CN113572022B - Laser frequency stabilization system based on improved double-path digital phase-locked amplifier - Google Patents

Abstract

The invention belongs to the technical field of laser frequency stabilization, and particularly discloses a laser frequency stabilization system based on an improved double-path digital phase-locked amplifier. The method gets rid of the limitation that the frequency stabilization of the laser depends on manual phase adjustment, and improves the intelligent degree of the frequency stabilization system. Meanwhile, due to the existence of a negative feedback loop in the phase-locked amplifier, the problem of error signal jitter caused by phase jitter in the working process of the system can be reduced, and the quality of an error signal is improved.

Description

Laser Frequency Stabilization System Based on Improved Dual-channel Digital Lock-in Amplifier

technical field

The invention belongs to the technical field of laser frequency stabilization, in particular to a laser frequency stabilization system based on an improved dual-channel digital phase-locked amplifier. The laser frequency stabilization is realized by using an improved dual-channel digital lock-in amplifier.

Background technique

Laser has the characteristics of good monochromaticity and strong coherence, and has a wide range of applications in the fields of precision measurement, gravitational wave detection and information science. However, the frequency stability of the traditional free-running laser is poor. After the passive frequency stabilization (temperature control, constant current source drive, etc.) design is adopted, the frequency drift still cannot meet the requirements of modern precision measurement for the laser frequency stability. The long-term accuracy of the laser frequency must take certain measures to actively improve the stability of the laser frequency.

There are many kinds of existing laser frequency stabilization methods, some of which need to modulate the laser frequency (with internal or external modulation) to generate an error signal, and then use a lock-in amplifier to extract the error signal from the clutter to achieve laser stabilization. frequency purpose. However, the current method of extracting the error signal generally uses a single-channel lock-in amplifier to extract the error signal, and then adjusts the phase and observes the size of the error signal until the maximum peak value of the error signal is observed by the naked eye to determine the phase optimal point. The above-mentioned manual phase adjustment has a certain subjectivity and is not real-time, and cannot solve the problem of the amplitude jitter of the error signal caused by the phase jitter in the frequency stabilization process. In addition, although the traditional dual-output digital correlation demodulator can obtain the signal amplitude and phase, the output amplitude is always positive and has no directionality, so it cannot be directly used for laser frequency stabilization.

SUMMARY OF THE INVENTION

In view of the problems existing in the prior art, the purpose of the present invention is to provide a laser frequency stabilization system based on an improved dual-channel digital lock-in amplifier. By improving the dual-channel digital lock-in amplifier, the magnitude of the error signal can be directly extracted At the same time, it can directly output the phase amount, and use the phase amount to adjust the reference signal in real time, so that the error signal is always at the theoretical value, thereby improving the signal-to-noise ratio of the error signal and improving the frequency stabilization accuracy. In addition, the phase automatic adjustment function of the frequency stabilization system improves the The degree of automation of frequency stabilization.

In order to achieve the above objects, the present invention adopts the following technical solutions to achieve.

Laser frequency stabilization system based on improved dual-channel digital lock-in amplifier, including: laser, saturable absorption spectrum generating optical circuit unit, photodetector, transimpedance amplifier, ADC, improved dual-channel digital lock-in amplifier, first frequency synthesizer , digital adder, triangular wave generator, DAC and adding circuit;

The laser emits laser light, and the laser passes through the saturable absorption spectrum generating optical circuit unit to generate a saturable absorption spectrum optical signal; the photodetector receives the saturable absorption spectrum optical signal and converts it into a saturated spectrum current signal, and the saturated spectrum current signal passes through a transimpedance amplifier. Converted into a voltage signal that can be collected by the ADC, the voltage signal is converted into a saturated spectrum digital signal by the ADC and then enters the improved dual-channel digital lock-in amplifier;

The improved dual-channel digital lock-in amplifier extracts the error signal and phase information in the saturation spectrum digital signal, and outputs the error signal to the digital PID circuit to obtain the feedback control voltage digital signal; at the same time, the phase information is in the improved dual-channel digital lock. The negative feedback is formed inside the phase amplifier, and the phase difference is dynamically adjusted so that the phase difference is always 0°;

When determining the initial frequency stabilization point, narrow the frequency scanning range of the laser until only the error signal of the target spectral peak can be observed. At this time, the three input ends of the digital adder are respectively input to the first frequency synthesizer to generate modulation The signal, the frequency sweep signal generated by the triangular wave generator and the error signal output by the improved dual-channel digital lock-in amplifier, combined with the bias point of the laser frequency to determine the initial frequency stabilization point voltage, and send it to the summing circuit;

The feedback control voltage digital signal is converted into an analog voltage signal by a DAC and then sent to an addition circuit, and the addition circuit outputs a frequency stabilization voltage signal according to the received initial frequency stabilization point voltage and the feedback control voltage; the frequency stabilization voltage signal is used to realize the laser frequency stabilization.

Further, the improved dual-channel digital lock-in amplifier includes a second frequency synthesizer, a third frequency synthesizer, two low-pass filters and two digital multipliers; wherein, the second frequency synthesizer generates a reference Sine demodulation signal, which is multiplied with the input saturation spectrum digital signal by a digital multiplier and filtered by a low-pass filter to extract the error signal, that is, the I-channel signal;

The third frequency synthesizer generates a reference cosine demodulation signal, which is multiplied by another digital multiplier and filtered by another low-pass filter to obtain a Q-channel signal;

Calculate the phase information according to the I channel signal and the Q channel signal, and feed back the phase information to the phase input ports of the second frequency synthesizer and the third frequency synthesizer, and dynamically adjust the phase to make it 0° to maximize the output error signal.

Further, it also includes a PC, which transmits the error signal and the phase to the PC through the serial port, and displays the data in the PC in real time.

Further, the digital signal processing process is realized in FPGA, that is, the improved dual-channel digital lock-in amplifier, digital PID circuit, first frequency synthesizer, second frequency synthesizer, third frequency synthesizer and triangular wave generator are realized through FPGA .

Further, a voltage-current converter is also included, which is used for generating the driving current of the laser according to the frequency-stabilized voltage signal.

Further, a Faraday isolator is arranged between the laser and the saturable absorption spectrum generating optical circuit unit, and a temperature control module is connected to the laser.

Further, the saturable absorption spectrum generating optical path unit includes a first beam splitter, a half-wave plate, a polarization beam splitter, a mirror pair, a second beam splitter and a cesium bubble, wherein the laser is split through the first beam splitter. For frequency-stabilized light and experimental light, the frequency-stabilized light is divided into strong pump light and weak light by a half-wave plate and a polarization beam splitter in turn. The saturable absorption spectrum signal is generated at the bubble, and the weak light is divided into the reference light and the probe light by the second beam splitter and received by the photodetector.

Compared with the prior art, the beneficial effects of the present invention are:

(1) The present invention improves the dual-channel digital phase-locked amplifier, so that it can obtain the error signal and the phase at the same time. According to the phase, a feedback loop can be formed inside the phase-locked loop to dynamically adjust the phase difference between the two, so that the phase difference is always is 0°. It gets rid of the limitation of relying on manual adjustment and improves the intelligence of the frequency stabilization system. At the same time, due to the existence of the negative feedback loop inside the lock-in amplifier, it can reduce the problem of error signal jitter caused by phase jitter in the working process of the system, and improve the stability of the error signal. From this aspect, this method reduces the phase The difference indirectly improves the signal-to-noise ratio of the error signal, which has a certain significance for improving the laser frequency.

(2) The phase difference extracted by the improved dual-channel digital lock-in amplifier of the present invention can not only form a feedback loop internally, but also can be sent to a PC through the serial port function, and then displayed in real time through the PC. Compared with the method of observing the amplitude of the error signal, this method is more intuitive. Even if the internal feedback loop of the lock-in amplifier is closed, it is still more intuitive to manually adjust the phase.

(3) When using the system of the present invention to stabilize the frequency, when modulating the laser with an external signal, there is no need to use an additional phase-locked loop circuit to synchronize the phases of the reference signal and the modulated signal, which simplifies the circuit design and improves the flexibility of the frequency stabilization system. sex.

Description of drawings

The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

Fig. 1 is the system structure schematic diagram of the present invention;

2 is an optical path structure diagram of a saturable absorption spectrum generating optical path unit according to an embodiment of the present invention;

3 is a schematic diagram of an improved dual-channel digital lock-in amplifier according to an embodiment of the present invention;

In the above figure, 1 laser; 2 saturable absorption spectrum generating optical path unit; 21 first beam splitter; 22 half-wave plate; 23 polarization beam splitter; 24 mirror pair; 25 second beam splitter; 26 cesium bubble; 3 Photodetector; 4 Transimpedance Amplifier; 5ADC; 6 Improved Dual-Channel Digital Lock-in Amplifier; 61 Second Frequency Synthesizer; 62 Third Frequency Synthesizer; 63 Low Pass Filter; 64 Digital Multiplier; 7 First Frequency Synthesizer; 8 digital adders; 9 triangle wave generators; 10DAC; 11 summing circuits; 12 Faraday isolators; 13 pressure-current converters; 14 temperature control modules.

Detailed ways

The embodiments of the present invention will be described in detail below in conjunction with the examples, but those skilled in the art will understand that the following examples are only used to illustrate the present invention and should not be regarded as limiting the scope of the present invention.

Referring to FIG. 1, a laser frequency stabilization system based on an improved dual-channel digital lock-in amplifier 6 provided by the present invention includes: a laser 1, a saturable absorption spectrum generating optical circuit unit 2, a photodetector 3, a transimpedance amplifier 4, an ADC5 , an improved dual-channel digital lock-in amplifier 6, a first frequency synthesizer 7, a digital adder 8, a triangular wave generator 9, a DAC10 and an adding circuit 11;

The laser 1 emits laser light, and the laser passes through the saturable absorption spectrum generating optical circuit unit 2 to generate a saturable absorption spectrum optical signal; the photodetector 3 receives the saturable absorption spectrum optical signal and converts it into a saturated spectrum current signal, and the saturated spectrum current signal passes through. The transimpedance amplifier 4 is converted into a voltage signal that can be collected by the ADC5, and the voltage signal is converted into a saturated spectrum digital signal by the ADC5 and then enters the improved dual-channel digital lock-in amplifier 6;

The improved dual-channel digital lock-in amplifier 6 extracts the error signal and phase information in the saturation spectrum digital signal, and outputs the error signal to the digital PID circuit to obtain the feedback control voltage digital signal; at the same time, the phase information is in the improved dual-channel digital signal. A negative feedback is formed inside the lock-in amplifier 6, and the phase difference is dynamically adjusted so that the phase difference is always 0°;

When determining the initial frequency stabilization point, reduce the frequency scanning range of the laser 1 until only the error signal of the target spectral peak can be observed. At this time, the three input ends of the digital adder 8 are respectively input to the first frequency synthesizer 7. Generate the modulation signal, the frequency sweep signal generated by the triangular wave generator 9 and the error signal output by the improved dual-channel digital lock-in amplifier 6, and then determine the initial frequency stabilization point voltage in combination with the bias point of the laser frequency, and transmit it to the adding circuit 11;

The feedback control voltage digital signal is converted into an analog voltage signal by the DAC10 and then sent to the addition circuit 11, and the addition circuit 11 outputs the frequency stabilization voltage signal according to the received initial frequency stabilization point voltage and the feedback control voltage; using the frequency stabilization voltage signal The frequency stabilization of laser 1 is realized.

The system of the present invention includes a DFB laser 1 (distributed feedback laser 1), a Faraday isolator 12, a saturable absorption spectrum generating optical circuit unit 2 and a balanced photodetector 3, a transimpedance amplifier 4, an ADC5 circuit, a DAC10 circuit, and a temperature control module 14. , a pressure-to-current converter 13, an FPGA chip, and an improved dual-channel digital lock-in amplifier 6 realized by FPGA, a digital PID, a first frequency synthesizer 7, a second frequency synthesizer 61, a third frequency synthesizer 62 (DDS )Wait. The saturable absorption spectrum generating light path unit 2 includes a beam splitter (BS), a half-wave plate 22 , a polarization beam splitter 23 (PBS), a mirror pair 24 , and a cesium bubble 26 .

The DFB laser 1 is driven by the pressure-current converter 13, and the temperature control module 14 is added to set the temperature. The laser is sequentially irradiated to the input end of the photodetector 3 through the Faraday isolator 12 and the saturable absorption spectrum generating optical circuit unit 2. The photodetector 3 The output terminal of the DAC10 is connected to the input port of ADC5 (digital-to-analog conversion chip) through the transimpedance amplifier 4, the output of ADC5 is connected to the FPGA, the FPGA output digital signal controls the output of DAC10 (digital-to-analog conversion chip), and the output of DAC10 is connected to the addition circuit 11 and the DC bias point. After the addition, it is connected to the input end of the pressure-current conversion, and the output of the pressure-current conversion circuit drives the DFB laser 1 to output laser light, thereby forming a frequency stabilization loop.

Referring to FIG. 2 , the optical path structure of the saturable absorption spectrum generating optical path unit 2 of the present invention will be further described in detail.

The saturable absorption spectrum generating light path unit 2 includes a first beam splitter 21, a half-wave plate 22, a polarization beam splitter 23, a mirror pair 24, a second beam splitter 25 and a cesium bubble 26, wherein the laser passes through the first beam splitter The mirror 21 is divided into frequency-stabilized light and experimental light, and the frequency-stabilized light is divided into strong pump light and weak light through the half-wave plate 22 and the polarization beam splitter 23 in turn. , and the probe light is opposed to the cesium bubble 26 to generate a saturated absorption spectrum signal; the weak light is divided into the reference light and the probe light by the second beam splitter 25 and received by the photodetector 3 .

Specifically, the laser S is divided into the experimental light and the laser participating in frequency stabilization through a BS (spectroscopy ratio of 9:1). The polarization of the frequency-stabilized light is adjusted by the half-wave plate 22, so that the laser light passing through the polarization beam splitter 23PBS is divided into pump light and weak light. At the opposite side, a saturable absorption spectrum signal is generated. The weak light is divided into reference light and probe light by the second beam splitter 25 (split ratio 50:50) and received by the balanced detector. Adjust the driving current of laser 1 to the vicinity of the absorption peak, turn on the frequency sweep switch of laser 1, and the pump light will saturate the cesium atoms. At this time, when the probe light passes through the saturated cesium atoms, a saturated absorption spectrum signal will be generated due to the Lamb effect.

Referring to FIG. 3 , the improved dual-channel digital lock-in amplifier 6 of the present invention will be described in further detail.

The improved dual-channel digital lock-in amplifier 6 includes a second frequency synthesizer 61, a third frequency synthesizer 62, two low-pass filters 63 and two digital multipliers 64; wherein, the second frequency synthesizer 61 generates a reference sinusoidal demodulation signal, which is multiplied by a digital multiplier 64 and filtered by a low-pass filter 63 to extract an error signal, i.e., the 1-way signal;

The third frequency synthesizer 62 generates a reference cosine demodulation signal, which is multiplied by another digital multiplier 64 and filtered by another low-pass filter 63 to obtain a Q-channel signal;

Calculate the phase information according to the I-channel signal and the Q-channel signal, and feed back the phase information to the phase input ports of the second frequency synthesizer 61 and the third frequency synthesizer 62, and dynamically adjust the phase to make it 0°, so that the output error signal maximum.

其中n(k)为高斯噪声，k表示采样点，N为一个信号周期内的采样点数，A(k)为信号幅度，θ(k)为相位信息。Specifically, the laser modulation frequency is f ₀ , the sampling frequency is f _s , and f _s =N*f ₀ (N>=2), then the expression of the signal collected by ADC5 is:

where n(k) is Gaussian noise, k is the sampling point, N is the number of sampling points in a signal cycle, A(k) is the signal amplitude, and θ(k) is the phase information.

与s(k)通过数字乘法器64相乘，此时利用积化和差公式可以得出

The sinusoidal demodulation signal output by the second frequency synthesizer 61 (DDS)

Multiply with s(k) by the digital multiplier 64, at this time, the product sum difference formula can be used to obtain

与倍频项

经过低通滤波器63后倍频项被滤除，只剩下差频项输出

因为

是一个固定的数字值，可令其为1，此时I(k)＝e(k)，e(k)表示误差信号，θ(k)是参考信号与输入信号的相位差。According to the above expression, the product of the two will have a difference frequency term

with the octave term

After the low-pass filter 63, the multiplier term is filtered out, leaving only the difference frequency term output.

because

is a fixed digital value, which can be set to 1. At this time, I(k)=e(k), e(k) represents the error signal, and θ(k) is the phase difference between the reference signal and the input signal.

相乘后经过低通滤波器63后，同理可得到

将I(k)与Q(k)送入FPGA的cordic IP核的输入口可以求得

最后再将θ(k)反馈到FPGA内部的第二频率合成器61和第三频率合成器62的相位输入口，动态调整相位θ(k)使其为0°(具有动态调整输入信号与参考信号相位差的特性是改进锁相放大器区别于其他锁相放大器的本质特征，而这一特性特别适用于稳定激光频率)，则此时输出的误差信号e(k)最大，其值为A(k)。此外可将相位差θ(k)与e(k)通过串口发送到主机端，在PC机中实时显示数据。Another way, when s(k) is equal to

After multiplying and passing through the low-pass filter 63, the same can be obtained

Sending I(k) and Q(k) to the input port of the cordic IP core of the FPGA can be obtained

Finally, θ(k) is fed back to the phase input ports of the second frequency synthesizer 61 and the third frequency synthesizer 62 inside the FPGA, and the phase θ(k) is dynamically adjusted to make it 0° (with dynamic adjustment of the input signal and reference The characteristic of the signal phase difference is the essential characteristic of improving the lock-in amplifier to distinguish it from other lock-in amplifiers, and this characteristic is especially suitable for stabilizing the laser frequency), then the output error signal e(k) is the largest at this time, and its value is A ( k). In addition, the phase difference θ(k) and e(k) can be sent to the host through the serial port, and the data can be displayed in the PC in real time.

In the present invention, the first frequency synthesizer 7, the second frequency synthesizer 61 and the third frequency synthesizer 62 correspondingly generate three sinusoidal signals with the same frequency and different phases. The second frequency synthesizer 61 and the third frequency synthesizer 62 have Fixed phase difference of 90° (generates sine and cosine signals respectively).

The present invention proposes a new type of digital lock-in amplifier that is more intelligent and more suitable for laser frequency stabilization. The method is not limited to saturation spectrum frequency stabilization, but is also applicable to some frequency stabilization methods that require the lock-in amplifier to extract error signals.

The specific implementation steps of the system are described in detail below by taking the saturation spectrum frequency stabilization as an example:

Step 1, build the light path:

Drive laser 1, let the laser build an optical path according to the path shown in Figure 2, make the pump light and the probe light overlap as much as possible at the cesium bubble 26, and adjust the light intensity of the reference light and probe light at the same time, so that they reach the balanced photodetector The light intensity at 3 is as the same as possible.

Step 2, generate saturation spectrum signal and receive

The pump light and the probe light with the same intensity are irradiated on the balanced photodetector 3. At this time, the frequency sweep button of the laser 1 is turned on, and the driving current is slowly adjusted until the 6 spectral peaks can be clearly seen on the oscilloscope.

Step 3, obtain the error signal e(k) and phase θ(k)

The power absorbed by the cesium atom can be expressed as follows: P _T (w)=P ₀ exp[-α(w)x]

Among them, P ₀ is the incident light intensity, α(w) is the absorption coefficient, w is the laser frequency, and x is the length of the cesium atomic region that the laser passes through.

When the absorption is very small (α(w)x<<1), the approximate formula P _T (w)≈P ₀ [1-α(w)x] can be obtained by using Taylor's formula _. Further arrangement can be obtained cesium atomic absorption power ΔP(w)=P ₀ -P _T (w)=P ₀ α(w)x.

At this time, the absorbed power ΔP after adding Δw(t) modulation to the output frequency of the laser 1 is expressed as follows:

△P(w+△w(t))=P ₀ α(w+△w(t))x

And because the modulation amplitude Δw(t) is very small, much smaller than the laser frequency w, the above expression can be further expanded by Taylor's formula,

Its expansion is as follows:

Let Δw(t)=Asin(wt), the expression of ΔP(w+Δw(t)) of absorbed power is as follows:

Because the absorption coefficient α(w) of cesium atom is Lorentzian type and has even symmetry, its odd derivative has the same size and opposite direction on both sides of the transition point, which can be used to stabilize the output frequency of laser 1. .

The present invention takes the first-order differential method for frequency stabilization as an example. At this time, the main useful signal generated by the saturation spectrum is s(t)=P ₀ xAα′(w) sin(wt), and the rest can be represented by noise s(t). Convert the signal s(t) into a voltage signal and then convert it into a digital quantity through ADC5, use the lock-in amplifier to extract the characteristics of the weak signal, extract the error signal e(k) and the phase difference θ(k), and then follow the flow chart in Figure 3. The error signal e(k) is output and the phase signal is sent to the DDS phase adjustment port inside the phase-locked loop to adjust the phase of the demodulated signal. At the same time, e(k) and θ(k) can be sent to the PC for display through the serial port.

Step 4, determine the initial point of the laser frequency

Reduce the frequency scanning range of laser 1 until the PC can only observe the error signal of the target spectral peak, record the digital quantities corresponding to the maximum and minimum values of the error signal, and average the two as the initial point of the laser frequency.

Step 5, generate a feedback control signal

The error signal output by the improved two-way lock-in amplifier is sent to the digital PID circuit, and the feedback control quantity is obtained.

Step 6, obtain the amount of voltage used for frequency stabilization

The feedback control amount and the modulation amount are added by the digital adder 8 in the FPGA, and then the output voltage is output to the input port of the adding circuit 11 through the DAC10. At the other input end, the output of the adding circuit 11 is the voltage used for frequency stabilization.

Step 7, convert into a frequency-stabilized current for driving laser 1:

The frequency-stabilized voltage signal is sent to the input end of the voltage-current converter 13, and a driving current capable of stabilizing the frequency of the laser 1 is obtained through a 15-ohm conversion resistor.

The invention improves the dual-channel digital phase-locked amplifier, so that it can acquire the error signal and the phase at the same time, and according to the phase, a feedback loop can be formed inside the phase-locked loop to dynamically adjust the phase difference between the two, so that the phase difference is always 0° . It gets rid of the limitation of relying on manual adjustment and improves the intelligence of the frequency stabilization system. At the same time, because of the existence of the negative feedback loop inside the lock-in amplifier, the problem of the jitter of the error signal caused by the phase jitter in the working process of the system can be reduced, and the stability of the error signal can be improved.

Although the present invention has been described in detail with general description and specific embodiments in this specification, some modifications or improvements can be made on the basis of the present invention, which will be obvious to those skilled in the art. Therefore, these modifications or improvements made without departing from the spirit of the present invention fall within the scope of the claimed protection of the present invention.

Claims (8)

1. Laser frequency stabilization system based on improved two-way digital lock-in amplifier, characterized in that includes: the device comprises a laser, a saturated absorption spectrum generation optical path unit, a photoelectric detector, a trans-impedance amplifier, an ADC (analog to digital converter), an improved two-way digital phase-locked amplifier, a first frequency synthesizer, a digital adder, a triangular wave generator, a DAC (digital to analog converter) and an addition circuit;

the laser emits laser, and the laser generates a saturated absorption spectrum optical signal through a saturated absorption spectrum generation optical path unit; the photoelectric detector receives a saturated absorption spectrum optical signal and converts the saturated absorption spectrum optical signal into a saturated spectrum current signal, the saturated spectrum current signal is converted into a voltage signal which can be collected by the ADC through the trans-impedance amplifier, and the voltage signal is converted into a saturated spectrum digital signal through the ADC and then enters the improved two-way digital phase-locked amplifier;

the improved double-path digital phase-locked amplifier extracts an error signal and phase information in a saturation spectrum digital signal and outputs the error signal to a digital PID circuit to obtain a feedback control voltage digital signal; meanwhile, negative feedback is formed by the phase information in the improved two-way digital phase-locked amplifier, and the phase difference is dynamically adjusted to be 0 degree all the time;

when an initial frequency stabilizing point is determined, the frequency scanning range of the laser is narrowed until only an error signal of a target spectrum peak can be observed, at the moment, three input ends of the digital adder are respectively input into the first frequency synthesizer to generate a modulation signal, a frequency sweeping signal generated by the triangular wave generator and an error signal output by the improved two-way digital phase-locked amplifier, and then the initial frequency stabilizing point voltage is determined by combining a bias point of laser frequency and is transmitted to the adding circuit;

the feedback control voltage digital signal is converted into an analog voltage signal through a DAC and then is sent to an addition circuit, and the addition circuit outputs a frequency stabilization voltage signal according to the received initial frequency stabilization point voltage and the feedback control voltage; and realizing the frequency stabilization of the laser by using the frequency stabilization voltage signal.

2. The improved two-way digital lock-in amplifier based laser frequency stabilization system according to claim 1, wherein the improved two-way digital lock-in amplifier comprises a second frequency synthesizer, a third frequency synthesizer, two low pass filters and two digital multipliers; the second frequency synthesizer generates a reference sine demodulation signal, the signal is multiplied by an input saturation spectrum digital signal through a digital multiplier and then filtered through a low-pass filter, and an error signal, namely an I-path signal, is extracted;

the third frequency synthesizer generates a reference cosine demodulation signal, and the signal is multiplied by an input saturated spectrum digital signal through another digital multiplier and then filtered through another low-pass filter to obtain a Q-path signal;

and calculating phase information according to the I path signal and the Q path signal, feeding the phase information back to phase input ports of the second frequency synthesizer and the third frequency synthesizer, and dynamically adjusting the phase to 0 degree to maximize an output error signal.

3. The improved two-way digital lock-in amplifier based laser frequency stabilization system according to claim 1, further comprising a PC, wherein the error signal and the phase are transmitted to the PC through a serial port, and data are displayed in the PC in real time.

4. The laser frequency stabilization system based on the improved two-way digital phase-locked amplifier according to claim 2, wherein the digital signal processing process of the system is realized in an FPGA, that is, the improved two-way digital phase-locked amplifier, the digital PID circuit, the first frequency synthesizer, the second frequency synthesizer, the third frequency synthesizer and the triangular wave generator are realized by the FPGA.

5. The improved two-way digital lock-in amplifier based laser frequency stabilization system of claim 1, further comprising a voltage-to-current converter for generating a laser drive current according to the frequency stabilization voltage signal.

6. The laser frequency stabilization system based on the improved two-way digital lock-in amplifier is characterized in that the saturated absorption spectrum generation optical path unit comprises a first beam splitter, a half-wave plate, a polarization beam splitter, a reflector pair, a second beam splitter and a cesium bulb, wherein laser light is divided into frequency stabilization light and experimental light through the first beam splitter, the frequency stabilization light is divided into strong pump light and weak light through the half-wave plate and the polarization beam splitter in sequence, and the strong pump light is opposite to probe light at the cesium bulb after the direction of the strong pump light is adjusted through the reflector pair to generate a saturated absorption spectrum signal; the weak light is split into reference light and detection light by the second beam splitter and received by the photodetector.

7. The laser frequency stabilization system based on the improved two-way digital phase-locked amplifier as claimed in claim 1, wherein a faraday isolator is arranged between the laser and the saturated absorption spectrum generation optical path unit, and the laser is connected with a temperature control module.

8. The improved two-way digital lock-in amplifier based laser frequency stabilization system according to claim 2, wherein the three modulation signals generated by the first frequency synthesizer, the second frequency synthesizer and the third frequency synthesizer respectively have the same frequency and different phases.

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