A real-time measurement device and method for pressure broadening based on the optical rotation effect

Abstract

A real-time measurement device and method for pressure broadening based on the optical rotation effect, which can respectively measure the polarization ellipse rate and the optical rotation angle of polarized light by using the phase delay characteristic of an optical elastic modulator, and realize the measurement of the pressure broadening of an atomic gas cell according to the ratio relationship between the polarization ellipse rate and the optical rotation angle. Compared with the conventional method, the real-time measurement of pressure broadening in the present invention is not affected by other external factors such as the intensity of the detection light, the intensity of the pumping light, and the temperature of the gas cell, and can stably, accurately and rapidly measure the pressure broadening of the atomic gas cell under the conditions of large detuning measurement and interference in the experimental environment.

Description

Technical Field

[0001] The present invention relates to an apparatus and method for measuring the pressure broadening of an atomic gas cell by photoelastic modulation, belonging to the technical field of optical detection. Background Art

[0002] The traditional method for measuring the pressure broadening of an atomic gas cell is the optical absorption method. During the test process, it is necessary to measure the optical power before and after passing through the atomic gas cell and the calibration voltage of the shielding coil multiple times, and obtain the pressure broadening value of the atomic gas cell through Lorentz line shape fitting (Lorentz is the Lorentz). The operation process is complex and the measurement time is long. During the test process, it is susceptible to the influence of laser power and wavelength fluctuations as well as temperature fluctuations. The complexity of the optical absorption method and the external influence during the long measurement process result in its not meeting the current requirements for ultra-high sensitive and extremely weak magnetic field measurement. Therefore, a fast and stable method for measuring the pressure broadening of an atomic gas cell needs to be proposed. By studying the interaction between light and atoms, it can be known that the ratio of the ellipticity of polarized light to the rotation angle of polarized light is only related to the frequency detuning of the detection light and the pressure broadening of the D1 line, and is not affected by factors such as the intensity of the detection light, the intensity of the pumping light, and the temperature of the gas cell. In a closed gas cell, different buffer gases 4 The densities of He and the quenching gas N2 correspond to different pressure broadenings of the D1 line. Therefore, the pressure broadening of the D1 line of alkali metal atoms in the atomic gas cell can be measured by the ratio of the rotation angle of polarized light to the ellipticity of polarized light.

[0003] After measuring the ellipticity and rotation angle of polarized light, the pressure broadening of the D1 line of alkali metal atoms in the atomic gas cell can be obtained by fitting the ratio of the two. Since the rotation angle and ellipticity of polarized light can still be measured with high precision when the frequency detuning of the detection light is large, this method can achieve accurate measurement of the pressure broadening of the atomic gas cell under large detuning measurement conditions. And compared with other measurement methods, this measurement method has rapidity and simplicity.

[0004] For example, in an atomic magnetometer, the interaction between polarized light and an atomic ensemble is used to measure the magnetic field. The rotation angle of polarized light reflects the magnitude of the atomic spin precession signal, and the ellipticity of polarized light reflects important parameters such as the optical pumping rate and the optical absorption rate. It is necessary to measure the rotation angle and ellipticity with high sensitivity to reflect the interaction strength between polarized light and the atomic ensemble. And the measurement and calibration of pressure broadening, as a key pre-technology for magnetic field measurement, are very important for magnetic field measurement. Thus, it is of great significance to measure the pressure broadening quickly and with high precision using the rotation angle and ellipticity. However, the usual pressure broadening measurement techniques are difficult to achieve high-precision and fast measurement under large detuning measurement conditions. Therefore, this pressure broadening measurement technique is of great significance in atomic magnetometers. Summary of the Invention

[0005] Technical problem solved by the present invention: Provide a real-time measurement device and method for pressure broadening based on the optical rotation effect, propose a fine adjustment scheme for optical elements to reduce the installation error angle of optical elements, and propose a calculation method based on the ratio of ellipticity and optical rotation angle to measure the pressure broadening of the atomic gas cell, improving the stability, accuracy, rapidity, and simplicity of the measurement of the pressure broadening of the atomic gas cell.

[0006] The technical solution of the present invention is as follows:

[0007] A real-time measurement device for pressure broadening based on the optical rotation effect, comprising a linearly polarized light generation module arranged on the incident side of the detection light of the atomic gas cell, a polarization characteristic detection module arranged on the outgoing side of the detection light of the atomic gas cell, and a circularly polarized light generation module arranged on the incident side of the pumping light of the atomic gas cell. The polarization characteristic detection module is connected to a pressure broadening measurement module. The pressure broadening measurement module includes a lock-in amplifier respectively connected to an atomic gas cell pressure broadening calculation system, a controller, and a photodetector in the polarization characteristic detection module. The photodetector is sequentially connected to a photoelastic modulator through a polarizer. The photoelastic modulator is selectively connected to the outgoing side of the detection light of the atomic gas cell through or without passing through a second quarter-wave plate. The photoelastic modulator is connected to the controller. The modulation frequency of the photoelastic modulator is set by the controller. The controller provides a reference frequency for the lock-in amplifier. The lock-in amplifier demodulates the output signal of the photodetector according to the reference frequency. The atomic gas cell pressure broadening calculation system calculates the pressure broadening of the atomic gas cell through the fundamental frequency voltage signal and the second harmonic voltage signal output by the lock-in amplifier.

[0008] Including the following expressions:

[0009] ,

[0010] where U1 is the fundamental frequency voltage signal output by the lock-in amplifier when the detection light passes through the second quarter-wave plate, U2 is the second harmonic voltage signal output by the lock-in amplifier when the detection light passes through the second quarter-wave plate, U3 is the fundamental frequency voltage signal output by the lock-in amplifier when the detection light does not pass through the second quarter-wave plate, U4 is the second harmonic voltage signal output by the lock-in amplifier when the detection light does not pass through the second quarter-wave plate, Γ / 2 is the pressure broadening of the atomic gas cell, and Δv is the frequency detuning of the detection light.

[0011] The atomic gas cell is located in a heating and magnetic shielding system.

[0012] The circularly polarized light generation module includes a pumping laser, a pumping laser light intensity control module, a pumping laser beam expanding system, a reflecting mirror, a polarizer, and a first quarter-wave plate connected in sequence. The pumping light irradiates the atomic gas cell from the incident side of the pumping light of the atomic gas cell after passing through the first quarter-wave plate.

[0013] The linearly polarized light generating module includes a detection laser, a detection laser light intensity control module, a detection laser beam expander system, and a polarizer connected in sequence. After passing through the polarizer, the detection light irradiates the atomic gas cell from the detection light incident side of the atomic gas cell.

[0014] A real-time measurement method for pressure broadening based on the optical rotation effect, using the above-mentioned real-time measurement device for pressure broadening based on the optical rotation effect, includes the following steps:

[0015] Step 1: Install the atomic gas cell, install the linearly polarized light generating module, install the polarization characteristic detection module, install the circularly polarized light generating module, install the pressure broadening measurement module. Rotate the polarizer until the output signal of the photodetector is at a minimum value. Rotate the second quarter-wave plate until the angle between its fast axis and the transmission axis of the polarizer is 0°. Rotate the photoelastic modulator until the angle between its fast axis and the transmission axis of the polarizer is 45°. Turn on the magnetic shielding system, heat the atomic gas cell to 200 °C and keep it stable for one hour.

[0016] Step 2: Set the reference frequency of the lock-in amplifier to the modulation frequency of the photoelastic modulator, so that the lock-in amplifier outputs the fundamental frequency and the second harmonic frequency in the voltage signal of the photodetector, in order to utilize the fundamental frequency sensitive ellipticity when the second quarter-wave plate is added to the detection optical path, and the fundamental frequency sensitive optical rotation angle when the quarter-wave plate is removed from the detection optical path.

[0017] Step 3: Record the fundamental frequency voltage U 1 and U 3, and the second harmonic frequency voltage U 2 and U 4 output by the lock-in amplifier after the detection light passes through the atomic gas cell, and calculate the ratio of the ellipticity to the optical rotation angle.

[0018] Step 4: Through the ratio of the ellipticity to the optical rotation angle and the calibration of the detuning frequency of the detection light, fit to obtain the pressure broadening expression of the atomic gas cell, and realize the real-time measurement of pressure broadening based on the optical rotation effect.

[0019] In Step 1, it includes adjusting the angle β QWP between the fast axis of the second quarter-wave plate and the transmission axis of the polarizer to vary within the range of -14° to 14°.

[0020] The technical effects of the present invention are as follows: A real-time measurement device and method for pressure broadening based on the optical rotation effect according to the present invention can respectively measure the polarization ellipse rate and the optical rotation angle of polarized light by utilizing the phase delay characteristic of a photoelastic modulator, and realize the measurement of the pressure broadening of an atomic gas cell according to the ratio relationship between the polarization ellipse rate and the optical rotation angle. Compared with the conventional method, the real-time measurement of pressure broadening in the present invention is not affected by other external factors such as the detection light intensity, the pumping light intensity, and the gas cell temperature, and can stably, accurately, and rapidly measure the pressure broadening of the atomic gas cell under the conditions of large detuning measurement and interference in the experimental environment.

[0021] The features of the present invention are as follows:

[0022] (1) The present invention uses a single photoelastic modulator to realize the modulation of polarized light, fully suppressing the interference of low-frequency technical noise in the signal to be measured, and effectively realizing noise suppression.

[0023] (2) The fine-tuning scheme of optical elements is adopted to reduce the installation error angle of optical elements, and the calibration scheme and the calculation method of the ellipse rate and the optical rotation angle are used to suppress the measurement error. In the present invention, the quarter-wave plate is finely tuned respectively based on the first harmonic and the second harmonic of the output voltage signal of the lock-in amplifier, and a calibration scheme and a calculation method of the ellipse rate and the optical rotation angle are proposed to reduce the error of optical elements, improve the measurement accuracy of the ellipse rate and the optical rotation angle, and ensure the more accurate realization of the measurement of the pressure broadening of the atomic gas cell.

[0024] (3) The accurate value of the pressure broadening is obtained from the experimental data by the fitting method. The error source of this measurement method itself is small, and this fitting method eliminates other errors in the experiment, ensuring the accuracy of the measurement result.

[0025] (4) It can stably, accurately, and rapidly measure the pressure broadening of the atomic gas cell under conditions such as large detuning. Description of the Drawings

[0026] Figure 1 It is a schematic structural diagram of a real-time measurement device for pressure broadening based on the optical rotation effect according to the present invention.

[0027] Description of the reference numerals in the drawings is as follows: 1 - pumping laser; 2 - pumping laser light intensity control module; 3 - pumping laser beam expanding system; 4 - reflector; 5 - polarizer; 6 - first quarter-wave plate; 7 - detection laser; 8 - detection laser light intensity control module; 9 - detection laser beam expanding system; 10 - polarizer; 11 - second quarter-wave plate; 12 - photoelastic modulator; 13 - analyzer; 14 - photodetector; 15 - lock-in amplifier; 16 - controller; 17 - atomic gas cell pressure broadening calculation system. Detailed Embodiment

[0028] The following combines the drawings ( Figure 1The present invention will be described with reference to the embodiments.

[0029] Figure 1 It is a schematic structural diagram of a real-time measurement device for pressure broadening based on the optical rotation effect of the present invention. Refer to Figure 1 As shown, a real-time measurement device for pressure broadening based on the optical rotation effect includes a linearly polarized light generation module arranged on the incident side of the detection light of the atomic gas cell, a polarization characteristic detection module arranged on the outgoing side of the detection light of the atomic gas cell, and a circularly polarized light generation module arranged on the incident side of the pumping light of the atomic gas cell. The polarization characteristic detection module is connected to the pressure broadening measurement module. The pressure broadening measurement module includes a lock-in amplifier 15 respectively connected to the atomic gas cell pressure broadening calculation system 17, the controller 16, and the photodetector 14 in the polarization characteristic detection module. The photodetector 14 is sequentially connected to the photoelastic modulator 12 through the analyzer 13. The photoelastic modulator 12 is selectively connected to the outgoing side of the detection light of the atomic gas cell through or without passing through the second quarter-wave plate 11. The photoelastic modulator 12 is connected to the controller 16. The modulation frequency of the photoelastic modulator 12 is set by the controller 16. The controller 16 provides a reference frequency for the lock-in amplifier 15. The lock-in amplifier 15 demodulates the signal output by the photodetector 14 according to the reference frequency. The atomic gas cell pressure broadening calculation system 17 calculates the atomic gas cell pressure broadening through the fundamental frequency voltage signal and the second harmonic voltage signal output by the lock-in amplifier 15.

[0030] It includes the following expressions:

[0031] ,

[0032] where U 1 is the fundamental frequency voltage signal output by the lock-in amplifier 15 when the detection light passes through the second quarter-wave plate 11, U 2 is the second harmonic voltage signal output by the lock-in amplifier 15 when the detection light passes through the second quarter-wave plate 11, U 3 is the fundamental frequency voltage signal output by the lock-in amplifier 15 when the detection light does not pass through the second quarter-wave plate 11, U 4 is the second harmonic voltage signal output by the lock-in amplifier 15 when the detection light does not pass through the second quarter-wave plate 11, Γ / 2 is the atomic gas cell pressure broadening, and Δv is the detection light frequency detuning.

[0033] The atomic gas cell is located in the heating and magnetic shielding system. The circularly polarized light generation module includes a pumping laser 1, a pumping laser intensity control module 2, a pumping laser beam expanding system 3, a reflecting mirror 4, a polarizer 5, and a first quarter-wave plate 6 that are connected in sequence. The pumping light irradiates the atomic gas cell from the pumping light incident side of the atomic gas cell after passing through the first quarter-wave plate 6. The linearly polarized light generation module includes a detection laser 7, a detection laser intensity control module 8, a detection laser beam expanding system 9, and a polarizer 10 that are connected in sequence. The detection light irradiates the atomic gas cell from the detection light incident side of the atomic gas cell after passing through the polarizer 10.

[0034] A real-time measurement method for pressure broadening based on the optical rotation effect, using the above-mentioned real-time measurement device for pressure broadening based on the optical rotation effect, includes the following steps: Step 1, install the atomic gas cell, install the linearly polarized light generation module, install the polarization characteristic detection module, install the circularly polarized light generation module, install the pressure broadening measurement module, rotate the polarizer until the output signal of the photodetector is at a minimum value, rotate the second quarter-wave plate until the angle between its fast axis and the transmission axis of the polarizer is 0°, rotate the photoelastic modulator until the angle between its fast axis and the transmission axis of the polarizer is 45°, turn on the magnetic shielding system, and heat the atomic gas cell to 200 °C and stabilize for one hour; Step 2, set the reference frequency of the lock-in amplifier to the modulation frequency of the photoelastic modulator, so that the lock-in amplifier outputs the first harmonic and the second harmonic in the voltage signal of the photodetector, in order to utilize the first harmonic sensitive ellipticity when the second quarter-wave plate is added to the detection optical path, and the first harmonic sensitive optical rotation angle when the quarter-wave plate is removed from the detection optical path; Step 3, respectively record the first harmonic voltage U 1 and U 3, and the second harmonic voltage U 2 and U 4 output by the lock-in amplifier after the detection light passes through the atomic gas cell, and calculate the ratio of the ellipticity and the optical rotation angle; Step 4, through the ratio of the ellipticity and the optical rotation angle, and calibrating the detuning frequency of the detection light, fit to obtain the pressure broadening expression of the atomic gas cell, and realize the real-time measurement of pressure broadening based on the optical rotation effect.

[0035] Step 1 includes adjusting the angle between the fast axis of the second quarter-wave plate and the transmission axis of the polarizer β QWP to vary within the range of -14° to 14°.

[0036] The present invention relates to a real-time measurement device and method for pressure broadening based on the optical rotation effect. This method uses the phase delay characteristics of a photoelastic modulator to measure the polarization ellipse rate and the optical rotation angle of polarized light respectively. Based on the ratio relationship between the polarization ellipse rate and the optical rotation angle, the pressure broadening of the atomic gas cell is measured. Compared with the conventional method, the measurement method proposed by the present invention is not affected by other external factors such as the detection light intensity, the pumping light intensity, and the gas cell temperature, and can achieve stable, accurate and rapid measurement of the pressure broadening of the atomic gas cell under the conditions of large detuning measurement and interference in the experimental environment.

[0037] A real-time measurement device for pressure broadening based on the optical rotation effect, where the optical rotation effect includes the circular birefringence effect and the circular dichroism effect, and can achieve rapid and anti-interference measurement of the pressure broadening of the atomic gas cell. The device includes a pumping laser, a pumping laser light intensity control module, a pumping laser beam expanding system, a reflecting mirror, a polarizer, a quarter-wave plate, a detection laser, a detection laser beam expanding system, a detection laser light intensity control module, a quarter-wave plate, a photoelastic modulator, a photodetector, a lock-in amplifier, a controller, an atomic gas cell pressure broadening calculation system, an atomic gas cell, and a heating and magnetic shielding system; the pumping laser, the pumping laser light intensity control module, the pumping laser beam expanding system, the reflecting mirror, the polarizer, and the quarter-wave plate form a standard parameter pumping polarized light generation module; the polarization state of the standard parameter pumping polarized light changes after interacting with the atomic gas cell; the detection laser, the detection laser beam expanding system, the detection laser light intensity control module, and the polarizer form a standard parameter detection polarized light generation module; in the atomic gas cell and the heating and magnetic shielding system, the atomic gas cell is heated to 200 °C and stabilized for one hour to fully polarize the atoms in the gas cell, and the magnetic shielding shields the external magnetic field to make the atomic gas cell in an extremely weak magnetic environment; the polarization state of the standard parameter detection polarized light changes after interacting with the atomic gas cell; the detection light carries information about the circular birefringence effect and the circular dichroism effect of the interaction between light and atoms, and based on this, the pressure broadening of the atomic gas cell is measured. Among them, the information of the circular birefringence effect is reflected by the optical rotation angle parameter, and the information of the circular dichroism effect is reflected by the ellipse rate parameter; the detection light passes through the quarter-wave plate and the photoelastic modulator and then enters the photodetector, and the phase delay characteristic of the photoelastic modulator is used to realize the detection of the ellipse rate information The detection light passes through the photoelastic modulator and then enters the photodetector, and the phase delay characteristic of the photoelastic modulator is used to realize the detection of the optical rotation angle information The modulation frequency of the photoelastic modulator is set by the photoelastic modulator controller. The photoelastic modulator controller provides a reference frequency for the lock-in amplifier. The lock-in amplifier demodulates the signal according to the reference frequency, and calculates the ratio of the calibration results of the ellipse rate and the optical rotation angle measurements when stable through the atomic gas cell pressure broadening calculation system , the exact value of the pressure broadening of the atomic gas cell can be quickly calculated.

[0038] The angle between the fast axis of the quarter-wave plate and the transmission axis of the polarizer is 0°, and the angle between the fast axis of the photoelastic modulator and the transmission axis of the polarizer is 45°.

[0039] Using the atomic gas cell pressure broadening measurement device to measure the pressure broadening of the atomic gas cell, including the following steps:

[0040] a. Install the atomic gas cell and the heating and magnetic shielding system; install the pumping laser, the pumping laser intensity control module, the pumping laser beam expanding system, the mirror, the polarizer and the quarter-wave plate; install the detection laser, the detection laser beam expanding system, the detection laser intensity control module, the polarizer and the photodetector, and rotate the polarizer until the output of the photodetector is at a minimum. Install the quarter-wave plate and the photoelastic modulator, rotate the quarter-wave plate until the angle between its fast axis and the transmission axis of the polarizer is 0°, and rotate the photoelastic modulator until the angle between its fast axis and the transmission axis of the polarizer is 45°; turn on the magnetic shielding system, heat the atomic gas cell to 200 °C and stabilize for one hour;

[0041] b. Set the reference frequency of the lock-in amplifier to the modulation frequency of the photoelastic modulator to make the lock-in amplifier output the first harmonic and the second harmonic in the voltage signal of the photodetector;

[0042] c. Record the first harmonic voltage U 1 and the second harmonic voltage U 2 output by the lock-in amplifier; after removing the half-wave plate, repeat the fine adjustment operation of the quarter-wave plate in step a, and repeat the steps in step b to measure the first harmonic voltage U 3 and the second harmonic voltage U 4 of the ellipticity signal;

[0043] d. Record the first harmonic voltage U 1 and U 3, the second harmonic voltage U 2 and U 4 output by the lock-in amplifier after passing through the atomic gas cell respectively, and calculate the ratio of the ellipticity to the optical rotation angle;

[0044] e. Calculate the ratio of the ellipticity to the optical rotation angle and calibrate the frequency detuning amount Δv of the detection light, and then obtain the pressure broadening value through fitting and solving.

[0045] In step b, finely adjust the quarter-wave plate until the first harmonic of the demodulation signal is at a minimum to achieve the fine adjustment of the quarter-wave plate.

[0046] In step c, rotate the quarter-wave plate so that the included angle between its fast axis and the transmission axis of the polarizer ranges from -14° to 14°.

[0047] In step e, experimentally measure the fundamental frequency and second harmonic signals, and use the frequency detuning of the calibration detection light as the independent variable and the pressure broadening value as the dependent variable. The pressure broadening value is used as the proportional scale factor for fitting.

[0048] A real-time pressure broadening measurement device based on the optical rotation effect according to the present invention has an optical path core including a quarter-wave plate, a photoelastic modulator, a polarizer, and a photodetector. The quarter-wave plate and the photoelastic modulator realize polarization light modulation at a specific phase. The modulated detection light enters the photodetector after passing through the polarizer. The circuit includes a lock-in amplifier, a controller, and an atomic gas cell pressure broadening calculation system, etc. The reference frequency of the lock-in amplifier is provided by the photoelastic modulator controller. The lock-in amplifier demodulates the light intensity signal. The ratio information of the optical rotation angle and ellipticity of the polarized light can be obtained from the fundamental frequency and second harmonic of the signal when the quarter-wave plate exists and does not exist in the demodulation optical path.

[0049] The optical rotation angle is obtained by solving the density matrix α and the ellipticity ε The expressions can be represented as:

[0050] ,

[0051] where n is the number density of alkali metal atoms in the atomic gas cell, l is the propagation length of the detection light in the alkali metal atom ensemble in the gas cell, r e is the classical electron radius, c is the classical light speed, S x is the polarizability of the alkali metal atom ensemble in the gas cell in the x-axis direction, f D1 is the oscillation intensity factor of the D1 line of the alkali metal atom, is the complex Lorentz line shape, represents the real part of the Lorentz line shape, represents the imaginary part of the Lorentz line shape.

[0052] From equations (1) and (2), the ratio expression of the ellipticity to the optical rotation angle is:

[0053] ,

[0054] where Γ / 2 is the pressure broadening of alkali metal atoms in the gas cell, and Δv is the frequency detuning of the detection light.

[0055] As can be seen from Equation (3), the ratio of the ellipticity to the optical rotation angle is only related to the pressure broadening of the alkali metal atoms and the frequency detuning of the detection light, and has nothing to do with any other external factors. Given any two of the ratio of the ellipticity to the optical rotation angle, the pressure broadening of the alkali metal atoms, and the frequency detuning of the detection light, the remaining unknown term can be obtained. Therefore, we can calibrate the frequency detuning of the detection light and measure the optical rotation angle and ellipticity experimentally to obtain the pressure broadening of the alkali metal atoms in the atomic gas cell.

[0056] The theory of this method is accurate and complete. There are no other error terms theoretically, and it is an in-situ measurement method. Moreover, this method is simple and easy to implement experimentally, and the time required for the measurement process is very short, reducing the impact on subsequent experiments caused by the long time of other experimental methods. At the same time, the error sources of this method are also small experimentally.

[0057] The experimental principle of this method is as follows. Let the optical rotation angle obtained after passing through the atomic gas cell be α , and the ellipticity be ε of the elliptically polarized light. Then the Jones vector E0’ of this elliptically polarized light can be expressed as:

[0058] ,

[0059] where E0 is the amplitude of this elliptically polarized light, i is the imaginary unit, and the Jones matrix G 1 / 4 of the quarter-wave plate can be expressed as:

[0060] ,

[0061] The phase delay δ of the photoelastic modulator can be expressed as:

[0062] ,

[0063] where δ 0 is the peak phase delay of the photoelastic modulator, ω is the modulation frequency of the photoelastic modulator, t is the modulation time of the photoelastic modulator, δ s is the static phase delay of the photoelastic modulator. Therefore, the Jones matrix G PEM of the photoelastic modulator can be expressed as:

[0064] ,

[0065] where θ PEM is the mounting error angle of the photoelastic modulator. The Jones vector of the light emerging from the photoelastic modulator can be expressed as E = G PEMG 1 / 4 If it is E0’, then the light intensity I received by the photodetector is:

[0066] ,

[0067] where E * represents the complex conjugate of the Jones vector of the light emitted by the photoelastic modulator. Since δ s and θ PEM are zeroed during the fine adjustment of the optical element. Finally, when there is a quarter-wave plate in the detection optical path, the photodetector converts the received light intensity I into an electrical signal, and then the first-harmonic voltage signal U 1 and the second-harmonic voltage signal U 2 are:

[0068] ,

[0069] where η is a coefficient related to the gain of the balanced photodetector and the lock-in amplifier, J 1( δ 0) and J 2( δ 0) are the first-order and second-order Bessel functions.

[0070] Similarly, it can be obtained that when there is no quarter-wave plate in the detection optical path, the first-harmonic voltage signal U 3 and the second-harmonic voltage signal U 4 can be expressed as:

[0071] ,

[0072] It is easy to know that when there is a half-wave plate in the detection optical path, the first-harmonic U 1 is sensitive to the ellipticity ε , and when there is no half-wave plate in the detection optical path, the first-harmonic U 3 is sensitive to the optical rotation angle α .

[0073] It can be seen that the experimental fitting formula for measuring the pressure broadening of the atomic gas cell can be expressed as:

[0074] ,

[0075] where the pressure broadening Γ / 2 of the alkali metal atoms in the gas cell is the dependent variable of the fitting function for fitting, and the rest are experimental measurement data and serve as the independent variables of the fitting function.

[0076] Such as Figure 1As shown in the figure, a real-time measurement device for pressure broadening based on the optical rotation effect according to the present invention includes four parts: a polarized light generation module, an atomic gas cell, a polarization characteristic detection module, and a pressure broadening measurement module. The polarized light generation module is composed of a pump laser 1, a pump laser intensity control module 2, a pump laser beam expander system 3, a mirror 4, a polarizer 5, and a quarter-wave plate 6. After the generated polarized light interacts with the atomic gas cell, the polarization state changes. The polarization characteristic detection module is composed of a detection laser 7, a detection laser intensity control module 8, a detection laser beam expander system 9, a polarizer 10, a quarter-wave plate 11, a photoelastic modulator 12, an analyzer 13, and a photodetector 14. The pressure broadening measurement module is composed of a lock-in amplifier 15, a controller 16, and an atomic gas cell pressure broadening calculation system 17. The device description is as follows. The pump laser 1 emits laser light, which generates circularly polarized light after passing through the pump laser intensity control module 2, the pump laser beam expander system 3, the mirror 4, the polarizer 5, and the quarter-wave plate 6. After this circularly polarized light interacts with the atomic gas cell, the polarization characteristic changes.

[0077] The detection laser 7 emits laser light, which generates linearly polarized light after passing through the detection laser intensity control module 8, the detection laser beam expander system 9, and the polarizer 10. After this linearly polarized light interacts with the atomic gas cell, the polarization characteristic changes. When the polarized light passes through the quarter-wave plate 11 and then through the photoelastic modulator 12, it becomes polarized light after modulation. The polarized light after modulation directly enters the photodetector 14 after passing through the analyzer 13, and then the photodetector 14 outputs a voltage signal. The lock-in amplifier 15 with a reference frequency of the modulation frequency of the photoelastic modulator extracts the first-harmonic and second-harmonic components in the voltage signal, and then processes them through the photoelastic modulator controller 16. The photoelastic modulator controller 16 is used to set the modulation frequency of the photoelastic modulator on the one hand and to provide a reference frequency for the lock-in amplifier on the other hand.

[0078] The angle between the fast axis of the quarter-wave plate and the transmission axis of the polarizer is 0°, and the angle between the fast axis of the photoelastic modulator and the transmission axis of the polarizer is 45°.

[0079] The specific implementation method of the present invention is as follows:

[0080] a. Install the pump laser, pump laser intensity control module, pump laser beam expansion system, reflector, polarizer, quarter-wave plate, detection laser, detection laser intensity control module, detection laser beam expansion system, polarizer, quarter-wave plate, photoelastic modulator, analyzer, photodetector, lock-in amplifier, controller, and atomic gas chamber pressure broadening solution system. First, block the reflected light from the reflector and adjust the polarizer until the output of the photodetector is at a minimum. The spot size must be determined by the optical components and the detection area of the photodetector. According to the fast axis position marked on the device, rotate the quarter-wave plate until the angle between its fast axis and the transmission axis of the polarizer is 0°, and rotate the photoelastic modulator until the angle between its fast axis and the transmission axis of the polarizer is 45°. It should be noted that the laser wavelength of the laser needs to match the optical components, especially the settings of the photoelastic modulator.

[0081] b. Set the reference frequency of the lock-in amplifier to the modulation frequency of the photoelastic modulator, so that the lock-in amplifier outputs the first and second harmonics of the photodetector voltage signal. Fine-tune the quarter-wave plate until the first harmonic is at its minimum value to achieve fine tuning of the quarter-wave plate. Because the presence of the quarter-wave plate in the detection optical path determines how the optical rotation angle and ellipticity of polarized light are measured, fine adjustment is required to reduce measurement errors caused by optical components.

[0082] c. Record the frequency-doubled voltage output by the lock-in amplifier when there is a quarter-wave plate in the detection light path and when there is no quarter-wave plate after the detection light passes through the atomic gas chamber. U 1 and U 3. Double frequency voltage U 2 and U 4. Calculate the ratio of the ellipticity and the optical rotation angle caused by the atomic gas cell.

[0083] d. The frequency detuning of the detection light is calibrated by the ratio of the calculated ellipticity and the optical rotation angle. When the system is stable, the pressure broadening of the atomic gas chamber can be obtained by fitting the ratio of the calculated ellipticity and the optical rotation angle.

[0084] Thus, the pressure broadening measurement of the atomic gas cell has been realized.

[0085] Based on the functional relationships of the output voltage with the first harmonic, second harmonic, optical rotation angle, and ellipticity, as well as the calibration for detecting the optical frequency detuning amount, the present invention realizes the measurement of the pressure broadening in the atomic gas cell. A fine adjustment scheme for optical elements is proposed to reduce the installation error angle of the optical elements. A method for calculating the optical rotation angle and ellipticity is proposed to reduce the error in the pressure broadening measurement and improve the measurement accuracy. The application of the modulation detection method improves the sensitivity of the present device. The fitting method applied eliminates other error factors in the experimental process. The rapid and accurate measurement of the pressure broadening in the atomic gas cell under large detuning conditions is realized.

[0086] The content not detailedly described in the specification of the present invention belongs to the prior art well-known to those skilled in the art. It is hereby specified that the above description helps those skilled in the art to understand the present invention, but does not limit the protection scope of the present invention. Any implementation that is an equivalent replacement, modification and improvement, and / or simplification of the above description without departing from the substantial content of the present invention falls within the protection scope of the present invention.

Claims

1. A real-time measurement device for pressure broadening based on the optical rotation effect, characterized in that It includes setting a linearly polarized light generation module on the incident side of the detection light of the atomic gas cell, setting a polarization characteristic detection module on the outgoing side of the detection light of the atomic gas cell, and setting a circularly polarized light generation module on the incident side of the pumping light of the atomic gas cell. The polarization characteristic detection module is connected to a pressure broadening measurement module. The pressure broadening measurement module includes a lock-in amplifier that is respectively connected to an atomic gas cell pressure broadening calculation system, a controller, and a photodetector in the polarization characteristic detection module. The photodetector is sequentially connected to a photoelastic modulator through a polarizer. The photoelastic modulator is selectively connected to the outgoing side of the detection light of the atomic gas cell through or without passing through a second quarter-wave plate. The photoelastic modulator is connected to the controller. The modulation frequency of the photoelastic modulator is set by the controller. The controller provides a reference frequency for the lock-in amplifier. The lock-in amplifier demodulates the output signal of the photodetector according to the reference frequency. The atomic gas cell pressure broadening calculation system calculates the atomic gas cell pressure broadening through the first harmonic voltage signal and the second harmonic voltage signal output by the lock-in amplifier; It includes the following expressions: ， where U1 is the first harmonic voltage signal output by the lock-in amplifier when the detection light passes through the second quarter-wave plate, U2 is the second harmonic voltage signal output by the lock-in amplifier when the detection light passes through the second quarter-wave plate, U3 is the first harmonic voltage signal output by the lock-in amplifier when the detection light does not pass through the second quarter-wave plate, U4 is the second harmonic voltage signal output by the lock-in amplifier when the detection light does not pass through the second quarter-wave plate, Γ / 2 is the atomic gas cell pressure broadening, and Δv is the detection light frequency detuning.

2. The real-time measurement device for pressure broadening based on the optical rotation effect according to claim 1, characterized in that, The atomic gas cell is located in a heating and magnetic shielding system.

3. The real-time measurement device for pressure broadening based on the optical rotation effect according to claim 1, characterized in that, The circularly polarized light generation module includes a pumping laser, a pumping laser light intensity control module, a pumping laser beam expander system, a reflector, a polarizer, and a first quarter-wave plate that are sequentially connected. The pumping light irradiates the atomic gas cell from the incident side of the pumping light of the atomic gas cell after passing through the first quarter-wave plate.

4. The real-time measurement device for pressure broadening based on the optical rotation effect according to claim 1, characterized in that The linearly polarized light generation module includes a detection laser, a detection laser light intensity control module, a detection laser beam expander system, and a polarizer that are sequentially connected. The detection light irradiates the atomic gas cell from the incident side of the detection light of the atomic gas cell after passing through the polarizer.

5. A real-time measurement method for pressure broadening based on the optical rotation effect, characterized in that, Using the real-time pressure broadening measurement device based on the optical rotation effect according to any one of claims 1-4, it includes the following steps: Step 1, install the atomic gas cell, install the linearly polarized light generation module, install the polarization characteristic detection module, install the circularly polarized light generation module, install the pressure broadening measurement module, rotate the polarizer until the output signal of the photodetector is at a minimum value, rotate the second quarter-wave plate until the angle between its fast axis and the transmission axis of the polarizer is 0°, rotate the photoelastic modulator until the angle between its fast axis and the transmission axis of the polarizer is 45°, turn on the magnetic shielding system, and heat the atomic gas cell to 200 °C and stabilize for one hour; Step 2: Set the reference frequency of the lock-in amplifier to the modulation frequency of the photoelastic modulator, so that the lock-in amplifier outputs the fundamental frequency and the second harmonic frequency in the voltage signal of the photodetector, in order to utilize the fundamental frequency sensitive ellipticity when the second quarter-wave plate is added to the detection optical path, and the fundamental frequency sensitive optical rotation angle when the quarter-wave plate is removed from the detection optical path; Step 3: Record the fundamental frequency voltage and the second harmonic voltage output by the lock-in amplifier after the detection light passes through the atomic gas cell respectively U 1 and U 3, and the second harmonic voltage U 2 and U 4, and calculate the ratio of the ellipticity to the optical rotation angle; Step 4: By using the ratio of the ellipticity and the optical rotation angle, and calibrating the detuning frequency of the detection light, fit to obtain the pressure broadening expression of the atomic gas cell, and realize the real-time measurement of pressure broadening based on the optical rotation effect.

6. The real-time measurement method for pressure broadening based on the optical rotation effect according to claim 5, characterized in that, In Step 1, the adjustment range of the angle between the fast axis of the second quarter-wave plate and the light-transmitting axis of the polarizer β QWP varies within the range of -14° to 14°.

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