Single-line configuration modulation transfer spectroscopy laser frequency stabilization system and method

Abstract

The application relates to a single-line configuration modulation transfer spectrum laser frequency stabilization system and method, and relates to the technical field of laser frequency stabilization. The system comprises a laser source, a polarization selection element, an atomic gas chamber, an electro-optic modulator, a polarization rotation element, a reflection or light splitting element, a photoelectric detector and an electric control module. The laser source outputs laser to form forward propagation light which passes through the atomic gas chamber; the light beam which passes out is divided into detection light and return light through the reflection or light splitting element; the return light forms return modulation light after polarization rotation and modulation, and returns to the atomic gas chamber along the same main optical axis, and the forward propagation light and the return light are reversely spatially overlapped to generate a modulation transfer effect; the photoelectric detector receives the detection light to output an electric signal, and the electric control module demodulates an error signal and feeds back and adjusts the frequency of the laser source. The application integrates the optical path on a single main optical axis, eliminates independent alignment of double light beams, improves the anti-interference ability and long-term stability, and is convenient for system miniaturization integration.

Description

Technical Field

[0001] This application relates to the field of laser frequency stabilization technology, and in particular to a single-line configuration modulated transfer spectrum laser frequency stabilization system and method. Background Technology

[0002] Modulation transfer spectroscopy (MTS) is a spectroscopic technique that utilizes the nonlinear interactions of atomic or molecular media to generate a Doppler-free background error signal. This technique is commonly used in laser frequency stabilization systems that use atomic transition spectral lines as frequency references, enabling the generation of error signals suitable for closed-loop feedback control near the target transition frequency. This error signal exhibits significant frequency detuning near the zero-crossing point, and therefore can be used to characterize the degree and direction of deviation of the laser frequency from the reference transition.

[0003] In existing mature modulation-transfer spectral laser frequency stabilization systems, a classic dual-beam spatially opposed structure is typically employed. Specifically, the system splits the laser beam from the same source into two completely independent spatial paths: a pump beam and a probe beam. Multiple mirrors guide these two beams into the target atomic gas cell, allowing them to propagate in opposite directions within the cell and achieve precise spatial overlap as much as possible. The pump beam is loaded with an radio frequency modulation signal via an electro-optic modulator, while the probe beam is not directly loaded with this signal. When the two beams interact nonlinearly in the atomic medium, the modulation information on the pump beam is coupled to the probe beam through a modulation-transfer process. Subsequently, the probe beam carrying spectral information is received by a photodetector and converted into an electrical signal. After coherent demodulation with a reference signal of the same frequency, an error signal is obtained. Finally, a closed-loop feedback circuit controls the laser's frequency tuning end, achieving laser frequency locking.

[0004] The aforementioned traditional two-beam structure has been widely used, but its optical path configuration relies on two independent spatial beams achieving high-quality reverse overlap within the atomic gas chamber. Therefore, during setup and debugging, it is usually necessary to adjust the two optical paths separately and control the position, angle, and overlap area of ​​the two beams. Under conditions such as temperature changes, vibration, impact, or slow deformation of the mechanical structure, the relative positional relationship of the two beams may drift, thereby affecting the error signal amplitude, zero-point position, and long-term lock-in state.

[0005] Furthermore, dual-beam spatial through-beam structures typically require numerous discrete optical components and mechanical adjustment mechanisms, increasing optical path length, mounting dimensions, and packaging complexity. For benchtop experimental systems, these issues can be mitigated through optical platforms and mechanical supports; however, for laser frequency reference devices requiring miniaturization, modularity, long-term unattended operation, or industrial field applications, existing structures still offer room for improvement in terms of ease of assembly and adjustment, environmental adaptability, and engineering integration. Summary of the Invention

[0006] In view of the above problems, this application provides a single-line configuration modulation transfer spectral laser frequency stabilization system and method. By integrating the forward propagating light and the return modulation light into a single principal optical axis, the reverse spatial coincidence is naturally achieved, eliminating the traditional dual-beam spatial alignment process and improving the anti-interference capability, long-term stability and engineering integration of the frequency stabilization system.

[0007] In a first aspect, embodiments of this application provide a single-line configuration modulation-transfer spectral laser frequency stabilization system, including a laser source, a polarization selection element, an atomic gas cell, an electro-optic modulator, a polarization rotation element, a reflection or beam splitting element, a photodetector, a coherent radio frequency source, a demodulation module, and a feedback control module. The laser output from the laser source passes through the polarization selection element to form forward propagation light, which passes through the atomic gas cell along the principal optical axis. The beam exiting the atomic gas cell is split into detection light and return light by the reflection or beam splitting element. The return light passes through the polarization rotation element at least once in the round-trip optical path. The light returns to the atomic gas cell after passing through the device and the electro-optic modulator, forming a return-modulated light. The forward-propagating light and the return-modulated light propagate in opposite directions along the same principal optical axis and spatially overlap within the effective area of ​​the atomic gas cell, thereby generating a modulation transfer effect in the atomic medium, so that the detection light carries modulation transfer spectral information. The photodetector receives the detection light and outputs an electrical signal. The demodulation module performs coherent demodulation on the electrical signal based on the demodulation reference signal provided by the coherent radio frequency source to obtain an error signal. The feedback control module adjusts the frequency of the laser source according to the error signal.

[0008] Preferably, the polarization selection element is a polarization beam splitter prism; the polarization rotation element is a quarter-wave plate, the fast axis of the quarter-wave plate is at 45° to the linear polarization direction of the forward propagating light; after the returning light passes through the quarter-wave plate, its linear polarization direction is rotated by 90° relative to the initial incident polarization direction, and when it returns to the polarization beam splitter prism, it is deflected away from the laser source direction and discarded.

[0009] Preferably, the reflecting or beam-splitting element is a partial reflector; along the propagation direction of the forward-propagating light, the electro-optic modulator, the polarization rotation element, and the partial reflector are sequentially arranged on the main optical axis after the atomic gas cell; the photodetector is arranged on the transmission side of the partial reflector; part of the light incident on the partial reflector passes through the partial reflector to form the detection light, and the other part is reflected by the partial reflector and returns along the original optical path to form the return light.

[0010] Preferably, when the forward propagating light first passes through the electro-optic modulator, its polarization direction is perpendicular to the effective modulation polarization direction of the electro-optic modulator and no modulation occurs; when the return light, after being reflected by the partial reflector and rotated by the polarization rotation element, passes through the electro-optic modulator again, its polarization direction is parallel to the effective modulation polarization direction of the electro-optic modulator and is modulated.

[0011] Preferably, the reflecting or splitting element is a beam splitter; the beam splitter is disposed on the main optical axis between the atomic gas cell and the electro-optic modulator, and is used to split the light beam after passing through the atomic gas cell into a detection branch and a return modulation branch; the detection branch is connected to the photodetector, and the return modulation branch is arranged along the main optical axis in sequence with the electro-optic modulator, the polarization rotation element and the total reflection mirror.

[0012] Preferably, the light beam reflected by the total reflection mirror returns along the original optical path to form the return light; the return light passes through the polarization rotation element and the electro-optic modulator again, and is transmitted back to the atomic gas cell by the beam splitter, and propagates in the opposite direction to the forward propagating light and spatially coincides within the effective working area of ​​the atomic gas cell.

[0013] Preferably, the coherent RF source outputs a modulation drive signal for driving the electro-optic modulator and a demodulation reference signal for inputting the demodulation module. The modulation drive signal and the demodulation reference signal are coherent RF signals with the same frequency, and the relative phase between the modulation drive signal and the demodulation reference signal is adjustable.

[0014] Preferably, the electrical signal output by the photodetector is processed by a bias T-network, an AC coupling circuit, an amplification circuit, a filtering circuit, or an impedance matching circuit before being input to the demodulation module; the feedback control module is a PID controller or a digital control module containing a PID control algorithm, and outputs a feedback signal to the current tuning terminal, piezoelectric tuning terminal, or other frequency tuning terminal of the laser source.

[0015] Secondly, embodiments of this application provide a single-line configuration modulation transfer spectral laser frequency stabilization method, comprising the following steps: The laser output from the laser source is polarized by a polarization selection element to form forward propagating light, which is then guided into the atomic gas cell along the principal optical axis. The beam of light that has passed through the atomic gas cell is split into detection light and return light by a reflection or beam splitter; The returned light is made to return to the atomic gas cell after passing through at least a polarization rotation element and an electro-optic modulator in the round-trip optical path, forming a returned modulated light, wherein the electro-optic modulator is driven by a modulation drive signal; The back-modulated light and the forward-propagating light are made to propagate in opposite directions along the same principal optical axis and spatially overlap within the effective area of ​​the atomic gas cell, so as to generate a modulation transfer effect and make the detection light carry modulation transfer spectral information. The detection light is received by a photodetector and an electrical signal is output. An error signal is obtained by coherently demodulating the electrical signal using a demodulation reference signal that is coherent with the modulation drive signal. The laser source frequency is controlled by closed-loop feedback based on the error signal.

[0016] Preferably, the method further includes the step of: Before closed-loop feedback control, the laser frequency is scanned, and the modulation depth, the relative phase between the demodulation reference signal and the modulation drive signal, the detector gain, and the power distribution between the detection light and the return modulation light are adjusted to obtain the error signal near the target atomic spectral line. After obtaining the error signal, the working point is locked near the zero crossover point of the error signal. Closed-loop feedback control is activated and the feedback parameters are adjusted to stabilize the laser frequency near the target atomic spectral line.

[0017] The single-line configuration modulation-transfer spectral laser frequency stabilization system and method provided in this application integrates the forward propagating light and the return modulation light onto a single principal optical axis. By combining reflection or beam splitting elements with polarization rotation elements, beam folding and polarization conversion are achieved, allowing the beams to naturally reverse and coincide within the atomic gas chamber, eliminating the alignment process required for traditional dual-beam independent optical paths. This structure reduces the system's sensitivity to temperature drift and mechanical vibration, improving long-term operational stability. Simultaneously, it reduces the number of optical components and the optical path length, lowering assembly and adjustment difficulty and equipment cost, and facilitating miniaturized packaging and modular integration of the system. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments of this application will be briefly described below. Obviously, the drawings described below are only for illustrating preferred embodiments and are not intended to limit the scope of this application. In the drawings: Figure 1 A schematic diagram of the single-line configuration modulation-transfer spectral laser frequency stabilization system provided for an embodiment of this application.

[0019] Figure 2 A schematic diagram of a single-line configuration modulation-transfer spectral laser frequency stabilization system provided for another embodiment of this application.

[0020] The components include: laser source 10, polarization selection element 20, atomic gas cell 30, electro-optic modulator 40, polarization rotation element 50, partial reflector 61, beam splitter 62, total reflection mirror 70, photodetector 80, coherent radio frequency source 90, demodulation module 100, and feedback control module 110. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this application without creative effort are within the scope of protection of this application.

[0022] It should be noted that, unless otherwise expressly specified and limited, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit this application.

[0023] In its terminology, the terms "comprising" and "having," and any variations thereof, in this specification and claims are intended to cover non-exclusive inclusion. The terms "first," "second," etc., are used only to distinguish different objects and should not be construed as indicating or implying relative importance, or implicitly specifying the number, particular order, or primary / secondary relationship of the indicated technical features. Furthermore, "a plurality of" means two or more, unless otherwise explicitly defined.

[0024] In terms of spatial orientation, the technical terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. This is only for the convenience of describing the embodiments of this application and simplifying the description, and is not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this application.

[0025] Regarding connection relationships, the technical terms "installation," "connection," "joining," and "fixing" should be interpreted broadly. For example, it can refer to a fixed connection, a detachable connection, or an integral part; it can refer to a mechanical connection or an electrical connection; it can refer to a direct connection or an indirect connection through an intermediate medium; it can refer to the internal connection of two components or the interaction between two components. For those skilled in the art, the specific meaning of the above terms in this application can be understood according to the specific circumstances.

[0026] Firstly, embodiments of this application provide a single-line configuration modulation-transfer spectroscopy laser frequency stabilization system. As used in this specification, single-line configuration refers to the system's main working optical fields being arranged around the same principal optical axis. The forward-propagating light and the returning modulation light propagate in opposite directions along the same principal optical axis or a near-coaxial path within the effective working area of ​​the atomic gas cell 30 and spatially overlap, thus eliminating the need for two independent optical paths in traditional dual-beam modulation-transfer spectroscopy systems that require separate adjustments to their spatial overlap.

[0027] This single-line configuration modulation-transfer spectral laser frequency stabilization system is physically divided into an optical module and an electronic control module. The optical module is used to complete laser output, polarization selection, atomic interaction, backlight construction, modulation transfer, and photoelectric detection; the electronic control module is used to provide the modulation drive signal required for electro-optic modulation and the demodulation reference signal required for coherent demodulation, and to perform demodulation, error signal extraction, and feedback control calculations on the electrical signal output by the photodetector 80.

[0028] Specifically, the system mainly includes a laser source 10, a polarization selection element 20, an atomic gas cell 30, an electro-optic modulator 40, a polarization rotation element 50, a reflection or beam splitting element, a photodetector 80, a coherent radio frequency source 90, a demodulation module 100, and a feedback control module 110.

[0029] In this embodiment, the laser source 10 is a narrow-linewidth tunable laser source capable of covering the wavelength band of the transition spectral lines of the atoms or molecules to be locked. The atomic gas chamber 30 contains an alkali metal atomic medium or other target atomic medium suitable for forming a modulation transfer spectral error signal, and the atomic gas chamber 30 is used in conjunction with a temperature control structure to maintain stable atomic interaction conditions.

[0030] The laser output from laser source 10 forms forward propagating light after passing through polarization selection element 20. Furthermore, a half-wave plate can be placed in the optical path between laser source 10 and polarization selection element 20 to pre-adjust the polarization state of the laser before it enters the polarization selection element 20, maximizing the transmission power of the forward propagating light. The forward propagating light passes through atomic gas cell 30 along the principal optical axis. After exiting atomic gas cell 30, the beam is split into detection light and return light by reflection or beam splitting elements. The return light returns to atomic gas cell 30 after passing through polarization rotation element 50 and electro-optic modulator 40 at least in the round-trip optical path, forming return modulated light. The forward propagating light and return modulated light propagate in opposite directions along the same principal optical axis and spatially overlap within the effective working area of ​​atomic gas cell 30, generating a modulation transfer effect in the atomic medium, causing the detection light to carry modulation transfer spectral information. The photodetector 80 receives the detection light and outputs an electrical signal. The demodulation module 100 performs coherent demodulation on the electrical signal based on the demodulation reference signal provided by the coherent radio frequency source 90 to obtain an error signal. The feedback control module 110 adjusts the frequency of the laser source 10 according to the error signal.

[0031] In some embodiments, the polarization selection element 20 is a polarization beam splitter prism; in other alternative embodiments, a polarizer or other optical element capable of polarization selection and separation may also be used. The polarization rotation element 50 is a quarter-wave plate, or a polarization control element capable of equivalent polarization rotation may be used. The fast axis of the quarter-wave plate is at 45° to the linear polarization direction of the forward-propagating light. After the returning light passes through the quarter-wave plate, its linear polarization direction is rotated by 90° relative to the initial incident polarization direction, and when it returns to the polarization beam splitter prism, it is deflected away from the laser source 10 and discarded, thereby effectively avoiding optical feedback interference to the laser source 10 caused by the returning light.

[0032] In the first specific optical path implementation based on partial reflectors, such as Figure 1As shown, the reflecting or splitting element is a partial reflector 61. Along the propagation direction of the forward-propagating light, the electro-optic modulator 40, the polarization rotation element 50, and the partial reflector 61 are sequentially arranged on the principal optical axis after the atomic gas cell 30. The photodetector 80 is located on the transmission side of the partial reflector 61. Part of the light incident on the partial reflector 61 passes through the partial reflector 61 to form the detection light, while the other part is reflected by the partial reflector 61 and returns along the original optical path to form the return light. The reflectivity and transmittance of the partial reflector 61 are selected according to the distribution requirements of the return light intensity and the detection light power; for example, a ratio of 9:1 or 7:3 can be used. In this optical path structure, when the forward-propagating light first passes through the electro-optic modulator 40, its polarization direction is perpendicular to the effective modulation polarization direction of the electro-optic modulator 40 and no modulation occurs; when the return light, after being reflected by the partial reflector 61 and rotated by the polarization rotation element 50, passes through the electro-optic modulator 40 again, its polarization direction is parallel to the effective modulation polarization direction of the electro-optic modulator 40 and is modulated.

[0033] In the second specific optical path implementation based on the beam splitter, such as Figure 2 As shown, the reflecting or splitting element is a beam splitter 62. Beam splitter 62 is positioned on the main optical axis between the atomic gas cell 30 and the electro-optic modulator 40, and is used to split the beam exiting the atomic gas cell 30 into a detection branch and a return modulation branch. The splitting ratio of beam splitter 62 is selected according to the distribution requirements of the detection power and the return modulation light power, for example, a ratio of 1:9 or 3:7. The detection branch is connected to a photodetector 80, and the return modulation branch is sequentially arranged along the main optical axis with the electro-optic modulator 40, polarization rotation element 50, and total reflection mirror 70. The beam reflected by the total reflection mirror 70 returns along the original optical path to form return light. After passing through the polarization rotation element 50 and the electro-optic modulator 40 again, the return light is transmitted back to the atomic gas cell 30 via beam splitter 62, and propagates in the opposite direction to the forward propagating light within the effective working area of ​​the atomic gas cell 30, spatially coinciding with it.

[0034] Regarding the electrical and control structure of the above system embodiment, the coherent RF source 90 outputs two coherent RF signals. One signal serves as a modulation drive signal to drive the electro-optic modulator 40, and an RF power adjustment circuit, such as an RF amplifier or attenuator, can be included in this modulation drive branch to obtain an RF amplitude suitable for driving the electro-optic modulator 40. The other signal serves as a demodulation reference signal input to the demodulation module 100. In specific implementation, the modulation frequency is mainly selected based on the target atom transition linewidth, preferably set to half of the target atom transition linewidth, and matched in conjunction with the system operating conditions. The modulation drive signal and the demodulation reference signal are coherent RF signals with the same frequency, and the relative phase between them is adjustable. The electrical signal output by the photodetector 80 is processed by a bias T-network, AC coupling circuit, amplification circuit, filtering circuit, or impedance matching circuit before being input to the demodulation module 100. The demodulation module 100 performs coherent demodulation on the electrical signal based on the demodulation reference signal and outputs an error signal. The feedback control module 110 adopts an analog PID controller, a digital PID controller, a field programmable gate array (FPGA), a digital signal processor (DSP), or other closed-loop control structures, and outputs feedback signals to the current tuning terminal, piezoelectric tuning terminal, or other frequency tuning terminal of the laser source 10.

[0035] Secondly, embodiments of this application also provide a single-line configuration modulation-transfer spectral laser frequency stabilization method, which is implemented based on the above-mentioned single-line configuration modulation-transfer spectral laser frequency stabilization system and specifically includes the following steps.

[0036] S1. The laser output from the laser source 10 is polarized by the polarization selection element 20 to form forward propagating light, and then introduced into the atomic gas cell 30 along the main optical axis.

[0037] S2. The light beam that has passed through the atomic gas cell 30 is divided into detection light and return light by a reflection or beam splitting element.

[0038] S3. The returning light returns to the atomic gas cell 30 after passing through the polarization rotation element 50 and the electro-optic modulator 40 at least in the round-trip optical path, forming returning modulated light, wherein the electro-optic modulator 40 is driven by a modulation drive signal.

[0039] S4. Make the returning modulation light and the forward propagating light propagate in opposite directions along the same principal optical axis and spatially overlap within the effective working area of ​​the atomic gas cell 30 to produce a modulation transfer effect, so that the detection light carries modulation transfer spectral information.

[0040] S5. The photodetector 80 receives the detection light and outputs an electrical signal.

[0041] S6. The electrical signal is coherently demodulated using a demodulation reference signal that is coherent with the modulation drive signal to obtain the error signal.

[0042] S7. Perform closed-loop feedback control on the frequency of laser source 10 based on the error signal.

[0043] In some embodiments, prior to the step of performing closed-loop feedback control described above, the method further includes a system scan and lock preparation step.

[0044] Specifically, before closed-loop feedback control, the laser frequency is scanned by current tuning or piezoelectric tuning. The output signals of the photodetector 80 and demodulation module 100 are observed to find modulation transfer spectral error signals near the target atomic spectral lines. During this process, the modulation depth, the relative phase between the demodulation reference signal and the modulation drive signal, the detector gain, and the power distribution between the detection light and the return modulation light are adjusted to ensure that the demodulated output obtains an error signal with correct polarity and acceptable slope and symmetry near the target spectral lines. After obtaining the error signal, the frequency scan is stopped and the laser frequency is tuned to the vicinity of the target atomic spectral lines. The zero-crossing point of the error signal is used as the locking operating point. Closed-loop feedback control is then activated, and the feedback parameters are adjusted to enable the feedback loop to perform closed-loop adjustment of the laser source 10 frequency, thereby stabilizing the laser frequency near the target atomic spectral lines.

[0045] It should be noted that the technical solution provided in this application focuses on optimizing the optical path configuration, reducing assembly complexity, improving structural stability, and enhancing engineering implementation convenience, rather than being limited to necessarily being superior to traditional two-beam solutions in terms of error signal intensity or spectral performance. While retaining the basic principle of modulation-transfer spectral frequency stabilization, this solution provides a physical implementation method for laser frequency stabilization systems that is easy to build, package, and apply in engineering by adopting a single-line configuration that differs from traditional two-beam structures.

[0046] The single-line configuration modulation-transfer spectral laser frequency stabilization system and method provided in this application integrates the forward propagating light and the return modulation light onto a single principal optical axis. By combining reflection or beam splitting elements with polarization rotation elements, beam folding and polarization conversion are achieved, allowing the beams to naturally reverse and coincide within the atomic gas chamber, eliminating the alignment process required for traditional dual-beam independent optical paths. This structure reduces the system's sensitivity to temperature drift and mechanical vibration, improving long-term operational stability. Simultaneously, it reduces the number of optical components and the optical path length, lowering assembly and adjustment difficulty and equipment costs, and facilitating miniaturized packaging and modular integration of the system.

[0047] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and not to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application, and they should all be covered within the scope of the claims and specification of this application.

[0048] In particular, as long as there is no structural conflict, the various technical features mentioned in the various embodiments of this application can be combined in any way. This application is not limited to the specific embodiments disclosed herein, but includes all technical solutions falling within the scope of the claims.

Claims

1. A single-line configuration modulated transfer spectral laser frequency stabilization system, comprising a laser source, a polarization selection element, an atomic gas cell, an electro-optic modulator, a polarization rotation element, a reflection or beam splitting element, a photodetector, a coherent radio frequency source, a demodulation module, and a feedback control module, characterized in that, The laser beam output from the laser source passes through the polarization selection element to form forward propagation light, which passes through the atomic gas cell along the principal optical axis. After exiting the atomic gas cell, the beam is split into detection light and return light by the reflection or beam splitting element. The return light returns to the atomic gas cell after passing through at least the polarization rotation element and the electro-optic modulator in the round-trip optical path, forming return modulated light. The forward propagation light and the return modulated light propagate in opposite directions along the same principal optical axis and spatially overlap within the effective working area of ​​the atomic gas cell to generate a modulation transfer effect in the atomic medium, so that the detection light carries modulation transfer spectral information. The photodetector receives the detection light and outputs an electrical signal. The demodulation module performs coherent demodulation on the electrical signal based on the demodulation reference signal provided by the coherent radio frequency source to obtain an error signal. The feedback control module adjusts the frequency of the laser source according to the error signal.

2. The single-line configuration modulation-transfer spectral laser frequency stabilization system according to claim 1, characterized in that, The polarization selection element is a polarization beam splitter prism; the polarization rotation element is a quarter-wave plate, the fast axis of the quarter-wave plate is at 45° to the linear polarization direction of the forward propagating light; after the returning light passes through the quarter-wave plate, its linear polarization direction is rotated by 90° relative to the initial incident polarization direction, and when it returns to the polarization beam splitter prism, it is deflected away from the laser source direction and discarded.

3. The single-line configuration modulation-transfer spectral laser frequency stabilization system according to claim 1 or 2, characterized in that, The reflecting or splitting element is a partial reflector; along the propagation direction of the forward propagating light, the electro-optic modulator, the polarization rotation element, and the partial reflector are sequentially arranged on the main optical axis after the atomic gas cell; the photodetector is arranged on the transmission side of the partial reflector; part of the light incident on the partial reflector passes through the partial reflector to form the detection light, and the other part is reflected by the partial reflector and returns along the original optical path to form the return light.

4. The single-line configuration modulation-transfer spectral laser frequency stabilization system according to claim 3, characterized in that, When the forward-propagating light first passes through the electro-optic modulator, its polarization direction is perpendicular to the effective modulation polarization direction of the electro-optic modulator and no modulation occurs; when the return light, after being reflected by the partial reflector and rotated by the polarization rotation element, passes through the electro-optic modulator again, its polarization direction is parallel to the effective modulation polarization direction of the electro-optic modulator and is modulated.

5. The single-line configuration modulation-transfer spectral laser frequency stabilization system according to claim 1 or 2, characterized in that, The reflecting or splitting element is a beam splitter; the beam splitter is disposed on the main optical axis between the atomic gas cell and the electro-optic modulator, and is used to split the light beam after passing through the atomic gas cell into a detection branch and a return modulation branch; the detection branch is connected to the photodetector, and the return modulation branch is arranged along the main optical axis in sequence with the electro-optic modulator, the polarization rotation element and the total reflection mirror.

6. The single-line configuration modulation-transfer spectral laser frequency stabilization system according to claim 5, characterized in that, The beam reflected by the total reflection mirror returns along the original optical path to form the return light; the return light passes through the polarization rotation element and the electro-optic modulator again, and is transmitted back to the atomic gas cell by the beam splitter, and propagates in the opposite direction to the forward propagating light and spatially coincides within the effective working area of ​​the atomic gas cell.

7. The single-line configuration modulation-transfer spectral laser frequency stabilization system according to claim 1, characterized in that, The coherent RF source outputs a modulation drive signal for driving the electro-optic modulator and a demodulation reference signal for inputting the demodulation module. The modulation drive signal and the demodulation reference signal are coherent RF signals with the same frequency, and the relative phase between the modulation drive signal and the demodulation reference signal is adjustable.

8. The single-line configuration modulation-transfer spectral laser frequency stabilization system according to claim 1, characterized in that, The electrical signal output by the photodetector is processed by a bias T-network, an AC coupling circuit, an amplification circuit, a filtering circuit, or an impedance matching circuit before being input into the demodulation module. The feedback control module is a PID controller or a digital control module containing a PID control algorithm, and outputs a feedback signal to the current tuning terminal, piezoelectric tuning terminal, or other frequency tuning terminal of the laser source.

9. A method for stabilizing the frequency of a single-line configuration modulated transfer spectrum laser, characterized in that, Includes the following steps: The laser output from the laser source is polarized by a polarization selection element to form forward propagating light, which is then guided into the atomic gas cell along the principal optical axis. The beam of light that has passed through the atomic gas cell is split into detection light and return light by a reflection or beam splitter; The returned light is made to return to the atomic gas cell after passing through at least a polarization rotation element and an electro-optic modulator in the round-trip optical path, forming a returned modulated light, wherein the electro-optic modulator is driven by a modulation drive signal; The back-modulated light and the forward-propagating light are made to propagate in opposite directions along the same principal optical axis and spatially overlap within the effective area of ​​the atomic gas cell, so as to generate a modulation transfer effect and make the detection light carry modulation transfer spectral information. The detection light is received by a photodetector and an electrical signal is output. An error signal is obtained by coherently demodulating the electrical signal using a demodulation reference signal that is coherent with the modulation drive signal. The laser source frequency is controlled by closed-loop feedback based on the error signal.

10. The single-line configuration modulation transfer spectrum laser frequency stabilization method according to claim 9, characterized in that, It also includes the following steps: Before closed-loop feedback control, the laser frequency is scanned, and the modulation depth, the relative phase between the demodulation reference signal and the modulation drive signal, the detector gain, and the power distribution between the detection light and the return modulation light are adjusted to obtain the error signal near the target atomic spectral line. After obtaining the error signal, the working point is locked near the zero crossover point of the error signal. Closed-loop feedback control is activated and the feedback parameters are adjusted to stabilize the laser frequency near the target atomic spectral line.

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