A frequency stabilization device based on combination of F-P cavity and reference cell

Abstract

The application provides a frequency stabilizing device based on combination of an F-P cavity and a reference cell, wherein atomic and molecular gas is filled in a traditional F-P cavity, a feedback signal is applied to the F-P cavity length by using a saturated absorption frequency stabilizing technology, the cavity length is locked on an atomic and molecular energy level transition frequency, disturbance of the cavity length caused by the environment is overcome, and the F-P cavity with the locked cavity length is applied to PDH frequency stabilization of a laser. The device has compact structure, can consider short-term frequency stability of the PDH frequency stabilization and long-term frequency stability of the saturated absorption frequency stabilization, and improves environmental adaptability of the frequency stabilized laser.

Description

Technical Field

[0001] This application relates to the field of laser frequency stabilization, specifically to a frequency stabilization device based on a combination of an FP cavity and a reference cell. Background Technology

[0002] Frequency-stabilized lasers have broad application prospects in fields such as atomic manipulation, laser communication, lidar, and precision spectroscopy. Currently, the mainstream laser frequency stabilization methods include Proud-Drever-Hall (PDH) stabilization, which uses a Proud-Drever-Hall (FP) cavity as the reference source, and saturated absorption stabilization, which uses the energy level transition frequencies of atoms and molecules as the reference source. The former achieves better short-term frequency stability, but the long FP cavity makes it susceptible to environmental interference, leading to a deterioration in long-term stabilization performance. The latter, on the other hand, achieves better long-term frequency stability. The main technical challenge of PDH stabilization lies in the stability of the FP cavity. Although good stabilization can be achieved in a laboratory environment, it is difficult to achieve portability and multi-environment applications.

[0003] PDH frequency stabilization technology combines frequency modulation and optical heterodyne detection. An electro-optic modulator modulates the laser, while a photodetector detects the reflected signal after the laser is perpendicularly incident on the FP cavity. This reflected signal is multiplied by a demodulated signal of the same frequency to obtain an error signal, which is fed back to the laser, ultimately locking the laser frequency to the resonant frequency of the FP cavity. The resonant frequency of the FP cavity is ν = pc / (2nL), where p is any positive integer, c is the speed of light, n is the refractive index, and L is the cavity length. It is clear that the stability of the resonant frequency largely depends on the stability of the cavity length. Currently, ultra-stable lasers obtained using PDH technology in the laboratory often use materials with low coefficients of thermal expansion for the FP cavity cavity and its support, while placing the cavity in a thermally insulated and vibration-isolated environment to ensure the stability of the cavity length as much as possible.

[0004] In portable laser applications, various environments are inevitably encountered, making it difficult to achieve the frequency stability required in a laboratory setting. In such cases, the stability of the FP cavity length becomes particularly important. Saturated absorption frequency stabilization technology utilizes the energy level transition frequencies of atoms and molecules as a frequency reference source. The modulated error signal is fed back to the laser to achieve frequency stabilization. Under normal conditions, the energy level transition frequencies exhibit excellent stability, which contributes to the good long-term frequency stability of saturated absorption frequency stabilization.

[0005] Therefore, based on the disadvantages of the two methods mentioned above, this application proposes a laser frequency stabilization device suitable for use in handling and multi-environment application scenarios. Summary of the Invention

[0006] The purpose of this invention application is to propose a frequency stabilization device based on the combination of FP cavity and reference cell to meet the needs of portable and multi-environment laser applications. The device combines the advantages of the two frequency stabilization methods and can be integrated, providing a solution for achieving laboratory-level performance for future portable and multi-environment applications of frequency-stabilized lasers.

[0007] This invention discloses a frequency stabilization device and its working method based on a combination of a FP cavity and a reference cell. The frequency stabilization device includes a laser, an electro-optic modulator, a signal generator, a mixer, a servo control system, a photodetector, thick glass, a polarizing beam splitter, a quarter-wave plate, an FP cavity filled with atomic and molecular gas, and piezoelectric ceramics.

[0008] During operation, after the laser generates light, its operating point is fine-tuned to ensure a large optical signal is detected at the transmission point of the FP cavity filled with atomic and molecular gas. A sawtooth wave signal generator is activated to produce signals of the same frequency for controlling both the laser and the piezoelectric ceramic, ensuring the laser frequency is always synchronized with the FP cavity's resonant frequency. The laser frequency periodically traverses within a certain range while also transmitting through the FP cavity, at which point a saturated absorption spectral line is observed at the FP cavity's transmission point. The laser's operating point and the piezoelectric ceramic voltage are fine-tuned so that the saturated absorption spectral line has only one peak centered on the screen. Piezoelectric ceramic modulation is activated, and the signal is multiplied and demodulated with the transmitted signal at the mixer; the resulting error signal is fed back to the piezoelectric ceramic. An electro-optic modulator is activated to modulate the laser, and the signal is multiplied and demodulated with the monitoring signals reflected from the quarter-wave plate and beam splitter prism; the resulting error signal is fed back to the laser. The sawtooth wave is deactivated, and two servo control systems are simultaneously activated to lock the cavity length of the FP cavity with a stable energy level transition spectral line and the laser frequency with the FP cavity's resonant frequency.

[0009] The device has a compact structure and can balance the short-term frequency stability of PDH frequency stabilization and the long-term frequency stability of saturable absorption frequency stabilization, thereby improving the environmental adaptability of frequency-stabilized lasers and providing a new solution for making frequency-stabilized lasers portable and applicable to various environments.

[0010] Compared with existing frequency stabilization solutions, the beneficial effects of this invention are:

[0011] (1) The atomic and molecular gas is filled into the FP cavity, and the structure can be integrated, which is conducive to realizing a portable design; (2) The design combines the advantages of saturated absorption frequency stabilization and PDH frequency stabilization, locking the cavity length of the FP cavity, which is easily affected by environmental interference, at a stable atomic and molecular energy level transition frequency, thus ensuring the stability of the FP cavity; (3) Two servo control systems are adopted, and the PDH frequency stabilization achieved by the FP cavity with stable cavity length ensures the stability of the frequency stabilized laser, which can ensure that the laser performs well in various environments. Attached Figure Description

[0012] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0013] Figure 1 This is a schematic diagram of a frequency stabilization device based on the combination of FP cavity and reference cell according to the present invention;

[0014] Figure 2 This is a flowchart of a frequency stabilization method based on the combination of FP cavity and reference cell according to the present invention. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. It should be noted that, in the absence of conflict, the various features in the embodiments of this application can be combined with each other, and the combined embodiments are still within the protection scope of this application.

[0016] In the description of this application, it should be noted that, unless otherwise specified and limited, the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this application is in use. They are only for the convenience of describing this application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this application. In addition, the terms "first," "second," and "third," etc., are only used to distinguish descriptions and should not be construed as indicating or implying relative importance.

[0017] The present invention will now be described in further detail with reference to the accompanying drawings.

[0018] See appendix Figure 1This application proposes a frequency stabilization device based on a combination of a FP cavity and a reference cell, comprising: 1. a laser source; 2. a servo system one; 3. a signal generator one; 4. an electro-optic modulator; 5. a signal generator two; 6. a phase shifter one; 7. a mixer one; 8. a photodetector one; 9. a signal generator three; 10. a phase shifter two; 11. a servo system two; 12. a mixer two; 13. a thick glass one; 14. a polarizing beam splitter prism; 15. a quarter-wave plate; 16. an FP cavity for injecting atomic and molecular gas; 17. a piezoelectric ceramic; 18. a thick glass two; 19. a photodetector two; and 20. a reflector.

[0019] After passing through the electro-optic modulator 4, the laser beam is split into two beams by the high-transmittance, low-reflection thick glass 13: a transmitted beam and a reflected beam. The transmitted beam is reflected by the reflector 20 and the thick glass 18 as the pump beam, while the reflected beam serves as the probe beam. The probe beam enters the FP cavity. The reflected signal is detected by the photodetector (PD) 8 through the quarter-wave plate 15 and the beam splitter prism 14, while the transmitted signal is detected by the photodetector 19. These signals serve as the signal sources for PDH frequency stabilization and saturable absorption frequency stabilization, respectively. The signal generator 3 generates a sawtooth wave sweep signal with the same frequency to control the laser current and the piezoelectric ceramic PZT voltage. The signal generators 5 and 9 use different frequency stabilization methods, resulting in different modulation signal frequencies. They modulate and demodulate the laser and the piezoelectric ceramic PZT, respectively. The demodulated and filtered error signals are fed back to the laser source 1 and the piezoelectric ceramic PZT 17 by the servo system 2 and servo system 11, respectively, to ensure the stability of the FP cavity 16 cavity length while achieving PDH frequency stabilization.

[0020] See appendix Figure 2 The flowchart of the frequency stabilization method based on a frequency stabilization device combining an FP cavity and a reference cell proposed in this application includes the following steps:

[0021] Step 1: Turn on the laser and fine-tune the laser operating point so that a large optical signal can be detected at the FP cavity transmission point when the laser is running freely, even if the laser is near the FP cavity resonant frequency.

[0022] Step 2: Apply sawtooth wave signals of the same frequency to the laser and the piezoelectric ceramic PZT, and fine-tune the amplitude of the sawtooth wave so that the laser can observe a relatively clear and complete saturated absorption spectrum at the transmission point of the FP cavity, that is, the outgoing light emitted by the laser can always pass through the FP cavity.

[0023] Step 3: Simultaneously fine-tune the laser operating point, the piezoelectric ceramic PZT control voltage, and the sawtooth wave amplitude value so that only one peak of the saturated absorption spectrum can be observed, and it is located in the center of the observation screen.

[0024] Step 4: A sinusoidal signal is generated to modulate the piezoelectric ceramic PZT. At the same time, another identical signal is phase-shifted and connected to a mixer. It is multiplied with the signal transmitted at the FP cavity and demodulated to generate an error signal. The error signal is applied to the piezoelectric ceramic PZT.

[0025] Step 5: Start the electro-optic modulator EOM to generate a sinusoidal signal with a frequency much higher than the linewidth of the FP cavity itself. Use the electro-optic modulator to modulate the laser. Another identical demodulated signal is phase-shifted and multiplied with the FP strong reflected light photodetector signal for demodulation and error signal is observed. This error signal acts on the laser.

[0026] Step 6: Turn off the sawtooth wave signal and simultaneously turn on the two servo control systems to simultaneously control the FP cavity length stabilization and the laser PDH frequency stabilization.

[0027] Implementation

[0028] Combined with appendix Figure 1 Appendix Figure 2 A DFB laser with a center wavelength of 780 nm and a power of 30 mW originates from laser source 1, passes through EOM4, and is split into two beams by thick glass 13. The transmitted light serves as the pump light, and the reflected light serves as the probe light, which is perpendicularly incident on the FP cavity 16 injected with rubidium atomic gas. Fine-tuning the laser's operating point allows the optical signal to be detected on photodetector 19, at which point the laser frequency is near its resonant frequency. Signal generator 3 is activated to apply a sawtooth wave of the same frequency to laser source 1 and piezoelectric ceramic PZT17, ensuring that the laser frequency can transmit through the FP cavity even during periodic traversal, thus allowing the saturation absorption spectrum to be observed on photodetector 19. Fine-tuning the operating point of laser source 1 and the control voltage of piezoelectric ceramic PZT17 ensures that a specific peak in the absorption spectrum is centered in the observation area. Fine-tuning the amplitude of the sawtooth wave in signal generator 3 ensures that only this single peak is visible in the center of the observation screen. The signal generator 39 generates a 10kHz sine wave signal to modulate the piezoelectric ceramic PZT17. The same signal is processed by phase shifter 210 and then multiplied and demodulated at the photodetector 219 and mixer 212. The servo system 211 is used to control the piezoelectric ceramic PZT17.

[0029] The signal generator 25 generates a 20MHz sine wave signal, which is modulated by EOM4. ​​The modulated laser is reflected back by the F-P16 cavity and then monitored by the photodetector 8 through the quarter-wave plate 15 and the polarization beam splitter 14. The signal is multiplied and demodulated at the mixer 7. The resulting error signal is fed back to the laser source 1 through the servo system 2 to achieve PDH frequency stabilization.

[0030] While the present invention has been disclosed above with reference to preferred embodiments, these embodiments are not intended to limit the scope of the present invention. Any equivalent changes or modifications made without departing from the spirit and scope of the present invention are also within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the content defined in the claims.

Claims

1. A frequency stabilization device based on a combination of FP cavity and reference cell, characterized in that: The device includes a laser source (1), a servo system (2), a signal generator (3), an electro-optic modulator (EOM) (4), a signal generator (5), a phase shifter (6), a mixer (7), a photodetector (8), a signal generator (9), a phase shifter (10), a servo system (11), a mixer (12), a thick glass (13), a polarizing beam splitter (14), a quarter-wave plate (15), an FP cavity for injecting atomic and molecular gas (16), a piezoelectric ceramic PZT (17), a thick glass (18), a photodetector (19), and a reflector (20). The laser emitted from the laser source is split into two beams after passing through the electro-optic modulator (4) and the high-transmittance, low-reflection thick glass (13), one transmitted beam and one reflected beam. The transmitted beam is reflected by the reflector (20) and the thick glass (18) as pump light, and the reflected beam is used as probe light. The probe light is incident on the FP cavity, and the reflected signal is detected by photodetector one (8) through a quarter-wave plate (15) and a beam splitter (14). The transmitted signal is detected by photodetector two (19), which serve as signal sources for PDH frequency stabilization and saturable absorption frequency stabilization, respectively. Signal generator 1 (3) generates a sawtooth wave sweep signal with the same frequency to control the laser current and the piezoelectric ceramic PZT voltage; Signal generator 2 (5) and signal generator 3 (9) have different modulation signal frequencies to modulate and demodulate the laser and the piezoelectric ceramic PZT respectively. The demodulated and filtered error signals are fed back to the laser source (1) and the piezoelectric ceramic PZT (17) by servo system 1 (2) and servo system 2 (11) respectively, to ensure the stability of the cavity length of the FP cavity (16) and realize the PDH frequency stabilization.

2. The frequency stabilization device based on the combination of FP cavity and reference cell according to claim 1, characterized in that: The FP cavity is filled with atomic and molecular gas. During frequency stabilization, reflected light and transmitted light are used simultaneously, which are used for PDH frequency stabilization and saturated absorption frequency stabilization, respectively. The transmitted light is fed back to the piezoelectric ceramic PZT to lock the cavity length of the FP cavity at the atomic and molecular energy level transition frequency, thus ensuring the stability of the cavity length.

3. The frequency stabilization device based on the combination of FP cavity and reference cell according to claim 2, characterized in that: The modulation and demodulation feedback system for PDH frequency stabilization, consisting of servo system 1 (2), signal generator 2 (5), phase shifter 1 (6), mixer 1 (7), and photodetector 1 (8), is the same as the system for saturated absorption frequency stabilization, consisting of signal generator 3 (9), phase shifter 2 (10), servo system 2 (11), mixer 2 (12), and photodetector 2 (19).

4. A frequency stabilization method for the frequency stabilization device according to any one of claims 1-3, characterized in that, It includes the following steps: Step 1: Turn on the laser and fine-tune the laser operating point so that a large optical signal can be detected at the FP cavity transmission point when the laser is running freely, even if the laser is near the FP cavity resonant frequency. Step 2: Apply sawtooth wave signals of the same frequency to the laser and the piezoelectric ceramic PZT, and fine-tune the amplitude of the sawtooth wave so that the laser can observe a relatively clear and complete saturated absorption spectrum at the transmission point of the FP cavity, that is, the outgoing light emitted by the laser can always pass through the FP cavity. Step 3: Simultaneously fine-tune the laser operating point, the piezoelectric ceramic PZT control voltage, and the sawtooth wave amplitude value so that only one peak of the saturated absorption spectrum can be observed, and it is located in the center of the observation screen. Step 4: A sinusoidal signal is generated to modulate the piezoelectric ceramic PZT. At the same time, another identical signal is phase-shifted and connected to a mixer. It is multiplied with the signal transmitted at the FP cavity and demodulated to generate an error signal. The error signal is applied to the piezoelectric ceramic PZT. Step 5: Start the electro-optic modulator EOM to generate a sinusoidal signal with a frequency much higher than the linewidth of the FP cavity itself. Use the electro-optic modulator to modulate the laser. Another identical demodulated signal is phase-shifted and multiplied with the FP strong reflected light photodetector signal for demodulation and error signal is observed. This error signal acts on the laser. Step 6: Turn off the sawtooth wave signal and simultaneously turn on the two servo control systems to simultaneously control the FP cavity length stabilization and the laser PDH frequency stabilization.

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