Laser frequency locking and coherent population trapping resonance polarization selection detection system and method
By employing a polarization-selective detection system in the CPT atomic clock, and using λ/4 and λ/2 waveplates to separate the beam, interference from fundamental and higher-order sideband light is eliminated, improving the contrast of the CPT signal. This solves the problem of poor signal quality in traditional circularly polarized light schemes and realizes a high-performance CPT atomic clock.
Patent Information
- Application Number
- CN202511046481.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-07-29
- Publication Date
- 2025-11-04
AI Technical Summary
Traditional circularly polarized light schemes cause most atoms to fall into the polarized dark state, resulting in poor CPT signal quality. Furthermore, CPT signals are susceptible to laser frequency and phase jitter, leading to low signal contrast and making it difficult to achieve high-performance CPT atomic clocks.
A polarization-selective detection system employing laser frequency locking and coherent population trapping resonance utilizes λ/4 and λ/2 waveplate beam splitters to separate the beam. Combined with a polarization beam splitter and a photodetector, the polarization is selected by adjusting the waveplate angle, eliminating interference from fundamental and higher-order sideband light and improving signal contrast.
A high-contrast CPT resonance signal was achieved, improving the performance of the CPT atomic clock, making it suitable for miniaturized applications, and demonstrating significant frequency stability and signal quality in miniaturized CPT atomic clocks.
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Figure CN120890554A_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The present application relates to the field of optoelectronics, more particularly to a polarization selective detection system and method for laser frequency locking and coherent population trapping resonance. BACKGROUND
[0002] Coherent driving light field with several GHz frequency difference couples two ground state levels to an excited state, which makes the transmission light through the atomic bubble narrow, this phenomenon is called coherent population trapping, coherent population trapping (CPT) resonance is a quantum interference phenomenon, and a miniaturized and micro-sized atomic clock based on the CPT principle has gradually become a research hotspot in recent years. Due to its small size and low power consumption, it has wide application prospects in global positioning system, network communication industry, underwater navigation and other fields.
[0003] The passive CPT atomic clock uses a microwave modulated vertical cavity surface emitting laser (VCSEL) as a light source, and uses two sidebands in the multicolor light sideband to interact with the atoms to prepare a CPT state in a left-handed or right-handed circularly polarized state. This scheme is simple in optical path and easy to miniaturize, but the use of circularly polarized light causes most of the atoms to fall into the polarization dark state, thereby not contributing to the resonance signal; in addition, only the ±1 order sideband in the coherent multicolor light carries the CPT information, and other frequency components are detected as optical background noise. These two shortcomings make the obtained CPT signal quality poor, which affects the performance of the CPT clock.
[0004] Although this traditional scheme has advantages in simplicity and miniaturization potential, it has several drawbacks. Circularly polarized light will extract most of the atoms into the Zeeman magnetic levels that cannot participate in CPT resonance. In addition, the CPT atomic clock is susceptible to fluctuations in the injection current and environmental disturbances. These disturbances cause frequency and phase jitter of the laser, which manifests as significant frequency noise. This noise, after frequency-amplitude conversion, ultimately leads to changes in the atomic energy levels and the optical frequency. Only the ±1 order sideband is excited for CPT resonance, and the carrier and high-order sidebands of the same laser will not only produce AC Stark shift, but also be detected as background noise. Contrast, defined as the ratio of the amplitude of CPT resonance to the background signal, is a key indicator of atomic clock performance, so the contrast is usually less than 10% when circularly polarized light is used.
[0005] Therefore, stabilizing the laser frequency is crucial to reducing the impact of frequency fluctuations on the stability of the atomic clock. In a compact CPT atomic clock, the Doppler-broadened spectrum of the atoms absorbing the multicolor laser is extracted to stabilize the laser frequency; the non-Doppler CPT resonance spectrum caused by the interaction of the atoms with the ±1 sideband of the multicolor laser output is extracted to stabilize the microwave frequency.
[0006] Elliptical polarization light plus differential detection technique has been proposed for compact CPT atomic clocks. Although this method effectively suppresses the background signal from unwanted optical elements, it also reduces the magneto-optical rotation (MOR) signal in the CPT spectrum. To address this limitation, a method was previously introduced to detect the MOR signal using elliptical polarization light, which has the advantage of laser noise suppression. However, this technique presents challenges in extracting the Doppler spectrum line required for laser frequency stabilization. SUMMARY
[0007] Therefore, the present application provides a polarization selection detection system and method for laser frequency locking and coherent population trapping resonance, which can simultaneously realize laser frequency locking and high-contrast CPT resonance observation.
[0008] To achieve the above object, the present application adopts the following technical solutions:
[0009] The polarization selection detection system for laser frequency locking and coherent population trapping resonance comprises a vertical cavity surface emitting laser, a first λ / 4 wave plate, an atomic cell, a second λ / 4 wave plate, a λ / 2 wave plate, a polarization beam splitter, a first photodetector, and a second photodetector. Linearly polarized light emitted by the vertical cavity surface emitting laser becomes elliptical polarized light after passing through the first λ / 4 wave plate, and then interacts with the Rb atom in the atomic cell. Polarization selection is achieved by adjusting the optical axis angle of the first λ / 4 wave plate and the second λ / 4 wave plate. The light beams after polarization selection pass through the second λ / 4 wave plate, the λ / 2 wave plate, and the polarization beam splitter to be divided into two beams, which are detected by the first photodetector and the second photodetector to obtain photodetector signals. 87 Rb atom interaction, polarization selection is achieved by adjusting the optical axis angle of the first λ / 4 wave plate and the second λ / 4 wave plate, and the light beams after polarization selection pass through the second λ / 4 wave plate, the λ / 2 wave plate, and the polarization beam splitter to be divided into two beams, which are detected by the first photodetector and the second photodetector to obtain photodetector signals.
[0010] Optionally, the atomic cell is placed in a solenoid coil, the solenoid is used to provide a magnetic field in the direction of light propagation and to provide a quantization axis for the system and separate the atomic magnetic energy levels; a multi-layer magnetic shielding barrel is placed outside the solenoid coil to shield the interference of the environmental magnetic field on the system.
[0011] Optionally, a lens and a neutral attenuator are sequentially arranged between the vertical cavity surface emitting laser and the first λ / 4 wave plate.
[0012] Optionally, polarization selection is achieved by adjusting the optical axis angle of the first λ / 4 wave plate and the second λ / 4 wave plate, wherein the angle corresponding to the position of maximum contrast of the CPT resonance signal is 90°.
[0013] Optionally, the system further comprises a first lock-in amplifier and a second lock-in amplifier, which are connected to the first photodetector and the second photodetector, respectively; the first lock-in amplifier is connected to a frequency modulation module and a radio frequency module in sequence and input into a bias generator, and the second lock-in amplifier is connected to a data acquisition module and a direct current servo module in sequence and input into the bias generator.
[0014] Optionally, the biasing device is used to superimpose microwaves and input into the vertical cavity surface emitting laser.
[0015] A polarization selection detection method of coherent population trapping resonance, using the laser frequency locking and polarization selection detection system of coherent population trapping resonance, comprises the following steps:
[0016] When linearly polarized light emitted from the laser becomes elliptically polarized light after passing through the first lambda / 4 wave plate, and interacts with the atomic bubble 87 Rb atoms, the polarization is achieved by adjusting the angle between the first lambda / 4 wave plate and the optical axis direction of the first lambda / 4 wave plate; the optical signals are obtained by detecting the two beams after the second lambda / 4 wave plate, the lambda / 2 wave plate and the polarization beam splitter with the first photodetector and the second photodetector respectively.
[0017] Compared with the prior art, the laser frequency locking and polarization selection detection system and method of coherent population trapping resonance provided by the technical scheme can remove the background signal caused by the VCSEL base frequency and high-order sideband light in the CPT signal, and improve the signal contrast. The influence of the relative angle between the two quarter-wave plates on the contrast is studied, and compared with the traditional circularly polarized light excitation CPT resonance scheme, the CPT resonance signal with a clock transition signal contrast of up to 31% is obtained according to the theoretical model. This scheme only needs to make a slight change in the optical path, but the signal quality is obviously improved, and is very suitable for miniaturized CPT atomic clock application. By using microelectromechanical processing (MEMS) integration, it can also be applied to miniaturized CPT atomic clocks. At the same time, it is proposed that the polarization selection CPT resonance scheme can be realized by using pulsed light to obtain high-contrast and narrow-line-width signals, which can be applied to atomic clocks, and the performance index has a great space for improvement. BRIEF DESCRIPTION OF DRAWINGS
[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description. Obviously, the drawings in the following description are only embodiments of the present application, and those skilled in the art can obtain other drawings according to the provided drawings without creating any creative labor.
[0019] Figure 1 To participate 87 Rb coherent population trapping excited energy level diagram; wherein, a is the ground state energy level diagram of clock transition, b is the actual energy level simplified diagram;
[0020] Figure 2 The experimental device diagram of the present application;
[0021] Wherein, 1-Vertical cavity surface emitting laser, 2-Lens, 3-Neutral attenuator, 4-First λ / 4 wave plate, 5-Solenoid, 6-Second λ / 4 wave plate, 7-λ / 2 wave plate, 8-Polarization beam splitter, 9-First photodetector, 10-Second photodetector, 11-Atomic bubble;
[0022] Figure 3a It is the first laser frequency stabilization scheme diagram of the application;
[0023] Figure 3b It is the second laser frequency stabilization scheme diagram of the application;
[0024] Figure 4 It is the contrast and angle θ curve diagram of the application;
[0025] Figure 5 It is the coherent population trapping (CPT) resonance signal contrast and total laser intensity dependence diagram of the application under the polarization selection excitation scheme (square) and (circular) excitation scheme;
[0026] Figure 6a It is the CPT signal diagram and the modulation signal diagram of the phase-locked amplifier of the application;
[0027] Figure 6b It is the intermediate CPT signal amplification diagram of the application;
[0028] Figure 7 It is the noise spectrum diagram of two schemes of the application;Wherein a is a circular polarization scheme, b is a polarization filtering scheme, c is an electronic noise spectrum of a light-free photodiode. DETAILED DESCRIPTION
[0029] The technical solutions in the embodiments of the application will be apparently and completely described below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative work belong to the protection scope of the application.
[0030] The embodiment of the application discloses a laser frequency locking and coherent population trapping resonance polarization selection detection system, as shown in Figure 2 The vertical cavity surface emitting laser 1, the first λ / 4 wave plate 4, the second λ / 4 wave plate 6, the λ / 2 wave plate 7, the polarization beam splitter 8, the first photodetector 9, the second photodetector 10 and the atomic bubble 11 are included. 87The Rb atom acts as a polarization selector, which is achieved by adjusting the optical axis angle between the first λ / 4 waveplate 4 and the second λ / 4 waveplate 6. The polarization selector beam is then split into two beams by the second λ / 4 waveplate 6, the λ / 2 waveplate 7 and the polarization beam splitter 8, respectively. The photoelectric signals are then detected by the first photodetector 9 and the second photodetector 10.
[0031] Furthermore, the atomic bubble is placed outside the coil of solenoid 5 to provide a magnetic field along the direction of light propagation, while providing a quantization axis for the system and separating atomic magnetic energy levels; a multi-layer magnetic shielding barrel is placed outside the coil of solenoid 5 to shield the system from interference from the ambient magnetic field.
[0032] Furthermore, a lens 2 and a neutral density attenuator 3 are sequentially arranged between the vertical cavity surface-emitting laser and the first λ / 4 waveplate.
[0033] Furthermore, polarization selection is achieved by adjusting the angle between the optical axes of the first λ / 4 waveplate and the second λ / 4 waveplate, wherein the angle corresponding to the position of maximum contrast of the CPT resonance signal is 90°.
[0034] Furthermore, it also includes a first lock-in amplifier and a second lock-in amplifier, which are connected to the first photodetector and the second photodetector, respectively; the first lock-in amplifier is connected in sequence to the frequency modulation module and the radio frequency module, and input to the bias circuit; the second lock-in amplifier is connected in sequence to the data acquisition module and the DC servo module, and input to the bias circuit.
[0035] Furthermore, the bias is used to superimpose microwaves and input them into a vertical cavity surface-emitting laser.
[0036] More specifically, this embodiment uses a wavelength of 87 The elliptically polarized light of the Rb D1 line, with the excited state Fe=2 as the upper energy level coupling level, can excite four CPT-Λ configurations for the ground state energy level of the clock transition, such as... Figure 1 As shown by the solid line in the middle, due to the ground state m F =0-m F There are many atoms with =0, and the actual energy level simplifies to: Figure 1 As shown in b. Wherein is the excited state m F =+1 coupling is called right-handed light (σ) + ), and excited state m F =-1 coupling is called left-handed light (σ) - ).
[0037] Assuming the laser beam propagates along the quantum axis defined by an external magnetic field, the intensity of the CPT produced by the interaction of light with atoms, when detected in the θ direction, can be written as:
[0038]
[0039] where α + and α - are the absorption coefficients of left and right circularly polarized light, respectively, l c is the length of the atomic vapor cell, λ is the wavelength of the light field in vacuum, ψ is the angle of magnetic optical rotation, ω is the optical frequency, n ± is the refractive index, Δν is the linewidth of the CPT resonance, χ0 ± is the amplitude of the linear susceptibility, and δ is the Raman detuning. According to the density matrix equation, we have
[0040]
[0041] where n is the particle density, γ2 is the coherence relaxation rate of the ground state, d is the optical transition coefficient, ε0 is the vacuum permeability, h is the Planck constant, Ω p = Ω 2 / Γ, and Γ is the spontaneous emission rate of the excited state, and Ω is the Rabi frequency, which measures the strength of the interaction between the laser and the atom.
[0042] Since the light intensity that contributes to the CPT resonance varies with the angle θ, while the light that does not produce CPT resonance does not vary with θ. Since 2πα ± l c / λ < < 1, -2π(α + + α - )l c / λ < < 1, from equation (1) we can see that the transmitted light is:
[0043] I = I c sin 2 (θ + ψ) + I nc sin 2 (θ) (5)
[0044] Since the angle of magnetic optical rotation is very small, the amplitude of the light background I DC of the CPT resonance information is:
[0045] I DC = I C sin 2 (θ) + I nc sin 2 (θ) (6) The amplitude of the CPT signal Ipp:
[0046]
[0047] The contrast is defined as:
[0048] Contrast = I pp / (I DC + Ipp (8)
[0049] Once θ is determined, the relationship between the CPT resonance amplitude and the degree of polarization of light is as follows:
[0050]
[0051] E- and E+ are the amplitudes of left- and right-hand circularly polarized light.
[0052] This embodiment uses an integrable VCSEL as the laser source, outputting a divergent beam with a bandwidth of approximately 100MHz. A lens collimates it into a parallel beam. Working atom 87 Rb, a buffer gas of N2 and CH4 at a pressure of 20 Torr, are sealed in a 1:2 ratio within an atomic bubble 40 mm in length and 20 mm in diameter. The atomic bubble is placed outside a solenoid coil, which provides a magnetic field of B = 24 μT along the direction of light propagation, while also providing the quantization axis and isolating the light source. Figure 1 The atomic magnetic energy level of b. A three-layer magnetic shielding barrel is placed outside the solenoid coil; its function is to shield the system from interference from the ambient magnetic field. To obtain the coherent two-color light required for the experiment, a microwave with a power of 6.5 dBm and a frequency of ν was used. hfs Microwaves ≈ 3.417 GHz are superimposed on the DC current input to the VCSEL via a Bias-T circuit. Driven by this current, the VCSEL emits frequency-modulated multicolor line-polarized light, using ±1st order sidebands as coherent two-color light for preparing the CPT state. The VCSEL and the atomic bubble are each controlled by a temperature control system at their respective operating temperatures: the VCSEL temperature is controlled at 25°C, and the sample bubble temperature is controlled at 60°C.
[0053] The output light from the VCSEL passes through a lens and an attenuator sequentially, becoming linearly polarized light with a spot diameter of approximately 4 mm. When an injected current of 1.5 mA is applied, the VCSEL emits 795 nm laser light. After the linearly polarized light emitted from the laser passes through the first λ / 4 waveplate and becomes elliptically polarized, it interacts with the atomic bubble... 87 The polarization is selected by adjusting the angle θ between the optical axes of the first and second λ / 4 waveplates through the action of Rb atoms. Then, the beam is split into two beams by the second λ / 4 waveplate, λ / 2 waveplate and polarization beam splitter PBS, and the photoelectric signals are obtained by detecting the beams separately using photodetectors (Thorlabs, PDA36A-EC).
[0054] This embodiment also discloses a polarization selection detection method for coherent population trapping resonance, utilizing the aforementioned laser frequency locking and coherent population trapping resonance polarization selection detection system, including the following steps:
[0055] When the linearly polarized light emitted from the laser passes through the first λ / 4 waveplate and becomes elliptically polarized, it interacts with the atomic bubble... 87The Rb atoms act as a means to achieve polarization by adjusting the angle between the optical axis directions of the first λ / 4 waveplate and the second λ / 4 waveplate. The beam is then split into two beams by the second λ / 4 waveplate, the λ / 2 waveplate, and the polarization beam splitter, and the photoelectric signals are obtained by the first photodetector and the second photodetector, respectively.
[0056] To achieve a high-performance CPT atomic clock, precise laser frequency stabilization is essential. A comparative study between the MOR detection scheme and the method of this invention is presented. Figure 3a It shows when polychromatic light and 87 The microwave-modulated D1 line transmission spectrum is recorded by the same photodetector (PD) during the interaction of Rb atoms. For laser frequency locking, modulation and demodulation techniques are used to generate an error signal from the absorption spectrum. The frequency discrimination slope of this error signal is an important indicator for evaluating frequency stabilization performance. The steeper the frequency discrimination slope, the better the laser frequency locking capability. Figure 3b In this study, the proposed method achieved a significant center slope of 0.175 mV / MHz in the error signal, which is 9 times higher than the 0.0197 mV / MHz slope obtained by the MOR detection method. This significant enhancement in the frequency discrimination slope demonstrates the superior performance of the proposed method in CPT atomic clock laser frequency stabilization.
[0057] In VCSELs, only the ±1st order sideband light interacts with atoms to produce CPT resonance, while the fundamental and higher order sideband light do not interact with atoms, and their intensity and polarization remain essentially unchanged after passing through the atomic bubble. However, due to birefringence and magneto-optical rotation, Rb atoms absorb the left-hand and right-hand circularly polarized components of the ±1st order sideband light differently, leading to changes in the relative intensity and polarization of the two types of light after passing through the bubble. The four transition coefficients differ; the dark CPT state prepared by the right-hand circularly polarized light is a bright state for the left-hand circularly polarized light. Therefore, the absorption of the left-hand circularly polarized light is more severe, resulting in lower background noise. The absorption of the weaker left-hand circularly polarized light is even more severe.
[0058] By passing through the second λ / 4 waveplate, the fundamental frequency and higher-order sideband light are restored to their polarization state before passing through the first λ / 4 waveplate. However, the ±1st-order sideband light cannot be restored to linearly polarized light and becomes approximately linearly polarized elliptically polarized light. The λ / 2 waveplate and PBS reflect almost all of the fundamental frequency and higher-order sideband light. The σ+ component in the ±1st-order sideband light is filtered out and reflected by the PBS and detected by PD2. Only the σ- component in the ±1st-order sideband light is transmitted through the PBS and detected by PD1. Therefore, the polarization filtering scheme eliminates the adverse effects of the fundamental frequency, higher-order sideband light, and the strong amplitude components in the ±1st-order sideband light on the CPT signal, greatly reducing the signal background and improving contrast.
[0059] Rotate the optical axis of the second λ / 4 waveplate to the angle θ between it and the first λ / 4 waveplate, at an optical power of 0.13 mW / cm. 2 Experimental results of time contrast versus polarization angle θ are as followsFigure 3a and Figure 3b As shown, Figure 3a and Figure 3b The curve in the figure is the theoretical curve based on equation (8). In the polarization selection scheme, the contrast of the CPT resonance signal is very sensitive to the angle of the first λ / 4 waveplate and the change in the relative angle between the first λ / 4 waveplate and the second λ / 4 waveplate. The included angle θ corresponding to the position of maximum contrast is 90°.
[0060] The CPT signal is optimal when the amplitude ratio of the left-hand and right-hand circularly polarized light is 0.4. Therefore, the optical axis of the emitted ray-polarized light is rotated by 21.8° relative to the first λ / 4 waveplate, and the second λ / 4 waveplate is rotated so that the angle θ between it and the first λ / 4 waveplate is 90°. By adjusting the neutral density filter, the relationship between the CPT resonance amplitude and the light intensity variation is obtained, as follows: Figure 4 As shown, the curve is drawn according to formula (9).
[0061] Short-term frequency stability is an important indicator of atomic clocks, and it is represented by the Allen variance σ(τ).
[0062]
[0063] k is the scaling factor, ν hfs It represents the hyperfine level difference of rubidium atoms, Δv is the linewidth of the CPT resonance line, and S is the CPT amplitude (corresponding to...). Figure 5 ), N is the signal-to-noise ratio (corresponding to...) Figure 7 ).from Figure 5 As can be seen, the CPT amplitude increases linearly with increasing light intensity. Moreover, under the same light intensity, the amplitude of the present invention is half that of the traditional scheme. However, the traditional scheme first decreases as the light intensity reaches saturation, while the present invention continues to increase with increasing light intensity. Figure 7 The noise diagrams of the two methods are shown at the point where the CPT amplitude is the largest. It can be seen that the noise of the present invention is more than ten times that of the traditional method. Therefore, the short-term stability will be greatly improved by using the present invention.
[0064] By changing the microwave detuning range to satisfy the Raman resonance condition, three CPT resonance peaks can be obtained, with the left one being m. F =-1-m F =-1, the middle CPT peak is the ground state m. F =0-m F =0 is generated, and the right side is m. F =+1-m F The signal generated by +1 shows that the amplitude of the middle CPT peak is an order of magnitude higher than the resonance peaks on both sides. Furthermore, the amplitude of right-handed light is greater than that of left-handed light, resulting in a larger resonance amplitude of the CPT on the right side compared to the left.
[0065] Figure 6a andFigure 6b In order to lock the microwave frequency, a frequency modulation signal is superimposed on the microwave signal, and the CPT resonance spectrum obtained by the photodetector 2 is synchronously demodulated to obtain a correction signal. The modulation depth of the modulation signal is 160 Hz, and the modulation frequency is 136 Hz.
[0066] As shown in Figure 7 the polarization selection scheme only needs to make a slight change in the optical path compared with the traditional scheme. Compared with the desktop system of the traditional scheme that has been realized, the scheme only adds a λ / 4 and a photodetector on the basis of the original optical path. Therefore, when applied to a small CPT atomic clock, the scale of the physical system in the direction of light propagation remains basically unchanged. When applied to a micro CPT atomic clock, the slight increase in the volume of the physical system manufactured by micro-electromechanical machining technology has negligible effect on the volume of the whole machine.
[0067] The various embodiments in the specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other. For the device disclosed by the embodiments, since it corresponds to the method disclosed by the embodiments, the description is relatively simple, and the related parts can be referred to the method part.
[0068] The above description of the disclosed embodiments enables a person skilled in the art to implement or use the present application. Various modifications to the embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.
[0069] The present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.
[0070] The present application will not be limited to the embodiments shown herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.
Claims
1. A polarization selection detection system for laser frequency locking and coherent population trapping resonance, characterized in that, include: Vertical cavity surface-emitting laser, first λ / 4 waveplate, atomic bubble, second λ / 4 waveplate, λ / 2 waveplate, polarization beam splitter, first photodetector and second photodetector; The linearly polarized light emitted from the vertical-cavity surface-emitting laser becomes elliptically polarized after passing through the first λ / 4 waveplate, and then interacts with the atomic bubbles inside. 87 The Rb atom acts as a polarization selector, which is achieved by adjusting the optical axis angle between the first λ / 4 waveplate and the second λ / 4 waveplate. The polarized beam is then split into two beams by the second λ / 4 waveplate, the λ / 2 waveplate, and the polarization beam splitter, respectively. The photoelectric signals are then detected by the first photodetector and the second photodetector, respectively.
2. The polarization selection detection system for laser frequency locking and coherent population trapping resonance according to claim 1, characterized in that, An atomic bubble is placed inside a solenoid coil. The solenoid provides a magnetic field for the atomic bubble along the direction of light propagation, while also providing a quantization axis for the system and separating atomic magnetic energy levels. A multi-layer magnetic shielding barrel is placed outside the solenoid coil to shield the system from interference from the ambient magnetic field.
3. The polarization selection detection system for laser frequency locking and coherent population trapping resonance according to claim 1, characterized in that, A lens and a neutral density filter are sequentially arranged between the vertical cavity surface-emitting laser and the first λ / 4 waveplate.
4. The polarization selection detection system for laser frequency locking and coherent population trapping resonance according to claim 1, characterized in that, Polarization selection is achieved by adjusting the angle between the optical axes of the first λ / 4 waveplate and the second λ / 4 waveplate, wherein the angle corresponding to the position of maximum contrast of the CPT resonance signal is 90°.
5. The polarization selection detection system for laser frequency locking and coherent population trapping resonance according to claim 1, characterized in that, It also includes a first lock-in amplifier and a second lock-in amplifier, which are connected to the first photodetector and the second photodetector, respectively; the first lock-in amplifier is connected to the frequency modulation module and the radio frequency module in sequence and input to the bias circuit; the second lock-in amplifier is connected to the data acquisition module and the DC servo module in sequence and input to the bias circuit.
6. The polarization selection detection system for laser frequency locking and coherent population trapping resonance according to claim 5, characterized in that, The bias is used to superimpose microwaves and input them into a vertical cavity surface-emitting laser.
7. A method for detecting the polarization selection of laser frequency locking and coherent population trapping resonance, utilizing the polarization selection detection system for laser frequency locking and coherent population trapping resonance as described in any one of claims 1-6, characterized in that, Includes the following steps: When the linearly polarized light emitted from the laser passes through the first λ / 4 waveplate and becomes elliptically polarized, it interacts with the atomic bubble... 87 The Rb atoms act as a means to achieve polarization by adjusting the angle between the optical axis directions of the first λ / 4 waveplate and the second λ / 4 waveplate. The beam is then split into two beams by the second λ / 4 waveplate, the λ / 2 waveplate, and the polarization beam splitter, and the photoelectric signals are obtained by the first photodetector and the second photodetector, respectively.