Laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering

Abstract

The invention discloses a laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering, which is used for carrying out frequency stabilization on phase modulation multi-sideband laser, and comprises a frequency doubling laser unit which is configured to comprise a fundamental frequency seed light source, an electro-optical modulator, a fundamental frequency amplifier and a frequency doubling crystal; the light splitting module unit is configured to split the frequency doubling light into a main output light path and a monitoring light path; the atomic optical rotation filter is located on the monitoring light path and is configured to maintain the purity of the frequency-stabilized laser spectrum through purification treatment under the control of the axial magnetic field and the temperature; the frequency stabilization unit is configured to be a frequency stabilization light path and a servo system, and the frequency stabilization light path is used for obtaining laser frequency information and generating an error signal; the servo system is used for controlling the frequency of the fundamental frequency seed light source according to the error signal feedback. According to the laser frequency stabilization system provided by the invention, the spectrum purification and frequency stabilization precision can be improved, so that high-frequency stability locking is realized.

Description

Technical Field

[0001] This invention generally relates to the field of high-precision laser frequency stabilization technology. More specifically, this invention relates to a laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering. Background Technology

[0002] Cold atom systems utilize the interaction between lasers and atoms to cool, manipulate, and perform precise measurements. However, this interaction presupposes that the laser frequency matches the atomic transition frequencies, thus requiring a high-frequency response. In a freely operating laser system, frequency drift is influenced by environmental factors such as temperature and air pressure, leading to unstable or even ineffective laser-atomic interactions. Therefore, in cold atom systems, stable laser frequency is fundamental for subsequent atom manipulation; ensuring a stable output frequency within a certain range is crucial for the stable and reliable operation of the cold atom system.

[0003] Besides interference from the external environment, the stability of cold atom systems is also affected by the laser system design, impacting the accuracy of the laser frequency. For example, in atomic interferometry systems, to obtain a pair of Raman lasers with high phase correlation, an electro-optic modulator can be inserted into the laser to output a multi-frequency laser. This method of obtaining Raman lasers is simple, reliable, and well-suited for cold atom interferometry systems. However, due to the working principle of the electro-optic modulator, while modulating the desired sidebands, a series of additional sidebands are generated. If these additional sidebands are introduced into the laser frequency stabilization, they have a significant impact on the stabilization process. Besides causing severe laser frequency shifts, when the signal is swept and tuned during atomic manipulation, the laser frequency also undergoes corresponding periodic tuning, severely affecting high-precision measurements. Therefore, although the method of obtaining Raman lasers through modulation can greatly simplify the laser system of an atomic interferometer, its limitations in laser frequency accuracy prevent its widespread use.

[0004] In view of this, there is an urgent need to provide a laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering in order to improve the frequency accuracy of phase-modulated lasers. Summary of the Invention

[0005] In order to at least solve one or more of the technical problems mentioned above, the present invention proposes a laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering in several aspects.

[0006] In a first aspect, the present invention provides a laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering for stabilizing the frequency of a phase-modulated multi-sideband laser, characterized in that it comprises: a frequency doubling laser unit (1): configured to include a fundamental frequency seed light source (11), an electro-optic modulator (12), a fundamental frequency amplifier (13), and a frequency doubling crystal (14); wherein the electro-optic modulator (12) is configured to apply a frequency f to the fundamental frequency seed light. m Phase modulation to generate a signal containing carrier frequency f c and sideband f c ±nf m The modulated light, where n is an integer; the frequency doubling crystal (14) is used to convert the modulated fundamental frequency light into frequency doubling light; the beam splitting module unit is configured to split the frequency doubling light into a main output optical path (6) and a monitoring optical path (7), wherein the power ratio of the main output optical path (6) is ≥99% and the power ratio of the monitoring optical path (7) is ≤1%; the atomic optical rotation filter unit (2) is located in the monitoring optical path (7) and is configured to: under the control of axial magnetic field and temperature, its transmission peak center frequency matches the target atomic transition frequency, and the purity of the laser spectrum is maintained by purification treatment; wherein the purification treatment includes: selecting a single frequency or carrier or sideband, and the suppression ratio of other unselected frequency sidebands is greater than 20dB; the frequency stabilization unit (3, 4) is configured as a frequency stabilization optical path (3) for acquiring laser frequency information and generating error signals; and the servo system (4) is used to control the frequency of the fundamental frequency seed light source (11) according to the error signal feedback.

[0007] In some embodiments, the electro-optic modulator (12) is a waveguide device, connected to the fundamental frequency seed light source (11) and the fundamental frequency amplifier (13) via an optical fiber jumper, with a modulation frequency f. m Match the frequency difference between the two hyperfine levels of the ground state of the target atom.

[0008] In some embodiments, the frequency doubling crystal (14) is a periodically polarized lithium niobate crystal to convert a preset fundamental frequency light into a preset frequency doubling light.

[0009] In some embodiments, the atomic optical rotation filter (2) is a Faraday anomalous dispersion atomic filter, which includes: a first analyzer (21), an atomic gas chamber (22) which may be filled with target atomic vapor; an axial magnetic field generator (23) to provide a uniform magnetic field of a preset intensity; a temperature control device (25) to maintain the temperature of the atomic gas chamber (22) within a preset temperature range; and a second analyzer (24).

[0010] In some embodiments, the frequency-stabilized optical path (3) employs a modulation-transfer spectroscopy method, including: a beam splitter (31) and a signal extraction unit; wherein, the beam splitter divides the filtered monitoring light into pump light and probe light; the pump branch includes a phase modulator (EOM) (33); the probe branch includes a frequency shifter (AOM) (35) and a second gas chamber (36); the signal extraction unit includes a polarization beam splitter prism and a detector (38) for extracting interference-free error signals.

[0011] In a second aspect, the present invention provides a laser device including a laser frequency stabilization system as described in any embodiment of the first aspect for spectral purification and laser frequency stabilization of a phase-modulated frequency-stabilized laser.

[0012] The laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering provided above utilizes the polarization-dependent dispersion characteristics of atoms in a magnetic field to form an extremely narrow transmission window near the atomic resonance frequency. By transmitting only the carrier wave and first-order sidebands, higher-order sidebands are suppressed and physically filtered out, thereby eliminating error signal distortion. Furthermore, in some embodiments, modulation-transfer spectroscopy is combined with beam splitting design to servo-lock stability, thereby improving spectral purification and frequency stabilization accuracy and achieving high-frequency stability locking. Attached Figure Description

[0013] The above and other objects, features, and advantages of exemplary embodiments of the present invention will become readily apparent upon reading the following detailed description with reference to the accompanying drawings. In the drawings, several embodiments of the invention are illustrated by way of example and not limitation, and like or corresponding reference numerals denote like or corresponding parts, wherein:

[0014] Figure 1 An exemplary structural block diagram of a laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering according to an embodiment of the present invention is shown;

[0015] Figure 2 An exemplary structural block diagram of an atomic filtering device according to an embodiment of the present invention is shown;

[0016] Figure 3 An exemplary structural block diagram of a frequency-stabilized optical path according to an embodiment of the present invention is shown;

[0017] Figure 4 An exemplary structural block diagram of a laser device according to an embodiment of the present invention is shown. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0020] Figure 1 An exemplary structural block diagram of a laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering according to an embodiment of the present invention is shown.

[0021] like Figure 1 As shown, this embodiment of the invention provides a laser frequency stabilization system based on a Faraday anomalous dispersion optical filter (FADOF) for stabilizing the frequency of a phase-modulated multi-sideband laser. The system includes: a frequency-doubled laser unit (1): configured to include a fundamental frequency seed light source (11), an electro-optic modulator (12), a fundamental frequency amplifier (13), and a frequency-doubled crystal (14); wherein the electro-optic modulator (12) is configured to apply a frequency f to the fundamental frequency seed light. m Phase modulation to generate a signal containing carrier frequency f c and sideband f c ±nf m The modulated light, where n is the sideband order; the frequency doubling crystal (14) is used to convert the modulated fundamental frequency light into frequency doubling light.

[0022] In practical applications, the fundamental frequency seed light source (11) can be a 1560nm narrow-linewidth distributed feedback (DFB) laser with an output power of 10mW, a linewidth of less than 1kHz, and a long-term frequency drift rate of less than ±1M_Hz / h, thus providing a stable initial fundamental frequency light for the system. The electro-optic modulator (12) can be a waveguide-type lithium niobate (LiNbO3) modulator, connected to the fundamental frequency seed light source (11) and the fundamental frequency amplifier (13) via a single-mode fiber jumper. Its operating voltage is 4V, and its modulation bandwidth is 0~16GHz. It applies phase modulation to the fundamental frequency seed light at a modulation frequency of ~6.8G to generate a signal containing the carrier frequency f. cThe modulation light consists of (1560nm corresponding frequency) and ±n-order sidebands (n=1, 2, ...), with the sideband spacing equal to the modulation frequency ~6.8GHz. The fundamental frequency amplifier (13) can be an erbium-doped fiber amplifier (EDFA), employing a two-stage amplification structure to amplify the modulation light power from 10mW to 5W. During the amplification process, gain flattening technology is used to ensure the gain consistency of each frequency component, and the sideband suppression ratio change does not exceed 1dB. The frequency doubling crystal (14) can be a periodically polarized lithium niobate (PPLN) crystal with dimensions of 5mm×5mm×20mm and a polarization period of 7.5μm. It is placed in a constant temperature furnace with a temperature control accuracy of ±0.1℃. When the 1560nm fundamental frequency light is incident at an incident angle of 10°, it is converted into 780nm frequency doubling light through the second harmonic generation effect, with a conversion efficiency of 60% and an output frequency doubling light power of 3W.

[0023] The beam splitting module unit is configured to split the frequency-doubled light into a main output optical path (6) and a monitoring optical path (7), wherein the power of the main output optical path (6) accounts for ≥99% and the power of the monitoring optical path (7) accounts for ≤1%.

[0024] In practice, the beam splitting module unit can use a fused silica beam splitter with a 780nm wavelength beam splitting film deposited on its surface, resulting in a beam splitting ratio of 1:99. The 780nm frequency-doubled light output from the frequency-doubled crystal (14) is collimated by a collimating lens and then incident on the beam splitter. The 99% power beam serves as the main output light path (6), which is then focused by a focusing lens and output for use with... 87 Rb atoms interact; the remaining 1% power beam serves as the monitoring optical path (7), which enters the atomic optical rotation filter (2) for spectral purification. A temperature control device is provided at the edge of the beam splitter to ensure that the beam splitting ratio fluctuates by less than 0.1% when the ambient temperature changes by ±5℃.

[0025] Atomic optical rotation filter (2): Located in the monitoring optical path (7), configured to maintain the purity of the frequency-stable laser spectrum through purification processing under axial magnetic field and temperature control. The purification processing includes matching the center frequency of the transmission peak of the atomic optical rotation filter with the transition frequency of the target atom to select a single frequency, carrier, or sideband, and the suppression ratio of other unselected frequency sidebands is greater than 20dB.

[0026] In practice, the atomic optical rotation filter (2) is sequentially configured with a first analyzer (21), an atomic gas chamber (22), an axial magnetic field generator (23), a second analyzer (24), and a temperature control device (25) along the monitoring optical path (7). The first analyzer (21) and the second analyzer (24) can both be Glan-Taylor prisms, orthogonally arranged in polarization directions, with an extinction ratio greater than 1000:1. The atomic gas chamber (22) can be a cylindrical borosilicate glass chamber with an inner diameter of 20 mm and a length of 100 mm, filled with 99.999% pure Rb atomic vapor, operating at a pressure of 10 Pa. The windows at both ends of the chamber are coated with a 780 nm antireflection film, with a transmittance >99%. The axial magnetic field generator (23) can be composed of a pair of coaxial Helmholtz coils with 500 turns. When a 1.2 A DC current is applied, it generates a 1000 Gs axial uniform magnetic field, with magnetic field uniformity better than ±1% within the effective area of ​​the chamber. The temperature control device (25) can employ a Peltier effect temperature controller to stabilize the temperature of the atomic gas chamber (22) at 73°C through PID control, with temperature fluctuations less than ±0.05°C. Under these parameters, the center frequency of the transmission peak of the filter can be precisely matched. 87 The Rb atom D2 spectral line changes from 5S1 / 2 (F=2) to 5P. a / 2(F′=3) transition frequency (780.241nm), transmittance to the carrier >80%, transmittance to ±1st order sideband <5%, sideband suppression ratio >30dB, thereby achieving spectral purification of the monitoring light.

[0027] Frequency stabilization unit (3, 4): configured as frequency stabilization optical path (3) and servo system (4), wherein the frequency stabilization optical path is used to acquire laser frequency information and generate error signal; the servo system is used to control the frequency of the fundamental frequency seed light source (11) according to the feedback of the error signal.

[0028] In practice, the frequency stabilization unit (3, 4) includes a frequency stabilization optical path (3) and a servo system (4). The frequency stabilization optical path (3) adopts the modulation transfer spectroscopy method and consists of a beam splitter (31), a signal extraction unit, and pump and probe branches. The beam splitter (31) is a 1:1 non-polarization beam splitter, which divides the monitoring light purified by the atomic rotation filter (2) into pump light and probe light. The pump branch is equipped with a phase modulator (33) in sequence. The phase modulator (33) can be a lithium niobate modulator with a working frequency of 5MHz, which is synchronized with the demodulation signal in the circuit and applies a phase modulation of ±0.5π to the pump light. The probe branch is equipped with a frequency shifter (35) and a second gas chamber (36) in sequence. The frequency shifter (35) is an acousto-optic modulator (AOM) with a working frequency of 80MHz, which can shift the frequency of the probe light by 80MHz. The second gas chamber (36) is filled with Rb atomic vapor. The signal extraction unit includes a polarization beam splitter prism and a detector (38). The polarization beam splitter prism is used to separate the polarization directions of the pump light and the probe light. The detector (38) can be a silicon photodetector with a bandwidth of 150MHz, used to detect the interference signal of the two lights and extract the error signal. The servo system (4) can use a digital PID controller with a sampling frequency of 1MHz, a proportional coefficient of 10, an integral time constant of 0.1s, and a derivative time constant of 0.01s. It can output a control voltage (0-10V) according to the error signal. The resonant cavity length of the fundamental frequency seed light source (11) is adjusted by the piezoelectric ceramic driver to achieve frequency feedback control with a control accuracy of 10kHz.

[0029] In another embodiment of the laser frequency stabilization system of the present invention, a frequency-doubled laser may be included, comprising a fundamental frequency seed beam, an amplifier, and a frequency-doubled crystal (e.g., a 1560nm fundamental frequency seed beam, a 1560nm amplifier, and a PPLN frequency-doubled crystal). An electro-optic modulator (a waveguide-type electro-optic modulator with pigtails for both input and output) is inserted between the seed beam and the amplifier via an optical fiber jumper. A frequency signal (e.g., a corresponding frequency signal) is applied to the electro-optic modulator. 87A 6.8 GHz signal (corresponding to the frequency of the Rb atom hyperfine level structure) is used to modulate the phase of the fundamental laser beam, generating a series of equally spaced laser frequency components (for the 6.8 GHz signal modulation, this generates a +6.8 GHz sideband corresponding to the +1st level, a +13.6 GHz sideband corresponding to the +2nd level, ... a series of sidebands corresponding to the -6.8 GHz sideband corresponding to the -1st level, a -13.6 GHz sideband corresponding to the -2nd level, etc.). This beam is then input into a fundamental frequency optical amplifier for power amplification. The laser is then incident on a frequency-doubling crystal (such as a PPLN crystal) to achieve frequency doubling, generating frequency-doubled light outputs with multiple equally spaced frequency components (for the 6.8 GHz signal modulation, the 780 nm laser generates a series of sidebands corresponding to the +6.8 GHz sideband corresponding to the +1st level, a +13.6 GHz sideband corresponding to the +1st level, ... a series of sidebands corresponding to the -6.8 GHz sideband corresponding to the -1st level, a -13.6 GHz sideband corresponding to the -2nd level, etc.). These are used to match the atom hyperfine level structure for the interaction between atoms and the laser.

[0030] In another embodiment, the implementation steps may include:

[0031] Step 1: Output 10mW fundamental frequency light through the 1560nm DFB seed source (11), apply 6.8GHz phase modulation through the waveguide optical modulator (EOM) (12) to generate sidebands fc,fc±6.8GHz,….

[0032] Step 2: The modulated light is amplified to 2W by the fundamental frequency amplifier (EDFA) (13), and then frequency doubled by the frequency doubler crystal (PPLN) (14) to output 780nm laser (power 1.2W).

[0033] Step 3: The beam splitting module unit splits the beam at a ratio of 99:1. After beam splitting, the main output optical path (6) outputs 1.188W to the atomic interferometer (5), and the monitoring optical path (7) outputs 12mW to the atomic optical rotation filter (2).

[0034] Step 4: The monitoring light is filtered by an atomic optical rotation filter (FADOF) (2) to remove all sidebands (suppression ratio >35dB), retaining only the carrier, and the clean light is input into the MTS frequency-stabilized optical path (3).

[0035] Step 5: The servo system (4) adjusts the seed source frequency according to the error signal to achieve high stability locking of the laser frequency.

[0036] The technical effects achieved through the above-described embodiments of the present invention include: 1. Excellent frequency stability: By combining modulation transfer spectroscopy with PID closed-loop control, the laser frequency stability reaches ±1kHz / 100s, and the long-term (24h) frequency drift is less than ±50kHz, meeting the stringent requirements for laser frequency stability in fields such as atomic clocks and precision spectral measurements; 2. High spectral purity: The atomic rotation filter has a stray sideband suppression ratio exceeding 30dB, and the sidemode suppression ratio of the main output laser is >50dB, effectively avoiding interference from multi-sideband lasers on atomic interactions; 3. Balancing high power output with low loss: The main output optical path power is >2W, and the high-proportion beam splitting design of the beam splitting module results in a monitoring optical path loss of only 1%, which does not affect the practical power of the main optical path; 4. Strong system synergy: The parameters of each unit (such as modulation frequency and atomic energy level, magnetic field and temperature) are precisely matched, and the system can still work stably in an ambient temperature range of -10℃ to 40℃, with a mean time between failures (MTBF) of >1000 hours, providing a reliable laser source guarantee for cutting-edge research in quantum information processing, cold atom physics, and other fields.

[0037] In one embodiment, the electro-optic modulator (12) is a waveguide device, which connects the fundamental frequency seed light source (11) and the fundamental frequency amplifier (13) via an optical fiber jumper, and its modulation frequency f m Match the frequency difference between the two hyperfine levels of the ground state of the target atom.

[0038] In the specific implementation process, the electro-optic modulator (12) can be a waveguide-type lithium niobate (LiNbO) a The integrated optical components have a chip size of 10mm × 3mm × 0.5mm. The core waveguide is a single-mode ridge structure with a waveguide width of 4μm and a ridge height of 2μm, fabricated using a proton exchange process to ensure an optical field confinement efficiency >95%. The modulator incorporates RF electrodes with a 50μm electrode spacing, employing a coplanar waveguide structure with a characteristic impedance of 50Ω, matched to the RF driver impedance to reduce signal reflection loss (return loss <-25dB@3.4GHz). The modulator's half-wave voltage (Vπ) is 3.5V, with a modulation bandwidth of 0-6GHz. Within a 3dB bandwidth, the phase modulation linearity is better than 99.5%, and it can stably output modulated light including the carrier and ±n-order sidebands with a sideband amplitude consistency error <2%.

[0039] The fiber optic patch cord connection can use single-mode polarization-maintaining fiber with a core diameter of 9μm, a cladding diameter of 125μm, and a numerical aperture of 0.14. FC / APC connectors are used at both ends of the fiber, with an end-face grinding angle of 8°, and are coupled to the input / output waveguides of the electro-optic modulator (12) through a precision alignment process. The input end connects to the output pigtail of the fundamental frequency seed light source (11), and the output end connects to the input pigtail of the fundamental frequency amplifier (13). The overall insertion loss is <0.5dB, the return loss is <-60dB, and the attenuation fluctuation of the fiber optic link is <0.1dB within a temperature range of -40℃ to 85℃, effectively avoiding optical power fluctuations caused by mechanical vibration or temperature changes. Simultaneously, to enhance connection stability, a stainless steel armor sleeve with a bending radius ≥30mm can be fitted onto the outside of the fiber optic patch cord to ensure that bending does not cause additional losses during system integration.

[0040] Furthermore, the modulation frequency f m The frequency difference between the two hyperfine levels of the target atom's ground state is an important setting in this embodiment of the invention. 87 The Rb atom has a D2 spectral line at 780.2 nm. The frequency difference between the two hyperfine levels of the ground state of the atom corresponding to the D2 line is about 6.8 GHz. Therefore, the modulation frequency of the electro-optic modulator (12) is set to 6.8 GHz so that the difference between the generated first-order sideband frequency and the carrier frequency exactly matches the frequency difference between the two hyperfine levels of the ground state, which can be used for atomic two-photon Raman transitions.

[0041] Furthermore, the temperature stability of the waveguide-type electro-optic modulator (12) can be achieved through the built-in Peltier temperature control module. The operating temperature can be set to 25℃±0.01℃, and the modulation frequency change rate caused by temperature drift is <1kHz / ℃, ensuring that the matching accuracy between the sideband and the atomic energy level can still be maintained when the ambient temperature fluctuates. The RF drive circuit of the modulator adopts a low-noise amplifier (noise figure <1.5dB) with an output power of 15dBm, which stabilizes the phase modulation depth at 1.2x, ensuring that the power of the ±1st order sideband reaches 80% of the carrier power, thereby meeting the subsequent frequency doubling and filtering requirements.

[0042] Through the above embodiments, the following technical effects can be achieved: 1. The waveguide structure significantly improves modulation efficiency. Compared with the traditional bulk modulator, the phase modulation depth can be increased by 40% under the same driving voltage, and the volume is reduced to 1 / 5, which is beneficial for system miniaturization and integration. 2. The low-loss connection (<0.5dB) of the fiber optic patch cord reduces the attenuation and distortion of the optical signal during transmission, thereby increasing the signal-to-noise ratio of the input light of the fundamental frequency amplifier (13) by 10dB and improving the sideband suppression ratio stability to ±0.5dB / hour. 3. The modulation frequency and 87Precise matching of the Zeeman levels of the Rb atoms (error < 1 kHz) further improves the suppression ratio of the ±1st-order sidebands of the atomic optical rotation filter (2) to 35 dB, providing a cleaner reference signal for the subsequent frequency stabilization units (3, 4). Ultimately, the system's frequency stability reaches ±500 Hz within 100 s, an order of magnitude improvement over systems without precise frequency matching. This design is particularly suitable for fields requiring extremely high frequency accuracy, such as atomic interferometers and optical clocks.

[0043] In one embodiment, the modulation frequency corresponds to 87 The energy levels of the D2 spectral line of Rb atoms are described in detail below:

[0044] 87 The D2 spectral line of Rb atoms originates from the electronic transition from the ground state 5S¹ / ² to the excited state 5P³ / ², with a central wavelength of 780.241 nm (corresponding to a frequency of approximately 384.23 THz). Due to the coupling effect between nuclear spin and electron spin, this spectral line exhibits hyperfine structure splitting: the ground state 5S¹ / ² splits into two energy levels, F = 1 (lower energy) and F = 2 (higher energy), with an energy level interval of approximately 6.835 GHz; the excited state 5P³ / ² splits into four energy levels, F′ = 0, 1, 2, and 3. Among these, the F′ = 3 energy level has the highest transition probability to the ground state F = 2 energy level, and the transition frequency between these two levels is the main transition frequency of the D2 spectral line (780.241 nm).

[0045] Modulation frequency and 87 The specific correspondence of the D2 spectral line energy levels of the Rb atom is as follows: The modulation frequency of the electro-optic modulator (12) is precisely set to 6.8 GHz, so that the ±1st order sidebands generated after modulation of the fundamental seed light (1560 nm) correspond exactly to the frequency difference of the two hyperfine energy level structures of the ground state (ground state 5S1 / 2F=1 and F=2) after frequency doubling to 780 nm laser. For example, the carrier frequency corresponds to the transition F=2→F′=3, and the +1st order sideband corresponds to the transition F=1→F′=3. The frequency interval between the sideband and the carrier is strictly equal to the frequency difference of the two hyperfine energy level structures of the ground state, 6.8 GHz.

[0046] To achieve the above precise correspondence, the following calibration steps can be taken: 1) Measure using a high-resolution grating spectrometer (resolution <0.001nm). 87 1) Identify the central peak position of the F=2→F′=3 transition in the D2 spectral line by saturating absorption spectrum of Rb atoms; 2) Adjust the output frequency of the radio frequency signal generator to this value and lock the driving frequency of the electro-optic modulator (12) with the reference signal; 3) Monitor the transmission spectrum of the atomic optical rotation filter (2) and fine-tune the modulation frequency to reduce the transmittance of the ±1st order sideband to the minimum (<5%) to ensure that the sideband and non-target energy level transitions are completely mismatched.

[0047] In addition, the system needs to compensate for the influence of environmental factors on the energy level: when the ambient temperature changes by ±1℃, 87 Changes in Rb atom vapor density cause a drift of approximately 1 kHz in the energy level transition frequency. By stabilizing the temperature of the atomic gas chamber (22) at 73℃ ± 0.05℃ using a temperature control device (25), this drift can be suppressed to < 0.1 kHz. When the axial magnetic field fluctuates by ± 10 Gs, the Zeeman sublevel spacing changes by approximately 34 kHz. Through the current feedback control (stability ± 0.1 mA) of the magnetic field generator (23), the magnetic field fluctuation can be controlled within ± 5 Gs, corresponding to a change in energy level spacing of < 17 kHz. With the PID adjustment of the modulation frequency (response time < 1 ms), the matching error between the modulation frequency and the energy level spacing is finally stabilized at < 1 kHz.

[0048] In one embodiment, the frequency doubling crystal (14) is a periodically polarized lithium niobate crystal to convert a preset fundamental frequency light into a preset frequency-doubled light. The specific description is as follows:

[0049] The core parameters of the PPLN crystal must match the characteristics of the fundamental frequency light and the target frequency-doubled light. In this embodiment of the invention, the fundamental frequency light is preset to 1560nm, and the frequency-doubled light is preset to 780nm. Therefore, the selected PPLN crystal size is set to 6mm×5mm×25mm (length×width×thickness), where the length direction is the light propagation direction. The polarization period of the crystal is the key to achieving efficient frequency doubling. Through quasi-phase matching theory calculations, when the 1560nm fundamental frequency light is incident at a 10° incident angle, a polarization period of 7.6μm can satisfy the phase matching condition for second harmonic generation. At this time, the second-order nonlinear polarization intensity of the fundamental frequency light is maximized, providing the optimal phase condition for frequency doubling. A dual-band antireflection film of 1560nm and 780nm is deposited on the light-transmitting surface of the crystal. The film layer is prepared by electron beam evaporation process to ensure that the transmittance of 1560nm light is >98% and the transmittance of 780nm light is >97%, reducing the reflection loss of light on the crystal surface.

[0050] The conversion process from fundamental frequency light to frequency-doubled light can be achieved as follows: The 1560nm modulated light, amplified by the fundamental frequency amplifier (13), is first focused by a convex lens with a focal length of 50mm, compressing the spot diameter to 150μm before being incident on the input end of the PPLN crystal. To avoid beam divergence affecting the conversion efficiency, a concave lens with a focal length of 50mm is set at the output end of the crystal to collimate the beam into parallel light, forming a "focus-collimation" optical path structure, so that the fundamental frequency light maintains a high light intensity density (approximately 150W / mm) within the optical path of the crystal. 2Meanwhile, the PPLN crystal is fixed by a precision five-dimensional adjustment frame (adjustment accuracy ±0.01°) to ensure that the incident angle of the fundamental frequency light is stable within the range of 10°±0.05°. Excessive angle deviation will cause the quasi-phase matching condition to be destroyed, and the conversion efficiency will decrease significantly (the efficiency will decrease by about 30% when the angle deviation is 0.5°).

[0051] To ensure stable conversion efficiency, the PPLN crystal in this embodiment can be equipped with a high-precision temperature control device. This device uses a closed-loop control system consisting of a thermoelectric cooler (TEC) and a platinum resistance temperature sensor (accuracy ±0.01℃) to stabilize the crystal's operating temperature at 45℃ ±0.05℃. This is because the polarization period of the PPLN crystal is temperature-sensitive; a 1℃ change in temperature results in an equivalent change of approximately 0.01μm in the effective polarization period, thus affecting the quasi-phase matching effect. Temperature control can limit the resulting conversion efficiency fluctuations to within ±1%. Furthermore, the crystal is externally wrapped with an oxygen-free copper heat sink, which fits tightly against the TEC cold end, with a thermal resistance <0.5℃ / W. This ensures that the heat generated during crystal operation (mainly from light absorption, approximately 0.5W) can be quickly dissipated, preventing localized temperature rise.

[0052] The converted 780nm frequency-doubled light still needs to undergo beam purification. Specifically, a bandpass filter (center wavelength 780nm, bandwidth 10nm) is set at the crystal output end to filter out the unconverted 1560nm fundamental frequency light and other harmonic components (such as fourth-harmonic light) that may be generated by the crystal, so that the purity of the frequency-doubled light reaches more than 99.9%. At the same time, a polarizer is used to screen out the frequency-doubled light whose linear polarization direction matches the subsequent optical path, with a polarization extinction ratio >1000:1, to meet the requirements of the atomic optical rotation filter (2) for the polarization state of the incident light.

[0053] Through the above-described design embodiments, the frequency doubling conversion efficiency of the PPLN crystal can reach 65%, meaning that when the input 1560nm fundamental frequency optical power is 5W, the output 780nm frequency-doubled optical power can reach 3.25W, with long-term (24h) power fluctuation <±2%. The linewidth of the frequency-doubled light inherits the narrow linewidth characteristic of the fundamental frequency seed light source, with a measured linewidth <1.5kHz, thus meeting the requirements of... 87 The spectral width requirement for the interaction of Rb atom energy level transitions.

[0054] The main technical advantages of this embodiment are as follows: 1. Compared with the angle phase matching of traditional bulk crystals, the quasi-phase matching mechanism of PPLN crystals has higher conversion efficiency (more than 40% higher than that of lithium niobate bulk crystals of the same size), and can achieve efficient frequency doubling over a wider temperature and wavelength range, enhancing the system's tolerance to fundamental frequency power fluctuations. 2. Precise polarization period and temperature control design enable the wavelength stability of the frequency-doubled light to reach ±0.001nm / ℃, ensuring that the 780nm frequency-doubled light is always aligned. 87The transition frequency of the D2 spectral line of Rb atoms provides a stable optical frequency reference for subsequent atomic optical rotation filtering and frequency stabilization. 3. The dual-band antireflection coating and beam purification design reduce stray light interference, increasing the signal-to-noise ratio of the frequency-doubled light to 60dB, preventing stray light from causing non-resonant excitation of the atomic system, and ensuring the specificity of atomic interactions. 4. The integrated design of the PPLN crystal (volume only 750mm²) 3 It facilitates system miniaturization, reducing the size by 60% compared to traditional frequency multiplier modules, making it more suitable for space-constrained precision measurement equipment.

[0055] In one embodiment, the fundamental frequency seed light source (11) can be a 1560nm laser, and the 780nm laser output after frequency doubling is used for... 87 Rb atoms interact, resulting in a main output optical power >2W. This is explained in detail below:

[0056] The fundamental frequency seed light source (11) can be a 1560nm narrow linewidth fiber laser, specifically a distributed feedback (DFB) type, with the following core parameters: initial output power of 10mW, which can be continuously adjusted in the range of 5-15mW through the internal power adjustment module; spectral linewidth <1kHz (100ms integration time), long-term frequency drift rate <±1MHz / h (at an ambient temperature of 25℃); output light polarization degree >98%, outputting in a linear polarization state (horizontal direction), which facilitates maximizing the phase modulation efficiency of the subsequent electro-optic modulator.

[0057] The laser can use erbium-doped fiber as the gain medium and stabilize its operating temperature at 25℃±0.01℃ using a Peltier temperature control device. The wavelength drift caused by temperature fluctuation is <0.001nm / ℃, ensuring the stability of the 1560nm center wavelength. Its output end is connected to a single-mode fiber (SMF-28e+) via an FC / APC connector. The fiber is 1 meter long and has an insertion loss of <0.2dB, providing a low-noise, high-stability fundamental frequency optical input for the subsequent electro-optic modulator (12). In addition, the laser has a built-in current feedback module. When the input current fluctuates by ±0.1mA, the output power fluctuation can be suppressed to <±0.1%, avoiding the interference of power jitter on the subsequent amplification and frequency doubling process.

[0058] In the frequency doubling process and 780nm laser generation, the 1560nm fundamental frequency light is phase-modulated by the electro-optic modulator (12) and then enters the fundamental frequency amplifier (13) for power amplification. The fundamental frequency amplifier adopts a two-stage erbium-doped fiber amplification structure: the first stage is a preamplifier, which amplifies the 10mW seed light to 100mW, with a gain flatness of <1dB (1550-1570nm band); the second stage is the main amplifier, which boosts the power to 8W through double-clad erbium-doped fiber. During the amplification process, polarization-maintaining fiber is used to suppress polarization state degradation and ensure that the polarization degree of the output light remains >95%. An optical isolator (isolation degree >40dB) is set between the two amplifier stages to prevent backscattered light from interfering with the stability of the seed light source.

[0059] The amplified 1560nm modulated light enters the frequency doubling crystal (14) – a periodically polarized lithium niobate (PPLN) crystal (parameters as described in the previous embodiments of this invention: polarization period 7.6μm, size 6mm×5mm×25mm), achieving second harmonic conversion under quasi-phase matching conditions. By optimizing the incident angle of the fundamental frequency light (10°) and the crystal operating temperature (45℃), the frequency doubling conversion efficiency is stabilized at over 65%, meaning that 8W of fundamental frequency light can generate ≥5.2W of 780nm frequency-doubled light. After the residual fundamental frequency light is filtered out by a bandpass filter (center wavelength 780nm, bandwidth 10nm), the frequency-doubled light enters the beam splitting module unit.

[0060] The beam splitter of the beam splitter unit distributes the frequency-doubled optical power at a ratio of 99:1, with 99% of the beam serving as the main output optical path (6). Based on the total power of 5.2W after frequency doubling, the theoretical power of the main output optical path can reach 5.148W. In practical applications, the power can be adjusted to the range of 2-5W through attenuators to meet the power requirements of different scenarios.

[0061] The main output 780nm laser and 87 The interactions of Rb atoms exhibit a clear energy level correspondence: the wavelength is precisely matched. 87 The Rb atom's D2 spectral line exhibits a 5S¹ / ² (F=3)→5P³ / ² (F′=4) transition frequency (780.241 nm) and a linewidth <1.5 kHz, enabling resonant coupling with atomic energy levels. To ensure efficient interaction, a beam shaping system is implemented in the main output optical path: a beam expander enlarges the spot diameter from 1 mm to 5 mm (suitable for atomic gas cells), achieving a beam divergence angle <1 mrad and forming a uniform optical field in the central region of the atomic gas cell with a power density of 100 mW / cm². 2 It meets the light intensity requirements for experiments such as atomic trapping and laser cooling.

[0062] The following technical effects can be achieved through this embodiment: 1. The selection of a 1560nm fundamental frequency light source has significant advantages. This band falls within the low-loss window of optical fiber communication (0.2dB / km), facilitating transmission and integration via optical fiber links. Compared to traditional 1064nm fundamental frequency light, optical fiber transmission loss is reduced by 60%, and system wiring flexibility is greatly improved; 2. High-power main output light (>2W) provides sufficient optical energy for large-scale atomic physics experiments, capable of simultaneously driving multiple atomic gas chambers or expanding the interaction region. Compared to low-power systems (<1W), experimental data acquisition efficiency is improved by more than 3 times; 3. 780nm laser and 87 The precise matching of Rb atomic energy levels (wavelength deviation <0.001nm), combined with narrow linewidth characteristics, enables atomic transition excitation efficiency exceeding 90%, avoiding background noise caused by non-resonant excitation. This provides a high signal-to-noise ratio basis for precision measurements (such as atomic clock frequency calibration). Furthermore, the synergistic effect of the system's power stability (±2% / 24h) and frequency stability (±500Hz / 100s) allows it to be directly applied to cutting-edge fields with stringent laser performance requirements, such as quantum entanglement experiments and atomic interferometers.

[0063] Figure 2 An exemplary structural block diagram of an atomic filtering device according to an embodiment of the present invention is shown.

[0064] like Figure 2 As shown, the atomic optical rotation filter (2) is a Faraday anomalous dispersion atomic filter. Its overall structure includes: a first analyzer (21), an atomic gas chamber (22) which can be filled with target atomic vapor; an axial magnetic field generator (23) for providing a uniform magnetic field of a preset intensity; a temperature control device (25) for maintaining the temperature of the atomic gas chamber (22) within a preset temperature range; and a second analyzer (24). The gas in the atomic gas chamber is situated within the magnetic field generated by the axial magnetic field generator or a permanent magnet. The magnetic field surrounds the atomic gas chamber, thus forming a uniform and strong magnetic field. Since the atomic gas chamber is installed in the temperature control device, precise temperature control of the gas chamber can be achieved.

[0065] One embodiment of the filtering process is as follows: the laser is modulated by a polarizer, and then the laser with the same polarization state is transmitted through the first analyzer (21) and incident into the atomic gas chamber (22). By optimizing the temperature and magnetic field of the atomic gas chamber, the atoms in the atomic gas chamber cause optical rotation, so that the polarization state of the laser with the same frequency as the carrier frequency is different from that of the laser with the non-corresponding frequency (positive and negative 1st level modulation sideband, positive and negative 2nd level modulation sideband, etc.). Finally, the unwanted frequency laser is filtered out or reflected by the second analyzer (24), while the laser with the carrier frequency is retained as the output of the atomic optical rotation filter.

[0066] In another embodiment, the parameters of the Faraday anomalous dispersion atomic filter can be: magnetic field strength 1000 Gs; gas cell temperature 73 °C; carrier transmittance > 80%, ±1st order sideband transmittance < 5%, sideband suppression ratio > 30 dB, where n is the sideband order.

[0067] In another embodiment, both the first analyzer (21) and the second analyzer (24) can be Gran-Foucault prisms made of natural calcite crystals with a light-transmitting aperture of 20 mm and a working wavelength range of 770-790 nm. The polarization directions of the two prisms are orthogonally set (angle 90°), with an extinction ratio of >10000:1 and a transmittance of >90% at a wavelength of 780 nm, ensuring that unpolarized light is converted into linearly polarized light after passing through the first analyzer, and only light with a specific polarization direction is allowed to pass through the second analyzer. The atomic gas chamber (22) can be a cylindrical borosilicate glass container with an outer diameter of 25 mm and a length of 100 mm. The windows at both ends are made of optical-grade fused silica material, with the window plane perpendicular to the axis and the surface coated with a 780 nm antireflection film (transmittance >99.5%). The chamber is filled with pure Rb atomic vapor and mixed with 10 Torr of nitrogen as a buffer gas to suppress atomic diffusion. The working pressure is maintained at 1 Pa ± 0.1 Pa, and the pressure is regulated by a miniature vacuum valve on the side wall of the chamber.

[0068] The axial magnetic field generator (23) can be composed of two sets of coaxial Helmholtz coils with a diameter of 150 mm and 600 turns. The coils are wound with enameled copper wire (0.5 mm in diameter) and the coil spacing is 75 mm (equal to the coil radius). When a DC current of 1.2 A is applied, a uniform axial magnetic field of 500-1500 Gs can be generated in the central region of the gas chamber. Calibrated with a gaussmeter (measurement accuracy ±1 Gs), the magnetic field uniformity is better than ±1% within the effective optical path range of the gas chamber (±40 mm), and the magnetic field attenuation rate in the edge region is <0.5 Gs / mm.

[0069] The temperature control device (25) can adopt a double-layer temperature control structure: the inner layer is a copper heating jacket that wraps the gas chamber, with 4 built-in 10W Peltier elements; the outer layer is a heat insulation layer (10mm thick, thermal conductivity <0.03W / m·K). The temperature sensor is a platinum resistance thermometer (PT1000, accuracy ±0.01℃), which is embedded between the heating jacket and the gas chamber. The temperature of the gas chamber is stabilized at 73℃±0.05℃ by a PID controller (sampling frequency 1kHz). The change in atomic number density caused by temperature fluctuation is <0.5% / h, avoiding the effect of vapor density fluctuation on the filtering characteristics.

[0070] The workflow and performance tuning are described in detail below:

[0071] The 780nm frequency-doubled light in the monitoring optical path (7) is first incident on the first analyzer (21) and converted into linearly polarized light (the polarization direction makes an angle of 45° with the horizontal direction), and then enters the atomic gas chamber (22). Under the action of a preset 1000Gs axial magnetic field, the absorption line of the D2 spectrum of the Rb atom produces a Faraday rotation effect—the polarization direction of the linearly polarized light will rotate with the degree of matching between the light frequency and the atomic transition frequency. When the light frequency is exactly equal to the target transition frequency, the polarization rotation angle reaches its maximum value (about 45°).

[0072] Linearly polarized light after passing through the gas cell is incident on the second analyzer (24). Since its polarization direction is orthogonal to that of the first analyzer, light with a polarization direction rotated by 90° can pass through efficiently (theoretical transmittance 100%). By adjusting the magnetic field strength and temperature, the polarization rotation angle of the target carrier frequency (corresponding to the atomic transition center frequency) is stabilized at 45°. At this time, after two rotations, the carrier light is aligned with the polarization direction of the second analyzer, and a transmittance >80% can be achieved. However, the ±1st order sideband frequency is mismatched with the atomic transition frequency, and the polarization rotation angle is <5°. After passing through the second analyzer, a transmittance <5% and a sideband suppression ratio >30dB can be achieved.

[0073] To maintain long-term stability of filtering performance, this embodiment of the invention employs a dual feedback mechanism: Magnetic field feedback uses a Hall sensor to monitor the coil current in real time. When the current fluctuates by ±0.01A (corresponding to a magnetic field fluctuation of ±8Gs), the PID control module is triggered to compensate for the current deviation. Temperature feedback compares the measured value from the sensor with a set value of 73℃ to drive the Peltier element for heating / cooling, with a response time of <100ms. Through this mechanism, the carrier transmittance fluctuation of the filter can be controlled within ±2% / 24h, and the sideband suppression ratio fluctuation can be <±1dB / 24h.

[0074] Through structural optimization and parameter matching in the above embodiments, the following technical effects can be achieved: 1. High selective filtering performance: a sideband rejection ratio of over 30dB can effectively eliminate ±1st order and higher order sidebands, ensuring that the monitoring light entering the frequency stabilization unit retains only the carrier component, avoiding interference from sideband signals on error extraction. Compared with traditional Fabry-Perot filters, frequency selectivity is improved by more than 10 times. 2. Strong environmental anti-interference capability: through high-precision closed-loop control of magnetic field and temperature, the filter can maintain stable performance in ambient temperatures ranging from -10℃ to 40℃, solving the filter drift problem caused by changes in atomic vapor density with the environment. 3. Low insertion loss characteristics: the total transmission loss of the entire filter is <5% (including analyzer, gas cell window, and atomic absorption), ensuring that the optical power of the monitoring optical path is sufficient to drive the subsequent frequency stabilization unit (detector saturation power >1mW). 4. With 87The precise matching of Rb atomic energy levels, combined with a 1000 Gs magnetic field and a 73°C temperature, compresses the full width at half maximum (FWHM) of the transmission peak to <150 MHz, precisely covering the frequency drift range of the fundamental laser (±100 MHz). This provides a high signal-to-noise ratio reference for the frequency stabilization system, ultimately improving the laser frequency stability to ±500 Hz / 100 s. This design is particularly suitable for scenarios with stringent requirements for laser spectral purity, such as atomic physics experiments and quantum communication.

[0075] Figure 3 An exemplary structural block diagram of a frequency-stabilized optical path according to an embodiment of the present invention is shown.

[0076] like Figure 3 As shown, in one embodiment, the frequency-stabilized optical path (3) employs modulation-transfer spectroscopy and includes: a beam splitter (31) and a signal extraction unit. After being filtered by an atomic rotation filter (2), the filtered monitoring light is split into pump light and probe light by a beam splitter (PBS); the pump branch includes a phase modulator (EOM) (33); the probe branch includes a frequency shifter (AOM) (35) and a second gas chamber (36); the signal extraction unit includes a polarization beam splitter prism and a detector (38), and the probe light passes through the second gas chamber (36) in a direction almost completely antiparallel to the pump light. In the gas chamber, the pump light transfers its modulation information to the probe light through four-wave mixing. The probe light then strikes the detector (38) to extract interference-free error signals.

[0077] In one embodiment, the output light from the laser beam splitter is divided into two parts: one part serves as the main output light for interaction with atoms, and the other part serves as the monitoring light, accounting for 1% of the total output, for laser frequency stabilization and monitoring.

[0078] First, the detection light is passed through a specific-frequency atomic optical rotation filter. By optimizing the temperature and magnetic field, the frequency corresponding to its transmission peak is adjusted to match the required atomic transition frequency, while ensuring maximum suppression of extraneous sidebands. This means the transmittance of the atomic optical rotation filter is minimized, thus guaranteeing a pure laser spectrum entering the frequency stabilization system, free from the influence of extraneous sidebands. For example, if the temperature is optimized to 73℃ and the magnetic field to 1000 Gs, the carrier output transmittance of the atomic optical rotation filter is better than 80%, and the transmittance of the positive and negative one-order sidebands is less than 5%, achieving the filtering effect suitable for atomic frequency stabilization.

[0079] Taking the modulation-transfer method as an example, the laser beam is incident into the frequency stabilization system and split into two paths by a beam splitter: one as the pump beam and the other as the probe beam. The pump beam undergoes phase modulation by an electro-optic modulator before entering the gas chamber. The other beam, serving as the probe beam, is frequency-shifted by an acoustic-optic modulator and then incident into the gas chamber in the opposite direction to the pump beam. After exiting the gas chamber, it is reflected by a polarizing beam splitter and enters the detector for detection. Through demodulation and feedback systems, the feedback signal is input into the laser seed beam, achieving laser frequency stabilization. Because the monitoring light has excellent spectral purity, it generates an interference-free error curve, which is used to achieve laser frequency stabilization.

[0080] To help those skilled in the art better understand the technical solution of the present invention, the embodiments of the above solution are further described in detail below:

[0081] The beam splitter (31) is a 1:1 broadband unpolarized beam splitting cube, made of fused silica, with a light-transmitting aperture of 20 mm. The beam splitting ratio error at 780 nm wavelength is <1%, and the insertion loss is <0.3 dB. Its function is to split the monitoring light (power about 3 mW) purified by the atomic rotation filter (2) into pump light (1.5 mW) and probe light (1.5 mW), and the polarization directions of the two beams are consistent (45° with the horizontal direction).

[0082] A phase modulator (33) is installed along the optical path in the pump branch. The phase modulator (33) is an optical fiber coupled lithium niobate modulator with a working wavelength of 780nm, a modulation bandwidth of 5MHz, and a half-wave voltage of 3.0V. It can generate a phase modulation depth of ±0.8π at a modulation frequency of 5MHz.

[0083] A frequency shifter (35) and a second gas chamber (36) are sequentially arranged along the optical path in the detection branch. The frequency shifter (35) is an acousto-optic modulator (AOM) with a center frequency of 80MHz, a bandwidth of 10MHz, and a diffraction efficiency of >85%. It can accurately shift the frequency of the probe light by 80MHz with a shift stability of <1kHz / h. The second gas chamber (36) adopts a cylindrical borosilicate glass structure with a length of 100mm and an inner diameter of 20mm. It is filled with Rb atomic vapor and 10Torr nitrogen buffer gas. The signal extraction unit consists of a polarizing beam splitter and a detector (38). The polarizing beam splitter can separate the incident light into vertical and parallel components according to the polarization direction, with an extinction ratio of >1000:1 and a transmittance of >90% at 780nm. The detector (38) is a silicon photomultiplier detector with a bandwidth of 150MHz and a noise equivalent power of <29.2pW / √Hz. It can simultaneously detect the intensity difference between two polarized beams and effectively suppress common-mode noise.

[0084] The specific workflow of the embodiment is described in detail below:

[0085] The pure carrier light (780nm) output from the atomic optical rotation filter (2) first enters the beam splitter (31) and is divided into pump light and probe light. After the pump light is subjected to 5MHz phase modulation by the phase modulator (33), it becomes modulated light containing the carrier and ±1st order sidebands. After the probe light is shifted by 80MHz by the frequency shifter (35), it is incident into the second gas chamber (36) in the opposite direction to the pump light. At this time, in the gas chamber, the pump light transfers its modulation information to the probe light through four-wave mixing. Then the probe light enters the detector (38).

[0086] The output electrical signal is demodulated by a lock-in amplifier (reference frequency 5MHz) to obtain an error signal, which is free from background light interference (common mode rejection ratio >60dB).

[0087] In another embodiment, the operating frequency of the phase modulator (33) is a preset multiple of the corresponding spectral linewidth, so that no additional phase shift is introduced when the filtered monitoring light is used as the frequency-stabilized laser to obtain the error signal. Specifically, it can be understood as follows: the phase modulator (33) in the frequency-stabilized optical path adopts an optical fiber coupled lithium niobate device, with an operating frequency set to 5MHz, a modulation bandwidth of 0-100MHz, a half-wave voltage of 3V, and a modulation depth of ±0.3π. This frequency matches the transmission bandwidth (150MHz) and suppression bandwidth (±2GHz) of the atomic optical rotation filter (2). 5MHz is much smaller than the transmission bandwidth, ensuring that the modulation sideband falls completely within the filter passband. The driving signal adopts a low-noise crystal oscillator with a frequency stability of <1kHz / h, synchronized with the system clock through a phase-locked loop. The ±1st order sideband power of the modulated pump light is only 4.5% of the carrier. When the filtered monitoring light enters the frequency-stabilized optical path, the 5MHz modulation frequency achieves modulation of the frequency-stabilized light, and after demodulation, the error signal of the laser frequency noise is obtained.

[0088] Through the above-described embodiments, the 5MHz low-frequency modulation improves the signal-to-noise ratio of the error signal by 15dB; the sidebands fall completely within the passband, eliminating phase mismatch and optimizing frequency stability to ±300Hz / 100s; the power consumption of the low-frequency drive circuit is reduced by 60%, improving system integration, and is especially suitable for scenarios such as atomic interferometers that are sensitive to phase noise.

[0089] In summary, the technical solutions provided by this invention can achieve the following technical effects:

[0090] 1. By innovatively employing a Faraday anomalous dispersion atom filter, combined with a 6.8GHz modulation frequency and the target atoms, such as... 87Precise matching of the two hyperfine energy levels of the Rb atom ground state achieves a sideband suppression ratio >30dB. Combined with the background error-free extraction of modulation transfer spectroscopy, the laser frequency stability is controlled within ±500Hz / 100s, and the long-term drift is <±500Hz / 24h. This solves the key contradiction between multi-sideband interference and frequency drift in traditional systems, thereby achieving a synergistic improvement in spectral purity and frequency stability.

[0091] 2. Based on the quasi-phase-matched frequency doubling technology of PPLN crystal, a high-efficiency conversion from 1560nm to 780nm is achieved (efficiency >65%). Combined with a 99:1 high-ratio beam splitting design, the main output optical power reaches 2-5W, and the monitoring optical path loss is only 1%. This not only meets the requirements of atomic physics experiments for high-power lasers, but also ensures the signal strength of the frequency stabilization unit, thus achieving a balance between high power output and low loss design.

[0092] 3. Through fiber-optic coupling design of the waveguide-type electro-optic modulator (insertion loss <0.5dB) and modular temperature / magnetic field control (stability ±0.05℃ / ±5Gs), the system maintains stable operation in an environment of -10℃ to 40℃, with a mean time between failures (MTBF) >1000 hours. This system provides a laser source with high purity, high power, and high stability for fields such as atomic clocks and quantum communication, promoting the practical application of precision measurement technology and thus optimizing system integration and environmental adaptability.

[0093] Figure 4 An exemplary structural block diagram of a laser device according to an embodiment of the present invention is shown.

[0094] like Figure 4 As shown, this embodiment of the invention also provides a laser device 41, including a laser frequency stabilization system 42 as described in any of the embodiments above, for spectral purification and laser frequency stabilization of a phase-modulated frequency-stabilized laser. It is understood that this laser device is a specific implementation of the laser frequency stabilization system implemented in the foregoing embodiments; therefore, the foregoing description, in conjunction with... Figure 1-3 The characteristics described above can be similarly applied here, and will not be elaborated further.

[0095] While numerous embodiments of the invention have been shown and described herein, it will be apparent to those skilled in the art that such embodiments are provided by way of example only. Many modifications, alterations, and alternatives will occur to those skilled in the art without departing from the spirit and essence of the invention. It should be understood that various alternatives to the embodiments of the invention described herein may be employed in the practice of the invention. The appended claims are intended to define the scope of protection of the invention and therefore cover equivalents or alternatives within the scope of these claims.

Claims

1. A laser frequency stabilization system based on Faraday anomalous dispersion atomic filtering, used for frequency stabilization of phase-modulated multi-sideband lasers, characterized in that, include: Frequency-doubled laser unit (1): configured to include a fundamental frequency seed light source (11), an electro-optic modulator (12), a fundamental frequency amplifier (13), and a frequency-doubled crystal (14); wherein the electro-optic modulator (12) is configured to apply a frequency f to the fundamental frequency seed light. m Phase modulation to generate a signal containing carrier frequency f c and sideband f c ±nf m The modulated light, where n is the sideband order; the frequency doubling crystal (14) is used to convert the modulated fundamental frequency light into frequency doubling light; The beam splitting module unit is configured to split the frequency-doubled light into a main output optical path (6) and a monitoring optical path (7), wherein the power of the main output optical path (6) accounts for ≥99% and the power of the monitoring optical path (7) accounts for ≤1%. Atomic optical rotation filter (2): located in the monitoring optical path (7), configured to maintain the purity of the laser spectrum with a stable frequency through purification treatment under axial magnetic field and temperature control; wherein the purification treatment includes: matching the target atom transition frequency with the center frequency of the transmission peak of the atomic optical rotation filter to select a single frequency or carrier or sideband, and the suppression ratio of other unselected frequency sidebands is greater than 20dB; Frequency stabilization unit (3, 4): configured as frequency stabilization optical path (3) and servo system (4), wherein the frequency stabilization optical path is used to acquire laser frequency information and generate error signal; the servo system is used to control the frequency of the fundamental frequency seed light source (11) according to the error signal feedback.

2. The laser frequency stabilization system according to claim 1, characterized in that, The electro-optic modulator (12) is a waveguide device, connected to the fundamental frequency seed light source (11) and the fundamental frequency amplifier (13) via an optical fiber jumper, with a modulation frequency f. m Match the frequency difference between the two hyperfine levels of the ground state of the target atom.

3. The laser frequency stabilization system according to claim 1, characterized in that, The frequency doubling crystal (14) is a periodically polarized lithium niobate crystal, which converts the preset fundamental frequency light into the preset frequency doubling light.

4. The laser frequency stabilization system according to claim 1, characterized in that, The atomic optical rotation filter (2) is a Faraday anomalous dispersion atomic filter, which includes: a first analyzer (21); an atomic gas chamber (22) which may be filled with target atomic vapor; an axial magnetic field generator (23) for providing a uniform magnetic field of a preset intensity; a temperature control device (25) for maintaining the temperature of the atomic gas chamber (22) within a preset temperature range; and a second analyzer (24).

5. The laser frequency stabilization system according to claim 4, characterized in that, The parameters of the Faraday anomalous dispersive atomic filter are: magnetic field strength 1000 Gs; gas cell temperature 73℃; carrier transmittance > 80%, ±1st order sideband transmittance < 5%, |n| ≥ 2nd order sideband suppression ratio > 30dB, where n is the sideband order.

6. The laser frequency stabilization system according to claim 1, characterized in that, The frequency-stabilized optical path (3) adopts the modulation transfer spectroscopy method and includes: a beam splitter (31) and a signal extraction unit; wherein, the beam splitter divides the filtered monitoring light into pump light and probe light; the pump branch includes a phase modulator (33); the probe branch includes a frequency shifter (35) and a second gas chamber (36); the signal extraction unit includes a polarization beam splitter prism and a detector (38) for extracting interference-free error signals.

7. The laser frequency stabilization system according to claim 6, characterized in that, The operating frequency of the phase modulator (33) is a preset multiple of the corresponding spectral linewidth, so that no additional phase shift is introduced when the filtered monitoring light is used as a frequency-stabilized laser to obtain an error signal.

8. The laser frequency stabilization system according to claim 1, characterized in that, The fundamental frequency seed light source (11) is a 1560m laser, and the 780m laser output after frequency doubling is used for... 87 Rb atoms interact, resulting in a main output optical power >2W.

9. A laser device, characterized in that, Includes the laser frequency stabilization system as described in any one of claims 1-8, for spectral purification and laser frequency stabilization of phase-modulated frequency-stabilized lasers.

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