Ultra-high stability laser system based on two-path independent composite frequency stabilization and sum frequency

Abstract

This invention discloses an ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency modulation, belonging to the field of precision laser spectroscopy and frequency control technology. The system includes two seed lasers (1560 nm and 1064 nm), a dual-wavelength common-cavity ultra-stable module, two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules, and a nonlinear sum-frequency modulation (SFG) module. Full-time-scale frequency noise suppression is achieved through a three-stage cascaded closed loop consisting of a MHz fast loop, a kHz slow loop, and a Hz extremely slow loop. An acousto-optic modulator is placed in the sampling optical path fed into the ultra-stable cavity, and physical decoupling of the fast and slow loops is achieved through radio frequency bridging. The extremely low-frequency error signal detected by NICE-OHMS is fed back to the PPMgOLN crystal temperature or AOM driving frequency to compensate for the thermally induced drift of the nonlinear crystal. The system ultimately outputs a 633 nm laser power of not less than 50 mW, with a short-term frequency instability better than 5 × 10⁻⁶ mW. ‑15 @1 s.

Description

Technical Field

[0001] This invention relates to the fields of precision laser spectroscopy and frequency control technology, and in particular to an ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum frequency. Background Technology

[0002] The 633nm wavelength is one of the wavelengths recommended by the International Bureau of Weights and Measures for achieving the meter standard, and it plays a crucial role in cutting-edge science and high-end equipment fields such as large-scale precision interferometric length measurement, space gravitational wave detection, and quantum sensing. Traditionally, high-stability light sources in this band have mainly relied on He-Ne gas lasers. 127 I2 saturated absorption frequency stabilization technology. However, He-Ne lasers are limited by the gas discharge emission mechanism, resulting in shortcomings such as low output power, large size, high power consumption, and limited vibration resistance, making it difficult to meet the engineering requirements of modern high-end manufacturing and space exploration.

[0003] To overcome the aforementioned limitations, academia and industry have explored the use of all-solid-state lasers combined with nonlinear optical frequency conversion technology to generate 633nm lasers. Among these, the sum-frequency generation (SFG) of 1560nm and 1064nm lasers is a promising approach. 1560nm corresponds to the C-band in optical communication, and erbium-doped fiber laser technology is mature. 1064nm is the fundamental frequency of Nd:YAG / Nd:YVO4, and non-planar ring cavity lasers (NPROs) exhibit excellent noise performance. The sum-frequency generation satisfies the energy conservation relationship 1 / λ_SFG = 1 / 1560 + 1 / 1064, yielding λ_SFG ≈ 632.6nm, consistent with the He-Ne / I2 standard wavelength. However, existing technologies have significant limitations in the frequency stabilization physical mechanism and control topology, failing to meet the higher stability requirements of next-generation high-precision applications.

[0004] 1. Insufficient superposition of phase noise and broadband suppression in nonlinear conversion: The frequency drift and phase noise of the sum-frequency output laser are equal to the linear superposition of the two seed lasers. Existing SFG schemes typically only provide single-loop feedback at the 633nm output. The servo bandwidth is limited by the signal-to-noise ratio of the molecular spectrum, usually <1kHz, making it difficult to effectively suppress the phase noise of the seed laser in the mid-to-high frequency range, such as >10kHz. This results in a large short-term linewidth of the laser, which cannot meet the requirements of precision interferometry for sub-Hz linewidth. 2. Physical mismatch and loop conflict of servo control bandwidth: Ultra-low expansion glass Fabry-Perot cavities (FP) can provide MHz-level feedback bandwidth, narrowing the laser linewidth to sub-Hz. However, ultra-stable cavities suffer from long-term thermophysical creep, with a typical drift rate of ~1Hz / s, requiring calibration using an atomic / molecular absolute frequency reference. If the broadband fast loop (i.e., the high-frequency signal extracted by the cavity lock, with a bandwidth in the MHz range) and the narrowband slow loop (i.e., the error signal of the low-frequency signal extracted by the atomic / molecular lock, with a bandwidth in the kHz range) are directly superimposed on the control port inside the same laser, it is very easy to cause servo loop conflict and system oscillation. Existing cascaded frequency stabilization approaches are mostly designed for single wavelengths and have not been extended to complex system architectures involving dual-path independent frequency stabilization followed by summation. 3. Low-frequency phase drift introduced by the nonlinear crystal itself: The main optical path used for summation typically has high power ranging from tens to hundreds of milliwatts. Under high-power continuous pumping, periodically polarized lithium niobate crystals, such as those with thermally induced refractive index changes (e.g., dn / dT effect) and slight thermal expansion, introduce additional optical path difference drift in the 0.01–10 Hz frequency band. Even if the two seed lasers have been stabilized, the summation output light may still deviate from the target frequency due to the physical drift of the crystal itself. Existing solutions lack an end-to-end compensation mechanism for this residual noise source.

[0005] Therefore, there is an urgent need for an ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency stabilization. Through a decoupled composite servo control architecture with multiple time scales and multiple physical levels, phase noise and frequency drift can be eliminated across the entire frequency band, while providing tens to hundreds of milliwatts of practical optical power to meet the needs of precision metrology and cutting-edge physics. Summary of the Invention

[0006] In view of this, the present invention proposes an ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency modulation. The system uses a dual-wavelength all-solid-state laser as a seed source, and places an acousto-optic modulator on the sampling optical path fed into the ultra-stable cavity to achieve radio frequency bridging. The two seed lasers are locked to dual wavelengths broadband by sharing the same physical ultra-stable cavity. The main optical path is indirectly calibrated with atomic / molecular absolute frequency reference. The sum-frequency modulation is performed through a periodically polarized nonlinear crystal with high resistance to optical loss. The system eliminates noise and drift across the entire frequency band through a three-stage cascaded closed-loop adjustment of MHz fast loop, kHz slow loop, and Hz extremely slow loop.

[0007] This invention provides an ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency stabilization, comprising: The first seed laser is used to output a 1560nm continuous wave seed laser. The second seed laser is used to output a 1064nm continuous wave seed laser. The dual-wavelength common cavity ultra-stable module is connected to the optical paths of the first seed laser and the second seed laser. It is used to extract the high-bandwidth error signal PDH from the seed lasers output by the first seed laser and the second seed laser, and feed the high-bandwidth error signal PDH back to the first seed laser and the second seed laser to suppress the frequency noise of the two seed lasers and realize the first-stage optical frequency closed-loop control. The two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules each include an acousto-optic modulator (AOM). The AOMs are positioned one-to-one between the first seed laser, the second seed laser, and the dual-wavelength co-cavity ultra-stable module in the main optical path. They are used to separate the first branch beams from the sampled light diffracted by the AOMs, and to generate frequency-doubled lasers by frequency doubling the two first branch beams respectively. The error signals of the transition frequencies of the frequency-doubled lasers are extracted respectively. Based on the error signals of the transition frequencies, the RF reference bridging is achieved by changing the RF drive frequency of the AOMs to correct the physical drift of the dual-wavelength co-cavity ultra-stable module caused by material creep and temperature drift, and to achieve second-level optical frequency closed-loop control with a total unity gain bandwidth not exceeding 10kHz. The nonlinear sum-frequency (SFG) module includes a TEC temperature control circuit. The SFG module connects to the main optical path of either the first or second seed laser, amplifying the signal from the main optical path. The amplified laser signal is then summed and frequency-controlled at the operating temperature set by the TEC temperature control circuit, outputting a sum-frequency laser. A second branch beam is then separated from the output optical path of the sum-frequency laser. An extremely low-frequency error signal in the 0.01Hz to 10Hz frequency band is demodulated and fed back to the TEC temperature control circuit and the acousto-optic modulator (AOM) of the two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules, achieving a third-level optical frequency closed-loop control. This third-level optical frequency closed-loop control is orthogonal to each other in terms of frequency band and hardware physical operation.

[0008] Based on the above technical solutions, preferably, the dual-wavelength common-cavity ultra-stable module includes two optical isolators, two electro-optic modulators (EOMs), two mode-matching lens groups, two mirror groups, an ultra-low expansion glass Fabry-Perot ultra-stable cavity, two broadband photodetectors, and two independent first PID controllers. The first and second seed lasers are each processed by an optical isolator before entering an electro-optic modulator (EOM) in a corresponding manner. The output optical path of the EOM then passes through the mode-matching lens group to adjust the wavefront curvature and beam waist position of the Gaussian beam before being fed into a mirror group in a corresponding manner to adjust the optical axis attitude. Then, different longitudinal modes are coupled into a shared ultra-low expansion glass Fabry-Perot ultra-stable cavity. Two broadband photodetectors acquire reflected light signals from the cavity reflection port of the ultra-low expansion glass Fabry-Perot ultra-stable cavity, respectively. The acquired reflected light signals are demodulated to obtain two high-bandwidth error signals (PDHs). Two first PID controllers are used to acquire the two high-bandwidth error signals (PDHs) respectively and output high-frequency control signals. The high-frequency control signals are fed back to the first seed laser and the second seed laser. An overlapping frequency division network is set between the two high-bandwidth error signals (PDHs), with the division point set at 100kHz and the overall phase margin not less than 45°.

[0009] Preferably, the ultra-low expansion glass Fabry-Perot ultra-stable cavity has a length of 100 mm, a dual-band precision of 150,000, and is suspended in a heat-shielded vacuum chamber with a vacuum level of less than 2 × 10⁻⁶. -6 Pa, constant temperature fluctuation <0.5mK.

[0010] Preferably, the closed-loop cutoff frequency of the first PID controller f c >1MHz, phase margin not less than 45°; after suppression of the entire ring white noise substrate, the instantaneous linewidth of the two seed lasers is not greater than 1Hz.

[0011] Preferably, the acousto-optic frequency shifting and dual-reference absolute frequency stabilization module connected to the main optical path of the first seed laser includes a first acousto-optic modulator AOM1, a first PPLN crystal, a rubidium atom gas cell, and a second PID controller arranged sequentially. The 1560nm laser output from the first seed laser is first injected into the first acousto-optic modulator AOM1 for physical frequency shifting. One path of the frequency-shifted diffracted light is sent as sampling light into the dual-wavelength co-cavity ultra-stable module, and the other path of the diffracted light is injected into the first PPLN crystal as the first branch beam of 1560nm. The polarization period of the first PPLN crystal is... Matching a 1560nm frequency doubling, the first PPLN crystal generates a 780nm frequency-doubled laser and injects it into the rubidium atom gas cell. The error signal of the transition frequency of the 780nm frequency-doubled laser is extracted using saturable absorption spectroscopy. The error signal of the transition frequency of the 780nm frequency-doubled laser is sent to the second PID controller. The output of the second PID controller is connected to the radio frequency signal source of the first acousto-optic modulator AOM1. The second PID controller feeds back the output signal to the radio frequency signal source of the first acousto-optic modulator AOM1 to change the driving frequency of the first acousto-optic modulator AOM1 in real time. The acousto-optic frequency shifting and dual-reference absolute frequency stabilization module, connected to the main optical path of the second seed laser, includes a second acousto-optic modulator AOM2, a second PPLN crystal, an iodine molecule gas cell, and a third PID controller arranged sequentially. The 1064nm laser output from the second seed laser is first injected into the second acousto-optic modulator AOM2 for physical frequency shifting. One path of the frequency-shifted diffracted light is sent as sampling light into the dual-wavelength co-cavity ultra-stable module, and the other path of the diffracted light is injected into the second PPLN crystal as the first branch beam of 1064nm. The polarization period of the second PPLN crystal is matched to the 1064nm frequency doubling, and the second PPLN crystal generates a 532nm frequency-doubled laser, which is then injected into the iodine molecule gas cell. The error signal of the transition frequency of the 532nm frequency-doubled laser is extracted using modulation transfer spectroscopy (MTS) and locked to the α-axis of the iodine molecule. 10 The component sends the error signal of the transition frequency of the 532nm frequency-doubled laser to the third PID controller. The output of the third PID controller is connected to the radio frequency signal source of the second acousto-optic modulator. The third PID controller feeds back the output signal to the radio frequency signal source of the second acousto-optic modulator AOM2 to change the driving frequency of the second acousto-optic modulator AOM2 in real time. By changing the radio frequency driving frequencies of the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2, the absolute frequency of the main optical path after being locked by the first-stage optical frequency closed-loop control is equal to the intrinsic longitudinal mode frequency of the ultra-low expansion glass Fabry-Perot ultra-stable cavity minus the driving frequencies of the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2. This allows the effective locking point to be dragged in a radio frequency reference bridging manner, correcting the long-term physical drift of the ultra-low expansion glass Fabry-Perot ultra-stable cavity caused by material creep and temperature drift, thus achieving the second-stage optical frequency closed-loop control.

[0012] Preferably, the modulation frequency of the electro-optic modulator EOM used in the first seed laser is 20MHz; the modulation frequency of the electro-optic modulator EOM used in the second seed laser is 25MHz. The two modulation frequencies are incompatible with each other and are different from the radio frequency driving frequencies of the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2.

[0013] Preferably, the center frequency of the first acousto-optic modulator AOM1 is 80MHz and the tuning range is ±20MHz; the bandwidth of the second acousto-optic modulator AOM2 is 200MHz and the tuning range is ±50MHz.

[0014] Preferably, the nonlinear sum-frequency SFG module further includes two polarization-maintaining fiber amplifiers, a PPMgOLN crystal, an external Fabry-Perot iodine cell, and a NICE-OHMS detection module. The inputs of the two polarization-maintaining fiber amplifiers are connected one-to-one to the main optical paths of the first and second seed lasers to increase the laser power. The outputs of both polarization-maintaining fiber amplifiers are connected to the optical path of the PPMgOLN crystal. The TEC temperature control circuit stabilizes the temperature of the PPMgOLN crystal to meet the phase-matching temperature requirement, resulting in a 633nm sum-frequency laser. A second branch beam is separated from the output optical path of the PPMgOLN crystal and injected into the external Fabry-Perot iodine cell. The external Fabry-Perot iodine cell includes an external resonant cavity filled with... 127 I2 is subjected to radio frequency phase modulation through the NICE-OHMS detection module to obtain an extremely low frequency error signal in the 0.01Hz to 10Hz frequency band, which is proportional to the detuning of the iodine transition relative to the optical frequency of the 633nm sum-frequency laser. The extremely low frequency error signal is fed back to the TEC temperature control circuit through the main feedback channel, covering the 0.01Hz to 0.1Hz frequency band; or it is fed back to the radio frequency signal source of the first acousto-optic modulator AOM1 in the two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules through the auxiliary feedback channel, covering the 0.1Hz to 10Hz frequency band. When the output of the TEC temperature control circuit exceeds 90% of the dynamic range, the channel switching is triggered.

[0015] Preferably, the free spectral range (FSR) of the external resonant cavity of the external Fabry-Perot iodine gas cell is exactly equal to the frequency of the phase modulation of the built-in electro-optic modulator of the NICE-OHMS detector module.

[0016] Preferably, the power of the second branch beam is 1 to 2 mW to ensure that the light intensity in the external resonant cavity does not cause [problems]. The saturation effect is achieved while simultaneously meeting the shot noise limit signal-to-noise ratio requirements of the NICE-OHMS detection module.

[0017] The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency stabilization provided by this invention has the following advantages compared with the prior art: 1. Noise Suppression Across the Entire Time Scale. This invention constructs a three-loop cascaded architecture. The first stage is a fast loop with a unity-gain bandwidth of not less than 1MHz, used to suppress seed laser white noise and high-frequency phase noise. The second stage is a slow loop with a unity-gain bandwidth not exceeding 10kHz, used to eliminate long-term physical creep of the ultrastable cavity. The third stage is an extremely slow loop with a unity-gain bandwidth not exceeding 10Hz, used to suppress thermally induced optical path difference drift of the nonlinear crystal. The three loops are staggered in frequency band and completely orthogonal in physical hardware operation, thereby achieving effective suppression of frequency noise and drift across the entire time scale from microseconds to tens of thousands of seconds. It is expected that the short-term frequency instability of the 633nm laser, characterized by Allan deviation, will not exceed 5×10⁻⁶. -15 @1s, long-term drift is no greater than 1×10 -15 @1000s, significantly outperforming existing single-loop and frequency solutions.

[0018] 2. Complete physical decoupling of the fast and slow loops. Traditional solutions directly feed the low-frequency signal of the atomic spectrum to the internal structure of the laser, which is prone to preemption and conflict with the high-frequency signal of the cavity locking mechanism in physical execution. This invention places the acousto-optic modulator in the sampling optical path fed into the ultrastable cavity, so that the change of DDS frequency can truly drive the effective locking point of the ultrastable cavity. The slow loop signal does not act on the high-frequency actuation port of the laser throughout the entire process. Combined with the PZT and current / EOM dual-channel frequency division feedback inside the fast loop, the contradiction that the physical bandwidth of a single PZT is insufficient to support a MHz closed loop is solved. The fast and slow loops are completely orthogonal in the frequency band of the mathematical transfer function and the physical execution hardware, which greatly improves the robustness and locking margin of the system.

[0019] 3. End-to-end compensation for residual drift of nonlinear crystals. To address residual phase noise during nonlinear conversion, this invention proposes an end-to-end compensation mechanism for the thermally induced optical path difference drift of the nonlinear crystal itself within the sum-frequency architecture. By distinguishing the physical characteristics of the low-light frequency doubling PPLN used for second-stage absolute frequency stabilization and the PPMgOLN responsible for high-power sum-frequency conversion, and in conjunction with the dual-channel frequency band division of labor feedback of the third-stage ultra-slow loop—the main channel controls the TEC temperature of the PPMgOLN covering the 0.01 to 0.1 Hz frequency band, and the auxiliary channel controls the DDS1 frequency word of the AOM1 covering the 0.1 to 10 Hz frequency band—the physical limitation that the seed light is stable but the nonlinear output light is not necessarily stable is overcome. The three absolute references, namely Rb@780nm, I2@532nm, and I2@633nm, are designed with master-slave priority to avoid loop over-constraint caused by frequency incommensurability.

[0020] 4. Engineering and Practical Advantages. The all-solid-state continuous wave seed source, polarization-maintaining fiber amplifier, and periodically polarized crystal architecture enable a final output power of 50 to 100 mW, more than an order of magnitude higher than traditional He-Ne lasers. The two seed paths share the same physical cavity and the same vacuum thermal shielding environment, ensuring that the frequency noise caused by cavity vibration and temperature drift maintains a highly correlated spectrum between the two paths. This facilitates unified calibration via a third-level single reference and simplifies the mechanical structure and environmental isolation design. The system possesses excellent resistance to environmental vibration, with an MTBF of no less than 20,000 hours for all-solid-state devices, making it highly suitable for cutting-edge engineering applications such as length measurement in high-end lithography dual-frequency laser interferometers, space gravitational wave detection (LISA, Tianqin), absolute gravimeters, and cold atom physics. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is an overall structural diagram of the ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency stabilization of the present invention; Figure 2 This is a schematic diagram of the dual-wavelength common cavity ultra-stable module of the ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum frequency stabilization according to the present invention; Figure 3 This is a schematic diagram of the structure of the two acoustic-optical frequency shift and dual-reference absolute frequency stabilization modules of the ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum frequency stabilization of the present invention. Figure 4 This invention relates to a nonlinear sum-frequency (SFG) module for an ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency generation. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] The existing sum-frequency schemes have the following shortcomings in the frequency stabilization physical mechanism: 1. Insufficient phase noise superposition and broadband suppression in nonlinear conversion: The frequency drift and phase noise of the sum-frequency output laser are equal to the linear superposition of the two seed lasers. The existing SFG scheme usually only performs single-loop feedback at the 633nm output end. The servo bandwidth is limited by the signal-to-noise ratio of the molecular spectrum, usually <1kHz, which makes it difficult to effectively suppress the phase noise of the seed laser in the mid-to-high frequency range, such as >10kHz; 2. Physical mismatch and loop conflict of servo control bandwidth: Ultra-low expansion glass Fabry-Perot cavity (FP) can provide MHz-level feedback bandwidth, narrowing the laser linewidth to sub-Hz, but the ultra-stable cavity has long-term thermophysical creep; 3. Low-frequency phase drift introduced by the nonlinear crystal itself: The main optical path used for sum-frequency conversion usually has high power of tens to hundreds of milliwatts. Crystals such as periodically polarized lithium niobate will introduce additional optical path difference drift in the 0.01–10Hz frequency band due to thermally induced refractive index changes under high-power continuous pumping.

[0025] In view of this, such as Figure 1 As shown, this invention provides an ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency stabilization, the overall architecture of which includes: The first seed laser is used to output a 1560nm continuous wave seed laser. In this embodiment, a low-noise erbium-doped fiber laser, such as the NKT Koheras ADJUSTIK, is used, with an output power of about 10mW and a free-running linewidth of less than 1kHz. The first seed laser also has a current fast tuning port with a bandwidth of about 10MHz and a PZT slow tuning port with a bandwidth of about 10kHz, providing two independent operating channels.

[0026] The second seed laser is used to output a 1064nm continuous wave seed laser. In this embodiment, a non-planar ring cavity laser (NPRO), such as Coherent / Innolight Mephisto, is used, with an output power of about 500mW and a free-running linewidth of less than 1kHz. The laser has a built-in PZT slow tuning port with a bandwidth of 10 to 50kHz, and an external cavity EOM device with a bandwidth of about 10MHz is connected in series at the output end as a fast actuation channel.

[0027] It should be noted that 1560nm and 1064nm refer to the wavelengths of the two seed lasers, while the precise nominal wavelengths of the two seed lasers are locked to [specific wavelength range] by the system and frequency modulation. 127 I2R(127)11-5 transition a 13 The constraints of the hyperfine components are uniquely determined by inverse reasoning. Specifically, I2R(127)11-5 a 13The center frequency is 473,612,214,705 kHz, which is the recommended value by BIPM, corresponding to a vacuum wavelength of approximately 632.991 nm. This is determined by the energy conservation relationship 1 / λ_SFG = 1 / λ1 + 1 / λ2. 87 Rb D2 line With a transition constraint of 780.246 nm, the precise nominal wavelength λ1 of the first seed laser can be deduced to be approximately 1560.49 nm, and the precise nominal wavelength λ2 of the second seed laser to be approximately 1064.49 nm. The two seed lasers will be referred to as 1560 nm and 1064 nm respectively in the following text of this specification, without repeating their precise values.

[0028] The dual-wavelength common-cavity ultra-stable module is connected to the optical paths of the first and second seed lasers. It is used to extract the high-bandwidth error signal (PDH) from the seed lasers output by the first and second seed lasers and feed the PDH back to the fast tuning ports of the first and second seed lasers respectively. After extracting the PDH, the dual-wavelength common-cavity ultra-stable module decomposes the PDH into fast and slow paths through a frequency division network: the fast path is fed back to the current port of the first seed laser and the external EOM device at the output of the second seed laser, respectively, and the slow path is fed back to the PZT slow tuning ports of the two lasers to suppress the frequency noise of the two seed lasers, thereby achieving the first-stage optical frequency closed-loop control with a unity-gain bandwidth of not less than 1MHz.

[0029] The two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules each include an acousto-optic modulator (AOM). The AOMs are positioned one-to-one between the first seed laser, the second seed laser, and the dual-wavelength co-cavity ultra-stable module on the main optical path. They are used to separate the first branch beams from the sampled light diffracted by the AOMs, and to generate frequency-doubled lasers by frequency doubling the two first branch beams respectively. The error signals of the transition frequencies of the frequency-doubled lasers are extracted respectively. Based on the error signals of the transition frequencies, the RF reference bridging is achieved by changing the RF drive frequency of the AOMs to correct the physical drift of the dual-wavelength co-cavity ultra-stable module caused by material creep and temperature drift, and to achieve second-level optical frequency closed-loop control with a total unity gain bandwidth not exceeding 10kHz.

[0030] The nonlinear sum-frequency (SFG) module includes a TEC temperature control circuit. The SFG module connects to the main optical path of either the first or second seed laser, amplifying the signal from the main optical path. The amplified laser signal is then summed and frequency-controlled at the operating temperature set by the TEC temperature control circuit, outputting a sum-frequency laser. A second branch beam is then separated from the output optical path of the sum-frequency laser. An extremely low-frequency error signal in the 0.01Hz to 10Hz frequency band is demodulated and fed back to the TEC temperature control circuit and the acousto-optic modulator (AOM) of the two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules, achieving a third-level optical frequency closed-loop control. This third-level optical frequency closed-loop control is orthogonal to each other in terms of frequency band and hardware physical operation.

[0031] like Figure 2 As shown, the dual-wavelength common-cavity ultrastable module includes two optical isolators, two electro-optic modulators (EOMs), two mode-matching lens groups, two mirror groups, an ultra-low expansion glass Fabry-Perot ultrastable cavity, two broadband photodetectors, and two independent first PID controllers. The first and second seed lasers are processed by an optical isolator and then enter an electro-optic modulator (EOM) in a one-to-one correspondence. The output optical path of the EOM is then adjusted by the mode-matching lens group to change the wavefront curvature and beam waist position of the Gaussian beam, and then fed into a mirror group in a one-to-one correspondence to adjust the optical axis attitude, before being coupled into the common-cavity common-cavity module. Different longitudinal modes are constructed within an ultra-low expansion glass Fabry-Perot ultra-stable cavity. Two broadband photodetectors acquire reflected light signals from the cavity reflection port of the ultra-low expansion glass Fabry-Perot ultra-stable cavity, respectively. The acquired reflected light signals are demodulated to obtain two high-bandwidth error signals (PDHs). Two first PID controllers are used to acquire the two high-bandwidth error signals (PDHs) respectively and output high-frequency control signals. The high-frequency control signals are fed back to the first seed laser and the second seed laser. An overlapping frequency division network is set between the two high-bandwidth error signals (PDHs), with the division point set at 100kHz and the overall phase margin not less than 45°.

[0032] The output lasers from the first and second seed lasers are processed by optical isolators and then injected into the acousto-optic modulators (AOMs) of the two acousto-optic frequency shifting and dual-reference absolute frequency stabilization modules for physical frequency shifting. The first-order diffraction sampling portion of the AOM then enters the two electro-optic modulators (EOMs) of the dual-wavelength common-cavity ultra-stable module. The two EOMs perform electro-optic phase modulation on the sampled lasers at 1560nm and 1064nm, respectively, at modulation frequencies of 20MHz and 25MHz. The two frequencies are incompatible and are spectrally completely separated from the RF drive frequency of their respective AOMs, i.e., the modulation frequencies of the two EOMs are far from the RF drive frequencies of their respective AOMs, thus avoiding RF crosstalk.

[0033] Two independent first PID controllers process two high-bandwidth error signals PDH respectively. The Laplace transfer function of the first PID controller is: , Here are the proportional, integral, and differential coefficients, and τ is the low-pass time constant of the differential element. s It is a Laplace operator, and the closed-loop cutoff frequency is... f c The frequency is >1MHz, but is set to 1.5MHz in this embodiment. Long-term frequency drift is compensated for by a slow channel to ensure the laser frequency is locked to the intrinsic frequency of the ultra-low expansion glass Fabry-Perot ultra-stable cavity. The output of each first PID controller is decomposed into fast and slow control signals via an overlapping frequency divider network:

[0034] 1. Fast path > 100kHz: For the 1560nm path, after RF power amplification, it is fed into the current tuning port of the first seed laser; for the 1064nm path, it is fed into the external cavity EOM device at the output of the second seed laser. 2. Slow path <100kHz: After integration, the signal is fed into the PZT ports of the two seed lasers to keep the operating point at the center of the range and avoid long-term drift that could lead to PZT saturation.

[0035] The frequency division point is set around 100kHz, and a -3dB overlap band is configured to ensure that the overall ring phase margin is not less than 45°. In this embodiment, the overall ring unity-gain bandwidth is set to 1.5MHz, thereby narrowing the instantaneous linewidth of the two seed lasers to the sub-Hz level and locking them to the resonance peaks of two different longitudinal modes in the same ultrastable cavity.

[0036] In one embodiment, the ultra-low expansion glass Fabry-Perot ultrastable cavity has a length L = 100 mm and a free spectral range FSR = c / (2L) = 1.5 GHz. The cavity mirror is coated with dual-band high-reflectivity dielectric films for the 1560 nm and 1064 nm wavelength bands, respectively, with a fineness of not less than 150,000 for each band, corresponding to a cavity mode linewidth of approximately 10 kHz. The fineness is measured using the cavity ring-down method. This ultrastable cavity is suspended in a thermally shielded vacuum chamber with a vacuum level better than 2 × 10⁻⁶. -6 The system employs a two-stage active temperature control system, such as a combination of an outer TEC layer and an inner heating wire, to control cavity temperature fluctuations to less than 0.5 mK. The temperature setpoint is selected near the zero expansion temperature of the ULE material, with typical temperatures ranging from 25 to 35°C. The cavity thermal noise limit corresponds to a frequency instability better than 4 × 10⁻⁶ mK. -15 @1s.

[0037] The first-stage fast loop satisfies a unity-gain bandwidth of not less than 1MHz and a phase margin of not less than 45°; after suppressing the white noise substrate of the entire loop, the instantaneous linewidth of the two seed lasers is not greater than 1Hz.

[0038] The electro-optic modulator (EOM) performs electro-optic phase modulation on the 1560nm and 1064nm seed lasers respectively, generating radio frequency sidebands to facilitate the subsequent extraction of the high-bandwidth error signal (PDH). The ultra-low expansion glass Fabry-Perot ultra-stable cavity is used for PDH frequency locking. By incidenting the laser adjusted by the electro-optic modulator (EOM) onto the fiber resonator (i.e., different longitudinal modes of the ultra-low expansion glass Fabry-Perot ultra-stable cavity), and the fiber resonator being coated with a dual-band high-reflectivity film, the reflected light signal is acquired and demodulated to extract the high-bandwidth error signal (PDH) proportional to the difference between the laser frequency and the cavity resonant frequency. This high-bandwidth error signal (PDH) is used in conjunction with the first PID controller to adjust the frequency of the seed laser. In this embodiment, piezoelectric ceramic (PZT) is selected as the fast tuning port for each of the two seed lasers to narrow the common mode of the two seed sources, suppress instantaneous frequency noise, and achieve closed-loop feedback of the fast loop.

[0039] like Figure 3 The upper part shows the acousto-optic frequency shift and dual-reference absolute frequency stabilization module connected to the main optical path of the first seed laser, which includes a first acousto-optic modulator AOM1, a first polarization beam splitter, a first PPLN crystal, a rubidium atom gas cell, and a second PID controller; wherein: The 1560nm laser output from the first seed laser is first injected into the first acousto-optic modulator AOM1 for physical frequency shifting. The center frequency of the first acousto-optic modulator AOM1 is 80MHz, and the tuning range is ±20MHz. The first-order diffracted light from the first acousto-optic modulator AOM1 is split into two paths after passing through the first polarization beam splitter:

[0040] The first path is the sampling light, which enters the aforementioned dual-wavelength co-cavity ultra-stable module for PDH fast loop locking; The second 1560nm first branch beam, with a power of not less than 50mW, is injected into the first PPLN crystal. The polarization period of the first PPLN crystal is matched with a 1560nm frequency doubling, and the first PPLN crystal generates a 780nm frequency-doubled laser, which is then injected into the rubidium atom gas cell. The 780nm frequency-doubled laser is extracted relative to the rubidium atom gas cell using saturable absorption spectroscopy. 87 Rb D2 line The error signal of the transition frequency is sent to the second PID controller. The output of the second PID controller is connected to the radio frequency signal source of the first acousto-optic modulator AOM1. The second PID controller feeds back the output signal to the radio frequency signal source of the first acousto-optic modulator AOM1, that is, the frequency control word of the direct digital frequency synthesizer DDS1 of the first acousto-optic modulator AOM1, which is used to change the driving frequency of the first acousto-optic modulator AOM1 in real time.

[0041] To ensure the frequency doubling signal-to-noise ratio, the first PPLN crystal in this embodiment is a waveguide-type periodically polarized lithium niobate crystal with a length of 50 mm, a polarization period Λ≈19.5 μm, and a waveguide cross-section of approximately 6 μm × 6 μm. It is matched for frequency doubling from 1560 nm to 780 nm, and its operating temperature is stabilized at around 80°C by an independent TEC. The normalized frequency doubling efficiency of the waveguide-type first PPLN crystal can reach 500% / (W·cm). 2 The single-pass frequency doubling efficiency is about 5%, and the output 780nm frequency doubling laser power is about 2 to 5mW.

[0042] As an alternative implementation, a standard-sized PPLN with a length of 30mm can be used, along with a first-stage pre-EDFA to amplify the first branch beam to over 200mW, obtaining a 780nm frequency-doubled beam of equivalent power. The 780nm frequency-doubled laser, after isolation and attenuation, enters the filling beam. 87 The rubidium atom gas cell for Rb is 50 mm in length, operates at room temperature, and is locked to saturable absorption spectroscopy (SAS). 87 Rb D2 line The transition frequency is 384,228,115 MHz, and the error signal of the transition frequency is extracted. This error signal is sent to the second PID controller. The output of the second PID controller controls the frequency control word of the direct digital frequency synthesizer (DDS1) via an analog interface. The DDS1 is a 48-bit frequency word with a frequency resolution of approximately μHz. The output of the DDS1 is amplified by RF power and then drives the first acousto-optic modulator (AOM1). The full-loop unity-gain bandwidth of the second PID controller is set to no more than 10 kHz; in this embodiment, a typical value is set to 5 kHz. This bandwidth is limited by the saturated absorption spectrum signal-to-noise ratio and the DDS frequency update rate, and is matched to the physical characteristics of the slow loop.

[0043] like Figure 3 The lower half shows the acousto-optic frequency shift and dual-reference absolute frequency stabilization module connected to the main optical path of the second seed laser, which includes a second acousto-optic modulator AOM2, a second polarization beam splitter, a second PPLN crystal, an iodine molecule gas cell, and a third PID controller; wherein: The 1064nm laser output from the second seed laser is first injected into the second acousto-optic modulator AOM2 for physical frequency shifting. The center frequency of the second acousto-optic modulator AOM2 is 200MHz, with a tuning range of ±50MHz. The first-order diffraction beam from the second acousto-optic modulator AOM2 is also split into two paths: one is a sampling beam sent to the ultrastable cavity, and the other is a first branch beam of 1064nm with a power of not less than 50mW. The first branch beam of 1064nm is injected into the second PPLN crystal. In this embodiment, the second PPLN crystal is a waveguide-type PPLN with a length of 50mm, a polarization period Λ≈7.0μm, an operating temperature of 40°C, and an output 532nm frequency-doubled laser power of approximately 5 to 10mW.

[0044] 532nm frequency-doubled laser enters the filling 127 The iodine molecule gas cell of I2, 150 mm in length, has a cold finger temperature range of -10°C to 0°C and is locked to [the cell's location] using modulation transfer spectroscopy (MTS). 127 I2R(56)32-0 transition a 10 Ultra-fine components, center frequencies 563,260,223,513kHz, which is the recommended BIPM value.

[0045] The error signal is fed into the third PID controller, and the output controls the frequency control word of the direct digital frequency synthesizer DDS2. The full-loop unity-gain bandwidth of the third PID controller is set to no more than 10kHz, and a typical value of 3kHz is used in this embodiment. The radio frequency signal sources of the two acousto-optic modulators AOM, namely the direct digital frequency synthesizers DDS1 and DDS2, are both 48-bit direct digital frequency synthesizers with a frequency resolution on the order of approximately μHz.

[0046] The functions of the two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules are: 1. To split the first branch beam from the sampled light after diffraction by the acousto-optic modulator (AOM) and perform frequency doubling, then inject the frequency-doubled laser into the atomic / molecular gas cell to extract the error signal of the transition frequency; 2. To change the radio frequency drive frequency of the acousto-optic modulator (AOM) so that the absolute frequency of the main optical path after fast-path locking of the dual-wavelength co-cavity ultra-stable module is equal to the intrinsic longitudinal mode frequency of the ultra-stable cavity minus the AOM drive frequency, thereby dragging the effective locking point in the radio frequency reference bridging mode to correct the long-term physical drift of the ultra-low expansion glass Fabry-Perot ultra-stable cavity caused by material creep and temperature drift; 3. To achieve second-level optical frequency closed-loop control with a total loop unity gain bandwidth not exceeding 10kHz.

[0047] In the two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules, the 1064nm path corresponding to the second seed laser is hard-locked to... 127The absolute transition of I2 at 532nm serves as the primary reference; the absolute detuning of the 1560nm path corresponding to the first seed laser is uniquely determined by the 633nm I2 transition through the third-level feedback; the rubidium atom frequency stabilization circuit corresponding to the first seed laser is only used for initial capture and long-term monitoring to avoid loop conflicts between the three absolute references due to frequency incommensurability.

[0048] Since both the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2 are located in the sampling optical path fed into the ultra-stable cavity, the absolute frequency of the main optical path satisfies and ,in f cavity and The eigenfrequency of the longitudinal mode corresponding to the ultra-low expansion glass Fabry-Perot ultrastable cavity. These are the driving frequencies of the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2, respectively. When the frequency words of DDS1 and DDS2 are changed by the second PID controller and the third PID controller, the PDH fast loop automatically adjusts the laser frequency to maintain lock, thereby dragging the effective lock point through the RF reference bridge method to achieve indirect calibration of the absolute frequency of the main optical path.

[0049] The physical mechanism of the RF reference bridge is explained as follows: Since both the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2 are located in the sampling optical path fed into the low-expansion glass Fabry-Perot ultrastable cavity, the PDH fast loop locking conditions are as follows: , When the second PID controller changes the driving frequency of the first acousto-optic modulator AOM1 f DDS1 At that time, the sampling light frequency shifts → the PDH error signal changes → the fast loop automatically adjusts the laser frequency to maintain lock → the absolute frequency of the main optical path changes. f 1560 The situation has changed; the slow-loop signal did not enter the high-frequency actuation port of the laser throughout the entire process.

[0050] Taking the 1560nm branch as an example, the derivation is as follows: The sampled optical frequency after frequency shifting of the first acousto-optic modulator AOM1 is: PDH Fast Loop Forced f sample = f cavity Therefore Frequency after frequency multiplication The error signal extracted from saturated absorption spectroscopy is , f RB For selected The ultra-fine transition center frequency, the second PID controller adjusts DDS1 to reduce the error signal extracted from the saturated absorption spectrum. ,at this time That is, the driving frequency of the first acousto-optic modulator AOM1. f DDS1 Automatic tracking of the drift of the ultrastable cavity relative to the rubidium transition. It is important to note the absolute frequency of the main optical path. f 1560 The change is not achieved by the second PID controller directly driving the laser's fast actuation port, but through the first-stage fast loop closed feedback. The fast loop locks the laser in the ultra-stable cavity, and the effective locking point of the ultra-stable cavity is dragged by DDS1. This RF bridging process is used to compensate for long-term creep. The slow loop signal and the fast loop signal are completely decoupled in physical hardware.

[0051] Similarly, for the 1064nm branch, the following conditions are met: By adjusting the second-stage slow loop, the absolute frequency of the 1064nm main optical path is... f 1064 Hard-locked to the selected 127 I2@532nm a 10 The center frequency of the transition f I2(532) Meanwhile, it indirectly compensates for the long-term creep of the ultra-stable cavity. The 1560nm path is initially pulled by the Rb atomic ring only in the second-level optical frequency closed-loop control. During normal operation, its absolute detuning is solely determined by the NICE-OHMS detection module of the third-level optical frequency closed-loop control, which will be described later.

[0052] By adjusting the RF signal source of the first or second acousto-optic modulator, it is equivalent to dragging the effective locking point of the ultra-low expansion glass Fabry-Perot ultra-stable cavity, thereby stabilizing the optical frequency of the main optical path. Due to the existence of the RF reference bridging method, the effective locking point of the ultra-low expansion glass Fabry-Perot ultra-stable cavity is dragged along by the RF reference bridging, and the absolute frequency reference of the main optical path and the transition frequency are anchored, realizing slow-loop physical bridging.

[0053] In this invention, the first and second PPLN crystals are periodically polarized lithium niobate PPLN crystals. To ensure sufficient power for the branch beams, polarization-maintaining fibers can be connected in series before the two first branch beams are injected into their respective PPLN crystals, amplifying the power of the corresponding first branch beams to no less than 200mW. This ensures that the 780nm laser power after frequency doubling is no less than 500μW and the 532nm laser power is no less than 5mW, satisfying the signal-to-noise ratio required for the saturated absorption spectrum and modulation transfer spectrum.

[0054] In one embodiment, the center frequency of the first acousto-optic modulator AOM1 is 80MHz, and the bandwidth of the second PID controller is 100MHz; the center frequency of the second acousto-optic modulator AOM2 is 200MHz, and the bandwidth of the third PID controller is 50MHz.

[0055] like Figure 4 As shown, the nonlinear sum-frequency SFG module specifically includes two polarization-maintaining fiber amplifiers, a PPMgOLN crystal, a TEC temperature control circuit, an external Fabry-Perot iodine gas chamber, a NICE-OHMS detection module, and a fourth PID controller; wherein: Two polarization-maintaining fiber amplifiers are connected one-to-one with the main optical paths of the first and second seed lasers, respectively, to amplify the power of the 1560nm laser to 100 to 300mW and the power of the 1064nm laser to 200 to 500mW. The main optical path here refers to the zero-order or sampled residual light diffracted by the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2. In this embodiment, the 1560nm laser uses an erbium-doped fiber amplifier with an amplification value of 200mW; the 1064nm laser uses a ytterbium-doped fiber amplifier with an amplification value of 300mW.

[0056] PPMgOLN crystals, 25 mm in length, are doped with 5 mol% MgO to raise the photorefractive damage threshold to no less than 500 MW / cm². 2 The polarization period Λ≈11.0μm, and the polarization period matches the sum-frequency process from 1560nm and 1064nm to 633nm. The output beams of the two polarization-maintaining fiber amplifiers are collinearly injected into the crystal after spatial mode matching. The TEC temperature control circuit is used to stabilize the temperature of the PPMgOLN crystal within the phase matching temperature range of ±0.01K, with a sum-frequency efficiency of not less than 30% / W and an output 633nm sum-frequency laser power of not less than 50mW. The external Fabry-Perot iodine gas chamber includes an external resonant cavity and a cavity filled with iodine gas. 127 I2 and independent PDH locking chamber ring; wherein: The free spectral range (FSR) of the external resonant cavity is precisely equal to the phase modulation frequency of the built-in electro-optic modulator of the NICE-OHMS detection module; a cavity length of 75 mm, a free spectral range (FSR) of 2 GHz, a cavity precision of not less than 5000 can be selected, and the cavity mirror has high reflectivity of 633 nm and partial transmission of the MHz-level cavity-locked modulation frequency. 127The I2 cold finger temperature is controlled between -10°C and 0°C. The independent PDH cavity-locking sub-ring superimposes MHz-level phase modulation on the second branch beam, and the resulting independent PDH error signal is fed back to the piezoelectric ceramic device (PZT) in the external resonant cavity, locking the external resonant cavity to a 633nm sum-frequency laser. As an alternative implementation, a cavity length of 375mm can be selected, corresponding to a free spectral range (FSR) of 400MHz for the external resonant cavity, achieving a narrower cavity mode linewidth with a longer cavity, but requiring higher mechanical stability. One of the two methods should be chosen.

[0057] The independent PDH cavity-locking sub-loop generates a radio frequency sideband by superimposing a 5MHz phase modulation (EOM) with a modulation depth of 0.3 onto the second branch beam. The reflected light is demodulated by a broadband detector at 5MHz to obtain the PDH error signal for the external cavity. This signal is then fed back to the piezoelectric ceramic device (PZT) in the external resonant cavity via an independent PID controller with a bandwidth of 10kHz, precisely locking the longitudinal mode of the external resonant cavity to the 633nm sum-frequency laser. This independent PDH cavity-locking sub-loop ensures the laser frequency... External resonant cavity resonance The resonance condition among the three components of the NICE-OHMS detection module is a prerequisite for the NICE-OHMS detection module to function.

[0058] The NICE-OHMS detection module includes a built-in electro-optic modulator and a broadband photodetector. The built-in electro-optic modulator applies phase modulation to the 633nm laser injected into the external resonant cavity, precisely equal to the FSR of the external cavity's free spectral range, with a modulation depth of 1.0. This causes the carrier wave and ±1st-order sidebands to resonate simultaneously in adjacent longitudinal modes of the external cavity and... 127 I2 absorption; after the transmitted light is received by a broadband photodetector, heterodyne demodulation is performed at the modulation frequency to extract the relative frequency of the 633nm sum-frequency laser. 127 I2R(127)11-5 transition a 13 Extremely low frequency error signals in the 0.01Hz to 10Hz frequency band, proportional to the ultra-fine component detuning.

[0059] Since the free spectral range (FSR) of the external resonant cavity is strictly equal to the modulation frequency of the NICE-OHMS detection module, the AM noise of the carrier and sidebands, the amplitude and phase drift of the EOM, and the laser intensity noise are all canceled out after demodulation, resulting in a pure fundamental frequency signal and a signal-to-noise ratio improvement of 2 to 3 orders of magnitude compared to traditional FM spectra. After low-pass filtering, with a cutoff frequency of 10Hz, the signal obtained is comparable to the 633nm sum-frequency laser. 127 I2R(127)11-5 transition a 13The ultra-fine component detuning results in an extremely low frequency error signal in the 0.01Hz to 10Hz frequency band. The transition center frequency is 473,612,214,705kHz, which is the BIPM recommended value and precisely matches the He-Ne / I2 standard wavelength, ensuring that the output of this solution is directly traceable to the International Bureau of Weights and Measures length standard. It should be noted that the 2GHz or 400MHz modulation frequency of the NICE-OHMS probe module is fully isolated spectrally from the 5MHz modulation frequency of the independent PDH cavity-locked sub-loop; the two are completely decoupled through bandpass filtering and do not interfere with each other.

[0060] The fourth PID controller receives the NICE-OHMS extremely low frequency error signal, with a total loop unity gain bandwidth not exceeding 10Hz. The output signal achieves third-level optical frequency closed-loop control through two feedback paths: the main feedback channel feeds back to the TEC temperature control circuit of the PPMgOLN crystal, covering the frequency band from 0.01Hz to 0.1Hz, and compensates for the crystal's thermally induced optical path difference drift through the dn / dT effect; the auxiliary feedback channel feeds back to the frequency word of the direct digital frequency synthesizer DDS1 of the first acousto-optic modulator AOM1, covering the frequency band from 0.1Hz to 10Hz, and takes over the main channel when the TEC dynamic range approaches its boundary. At this time, the rubidium atom frequency stabilization loop corresponding to the first seed laser switches to monitoring mode and does not participate in hard locking.

[0061] The second and fourth PID controllers employ a mutually exclusive switching mechanism to control the direct digital frequency synthesizer (DDS1): during normal operation, the second PID controller maintains the driving frequency of the first acousto-optic modulator (AOM1). f DDS1 The control is transferred to the fourth PID controller, whose auxiliary feedback channel is in standby mode. When the output of the TEC temperature control circuit of the PPMgOLN crystal exceeds 90% of the dynamic range, the control is switched. The auxiliary feedback channel of the fourth PID controller takes over the direct digital frequency synthesizer DDS1 of the first acousto-optic modulator AOM1, and the second PID controller switches to monitoring mode synchronously. The switching process adopts a non-disturbance algorithm. The initial output value of the new controller inherits the final output value of the old controller. The switching time does not exceed 100ms to avoid frequency jumps.

[0062] Here, the output signal of the fourth PID controller achieves the third-level optical frequency closed-loop control through two feedback paths divided by frequency band: Main feedback channel: Covering the 0.01 to 0.1 Hz frequency band, it feeds back to the TEC temperature control circuit of the PPMgOLN crystal. A small temperature bias within ±5 mK is superimposed on the phase-matching temperature, changing the crystal optical path length through the dn / dT effect to compensate for the thermally induced optical path difference drift under high-power pumping. The physical response bandwidth of the TEC is approximately 0.1 Hz, which perfectly matches the main channel compensation frequency band.

[0063] Auxiliary feedback channel: Covering the 0.1 to 10 Hz frequency band, it takes over the main channel when the dynamic range of the TEC temperature control circuit approaches its limit, feeding back the DDS1 frequency word of the first acousto-optic modulator AOM1. Through RF bridging, it corrects residual drift over a wider bandwidth and a larger dynamic range. At this time, the 1560nm rubidium atom frequency stabilization loop switches to monitoring mode and does not participate in hard locking, avoiding loop conflicts between the three absolute references.

[0064] In one embodiment, the power of the second branch beam split from the 633nm sum-frequency laser output from the PPMgOLN crystal is 1 to 2 mW, ensuring that the light intensity within the external resonant cavity does not cause [unclear - possibly a problem]. 127 The I2 saturation effect is satisfied, while also meeting the shot noise limit signal-to-noise ratio requirements of the NICE-OHMS detection module.

[0065] Absolute reference master-slave priority: This invention sets the I2@532nm of the 1064nm path as the master reference, which has the highest signal-to-noise ratio and complete BIPM traceability. The absolute detuning of the 1560nm path is uniquely determined by the third-level optical frequency closed-loop control based on the 633nm I2NICE-OHMS stage. Its corresponding Rb@780nm frequency stabilization stage is only used for initial acquisition and long-term monitoring and does not participate in normal hard locking, thereby avoiding loop conflicts caused by the incommensurability of the three absolute references.

[0066] The three-level closed-loop frequency band separation and hardware decoupling mechanism: The three-level closed-loop system achieves decoupling through frequency band division and physical actuation hardware separation, avoiding direct conflicts in control signals. Specifically, it includes the following: 1. First-stage optical frequency closed-loop control, i.e., fast loop, hereinafter referred to as first stage, corresponds to noise >100kHz: Actuation hardware: the current tuning port of the first seed laser and the PZT, and the external EOM of the second seed laser and the PZT; Control objective: Suppress the high-frequency phase noise of the seed laser and narrow the instantaneous linewidth to <1Hz; Reference benchmark: High bandwidth error signal PDH of ultra-low expansion glass Fabry-Perot ultra-stable cavity; Full-loop bandwidth: >1MHz, phase margin ≥45°; 2. Second-stage optical frequency closed-loop control, slow loop, hereinafter referred to as the second stage, corresponds to noise from 0.1Hz to 10kHz: Operating hardware: Direct digital frequency synthesizer DDS1 of the first acousto-optic modulator AOM1, and direct digital frequency synthesizer DDS2 of the second acousto-optic modulator AOM2; Control objective: To compensate for the long-term creep and temperature drift of the ultrastable cavity through radio frequency bridging, lock the main optical path, and enable atomic / molecular transitions at absolute frequencies; Reference benchmark: 87 Rb D2 line transition @780nm as an auxiliary reference, 127 I2 transition at 532nm is used as the primary reference. Full-loop bandwidth: ≤10kHz, overlapping with the first stage at 100kHz; 3. Third-level optical frequency closed-loop control, also known as the extremely slow loop, hereinafter referred to as the third level, corresponds to noise levels from 0.01Hz to 10Hz: Actuation hardware: TEC temperature control circuit of PPMgOLN crystal, where the main channel corresponds to the frequency band of 0.01 to 0.1 Hz; DDS1 frequency word, the auxiliary channel corresponds to the frequency band of 0.1 to 10 Hz; Control objectives: Compensate for the thermally induced optical path difference drift of the nonlinear crystal, lock the 633 nm sum-frequency laser; absolute frequency to 127 I2R(127)11-5 a 13 Transition; Reference Baseline: 127 I-transition @ 633nm; Total loop bandwidth: ≤ 10Hz, naturally separated from the second stage at 0.1Hz; Control switching: During normal operation, the second-level control DDS1 is activated. When the TEC output exceeds 90% of the dynamic range, the third-level auxiliary channel takes over DDS1, and the second-level synchronous switch is switched to monitoring mode. The switching adopts a non-disruptive algorithm to avoid frequency jumps.

[0067] Key design considerations for decoupling mechanisms: Frequency band separation: The cutoff frequencies of adjacent loops differ by at least one order of magnitude to ensure frequency domain isolation; Hardware separation: Different loops operate on different physical actuators, and the control of DDS1 by the second and third stages adopts mutually exclusive switching rather than superposition to avoid control conflicts. Cascaded transmission: The low-frequency loop indirectly acts by changing the reference point of the high-frequency loop, such as changing the DDS frequency → the effective locking point of the ultra-stable cavity moves → the PDH fast loop automatically follows, rather than directly injecting control signals; Feedback signals from any stage are not directly superimposed on the actuation hardware used by another stage, ensuring the stability and maintainability of the system.

[0068] System performance metrics: Through the above three-stage cascaded closed-loop and hierarchical priority strategy, the system achieves frequency noise suppression and drift compensation across the entire time scale. Specific performance metrics are as follows: 1. Short-term frequency instability, Allan deviation ≤ 5 × 10⁻⁶ -15 @1s; 2. Long-term frequency drift, Allan deviation ≤ 1×10 -15 @1000s; 3. Absolute frequency accuracy: ≤10kHz, relative to BIPM recommendations.127 I2R(127)11-5 a 13 Leap; 4. Output power: ≥50mW@633nm; 5. Instantaneous linewidth after first-stage fast ring compression: <1Hz; This performance is significantly better than that of traditional He-Ne / I2 frequency-stabilized lasers, with a typical instability of ~1×10⁻⁶. -13 @1s, meeting the needs of cutting-edge applications such as length measurement with dual-frequency laser interferometers for high-end lithography machines, space gravitational wave detection such as LISA and Tianqin, and absolute gravimeters.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-stability laser system based on dual-path independent composite frequency stabilization and sum-frequency stabilization, characterized in that, include: The first seed laser is used to output a 1560nm continuous wave seed laser. The second seed laser is used to output a 1064nm continuous wave seed laser. The dual-wavelength common cavity ultra-stable module is connected to the optical paths of the first seed laser and the second seed laser. It is used to extract the high-bandwidth error signal PDH from the seed lasers output by the first seed laser and the second seed laser, and feed the high-bandwidth error signal PDH back to the first seed laser and the second seed laser to suppress the frequency noise of the two seed lasers and realize the first-stage optical frequency closed-loop control. The two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules each include an acousto-optic modulator (AOM). The AOMs are positioned one-to-one between the first seed laser, the second seed laser, and the dual-wavelength co-cavity ultra-stable module in the main optical path. They are used to separate the first branch beams from the sampled light diffracted by the AOMs, and to generate frequency-doubled lasers by frequency doubling the two first branch beams respectively. The error signals of the transition frequencies of the frequency-doubled lasers are extracted respectively. Based on the error signals of the transition frequencies, the radio frequency reference bridging is achieved by changing the radio frequency drive frequency of the AOMs to correct the physical drift of the dual-wavelength co-cavity ultra-stable module caused by material creep and temperature drift, and to achieve second-level optical frequency closed-loop control with a total unity gain bandwidth not exceeding 10kHz. The nonlinear sum-frequency (SFG) module includes a TEC temperature control circuit. The SFG module is used to connect to the main optical path of the first or second seed laser, respectively, to amplify the signal of the main optical path. The amplified laser signal is summed and frequency-controlled at the operating temperature set by the TEC temperature control circuit to output a sum-frequency laser. Then, a second branch beam is separated from the output optical path of the sum-frequency laser. The extremely low frequency error signal in the 0.01Hz to 10Hz frequency band is separated by demodulation. The extremely low frequency error signal is fed back to the TEC temperature control circuit and the acousto-optic modulator (AOM) of the two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules to realize the third-level optical frequency closed-loop control. The three-level optical frequency closed-loop control is orthogonal to each other in terms of frequency band and hardware physical operation.

2. The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency as described in claim 1, characterized in that, The dual-wavelength cocavity ultra-stable module includes two optical isolators, two electro-optic modulators (EOMs), two mode-matching lens groups, two mirror groups, an ultra-low expansion glass Fabry-Perot ultra-stable cavity, two broadband photodetectors, and two independent first PID controllers. The first and second seed lasers are each processed by an optical isolator before entering an electro-optic modulator (EOM). The output optical path of the EOM is then adjusted by the mode-matching lens groups to regulate the wavefront curvature and beam waist position of the Gaussian beam, and then fed into a mirror group to adjust the optical axis attitude before being coupled into the... Different longitudinal modes are introduced into a shared ultra-low expansion glass Fabry-Perot ultra-stable cavity. Two broadband photodetectors acquire reflected light signals from the cavity reflection port of the ultra-low expansion glass Fabry-Perot ultra-stable cavity, respectively. The acquired reflected light signals are demodulated to obtain two high-bandwidth error signals (PDHs). Two first PID controllers are used to acquire the two high-bandwidth error signals (PDHs) respectively and output high-frequency control signals. The high-frequency control signals are fed back to the first seed laser and the second seed laser. An overlapping frequency division network is set between the two high-bandwidth error signals (PDHs), with the division point set at 100kHz and the overall phase margin not less than 45°.

3. The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency as described in claim 2, characterized in that, The ultra-low expansion glass Fabry-Perot ultra-stable cavity has a length of 100 mm, a dual-band precision of 150,000, and is suspended in a heat-shielded vacuum chamber with a vacuum level of less than 2 × 10⁻⁶. -6 Pa, constant temperature fluctuation <0.5mK.

4. The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency as described in claim 2, characterized in that, The closed-loop cutoff frequency of the first PID controller f c >1MHz, phase margin not less than 45°; after suppression of the entire ring white noise substrate, the instantaneous linewidth of the two seed lasers is not greater than 1Hz.

5. The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency as described in claim 2, characterized in that, The acousto-optic frequency shifting and dual-reference absolute frequency stabilization module, connected to the main optical path of the first seed laser, includes a first acousto-optic modulator (AOM1), a first PPLN crystal, a rubidium atom gas cell, and a second PID controller arranged sequentially. The 1560nm laser output from the first seed laser is first injected into the first acousto-optic modulator (AOM1) for physical frequency shifting. One path of the frequency-shifted diffracted light is sent as sampling light into the dual-wavelength co-cavity ultra-stable module, and the other path of the diffracted light is injected into the first PPLN crystal as the first branch beam of 1560nm. The polarization period of the first PPLN crystal is matched to 1. A 560nm frequency doubling method is used. The first PPLN crystal generates a 780nm frequency doubling laser and injects it into the rubidium atom gas cell. The error signal of the transition frequency of the 780nm frequency doubling laser is extracted by saturated absorption spectroscopy. The error signal of the transition frequency of the 780nm frequency doubling laser is sent to the second PID controller. The output of the second PID controller is connected to the radio frequency signal source of the first acousto-optic modulator (AOM1). The second PID controller feeds back the output signal to the radio frequency signal source of the first acousto-optic modulator (AOM1) to change the driving frequency of the first acousto-optic modulator (AOM1) in real time. The acousto-optic frequency shifting and dual-reference absolute frequency stabilization module, connected to the main optical path of the second seed laser, includes a second acousto-optic modulator (AOM2), a second PPLN crystal, an iodine molecule gas cell, and a third PID controller arranged sequentially. The 1064nm laser output from the second seed laser is first injected into the second acousto-optic modulator (AOM2) for physical frequency shifting. One path of the frequency-shifted diffracted light is sent as sampling light into the dual-wavelength co-cavity ultra-stable module, while the other path of the diffracted light is injected into the second PPLN crystal as the first branch beam of 1064nm. The polarization period of the second PPLN crystal is matched to the 1064nm frequency doubling, and the second PPLN crystal generates a 532nm frequency-doubled laser, which is then injected into the iodine molecule gas cell. The error signal of the transition frequency of the 532nm frequency-doubled laser is extracted using modulation transfer spectroscopy (MTS) and locked to the α-axis of the iodine molecule. 10 The component sends the error signal of the transition frequency of the 532nm frequency-doubled laser to the third PID controller. The output of the third PID controller is connected to the radio frequency signal source of the second acousto-optic modulator. The third PID controller feeds back the output signal to the radio frequency signal source of the second acousto-optic modulator (AOM2) to change the driving frequency of the second acousto-optic modulator (AOM2) in real time. By changing the radio frequency driving frequencies of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2), the absolute frequency of the main optical path after being locked by the first-stage optical frequency closed-loop control is equal to the intrinsic longitudinal mode frequency of the ultra-low expansion glass Fabry-Perot ultra-stable cavity minus the driving frequencies of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2). This allows the effective locking point to be dragged in a radio frequency reference bridging manner, correcting the long-term physical drift of the ultra-low expansion glass Fabry-Perot ultra-stable cavity caused by material creep and temperature drift, thus achieving the second-stage optical frequency closed-loop control.

6. The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency as described in claim 5, characterized in that, The first seed laser uses an electro-optic modulator (EOM) with a modulation frequency of 20MHz; The second seed laser uses an electro-optic modulator (EOM) with a modulation frequency of 25MHz. The two modulation frequencies are incompatible and different from the radio frequency drive frequencies of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2).

7. The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency as described in claim 5, characterized in that, The first acousto-optic modulator (AOM1) has a center frequency of 80MHz and a tuning range of ±20MHz; the second acousto-optic modulator (AOM2) has a bandwidth of 200MHz and a tuning range of ±50MHz.

8. The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency as described in claim 6, characterized in that, The nonlinear sum-frequency (SFG) module further includes two polarization-maintaining fiber amplifiers, a PPMgOLN crystal, an external Fabry-Perot iodine cell, and a NICE-OHMS detection module. The inputs of the two polarization-maintaining fiber amplifiers are connected one-to-one to the main optical paths of the first and second seed lasers to boost laser power. The outputs of both amplifiers are connected to the PPMgOLN crystal optical path. A TEC temperature control circuit stabilizes the temperature of the PPMgOLN crystal to meet the phase-matching temperature requirement, resulting in a 633nm sum-frequency laser. A second branch beam is separated from the output optical path of the PPMgOLN crystal and injected into the external Fabry-Perot iodine cell. The external Fabry-Perot iodine cell includes an external resonant cavity filled with... 127 I2 is subjected to radio frequency phase modulation through the NICE-OHMS detection module to obtain an extremely low frequency error signal in the 0.01Hz to 10Hz frequency band, which is proportional to the detuning of the iodine transition relative to the optical frequency of the 633nm sum-frequency laser. The extremely low frequency error signal is fed back to the TEC temperature control circuit through the main feedback channel, covering the 0.01Hz to 0.1Hz frequency band; or it is fed back to the radio frequency signal source of the first acousto-optic modulator (AOM1) in the two acousto-optic frequency shift and dual-reference absolute frequency stabilization modules through the auxiliary feedback channel, covering the 0.1Hz to 10Hz frequency band. When the output of the TEC temperature control circuit exceeds 90% of the dynamic range, the channel switching is triggered.

9. The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency as described in claim 8, characterized in that, The free spectral range (FSR) of the external resonant cavity of the external Fabry-Perot iodine gas chamber is exactly equal to the frequency of phase modulation of the built-in electro-optic modulator of the NICE-OHMS detector module.

10. The ultra-high stability laser system based on dual-path independent composite frequency stabilization and sum-frequency as described in claim 8, characterized in that, The power of the second branch beam is 1 to 2 mW to ensure that the light intensity inside the external resonant cavity does not cause [problems]. The saturation effect is achieved while simultaneously meeting the shot noise limit signal-to-noise ratio requirements of the NICE-OHMS detection module.

Images