Laser frequency stabilization method based on double-color circularly polarized light Doppler-free absorption spectrum

Abstract

A laser frequency stabilization method based on a double-color circularly polarized light Doppler-free absorption spectrum is characterized in that on the basis of a Doppler-free absorption light path, coherent population is generated through double-color light to capture a CPT effect, and the line width of an absorption peak at a frequency locking point is reduced, so that the slope of a dispersion signal is improved, and the sensitivity and stability of a frequency system are improved; the circularly polarized light spectrum signal absorption peak still has a larger amplitude under the geomagnetic field, and no additional magnetic shielding measure is needed, so that the light path structure is simplified, and a foundation is laid for miniaturization.

Description

Technical Field

[0001] The present invention relates to the technical field of laser frequency stabilization, and in particular to a laser frequency stabilization method based on a two-color circularly polarized light Doppler-free absorption spectrum. On the basis of a Doppler-free absorption optical path, a coherent population capture CPT effect is generated by two-color light, so that the absorption peak line width at a frequency locking point is reduced, thereby increasing the slope of a dispersion signal, and improving the sensitivity and stability of a frequency system. The absorption peak of a circularly polarized light spectrum signal still has a large amplitude under a geomagnetic field, and no additional magnetic shielding measures are required, thereby simplifying the optical path structure and laying a foundation for miniaturization. Background Art

[0002] Laser frequency stabilization is of great significance in optical experiments. Atomic spectral lines are usually used as references for laser frequency. Saturated absorption spectroscopy is a common frequency stabilization spectroscopy. The transmission peak that eliminates the Doppler effect is obtained by the reverse propagation of pump light and detection light. However, the transmission peak line width in the saturated absorption spectrum is large, resulting in a small change in the slope of the dispersion signal at the stabilization point, and the response to frequency fluctuations is not sensitive enough.

[0003] The CPT effect is a quantum coherent effect (CPT, coherent population trapping), which is a transmission peak that carries magnetic field and frequency information obtained by the reaction of two-color laser and alkali metal, and its line width is much smaller than the line width corresponding to the saturated absorption spectrum. Based on the Doppler-free absorption optical path, we replaced the pump light and detection light with two-color light of opposite rotation, and used the CPT effect to obtain an absorption peak with a narrow line width, increase the slope of the dispersion signal at the frequency locking point, improve the sensitivity of the frequency stabilization system to frequency fluctuations, and improve the frequency stabilization performance.

[0004] The CPT effect weakens as the magnetic field increases. However, from the perspective of energy levels, compared to linearly polarized light, circularly polarized light can F = 0 corresponding to the ground state energy level produces a CPT effect that is not affected by the magnetic field. Therefore, the two-color circularly polarized light Doppler-free absorption spectrum still has an absorption peak with a small line width under the geomagnetic field as a stabilization point for laser frequency stabilization. Summary of the invention

[0005] In view of the defects or shortcomings of the prior art, the present invention proposes a laser frequency stabilization method based on a two-color circularly polarized light Doppler-free absorption spectrum. On the basis of a Doppler-free absorption optical path, the coherent population capture CPT effect is generated by two-color light, the absorption peak line width at the locking point is reduced, and the slope of the dispersion signal is increased, thereby improving the sensitivity and stability of the frequency system; the absorption peak of the circularly polarized light spectrum signal still has a large amplitude under the geomagnetic field, and no additional magnetic shielding measures are required, thereby simplifying the optical path structure and laying a foundation for miniaturization.

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

[0007] The laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectrum is characterized by comprising the following steps:

[0008] Step 1, inputting a first-handed two-color circularly polarized light as a pump light into a first end of an atomic gas cell, the first-handed two-color circularly polarized light is emitted from a second end of the atomic gas cell and then changed into a second-handed two-color circularly polarized light as a detection light and then inputted into the atomic gas cell from the second end, the second handed light being opposite to the first handed light, and the second-handed two-color circularly polarized light being emitted from the first end and then entering a photoelectric detection system;

[0009] Step 2, performing laser frequency scanning so that the second-handed two-color circularly polarized light with different frequencies forms a two-color circularly polarized light Doppler-free absorption spectrum in the photoelectric detection system;

[0010] Step 3, determining a frequency locking point in the two-color circularly polarized light Doppler-free absorption spectrum, and obtaining a dispersion signal corresponding to the frequency locking point by demodulating the two-color circularly polarized light Doppler-free absorption spectrum;

[0011] Step 4: turn off the laser frequency scanning, input the dispersion signal as an error signal into the PID control module, control the laser frequency change through the PID output signal, and achieve laser frequency stabilization.

[0012] The first end of the atomic gas chamber is respectively connected to a photodetector and a first 1 / 4 wave plate through a depolarizing beam splitter, the first 1 / 4 wave plate is connected to a laser collimator through an optical isolator, the laser collimator is connected to an electro-optic modulator through a second polarization-maintaining optical fiber, the electro-optic modulator is respectively connected to a radio frequency signal generator and a first polarization-maintaining optical fiber, the first polarization-maintaining optical fiber is connected to a laser controller through a tunable external cavity semiconductor laser, the laser controller is connected to an output end of a phase-locked amplifier through a proportional-integral-differential controller, a first input end of the phase-locked amplifier is connected to the photodetector, a second input end of the phase-locked amplifier is connected to a sine wave signal generator, the sine wave signal generator is connected to a first input end of an adder, a second input end of the adder is connected to a triangular wave signal generator, an output end of the adder is connected to the laser controller, and a second end of the atomic gas chamber is connected to a reflector through a second 1 / 4 wave plate.

[0013] Step 1 includes: using a laser and an electro-optic modulator to generate a frequency that satisfies 87The Rb atomic D1 line satisfies the two-state hyperfine energy level frequency difference of 6.834 GHz and the two-color circularly polarized light, wherein the wavelength of the D1 line is 795 nm, and the 795 nm monochromatic linearly polarized light emitted by the tunable external cavity semiconductor laser is modulated into a two-color linearly polarized light with a frequency conforming to 377 THz±3.417 GHz by an electro-optical modulator controlled by a radio frequency signal with a frequency of 3.417 GHz, wherein 377 THz corresponds to a wavelength of 795 nm, and then the two-color linearly polarized light is converted into two-color circularly polarized light by a 1 / 4 wave plate.

[0014] Step 2 includes: turning on the triangle wave signal generator, inputting the triangle wave signal into the laser controller, scanning the laser frequency within a certain range, and observing the two-color circularly polarized light Doppler-free absorption spectrum generated by the coherent population capture CPT effect through an oscilloscope.

[0015] Step 3 includes: finding the peak of the absorption peak with large amplitude and small line width in the two-color circularly polarized light Doppler-free absorption spectrum as the stabilization point, and selecting 87 The ground state energy level F in the two-color non-Doppler absorption spectrum of RbD1 line g =1 and F g =2 transition to excited state energy level F e =2, the CPT three-level system consisting of two ground state energy levels and one excited state energy level is not affected by the magnetic field. When the pump light and the detection light are two-color circularly polarized light propagating in opposite directions, they interfere constructively because of their opposite rotation directions. The CPT absorption peak amplitude reaches the maximum. When it is used as the frequency locking point, the dispersion signal slope is the largest. Therefore, in the geomagnetic field environment, the two-color circularly polarized light Doppler absorption spectrum still has an absorption peak with a large amplitude that can be used for laser frequency stabilization. After the frequency stabilization point is determined, the laser center frequency is adjusted so that the frequency locking point is at the center of the periodic signal.

[0016] Step 4 includes: turning on the sine wave signal generator, superimposing the sine signal on the triangular wave scanning signal through an adder and then outputting it to the laser controller to modulate the two-color circularly polarized light Doppler-free absorption spectrum, thereby obtaining the modulated two-color circularly polarized light Doppler-free absorption spectrum, and at the same time inputting the sine wave signal as a reference signal into the phase-locked amplifier, and converting the Doppler-free absorption spectrum signal into a dispersion signal through the phase-locked amplifier demodulation, the zero-point frequency in the dispersion signal corresponds to the frequency locking point frequency, and after obtaining the dispersion signal, adjusting the amplitude and frequency of the reference signal so that the Doppler-free absorption spectrum signal and the dispersion signal at the frequency locking point are clearly visible.

[0017] Step 4 includes the following formula:

[0018]

[0019] Where u(t) is the PID output signal, e(t) and e(τ) are both PID input signals, i.e., dispersion signals serving as error signals, and K p is the proportional gain, K i is the integral gain, K d is the differential gain, t and τ are both time.

[0020] The technical effects of the present invention are as follows: the present invention provides a laser frequency stabilization method based on two-color circularly polarized light without Doppler absorption spectrum, which uses two-color circularly polarized light without Doppler absorption spectrum as an error signal to achieve the laser frequency stabilization under the geomagnetic field. 87 The frequency of the RbD1 line is locked. To ensure the feasibility of laser frequency stabilization, the advantages of the two-color circularly polarized light without Doppler absorption spectrum are theoretically analyzed from the perspective of energy level. The wavelength generated by the tunable external cavity diode laser (ECDL) is around 795nm (corresponding to 87 The monochromatic linearly polarized laser with the atomic transition frequency of the RbD1 line is used. The electro-optic modulator (EOM) controlled by the 3.417GHz radio frequency signal converts the 795nm monochromatic light polarization into two-color linearly polarized light with a frequency of 377THz (corresponding to 795nm) ±3.417GHz. The two-color linearly polarized light is converted into two-color circularly polarized light through a quarter wave plate (QWP). The two-color circularly polarized light is incident on the air chamber as the pump light, and after passing through the air chamber, it is converted into two-color linear polarized light through the quarter wave plate. After being reflected by the reflector, the linear polarized light is again converted into two-color circularly polarized light with the opposite rotation direction to the pump light through the quarter wave plate. The two-color circularly polarized light with the opposite rotation direction is input into the air chamber again as the detection light, and the photodetector receives the light signal to obtain the two-color circularly polarized light Doppler-free absorption spectrum. The triangle wave signal is input to the laser controller to scan the laser frequency to obtain 87 The two-color circularly polarized light of the RbD1 line has a Doppler-free spectrum. Adjust the center frequency of the laser so that the locking point is located at the center of the scanning signal. Output a sinusoidal wave signal to the laser controller to modulate the laser current. Input the sinusoidal wave signal as a reference signal into the phase-locked amplifier, and use the phase-locked amplifier to demodulate to obtain the dispersion signal corresponding to the spectral signal. Adjust the frequency and amplitude of the sinusoidal wave signal so that both the spectral signal and the dispersion signal are clearly visible. Turn off the triangular wave signal input to keep the laser frequency at the locking point. Input the dispersion signal as an error signal into the proportional integral differential (PID) module, and the PID module outputs a signal to the laser control system to dynamically adjust the laser frequency. When the laser frequency shifts, the laser controller receives the error signal to adjust the laser current and the resonant cavity piezoelectric ceramic voltage to return the laser frequency to the stable frequency point again, thereby achieving 87Frequency locking of the RbD1 line. This method uses two-color light to produce the coherent population trapping (CPT) effect on the basis of a Doppler-free absorption optical path, reduces the absorption peak line width at the frequency locking point, and then increases the slope of the dispersion signal, improving the sensitivity and stability of the frequency system; the absorption peak of the circularly polarized light spectrum signal still has a large amplitude under the geomagnetic field, and no additional magnetic shielding measures are required, thereby simplifying the optical path structure and laying the foundation for miniaturization. This technology can be used in CPT magnetometers, CPT atomic clocks and other fields, with strong applicability and high practical value. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 The present invention is a schematic flow chart of a laser frequency stabilization method based on a two-color circularly polarized light Doppler-free absorption spectrum. Figure 1 The method comprises the steps of: 1, using a laser, an electro-optic modulator and a quarter wave plate to obtain a two-color circularly polarized light; 2, using a polarization beam splitter and a reflector to realize the reverse propagation of the pump light and the detection light, and adjusting the circular polarization rotation direction of the detection light to be opposite to the pump light through the quarter wave plate, so as to obtain a two-color circularly polarized light Doppler-free absorption spectrum; 3, turning on the laser scanning and modulation, using a photodetector to receive the absorption spectrum and transmit it to a phase-locked amplifier for demodulation, obtaining a dispersion signal corresponding to the spectrum, adjusting the laser current and scanning range, finding the dispersion signal corresponding to the frequency locking point, adjusting the amplitude and frequency of the laser modulation signal, so that both the spectrum signal and the dispersion signal are clearly visible. 4, turning off the laser scanning, inputting the dispersion signal as an error signal into a PID control module (PID, proportional-integral-derivative, proportional integral differential), controlling the laser frequency change through the PID output signal, and realizing laser frequency stabilization.

[0022] Figure 2 The present invention is a schematic diagram of the structure of a laser frequency stabilization system involved in the laser frequency stabilization method based on a two-color circularly polarized light Doppler-free absorption spectrum.

[0023] Figure 3 This is a schematic diagram of a three-level system that does not change with the magnetic field in the CPT effect. CPT is coherent population trapping. Figure 3 m F is the magnetic quantum number; 5 2 S 1 / 2 and 5 2 P 1 / 2 express 87 Atomic energy level of Rb atom at D1 line; F g represents the ground state hyperfine level in a three-level system; F e represents the excited state hyperfine level in the three-level system; σ +represents left-handed circularly polarized light; σ - Represents right-handed circularly polarized light.

[0024] The reference numerals are as follows: 1-laser controller; 2-tunable external cavity semiconductor laser (ECDL); 3-first polarization-maintaining fiber; 4-RF signal generator; 5-electro-optic modulator (EOM, Electro-Optic Modulator, used to modulate monochromatic laser into dual-color laser with frequencies of 377THz (corresponding to 795nm) ± 3.417GHz); 6-second polarization-maintaining fiber; 7-laser collimator; 8-optical isolator; 9-first 1 / 4 wave plate (Quarter-waveplate, QWP); 10-reflector (its reflected light is incident into the atomic gas chamber again as detection light); 11-second 1 / 4 wave plate (used to reverse the circular polarization states of the lasers incident on the gas chamber twice, for example, one is left-handed circularly polarized light and the other is right-handed circularly polarized light); 12-atomic gas chamber; 13-depolarizing beam splitter (NPBS, Non-Polarizing Beam Splitter ... Splitter); 14-photodetector (PD, photodetector, used to obtain two-color circularly polarized light Doppler-free spectrum); 15-phase-locked amplifier; 16-proportional-integral-derivative controller (PID, proportional-integral-derivative); 17-triangular wave signal generator; 18-sine wave signal generator; 19-adder. DETAILED DESCRIPTION

[0025] Below is the attached figure ( Figure 1-Figure 3 ) and Examples illustrate the present invention.

[0026] Figure 1 The present invention is a schematic flow chart of a laser frequency stabilization method based on a two-color circularly polarized light Doppler-free absorption spectrum. Figure 2 The present invention is a schematic diagram of the structure of a laser frequency stabilization system involved in the laser frequency stabilization method based on a two-color circularly polarized light Doppler-free absorption spectrum. Figure 3 This is a schematic diagram of a three-level system that does not change with the magnetic field in the CPT effect. Figures 1 to 3As shown, a laser frequency stabilization method based on a two-color circularly polarized light Doppler-free absorption spectrum comprises the following steps: step 1, inputting a first-handed two-color circularly polarized light as a pump light into a first end of an atomic gas chamber, the first-handed two-color circularly polarized light is changed into a second-handed two-color circularly polarized light as a detection light after being emitted from a second end of the atomic gas chamber, the second handed direction is opposite to the first handed direction, and the second-handed two-color circularly polarized light enters a photoelectric detection system after being emitted from the first end; step 2, performing laser frequency scanning, so that two-color circularly polarized light of second handed directions with different frequencies forms a two-color circularly polarized light Doppler-free absorption spectrum in the photoelectric detection system; step 3, determining a frequency locking point in the two-color circularly polarized light Doppler-free absorption spectrum, and obtaining a dispersion signal corresponding to the frequency locking point by demodulating the two-color circularly polarized light Doppler-free absorption spectrum; step 4, closing the laser frequency scanning, inputting the dispersion signal as an error signal into a PID control module, controlling the laser frequency change through a PID output signal, and realizing laser frequency stabilization.

[0027] The first end of the atomic gas chamber 12 is respectively connected to the photodetector 14 and the first 1 / 4 wave plate 9 through the depolarizing beam splitter prism 13, the first 1 / 4 wave plate 9 is connected to the laser collimator 7 through the optical isolator 8, the laser collimator 7 is connected to the electro-optic modulator 5 through the second polarization-maintaining fiber 6, the electro-optic modulator 5 is respectively connected to the radio frequency signal generator 4 and the first polarization-maintaining fiber 3, the first polarization-maintaining fiber 3 is connected to the laser controller 1 through the tunable external cavity semiconductor laser 2, the laser controller 1 is connected to the output end of the phase-locked amplifier 15 through the proportional-integral-differential controller 16, the first input end of the phase-locked amplifier 15 is connected to the photodetector 14, the second input end of the phase-locked amplifier 15 is connected to the sine wave signal generator 18, the sine wave signal generator 18 is connected to the first input end of the adder 19, the second input end of the adder 19 is connected to the triangular wave signal generator 17, the output end of the adder 19 is connected to the laser controller 1, and the second end of the atomic gas chamber 12 is connected to the reflector 10 through the second 1 / 4 wave plate 11.

[0028] Step 1 includes: using a laser and an electro-optic modulator to generate a frequency that satisfies 87The Rb atom D1 line satisfies the two-state hyperfine energy level frequency difference of 6.834GHz, and the wavelength of the D1 line is 795nm. The 795nm monochromatic linear polarized light emitted by the tunable external cavity semiconductor laser is modulated into a frequency conforming to 377THz±3.417GHz two-color linear polarized light by an electro-optic modulator controlled by a radio frequency signal with a frequency of 3.417GHz, and 377THz corresponds to a wavelength of 795nm, and then the two-color linear polarized light is converted into two-color circular polarized light by a 1 / 4 wave plate. Step 2 includes: turning on the triangle wave signal generator, inputting the triangle wave signal to the laser controller, scanning the laser frequency within a certain range, and observing the two-color circular polarized light generated by the coherent population capture CPT effect through an oscilloscope without Doppler absorption spectrum.

[0029] Step 3 includes: finding the peak of the absorption peak with large amplitude and small line width in the two-color circularly polarized light Doppler-free absorption spectrum as the stabilization point, and selecting 87 The ground state energy level F in the two-color non-Doppler absorption spectrum of RbD1 line g =1 and F g =2 transition to excited state energy level F e =2, the CPT three-level system consisting of two ground state energy levels and one excited state energy level is not affected by the magnetic field. When the pump light and the detection light are two-color circularly polarized light propagating in opposite directions, they interfere constructively because of their opposite rotation directions. The CPT absorption peak amplitude reaches the maximum. When it is used as the frequency locking point, the dispersion signal slope is the largest. Therefore, in the geomagnetic field environment, the two-color circularly polarized light Doppler absorption spectrum still has an absorption peak with a large amplitude that can be used for laser frequency stabilization. After the frequency stabilization point is determined, the laser center frequency is adjusted so that the frequency locking point is at the center of the periodic signal.

[0030] Step 4 includes: turning on the sine wave signal generator, superimposing the sine signal on the triangular wave scanning signal through an adder and then outputting it to the laser controller to modulate the two-color circularly polarized light Doppler-free absorption spectrum, thereby obtaining the modulated two-color circularly polarized light Doppler-free absorption spectrum, and at the same time inputting the sine wave signal as a reference signal into the phase-locked amplifier, and converting the Doppler-free absorption spectrum signal into a dispersion signal through the phase-locked amplifier demodulation, the zero-point frequency in the dispersion signal corresponds to the frequency locking point frequency, and after obtaining the dispersion signal, adjusting the amplitude and frequency of the reference signal so that the Doppler-free absorption spectrum signal and the dispersion signal at the frequency locking point are clearly visible.

[0031] Step 4 includes the following formula:

[0032]

[0033] Where u(t) is the PID output signal, e(t) and e(τ) are both PID input signals, i.e., dispersion signals serving as error signals, and K pis the proportional gain, K i is the integral gain, K d is the differential gain, t and τ are both time.

[0034] The laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectroscopy uses two-color circularly polarized light as the frequency stabilization light source and combines it with the Doppler-free absorption optical path to obtain the two-color circularly polarized light Doppler-free absorption spectrum. The two-color Doppler-free absorption spectrum uses the coherent population trapping (CPT) effect at the frequency locking point ( 87 RbD1 line) to obtain an absorption peak with a narrow line width, and at the same time, the characteristics of circularly polarized light in the CPT effect are used to overcome the disadvantage of small signal amplitude under the geomagnetic field, so that the two-color circularly polarized light Doppler-free absorption spectrum is used for laser frequency stabilization under the geomagnetic field. After building the two-color circularly polarized light Doppler-free absorption optical path, the laser frequency is scanned using a triangular wave signal to obtain a two-color circularly polarized light Doppler-free absorption spectrum. The laser current is adjusted so that the frequency locking point is located at the center of the signal. On the basis of the triangular wave signal, the spectral signal is modulated by superimposing a sine wave signal with an adder. The sine wave signal is input as a reference signal into the phase-locked amplifier, and the dispersion signal corresponding to the absorption spectrum is obtained by demodulating the phase-locked amplifier. The frequency and amplitude of the sine signal are adjusted to make the spectral signal and the dispersion signal clearly visible. Turn off the laser frequency triangular wave scanning, input the dispersion signal as an error signal into the proportional integral differential (PID) module for processing, output the processed signal to the laser control module, and adjust the laser frequency in real time to return to the stable frequency point frequency again to achieve laser frequency stabilization.

[0035] The principle of the laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectroscopy, refer to Figure 1 , 1-Laser controller, used to adjust the frequency and intensity of the laser emitted by the laser. 2-Tunable external cavity semiconductor laser, to obtain monochromatic linear polarized light with a frequency of about 795nm. 3-Polarization-maintaining fiber, to transmit the laser from the laser to the electro-optic modulator. 4-RF signal generator, to generate a radio frequency signal with a frequency of about 3.417GHz. 5-Electro-optic modulator, to modulate the monochromatic laser into a two-color laser with frequencies of 377THz (corresponding to 795nm) ±3.417GHz. 6-Polarization-maintaining fiber, to transmit the laser from the electro-optic modulator to the laser collimator. 7-Laser collimator, to emit the laser into space, so that the laser changes from fiber light to spatial light. 8-Optical isolator, to make the light propagate in one direction only, to prevent the light from propagating in the opposite direction to the electro-optic modulator and causing damage to it. 9-1 / 4 wave plate, to convert linearly polarized light into circularly polarized light. 10-Reflector, which reflects the laser light and re-enters the atomic gas chamber as detection light. The laser light before reflection is marked in yellow, and the laser light after reflection is marked in blue.

[0036] 11-1 / 4 wave plate, which reverses the circular polarization state of the laser light entering the gas chamber twice (i.e. one is left-handed circularly polarized light and the other is right-handed circularly polarized light). 12-Atomic gas chamber, vacuum gas chamber, filled with natural rubidium ( 87 Rb accounts for 27.85, 85 Rb accounts for 72.15). 13-Depolarizing beam splitter, the laser is divided into two beams of equal size and unchanged polarization state after transmission and reflection, among which the reflected light of the yellow laser and the transmitted light of the blue laser are retained. 14-Photodetector, used to receive the detection light, convert the optical signal into an electrical signal, and obtain a two-color circularly polarized light Doppler-free spectrum. 15-Phase-locked amplifier, modulates the spectral signal using the sinusoidal wave signal as a reference signal to obtain a dispersion signal. 16-Proportional integral differential (PID) controller, performs proportional integral differential processing on the dispersion signal and outputs it to the laser controller. 17-Triangular wave signal generator, outputs a triangular wave signal to the laser controller to realize frequency scanning. 18-Sine wave signal generator, outputs a sine wave signal to the laser controller and the phase-locked amplifier to realize signal modulation and demodulation. 19-Adder, superimposes the triangular wave signal and the sine wave signal and inputs them to the laser controller.

[0037] When alkali metal atoms react with light of a specific frequency, the atoms absorb the energy of the light and undergo transitions. When two-color light reacts with alkali metal atoms, the coherent population trapping (CPT) effect occurs, and the atoms are trapped in the ground state energy level, so that the atoms no longer absorb the energy of light. This is reflected in the spectrum as a transmission peak at the atomic transition frequency point. However, when two beams of two-color circularly polarized light that propagate in opposite directions (i.e., one beam is left-handed light and the other is right-handed light) react with atoms at the same time to produce the CPT effect, the atoms trapped in the ground state due to the CPT effect have enhanced absorption relative to the other beam of light, resulting in a sharp absorption peak in the spectrum. The apex of the absorption peak corresponds to the frequency at the frequency locking point, so the Doppler-free absorption spectrum of two-color circularly polarized light can be used for laser frequency stabilization. This article adopts 87 Ground state F in the two-color non-Doppler absorption spectrum of Rb atom D1 line g =1 and F g =2 transition to excited state F e =2 is used as the stable frequency point, and the ECDL laser is used to generate 87 The monochromatic linearly polarized laser corresponding to the RbD1 line (795nm) is half-wave modulated by the EOM controlled by a 3.417GHz RF signal to obtain a two-color linearly polarized light containing ±1-order sideband light. The frequency difference of the sideband light is 6.834GHz, which roughly satisfies 87The ground state hyperfine energy level difference of RbD1 line. The obtained two-color linear polarized light is converted into two-color circular polarized light through a 1 / 4 wave plate. The two-color circular polarized light is incident into the gas chamber as pump light and detection light respectively through a reflector and a 1 / 4 wave plate. At this time, the rotation directions of the pump light and the detection light are opposite. Turn on the triangle wave signal generator to scan the laser frequency, and the optical signal of the detection light is received by the photodetector to obtain 87 The RbD1 line two-color circularly polarized light has no Doppler absorption spectrum. Adjust the laser current and the triangle wave scanning range to make the ground state F g =1 and F g =2 transition to excited state F e =2, the corresponding absorption peak vertex is located at the center of the periodic signal. The sinusoidal modulated signal is superimposed on the triangular wave scanning signal by an adder, and is modulated and demodulated into a dispersion signal by a phase-locked amplifier. The frequency and amplitude of the sinusoidal signal are adjusted to make both the spectral signal and the dispersion signal clearer. Turn off the laser frequency scanning, input the dispersion signal as an error signal into the PID module, and the PID outputs the error signal to the laser control module. The laser dynamically adjusts the laser current and the resonant cavity piezoelectric ceramic voltage according to the received error signal to make the laser frequency return to the locking point again, thereby achieving laser frequency stabilization.

[0038] This method requires five steps to realize a laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectroscopy.

[0039] Step 1: Two-color circularly polarized light generation

[0040] In order to find the corresponding frequency locking point, it is necessary to generate a frequency that satisfies 87 The Rb atomic D1 line (795nm) and the two-state hyperfine energy level difference (6.834GHz) are two-color circularly polarized light. Usually, the laser frequency of the ECDL laser is adjusted to 87 Near the D1 line of Rb atoms, the monochromatic linear polarized light that meets the requirements is emitted. The EOM controlled by the 3.417GHz radio frequency signal converts the monochromatic linear polarized light into two-color linear polarized light with the frequency difference that meets the requirements, and then the 1 / 4 wave plate converts the two-color linear polarized light into two-color circular polarized light.

[0041] Step 2: Build a two-color circularly polarized light path without Doppler absorption

[0042] According to the Doppler effect, the actual transition frequency when light and atoms react is affected by the relative speed of light and atoms, which leads to an increase in the line width of the corresponding absorption peak. In order to eliminate the influence of the Doppler effect, we refer to the saturated absorption optical path and use the two-color circularly polarized light as the pump light and the detection light to propagate into the gas chamber in opposite directions. Because the pump light and the detection light have the same frequency and propagate in opposite directions, only atoms with zero speed can react with the two groups of light at the same time, thereby eliminating the influence of the Doppler effect and reducing the absorption peak line width at the locking point.

[0043] When the two-color light reacts with the alkali metal atoms and the frequency matches the transition of the two ground state hyperfine energy levels to the same excited state, they form a three-level system, and the coherent population trapping (CPT) effect occurs, and the atoms are trapped in the ground state energy level. When the counter-propagating pump light and detection light simultaneously produce the CPT effect with the atoms, the atoms trapped by the pump light will have enhanced absorption of the detection light, and vice versa, resulting in a sharp absorption peak in the spectrum. The following is an analysis of the atomic energy levels involved in the CPT effect:

[0044] In a magnetic field, 87 The D1 line energy level of the Rb atom will undergo Zeeman splitting. F =0 ground state hyperfine level |F g ,m F =0>, the corresponding energy is:

[0045]

[0046] Where E(F g ,m F =0) represents the magnetic quantum number m F =0 87 The energy of the ground state hyperfine level of the D1 line of the Rb atom, F g express 87 The ground state hyperfine level of the D1 line of Rb atom, F g =1,2;m F represents the magnetic quantum number; h is Planck's constant, which is 6.626×10 -34 ;v hfs is the microwave frequency corresponding to the ground state energy level when there is no external magnetic field, and its value is 6.834×10 9 ; I is the nuclear spin quantum number, which is 3 / 2.

[0047] The energy result is independent of the magnetic field size, so 87 The number of magnetic field sub-elements in the D1 line of Rb atom m F = 0, the ground state Zeeman self-level does not move with the magnetic field. g =1,m F =0> and |F g =2,m F =0>The CPT effect composed of two ground state energy levels can exist in the geomagnetic environment. Below we analyze the CPT three-level system composed of these two ground state energy levels.

[0048] Figure 3 Medium F Represents the magnetic quantum number; 5 2 S 1 / 2 and 5 2 P1 / 2 represents the atomic energy level of 87Rb atom at D1 line; F g represents the ground state hyperfine level in a three-level system; F e represents the excited state hyperfine level in the three-level system; σ + represents left-handed circularly polarized light; σ - Represents right-handed circularly polarized light.

[0049] When left-handed and right-handed circularly polarized light 87 When Rb atoms interact with each other, they can form Figure 3 The two ground state energy levels in the three-level system are not affected by the magnetic field, and these two systems share the same ground state, so the left-right two-color light is respectively at |F g =1,m F =0>,|F g =2,m F =0> The dark state prepared at the energy level will interfere. Assume that the electric field vectors of the two sets of two-color light interacting with the atom are

[0050]

[0051] Represents the electric field vector of the two-color light, where the subscript j = 1, 2 is used to distinguish two-color light of different frequencies, and the positive and negative signs represent the left-handed and right-handed components, respectively. Represents the intensity of two sets of two-color light electric field; is a unit vector; e is a natural constant, with a value of approximately 2.71828; i is an imaginary unit; ω j is the corresponding two-color light frequency; t is the time; Represents two sets of two-color light wave vectors, in vacuum there are represents the optical path length of the light field; Represents the initial phase of the light field. Then the Rabi frequency corresponding to different light fields when they resonate at the corresponding energy level is:

[0052]

[0053] It represents the Rabi frequency corresponding to the resonance of different circularly polarized two-color light fields at the corresponding energy levels, where the subscript j = 1, 2 is used to distinguish two-color light of different frequencies, and the positive and negative signs represent the left-handed and right-handed components respectively; Represents the intensity of two sets of two-color light electric field; is a unit vector; represents the corresponding electric dipole moment; is the reduced Planck constant, e is a natural constant, with a value of approximately 2.71828; i is an imaginary unit; ω j is the corresponding two-color light frequency; t is the time; Represents two sets of two-color light wave vectors, in vacuum there are represents the optical path length of the light field; Represents the initial phase of the light field.

[0054] Two sets of two-color circularly polarized light in |F g =1,m F =0>,|F g =2,m F =0> The dark states formed by the energy levels are

[0055]

[0056] |D ± > indicates that two groups of two-color circularly polarized light are in |F g =1,m F =0>,|F g =2,m F =0> The dark state formed by the energy level, the positive and negative signs represent the left and right components respectively; Indicates different circularly biased two-color light fields in |F g =1,m F =0>The Rabi frequency corresponding to the energy level resonance; Indicates different circularly biased two-color light fields in |F g =2,m F =0>The Rabi frequency corresponding to the energy level resonance;

[0057]

[0058] for 87 The transition of the D1 line of Rb atom is

[0059]

[0060] is a unit vector; represents the corresponding electric dipole moment,

[0061] Assumptions Available

[0062]

[0063] represents the electric field intensity of two sets of two-color light; e is a natural constant, with a value of approximately 2.71828; i is an imaginary unit; represents the optical path length of the light field; Represents two sets of two-color light wave vectors, in vacuum there are The initial phase of the light field represents the optical path difference between the left-handed and right-handed two-color light. When , the CPT effects produced by the left-handed and right-handed three-level systems interfere constructively, and when When the two interfere with each other.

[0064] for Figure 3 In the three-level system that is not affected by the magnetic field, when the pump light and the detection light are two-color circularly polarized light propagating in opposite directions, the two have opposite rotation directions (i.e., one is left-handed circularly polarized light and the other is right-handed circularly polarized light), and constructive interference occurs. The amplitude of the CPT absorption peak reaches the maximum, and the dispersion signal slope is the largest when it is used as the frequency locking point. When the pump light and the detection light are two-color linearly polarized light, we regard the linearly polarized light as a mixture of left-handed circularly polarized light and right-handed circularly polarized light in equal proportions. At this time, the CPT effect caused by the left-handed light and the right-handed light interferes destructively, resulting in almost zero contribution to the CPT peak. Therefore, in the geomagnetic field environment, the Doppler absorption spectrum of two-color circularly polarized light still has an absorption peak with a large amplitude, which can be used for laser frequency stabilization; while the absorption peak amplitude of the Doppler absorption spectrum of two-color linearly polarized light decreases rapidly and cannot be used as a reference for frequency stabilization.

[0065] Based on the above theory, we set up an experimental optical path to obtain a two-color circularly polarized light Doppler-free absorption spectrum. The two-color circularly polarized light changes its optical path through the depolarizing beam splitter NPBS and is used as the pump light incident on a 87 The laser is then emitted and converted into two-color linear polarized light by a quarter wave plate. After being reflected by a reflector, it is then converted into two-color circular polarized light with a rotation direction opposite to that of the pump light. This light is then incident into the gas chamber again as the detection light. After the detection light exits the gas chamber, it is received by a photodetector PD to obtain 87 Two-color circularly polarized light Doppler-free absorption spectrum of the RbD1 line.

[0066] Step 3: Find the right frequency lock point

[0067] Turn on the laser frequency scan and find the absorption peak with large amplitude and small line width in the two-color circularly polarized light Doppler-free absorption spectrum as the stabilization point. Here we choose 87 The ground state energy level F in the two-color non-Doppler absorption spectrum of RbD1 line g =1 and F g =2 transition to excited state energy level F e = 2 corresponding absorption peak. After the stable frequency point is determined, adjust the center frequency of the laser so that the frequency locking point is at the center of the periodic signal.

[0068] Step 4: Modulate and demodulate the spectral signal into a dispersion signal

[0069] The sine wave signal used for modulation is superimposed on the scanning signal, and the two-color circularly polarized light Doppler-free absorption spectrum is modulated and demodulated by a phase-locked amplifier to obtain the corresponding dispersion signal.

[0070] The two-color light Doppler-free absorption spectrum is simplified to:

[0071] G(ω)=f(ω)+g(ω)

[0072] f(ω)=-(aω 2 +bω+c)

[0073]

[0074] G(ω) is the function expression of the absorption peak of the two-color light without Doppler spectrum. f(ω) represents the Doppler background in the spectrum line component, which is a binomial and is displayed as a Fokker profile. g(ω) represents the transition component of the hyperfine energy level of the spectrum line, which is a Lorentz profile. a, b, c, K are unknown coefficients. ω is the actual frequency of the laser. γ is the line width of the spectrum line. g(ω) is axially symmetrical, and its center point frequency ω0 corresponds to the atomic transition frequency, i.e., the reference frequency of the laser frequency stabilization, and corresponds to the extreme point of the spectrum line in the signal.

[0075] Add a sine wave signal Asin(Ωt) to the laser frequency ω0, which is called the modulation signal. Suppose the modulation frequency is Ω, then the frequency after modulation is:

[0076] ω′=ω0+Asin(Ωt) (8)

[0077] A is the modulation depth and |A|≤γ, γ is the spectral line width, Ω is the modulation frequency; t is time. Taylor expand G(ω) at ω0:

[0078]

[0079] Then multiply a sin(Ωt) (called the reference signal, note that it is different from the reference frequency) and G(ω′) to extract the DC term:

[0080]

[0081] Where T = 2π / Ω is the period of the modulation signal. It can be seen that this DC term is proportional to the product of the first harmonic (first differential) of the spectrum signal G(ω) and the modulation depth A, which is the first harmonic (first differential) signal of the spectrum, commonly known as the error signal, and the mathematical process of integration is realized in the circuit through a phase-locked amplifier.

[0082] The absorption peak in the spectrum signal corresponds to the zero-crossing point in the dispersion signal, which is convenient for locking and judging the frequency offset of the laser. Adjust the frequency and amplitude of the sine wave signal to make both the spectrum signal and the dispersion signal clearer.

[0083] Step 5: Implement closed-loop control of laser frequency stabilization

[0084] Turn off the scanning signal, input the dispersion signal as the error signal into the PID module, and the PID outputs a frequency-stabilized signal to the laser control system. The result of the PID module is the result of adding the corresponding three algorithms (proportional, integral, and differential), and its input is the error value e(t) (this article refers to the numerical change of the dispersion signal). Define u(t) as the output value, and the PID algorithm can be expressed as follows:

[0085]

[0086] Where K p is the proportional gain; K i is the integral gain; K d is the differential gain; e(t) is the error.

[0087] Proportional regulation (P) means that the output of the controller is proportional to the input error signal. When only proportional regulation is used, there is a steady-state error in the system output. Proportional regulation is a proportional response to the deviation of the system. Once the system has a deviation, proportional regulation immediately produces a regulating effect to reduce the deviation. A large proportional parameter can speed up regulation and reduce errors, but an excessively large proportional parameter will reduce the stability of the system and even cause system instability.

[0088] In integral control (I), the output of the controller is proportional to the integral of the input error signal. For an automatic control system, if there is a steady-state error after entering the steady state, it is called a differential system. In order to eliminate the steady-state error, an "integral term" must be introduced in the controller. The integral term is the integral of the error depending on time, and as time increases, the integral term continues to accumulate. In this way, even if the error is very small, the integral term will increase with time, which drives the controller's output to increase and further reduce the steady-state error until it is equal to zero. When the error is zero, the integral control output is constant. The strength of the integral effect depends on the integral time constant K i ,K i The smaller K is, the stronger the integral effect is. i If the integral is too large, the integral effect will be weak. Adding integral regulation will reduce the stability of the system and slow down the dynamic response. It is often combined with the other two types of regulation to form a PI regulator or PID regulator.

[0089] In differential regulation (D), the output of the controller is proportional to the differential of the input error signal (i.e., the rate of change of the error). The automatic control system may oscillate or even become unstable during the process of overcoming the error. The reason is that there are large inertia components (links) or hysteresis components, which have the function of suppressing the error, and their changes always lag behind the changes in the error. In order to solve this problem, the change of the error suppression function can be made "ahead of time". That is, it is often not enough to introduce only the proportional term in the controller. The role of the proportional term is only to amplify the amplitude of the error. What needs to be added now is the "differential term", which can predict the trend of the error change. In this way, the controller with proportional + differential can suppress the change of the error in advance, thereby avoiding serious overshoot of the controlled quantity. Therefore, for the controlled object with large inertia or hysteresis, the proportional + differential (PD) controller can improve the dynamic characteristics of the system during the adjustment process. Before the deviation is formed, it is suppressed or eliminated by differential regulation, thereby improving the dynamic performance of the system. Appropriate differential parameters can reduce overshoot and shorten the adjustment time. However, the differential action has an amplifying effect on noise interference, so too strong differential regulation is not conducive to the system's anti-interference. The differential action cannot be used alone, and needs to be combined with the other two regulation rules to form a PID controller.

[0090] The laser control system dynamically adjusts the laser current or piezoelectric ceramic voltage according to the size and positive and negative dynamics of the obtained frequency stabilization signal, adjusts the laser frequency to the locking frequency point, and thus dynamically locks the laser frequency.

[0091] The contents not described in detail in the specification of the present invention belong to the prior art known to the professional and technical personnel in the field. It is pointed out here that the above description helps those skilled in the art to understand the invention, but does not limit the protection scope of the invention. Any equivalent replacement, modification and / or simplification of the above description without departing from the essence of the invention falls within the protection scope of the invention.

Claims

1. A laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectrum, characterized in that: The following steps are involved: Step 1, inputting a first-handed two-color circularly polarized light as a pump light into a first end of an atomic gas cell, the first-handed two-color circularly polarized light is emitted from a second end of the atomic gas cell and then changed into a second-handed two-color circularly polarized light as a detection light and then inputted into the atomic gas cell from the second end, the second handed light being opposite to the first handed light, and the second-handed two-color circularly polarized light being emitted from the first end and then entering a photoelectric detection system; Step 2, performing laser frequency scanning so that the second-handed two-color circularly polarized light with different frequencies forms a two-color circularly polarized light Doppler-free absorption spectrum in the photoelectric detection system; Step 3, determining a frequency locking point in the two-color circularly polarized light Doppler-free absorption spectrum, and obtaining a dispersion signal corresponding to the frequency locking point by demodulating the two-color circularly polarized light Doppler-free absorption spectrum; Step 4: turn off the laser frequency scanning, input the dispersion signal as an error signal into the PID control module, control the laser frequency change through the PID output signal, and achieve laser frequency stabilization.

2. The laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectrum according to claim 1, characterized in that: The first end of the atomic gas chamber is respectively connected to a photodetector and a first 1 / 4 wave plate through a depolarizing beam splitter, the first 1 / 4 wave plate is connected to a laser collimator through an optical isolator, the laser collimator is connected to an electro-optic modulator through a second polarization-maintaining optical fiber, the electro-optic modulator is respectively connected to a radio frequency signal generator and a first polarization-maintaining optical fiber, the first polarization-maintaining optical fiber is connected to a laser controller through a tunable external cavity semiconductor laser, the laser controller is connected to an output end of a phase-locked amplifier through a proportional-integral-differential controller, a first input end of the phase-locked amplifier is connected to the photodetector, a second input end of the phase-locked amplifier is connected to a sine wave signal generator, the sine wave signal generator is connected to a first input end of an adder, a second input end of the adder is connected to a triangular wave signal generator, an output end of the adder is connected to the laser controller, and a second end of the atomic gas chamber is connected to a reflector through a second 1 / 4 wave plate.

3. The laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectrum according to claim 1, characterized in that: Step 1 includes: using a laser and an electro-optic modulator to generate a frequency that satisfies 87 The Rb atomic D1 line satisfies the two-state hyperfine energy level frequency difference of 6.834 GHz and the two-color circularly polarized light, wherein the wavelength of the D1 line is 795 nm, and the 795 nm monochromatic linearly polarized light emitted by the tunable external cavity semiconductor laser is modulated into a two-color linearly polarized light with a frequency conforming to 377 THz±3.417 GHz by an electro-optical modulator controlled by a radio frequency signal with a frequency of 3.417 GHz, wherein 377 THz corresponds to a wavelength of 795 nm, and then the two-color linearly polarized light is converted into two-color circularly polarized light by a 1 / 4 wave plate.

4. The laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectrum according to claim 1, characterized in that: Step 2 includes: turning on the triangle wave signal generator, inputting the triangle wave signal into the laser controller, scanning the laser frequency within a certain range, and observing the two-color circularly polarized light Doppler-free absorption spectrum generated by the coherent population capture CPT effect through an oscilloscope.

5. The laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectrum according to claim 1, characterized in that: Step 3 includes: finding the peak of the absorption peak with large amplitude and small line width in the two-color circularly polarized light Doppler-free absorption spectrum as the stabilization point, and selecting 87 The ground state energy level F in the two-color non-Doppler absorption spectrum of RbD1 line g =1 and F g =2 transition to excited state energy level F e =2, the CPT three-level system consisting of two ground state energy levels and one excited state energy level is not affected by the magnetic field. When the pump light and the detection light are two-color circularly polarized light propagating in opposite directions, they interfere constructively because of their opposite rotation directions. The CPT absorption peak amplitude reaches the maximum. When it is used as the frequency locking point, the dispersion signal slope is the largest. Therefore, in the geomagnetic field environment, the two-color circularly polarized light Doppler absorption spectrum still has an absorption peak with a large amplitude that can be used for laser frequency stabilization. After the frequency stabilization point is determined, the laser center frequency is adjusted so that the frequency locking point is at the center of the periodic signal.

6. The laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectrum according to claim 1, characterized in that: Step 4 includes: turning on the sine wave signal generator, superimposing the sine signal on the triangular wave scanning signal through an adder and then outputting it to the laser controller to modulate the two-color circularly polarized light Doppler-free absorption spectrum, thereby obtaining the modulated two-color circularly polarized light Doppler-free absorption spectrum, and at the same time inputting the sine wave signal as a reference signal into the phase-locked amplifier, and converting the Doppler-free absorption spectrum signal into a dispersion signal through the phase-locked amplifier demodulation, the zero-point frequency in the dispersion signal corresponds to the frequency locking point frequency, and after obtaining the dispersion signal, adjusting the amplitude and frequency of the reference signal so that the Doppler-free absorption spectrum signal and the dispersion signal at the frequency locking point are clearly visible.

7. The laser frequency stabilization method based on two-color circularly polarized light Doppler-free absorption spectrum according to claim 1, characterized in that: Step 4 includes the following formula: Where u(t) is the PID output signal, e(t) and e(τ) are both PID input signals, i.e., dispersion signals serving as error signals, and K p is the proportional gain, K i is the integral gain, K d is the differential gain, t and τ are both time.

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