阅读说明：本技术 激光偏频锁定中高精度鉴频鉴相信号处理方法与装置 (High-precision frequency and phase discrimination signal processing method and device in laser offset frequency locking ) 是由 陈本永 谢建东 严利平 于 2020-05-12 设计创作，主要内容包括：本发明公开了一种激光偏频锁定中高精度鉴频鉴相信号处理方法与装置。将可调谐激光和飞秒光频梳经拍频信号探测单元后得到拍频信号,经模数采样后进入FPGA开发板进行鉴频鉴相处理。将拍频信号与参考信号相乘并进行低通滤波,通过坐标旋转运算器求得对应的相位,相位经相位解缠绕后进行缩放处理,通过数模转换器输出误差电压信号至模拟PID控制器,模拟PID控制器产生控制信号至可调谐激光器进行闭环控制,将拍频信号频率锁定至参考信号,同时计算拍频信号信噪比、频率、幅值。本发明解决了激光器锁定至光频梳的偏频锁定技术中对拍频信号鉴频鉴相精度难以提高、捕获范围小的问题,可以广泛适用于激光精密计量领域。(The invention discloses a method and a device for processing high-precision phase frequency and phase discrimination signals in laser offset frequency locking. Tunable laser and femtosecond optical frequency comb pass through a beat frequency signal detection unit to obtain beat frequency signals, and the beat frequency signals are subjected to analog-to-digital sampling and then enter an FPGA development board for frequency and phase discrimination. Multiplying the beat frequency signal by a reference signal, performing low-pass filtering, obtaining a corresponding phase through a coordinate rotation arithmetic unit, performing scaling processing after phase unwrapping, outputting an error voltage signal to an analog PID controller through a digital-to-analog converter, generating a control signal by the analog PID controller to a tunable laser for closed-loop control, locking the frequency of the beat frequency signal to the reference signal, and calculating the signal-to-noise ratio, the frequency and the amplitude of the beat frequency signal. The invention solves the problems that the frequency discrimination and phase discrimination precision of a beat frequency signal is difficult to improve and the capture range is small in the offset frequency locking technology of locking a laser to an optical frequency comb, and can be widely applied to the field of laser precision metering.)

1. A high-precision frequency and phase discrimination signal processing method in laser offset frequency locking is characterized in that:

1) the method comprises the following steps of detecting a beat frequency signal by using tunable laser emitted by a tunable laser (17) and laser emitted by a femtosecond optical frequency comb (14), acquiring the beat frequency signal by analog-to-digital sampling, and entering an FPGA development board, wherein the beat frequency signal is expressed as follows:

wherein f is_aThe laser frequency difference between the comb teeth of the tunable laser and the femtosecond optical frequency comb is shown, A represents the signal amplitude,representing the phase of the beat signal, t representing time;

the beat signal S (t) is then combined with a pair of orthogonal reference signals (sin (2 π f)_bt)，cos(2πf_bt)) and low-pass filtered to obtain quadrature signals (P (t), Q (t)) as follows:

wherein, LPF [ alpha ], []Representing a low-pass filtering operation, f_bDenotes the frequency of the reference signal, Δ f denotes the frequency difference between the beat signal and the reference signal, A denotes the signal amplitude, sin (2 π f)_bt) and cos (2 π f)_bt) is a sine reference component and a cosine reference component of the reference signal, respectively, and P (t) and Q (t) are a cosine orthogonal component and a sine orthogonal component of the orthogonal signal, respectively, wherein t represents time;

2) performing arc tangent calculation on orthogonal signals (P (t), Q (t)) through coordinate rotation calculation to obtain a decimal phase difference theta_w(t) and performing a sum of squares and an evolution to obtain the amplitude a (t), expressed as follows:

therein, mod_2π[]Expressing the residue relation, and expressing the fractional phase difference theta in the formula_w(t) is equal to the phase differenceFor remainder of 2 pi, decimal phase difference theta_w(t) ranges from-pi to pi;

3) for fractional phase difference theta_w(t) performing a phase unwrapping operation when the fractional phase difference θ_w(t) when continuously changing, recording the times N of the change amplitude reaching 2 pi between two adjacent times, wherein the initial value of N is zero, when the decimal phase jumps from pi to-pi, the numerical value of N is added with one, when the decimal phase jumps from-pi to pi, the numerical value of N is subtracted with one, thereby obtaining the unwrapping phase difference theta_u(t), expressed as follows:

4) unwrapping phase difference θ by gain control_u(t) scaling by a factor of K, and converting the scaled signal into an error voltage signal e (t) by digital-to-analog conversion, which is expressed as follows:

E(t)＝VK·θ_u(t)

wherein K represents a scaling coefficient and V represents a digital-to-analog conversion coefficient;

5) after the error voltage signal E (t) is obtained, the analog PID controller carries out proportional-integral processing through an analog circuit, and outputs control voltage to a PZT control end of the tunable laser (17) to start closed-loop control;

in the closed-loop control, the error voltage signal E (t) is rapidly reduced to zero, and then the unwrapping phase difference is unwrapped_uAnd (t) is 0, namely the frequency difference and the phase difference between the beat frequency signal and the reference signal are zero, and the constant frequency difference is kept between the tunable laser (17) and the comb teeth of the femtosecond optical frequency comb (14) to realize offset frequency locking.

2. An apparatus for high precision frequency and phase detection signal processing in laser offset frequency locking, which implements the method of claim 1, wherein:

the device comprises an analog-digital sampler (1), a digital control oscillator (2), a first multiplier (3), a second multiplier (4), a first low-pass filter (5), a second low-pass filter (6), a coordinate rotation arithmetic unit (7), a phase unwrapping arithmetic unit (8), a gain controller (9), a digital-to-analog converter (10), a fast Fourier arithmetic unit (11), a frequency counter (12), an ARM processor (13), a femtosecond optical frequency comb (14), a beat frequency signal detection unit (15), a tunable laser (17), a computer (18) and an analog PID controller (19); the output end of a femtosecond optical frequency comb (14) of a tunable laser (17) is connected and input to a beat frequency signal detection unit (15), the output end of the beat frequency signal detection unit (15) is connected and input to an analog-to-digital sampler (1), the output end of the analog-to-digital sampler (1) is connected and input to a first multiplier (3) and a second multiplier (4) respectively, the output end of a digital control oscillator (2) is also connected and input to the first multiplier (3) and the second multiplier (4) respectively, the output ends of the first multiplier (3) and the second multiplier (4) are connected and input to a coordinate rotation arithmetic unit (7) after passing through a first low-pass filter (5) and a second low-pass filter (6) respectively, the output end of the coordinate rotation arithmetic unit (7) is input to one input end of a phase unwrapping arithmetic unit (8) and an ARM processor (13) in a stepping mode respectively, and the output end of the phase unwrapping arithmetic unit (8) passes through a gain controller (, The back of the digital-to-analog converter (10) is connected and input to one input end of an analog PID controller (19), and the output end of the analog PID controller (19) is connected and input to the control end of the tunable laser (17); meanwhile, the output end of the analog-to-digital sampler (1) is connected with the other two input ends of the ARM processor (13) through the fast Fourier operator (11) and the frequency counter (12), the output end of the ARM processor (13) is connected with the computer (18), and the two output ends of the computer (18) are connected with the tunable laser (17) and the analog PID controller (19) respectively.

3. The apparatus according to claim 2, wherein the apparatus for processing high precision phase frequency and phase detection signals in laser offset frequency locking comprises: the atomic clock mainly comprises a digital control oscillator (2), a first multiplier (3), a second multiplier (4), a first low-pass filter (5), a second low-pass filter (6), a coordinate rotation arithmetic unit (7), a phase unwrapping arithmetic unit (8), a gain controller (9), a digital-to-analog converter (10), a fast Fourier arithmetic unit (11), a frequency counter (12) and an ARM processor (13), wherein the FPGA development board is formed by the atomic clock (16), and the atomic clock (16) is connected to a femtosecond optical frequency comb (14) and the FPGA development board respectively.

4. The apparatus according to claim 2, wherein the apparatus for processing high precision phase frequency and phase detection signals in laser offset frequency locking comprises: the numerical control oscillator (2) is connected to the atomic clock (16), and the frequency of a reference signal output by the numerical control oscillator (2) is traced to the atomic clock (16).

5. The apparatus according to claim 2, wherein the apparatus for processing high precision phase frequency and phase detection signals in laser offset frequency locking comprises: laser of a tunable laser (17) and laser of a femtosecond optical frequency comb (14) are input into a beat frequency signal detection unit (15), the beat frequency signal detection unit (15) outputs signals to an analog-to-digital sampler (1), and the analog-to-digital sampler (11) collects beat frequency signals and then inputs the beat frequency signals into an FPGA development board; in an FPGA development board, multiplying a beat frequency signal by a reference signal output by a digital control oscillator (2) through a first multiplier (3) and a second multiplier (4), and performing low-pass filtering processing on a multiplied result through a first low-pass filter (5) and a second low-pass filter (6) to obtain an orthogonal signal; the coordinate rotation arithmetic unit (7) performs arc tangent operation and square sum and square root operation on the orthogonal signal to obtain decimal phase difference and amplitude; the decimal phase difference output by the coordinate rotation arithmetic unit (7) is transmitted to the phase unwrapping arithmetic unit (8) to obtain an unwrapping phase, and then is scaled by the gain controller (9), and then generates an error voltage signal by the digital-to-analog converter (10) and inputs the error voltage signal to the analog PID controller (19); meanwhile, the beat frequency signal is processed through a fast Fourier arithmetic unit (11) and a frequency counter (12) to obtain the signal-to-noise ratio and the frequency of the beat frequency signal, and the signal-to-noise ratio and the frequency of the beat frequency signal and the amplitude output by the coordinate rotation arithmetic unit (7) are transmitted to an ARM processor (13); the ARM processor (13) transmits the received data to the computer (18), the computer (18) controls the tunable laser (17) to perform frequency pre-adjustment, and controls the analog PID controller (19) to start closed-loop control after the pre-adjustment is completed; the analog PID controller (19) performs proportional-integral processing according to an error voltage signal output by the digital-to-analog converter (19) and a frequency pre-adjustment signal output by the computer (18), outputs a control voltage to the tunable laser (17) for closed-loop control, quickly locks the phase difference between the beat frequency signal and the reference signal to zero to realize offset frequency locking, and the comb frequency difference and the phase of the tunable laser (17) and the femtosecond optical frequency comb (14) are constant values after locking.

Technical Field

The invention belongs to a signal processing method and a device in the technical field of laser frequency stabilization, in particular to a high-precision frequency discrimination and phase discrimination signal processing method and a device in laser offset frequency locking.

Background

The laser interferometry has the advantages of high response speed, high measurement precision, strong anti-interference capability, capability of directly tracing to the meter definition and the like, and is widely applied to high-precision displacement measurement, ultra-precision machining and manufacture and instrument detection and calibration. In laser interferometry, the stability and traceability of the laser frequency are key factors affecting the measurement accuracy. The key point of the laser frequency stabilization technology is that a stable reference frequency standard is selected, the traditional laser frequency stabilization technology mainly locks the laser frequency to a physical reference datum (such as a gas absorption chamber and an FP cavity), the methods are easily influenced by conditions of environmental temperature, air pressure, vibration and the like, the laser frequency is easy to drift, the locked laser frequency can hardly be tuned, and the laser frequency is only suitable for laser locking with single frequency stabilization.

A femtosecond optical frequency comb is a broad-spectrum light source composed of a plurality of equally spaced frequency components (comb teeth) in the frequency domain, and each comb tooth can trace to a frequency reference (e.g., a rubidium atomic clock). The laser frequency is locked to the optical frequency comb, so that the problems that the frequency is easy to drift due to environmental influence, cannot trace to the source, is small in frequency locking range and the like are solved. The phase detection precision and the capture range of the phase frequency detector in the offset frequency locking technology play a key role in the offset frequency locking. There are two main methods of frequency and phase discrimination. The first method is to mix the beat signal and the reference signal via analog mixer and low pass filter to obtain sinusoidal signal equal to the frequency difference between the beat signal and the reference signal, and to input the sinusoidal signal as error signal to analog PID controller for closed loop control. This method is commonly referred to as a "sinusoidal phase detector" and the phase detection accuracy is susceptible to beat signal fluctuations (e.g., power fluctuations, insufficient signal-to-noise ratio). The capture bandwidth generally refers to the maximum frequency difference of the input signal allowed by the normal operation of the phase frequency detector; for a 'sine phase discriminator', the capture bandwidth is small, usually in the order of kHz, and the frequency difference needs to be adjusted to the capture bandwidth in advance during locking, whereas when the frequency difference is large or fluctuates rapidly, locking is difficult or lock losing is easy. In the second method, the beat frequency signal and the reference signal are converted into square waves, then the square waves are added and subtracted (one sine wave corresponds to one counting value), and the difference value of the counting result is converted into an equal proportion voltage as an error signal for closed-loop control. The method is generally called as a digital phase frequency detector, has good performance under the conditions of stable beat frequency signals and high signal-to-noise ratio, the capture bandwidth is generally in the magnitude of MHz, the capture range can be further improved through the pre-frequency division treatment, but the actual phase detection precision is 2 pi (namely a single sine period), burrs easily appear under the influence of noise in the process of converting sine waves into square waves, and errors of +/-1 easily exist in the process of adding, subtracting and counting.

In the prior art of offset frequency locking, due to the frequency fluctuation of the laser to be frequency stabilized before locking and other reasons, the beat frequency signal is easy to fluctuate (frequency, power and signal-to-noise ratio), and the phase frequency detector is required to have a larger capture range and higher phase detection precision. Therefore, the key technical problems to be solved for improving the laser frequency stability and realizing the quick locking are to consider and improve the frequency discrimination and phase discrimination precision, the capture range and the anti-interference capability of the beat frequency signal of the laser to be frequency stabilized and the optical frequency comb.

Disclosure of Invention

In order to solve the problems in the background art, the invention discloses a high-precision frequency and phase discrimination signal processing method and device in laser frequency offset locking, and solves the problems that the phase discrimination precision of a beat frequency signal is difficult to improve and the capture range is small in the technology of locking the laser frequency to an optical frequency comb.

The technical scheme adopted by the invention for solving the technical problems is as follows:

a high-precision frequency and phase discrimination signal processing method in laser offset frequency locking comprises the following steps:

1) the method comprises the following steps of detecting a beat frequency signal by using tunable laser emitted by a tunable laser and laser emitted by a femtosecond optical frequency comb, acquiring the beat frequency signal by using analog-to-digital sampling, and entering an FPGA development board, wherein the beat frequency signal is expressed as follows:

wherein f is_aThe laser frequency difference between the comb teeth of the tunable laser and the femtosecond optical frequency comb is shown, A represents the signal amplitude,

which is indicative of the phase of the beat signal,in the range-pi to pi, t represents time;

then, the beat signal S (t) is compared with a pair of orthogonal reference signals (sin (2 π f) generated by a numerically controlled oscillator_bt)，cos(2πf_bt)) and low-pass filtered to obtain quadrature signals (P (t), Q (t)) as follows:

wherein, LPF [ alpha ], []Representing a low-pass filtering operation, f_bDenotes the frequency of the reference signal, Δ f denotes the frequency difference between the beat signal and the reference signal, A denotes the signal amplitude, sin (2 π f)_bt) and cos (2 π f)_bt) is a sine reference component and a cosine reference component of the reference signal, respectively, and P (t) and Q (t) are a cosine orthogonal component and a sine orthogonal component of the orthogonal signal, respectively, wherein t represents time;

2) performing arc tangent calculation on orthogonal signals (P (t), Q (t)) through coordinate rotation calculation to obtain a decimal phase difference theta_w(t) and performing a sum of squares and an evolution to obtain the amplitude a (t), expressed as follows:

therein, mod_2π[]Expressing the residue relation, and expressing the fractional phase difference theta in the formula_w(t) is equal to the phase differenceFor remainder of 2 pi, decimal phase difference theta_w(t) ranges from-pi to pi;

3) when the laser is not locked, the frequency difference delta f between the beat signal and the reference signal is not equal to 0, and the decimal phase difference theta_w(t) will exhibit a sawtooth-like periodic variation with each period having a phase jump from pi to-pi or from-pi to pi. For fractional phase difference theta_w(t) performing a phase unwrapping operation when the fractional phase difference θ_w(t) when continuously changing, recording the times N of the change amplitude reaching 2 pi between two adjacent times, wherein the initial value of N is zero, when the decimal phase jumps from pi to-pi, the numerical value of N is added with one, when the decimal phase jumps from-pi to pi, the numerical value of N is subtracted with one, thereby obtaining the unwrapping phase difference theta_u(t), expressed as follows:

unwrapping phase difference theta_uAnd (t) is the frequency and phase discrimination result of the beat frequency signal.

4) Unwrapping phase difference θ by gain control_u(t) scaling by a factor of K, and converting the scaled signal into an error voltage signal e (t) by digital-to-analog conversion, which is expressed as follows:

E(t)＝VK·θ_u(t)

wherein K represents a scaling coefficient and V represents a digital-to-analog conversion coefficient;

5) after the error voltage signal E (t) is obtained, the analog PID controller carries out proportional-integral processing through an analog circuit, and outputs control voltage to a PZT control end of the tunable laser to start closed-loop control;

in the closed-loop control, the error voltage signal E (t) is rapidly reduced to zero, and then the unwrapping phase difference is unwrapped_uAnd (t) is 0, namely the frequency difference and the phase difference between the beat frequency signal and the reference signal are both zero, the constant frequency difference is kept between the tunable laser and the comb teeth of the femtosecond optical frequency comb, the phase difference is also constant to be zero, and the offset frequency locking is realized.

Drawings

FIG. 1 is a schematic block diagram of the method and apparatus of the present invention.

In the figure: 1. the system comprises an analog-digital sampler, 2, a digital control oscillator, 3, a first multiplier, 4, a second multiplier, 5, a first low-pass filter, 6, a second low-pass filter, 7, a coordinate rotation arithmetic unit (CORDIC), 8, a phase unwrapping arithmetic unit, 9, a gain controller, 10, a digital-to-analog converter, 11, a fast Fourier arithmetic unit (FFT), 12, a frequency counter, 13, an ARM processor, 14, a femtosecond optical frequency comb, 15, a beat frequency signal detection unit, 16, an atomic clock, 17, a tunable laser, 18, a computer, 19 and an analog PID controller.

In specific implementation, FFT analysis and frequency measurement are simultaneously carried out on the beat frequency signal in an FPGA development board, the signal-to-noise ratio of the beat frequency signal and the frequency stability after locking are obtained to be used for assisting in the process of controlling offset frequency locking, the quality of offset frequency locking can be detected, and finally the offset frequency locking is realized.

The phase difference obtained by the processing of the steps 1) to 2) has higher phase discrimination precision; in the low-pass filtering process of the step 1), the influence of noise in the beat frequency signal can be weakened; in the step 2), the amplitude values in the orthogonal signals are offset when the arctangent calculation is carried out to obtain the decimal phase difference, namely, the phase discrimination process has the capacity of resisting disturbance to the fluctuation of the beat frequency signal strength.

And in the steps 3) to 5), the fractional phase difference is subjected to unwrapping and then used for offset frequency locking, so that the phase discrimination precision is considered, and the capture range is improved.

Secondly, a device for processing high-precision frequency and phase discrimination signals in laser offset frequency locking:

the device comprises an analog-digital sampler, a digital control oscillator, a first multiplier, a second multiplier, a first low-pass filter, a second low-pass filter, a coordinate rotation arithmetic unit, a phase unwrapping arithmetic unit, a gain controller, a digital-to-analog converter, a fast Fourier arithmetic unit, a frequency counter, an ARM processor, a femtosecond optical frequency comb, a beat frequency signal detection unit, a tunable laser, a computer and an analog PID controller; the output end of the tunable laser femtosecond optical frequency comb is connected and input to the beat frequency signal detection unit, the output end of the beat frequency signal detection unit is connected and input to the analog-digital sampler, the output end of the analog-digital sampler is respectively connected and input to the first multiplier and the second multiplier, the output end of the digital control oscillator is also respectively connected and input to the first multiplier and the second multiplier, the output ends of the first multiplier and the second multiplier are respectively connected and input to the coordinate rotation arithmetic unit after passing through the first low-pass filter and the second low-pass filter, the output end of the coordinate rotation arithmetic unit is respectively input to one input end of the phase unwrapping arithmetic unit and one input end of the ARM processor in a training mode, the output end of the phase unwrapping arithmetic unit is sequentially connected and input to one input end of the analog PID controller after passing through the gain controller and the digital-to-analog converter, and the output end of the analog PID controller is connected and input to the control end of the tunable laser; meanwhile, the output end of the analog-digital sampler is connected and input to the other two input ends of the ARM processor through the fast Fourier arithmetic unit and the frequency counter, the output end of the ARM processor is connected with the computer, and the two output ends of the computer are connected to the tunable laser and the analog PID controller respectively.

The atomic clock mainly comprises a digital control oscillator, a first multiplier, a second multiplier, a first low-pass filter, a second low-pass filter, a coordinate rotation arithmetic unit, a phase unwrapping arithmetic unit, a gain controller, a digital-to-analog converter, a fast Fourier arithmetic unit, a frequency counter and an ARM processor, and is connected to the femtosecond optical frequency comb and the FPGA development board respectively.

The digital control oscillator is connected to the atomic clock, and the reference signal frequency output by the digital control oscillator is traced to the atomic clock.

Laser of a tunable laser and laser of a femtosecond optical frequency comb are input into a beat frequency signal detection unit, the beat frequency signal detection unit outputs signals to an analog-to-digital sampler, and the analog-to-digital sampler collects beat frequency signals and then inputs the beat frequency signals into an FPGA development board; in an FPGA development board, multiplying a beat frequency signal by a reference signal output by a digital control oscillator through a first multiplier and a second multiplier, and performing low-pass filtering processing on a multiplied result through a first low-pass filter and a second low-pass filter to obtain an orthogonal signal; the coordinate rotation arithmetic unit carries out arc tangent operation and square sum and square root operation on the orthogonal signal to obtain decimal phase difference and amplitude; the decimal phase difference output by the coordinate rotation arithmetic unit is transmitted to the phase unwrapping arithmetic unit to obtain an unwrapping phase, then is scaled by the gain controller, generates an error voltage signal by the digital-to-analog converter and inputs the error voltage signal to the analog PID controller; meanwhile, processing the beat frequency signal through a fast Fourier arithmetic unit and a frequency counter to obtain the signal-to-noise ratio and the frequency of the beat frequency signal, and transmitting the signal-to-noise ratio and the frequency of the beat frequency signal to an ARM processor together with the amplitude output by a coordinate rotation arithmetic unit; the ARM processor transmits the received data to a computer, the computer controls the tunable laser to perform frequency pre-adjustment, and after the pre-adjustment is completed, the computer controls the analog PID controller to start closed-loop control; the analog PID controller carries out proportional integral processing according to an error voltage signal output by the digital-to-analog converter and a frequency pre-adjustment signal output by a computer, outputs control voltage to the tunable laser for closed-loop control, quickly locks the phase difference between the beat frequency signal and the reference signal to zero, realizes offset frequency locking, and the comb tooth frequency difference and the phase of the locked tunable laser and the femtosecond optical frequency comb are constant values.

The invention has the beneficial effects that:

(1) the invention adopts high-precision phase-locked amplification processing to carry out frequency discrimination and phase discrimination processing on the beat frequency signal, and the obtained decimal phase difference has extremely high phase discrimination precision and certain noise reduction and anti-interference capability. The obtained decimal phase difference is further subjected to phase unwrapping processing, so that the phase discrimination precision is considered, and the capture range is enlarged.

(2) The invention can perform FFT fast Fourier analysis, frequency measurement and amplitude measurement on the beat frequency signal while finishing the frequency and phase discrimination processing on the beat frequency signal, and can be used for assisting in controlling the offset frequency locking and monitoring the stability of the offset frequency locking.