atoms move too fast and easily adhere to the glass window to block the light path. The glass window needs to be cleaned or replaced regularly, which seriously affects the long-term stability of the experiment.

[0006] The traditional saturation absorption frequency locking method (application number: CN201510110737.3; CN201520006897.9; CN202020778135.1) is to divide the laser output by the laser into three beams of light. Two weaker parallel beams of light pass through the atomic gas chamber, which are called reference light and detection light. There is also a stronger beam of light, called pump light. The pump light is injected into the atomic gas chamber from the opposite direction and is reflected by the detection light to form a saturated absorption spectrum structure. The reference light passing through the atomic gas chamber enters the photodiode and contains Doppler broadening background information. The saturated absorption spectrum information obtained by the detection light is compared with the Doppler background signal obtained by the reference light to obtain the saturated absorption spectrum information without Doppler broadening. However, this method is not ideal for engineering applications. First, multiple beams increase the complexity and volume of the device. Second, the degree of overlap of the beams depends on the thickness and surface reflectivity of the splitting glass sheet, and the overlap is not easy to adjust.

[0007] The dichroic atomic vapor laser frequency locking method (application number: CN201811302407.4; CN202111272869.8; CN202010886569.8; CN201510942254.X) uses the difference in the absorption of linearly polarized light by the atomic Zeeman energy level to detect the change in polarization to lock the frequency. Although it reduces the number of optical path components compared to traditional methods, it requires a magnetic field of hundreds to thousands of gauss to produce Zeeman splitting. This places very high demands on coil winding, heat dissipation, and power supply, reducing the ease of operation of engineering applications.

[0008] In addition to the two common laser frequency locking methods mentioned above, there are also methods to achieve laser frequency stabilization by using acousto-optic modulation or electro-optic modulation and ultra-stable cavity in the saturated optical path (application numbers: CN201510725416.4; CN202310492175.8; CN202211525315.9). However, the limited frequency deviation range, expensive modulators, complex frequency locking operations and high precision limit the possibility of its widespread application.

[0009] In summary, the glass window of the high-temperature, high-density lithium atom gas chamber is prone to blocking the laser optical path, and the glass window needs to be cleaned or replaced regularly, which seriously affects the long-term stability of the experiment. At the same time, the existing saturated absorption frequency-locked optical path is difficult to adjust, which limits the feasibility of engineering applications. In view of this, the patent of this invention proposes a corresponding solution. Summary of the invention

[0010] The purpose of the present invention is to propose a laser frequency locking device and a method for using it for lithium isotope separation in view of the problems in the prior art that the glass window of a high-temperature, high-density lithium atom gas chamber is easy to block the laser optical path and the saturation absorption frequency locking optical path is difficult to adjust.

[0011] In order to achieve the above-mentioned purpose, the present invention specifically adopts the following technical scheme: a laser frequency locking device for lithium isotope separation, comprising a laser module, an optical device, a lithium atom gas chamber, and a frequency locking circuit module. The laser module comprises a tunable semiconductor laser, an optical isolator, and a coupled shaping optical path device. The optical device comprises a half-wave plate, a polarization beam splitter prism (PBS), a reflector, a quarter-wave plate, and a photodiode. The optical device is used to split the laser beam emitted by the laser module and then inject it into a high-temperature and high-density lithium atom gas chamber, to collide the light beams, and to convert the optical signal into an electrical signal. The lithium atom gas chamber comprises a vacuum subsystem, a gas distribution subsystem, a water cooling subsystem, a temperature control subsystem, an air blowing subsystem, a four-way pipe, a screen, a glass window, and a rectangular positioning block.

[0012] Preferably, the central wavelength of the tunable semiconductor laser is 671nm. The tunable semiconductor laser is composed of a 671nm laser gain chip, a volume grating, a thermoelectric cooler and a thermistor element, and the components are all mounted on the same substrate. The overall structure is compact and has good environmental applicability.

[0013] Preferably, the vacuum subsystem includes a vacuum acquisition device and a measuring device, the vacuum acquisition device is composed of a molecular pump, a mechanical pump, and a vacuum pump, and the measuring device is composed of an ionization and resistance vacuum gauge.

[0014] Preferably, the gas distribution subsystem includes an argon gas cylinder, a pressure reducing valve, a connecting gas pipe, and a vacuum fine-tuning valve.

[0015] Preferably, the water cooling subsystem includes four stainless steel rings, four stainless steel tubes, and four pagoda joints. Each stainless steel tube head is welded with a pagoda joint with an outer diameter of 8 mm, which can be connected to an external water cooler to achieve the purpose of circulating cooling.

[0016] Preferably, the temperature control subsystem includes a nickel-chromium heating wire, a constant current power supply, a K-type thermocouple, a thermometer, and an insulation layer. The K-type thermocouple and the thermometer perform temperature measurement and feed back to the constant current power supply to complete temperature control.

[0017] Preferably, the blowing subsystem includes a blowing pipeline and a vacuum fine-tuning valve.

[0018] Preferably, the frequency locking circuit module includes a modulation signal source, a multiplier, a low-pass filter, and a proportional integral differential (PID) circuit. The error signal generated is observed by an oscilloscope and negatively fed back to the laser to modulate and lock the output light frequency of the laser.

[0019] A method for using a laser frequency locking device for lithium isotope separation comprises the following steps:

[0020] 1. Open one side flange of the four-way pipe, place the screen in the center of the four-way pipe, connect the argon bottle to one of the flanges of the four-way pipe through the connecting pipe and the vacuum fine-tuning valve, and then introduce argon protective gas. Place the lithium metal in the center of the cavity of the four-way pipe in the protective gas atmosphere, then re-tighten the original flange and close the gas distribution system;

[0021] 2. Start the vacuum pumping system and draw the vacuum degree in the cavity of the four-way pipe to the vacuum value required by the experiment;

[0022] 3. Connect the water cooling subsystem to the external water pipe and water cooler through the pagoda joint, and start the water cooler to cool the pipe wall;

[0023] 4. Start the laser module 1, set the temperature and current of the laser through the thermoelectric cooler and current controller, set it to room temperature and above the threshold current point, wait for the 671nm laser gain chip to emit light, repeatedly adjust the threshold and reduce the current, and turn on the wavelength meter to observe the laser wavelength at 670nm at room temperature. By slightly adjusting the current and slight temperature changes, find a target operating point that does not jump for several GHz continuously, and finally obtain a laser with an output power that meets the experimental requirements and an output wavelength covering the D-line transition of lithium isotopes;

[0024] 5. Adjust the optical path device. By adjusting the reflector behind the quarter-wave plate, the pump light can overlap with its original optical path after passing through the quarter-wave plate and the second reflector. With the help of two apertures as auxiliary tools, the pump light that first enters the atomic gas chamber and the light reflected by the second reflector can be adjusted to pass through the center of the two apertures, thus achieving the overlap of the optical paths.

[0025] 6. Turn off the vacuum system and start the gas distribution system. Argon gas enters the pipeline and the air blowing pipe through the argon gas bottle, the pressure reducing valve and the vacuum fine-tuning valve. At the same time, the temperature control subsystem heats the glass window at low temperature and the high temperature heating area in the center of the pipeline. Lithium metal evaporates into lithium metal vapor in the central high temperature heating area at a given temperature.

[0026] 7. Turn on the oscilloscope, add a low-frequency sinusoidal signal to the laser drive current through the modulation signal source in the frequency locking circuit module, and obtain the modulated differential error signal;

[0027] 8. Send the differential error signal to the differential error signal filter, use the proportional integral differential (PID) circuit to adjust the differential error signal to the appropriate feedback gain and bandwidth, and finally send the differential error signal to the laser diode drive current to modulate and lock the output light frequency of the laser.

[0028] Preferably, when argon protective gas is introduced in step one, the amplitude of the saturated absorption spectrum is observed by an oscilloscope to find that when the argon gas is injected to a vacuum degree displayed as a numerical value, the amplitude of the spectrum line is the highest, thereby achieving the buffering purpose while obtaining a good frequency-locking signal.

[0029] The beneficial effects of the present invention are as follows:

[0030] 1. Aiming at the problem that the glass window of the high-temperature and high-density lithium atom gas chamber is easy to block the laser optical path, the length of the lithium atom gas chamber is increased, the probability of interaction between light and steam is increased, and the intensity of the frequency locking signal is increased, thereby improving the frequency locking accuracy. Secondly, the entire gas chamber is divided into a central high-temperature heating zone-cooling zone-window low-temperature heating zone. Under the cooperation of water cooling, temperature control subsystems and wire mesh, the high-temperature lithium vapor in the central area condenses into lithium liquid in the cooling zone and is brought back to the center of the pipeline by the surface tension of the wire mesh. When approaching the first and fourth water cooling subsystems, the diameter of the pipeline becomes smaller and flared, which is more conducive to the reflux of lithium liquid. The three measures of argon environment in the pipeline, heated glass window, and argon blowing into the window avoid the possibility of the glass window being covered by residual lithium vapor escaping from the cooling zone to a greater extent. In addition, the heating zone is realized by 2080 nickel-chromium wire covered with mica wrapping and glass fiber braiding layer. Double wire winding is used during winding to effectively offset the magnetic field, thereby reducing the interference of Zeeman splitting of lithium atomic energy level on frequency locking. This solution completely solves the problem of laser blocking by the glass window of the high-temperature and high-density lithium atom gas chamber, which is beneficial to the long-term stability of the experiment and further improves the accuracy of laser frequency locking.

[0031] 2. The tunable semiconductor laser with a central wavelength of 671 nm, the internal volume holographic grating has the characteristics of narrow spectral response, small spatial acceptance angle, high optical damage threshold, and mechanical and thermal stability. It shows excellent stability at high temperature, and the frequency selection temperature drift coefficient is small, which can be used to make lasers with more reliable environmental adaptability. The laser is divided into two beams of low-power reference light and high-power pump light through a polarization beam splitter (PBS) and a half-wave plate. The reference light is directly injected into the high-temperature and high-density lithium atomic gas chamber after passing through the first reflector, and is detected by the second photodiode. The pump light is reflected by the PBS and incident on the atomic gas chamber. The laser emitted from the atomic gas chamber is detected by the first photodiode after it is self-contradicted by the quarter-wave plate and the second reflector. The signals obtained by the two photodiodes are input into the frequency locking circuit module for processing. In the overlap technology of the overlapping beams, two apertures are used as auxiliary tools. When the pump light that first enters the atomic gas chamber and the light reflected by the second reflector are adjusted to pass through the center of the two apertures, the optical path overlap can be achieved. Compared with the existing solutions, the number of lenses is reduced, and the optical paths are overlapped only by adjusting the second reflector, which reduces the difficulty of adjusting the frequency-locked optical path and improves the stability and operability of the optical path system. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 It is a schematic diagram of the laser frequency locking device system for lithium isotope separation of the present invention;

[0033] Figure 2 It is a schematic diagram of the high-temperature and high-density lithium atom gas chamber of the present invention.

[0034] Figure numerals: 1. Laser module; 2. Half-wave plate; 3. Polarization beam splitter (PBS); 4. First reflector; 5. First photodiode; 6. Second photodiode; 7. Quarter-wave plate; 8. Second reflector; 9. Lithium atom gas chamber; 10. Frequency-locked circuit module; 11. Oscilloscope; 12. Four-way pipe; 13. High borosilicate glass window on the left; 14. Window low-temperature heating zone; 15. CF35 inner welding flange on the left; 16. Air blowing port; 17. Blowing argon gas; 18. Rectangular positioning block; 19. First flange; 20. First water cooling subsystem; 21. Third flange; 22. CF35 to CF16 Double-sided flange; 23. Vacuum fine-tuning valve; 24. Connecting air pipe; 25. Pressure reducing valve; 26. Argon gas bottle; 27. Fourth flange; 28. Four-way flange; 29. ​​Ionization vacuum gauge; 30. Resistance vacuum gauge; 31. CF35 vacuum baffle valve; 32. Vacuum acquisition equipment; 33. Silk screen; 34. Second water-cooling subsystem; 35. Central high-temperature heating zone; 36. Constant current power supply; 37. K-type thermocouple; 38. Thermometer; 39. Insulation layer; 40. Third water-cooling subsystem; 41. Fourth water-cooling subsystem; 42. Second flange; 43. Right CF35 internal welding flange; 44. Right high borosilicate glass window. DETAILED DESCRIPTION

[0035] The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The exemplary embodiments and descriptions of the present invention are used to explain the present invention, but are not intended to limit the present invention.

[0036] See also Figure 1-2 , the present invention provides the following technical solutions:

[0037] A laser frequency locking device for lithium isotope separation comprises a laser module 1, an optical device, a lithium atom gas chamber 9, and a frequency locking circuit module 10.

[0038] 1. Laser module: Laser module 1 includes a tunable semiconductor laser with a central wavelength of 671 nm, an optical isolator, and a coupled shaping optical path device. The tunable semiconductor laser with a central wavelength of 671 nm is composed of a 671 nm laser gain chip, a volume grating, a thermoelectric cooler and a thermistor element. All components are mounted on a substrate. The overall structure is compact and has good environmental applicability. The optical isolator is a bipolar isolator with an isolation of 30 dB, and the coupled shaping optical path devices are a fast-axis collimator and a slow-axis collimator.

[0039] 2. Optical devices: The optical devices include a half-wave plate 2, a polarization beam splitter (PBS) 3, a first reflector 4, a second reflector 8, a quarter-wave plate 7, a first photodiode 5, and a second photodiode 6, which are used to split the laser beam emitted by the laser module and inject it into a high-temperature and high-density lithium atom gas chamber, collide the light beams, and convert the optical signal into an electrical signal.

[0040] 3. High-temperature and high-density lithium atom gas chamber: The lithium atom gas chamber 9 includes a vacuum subsystem, a gas distribution subsystem, a water cooling subsystem, a temperature control subsystem, an air blowing subsystem, a four-way pipe 12, a wire mesh 33, a glass window and a rectangular positioning block 18.

[0041] (1) The vacuum subsystem includes an ionization vacuum gauge 29, a resistance vacuum gauge 30, a CF35 vacuum baffle valve 31, and a vacuum acquisition device 32; the vacuum subsystem is connected to the four-way flange 28 through the fourth flange 27 and then connected to the four-way pipeline 12; the vacuum acquisition device 32 is connected to the four-way flange 28 through the CF35 vacuum baffle valve 31;

[0042] (2) The gas distribution subsystem includes a vacuum fine-tuning valve 23, a connecting gas pipe 24, a pressure reducing valve 25, and an argon gas cylinder 26; it is connected to the CF35 to CF16 double-sided flange 22 through a third flange 21 and then connected to the four-way pipeline 12;

[0043] (3) The water cooling subsystem includes a first water cooling subsystem 20, a second water cooling subsystem 34, a third water cooling subsystem 40, and a fourth water cooling subsystem 41; the four water cooling systems are composed of four 304 stainless steel rings, four 304 stainless steel pipes, and four pagoda joints. The stainless steel rings are welded to the outer wall of the four-way pipe 12, and each stainless steel ring has two stainless steel pipes with a diameter of 8 mm. Each stainless steel pipe head is welded with a pagoda joint with an outer diameter of 8 mm, which can be connected to an external water cooler to achieve the purpose of circulating cooling;

[0044] (4) The temperature control subsystem includes a temperature control window low temperature heating zone 14, a central high temperature heating zone 35, a constant current power supply 36, a K-type thermocouple 37, a thermometer 38, and a thermal insulation layer 39; the 2080 nickel-chromium heating wire is used to heat the window low temperature heating zone 14 and the central high temperature heating zone 35, and its outer layer has a mica wrapping and a glass fiber braiding layer. The wire is wound in double wires and wrapped in a thermal insulation layer 39. The K-type thermocouple 37 and the thermometer 38 complete the temperature measurement and feed back to the constant current power supply 36 to complete the temperature control;

[0045] (5) The air blowing subsystem includes an air blowing port 16, an argon gas blowing port 17, and a vacuum fine-tuning valve 23; the air blowing port 16 is welded to two CF35 internal welding flanges where the glass window is installed, and can be connected to an external argon gas system;

[0046] (6) The four-way pipe 12 is made of 304 stainless steel, which is resistant to baking, has a low outgassing rate, and is suitable for use in a vacuum environment. The four ends are all CF35 internal welding flanges. When the pipe approaches the first water cooling subsystem 20 and the fourth water cooling subsystem 41, the pipe diameter becomes smaller and becomes a bell-mouth shape;

[0047] (7) The wire mesh 33 is a 200-mesh stainless steel mesh, which is placed in the center of the cross-way pipe 12;

[0048] (8) The rectangular positioning blocks 18 are symmetrically welded on both sides of the outer wall of the first flange 19 and the second flange 42, and M6 threaded holes are opened in the middle of the four rectangular positioning blocks 18;

[0049] (9) The glass window includes a left high borosilicate glass window 13 and a right high borosilicate glass window 44. The glass window is made of high borosilicate glass. The left CF35 internal welding flange 15 and the right CF35 internal welding flange 43 are connected to the first flange 19 and the second flange 42 by bolts.

[0050] 4. Frequency locking circuit module: The frequency locking circuit module 10 is composed of a self-developed modulation signal source, a multiplier, a low-pass filter, and a proportional integral differential (PID) circuit. The error signal is observed by an oscilloscope 11 and negatively fed back to the laser to modulate and lock the output light frequency of the laser.

[0051] In this embodiment:

[0052] 1. In the laser module 1, the tunable semiconductor laser with a central wavelength of 671nm, the seed light emitted by the 671nm laser gain chip passes through the optical isolator and the coupled shaping optical path device, and then is split by the half-wave plate 2 and the polarization beam splitter prism (PBS) 3 into a high-power pump light and a relatively low-power reference light. The reference light enters the lithium atom gas chamber 9 through the first reflector 4, and the pump light only passes through the quarter-wave plate 7 and the second reflector 8 and overlaps with its original optical path to eliminate the Doppler background. Finally, the reflected light and the reference light enter the first photodiode 5 and the second photodiode 6 respectively, and are input into the frequency locking circuit module 10 for processing.

[0053] Second, in order to lock the frequency of the laser on the saturation absorption line of lithium atoms, the frequency of the emitted laser needs to be modulated. Through the modulation signal source in the frequency locking circuit module 10, a low-frequency sinusoidal signal is added to the emitted laser frequency and directly modulated on the driving current of the laser. The modulated scanning spectrum signal can be used as an error signal. The error signal is filtered out by a filter to remove high-frequency components and noise and then outputs a low-frequency signal. After passing through a proportional integral differential (PID) circuit, it is used as a control signal. The control signal is negatively fed back to the laser frequency controller, so that the frequency of the laser is locked on a certain transition line of the fine spectrum of lithium atoms, wherein the oscilloscope 11 is used to monitor the relevant signals in real time.

[0054] 3. The mechanical processing parts of the lithium atom gas chamber 9 are all made of stainless steel, resistant to baking, with low gas emission rate, and suitable for use in vacuum systems. The first flange 19, the second flange 42, the third flange 21, and the fourth flange 27 on the four-way pipe 12 all adopt standard CF35 flanges, and the four rectangular positioning blocks 18 are symmetrically welded on both sides of the outer wall of the first flange 19 and the second flange 42 with M6 through holes, which is convenient for the lithium atom gas chamber 9 to be connected to the optical platform. The length of the lithium atom gas chamber 9 is more than 50cm, and the probability of light and steam interaction increases, which increases the intensity of the frequency locking signal and improves the frequency locking accuracy. The wire mesh 33 is a 200-mesh stainless steel mesh, which is placed inside the cavity of the four-way pipe 12. The high-temperature lithium vapor condenses into lithium liquid in the cooling zone, and is brought back to the center of the pipeline by the surface tension of the wire mesh 33, which is convenient for the reflux and reuse of lithium. Because the vapor pressure of impurities is usually different from the vapor pressure of lithium at a specific temperature, the concentration of impurities with higher vapor pressure is low, and they will not condense again and separate from lithium. The high-temperature lithium vapor is continuously purified by condensation and reflux under the cooperation of the water cooling subsystem and the bell-shaped pipe. At the same time, the vacuum subsystem, gas distribution subsystem, water cooling subsystem, temperature control subsystem and air blowing subsystem work together to prevent the high-temperature and high-density lithium vapor from covering the glass window and blocking the laser light path.

[0055] A method for using a laser frequency locking device for lithium isotope separation comprises the following steps:

[0056] 1. Open one side of the CF35 inner welding flange, place the wire mesh 33 in the center of the four-way pipe 12, connect the argon bottle 26 to the third flange 21 through the connecting gas pipe 24 and the vacuum fine-tuning valve 23, introduce argon protective gas, place the lithium metal in the center of the cavity in the protective gas atmosphere, tighten the CF35 inner welding flange again, and close the gas distribution system;

[0057] 2. Start the vacuum pumping system and pump the vacuum degree in the chamber to the vacuum value required by the experiment;

[0058] 3. Connect the water cooling subsystem to the external water pipe and water cooler through the pagoda joint, and start the water cooler to cool the pipe wall;

[0059] 4. Start the laser module, set the temperature and current of the laser through the thermoelectric cooler and current controller, set it to room temperature and above the threshold current point, and after the 671 nm laser gain chip emits light, repeatedly adjust the threshold current as low as possible to obtain the best working state, that is, the best current-power curve. At the same time, turn on the wavelength meter to observe that the laser wavelength is around 670 nm at room temperature. By slightly adjusting the current and temperature, find a target working point that does not jump for several GHz. Finally, the output power meets the experimental requirements, and the output wavelength covers the D-line transition of lithium isotopes.

[0060] 5. Adjust the optical path components, especially pay attention to adjusting the reflector behind the quarter-wave plate 7, so that the pump light passes through the quarter-wave plate 7 and the second reflector 8 and overlaps with its original optical path. With the help of two apertures as auxiliary tools, adjust the pump light that first enters the atomic gas chamber and the light reflected by the second reflector 8 to pass through the center of the two apertures, thus achieving the overlap of the optical paths;

[0061] 6. Close the vacuum system and start the gas distribution system. Argon gas enters the pipeline and the air blowing pipe through the argon bottle 26, the pressure reducing valve 25 and the vacuum fine-tuning valve 23. At the same time, the temperature control subsystem heats the glass window low temperature and the pipeline central high temperature heating area 35. The lithium metal evaporates into lithium metal vapor in the central high temperature heating area 35 at a given temperature.

[0062] 7. Turn on the oscilloscope 11, add a low-frequency sinusoidal signal to the driving current of the laser through the modulation signal source in the frequency-locked circuit module 10, and obtain the modulated differential error signal;

[0063] 8. Send the differential error signal to the differential error signal filter, and use the proportional integral differential (PID) circuit to adjust the differential error signal to the appropriate feedback gain and bandwidth. Finally, send the differential error signal to the laser diode drive current to modulate and lock the laser's output light frequency;

[0064] 9. In order to achieve the purpose of buffering and obtain a good frequency-locking signal at the same time, the amplitude of the saturation absorption spectrum can be observed at any time through the oscilloscope 11 during the process of argon injection into the gas distribution system, and it is found that when the argon is injected to the vacuum degree displayed as a value, the amplitude of the spectrum line is the highest.

[0065] The technical solutions provided by the embodiments of the present invention are introduced in detail above. Specific examples are used herein to illustrate the principles and implementation methods of the embodiments of the present invention. The description of the above embodiments is only applicable to help understand the principles of the embodiments of the present invention. At the same time, for those skilled in the art, according to the embodiments of the present invention, there may be changes in the specific implementation methods and application scopes. In summary, the contents of this specification should not be understood as limiting the present invention.

Claims

1. A laser frequency locking device for lithium isotope separation, characterized in that: The invention comprises a laser module, an optical device, a lithium atom gas chamber and a frequency locking circuit module; the laser module comprises a tunable semiconductor laser, an optical isolator and a coupled shaping optical path device; the optical device comprises a half-wave plate, a polarization beam splitter prism (PBS), a reflector, a quarter-wave plate and a photodiode; the laser is divided into two beams of low-power reference light and high-power pump light through the polarization beam splitter prism (PBS) and the half-wave plate; the reference light is directly incident into the high-temperature and high-density lithium atom gas chamber after passing through the first reflector and is detected by the second photodiode; the pump light is incident into the atomic gas chamber after being reflected by the polarization beam splitter prism (PBS); the laser emitted from the atomic gas chamber is detected by the first photodiode after being self-contradicted by the quarter-wave plate and the second reflector, and the signals obtained by the two photodiodes are input into the frequency locking circuit module for processing; the lithium atom gas chamber comprises a vacuum subsystem, a gas distribution subsystem, a water cooling subsystem, a temperature control subsystem, an air blowing subsystem, a four-way pipe, a silk screen, a glass window and a rectangular positioning block.

2. The laser frequency locking device for lithium isotope separation according to claim 1, characterized in that: The central wavelength of the tunable semiconductor laser is 671nm. The tunable semiconductor laser is composed of a 671nm laser gain chip, a volume grating, a thermoelectric cooler and a thermistor element, and the components are all mounted on the same substrate.

3. The laser frequency locking device for lithium isotope separation according to claim 1, characterized in that: The vacuum subsystem includes a vacuum acquisition device and a measuring device. The vacuum acquisition device is composed of a molecular pump, a mechanical pump, and a vacuum pump. The measuring device is composed of an ionization and resistance vacuum gauge.

4. The laser frequency locking device for lithium isotope separation according to claim 1, characterized in that: The gas distribution subsystem includes an argon gas bottle, a pressure reducing valve, a connecting gas pipe, and a vacuum fine-tuning valve.

5. The laser frequency locking device for lithium isotope separation according to claim 1, characterized in that: The water cooling subsystem includes four stainless steel rings, four stainless steel pipes, and four pagoda joints.

6. The laser frequency locking device for lithium isotope separation according to claim 1, characterized in that: The temperature control subsystem comprises a nickel-chromium heating wire, a constant current power supply, a K-type thermocouple, a temperature measuring instrument and a thermal insulation layer.

7. The laser frequency locking device for lithium isotope separation according to claim 1, characterized in that: The air blowing subsystem comprises an air blowing pipeline and a vacuum fine-tuning valve.

8. The laser frequency locking device for lithium isotope separation according to claim 1, characterized in that: The frequency locking circuit module includes a modulation signal source, a multiplier, a low-pass filter, and a proportional integral differential (PID) circuit.

9. The method for using a laser frequency locking device for lithium isotope separation according to claim 1, characterized in that: The following steps are involved:

1. Open one side flange of the four-way pipe, place the screen in the center of the four-way pipe, connect the argon bottle to one of the flanges of the four-way pipe through the connecting pipe and the vacuum fine-tuning valve, and then introduce argon protective gas. Place the lithium metal in the center of the cavity of the four-way pipe in the protective gas atmosphere, then re-tighten the original flange and close the gas distribution system; 2. Start the vacuum pumping system and draw the vacuum degree in the cavity of the four-way pipe to the vacuum value required by the experiment; 3. Connect the water cooling subsystem to the external water pipe and water cooler through the pagoda joint, and start the water cooler to cool the pipe wall; 4. Start the laser module, set the temperature and current of the laser through the thermoelectric cooler and current controller, set it to room temperature and above the threshold current point, wait for the 671nm laser gain chip to emit light, repeatedly adjust the threshold and reduce the current, and turn on the wavelength meter to observe the laser wavelength at 670nm at room temperature. By slightly adjusting the current and slight temperature changes, find a target operating point that does not jump for several GHz continuously, and finally obtain a laser with an output power that meets the experimental requirements and an output wavelength covering the D-line transition of lithium isotopes; 5. Adjust the optical path device. By adjusting the reflector behind the quarter-wave plate, the pump light can overlap with its original optical path after passing through the quarter-wave plate and the second reflector. With the help of two apertures as auxiliary tools, the pump light that first enters the atomic gas chamber and the light reflected by the second reflector can be adjusted to pass through the center of the two apertures, thus achieving the overlap of the optical paths.

6. Turn off the vacuum system and start the gas distribution system. Argon gas enters the pipeline and the air blowing pipe through the argon gas bottle, the pressure reducing valve and the vacuum fine-tuning valve. At the same time, the temperature control subsystem heats the glass window at low temperature and the high temperature heating area in the center of the pipeline. Lithium metal evaporates into lithium metal vapor in the central high temperature heating area at a given temperature.

7. Turn on the oscilloscope, add a low-frequency sinusoidal signal to the laser drive current through the modulation signal source in the frequency locking circuit module, and obtain the modulated differential error signal; 8. Send the differential error signal to the differential error signal filter, use the proportional integral differential (PID) circuit to adjust the differential error signal to the appropriate feedback gain and bandwidth, and finally send the differential error signal to the laser diode drive current to modulate and lock the output light frequency of the laser.

10. The method for using a laser frequency locking device for lithium isotope separation according to claim 9, characterized in that: When the argon protective gas is introduced in step 1, the amplitude of the saturated absorption spectrum is observed through an oscilloscope to find that when the argon gas is injected to a vacuum degree displayed as a value, the amplitude of the spectrum line is the highest, thereby achieving the buffering purpose while obtaining a good frequency locking signal.

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