A laser frequency offset and stabilization device and method based on an optical fiber ring resonator

Abstract

The present invention discloses a frequency offset and frequency stabilization device and method for a laser based on an optical fiber ring resonator. Based on the scanning transfer cavity detuning frequency stabilization technology with the saturated absorption peak as the reference, the resonance point of the laser frequency is locked by using the saturated absorption spectrum signal as the frequency discrimination signal. A piezoelectric ceramic fixedly connected to the optical fiber resonator is used to drive and adjust the resonator length, so that the frequency-locked laser generates a Gaussian-shaped resonance transmission peak after passing through the optical fiber resonator. The cavity length of the ring resonator is locked through modulation and demodulation and PID closed-loop control. The operating current and temperature of the target laser are adjusted to tune the laser frequency to the operating point, and then the spectrum of the target laser passing through the optical fiber resonator is modulated and demodulated to obtain the feedback control current, realizing the locking of the target laser frequency. The present invention is applied to the stabilization of the non-resonant frequency of semiconductor lasers, as well as the frequency locking of the pumping light and detection light for high-sensitivity atomic magnetic fields / inertia.

Description

Technical Field

[0001] The present invention relates to the technical field of laser frequency stability control, in particular to the technical field of frequency stability control at non-resonance points of lasers in quantum sensing technology, and specifically relates to a laser frequency offset and stability control device and method based on a fiber optic ring resonator. Background Art

[0002] Laser frequency stability technology is widely applied in multiple fields such as laser coherent measurement, spaceborne laser communication, cold atom physics, etc. Especially in the field of atomic spin inertia / magnetic field measurement related to the present invention, the frequency stability of the pumping and detection lasers is directly related to the sensitivity of the inertia / magnetic field measurement system. Especially when using a high-pressure atomic gas cell, there are pressure broadening and frequency shift. Therefore, when a semiconductor laser is applied to atomic spin optical pumping and detection, its operating point needs to deviate from the alkali metal atomic absorption line. In view of the above application requirements, it is necessary to study a laser frequency offset and stability control device and method.

[0003] In the prior art [1] (Wencui P, Lin Z, Shitong L, et al. Locking laser frequency of up to 40GHz offset to a reference with a 10GHz electro-optic modulator [J]. Optics Letters, 2014, V39(10):2998-3001), it is mentioned that a phase modulator is used to modulate the phase of the laser, and the laser frequency offset spectrum corresponding to the driving frequency of the phase modulator is generated for locking to achieve detuned frequency locking. In the prior art [2] (Okubo S, Iwakuni K, Hasegawa T. Modulation-free laser frequency stabilization to a saturated sub-Doppler spectral line in a transverse magnetic field [J]. Optics Communications, 2012, 285(20):4107-4111.), it is mentioned that the Zeeman energy level transition of DAVLL is used to generate a resonance frequency offset for laser frequency detuning locking. For the former of these two methods, it is limited by the energy and frequency of the driving frequency of the phase modulator, and the maximum detuning amount can only reach dozens of GHz, which does not meet the broadening and frequency shift requirements corresponding to the high-pressure gas atomic gas cell; while for the second method, an external magnetic field needs to be added, which introduces magnetic noise to the atomic inertia / magnetic field measurement system and affects the measurement accuracy of the system, and the frequency shift range is small and cannot meet the system usage requirements either. Summary of the Invention

[0004] To overcome the deficiencies of the prior art, the present invention provides a laser frequency offset and frequency stabilization device and method based on a fiber optic ring resonator. The resonance point frequency of the laser is stably transmitted through the fiber optic ring resonator, and the flexible fiber optic ring resonator can achieve the characteristics of a small resonator volume and a wide free spectral range. The device adopts an all-fiber design and uses a fiber optic circulator to replace spatial optical devices such as a polarization beam splitter prism and a beam splitter. It has a smaller volume, a simpler optical path, and better stability. In addition, the method uses the differential signal of the transmission spectrum to lock the length of the ring resonator. Compared with the traditional PDH cavity locking method, it does not require phase modulation, has a simple optical path, and a simple circuit control system.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0006] A laser frequency offset and frequency stabilization device based on a fiber optic ring resonator according to the present invention includes three loops. Among them, the first loop is a laser resonance point frequency locking loop, the second loop is a ring resonator cavity length locking loop, and the third loop is a frequency offset frequency locking loop of the laser to be measured; the first loop includes a semiconductor laser, an optical isolator, a beam splitter prism, a λ / 2 wave plate, a polarization beam splitter prism, a plane mirror, a vacuum atomic gas cell, a beam splitter, a first photodetector, a fiber optic coupler, a first signal generator, a first lock-in amplifier, a first PID controller, and a first adder; the second loop includes a first fiber optic circulator, a fiber optic ring resonator, a second photodetector, a second signal generator, a second lock-in amplifier, a second PID controller, and a second adder; the third loop includes a slave laser, a fiber optic isolator, a second fiber optic circulator, a third photodetector, a third adder, a third signal generator, a third PID controller, and a third lock-in amplifier.

[0007] Further, in the first loop, the light beam emitted from the semiconductor laser is divided into two beams by the beam splitter prism after passing through the optical isolator. One beam enters the second loop through the fiber optic coupler, and the other beam passes through the λ / 2 wave plate and then the polarization beam splitter prism. Its transmitted beam enters the vacuum atomic gas cell through the plane mirror and the beam splitter, and is received by the first photodetector after multiple reflections in the vacuum atomic gas cell; the output of the first photodetector is connected to the input of the first lock-in amplifier, the output of the first signal generator is connected to the reference input of the first lock-in amplifier, the output of the first lock-in amplifier is connected to the input of the first PID controller, the output of the first PID controller and the other output of the first signal generator are respectively connected to the two inputs of the first adder, and the output of the first adder is connected to the controller of the semiconductor laser;

[0008] Further, in the second loop, the light beam emitted from the first loop enters the A-1 port of the first fiber optic circulator, exits from the A-2 port of the first fiber optic circulator, enters the fiber optic ring resonator, then passes through the B-2 port of the second fiber optic circulator, exits from the B-3 port of the second fiber optic circulator, enters the third photodetector and is connected to the input end of the second lock-in amplifier. The output of the second signal generator is connected to the reference input end of the second lock-in amplifier. The output of the second lock-in amplifier is connected to the input of the second PID controller. The output of the second PID controller and the other output end of the second signal generator are respectively connected to the two input ends of the second adder. The output end of the second adder is connected to the input of the piezoelectric ceramic driver controller of the piezoelectric ceramic of the fiber optic ring resonator.

[0009] Further, in the third loop, the light emitted from the laser passes through the fiber optic isolator and then enters the B-1 port of the second fiber optic circulator, then enters the fiber optic ring resonator through the B-2 port of the second fiber optic circulator, is received by the second photodetector and is connected to the input end of the third lock-in amplifier. The output of the third signal generator is connected to the reference input end of the third lock-in amplifier. The output of the third lock-in amplifier is connected to the input of the third PID controller. The output of the third PID controller and the other output end of the third signal generator are respectively connected to the input ends of the third adder. The output end of the third adder is connected to the controller of the laser.

[0010] The present invention also provides a method for frequency transfer using the resonance point frequency locking of the master laser and the fiber optic ring resonator, and then achieving the non-resonance point frequency locking of the slave laser, which mainly includes the following steps:

[0011] ① Adjust the working temperature and current of the semiconductor laser to near the atomic hyperfine energy level transition frequency point, and sweep the frequency of the laser through the sawtooth wave generated by the signal generating circuit, so that the semiconductor laser generates a saturated absorption spectrum signal after passing through the vacuum atomic gas cell;

[0012] ② Apply a modulation signal to the laser current. The first photodetector converts the saturated absorption spectrum signal containing the modulation information into a voltage signal and sends it into the first lock-in amplifier.

[0013] ③ Adjust the phase of the reference signal to make the demodulated differential error signal maximum, and the zero point of the differential error signal corresponds to the peak point of the saturated absorption spectrum signal;

[0014] ④ Adjust the proportional integral differential and other parameters of the first PID controller to achieve the frequency locking of the semiconductor laser at the atomic hyperfine transition resonance frequency point.

[0015] ⑤The frequency-locked semiconductor laser is incident on the fiber optic ring resonator through the first fiber optic circulator. A periodic signal is applied to the fiber optic ring resonator by the first signal generator for frequency sweeping. The spectral signal generates a quasi-Gaussian linear resonance transmission peak. The transmitted spectral signal is output from the port of the first fiber optic circulator to the port of the second fiber optic circulator and enters the third photodetector.

[0016] ⑥A modulation signal is applied to the ring resonator using the second signal generator. The second lock-in amplifier demodulates the modulated quasi-dispersive spectral signal to obtain an error signal. The piezoelectric ceramic drive voltage is adjusted using the second PID controller to achieve synchronous adjustment of the length of the fiber optic ring resonator, completing the resonance of the cavity length of the ring resonator with the frequency of the semiconductor laser and achieving locking of the cavity length.

[0017] ⑦The frequency of the slave laser is tuned near the target frequency point. The output beam of the slave laser enters the fiber optic ring resonator after passing through the fiber optic isolator and the second fiber optic circulator. A frequency sweeping signal is applied to the current of the slave laser. The resonance spectrum generated by the transmission of the ring resonator is output to the third photodetector after passing through the second fiber optic circulator.

[0018] ⑧A modulation signal is applied to the current of the slave laser. The third lock-in amplifier demodulates the modulated spectral signal to obtain an error signal. The current is adjusted using the third PID controller to finally lock the frequency of the slave laser at the target frequency point.

[0019] The technology of the present invention has the following advantages compared with the prior art:

[0020] The technology of the present invention utilizes the periodic characteristics of the spectral transmission peak passing through the ring resonator to achieve frequency locking of the laser with a frequency offset of up to hundreds of GHz. In addition, this method does not require external modulation devices, and the system has simple operation and reliable performance. At the same time, a fully fiber optic solution is adopted. Utilizing the characteristics that the beam transmission of the fiber optic circulator is irreversible and the incidence of two beams with different frequencies from different directions on the circulator does not affect its output, the optical path is simpler than that of spatial optical devices, and the volume of the optical path can be greatly reduced. According to the inverse relationship between the free spectral range and the cavity length, and utilizing the flexible winding characteristics of the cavity length of the fiber optic ring resonator, this kind of ring resonator can further compress the system volume while ensuring a small free spectral range, facilitating system integration. In addition, the master laser uses saturated absorption frequency stabilization to achieve resonance point frequency stabilization, and the slave laser uses resonator frequency stabilization to achieve non-resonance point frequency stabilization, that is, this system can be reused for a dual-beam system with resonance point frequency stabilization and non-resonance point frequency stabilization.

[0021] Specifically, the device of the present invention uses fiber optic devices, a fiber optic circulator and a fiber optic ring cavity unidirectional transmission multiplexing space, and the optical path structure is simple and compact. By designing the relevant parameters of the fiber optic ring resonator, any frequency point within the tunable range of the laser can be locked. Due to the use of the fiber optic ring cavity to transfer the stability of the frequency reference, the frequency stabilization performance of this system is comparable to that of saturated absorption frequency stabilization.

[0022] Moreover, it uses the hyperfine level transition frequency of alkali metal atoms as the frequency reference. The stabilized master laser can be used as the frequency reference for the fiber optic ring resonator. At the same time, the beam after frequency stabilization can also be applied separately to other experiments. For example, in an inertial magnetic field measurement device based on the principle of atomic spin, it provides a frequency reference for detecting optical detuning frequency stabilization and can also be used as the pumping light in the inertial magnetic field measurement device.

[0023] The loop of the present invention can effectively control the frequency detuning amount of the slave laser by designing relevant parameters such as the cavity length and reflectivity of the ring resonator, and achieve non-resonant frequency stabilization; by locking the cavity length of the fiber optic ring resonator, the influence of temperature and vibration on the traditional ring cavity length is reduced.

[0024] Combined with the existing technical solutions in the invention content: Compared with the existing detuning frequency stabilization technologies, such as the DAVLL frequency stabilization technology (detuning amount MHz), the AOM / EOM frequency offset locking control technology (MHz - 20 GHz), and the phase-locked loop frequency offset locking technology (< 40 GHz), the laser frequency offset stabilization device of the present invention can adjust the free spectral range through parameters such as the cavity length and optical intensity coupling ratio of the fiber optic ring resonator, and can achieve arbitrary detuning locking (hundreds of GHz) within the tunable range of the slave laser frequency. This laser frequency locking technology does not require external modulation devices, has low system cost, small volume, and good stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a structural block diagram of a laser frequency offset stabilization device and method based on a fiber optic ring resonator according to the present invention.

[0026] The meanings of the reference numerals in the figure are as follows: 1-1 is a semiconductor laser, 1-2 is an optical isolator, 1-3 is a beam splitting prism, 1-4 is a λ / 2 wave plate, 1-5 is a polarization beam splitting prism, 1-6 is a plane mirror, 1-7 is a vacuum atomic gas cell, 1-8 is a beam splitter, 1-9 is a first photodetector, 1-10 is a first adder, 1-11 is a first PID controller, 1-12 is a first signal generator, 1-13 is a first lock-in amplifier, 1-14 is an optical fiber coupler; 2-1 is a first optical fiber circulator, 2-2 is an optical fiber ring resonator, 2-3 is a second photodetector, 2-4 is a second signal generator, 2-5 is a second lock-in amplifier, 2-6 is a second PID controller, 2-7 is a second adder; 3-1 is a slave laser, 3-2 is an optical fiber isolator, 3-3 is a second optical fiber circulator, 3-4 is a third photodetector, 3-5 is a third adder, 3-6 is a third signal generator, 3-7 is a third PID controller, 3-8 is a third lock-in amplifier. Specific embodiments

[0027] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0028] A laser frequency offset and stabilization device based on an optical fiber ring resonator according to the present invention includes three laser frequency stabilization loops. Among them, the first loop is a laser resonance point frequency locking loop, the second loop is an optical fiber ring resonator cavity length locking loop, and the third loop is a frequency locking loop for the frequency offset (non-resonance point) of the laser to be measured. The first loop includes a semiconductor laser, an optical isolator, a beam splitting prism, a λ / 2 wave plate, a polarization beam splitting prism, a beam splitter, a plane mirror, a vacuum atomic gas cell, a photodetector, a lock-in amplifier, a signal generator, and a PID controller. The second loop includes an optical fiber ring resonator, an optical fiber coupler, an optical fiber circulator, a photodetector, a signal generator, a lock-in amplifier, and a PID controller. The third loop includes a semiconductor laser, an optical fiber isolator, an optical fiber circulator, a photodetector, a signal generator, a lock-in amplifier, and a PID controller.

[0029] The semiconductor lasers described above include two units. One of them serves as the frequency reference locked by the fiber optic ring cavity, and its frequency is locked to the atomic hyperfine level transition line, which is called the master laser. The other is the laser to be stabilized, called the slave laser. The master laser uses the alkali metal atomic hyperfine level transition line as the frequency reference and uses the laser current controller as the actuator for frequency closed-loop control. The slave laser uses the resonance transmission peak of the fiber optic ring resonator after locking the cavity length as the frequency reference and uses the laser current controller as the actuator for frequency closed-loop control.

[0030] The fiber optic ring resonator described above is made of a 2×2 fiber optic directional coupler. By adjusting the fiber optic ring cavity length, its free spectral range is adjusted, and further the frequency offset (i.e., detuning amount) of the slave laser is adjusted. By adjusting the optical intensity coupling ratio and splicing loss of the fiber optic ring resonator, the linewidth of the transmission peak after resonance is controlled, and further the cavity length locking accuracy is controlled. The fiber optic ring resonator realizes real-time control of its cavity length through the driving voltage of the wound piezoelectric ceramic.

[0031] The lock-in amplifier and PID controller described above can use commercial standard products, or can be replaced by an integrated circuit of a multiplier and a loop filter and a PID controller. However, no matter which one is adopted, in order to achieve the best error signal, phase adjustment of the reference signal and the modulated spectral signal is required. For example, if a commercial lock-in amplifier is adopted, the error signal amplitude and phase can be obtained through quadrature demodulation.

[0032] The method of the present invention for locking the frequency at the resonance point of the master laser and performing frequency transfer through the fiber optic ring resonator, and then realizing the non-resonance point frequency locking of the slave laser mainly includes the following steps:

[0033] (1) Adjust the working temperature and current of the master laser to near the atomic hyperfine level transition frequency point, and sweep the frequency of the master laser through the sawtooth wave generated by the signal generating circuit, so that the master laser generates a saturated absorption spectral signal after passing through the vacuum atomic gas cell;

[0034] (2) Apply a modulation signal to the current of the master laser. The photodetector converts the saturated absorption spectral signal containing the modulation information into a voltage signal and sends it into the lock-in amplifier.

[0035] (3) Adjust the phase of the reference signal to make the demodulated differential error signal maximum, and the zero point of the differential error signal corresponds to the peak point of the saturated absorption spectral signal;

[0036] (4) Adjust the parameters such as proportional integral and differential of the PID controller to realize the frequency locking of the master laser at the atomic hyperfine transition resonance frequency point.

[0037] (5) The frequency-locked master laser enters the fiber optic ring resonator through a fiber optic circulator. A periodic signal is applied to the fiber optic ring resonator by a signal generator for frequency sweeping. The spectral signal generates a quasi-Gaussian linear resonance transmission peak. The transmitted spectral signal is output from port 2 of the fiber optic circulator to port 3 of the fiber optic circulator and enters a photodetector.

[0038] (6) A modulation signal is applied to the ring resonator using a signal generator. A lock-in amplifier demodulates the modulated quasi-dispersive spectral signal to obtain an error signal. A PID controller adjusts the piezoelectric ceramic drive voltage to synchronously adjust the length of the fiber optic ring resonator, completing the resonance of the cavity length of the fiber optic ring resonator with the frequency of the master laser and achieving the locking of the cavity length.

[0039] (7) Tune the frequency of the slave laser to near the target frequency point. The output beam of the slave laser enters the fiber optic ring resonator after passing through a fiber optic isolator and a fiber optic circulator. A frequency sweeping signal is applied to the current of the slave laser. The resonance spectrum generated by the transmission of the ring resonator is output to a photodetector after passing through the fiber optic circulator.

[0040] (8) A modulation signal is applied to the current of the slave laser. A lock-in amplifier demodulates the modulated spectral signal to obtain an error signal. A PID controller adjusts the current to finally lock the frequency of the slave laser at the target frequency point.

[0041] The present invention will be specifically described below with reference to the accompanying drawings.

[0042] As Figure 1 shown, a laser frequency offset and frequency stabilization device based on a fiber optic ring resonator of the present invention includes a semiconductor laser 1-1, an optical isolator 1-2, a beam splitting prism 1-3, a λ / 2 wave plate 1-4, a polarization beam splitting prism 1-5, a plane mirror 1-6, a vacuum atomic gas cell 1-7, a beam splitter 1-8, a first photodetector 1-9, a first adder 1-10, a first PID controller 1-11, a first signal generator 1-12, a first lock-in amplifier 1-13, a fiber optic coupler 1-14, a first fiber optic circulator 2-1, a fiber optic ring resonator 2-2, a second photodetector 2-3, a second signal generator 2-4, a second lock-in amplifier 2-5, a second PID controller 2-6, a second adder 2-7, a slave laser 3-1, a fiber optic isolator 3-2, a second fiber optic circulator 3-3, a third photodetector 3-4, a third adder 3-5, a third signal generator 3-6, a third PID controller 3-7, and a third lock-in amplifier 3-8. The semiconductor laser 1-1 includes a laser current source and a temperature controller. Among them, the semiconductor laser 1-1 is used as the master laser.

[0043] The positions and signal transmission relationships of the components of a laser frequency offset and frequency stabilization device based on a fiber optic ring resonator of the present invention are as follows:

[0044] The emitted beam of the semiconductor laser 1-1 passes through the optical isolator 1-2 and is divided into two beams after passing through the beam-splitting prism 1-3. One beam is used to lock the fiber optic ring resonator 2-2 as the frequency reference, and the other beam is used to generate the frequency reference, i.e., frequency stabilization at the resonance point. The frequency stabilization at the resonance point adopts a saturated absorption frequency stabilization system. The intensity ratio of the light is adjusted by the combination of the λ / 2 wave plate 1-4 and the polarization beam-splitting prism 1-5. In this embodiment, the stronger beam passing through the polarization beam-splitting prism 1-5 enters the vacuum atomic gas cell 1-7 in the reverse direction after passing through the plane mirror 1-6 and the beam splitter 1-8, and this beam serves as the pumping light; the weaker light reflected by the polarization beam-splitting prism 1-5 enters the vacuum atomic gas cell 1-7 in the forward direction and is received by the first photodetector 1-9, and this beam serves as the detection light. By applying a triangular wave frequency sweep signal to the semiconductor laser l-1, the first photodetector 1-9 receives the saturated absorption spectrum signal generated by the vacuum atomic gas cell 1-7.

[0045] A 20 - 100 KHz sine modulation signal is applied to the semiconductor laser 1-1 by the first signal generator 1-12. The first photodetector 1-9 obtains the saturated absorption spectrum signal containing the modulation information. This spectrum signal and a reference signal with the same frequency and phase are sent to the first lock-in amplifier 1-13 for demodulation, and the cut-off frequency of the low-pass filter is adjusted by adjusting the time constant of the first lock-in amplifier 1-13 to obtain a differential error signal suitable for frequency stabilization. This signal generates a correction error signal through the first PID controller 1-11. This correction signal, together with the aforementioned frequency sweep signal and modulation signal, is superimposed on the current source of the semiconductor laser 1-1 through the first adder 1-10. The zero point of the error signal corresponds to the atomic hyperfine energy level transition frequency point, and this frequency point is used as the frequency stabilization reference. The error signal is input to the first PID controller 1-11 to generate a correction signal for this frequency point, and this signal is input into the controller of the semiconductor laser 1-1 to achieve the closed-loop locking of the resonance frequency point of the semiconductor laser 1-1. The frequency of the semiconductor laser 1-1 is locked to this frequency reference through the first lock-in amplifier 1-13 and the first PID controller 1-11.

[0046] The beam emitted by the frequency-stabilized semiconductor laser 1-1 enters the A-1 port of the first fiber optic circulator 2-1 through the fiber optic coupler 1-14, and is incident on the fiber optic ring resonator 2-2 from the A-2 port of the first fiber optic circulator 2-1. The beam passing through the fiber optic ring resonator 2-2 enters the 2 port of the second fiber optic circulator 3-3, and is incident on the third photodetector 3-4 from the 3 port of the second fiber optic circulator 3-3. A sweep signal is applied to the driving voltage of the piezoelectric ceramic driving controller bonded to the fiber optic ring resonator 2-2 to finely adjust the cavity length of the fiber optic ring resonator 2-2, so that the third photodetector 3-4 generates a resonant cavity transmission peak spectrum that resonates with the frequency of the semiconductor laser 1-1. The first fiber optic circulator 2-1 also has an A-3 port, which is connected to the second photodetector 2- .

[0047] A 1K - 2KHz modulation signal is applied to the piezoelectric ceramic driving controller of the fiber optic ring resonator 2-2 by the second signal generator 2-4. The transmission peak spectrum signal containing modulation information is obtained by the third photodetector 3-4. This spectrum signal and a reference signal with the same frequency and phase are sent to the second lock-in amplifier 2-5 for demodulation. The cut-off frequency of the low-pass filter is adjusted by adjusting the time constant of the second lock-in amplifier 2-5 to obtain a first-order differential spectrum signal suitable for frequency stabilization. This signal generates a correction error signal through the second PID controller 2-6. This correction signal, together with the aforementioned sweep signal and the modulation signal, are all superimposed on the driving voltage of the piezoelectric ceramic driving controller of the fiber optic ring resonator 2-2 through the second adder 2-7. Through the above operations, the cavity length of the fiber optic resonator is locked at the cavity length position corresponding to the resonance frequency of the semiconductor laser.

[0048] When the cavity length L and the loss of the fiber optic ring resonator satisfy the conditions of formulas (1) and (2), a resonant transmission peak is generated. Where β is the propagation constant of light in the fiber, n is the refractive index of the fiber, L is the length of the fiber optic ring resonator, τ is the splicing loss introduced by the splicing of the fiber optic circulator, γ0 is the insertion loss of the fiber optic circulator, and k is the optical intensity coupling coefficient of the fiber optic coupler.

[0049]

[0050] The modulation of the resonant transmission peak is achieved by modulating the length of the ring resonator through the second signal generator 2-4. The transmission spectrum is output to the third photodetector 3-4 through the second fiber optic circulator 3-3 and converted into a voltage signal with a Gaussian line shape. This voltage signal is input to the input terminal of the second lock-in amplifier 2-5 for demodulation to obtain a primary harmonic error signal containing the frequency information and cavity length information of the semiconductor laser 1-1. The zero point of the error signal corresponds to the cavity length at the peak of the resonant peak. This frequency point is used as the cavity length locking reference, and the error signal is input to the second PID controller 2-6 to generate a cavity length correction signal. This signal is input to the input terminal of the piezoelectric ceramic drive controller connected to the fiber optic ring resonator 2-2. The cavity length is locked by adjusting the drive voltage of the piezoelectric ceramic drive controller.

[0051] The cavity length L of the fiber optic ring resonator determines the free spectral range v of the transmission peak. FSR , where v FSR is calculated by Equation (3). By adjusting the cavity length L, the free spectral range v can be obtained. FSR :

[0052]

[0053] where c is the speed of light, n is the refractive index of the optical fiber, L is the cavity length of the fiber optic ring resonator, and v FSR is the free spectral range.

[0054] Through the above process, when the frequency of the semiconductor laser 1-1 is locked to the atomic resonance point and the cavity length of the fiber optic ring resonator is locked to the cavity length resonant with the frequency of the semiconductor laser 1-1, the operating points such as the current and temperature of the slave laser 3-1 are adjusted so that the beam of the slave laser 3-1 passes through the fiber optic isolator 3-2 and enters the fiber optic ring resonator 2-2 through the B-1 port of the second fiber optic circulator 3-3. After passing through the fiber optic ring resonator 2-2, it enters the A-2 port of the first fiber optic circulator 2-1 and is incident on the second photodetector 2-3 from the A-3 port of the first fiber optic circulator 2-1, generating a periodic Gaussian line-shaped transmission peak. A modulation signal is applied to the current source of the slave laser 3-1 through the third signal generator 3-6. The third photodetector 3-4 obtains the transmission peak spectral signal containing modulation information. The spectral signal and the reference signal with the same frequency and phase as the modulation signal are input to the third lock-in amplifier 3-8 for demodulation, and the time constant of the third lock-in amplifier 3-8 is adjusted to adjust the cut-off frequency of the low-pass filter, generating a primary error signal containing cavity length information and the frequency information of the slave laser 3-1. Since the first two loops achieve highly stable locking of the cavity length, using this transmission peak as the frequency reference can achieve frequency locking of the slave laser 3-1. This error signal is input to the third PID controller 3-7 to generate a correction signal and is input to the controller of the slave laser 3-1 through the adder 3-5 on the ground. Therefore, when the slave laser 3-1 operates at a frequency off the resonance point and passes through the fiber optic ring resonator, periodic Gaussian resonance peaks with a free spectral range interval are generated. By adjusting the free spectral range, the frequency detuning amount is achieved. Therefore, through the locking of the cavity length of the ring resonator, the stable transfer of the resonance point frequency reference to the detuned resonance point frequency reference is achieved, and finally, highly stable locking control with an adjustable frequency offset of the slave laser 3-1 is realized.

[0055] The working principle of the present invention is as follows:

[0056] Referring to Equation (3), the cavity length L of the fiber optic resonator determines the free spectral range v of the transmission peak FSR , and by adjusting the cavity length L, the free spectral range v can be obtained FSRThe cavity length designed by the present invention is adjustable, which ensures that the free spectral range can be flexibly adjusted within a certain range, and can meet the frequency offset application requirements of different laser frequencies. The spectral intensity and central frequency of the resonant transmission peak generated after the laser beam passes through the fiber resonator are both related to the cavity length of the resonator. Therefore, on the one hand, by finely adjusting the cavity length of the resonator, the frequency of the transmission peak can be made to coincide with the laser frequency of the incident fiber, and then the cavity length of the resonator can be locked at the position resonant with the frequency of the semiconductor laser; on the other hand, by adjusting the laser frequency, the central frequency of the transmission peak can be made to coincide with the resonant frequency of the resonator corresponding to the cavity length. Using this feature, the frequency of the slave laser can be locked at the resonant frequency point corresponding to the cavity length of the resonator. By means of three control loops, the frequency of the semiconductor laser is locked at the atomic absorption peak, the cavity length of the resonator is locked at the frequency corresponding to the atomic absorption peak, and then the frequency of the slave laser is locked at the resonant frequency of the locked cavity length, indirectly transferring the saturated absorption frequency stabilization characteristic of the semiconductor laser to the slave laser. Since the cavity length of the resonator has been locked, even if the fiber resonator is sensitive to factors such as temperature, vibration, and air humidity, the cavity length can still be guaranteed to be stable within a certain fluctuation range, that is, the frequency stability of the slave laser is achieved.

[0057] The content not described in detail in the specification of the present invention belongs to the prior art well-known to those skilled in the art. It is easy for those skilled in the art to understand that the above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A laser frequency stabilization device based on a fiber ring resonator, characterized by: The invention comprises three loops, wherein the first loop is a laser resonance point frequency locking loop, the second loop is a ring resonator cavity length locking loop, and the third loop is a bias frequency locking loop of the laser to be measured; the first loop comprises a semiconductor laser, an optical isolator, a beam splitter, a λ / 2 wave plate, a polarization beam splitter, a plane reflector, a vacuum atomic gas chamber, a beam splitter, a first photodetector, an optical fiber coupler, a first signal generator, a first lock-in amplifier, a first PID controller and a first adder; the second loop comprises a first optical fiber circulator, an optical fiber ring resonator, a second photodetector, a second signal generator, a second lock-in amplifier, a second PID controller and a second adder; the third loop comprises a slave laser, an optical fiber isolator, a second optical fiber circulator, a third photodetector, a third adder, a third signal generator, a third PID controller and a third lock-in amplifier; In the first loop, a light beam emitted from the semiconductor laser passes through an optical isolator and then split into two beams via a beam splitter prism. One beam enters the second loop through an incident fiber coupler, and the other beam passes through a λ / 2 wave plate and then undergoes a polarization beam splitter prism to adjust the light intensity. The transmitted light beam passes through a plane mirror and a beam splitter into a vacuum atomic gas chamber, and is received by a first photodetector after multiple reflections in the vacuum atomic gas chamber. The output of the first photodetector is connected to the input of a first lock-in amplifier, the output of the first signal generator is connected to the reference input of the first lock-in amplifier, the output of the first lock-in amplifier is connected to the input of a first PID controller, the output of the first PID controller and the other output of the first signal generator are respectively connected to the two inputs of a first adder, and the output of the first adder is connected to the controller of the semiconductor laser. In the second loop, the light beam emitted by the first loop enters the A-1 port of the first fiber circulator, is emitted from the A-2 port of the first fiber circulator, enters the fiber ring resonator, passes through the B-2 port of the second fiber circulator, and is emitted from the B-3 port of the second fiber circulator to enter the third photodetector and is connected to the input end of the second lock-in amplifier, the output of the second signal generator is connected to the reference input end of the second lock-in amplifier, the output of the second lock-in amplifier is connected to the input end of the second PID controller, the output of the second PID controller and the other output end of the second signal generator are respectively connected to the two input ends of the second adder, and the output end of the second adder is connected to the input end of the piezoelectric ceramic drive controller of the fiber ring resonator; In the third loop, light emitted from the laser passes through the optical fiber isolator and enters the B-1 port of the second optical fiber circulator, then enters the optical fiber ring resonator through the B-2 port of the second optical fiber circulator, is received by the second photodetector, and is connected to the input end of the third lock-in amplifier. The output of the third signal generator is connected to the reference input end of the third lock-in amplifier. The output of the third lock-in amplifier is connected to the input end of the third PID controller. The output of the third PID controller and the other output end of the third signal generator are respectively connected to the input end of the third adder. The output end of the third adder is connected to the controller of the slave laser.

2. A method for laser frequency stabilization based on a fiber ring resonator according to claim 1, characterized in that: The following steps are involved: ① Adjust the operating temperature and current of the semiconductor laser to near the atomic hyperfine energy level transition frequency point, and sweep the semiconductor laser through the sawtooth wave generated by the signal generation circuit, so that the semiconductor laser generates a saturated absorption spectrum signal after passing through the vacuum atomic gas chamber; ② Applying a modulation signal to the semiconductor laser current, the first photodetector converts the saturated absorption spectrum signal containing the modulation information into a voltage signal and sends it to the first lock-in amplifier; ③ Adjust the phase of the reference signal so that the demodulated differential error signal is maximized and the zero point of the differential error signal corresponds to the peak point of the saturated absorption spectrum signal; ④ Adjust the proportional-integral-differential parameters of the first PID controller to achieve the semiconductor laser being fixed at the atomic hyperfine transition resonance frequency point; ⑤ A frequency-locked semiconductor laser is incident on a fiber ring resonator through a first fiber circulator. A periodic signal is applied to the fiber ring resonator through a first signal generator to perform frequency sweeping. The spectrum signal generates a Gaussian-like linear resonant transmission peak. The transmission spectrum signal is output from the port of the first fiber circulator to the port of the second fiber circulator and enters the third photodetector. ⑥ Use a second signal generator to apply a modulation signal to the ring resonator, use a second lock-in amplifier to demodulate the modulated dispersion-like spectrum signal to obtain an error signal, and use a second PID controller to adjust the piezoelectric ceramic drive voltage to achieve synchronous adjustment of the fiber ring resonator length, complete the resonance of the fiber ring resonator length with the semiconductor laser frequency and achieve cavity length locking; ⑦ Tune the slave laser frequency to near the target frequency point. The output beam from the slave laser passes through the fiber isolator and the second fiber circulator and then enters the fiber ring resonator. A sweep frequency signal is applied to the slave laser current. The resonance spectrum generated by the ring resonator transmission passes through the second fiber circulator and is output to the third photodetector. ⑧ Apply a modulation signal to the slave laser current, use a third phase-locked amplifier to demodulate the modulated spectral signal to obtain an error signal, and use a third PID controller to adjust the current to ultimately lock the frequency of the slave laser at the target frequency point.

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