Laser sweep frequency quantity control and measuring device and method of locking confocal f-p cavity

Abstract

The application discloses a kind of based on locking confocal F-P cavity laser sweep quantity accurate control and determination device and method.The device is by the wave division multiplexing of low-frequency electro-optic phase modulator to realize the locking of confocal F-P cavity length relative to helium-neon laser and the locking of tunable laser frequency relative to confocal F-P cavity at the beginning and end of scanning;Before laser scanning, confocal F-P cavity free spectral range FSR is measured by high-frequency electro-optic phase modulator with microwave frequency reference, and the number of scanning resonance peaks between two laser frequency lockings is counted to realize sweep quantity control and determination.The application realizes the control and measurement of laser frequency scanning quantity in frequency scanning interference method, improves the stability of confocal F-P cavity FSR as scanning frequency reference, and realizes high-precision measurement of FSR with microwave frequency reference, so as to improve the frequency scanning measurement accuracy, so as to improve the accuracy of sweep absolute ranging.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of laser interferometry, and particularly relates to a laser frequency sweeping quantity control and measurement device and method based on a locking confocal F-P cavity. BACKGROUND

[0002] The laser frequency sweeping interferometric absolute distance measurement method is widely applied in the field of absolute distance interferometry due to the advantages of large measurement range, high relative accuracy and no measurement ambiguity range. The accurate measurement of the laser frequency sweeping quantity is one of the key technologies of the laser frequency sweeping interferometric distance measurement. The traditional method for measuring the laser frequency sweeping quantity is to directly measure the laser frequency by using a wavelength meter or a spectrometer. However, the measurement accuracy is low due to the resolution of the optical frequency measurement instrument, and the measurement speed cannot meet the requirement of the fast laser frequency sweeping. Therefore, an additional frequency reference is generally needed to measure the sweeping quantity, such as an auxiliary interferometer, a femtosecond optical frequency comb or a confocal F-P cavity. In the method using the auxiliary interferometer, the long optical fiber for generating a stable optical path difference is easily affected by the environmental temperature, vibration and fiber dispersion, thereby reducing the measurement accuracy of the sweeping quantity. Although the method using the femtosecond optical frequency comb can provide high-precision optical frequency measurement, the existing femtosecond optical frequency comb is high in cost and large in space, and is difficult to be widely applied in the measurement of the sweeping quantity in the industrial field. The confocal F-P cavity can take the free spectral range (FSR) related to the cavity length as the equidistant frequency reference, and is lower in cost. Therefore, the confocal F-P cavity is widely applied in the laser frequency sweeping interferometric absolute distance measurement. However, the confocal F-P cavity is also easily affected by the environmental factors such as temperature, thereby reducing the measurement accuracy of the sweeping quantity and the absolute distance measurement accuracy. Therefore, improving the stability of the confocal F-P cavity and the measurement accuracy of the FSR of the confocal F-P cavity are the key problems to be solved in the laser frequency sweeping interferometric absolute distance measurement method based on the confocal F-P cavity. SUMMARY

[0003] In view of the problems in the prior art, the application discloses a laser frequency sweeping quantity control and measurement method based on a locking confocal F-P cavity, which solves the problem of high-precision measurement of the laser frequency sweeping quantity based on the confocal F-P cavity, and can be widely applied in the field of laser frequency sweeping interferometric distance measurement.

[0004] The application adopts an electro-optic phase modulator, simultaneously constructs a confocal F-P cavity locking subsystem and an FSR accurate measurement subsystem, improves the stability of the confocal F-P cavity and the measurement accuracy of the FSR of the confocal F-P cavity, and thereby improves the control and measurement accuracy of the laser frequency sweeping quantity.

[0005] The present application achieves the above-mentioned purpose by adopting the following technical scheme:

[0006] The present application achieves the above-mentioned purpose by adopting the following technical scheme:

[0007] The device comprises a reference helium-neon laser, a fiber beam splitter, a first fiber collimator, an optical isolator, a first dichroic mirror, a polarizer, an inverted beam expander, a low-frequency spatial electro-optic phase modulator, a beam splitter, a high-frequency fiber electro-optic phase modulator, a second fiber collimator, a polarization beam splitter, a quarter-wave plate, a confocal F-P cavity, a second dichroic mirror, a first filter, a first convex lens, a first photodetector, a second filter, a second convex lens, and a second photodetector.

[0008] The measurement laser emitted by the tunable laser as the object to be measured is first split into two beams by the fiber beam splitter and is incident into the first fiber collimator and the high-frequency fiber electro-optic phase modulator respectively, one of the measurement lasers is converted into a second spatial light by the second fiber collimator after passing through the high-frequency fiber electro-optic phase modulator and is incident into the beam splitter to be reflected, and the other measurement laser is converted into a first spatial light by the first fiber collimator and is incident into the first dichroic mirror to be transmitted.

[0009] The original reference laser emitted by the reference helium-neon laser is incident into the first dichroic mirror to be reflected after passing through the optical isolator, and the reference laser reflected by the first dichroic mirror and the measurement laser transmitted by the first dichroic mirror are sequentially converted into a polarized state by the polarizer, are reduced in spot size by the inverted beam expander, and are sinusoidal phase modulated by the spatial electro-optic phase modulator to generate optical signals with equal frequency interval sidebands.

[0010] The measurement laser and the reference laser transmitted by the beam splitter and the measurement laser reflected by the beam splitter are all reflected by the polarization beam splitter and then transmitted through the quarter-wave plate and enter the confocal F-P cavity, and are reflected back and forth inside the confocal F-P cavity multiple times to attenuate the light intensity, the returned reference laser after the back-and-forth reflection inside the confocal F-P cavity is transmitted through the quarter-wave plate and the polarization beam splitter in reverse order and is incident into the second dichroic mirror to be transmitted and reflected, the reference laser part in the light reflected by the second dichroic mirror is filtered by the second filter of the same waveband as the original reference laser emitted by the tunable laser and is condensed by the second convex lens and is then detected and received by the second photodetector, and the measurement laser part in the light transmitted by the second dichroic mirror is filtered by the first filter of the same waveband as the original reference laser emitted by the tunable laser and is condensed by the first convex lens and is then detected and received by the first photodetector.

[0011] The object to be measured of the present application is a tunable laser, and the present application is used for accurately measuring the sweep frequency range of the tunable laser.

[0012] The reference helium-neon laser is a frequency stabilized laser.

[0013] The tunable laser and the reference helium-neon laser emit laser beams of different wave bands, the second filter has a filtering wave band identical to that of the reference helium-neon laser, and the first filter has a filtering wave band identical to that of the tunable laser.

[0014] The confocal F-P cavity is provided with a PZT piezoelectric ceramic for controlling the cavity length of the confocal F-P cavity.

[0015] The device further comprises a first high-frequency amplifier, a signal source, a phase shifter, a frequency mixer, a low-pass filter, a second high-frequency amplifier, a first low-frequency amplifier, a second low-frequency amplifier and a signal acquisition and processing system.

[0016] The signal source is electrically connected to the first high-frequency amplifier and the high-frequency fiber electro-optic phase modulator, and is connected to the phase shifter and the frequency mixer.

[0017] The signal acquisition and processing system is electrically connected to the first photoelectric detector and the second photoelectric detector, and is electrically connected to the second low-frequency amplifier and the PZT piezoelectric ceramic.

[0018] The signal acquisition and processing system comprises an AD conversion module, a DA conversion module and an FPGA, the AD conversion module and the DA conversion module are connected to the FPGA, the low-pass filter, the first photoelectric detector and the second photoelectric detector are connected to the AD conversion module, the DA conversion module is connected to the second low-frequency amplifier, the first low-frequency amplifier and the second high-frequency amplifier, and the FPGA can be connected to an external computer.

[0019] In a specific implementation, the two laser beams emitted between the tunable laser and the reference helium-neon laser are separated and extracted by wavelength division, the two laser beams emitted by the tunable laser through the fiber beam splitter are phase-modulated differently, and are separated and extracted by frequency division.

[0020] II. A laser sweep frequency quantity accurate control and measurement method

[0021] 1) The reference helium-neon laser is turned on, and the tunable laser is not turned on.

[0022] The reference laser emitted by the reference helium-neon laser is detected by a second photodetector to form a locking cavity beat signal, the locking cavity beat signal is input into a signal acquisition processing system to obtain a locking cavity error signal, the locking cavity error signal is fed back to a PZT piezoelectric ceramic according to PID control to adjust the cavity length of the confocal F-P cavity, so that the locking cavity error signal is zero, and then the cavity length of the confocal F-P cavity is matched to the stable working frequency of the reference laser emitted by the reference helium-neon laser, and then the cavity length of the fixed confocal F-P cavity is locked;

[0023] 2) The reference helium-neon laser and the tunable laser are both turned on and work:

[0024] The measurement laser emitted by the tunable laser and modulated by the first fiber collimator and the low-frequency spatial electro-optic phase modulator is detected by the first photodetector to form a low-frequency beat signal, the low-frequency beat signal is input into a signal acquisition processing system to obtain a frequency stabilization error signal, the frequency stabilization error signal is fed back to the tunable laser according to PID control to adjust the frequency of the measurement laser emitted by the tunable laser, so that the frequency stabilization error signal is zero, and then the frequency of the measurement laser emitted by the tunable laser is locked;

[0025] The phase of the beat signal is adjusted by the phase shifter to be opposite to that of the local oscillator signal The two error signal curves are input into two PID controllers, one is used for locking the cavity length of the confocal F-P cavity relative to the frequency stabilized reference laser, and the other is used for locking the laser frequency of the tunable laser relative to the confocal F-P cavity.

[0026] In the scheme, a strict front-rear relationship is required between the two, the locking of the cavity length of the confocal F-P cavity is realized by using the locking cavity error signal first, and then the locking of the laser frequency of the tunable laser ECDL is realized by using the frequency stabilization error signal.

[0027] 3) The reference helium-neon laser and the tunable laser are both turned on and work:

[0028] The measurement laser emitted by the tunable laser and modulated by the high-frequency fiber electro-optic phase modulator is detected by the first photodetector to form a high-frequency beat signal, the FSR measurement error signal is obtained according to the high-frequency beat signal, the FSR measurement error signal is input into a signal acquisition processing system and fed back to the high-frequency fiber electro-optic phase modulator according to PID control to adjust the modulation frequency of the high-frequency fiber electro-optic phase modulator, so that the FSR measurement error signal is zero, and at this time, the frequency spacing between adjacent laser frequency sidebands generated by the high-frequency fiber electro-optic phase modulator is the FSR free spectral range of the confocal F-P cavity;

[0029] The measurement of FSR is carried out by modulating the frequency of the laser with a high frequency sinusoidal phase modulator, when the frequency of the laser is close to the resonant frequency of the confocal F-P cavity, the sidebands are distributed around different resonant frequencies. The frequency interval between the resonant peaks is consistent and equal to FSR, and the frequency interval between the sidebands is consistent and equal to the modulation frequency.

[0030] The detected frequency equal to the high frequency beat signal of FSR, through the analog mixer and the signal source to give another phase shift after the local oscillator signal downmix, two signal phase difference is zero, the error signal of FSR measurement is obtained. When the tunable laser is locked to slightly deviate from the resonant frequency, the scanning modulation frequency obtains the error signal of FSR measurement, and the center zero point corresponds to the modulation frequency of the FSR of the confocal F-P cavity, so as to realize the accurate measurement of FSR.

[0031] 4) the reference helium neon laser is turned on, and the tunable laser is turned on:

[0032] The frequency of the measurement laser emitted by the unlocked tunable laser is unlocked, the frequency of the tunable laser is scanned, and the measurement laser emitted by the tunable laser is detected by the first photoelectric detector to form a detection signal; in the frequency scanning process, the number of scanning resonant peaks in the detection signal and the corresponding wave band of the tunable laser is counted, and then multiplied by the value of the free spectral range FSR of the confocal F-P cavity obtained by the measurement in 3), to obtain the scanning frequency range Δf of the tunable laser, and finally complete the control of laser sweep frequency and the accurate measurement of sweep frequency.

[0033] The lock cavity error signal in the 1) is processed according to the following formula:

[0034]

[0035] K H = 2H1E H 2 J0(β H )J1(β H )

[0036] Wherein, S 锁腔 (h) represents the lock cavity error signal for locking the cavity length h of the confocal F-P cavity obtained by demodulating the interference signal with a frequency of ω m1 emitted by the helium neon laser and generated at the second photoelectric detector, ω m1 is also the modulation frequency of the low frequency spatial electro-optic phase modulator, and ω Hrepresents the frequency of the reference laser emitted by the reference helium-neon laser, h represents the cavity length of the confocal F-P cavity, Re{} represents the real part of a complex number, F() and F*() represent the reflection coefficient of the confocal F-P cavity and its conjugate, exp() represents the exponential function with the natural constant e as the base, i represents the imaginary unit, represents the phase difference between the locking cavity beat frequency signal and the corresponding local oscillator signal, K H represents the maximum amplitude of the locking cavity error signal in the ideal state, H1 represents the electronic scaling multiple of the pre-processing of the locking cavity beat frequency signal, E H , β H respectively represent the laser amplitude and the modulation depth of the reference laser emitted by the reference helium-neon laser into the low-frequency spatial electro-optic phase modulator, J0(), J1() respectively represent the 0th and 1st order Bessel functions of the first kind.

[0037] The frequency stabilization error signal in the 2) is obtained by processing according to the following formula:

[0038]

[0039] K E1 = 2H E1 E E1 2 J0(β E1 )J1(β E1 )

[0040] Wherein, S 稳频 (ω E ) represents the frequency stabilization error signal for stabilizing the laser frequency ω m1 of the tunable laser obtained by demodulating the interference signal generated by the laser emitted by the tunable laser at the first photodetector with a frequency of ω E , ω m1 is also the modulation frequency of the low-frequency spatial electro-optic phase modulator, ω E represents the laser frequency of the tunable laser, h represents the cavity length of the confocal F-P cavity, K E1 represents the maximum amplitude of the frequency stabilization error signal in the ideal state, Re{} represents the real part of a complex number, F() and F*() represent the reflection coefficient of the confocal F-P cavity and its conjugate, exp() represents the exponential function with the natural constant e as the base, i represents the imaginary unit, represents the phase difference between the low-frequency beat frequency signal and the corresponding local oscillator signal, H E1 represents the corresponding electronic scaling multiple, E E1 , β E1 respectively represent the laser amplitude and the modulation depth of the measurement laser emitted by the tunable laser into the low-frequency spatial electro-optic phase modulator, J0(), J1() are respectively the 0th and 1st order Bessel functions of the first kind.

[0041] The FSR measurement error signal in 3) is processed according to the following formula:

[0042]

[0043] K E2 = 2H E2 E E2 2 J0(β E2 )J1(β E2 )

[0044] Wherein, S FSR (ω m2 ) represents the FSR measurement error signal for measuring the free spectral range of the confocal F-P cavity obtained by demodulating the interference signal with frequency ω m2 emitted by the tunable laser at the first photodetector, ω m2 is also the modulation frequency of the high-frequency fiber electro-optic phase modulator, h represents the cavity length of the confocal F-P cavity, Re{} represents the real part of a complex number, F() and F*() represent the reflection coefficient of the confocal F-P cavity and its conjugate, exp() represents the exponential function with base e, i represents the imaginary unit, represents the phase difference between the beat signal of the FSR measurement and the corresponding local oscillator signal, K E2 represents the maximum amplitude of the FSR measurement error signal in the ideal state, H E2 represents the electronic scaling multiple of the corresponding signal, E E2 , β E2 represents the amplitude and modulation depth of the ECDL laser entering the high-frequency fiber electro-optic phase modulator. J0(), J1() are the 0th and 1st order Bessel functions of the first kind, respectively.

[0045] The scanning frequency range Δf of the tunable laser in 4) is calculated according to the following formula:

[0046] Δf = m·FSR

[0047] Wherein, m is the number of scanning resonance peaks, and FSR is the free spectral range of the confocal F-P cavity.

[0048] The above process needs to be executed in a certain order. First, lock the cavity to keep the FSR stable; then complete the frequency stabilization of the tunable laser, and use the high-frequency EOM to measure the FSR; finally, unlock the laser frequency and start the frequency scanning again. The number of resonance peaks in the scanning process is counted, and then multiplied by the measured FSR value to obtain the scanning frequency range Δf.

[0049] The device of the present application realizes the locking of the cavity length of the confocal F-P cavity relative to the 633nm helium-neon laser and the locking of the laser frequency of the 780nm tunable laser relative to the confocal F-P cavity at the beginning and end of the scanning by the wavelength division multiplexing of a low-frequency electro-optic phase modulator. In addition, the measurement of the free spectral range (FSR) of the confocal F-P cavity is realized by another high-frequency electro-optic phase modulator with a microwave frequency reference before the laser scanning. Finally, the sweep range control and measurement are realized by counting the number of resonance peaks scanned between the two laser frequency lockings.

[0050] The present application has the beneficial effects of:

[0051] The present application, based on the optical wavelength division multiplexing and frequency division multiplexing technology, modulates the frequency-stabilized reference laser and the tunable laser by an electro-optic phase modulator and then enters the same confocal F-P cavity. By extracting the respective error signals, not only the locking of the cavity length of the confocal F-P cavity and the measurement of its FSR are realized, but also the precise control and measurement of the sweep range of the tunable laser are realized. At the same time, the utilization rate of optical devices is improved and the cost is reduced.

[0052] The present application realizes the control and measurement of the laser frequency scanning range in the frequency scanning interferometry, improves the stability of the FSR of the confocal F-P cavity as the scanning frequency reference, and realizes the high-precision measurement of the FSR with a microwave frequency reference, thereby improving the measurement precision of the frequency scanning range. In this way, the precision of the absolute range measurement by frequency sweeping can be improved. BRIEF DESCRIPTION OF DRAWINGS

[0053] Figure 1 Fig. 1 is the schematic diagram of the device and method of the present application.

[0054] Figure 2 Fig. 2 is the schematic diagram of the locking cavity error signal, the frequency stabilization error signal and the FSR measurement error signal, wherein (a) represents the locking cavity error signal, (b) represents the frequency stabilization error signal, and (c) represents the FSR measurement error signal.

[0055] In the diagram: 1. Tunable laser; 2. Reference helium-neon laser; 3. Fiber beam splitter; 4. First fiber collimator; 5. Optical isolator; 6. First dichroic mirror; 7. Polarizer; 8. Inverted beam expander; 9. Low-frequency spatial electro-optic phase modulator; 10. Beam splitter; 11. High-frequency fiber electro-optic phase modulator; 12. Second fiber collimator; 13. Polarizing beam splitter; 14. Quarter-wave plate; 15. Confocal FP cavity; 16. PZT piezoelectric ceramic. 17. Porcelain, 18. Second dichroic mirror, 19. First filter, 20. First convex lens, 21. First photodetector, 22. Second filter, 23. Second convex lens, 24. Second photodetector, 25. First high-frequency amplifier, 26. Signal source, 27. Phase shifter, 28. Mixer, 29. Low-pass filter, 30. Second high-frequency amplifier, 31. First low-frequency amplifier, 32. Second low-frequency amplifier, 33. Signal acquisition and processing system. Detailed Implementation

[0056] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0057] like Figure 1 As shown, the specific embodiments of the present invention and the implementation process of achieving laser frequency sweep range measurement by step are as follows:

[0058] The first step is to lock the cavity length of the confocal FP cavity 15 to improve its FSR stability:

[0059] First, the 633nm reference helium-neon laser 2 is reflected by the first dichroic mirror 6 after passing through the optical isolator 5. The reflected light is converted into S-polarized light by the polarizer 7 and then narrowed by the inverted beam expander 8. Finally, it undergoes sinusoidal phase modulation by the low-frequency spatial electro-optic phase modulator 9, with a modulation frequency of ω. m1 The modulated laser frequency includes the laser fundamental frequency (ω). H ) and the sideband frequencies (ω) centered thereon. H +a·ω m1 (where a is a natural number not equal to 0).

[0060] The sinusoidal phase-modulated signal of the low-frequency spatial electro-optic phase modulator 9 is generated internally by the FPGA of the signal processing system 32, output by the DA converter, and amplified by the amplifier 29. The modulation frequency is less than the full width at half maximum (FWHM) of the confocal FP cavity 15, and the modulation depth is less than 1 rad. Therefore, only the 0th and ±1st order sidebands are considered, as shown below: E HEOM ≈E H [J0(β H )·exp(iω H t)+J1(β H )·exp[i(ωH +ω m1 )t]-J1(β H )·exp[i(ω H -ω m1 )t]] (1)

[0062] wherein E HEOM represents the amplitude of the reference laser emitted by the helium-neon laser 2 after phase modulation, ω H , E H , β H are the laser frequency, the laser amplitude and the modulation depth of the reference laser emitted by the reference helium-neon laser 2 into the low-frequency spatial electro-optical phase modulator 9, ω m1 is the modulation frequency of the low-frequency spatial electro-optical phase modulator 9, J0(), J1() are the 0th and 1st order Bessel functions of the first kind, exp() represents the exponential function with the natural constant e as the base, i represents the imaginary unit, and t represents time.

[0063] The modulated laser is transmitted by the beam splitter 10 and irradiates the polarizing beam splitter 13. After being reflected by the polarizing beam splitter 13, the laser passes through the quarter-wave plate 14 and enters the confocal F-P cavity 15. The cavity length of the confocal F-P cavity 15 is controlled by the PZT piezoelectric ceramic 16 mounted on one of the cavity mirrors. The laser is reflected back and forth in the confocal F-P cavity 15, and the amplitude is attenuated and returned to the polarizing beam splitter 13. The quarter-wave plate 14 is mounted between the confocal F-P cavity 15 and the polarizing beam splitter 13. After the laser passes back and forth twice, the polarization state is converted from S polarization to P polarization. Therefore, the light returned from the confocal F-P cavity 15 is transmitted at the polarizing beam splitter 13, reflected by the second dichroic mirror 17, further filtered by the second filter 21 of the 633 nm waveband and condensed by the second convex lens 22, and then interferes at the second photodetector 23 and is detected.

[0064] In the following step, both the helium-neon laser and the tunable laser are turned on. The light from the low-frequency spatial electro-optical phase modulator 9 is transmitted at the beam splitter 10, and the light from the second fiber collimator 12 is reflected at the beam splitter 10. The two lights are combined and irradiate the subsequent optical path together.

[0065] In the optical path, the reflection coefficient of the confocal F-P cavity 15 to different laser frequencies is represented as:

[0066]

[0067] Where F(ω,h) represents the reflection coefficient when a laser with frequency ω is incident into a confocal FP cavity with cavity length h, h represents the cavity length of the confocal FP cavity 15, r represents the reflectivity of its two cavity mirrors (assuming they are the same), FSR represents its free spectral range, ω represents the laser frequency incident into the confocal FP cavity 15, n represents the refractive index of the air inside the cavity, and c represents the speed of light in a vacuum.

[0068] The cavity locking beat frequency signal detected by the second photodetector 23 is sampled by the AD of the signal processing system 32 and mixed with the local oscillator signal in the FPGA through a mixer. After passing through a low-pass filter, a cavity locking error signal is generated. The cavity locking error signal exhibits bipolar, linear, and center-zero crossing characteristics near the resonant frequency of the confocal FP cavity 15. It serves as the error input of the PID control module inside the FPGA for cavity length locking control.

[0069] Keeping the helium-neon laser frequency and spatial light phase modulation frequency fixed, and using the cavity length h of the confocal FP cavity 15 as the variable for scanning and control, the cavity-locking error signal is as follows: Figure 2 As shown in (a), it is represented as:

[0070]

[0071] K H =2H1E H 2 J0(β H )J1(β H )

[0072] Among them, S 锁腔 (h) represents the laser emitted by the helium-neon laser 2 and generated at the second photodetector 23 with a frequency of ω. m1 The cavity-locking error signal ω obtained by demodulating the interference signal is used to lock the cavity length h of the confocal FP cavity 15. m1 Also the modulation frequency of the low-frequency spatial electro-optic phase modulator 9, ω H Let represent the frequency of the reference laser emitted by reference helium-neon laser 2, h be the cavity length of confocal FP cavity 15, Re{} denotes taking the real part of a complex number, F() and F*() represent the reflection coefficients and their conjugates of confocal FP cavity 15, exp() denotes an exponential function with the natural constant e as the base, and i represent the imaginary unit. K represents the phase difference between the locked cavity beat frequency signal and the corresponding local oscillator signal. H H1 represents the maximum amplitude of the lock cavity error signal under ideal conditions, and E represents the electronic scaling factor of the lock cavity beat frequency signal preprocessing. H β HJ0() and J1() represent the laser amplitude and modulation depth of the reference laser emitted by the reference helium-neon laser 2 when it enters the low-frequency spatial electro-optic phase modulator 9, respectively. J0() and J1() represent the 0th and 1st order Bessel functions of the first kind, respectively.

[0073] The second step is to lock the laser frequency of the tunable laser 1 before starting the scan:

[0074] The laser emitted by the 780nm tunable laser 1 is split into two beams by the fiber beam splitter 3. One beam is converted into spatial light by the first fiber collimator 4 and transmitted through the first dichroic mirror 6. It is then combined with the 633nm laser emitted by the reference laser 2. After passing through the phase modulation and confocal FP cavity 15, the two beams are transmitted through the second dichroic mirror 17 and separated from the 633nm laser, thus achieving wavelength division multiplexing of the optical path. The laser transmitted through the second dichroic mirror 17 is further filtered by the first 780nm filter 18 and focused by the first convex lens 19. It then interferes with and is detected by the first photodetector 20.

[0075] The low-frequency beat frequency signal detected by the first photodetector 20 is also sampled by the AD converter of the signal processing system 32 and mixed with the low-frequency local oscillator signal in the FPGA via a mixer. After passing through a low-pass filter, a frequency stabilization error signal is generated, which is then used by another PID controller for ECDL laser frequency locking control. The modulation frequency and cavity length are kept fixed, and the variable for scanning and control is the laser frequency ω of the tunable laser 1. E The frequency stability error signal is as follows: Figure 2 As shown in (b), it is represented as follows:

[0076]

[0077] K E1 =2H E1 E E1 2 J0(β E1 )J1(β E1 )

[0078] Among them, S 稳频 (ω E The frequency ω represents the frequency generated at the first photodetector 20 by the laser emitted by the tunable laser 1. m1 The interference signal demodulation obtained is used to stabilize the tunable laser 1 laser frequency ω. E The frequency stability error signal, ω m1 Also the modulation frequency of the low-frequency spatial electro-optic phase modulator 9, ω E The laser frequency of tunable laser 1 is represented by h, the cavity length of confocal FP cavity 15 is represented by K. E1represents the maximum amplitude of the frequency stabilization error signal in ideal state, Re{} represents taking the real part of a complex number, F() and F*() represent the reflection coefficient of the confocal F-P cavity and its conjugate, exp() represents the exponential function with the natural constant e as the base, i represents the imaginary unit, represents the phase difference between the low-frequency beat signal and the corresponding local oscillator signal, H E1 represents the corresponding electronic scaling factor, E E1 , β E1 respectively represent the laser amplitude and the modulation depth of the measurement laser emitted by the tunable laser 1 into the low-frequency spatial electro-optic phase modulator 9, J0(), J1() respectively represent the 0th and 1st order Bessel functions of the first kind.

[0079] The third step realizes the measurement of the FSR of the confocal F-P cavity 15:

[0080] The other beam of laser split by the fiber beam splitter 3 passes through the high-frequency fiber electro-optic phase modulator 11, is converted into spatial light by the second fiber collimator 12, is reflected at the beam splitter 10 and is combined with the transmitted part of the laser modulated by the low-frequency spatial electro-optic phase modulator 9, enters the confocal F-P cavity 15 together, is transmitted at the second dichroic mirror 17 again and finally interferes at the first photodetector 20 and is detected.

[0081] The modulation signal of the high-frequency fiber electro-optic phase modulator 11 is provided by the signal source 25 and is amplified by the high-frequency amplifier 24, the modulation frequency is close to the FSR of the confocal F-P cavity 15 and is much larger than the full width at half maximum of the confocal F-P cavity 15, that is, much larger than the modulation frequency of the low-frequency spatial electro-optic phase modulator 9. Therefore, the two beat signals of different frequencies detected by the first photodetector 20 can be separated by a filter, so as to realize the radio frequency division multiplexing of part of the optical path.

[0082] The detected high-frequency beat signal is mixed with another signal split by the signal source 25 and phase-shifted by the phase shifter 26, and the FSR measurement error signal is generated after being filtered by the low-pass filter 28. In this part, by scanning the output frequency of the signal source 25 near the FSR, the modulation frequency corresponding to the center zero point of the FSR measurement error signal is the FSR of the confocal F-P cavity 15, so as to complete the high-precision measurement of the FSR based on the microwave frequency. The laser frequency is slightly detuned and locked, the confocal F-P cavity length is locked, and the variable to be scanned is the modulation frequency ω m2 of the high-frequency fiber electro-optic phase modulator 11. Figure 2 (c) as shown, which is:

[0083]

[0084] KE2 = 2H E2 E E2 2 J0(β E2 )J1(β E2 )

[0085] where S FSR (ω m2 ) represents the FSR measurement error signal obtained by demodulating the interference signal with frequency ω m2 at the first photodetector 20, which is generated by the laser emitted by the tunable laser 1, ω m2 is also the modulation frequency of the high-frequency fiber electro-optic phase modulator 11, h represents the cavity length of the confocal F-P cavity, Re{} represents the real part of a complex number, F() and F*() represent the reflection coefficient of the confocal F-P cavity and its conjugate, exp() represents the exponential function with base e, i represents the imaginary unit, represents the phase difference between the beat signal of the FSR measurement and the corresponding local signal, K E2 represents the maximum amplitude of the FSR measurement error signal in the ideal state, H E2 represents the electronic scaling multiple of the corresponding signal, E E2 , β E2 represents the amplitude and modulation depth of the ECDL laser entering the high-frequency fiber electro-optic phase modulator 11. J0(), J1() represent the 0th and 1st order Bessel functions of the first kind, respectively.

[0086] Fourthly, the laser frequency of the tunable laser is controlled to be scanned, and the laser scanning amount is measured:

[0087] After the tunable laser 1 is unlocked, the frequency scanning is started, and when the frequency scanning is finished, it is locked again to another resonance frequency of the confocal F-P cavity 15.

[0088] During the scanning process, the number of 780nm waveband laser scanning through the resonance peak detected by the first photodetector 20 is counted, and multiplied by the value of the FSR measured in the third step to obtain the frequency range Δf of the scanning, and finally the control of the laser frequency scanning and the accurate measurement of the scanning amount are completed, which is represented as:

[0089] Δf = m·FSR (6)

[0090] where m is the number of scanning through the resonance peak, and FSR is the free spectral range of the confocal F-P cavity 15.

[0091] In the embodiment, the laser of the 780nm waveband tunable laser and the laser emitted by the 633nm waveband frequency stabilized reference laser are multiplexed according to the spectrum, the beam combination and separation of the two lasers are realized by a dichroic mirror, and the influence of the non-ideal spectrum performance of the dichroic mirror is further eliminated by a filter. The dichroic mirror with complete reflection to the laser near 633nm and complete transmission to the laser near 780nm is selected to realize the fusion and separation of the two spectrum wavebands, so that the optical path wavelength division multiplexing is realized and the loss of optical power is reduced.

[0092] In addition, the interference signals generated by the 780nm waveband laser modulated by two EOMs with different modulation frequencies and the same confocal F-P cavity are separated by electric frequency division multiplexing.

[0093] In summary, the cavity length of the confocal F-P cavity, the laser frequency of the tunable laser and the measurement of the FSR of the confocal F-P cavity are locked and controlled by the wavelength and frequency division multiplexing of one optical path, which not only improves the utilization rate of the optical element and effectively controls the laser frequency scanning, but also improves the stability of the FSR of the confocal F-P cavity by locking the cavity length of the confocal F-P cavity, improves the measurement accuracy of the FSR by taking the microwave frequency as the reference, and further realizes the high-precision measurement of the laser frequency scanning from the two directions of improving the stability and measurement accuracy of the FSR.

[0094] The above embodiment is used to explain and illustrate the present application, but not to limit the present application, and any modification and change made to the present application within the spirit and protection scope of the claims of the present application falls into the protection scope of the present application.

Claims

1. A device for precise control and measurement of laser sweep frequency in a locked confocal FP cavity, characterized in that: Includes a reference helium-neon laser (2), an optical fiber beam splitter (3), a first optical fiber collimator (4), an optical isolator (5), a first dichroic mirror (6), a polarizer (7), an inverted beam expander (8), a low-frequency spatial electro-optic phase modulator (9), a beam splitter (10), a high-frequency optical fiber electro-optic phase modulator (11), a second optical fiber collimator (12), a polarizing beam splitter (13), a quarter-wave plate (14), a confocal FP cavity (15), a second dichroic mirror (17), a first filter (18), a first convex lens (19), a first photodetector (20), a second filter (21), a second convex lens (22), and a second photodetector (23); The measurement laser emitted by the tunable laser (1) is first split into two beams by the fiber beam splitter (3) and then incident on the first fiber collimator (4) and the high-frequency fiber electro-optic phase modulator (11) respectively. One beam of measurement laser passes through the high-frequency fiber electro-optic phase modulator (11) and then passes through the second fiber collimator (12) to be converted into second spatial light and incident on the beam splitter (10) for reflection. The other beam of measurement laser passes through the first fiber collimator (4) to be converted into first spatial light and incident on the first dichroic mirror (6) for transmission. The reference laser emitted by the reference helium-neon laser (2) is reflected after passing through the optical isolator (5) and then incident on the first dichroic mirror (6). The reference laser reflected by the first dichroic mirror (6) and the measurement laser transmitted by the first dichroic mirror (6) are both successively passed through the polarizer (7), the inverted beam expander (8), and the low-frequency spatial electro-optic phase modulator (9) to generate light with equal frequency interval sidebands after sinusoidal phase modulation. The light with equal frequency interval sidebands is incident on the beam splitter (10) and transmitted. The measurement laser and reference laser transmitted through the beam splitter (10), as well as the measurement laser reflected by the beam splitter (10), are all incident on the polarizing beam splitter (13) and reflected. They then pass through the quarter-wave plate (14) and enter the confocal FP cavity (15), where they are reflected back and forth. The reference laser, after being reflected back and forth inside the confocal FP cavity (15), is transmitted in reverse order through the quarter-wave plate (14) and the polarizing beam splitter (13) before being incident on the second dichroic mirror (17) where it is transmitted and reflected. The reference laser portion of the light reflected by the second dichroic mirror (17) is filtered by the second filter (21) and focused by the second convex lens (22) in the same wavelength band as the original reference laser emitted by the reference helium-neon laser (2), and then detected and received by the second photodetector (23). The measurement laser portion of the light transmitted by the second dichroic mirror (17) is filtered by the first filter (18) and focused by the first convex lens (19) in the same wavelength band as the original reference laser emitted by the tunable laser (1), and then detected and received by the first photodetector (20).

2. The device for precise control and measurement of laser sweep frequency of a locked confocal FP cavity according to claim 1, characterized in that: The tunable laser (1) and the reference helium-neon laser (2) emit lasers in different wavelengths. The second filter (21) has the same wavelength as the laser emitted by the reference helium-neon laser (2). The first filter (18) has the same wavelength as the laser emitted by the tunable laser (1).

3. The device for precise control and measurement of laser sweep frequency of a locked confocal FP cavity according to claim 1, characterized in that: The confocal FP cavity (15) is provided with a PZT piezoelectric ceramic (16) for controlling the cavity length of the confocal FP cavity (15).

4. The device for precise control and measurement of laser sweep frequency of a locked confocal FP cavity according to claim 3, characterized in that: The device also includes a first high-frequency amplifier (24), a signal source (25), a phase shifter (26), a mixer (27), a low-pass filter (28), a second high-frequency amplifier (29), a first low-frequency amplifier (30), a second low-frequency amplifier (31), and a signal acquisition and processing system (32); The signal source (25) is electrically connected to the first high-frequency amplifier (24) and the high-frequency fiber electro-optic phase modulator (11). The signal source (25) is connected to the mixer (27) via the phase shifter (26). The first photodetector (20) is connected to the mixer (27). The mixer (27) is connected to the signal acquisition and processing system (32) via the low-pass filter (28). The signal acquisition and processing system (32) is electrically connected to the first photodetector (20) and the second photodetector (23), respectively. The signal acquisition and processing system (32) is electrically connected to the second low-frequency amplifier (31) and the PZT piezoelectric ceramic (16). The signal acquisition and processing system (32) is electrically connected to the first low-frequency amplifier (30) and the tunable laser (1). The signal acquisition and processing system (32) is electrically connected to the second high-frequency amplifier (29) and the low-frequency spatial electro-optic phase modulator (9).

5. The device for precise control and measurement of laser sweep frequency of a locked confocal FP cavity according to claim 4, characterized in that: The signal acquisition and processing system (32) includes an AD conversion module, a DA conversion module and an FPGA. The AD conversion module and the DA conversion module are both connected to the FPGA. The low-pass filter (28), the first photodetector (20) and the second photodetector (23) are all connected to the AD conversion module. The DA conversion module is connected to the second low-frequency amplifier (31), the first low-frequency amplifier (30) and the second high-frequency amplifier (29) respectively.

6. A method for precise control and measurement of laser sweep frequency applied to the device described in any one of claims 4-5, characterized in that: 1) Reference helium-neon laser (2) on, tunable laser (1) off: The reference laser emitted by the reference helium-neon laser (2) is detected by the second photodetector (23) to form a cavity-locking beat frequency signal. The cavity-locking beat frequency signal is input into the signal acquisition and processing system (32) to obtain the cavity-locking error signal. The cavity-locking error signal is fed back to the PZT piezoelectric ceramic (16) to adjust the cavity length of the confocal FP cavity (15) so that the cavity-locking error signal is zero, thereby locking and fixing the cavity length of the confocal FP cavity (15). 2) Both the reference helium-neon laser (2) and the tunable laser (1) are turned on and operating: The measurement laser emitted by the tunable laser (1) is modulated by the first fiber collimator (4) and the low-frequency spatial electro-optic phase modulator (9) and detected by the first photodetector (20) to form a low-frequency beat signal. The low-frequency beat signal is input to the signal acquisition and processing system (32) for processing to obtain the frequency stabilization error signal. The frequency stabilization error signal is fed back to the tunable laser (1) to adjust the frequency of the measurement laser emitted by the tunable laser (1) so that the frequency stabilization error signal is zero, thereby locking and fixing the frequency of the measurement laser emitted by the tunable laser (1). 3) Both the reference helium-neon laser (2) and the tunable laser (1) are turned on and operating: The measurement laser emitted by the tunable laser (1) and modulated by the high-frequency fiber electro-optic phase modulator (11) is detected by the first photodetector (20) to form a high-frequency beat frequency signal. The FSR measurement error signal is obtained by processing the high-frequency beat frequency signal. The FSR measurement error signal is fed back to the high-frequency fiber electro-optic phase modulator (11) to adjust the modulation frequency of the high-frequency fiber electro-optic phase modulator (11) so that the FSR measurement error signal is zero. At this time, the frequency spacing between the adjacent laser frequency sidebands generated by the high-frequency fiber electro-optic phase modulator (11) is the FSR free spectrum range of the confocal FP cavity (15). 4) Refer to the helium-neon laser (2) to turn on, and the tunable laser (1) to turn on: Unlock the frequency of the measurement laser emitted by the tunable laser (1), start the frequency scanning of the tunable laser (1), and the measurement laser emitted by the tunable laser (1) is detected by the first photodetector (20) to form a detection signal; during the frequency scanning process, count the number of resonance peaks swept in the corresponding band of the tunable laser (1) in the detection signal, and multiply it by the value of the free spectral region FSR of the confocal FP cavity (15) obtained by measurement in 3) to obtain the scanning frequency range Δf of the tunable laser (1), and finally complete the control of laser frequency sweep and the accurate measurement of the sweep amount.

7. The method for precise control and measurement of laser sweep frequency according to claim 6, characterized in that: The lock cavity error signal in 1) is obtained by processing it according to the following formula: K H =2H1E H 2 J0(β H )J1(b H ) Among them, S 锁腔 (h) represents the cavity-locking error signal obtained by demodulating the interference signal with frequency ωm1 generated at the second photodetector (23) from the laser emitted by the reference helium-neon laser (2) to lock the cavity length h of the confocal FP cavity (15), ω m1 Also the modulation frequency of the low-frequency spatial electro-optic phase modulator (9), ω H Let represent the frequency of the reference laser emitted by the reference helium-neon laser (2), h represent the cavity length of the confocal FP cavity (15), Re{} represents taking the real part of the complex number, F() and F*() represent the reflection coefficients and their conjugates of the confocal FP cavity (15), exp() represents the exponential function with the natural constant e as the base, and i represents the imaginary unit. K represents the phase difference between the locked cavity beat frequency signal and the corresponding local oscillator signal. H H1 represents the maximum amplitude of the lock cavity error signal under ideal conditions, and E represents the electronic scaling factor of the lock cavity beat frequency signal preprocessing. H β H J0() and J1() represent the laser amplitude and modulation depth of the reference laser emitted by the reference helium-neon laser (2) entering the low-frequency spatial electro-optic phase modulator (9), respectively. J0() and J1() represent the first-order Bessel functions of the 0th and 1st order, respectively.

8. The method for precise control and measurement of laser sweep frequency according to claim 6, characterized in that: The frequency stabilization error signal in 2) is obtained by processing it according to the following formula: K E1 =2H E1 E E1 2 J0(β E1 )J1(β E1 )in, S 稳频 (ω E The frequency ω represents the frequency generated at the first photodetector (20) by the laser emitted by the tunable laser (1). m1 The interference signal demodulation obtained is used to stabilize the tunable laser (1) laser frequency ω. E The frequency stability error signal, ω m1 Also the modulation frequency of the low-frequency spatial electro-optic phase modulator (9), ω E The laser frequency of the tunable laser (1) is represented by h, the cavity length of the confocal FP cavity (15) is represented by K. E1 Let represent the maximum amplitude of the frequency stabilization error signal under ideal conditions, Re{} denotes taking the real part of the complex number, F() and F*() represent the reflection coefficients and their conjugates of the confocal FP cavity, exp() denotes an exponential function with the natural constant e as the base, and i denotes the imaginary unit. H represents the phase difference between the low-frequency beat frequency signal and the corresponding local oscillator signal. E1 E represents the corresponding electronic scaling factor. E1 β E1 J0() and J1() represent the laser amplitude and modulation depth of the measurement laser emitted by the tunable laser (1) entering the low-frequency spatial electro-optic phase modulator (9), respectively, and the first-order Bessel functions of the 0th and 1st order are respectively.

9. The method for precise control and measurement of laser sweep frequency according to claim 6, characterized in that: The FSR measurement error signal in 3) is obtained by processing it according to the following formula: K E2 =2H E2 E E2 2 J0(β E2 )J1(β E2 Among them, S FSR (ω m2 The frequency ω is generated at the first photodetector (20) by the laser emitted by the tunable laser (1). m2 The FSR measurement error signal obtained by demodulating the interference signal is used to measure the free spectral range of the confocal FP cavity (15), ω. m2 Also, the modulation frequency of the high-frequency fiber electro-optic phase modulator (11), h represents the cavity length of the confocal FP cavity, Re{} represents taking the real part of the complex number, F() and F*() represent the reflection coefficients of the confocal FP cavity and their conjugates, exp() represents the exponential function with the natural constant e as the base, and i represents the imaginary unit. K represents the phase difference between the beat frequency signal and the corresponding local oscillator signal measured by FSR. E2 H represents the maximum amplitude of the FSR measurement error signal under ideal conditions. E2 E represents the electronic scaling factor of the corresponding signal. E2 β E2 J0() and J1() represent the amplitude and modulation depth of the ECDL laser entering the high-frequency fiber electro-optic phase modulator (11), respectively, and are the 0th and 1st order Bessel functions of the first kind, ω. E This indicates the laser frequency of the tunable laser (1).

10. The method for precise control and measurement of laser sweep frequency according to claim 6, characterized in that: The scanning frequency range Δf of the tunable laser (1) in 4) is specifically calculated according to the following formula: Δf=m·FSR Where m is the number of resonant peaks swept, and FSR is the free spectral region of the confocal FP cavity (15).

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