Laser gyro frequency stabilization system and its frequency stabilization method

Abstract

The present invention belongs to the technical field of optoelectronic devices, and provides a frequency stabilization system and a frequency stabilization method for a laser gyroscope, including a large laser gyroscope to be frequency stabilized, an electro-optic modulator, an adjustment optical path, a signal generator, a mode matching mirror group, a PID regulator, a mixer, a photodetector, an ultra-stable cavity, a low-pass filter, a computer, and a data acquisition card. The ultra-stable cavity has a square transmission optical path inside. The present invention effectively avoids the problem of insufficient isolation caused by an optical isolator composed of a polarization beam splitter prism and a quarter-wave plate, reduces the influence of the optical feedback effect, and better adapts to the high-precision measurement field involved in large laser gyroscopes.

Description

Technical Field

[0001] The present invention belongs to the technical field of optoelectronic devices, and particularly relates to a frequency stabilization system for a laser gyroscope and a frequency stabilization method therefor. Background Art

[0002] As a typical application of the Sagnac effect in the field of inertial navigation and a typical representative of the first quantum technology revolution, a laser gyroscope can accurately measure angular velocity without the condition of inertial mass, bringing a disruptive change to fields such as ultra-precision inertial navigation and attitude control, and becoming an indispensable core component from inertial sensing control to the field of basic science. With the continuous progress of related process technologies, the measurement accuracy of laser gyroscopes has gradually approached the theoretical limit. People have increased the scale factor by increasing the size of the gyroscope, and large laser gyroscopes have thus been born, achieving a breakthrough in accuracy on the basis of traditional laser gyroscopes. Currently, the measurement accuracy of mainstream large laser gyroscopes internationally has reached 10 -13 to 10 -11 order of magnitude, and the lowest theoretical accuracy has even dropped to 2×10 -15 rad / s, which can basically meet the stringent requirements of a series of high-precision fields such as the test of relativistic effects.

[0003] As a large scientific device used in the field of high-precision measurement of weak signals, it is crucial to maintain the long-term stable operation of a large laser gyroscope at a high-precision measurement level. The core of this is to maintain the frequency stability in its resonant cavity, including short-term stability and long-term stability. Short-term stability ensures the accuracy and repeatability of instant signal measurement, while long-term stability requires reducing or even eliminating the influence of long-term drift. Currently, the mainstream frequency stabilization scheme for large laser gyroscopes widely adopted internationally is "external absolute frequency standard + PDH frequency stabilization technology". This scheme generates an error signal based on the beat frequency between the output frequency of the gyroscope and the absolute frequency standard. After a series of complex processing processes, the control signal is fed back to the piezoelectric ceramic to achieve precise regulation of the cavity length, and ultimately achieve the purpose of outputting a laser with a stable frequency. In this process, the frequency stability of the output laser largely depends on the stability of the absolute frequency standard. In traditional PDH frequency stabilization technology, the resonant transmission peak line of a straight F-P cavity is generally used as the absolute frequency standard. The reason is that by increasing the reflectivity of the two reflecting mirrors constituting the F-P cavity, the finesse of the F-P cavity can be increased, making the width of its resonant transmission peak line narrower; at the same time, the high stability of the microcrystalline glass ensures a very small offset of the center of the resonant transmission peak of the F-P cavity itself, making the straight F-P cavity an ideal external frequency standard.

[0004] In traditional PDH technology, an electro-optic modulator (EOM) driven by a certain local oscillator frequency is often added in front of a straight F-P cavity. The electro-optic modulator will apply two symmetric sideband frequencies to the original frequency of the laser to be frequency-stabilized, so as to reflect the error information between the frequency of the laser to be frequency-stabilized and the resonance transmission peak of the straight F-P cavity. In actual optical path design, in order to prevent the light reflected from the straight F-P cavity from returning along the incident optical path to the resonance cavities of the electro-optic modulator and the laser to be frequency-stabilized, causing an optical feedback effect, it is necessary to introduce an optical isolation means between the electro-optic modulator and the straight F-P cavity. A common method is to add a quarter-wave plate after a polarizing beam splitter (PBS) to form an optical isolation module. When incident in the forward direction, the quarter-wave plate will generate a phase difference of a quarter wavelength on the two orthogonal components of the linearly polarized light emitted by the polarizing beam splitter, converting the linearly polarized light into circularly polarized light; when incident in the reverse direction, the quarter-wave plate acts on the circularly polarized light reflected from the straight F-P cavity again, and the phase difference between the two orthogonal components becomes a half wavelength, and the circularly polarized light is converted back into linearly polarized light. At this time, the polarization direction of the linearly polarized light is already perpendicular to the polarization direction of the original incident light and cannot pass through the polarizing beam splitter, thus realizing the optical isolation effect.

[0005] However, for high-precision large scientific devices such as large laser gyroscopes, using the above optical isolation module will inevitably cause the following problems:

[0006] 1. Introduce insertion loss. The polarizing beam splitter and the quarter-wave plate will cause certain reflection and absorption of the laser, introducing insertion loss to the originally weak light intensity signal and affecting the subsequent signal processing effect;

[0007] 2. Poor isolation effect. The optical isolation module composed of a polarizing beam splitter and a quarter-wave plate has high requirements for the polarization degree of the incident light. The isolation effect for light with a low polarization degree will be greatly reduced; and even if the polarization degree of the incident light is high, its isolation degree is relatively limited, usually about 20 dB - 40 dB, and may not meet the isolation degree requirements in the application of some extremely high-precision measurement fields;

[0008] 3. The frequency stabilization system is complex to regulate. First, the phase delay characteristic of the quarter-wave plate is relatively sensitive to temperature changes, and temperature fluctuations will greatly affect the optical isolation performance; second, in order to achieve effective optical isolation, it is necessary to accurately design and align the relative positions and angles of the polarizing beam splitter and the quarter-wave plate, thus increasing the overall debugging difficulty of the frequency stabilization system; in addition, the quarter-wave plate has the best phase delay effect on light of a specific wavelength, and the optical isolation performance for light of other wavelengths will decline. Therefore, wavelength drift under different conditions will weaken the optical isolation effect. Summary of the Invention

[0009] To solve the technical problems existing in the optical isolation module composed of a polarization beam splitter prism and a quarter-wave plate in the PDH technology during actual application, the present invention provides a laser gyro frequency stabilization system and a frequency stabilization method therefor.

[0010] To solve the above technical problems existing in the prior art, the technical solution adopted by the present invention is as follows:

[0011] On the one hand, a laser gyro frequency stabilization system is proposed, which includes a large laser gyro to be frequency stabilized, an electro-optic modulator, an adjustment optical path, a signal generator, a mode matching mirror group, a PID regulator, a mixer, a photodetector, an ultra-stable cavity, a low-pass filter, a computer, and a data acquisition card;

[0012] The large laser gyro to be frequency stabilized outputs laser to be frequency stabilized;

[0013] The electro-optic modulator receives the laser to be frequency stabilized. The signal generator is connected to the electro-optic modulator and the mixer through cables respectively, and outputs two radio frequency signals with the same frequency. One radio frequency signal is used as the working signal to excite the electro-optic modulator, and the other radio frequency signal is used as the local oscillator signal during the mixing process of the mixer. The electro-optic modulator applies two symmetric sidebands at the frequency of the laser to be frequency stabilized under the action of the radio frequency signal generated by the signal generator.

[0014] The adjustment optical path transmits the laser to be frequency stabilized emitted by the electro-optic modulator to the mode matching mirror group;

[0015] The mode matching mirror group adjusts the mode and phase of the laser to be frequency stabilized and then outputs the laser to be frequency stabilized to the ultra-stable cavity;

[0016] The laser output from the ultra-stable cavity is incident on the photodetector. The photodetector is connected to the mixer and the data acquisition card, and the data acquisition card is connected to the computer through a network port / serial port;

[0017] The mixer is connected to the low-pass filter to perform low-pass filtering on the mixing signal from the mixer and extract the error signal. The low-pass filter is respectively connected to the PID regulator and the computer. The PID regulator is connected to the piezoelectric ceramic fixed behind one of the resonant cavity mirrors in the large laser gyro to be frequency stabilized, converts the error signal from the low-pass filter into a voltage signal to drive the piezoelectric ceramic, and transmits it to the piezoelectric ceramic. The movement of the piezoelectric ceramic synchronously drives the resonant cavity mirror fixed to the piezoelectric ceramic, thereby realizing precise control of the length of the resonant cavity inside the large laser gyro to be frequency stabilized, and thus maintaining the stability of the output laser frequency of the large laser gyro to be frequency stabilized.

[0018] Further, the large laser gyro to be frequency stabilized is placed on an air-bearing optical platform to reduce environmental vibration.

[0019] Further, the optical path adjustment includes a front mirror and a rear mirror. The reflecting surface of the front mirror receives the laser to be frequency-stabilized emitted from the light-transmitting aperture of the electro-optic modulator and reflects it onto the reflecting surface of the rear mirror. The rear mirror reflects the laser to be frequency-stabilized from the front mirror to the mode matching mirror group. The front mirror and the rear mirror cooperate to enable the laser to be frequency-stabilized to smoothly enter the ultra-stable cavity and coincide with the eigenoptical field in the ultra-stable cavity.

[0020] Further, the mode matching mirror group is composed of n mirrors, which are successively the first mirror M1...... the nth mirror M along the incident direction of the laser n , among the n mirrors, there are plane mirrors and spherical mirrors coated with high-reflection films. By adjusting the radius of curvature of the spherical mirrors and the relative positions of the plane mirrors and the spherical mirrors in the mode matching mirror group, the position and size of the laser to be frequency-stabilized incident on the mode matching mirror group can be matched with the position and size of the beam eigen waist spot when it exits the mode matching mirror group and enters the ultra-stable cavity. The mode and phase of the laser to be frequency-stabilized are adjusted through the mode matching mirror group so that the laser to be frequency-stabilized can effectively resonate after entering the ultra-stable cavity.

[0021] Further, the ultra-stable cavity is placed in a temperature control cover and fixedly connected to an air-bearing optical platform.

[0022] Further, the main body material of the ultra-stable cavity is microcrystalline glass. The ultra-stable cavity has a square transmission optical path inside, and the reflection of the laser in the square transmission optical path is realized by four high-reflectivity mirrors. Piezoelectric ceramics are installed on the outer surfaces of two adjacent mirrors among the four mirrors, and the other two mirrors are the incident end mirror S1 and the output end mirror S2. The incident end mirror S1 and the output end mirror S2 serve as the entrance and exit for the external laser injection. The laser to be frequency-stabilized with its mode and phase adjusted by the mode matching mirror group is incident on the incident end mirror S1. The incident end mirror S1 transmits the laser to be frequency-stabilized into the ultra-stable cavity, and the output end mirror S2 transmits the laser to be frequency-stabilized to the reflecting surface of the rear cavity mirror. The rear cavity mirror then reflects the laser to be frequency-stabilized to the photosensitive surface of the photodetector.

[0023] Further, the electrical signal generated by the photodetector includes three parts: a DC term reflecting the respective intensities of the carrier and the sidebands, an interference term between the carrier and the sidebands, and an interference term between the two sidebands. Among them, the interference term between the carrier and the sidebands contains the error information between the frequency of the laser to be frequency-stabilized and the resonance peak of the ultra-stable cavity.

[0024] The mixed signal output by the mixer includes three parts: the amplitude of the interference term between the carrier and the sidebands, the first harmonic term, and the second harmonic term. Among them, the amplitude of the interference term between the carrier and the sidebands is used as the error signal.

[0025] The computer is built with a data reading and visualization program and a driver adapted to the temperature regulator on the temperature control cover wall to achieve the following purposes:

[0026] Read out the electrical signals from the photodetector to observe the resonance situation inside the ultra-stable cavity. When resonance occurs inside the ultra-stable cavity, an obvious light intensity transmission peak will be seen on the computer. Correspondingly, the transmitted light emerging from the incident-end mirror S1 of the ultra-stable cavity will interfere destructively with the first-reflected light of the laser to be frequency-stabilized on the incident-end mirror S1, manifested as a depression in the light intensity signal;

[0027] Receive the error signal processed by the low-pass filter and display its curve;

[0028] Monitor and adjust the temperature inside the ultra-stable cavity to keep it stable.

[0029] The beneficial technical effects of the present invention are:

[0030] The present invention effectively avoids the problem of insufficient isolation degree brought by the optical isolator composed of a polarization beam splitter prism and a quarter-wave plate, reduces the influence of the optical feedback effect, and better adapts to the high-precision measurement field involved in large laser gyroscopes. The present invention has better thermal stability and anti-vibration performance. The ultra-stable cavity inside the present invention has a square transmission optical path. Compared with a straight cavity body, the thermal expansion is more uniform in all directions, the heat conduction path is more complex and dispersed, and there will be no local overheating or too large temperature gradient like a straight cavity body. Therefore, it has better thermal stability and temperature control effect; at the same time, the structure of the square cavity body makes its stiffness relatively uniform in all directions, and the distribution of external vibration energy inside the cavity is relatively more dispersed, so it can better resist the interference of external vibration and is more compact and stable.

[0031] The ultra-stable cavity inside the present invention has a square transmission optical path. During the electro-optic modulation process, the remaining amplitude modulation generated will be less than that of a straight cavity body, which helps to reduce the interference to the error signal and further improve the frequency stabilization accuracy of the system.

[0032] On the premise of ensuring the same length of the resonant cavity, compared with the straight cavity scheme, this solution can effectively reduce the scale of the optical path part of the frequency stabilization system and enhance the practicability and flexibility of the frequency stabilization system. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on the structures shown in these drawings.

[0034] Figure 1 FIG. 1 is a schematic structural diagram of a frequency stabilization system for a laser gyroscope in an embodiment, where: 1: a large laser gyroscope to be frequency stabilized; 2: an electro-optic modulator; 3: a front-side reflector; 4: a rear-side reflector; 5: a signal generator; 6: a mode matching mirror group; 7: a PID regulator; 8: a mixer; 9: a photodetector; 10: an ultra-stable cavity; 11: a temperature control cover; 12: a rear reflector of the cavity; 13: a low-pass filter; 14: a computer; 15: a data acquisition card;

[0035] Figure 2 is Figure 1 the composition of the mode matching mirror group and the relative positions of each mirror surface in FIG. 2, where: P1 and P2 are the first spot observation position and the second spot observation position; M1 to M n are the first reflector to the n reflector for adjusting the incident laser mode and phase.

[0036] Figure 3 is Figure 1 FIG. 3 is a schematic structural diagram of the ultra-stable cavity in FIG. 2, where: S1 is the incident-end reflector and S2 is the exit-end reflector; there are two laser beams propagating in the clockwise and counterclockwise directions simultaneously in the ultra-stable cavity. The beam propagating in the clockwise direction is the laser to be frequency stabilized injected from the outside, and the beam propagating in the counterclockwise direction is the backscattered beam of the laser to be frequency stabilized, and its intensity can be ignored relative to the laser to be frequency stabilized;

[0037] Figure 4 is Figure 1 the flowchart of the frequency stabilization method for the laser gyroscope frequency stabilization system shown in FIG. 4. Specific Embodiments

[0038] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts fall within the scope of protection of the present invention.

[0039] Referring to FIGS. Figure 1 to FIGS. Figure 4 , Figure 1 FIG. 1 is a schematic structural diagram of a frequency stabilization system for a laser gyroscope in an embodiment, Figure 2 is Figure 1 the composition of the mode matching mirror group and the relative positions of each mirror surface in FIG. 2; Figure 3 is Figure 1 FIG. 3 is a schematic structural diagram of the ultra-stable cavity in FIG. 2, Figure 4 is Figure 1 the flowchart of the frequency stabilization method for the laser gyroscope frequency stabilization system shown in FIG. 4.

[0040] As Figure 1 shown in the embodiments, a laser gyro frequency stabilization system is provided, including: a large laser gyro 1 to be frequency stabilized, an electro-optic modulator 2, an adjustment optical path, a signal generator 5, a mode matching mirror group 6, a PID regulator 7, a mixer 8, a photodetector 9, an ultra-stable cavity 10, a low-pass filter 13, a computer 14, and a data acquisition card 15;

[0041] The large laser gyro 1 to be frequency stabilized is placed on an air-bearing optical platform to reduce environmental vibration, and the large laser gyro 1 to be frequency stabilized outputs a laser to be frequency stabilized with a frequency of ω 0;

[0042] The electro-optic modulator 2 is located at the light output port of the large laser gyro 1 to be frequency stabilized. The electro-optic modulator 2 receives the laser to be frequency stabilized emitted from the large laser gyro 1 to be frequency stabilized. The electro-optic modulator 2 is connected to a signal generator 5, and two symmetric sidebands are applied to the frequency of the laser to be frequency stabilized under the action of the radio frequency signal generated by the signal generator 5. The signal generator 5 is used to generate the working signal for exciting the electro-optic modulator 2 and the local oscillator signal for the mixing process. The signal generator 5 is respectively connected to the electro-optic modulator 2 and the mixer 8 through cables and outputs two radio frequency signals with the same frequency of f 0. Among them, the radio frequency signal input to the electro-optic modulator 2 is used as the working signal for exciting the electro-optic modulator 2, and the radio frequency signal input to the mixer 8 is used as the local oscillator signal for the mixing process.

[0043] The adjustment optical path is used to receive the laser to be frequency stabilized emitted from the light passing aperture of the electro-optic modulator 2 and transmit the laser to be frequency stabilized to the mode matching mirror group 6; wherein the adjustment optical path includes a front-side reflector 3 and a rear-side reflector 4. The front-side reflector 3 is placed behind the light passing aperture of the electro-optic modulator 2, and the rear-side reflector 4 is placed at the rear end of the front-side reflector 3. The two are used to jointly adjust the optical path. The reflecting surface of the front-side reflector 3 receives the laser to be frequency stabilized emitted from the light passing aperture of the electro-optic modulator 2 and reflects it to the reflecting surface of the rear-side reflector 4. The rear-side reflector 4 reflects the laser to be frequency stabilized from the front-side reflector 3 to the mode matching mirror group 6. The front-side reflector 3 and the rear-side reflector 4 cooperate to enable the laser to be frequency stabilized to be smoothly incident into the ultra-stable cavity 10 and coincide with the eigen-optical field in the ultra-stable cavity 10.

[0044] The mode matching mirror group 6 is placed at the rear end of the rear-side reflector 4 and is used to adjust the mode and phase of the laser to be stabilized. After the mode matching mirror group 6 adjusts the mode and phase of the laser to be frequency stabilized, it outputs the laser to be frequency stabilized to the ultra-stable cavity 10. By adjusting the mode and phase of the laser to be frequency stabilized through the mode matching mirror group 6, the laser to be frequency stabilized can effectively resonate after entering the ultra-stable cavity 10.

[0045] Referring to Figure 2, the mode matching mirror group 6 is composed of n mirrors, which are successively the first mirror M1...... the nth mirror M along the incident direction of the laser n , among the n mirrors, there are plane mirrors and spherical mirrors coated with high-reflection films. By adjusting the curvature radius of the spherical mirror and the relative positions of the plane mirror and the spherical mirror in the mode matching mirror group 6, the position and size of the eigen waist spot of the beam can be matched when the frequency-stabilized laser to be incident on the mode matching mirror group 6 exits the mode matching mirror group 6 and enters the ultra-stable cavity 10. The mode and phase of the frequency-stabilized laser are adjusted by the mode matching mirror group 6 so that the frequency-stabilized laser can effectively resonate after entering the ultra-stable cavity 10.

[0046] The ultra-stable cavity 10 is placed in the temperature control cover 11 and fixedly connected to the air-bearing optical platform, where the temperature control cover 11 is used for heat insulation and temperature control. Refer to Figure 3 , the main material of the ultra-stable cavity 10 is microcrystalline glass. The ultra-stable cavity 10 has a square transmission optical path inside, and the laser is reflected in the square transmission optical path by four high-reflectivity mirrors. Piezoelectric ceramics are installed outside the cavity of two adjacent mirrors among the four mirrors, and the other two adjacent mirrors are the incident end mirror S1 and the output end mirror S2. The incident end mirror S1 and the output end mirror S2 are used as the inlet and outlet for the injection of external laser. The frequency-stabilized laser after the mode and phase are adjusted by the mode matching mirror group 6 is incident on the incident end mirror S1. The incident end mirror S1 transmits the frequency-stabilized laser into the ultra-stable cavity 10, and the output end mirror S2 transmits the frequency-stabilized laser to the reflecting surface of the rear cavity mirror 12. The rear cavity mirror 12 is placed behind the light outlet of the ultra-stable cavity 10. The photodetector 9 is placed behind the rear cavity mirror 12 to convert the light intensity signal into an electrical signal. The rear cavity mirror 12 reflects the frequency-stabilized laser to the photosensitive surface of the photodetector 9.

[0047] The laser output from the ultra-stable cavity 10 is incident on the photosensitive surface of the photodetector 9. The photodetector 9 is respectively connected to the mixer 8 and the data acquisition card 15 through cables, and converts the collected laser signal into an electrical signal. The electrical signal generated by the photodetector 9 includes three parts: a DC term reflecting the respective intensities of the carrier and the sidebands, an interference term between the carrier and the sidebands, and an interference term between the two sidebands. Among them, the interference term between the carrier and the sidebands contains the error information between the frequency of the frequency-stabilized laser and the resonance peak of the ultra-stable cavity 10.

[0048] After receiving the radio frequency signal from the signal generator 5 and the electrical signal from the photodetector 9, the mixer 8 mixes the two and outputs the mixed signal to the low-pass filter 13. The mixed signal output by the mixer 8 contains three parts: the amplitude of the interference term between the carrier and the sidebands, the first harmonic term, and the second harmonic term. Among them, the amplitude of the interference term between the carrier and the sidebands is used as the error signal. The amplitude of the interference term between the carrier and the sidebands is a DC term. And when the frequency of the laser to be stabilized is consistent with the resonant frequency of the cavity, the amplitude curve passes through the zero point. When the two are close but not equal, the amplitude curve is either positive or negative. Therefore, it fully meets the frequency discrimination characteristics and can be used as an error signal.

[0049] The low-pass filter 13 is connected to the mixer 8 and the PID regulator 7 through cables, and low-pass filters the mixed signal from the mixer 8, filters out the irrelevant high-frequency impurities in the mixed signal, and extracts the error signal. The PID regulator 7 is used to convert the error signal into a voltage signal to drive the piezoelectric ceramic. The PID regulator 7 is connected to the piezoelectric ceramic fixed to the rear side of one of the resonant cavity mirrors in the large laser gyro 1 to be frequency-stabilized through a cable, converts an error signal from the low-pass filter 13 into a voltage signal to drive the piezoelectric ceramic, and transmits it to the piezoelectric ceramic. The piezoelectric ceramic moves synchronously to drive the resonant cavity mirror fixed to the piezoelectric ceramic, thereby realizing precise regulation of the length of the resonant cavity inside the large laser gyro 1 to be frequency-stabilized, and thus maintaining the stability of the output laser frequency of the large laser gyro 1 to be frequency-stabilized.

[0050] The low-pass filter 13 is connected to the computer 14 through a cable. The computer 14 is used for resonant state observation, data reading, data processing, and temperature control drive. The data acquisition card 15 is connected to the computer 14 through a network port / serial port, receives the electrical signals from the photodetector 9 and the temperature sensor on the wall of the temperature control cover 11, and centrally transmits them to the computer 14. The computer 14 has built-in data reading and visualization programs and a driver program adapted to the temperature regulator on the wall of the temperature control cover 11 to achieve the following purposes:

[0051] Read out the electrical signal from the photodetector 9 to observe the resonance situation inside the ultra-stable cavity 10. When resonance occurs inside the ultra-stable cavity 10, an obvious light intensity transmission peak will be seen on the computer 14. Correspondingly, the transmitted light emerging from the incident end mirror S1 of the ultra-stable cavity 10 will interfere destructively with the first reflected light of the laser to be frequency-stabilized on the incident end mirror S1, manifested as a depression in the light intensity signal;

[0052] Receive the error signal processed by the low-pass filter 13 and display its curve;

[0053] Monitor and adjust the temperature inside the ultra-stable cavity 10 to keep it stable.

[0054] In the above-mentioned frequency stabilization system of a large laser gyroscope based on an ultra-stable cavity, considering the characteristics of the single-channel output laser of the large laser gyroscope, the device selection should meet the following requirements: The response ranges of the electro-optic modulator 2, the signal generator 5, and the photodetector 9 to the optical field wavelength should be consistent with those of the large laser gyroscope 1 to be frequency stabilized and the ultra-stable cavity 10 to the optical field wavelength. When the large laser gyroscope 1 to be frequency stabilized is fixedly connected to the air-bearing optical platform, the linear polarization direction of its emitted light beam should be consistent with the polarization direction of the ultra-stable cavity 10. The electro-optic modulator 2 used should have a low-frequency modulation range including direct current coupling and low insertion loss to reduce the further loss of the weak optical signal emitted by the large laser gyroscope. The mode matching mirror group 6 is composed of a plane mirror and a spherical mirror coated with a high-reflection film, and is designed according to the Gaussian beam self-reproducing transformation method described in [Zhou Bingkun et al., Laser Principles [M]. 7th Edition. Beijing: National Defense Industry Press, 2014: 81-91]. By adjusting the curvature radius of the spherical mirror inside the mode matching mirror group 6 and the relative positions of the plane mirror and the spherical mirror, the position and size of the beam waist of the light beam to be stabilized emitted by the large laser gyroscope 1 to be frequency stabilized can be matched with those of the eigen beam waist when it enters the ultra-stable cavity 10 after passing through the electro-optic modulator 2, the front-side mirror 3, the rear-side mirror 4, and the mode matching mirror group 6. The output radio frequency signal frequency range of the signal generator 5 should cover the minimum driving frequencies of the electro-optic modulator 2 and the mixer 8, and have good output signal accuracy and stability. The PID regulator 7 should have a high bandwidth to quickly respond to signal changes; at the same time, the output voltage range should be wide enough to ensure a large enough cavity length tuning range and a small enough cavity length tuning accuracy for the piezoelectric ceramic. The photodetector 9 should have high sensitivity to detect weak signals, and its response bandwidth should be greater than the free spectral range of the ultra-stable cavity 10. The ultra-stable cavity 10 should have excellent cavity length stability. Essentially, it is a small laser gyroscope. The reflectivities of the incident-end mirror S1 and the output-end mirror S2 should be as large as possible to improve the finesse of the ultra-stable cavity 10, but at the same time, it is necessary to ensure that it matches the cavity length tuning accuracy of the piezoelectric ceramic; in addition, the center of the resonance peak of the ultra-stable cavity 10 should match the frequency of the laser output by the large laser gyroscope 1 to be frequency stabilized. The temperature control cover 11 should have good thermal stability and heat insulation. The temperature sensor on the wall should respond quickly, and the supporting temperature regulator should support multi-channel input and output to ensure the spatial uniform regulation of the ambient temperature of the ultra-stable cavity 10, and at the same time, the regulation accuracy should match the ambient temperature change. The passband range of the low-pass filter 13 should cover the error signal frequency range output by the mixer 8, and the stopband frequency should be slightly greater than the maximum frequency in the error signal frequency range to filter out unnecessary high-frequency signals as much as possible. The sampling frequency and resolution of the data acquisition card 15 should match the electrical signal frequencies and resolutions from the photodetector 9 and the temperature sensor on the wall of the temperature control cover 11.

[0055] Furthermore, referring to Figure 4, a frequency stabilization method for a laser gyro frequency stabilization system is proposed, including the following steps:

[0056] Step 1: Turn on the excitation power supply of the large laser gyro 1 to be frequency stabilized, so that it lights up and smoothly emits the laser to be frequency stabilized;

[0057] Step 2: By adjusting the optical path, make the laser to be frequency stabilized emitted by the large laser gyro 1 to be frequency stabilized injected into the ultra-stable cavity 10 and resonate. The adjusted optical path includes a front-side mirror 3 and a rear-side mirror 4. The reflecting surface of the front-side mirror 3 receives the laser to be frequency stabilized emitted from the light passing aperture of the electro-optic modulator 2 and reflects it to the reflecting surface of the rear-side mirror 4. The rear-side mirror 4 reflects the laser to be frequency stabilized from the front-side mirror 3 to the mode matching mirror group 6;

[0058] Step 2.1: Adjust the position of the front-side mirror 3 so that the laser spot of the laser to be frequency stabilized emitted from the light passing aperture of the electro-optic modulator 2 falls on the center of the reflecting surface of the front-side mirror 3;

[0059] Step 2.2: Adjust the position of the rear-side mirror 4 so that the laser spot of the laser to be frequency stabilized reflected by the front-side mirror 3 falls on the center of the reflecting surface of the rear-side mirror 4;

[0060] Step 2.3: Adjust the deflection and pitch of the rear-side mirror 4 so that the laser spot of the laser to be frequency stabilized falls on the center of the reflecting surface of the first mirror M1 in the mode matching mirror group 6;

[0061] Step 2.4: Adjust the deflection and pitch of the nth mirror M in the mode matching mirror group 6 n so that the laser spot of the laser to be frequency stabilized emitted by it falls on the center of the incident end mirror S1 of the ultra-stable cavity 10;

[0062] Step 2.5: Turn on the excitation power supply of the ultra-stable cavity 10 so that it lights up and smoothly emits the reference laser;

[0063] Step 2.6: Adjust the nth mirror M in the mode matching mirror group 6 n so that the laser spot of the laser to be frequency stabilized output by the large laser gyro 1 and the laser spot of the reference laser output by the ultra-stable cavity 10 coincide at the first spot observation position P1;

[0064] Step 2.7: Adjust the first mirror M1 in the mode matching mirror group 6 so that the laser spot of the laser to be frequency stabilized output by the large laser gyro 1 and the laser spot of the reference laser output by the ultra-stable cavity 10 (i.e., the square cavity) coincide at the second spot observation position P2;

[0065] Step 2.8: Repeatedly adjust according to Step 2.6 and Step 2.7 until the laser spot of the laser to be frequency stabilized output by the large laser gyro 1 and the laser spot of the reference laser output by the ultra-stable cavity 10 coincide at both the first spot observation position P1 and the second spot observation position P2. At this time, the laser to be frequency stabilized is successfully injected into the ultra-stable cavity 10;

[0066] Step 2.9: Turn off the excitation power supply of the ultra-stable cavity 10, turn on the photodetector 9 and the data acquisition card 15, collect the light intensity signal transmitted by the ultra-stable cavity 10 on the computer 14, finely adjust the deflection and pitch of the front-side mirror 3, the rear-side mirror 4, and the mode matching mirror group 6, and manually adjust the piezoelectric ceramic to finely adjust the resonant cavity length of the large laser gyro 1 to be frequency-stabilized until the brightest transmitted light spot is observed on the cavity rear mirror 12, and at the same time the transmission peak of the light intensity signal in the computer 14 reaches the highest. At this time, the laser to be frequency-stabilized has achieved resonance in the ultra-stable cavity 10;

[0067] Step 3: Collect the error signal:

[0068] Step 3.1: Turn on the electro-optic modulator 2, the signal generator 5, the mixer 8, and the low-pass filter 13 in sequence. Use the signal generator 5 to generate two radio frequency signals with the same frequency of f 0, and apply them to the electro-optic modulator 2 and the mixer 8 respectively, and set the low-pass frequency of the low-pass filter 13 according to the radio frequency signal frequency;

[0069] Step 3.2: Convert the error signal output by the low-pass filter 13 into a voltage signal to drive the piezoelectric ceramic, use the piezoelectric ceramic to scan the cavity length of the large laser gyro 1 to be frequency-stabilized, and observe the error signal curve input by the low-pass filter 13 on the computer 14;

[0070] Step 4: Stop scanning the cavity length of the large laser gyro 1 to be frequency-stabilized, turn on the PID regulator 7, adjust the P, I, and D parameters of the PID regulator 7, observe the changes of the error signal and the transmitted light intensity signal on the computer 14. When the transmitted light intensity remains at the maximum value and the error signal disappears, it indicates that the frequency of the laser to be frequency-stabilized has been successfully locked at the resonant frequency of the ultra-stable cavity 10.

[0071] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0072] The above embodiments only represent several implementation manners of the present application. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several deformations and improvements can still be made, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the appended claims.

Claims

1. Laser gyro frequency stabilization system, characterized in that: include: The large laser gyro to be stabilized (1) outputs the laser to be stabilized; The electro-optic modulator (2) receives the laser to be stabilized, and the signal generator (5) is connected to the electro-optic modulator (2) and the mixer (8) through cables, and outputs two radio frequency signals with the same frequency, one radio frequency signal is used as a working signal to excite the electro-optic modulator (2), and the other radio frequency signal is used as a local oscillator signal in the mixing process of the mixer (8). Under the action of the radio frequency signal, the electro-optic modulator (2) applies two symmetrical sidebands to the frequency of the laser to be stabilized; Adjusting the optical path to transmit the laser light to be stabilized emitted by the electro-optic modulator (2) to the mode matching mirror group (6); The mode matching mirror group (6) adjusts the mode and phase of the laser to be stabilized and then outputs the laser to be stabilized to the ultra-stable cavity (10); The laser output from the ultrastable cavity (10) is incident on the photodetector (9), the photodetector (9) is connected to the mixer (8) and the data acquisition card (15), and the data acquisition card (15) is connected to the computer (14) via the network port / serial port; The mixer (8) is connected to a low-pass filter (13), and a mixing signal from the mixer (8) is low-pass filtered to extract an error signal; the low-pass filter (13) is respectively connected to a PID regulator (7) and a computer (14); the PID regulator (7) is connected to a piezoelectric ceramic fixedly connected to the rear side of one of the resonant cavity reflectors in the large laser gyroscope (1) to be stabilized, and the error signal from the low-pass filter (13) is converted into a voltage signal for driving the piezoelectric ceramic, and transmitted to the piezoelectric ceramic; the piezoelectric ceramic moves synchronously to drive the resonant cavity reflector fixedly connected to the piezoelectric ceramic, thereby achieving precise control of the resonant cavity length inside the large laser gyroscope (1) to be stabilized, thereby maintaining the stability of the output laser frequency of the large laser gyroscope (1) to be stabilized.

2. The laser gyro frequency stabilization system according to claim 1, characterized in that: The large laser gyroscope to be stabilized (1) is placed on an air-floating optical platform to reduce environmental vibrations; the ultra-stable cavity (10) is placed in a temperature-controlled cover (11) and fixedly connected to the air-floating optical platform; a data acquisition card (15) receives electrical signals from a photoelectric detector (9) and from a temperature sensor on the wall of the temperature-controlled cover (11), and transmits the electrical signals to a computer (14).

3. The laser gyro frequency stabilization system according to claim 2, characterized in that: The adjustment optical path comprises a front reflector (3) and a rear reflector (4); the reflective surface of the front reflector (3) receives the laser light to be frequency-stabilized emitted from the light aperture of the electro-optic modulator (2) and reflects it onto the reflective surface of the rear reflector (4); the rear reflector (4) reflects the laser light to be frequency-stabilized from the front reflector (3) to a mode matching mirror group (6); the front reflector (3) and the rear reflector (4) cooperate to enable the laser light to be frequency-stabilized to be smoothly incident into the ultra-stable cavity (10) and to coincide with the intrinsic light field in the ultra-stable cavity (10).

4. The laser gyro frequency stabilization system according to claim 1, 2 or 3, characterized in that: The mode matching mirror group (6) is composed of n reflectors, which are arranged in the order of the first reflector (M1), the nth reflector (M2), the nth reflector (M3), the nth reflector (M4), the nth reflector (M5), the nth reflector (M6), the nth reflector (M7), the nth reflector (M8), the nth reflector (M9), the nth reflector (M11), the nth reflector (M12), the nth reflector (M13), the nth reflector (M14), the nth reflector (M15), the nth reflector (M16), the nth reflector (M17), the nth reflector (M18), the nth reflector (M29), the nth reflector (M319) and the nth reflector (M411). n ), the n reflecting mirrors include a plane mirror and a spherical mirror coated with a high reflection film, and by adjusting the curvature radius of the spherical mirror in the mode matching mirror group (6) and the relative positions of the plane mirror and the spherical mirror, the laser to be frequency stabilized incident on the mode matching mirror group (6) can match the position and size of the intrinsic waist spot of the light beam when it is emitted into the ultra-stable cavity (10) after passing through the mode matching mirror group (6), and the mode and phase of the laser to be frequency stabilized are adjusted by the mode matching mirror group (6) so that the laser to be frequency stabilized can effectively resonate after entering the ultra-stable cavity (10).

5. The laser gyro frequency stabilization system according to claim 4, characterized in that: After receiving the radio frequency signal from the signal generator (5) and the electrical signal from the photodetector (9), the mixer (8) mixes the two and outputs the mixed signal to a low-pass filter (13).

6. The laser gyro frequency stabilization system according to claim 5, characterized in that: The main material of the ultra-stable cavity (10) is microcrystalline glass. The ultra-stable cavity (10) has a square transmission light path inside. Four high-reflectivity reflectors are used to realize the reflection of laser in the square transmission light path. Two adjacent reflectors of the four reflectors are installed with piezoelectric ceramics on the outside of the cavity. The other two reflectors are an incident end reflector (S1) and an output end reflector (S2). The incident end reflector (S1) and the output end reflector (S2) serve as an inlet and an output outlet for external laser injection. The laser to be stabilized after the mode and phase are adjusted by the mode matching mirror group (6) is incident on the incident end reflector (S1). The incident end reflector (S1) transmits the laser to be stabilized into the ultra-stable cavity (10). The output end reflector (S2) transmits the laser to be stabilized to the reflection surface of the cavity rear reflector (12). The cavity rear reflector (12) then reflects the laser to be stabilized to the photosensitive surface of the photodetector (9).

7. The laser gyro frequency stabilization system according to claim 6, characterized in that: The electrical signal generated by the photoelectric detector (9) includes three parts: a DC term reflecting the respective strengths of the carrier and the sideband, an interference term between the carrier and the sideband, and an interference term between the two sidebands, wherein the interference term between the carrier and the sideband includes error information between the frequency of the laser to be stabilized and the resonance peak of the ultra-stable cavity (10).

8. The laser gyro frequency stabilization system according to claim 7, characterized in that: The mixing signal output by the mixer (8) includes three parts: the amplitude of the interference term between the carrier and the sideband, the first-time frequency term, and the second-time frequency term, wherein the amplitude of the interference term between the carrier and the sideband is used as an error signal.

9. The laser gyro frequency stabilization system according to claim 8, characterized in that: The computer (14) has a built-in data reading and visualization program and a driver adapted to the temperature regulator on the wall of the temperature control cover (11) to achieve the following purposes: The electrical signal from the photodetector (9) is read out to observe the resonance inside the ultrastable cavity (10). When resonance occurs inside the ultrastable cavity (10), an obvious light intensity transmission peak will be seen on the computer (14). Correspondingly, the transmitted light emitted by the incident end reflector (S1) of the ultrastable cavity (10) will coherently cancel the primary reflected light of the laser to be stabilized on the incident end reflector (S1), which is manifested as a depression in the light intensity signal. receiving the error signal processed by the low-pass filter (13) and displaying its curve; The temperature inside the ultra-stable cavity (10) is monitored and adjusted to keep it stable.

10. The frequency stabilization method of the laser gyro frequency stabilization system as claimed in claim 1, characterized in that: include: Step 1: Turn on the excitation power supply of the large laser gyro (1) to be stabilized, so that it lights up and smoothly emits the laser to be stabilized; Step 2: by adjusting the optical path, the laser to be stabilized emitted from the large laser gyro to be stabilized (1) is injected into the ultra-stable cavity (10) and resonates, the adjustment optical path comprises a front side reflector (3) and a rear side reflector (4), the reflective surface of the front side reflector (3) receives the laser to be stabilized emitted from the clear aperture of the electro-optic modulator (2), and reflects it to the reflective surface of the rear side reflector (4), and the rear side reflector (4) reflects the laser to be stabilized from the front side reflector (3) to the mode matching mirror group (6); Step 2.1: adjusting the position of the front reflector (3) so that the laser spot to be stabilized emitted from the clear aperture of the electro-optic modulator (2) falls on the center of the reflective surface of the front reflector (3); Step 2.2: Adjust the position of the rear reflector (4) so ​​that the laser spot to be stabilized reflected by the front reflector (3) falls on the center of the reflective surface of the rear reflector (4); Step 2.3: Adjust the deflection and pitch of the rear reflector (4) so ​​that the laser spot to be stabilized falls on the center of the reflective surface of the first reflector (M1) in the mode matching mirror assembly (6); Step 2.4: Adjust the nth reflector (M n ) so that the emitted laser spot to be stabilized falls on the center of the incident end reflector (S1) of the ultra-stable cavity (10); Step 2.5: Turn on the excitation power supply of the ultrastable cavity (10) to light it up and emit the reference laser smoothly; Step 2.6: Adjust the nth reflector (M n ), so that the light spots of the laser to be stabilized output by the large laser gyro to be stabilized (1) and the reference laser output by the ultra-stable cavity (10) overlap at the first light spot observation position (P1); Step 2.7: adjusting the first reflector (M1) in the mode matching mirror assembly (6) so that the light spots of the laser to be stabilized outputted by the large laser gyro to be stabilized (1) and the reference laser outputted by the ultrastable cavity (10) at the second light spot observation position (P2) overlap; Step 2.8: Repeat the adjustment according to steps 2.6 and 2.7 until the laser to be stabilized outputted by the large laser gyro to be stabilized (1) and the reference laser outputted by the ultra-stable cavity (10) overlap at the first spot observation position (P1) and the second spot observation position (P2), and at this time, the laser to be stabilized is successfully injected into the ultra-stable cavity (10); Step 2.9: Turn off the excitation power supply of the ultrastable cavity (10), turn on the photodetector (9) and the data acquisition card (15), collect the light intensity signal transmitted from the ultrastable cavity (10) on the computer (14), fine-tune the deflection and pitch of the front side reflector (3), the rear side reflector (4), and the mode matching mirror group (6), and adjust the piezoelectric ceramic to achieve fine-tuning of the resonant cavity length of the large laser gyroscope (1) to be stabilized, until the brightest transmission light spot is observed on the rear reflector (12) of the cavity, and at the same time, the transmission peak of the transmission light intensity signal in the computer (14) reaches the highest, and at this time, the laser to be stabilized has achieved resonance in the ultrastable cavity (10); Step 3: Collect the error signal: Step 3.1: Turn on the electro-optic modulator (2), signal generator (5), mixer (8), and low-pass filter (13) in sequence, and use the signal generator (5) to generate two frequencies of f 0, respectively applied to the electro-optic modulator (2) and the mixer (8), and setting the low-pass frequency of the low-pass filter (13) according to the frequency of the radio frequency signal; Step 3.2: The error signal output by the low-pass filter (13) is converted into a voltage signal for driving the piezoelectric ceramic, and the piezoelectric ceramic is used to scan the cavity length of the large laser gyro (1) to be stabilized, and the error signal curve input by the low-pass filter (13) is observed on a computer (14); Step 4: Stop scanning the cavity length of the large laser gyro (1) to be stabilized, turn on the PID regulator (7), adjust the P, I, and D parameters of the PID regulator (7), and observe the changes in the error signal and the transmitted light intensity signal on the computer (14). When the transmitted light intensity remains at the maximum value and the error signal disappears, it means that the frequency of the laser to be stabilized has been successfully locked on the resonant frequency of the ultra-stable cavity (10).

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