Laser power locking system

Abstract

The invention discloses a laser power locking system which comprises an optical system device, a data acquisition device and an adjusting device. The data acquisition device comprises a reference voltage determination module, an optical detection module and an amplifier, the reference voltage determination module is connected with the positive input end of the amplifier, and the optical detection module is connected with the negative input end of the amplifier; wherein the optical detection module comprises a photodiode, and when the voltage at the output end of the photodiode is the same as the voltage at the output end of the reference voltage determination module, the power of the strong light signal is locked. When the voltage at the output end of the photodiode in the data acquisition module is the same as the voltage at the output end of the reference voltage determination module, the direct current signal output by the photodiode is offset, and only the alternating current fluctuation signal output by the photodiode is amplified, so that the noise caused by the direct current signal output by the photodiode is avoided; and the signal-to-noise ratio of the detection signal output by the data acquisition device is improved, so that the stability of laser output is improved.

Description

Technical Field

[0001] The present application relates to the technical field of lasers, and more specifically, to a laser power locking system. Background Art

[0002] The LD-pumped 1064nm single-frequency solid-state laser has characteristics such as high efficiency, high beam quality, high power, and low noise. It has important applications in fields such as coherent detection and optical measurement. Low-noise and high-stability single-frequency laser output is a necessary guarantee for achieving accurate measurement.

[0003] In order to obtain high-stability single-frequency laser output, a laser power locking system is required to lock the power of the laser. In the current laser power locking system, the signal-to-noise ratio of the error signal detected during the locking process is relatively low. Therefore, the stability of the laser is poor. Summary of the Invention

[0004] The present application provides a laser power locking system. When the output terminal voltage of the photodiode in the data acquisition module is the same as the output terminal voltage of the reference voltage determination module, the DC signal output by the photodiode is cancelled, and only the AC fluctuation signal output by the photodiode is amplified. Thus, the noise caused by the DC signal output by the photodiode is avoided, the signal-to-noise ratio of the detection signal output by the data acquisition device is improved, and the stability of the laser output is thereby improved.

[0005] The present application provides a laser power locking system, including an optical system device, a data acquisition device, and an adjustment device;

[0006] The optical system device includes a laser and an interferometer. The laser output by the laser enters the interferometer, and the interferometer outputs a strong light signal and a weak light signal at the same time. The strong light signal is used as the output of the laser power locking system;

[0007] The data acquisition device acquires the weak light signal and generates a detection signal to output to the adjustment device. The adjustment device generates a feedback signal based on the detection signal and outputs it to the piezoelectric ceramic in the interferometer for adjustment to achieve power locking of the strong light signal;

[0008] The data acquisition device includes a reference voltage determination module, a light detection module, and an amplifier. The reference voltage determination module is connected to the positive input terminal of the amplifier, and the light detection module is connected to the negative input terminal of the amplifier;

[0009] Among them, the light detection module includes a photodiode. When the output terminal voltage of the photodiode is the same as the output terminal voltage of the reference voltage determination module, the power of the strong light signal is locked.

[0010] Preferably, the interferometer includes a first highly reflective mirror, a second highly reflective mirror, a third highly reflective mirror, and a fourth highly reflective mirror. The first highly reflective mirror and the third highly reflective mirror are the input mirror and the output mirror respectively;

[0011] The weak light signal obtained after the laser is transmitted through the first highly reflective mirror enters the second highly reflective mirror. The second highly reflective mirror reflects the weak light signal and then reaches the third highly reflective mirror. The third highly reflective mirror transmits the weak light signal and outputs it from the interferometer;

[0012] The strong light signal obtained after the laser is reflected by the first highly reflective mirror enters the fourth highly reflective mirror. The fourth highly reflective mirror reflects the strong light signal and then reaches the third highly reflective mirror. The third highly reflective mirror reflects the strong light signal and outputs it from the interferometer.

[0013] Preferably, the adjustment device includes a data processing device and a feedback control device;

[0014] The data processing device includes an analog-to-digital conversion module and a data processing module. The analog-to-digital conversion module converts the analog signal output by the data acquisition device into a digital signal, and the data processing module converts the digital signal into a digital control signal.

[0015] Preferably, the feedback control device includes a digital-to-analog conversion module and a high-voltage amplifier. The digital-to-analog conversion module is used to convert the digital control signal into an analog control signal, and the high-voltage amplifier adjusts the bias voltage and gain according to the analog control signal and then acts on the piezoelectric ceramic.

[0016] Preferably, the data processing module is a field programmable gate array.

[0017] Preferably, after the data processing device processes the analog signal through PID regulation, a control signal is obtained.

[0018] Preferably, after the data processing device processes the analog signal, it outputs the control signal to the feedback control device, specifically including:

[0019] Converting the analog signal into a digital signal;

[0020] Taking the difference between the digital signal and the expected value as the first deviation value at the current sampling moment;

[0021] Passing the first deviation value through three D flip-flops triggered by the rising edge of the same clock in sequence to obtain three deviation values in time sequence: the first deviation value, the second deviation value, and the third deviation value;

[0022] Multiplying and adding the first deviation value, the second deviation value, and the third deviation value by the proportional coefficient, the integral coefficient, and the differential coefficient respectively to obtain a first control quantity;

[0023] Performing a limiting process on the first control quantity to obtain a second control quantity, and taking the second control quantity as the control signal and outputting it to the feedback control device.

[0024] Preferably, the proportionality coefficient is used to adjust the sensitivity of the controller to the error, the integral coefficient is used to eliminate the long-term error, and the differential coefficient is used to predict the subsequent error.

[0025] Other features and advantages of the present application will become clear from the following detailed description of the exemplary embodiments of the present application with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The drawings incorporated in and constituting a part of this specification illustrate embodiments of the present application and, together with the description, serve to explain the principles of the present application.

[0027] Figure 1 Structural diagram of the laser power locking system provided for the present application;

[0028] Figure 2 Circuit structural diagram of the data acquisition device provided for the present application;

[0029] Figure 3 Control schematic diagram of the data processing module provided for the present application;

[0030] Figure 4 Optical power fluctuation situation without interference signal provided for the present application;

[0031] Figure 5 Optical power of the locking gear provided for the present application. DETAILED DESCRIPTION OF THE INVENTION

[0032] Various exemplary embodiments of the present application will now be described in detail with reference to the accompanying drawings. It should be noted that: unless otherwise specifically stated, the relative arrangements of components and steps, numerical expressions, and numerical values set forth in these embodiments do not limit the scope of the present application.

[0033] The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the present application, its application, or its use.

[0034] Techniques, methods, and devices known to those of ordinary skill in the relevant art may not be discussed in detail, but where appropriate, the techniques, methods, and devices should be considered as part of the specification.

[0035] In all the examples shown and discussed herein, any specific values should be construed as merely exemplary and not as a limitation. Thus, other examples of the exemplary embodiments may have different values.

[0036] The present application provides a laser power locking system. When the output voltage of the photodiode in the data acquisition module is the same as the output voltage of the reference voltage determination module, the DC signal output by the photodiode is cancelled, and only the AC fluctuation signal output by the photodiode is amplified. Thus, the noise caused by the DC signal output by the photodiode is avoided, the signal-to-noise ratio of the detection signal output by the data acquisition device is improved, and the stability of the laser output is thereby enhanced.

[0037] As Figure 1 shown, the laser power locking system provided by the present application includes an optical system device, a data acquisition device, a power monitoring device, and an adjustment device.

[0038] The optical system device includes a laser 1 and an interferometer, and the laser output by the laser 1 enters the interferometer. Specifically, the laser emitted by the laser passes through a 532nm high reflector and a 1064nm high lens to filter out the 532nm green light, and the light intensity entering the interferometer is adjusted through a beam splitter prism, and then enters the interferometer after passing through a half-wave plate and a high reflector. The interferometer outputs a strong light signal and a weak light signal at the same time. The weak light signal enters the data acquisition device, and the strong light signal is monitored by the power monitoring device, and this strong light signal is used as the output of the laser power locking system.

[0039] Preferably, the output power of the laser 1 is greater than 10 watts (watt level).

[0040] As an embodiment, as Figure 1 shown, the interferometer includes a 1064nm high reflector (first high reflector) 2, a second high reflector (1064nm) 4, a third high reflector (1064nm) 5, and a fourth high reflector (1064nm) 3. The first high reflector 2 and the third high reflector 5 are the input mirror and the output mirror respectively. The weak light signal obtained after the laser is transmitted through the first high reflector 2 enters the second high reflector 4. The second high reflector 4 reflects the weak light signal and reaches the third high reflector 5. The third high reflector 5 transmits the weak light signal and outputs it from the interferometer, and then enters the data acquisition device after being transmitted through the high reflector 7. The data acquisition device converts the weak light interference signal into a detection signal and then enters the adjustment device. The adjustment device generates a feedback signal based on the detection signal and outputs it to the piezoelectric ceramic in the interferometer for adjustment to achieve the power locking of the strong light signal.

[0041] The strong light signal obtained after the laser is reflected by the first high reflector 2 enters the fourth high reflector 3. The fourth high reflector 3 reflects the strong light signal and reaches the third high reflector 5. The third high reflector 5 reflects the strong light signal and outputs it from the interferometer, and then outputs it after being reflected by the high reflector 7 and passing through the power monitoring device. The power monitoring device is used to monitor the actual value of the laser power.

[0042] In the prior art, the laser power of the power locking system is basically in the milliwatt level. In order to obtain sufficient interference light, the reflectivity and transmittance of the cavity mirrors in the interferometer are both about 50%, that is, nearly half of the light output by the laser is used for interference light, and nearly half of the light is used for power detection.

[0043] In this application, since the output power of the laser is high (in the watt level), a high reflector 2 and a third high reflector 5 in the interferometer can obtain sufficient interference light through a lower transmittance. Based on such considerations, preferably, the transmittance of the first high reflector 2 and the third high reflector 5 is 2%-6%, and the reflectivity is 94%-98%. The reflectivity of the second high reflector 4 and the fourth high reflector 3 is greater than 99%. Thus, although the transmitted light obtained by the small transmittance of the high-power laser through the first high reflector 2 is weak light, the high-power laser makes the weak light sufficient for the detection of interference light, and the reflected light obtained by the large reflectivity of the first high reflector 2 from most of the high-power laser forms strong light for power detection. Thus, the cooperation of the high-power laser and the high reflector with low transmittance and high reflectivity reduces the difficulty of laser power locking and improves the efficiency of laser power locking.

[0044] As an embodiment, as Figure 2 shown, the data acquisition device is a high-sensitivity photodetector, including a reference voltage determination module, a light detection module, and an amplifier (such as OPA2227). The reference voltage determination module is connected to the positive input terminal of the amplifier, the light detection module is connected to the negative input terminal of the amplifier, and the output signal of the amplifier is used as the output signal of the data acquisition device.

[0045] The reference voltage determination module includes a sliding rheostat R, a fourth resistor R4, and a capacitor C. The first end of the sliding rheostat R is grounded, the second end is connected to the positive input terminal of the operational amplifier (forming node B), a fourth resistor R4 is provided between the second end of the sliding rheostat R and the power supply VCC, and a capacitor is provided between the second end of the sliding rheostat R and the ground. Among them, the sliding rheostat R is used to adjust the amplitude range of the input optical signal of the data acquisition device. By sliding to change the resistance value of the sliding rheostat R, the reference voltage of the reference voltage determination module will change, thereby the amplitude range of the input optical signal of the data acquisition device, and further change the output power of the laser power locking system.

[0046] As an embodiment, the optical signal is amplified within the amplitude range of the input optical signal entering the data acquisition device; when the input optical signal does not enter the amplitude range of the data acquisition device and the light does not enter the data acquisition device, the maximum voltage 4.4V of the data acquisition device is displayed; when the input optical signal does not enter the amplitude range of the data acquisition device and the light has entered the detector, the minimum voltage 100mV of the data acquisition device is displayed. Thus, through the designed high-sensitivity data acquisition device, the fluctuation of the optical signal can be collected with higher precision.

[0047] The optical detection module includes a photodiode PD, a first resistor R2, and a third resistor R3. The first end of the first resistor R2 is connected to the output end of the photodiode PD (forming node A), the second end of the first resistor R2 is connected to the inverting input end of the operational amplifier, and a third resistor R3 is also provided between the first end of the first resistor R2 and the ground. The other end of the photodiode PD is connected to the power supply VCC. The two ends of the second resistor R1 are respectively connected to the inverting input end and the output end OUT of the operational amplifier. Among them, R1 and R2 are two resistors with resistance values of 200 kΩ and 1 kΩ, and R2 and R1 are connected to form a proportional amplification factor of 200 times. Among them, the photodiode PD is used to detect the weak optical signal transmitted through the high-reflection mirror 7, and amplify the signal through the amplifier and transmit it to the adjustment device for power adjustment.

[0048] In the data acquisition device of the present application, the signal of the photodiode PD is a large DC and AC fluctuation signal. When the voltage at the output end of the photodiode PD (i.e., the voltage at node A) is the same as the voltage at the output end of the reference voltage determination module (i.e., the voltage at node B), the power of the strong optical signal output by the interferometer is the same as the preset output power, and power locking can be performed at this time. At this time, the DC signal Idc output by the photodiode PD is canceled, and only the AC fluctuation signal output by the photodiode PD is amplified. Therefore, the amplification factor added to the second resistor R1 and the first resistor R2 only amplifies the AC fluctuation signal Ii of the photodiode PD, and the DC signal at the output end OUT of the data acquisition device remains unchanged, and the output signal is R3*Idc + amplification factor * R3*Ii. This avoids the noise brought by the DC signal output by the photodiode, improves the signal-to-noise ratio of the detection signal output by the data acquisition device, and thus improves the stability of the laser output.

[0049] The adjustment device includes a data processing device and a feedback control device. The data processing device receives the detection signal output by the data acquisition device and converts it into a control signal and outputs it to the feedback control device. The feedback control device sends a control signal to the piezoelectric ceramic 6 on the interferometer to adjust the cavity length of the interferometer and realize the control of the laser power. Among them, the data processing device and the feedback control device respectively perform analog-to-digital conversion and digital-to-analog conversion, and the data processing device performs digital signal processing internally.

[0050] As an embodiment, as Figure 1 shown, the data processing device includes an analog-to-digital conversion module (A / D module) and a data processing module. The analog-to-digital conversion module converts the analog signal output by the data acquisition device into a digital signal, and the data processing module converts the digital signal into a digital control signal.

[0051] As an embodiment, the analog-to-digital conversion module uses the high-speed AD module AN9238, which is a 2-channel, 65MSPS, 12-bit analog-to-digital signal conversion module with an accuracy of 2.4mv. It is used to convert the optical signal received by the highly sensitive photodetector into an electrical signal.

[0052] Preferably, the analog-to-digital conversion module uses the AD7606 module with a bit width of 16 bits and an accuracy that can reach 0.15mv, which is 16 times that of the currently used module.

[0053] As an embodiment, the data processing module is a field-programmable gate array (FPGA), and it uses a development board based on the XILINX Zynq7000 development platform. This development platform is a solution for XILINX's Zynq7000 SOC chip. It uses ARM+FPGA SOC technology to integrate the dual-core ARM Cortex-A9 and FPGA programmable logic on one chip. It uses XILINX's Zynq7000 series XCZ7020-2CLG400I as the core processor, which has rich hardware resources and peripheral interfaces on both the ARM and FPGA.

[0054] The data processing module is used to receive the actual voltage value of the detection signal, calculate the error value based on the set reference voltage value and the actual voltage value, and then use digital PID to calculate the digital signal of the voltage control amount according to the error value. That is, after the data processing device processes the analog signal through PID regulation, a control signal is obtained.

[0055] The data processing module is a host computer software with PID digital function, and the host computer software is used to adjust K P ,K I and K D The specific values of the three parameters, as well as the switching between the scanning gear and the locking gear and the execution of the PID function by the hardware FPGA. The hardware is used to implement the calculation of the digital PID algorithm. After calculating the control voltage signal, it is sent to the feedback control device.

[0056] Among them, the proportional coefficient K P is used to adjust the sensitivity of the controller to the error. A larger K P value means that the controller will respond to the error faster. An initial K P value can be set, and then each time the signal feedback changes, according to the difference between the feedback value and the set value, that is, e(k), the K P value is adjusted. Generally speaking, if the difference is greater than the pre-set threshold, the K P value is increased, otherwise the K P value is decreased.

[0057] The integral coefficient KI For eliminating long-term errors, K I The larger the value, the faster the controller eliminates the long-term accumulated errors. The value of K can be adjusted according to the integral of the difference between the current voltage acquisition value and the set value (i.e., the sum of the error accumulation). I If the integral value is greater than a preset threshold, increase the value of K I value; otherwise, decrease the value of K I value.

[0058] Differential coefficient K D For predicting subsequent errors, K D The larger the value, the earlier the controller reacts to future errors. The value of K can be adjusted according to the differential of the difference between the current voltage acquisition value and the set value (i.e., the rate of change of the error). D If the differential value has a significant positive or negative value, increase the value of K D value; otherwise, decrease the value of K D value.

[0059] As an embodiment, as Figure 3 shown, after the data processing device processes the analog signal, it outputs the control signal to the feedback control device, which specifically includes:

[0060] Set the initial values in the program (including the maximum control increment u(k)max, the minimum control increment u(k)min, and the deviation values e(k1) = e(k2) = 0), then manually switch to the locked gear and start the digital PID calculation. Convert the input analog signal c(t) of this time into a digital signal. Use the difference between the digital signal and the expected value as the first deviation value e(k) at the current sampling moment. Pass the first deviation value e(k) through three D flip-flops triggered by the rising edge of the same clock in sequence to obtain three deviation values in time sequence: the first deviation value e(k), the second deviation value e(k1), and the third deviation value e(k2). Subsequently, multiply and add the first deviation value e(k), the second deviation value e(k1), and the third deviation value e(k2) with the proportional coefficient K P , integral coefficient K I and differential coefficient K D respectively to obtain the first control quantity u(k) = K P (e(k) - e(k1)) + K I (e(k)) + K D(e(k) - 2e(k1) + e(k2)). Finally, perform a clipping process on the first control quantity u(k) to obtain a second control quantity, and output the second control quantity as the control signal pid_out to the feedback control device for digital-to-analog conversion and apply it to the piezoelectric ceramic 6. By changing the voltage applied across the piezoelectric ceramic 6, the contraction and elongation of the piezoelectric ceramic are adjusted, thereby changing the length of the interferometer cavity, compensating the output optical power, and completing the stable control of the laser power.

[0061] Among them, during the clipping process, if u(k) > u(k)max, the second control quantity is u(k)max; if u(k) < u(k)min, the second control quantity is u(k)min; if u(k)min < u(k) < u(k)max, the second control quantity is u(k).

[0062] The feedback control device includes a digital-to-analog conversion module and a high-voltage amplifier. The digital-to-analog conversion module is used to convert the digital control signal into an analog control signal, that is, receive the digital signal of the voltage control quantity and convert the digital signal of the voltage control quantity into an analog signal. The high-voltage amplifier adjusts the bias voltage and gain according to the analog control signal and then acts on the piezoelectric ceramic to achieve the locking of the laser power.

[0063] As Figure 4 shown is the optical power fluctuation when there is no interference signal, Figure 5 and the optical power fluctuation at the locking gear. The data variance without locking is 0.21044, and the data variance at the locking gear is 0.00175. At present, the present application has achieved the stable control of the laser power. The coefficient of variation during free operation is 0.1%, and the coefficient of variation during locking is 0.0026%. The power fluctuation (peak-to-peak value) is reduced from ±1% to ±0.032%.

[0064] According to the adjusted PID coefficients, observe the feedback control effect. If further optimization is needed, perform iterative adjustment. During the laser power locking process, repeat the steps of data acquisition, data processing, and feedback control, and adjust and optimize the PID parameters until the voltage value of the interference signal stabilizes at the set value. The locking program of the present application can currently automatically achieve re-locking within a small range.

[0065] Although some specific embodiments of the present application have been described in detail through examples, those skilled in the art should understand that the above examples are only for illustration and not for limiting the scope of the present application. Those skilled in the art should understand that the above embodiments can be modified without departing from the scope and spirit of the present application. The scope of the present application is defined by the appended claims.

Claims

1. A laser power locking system, characterized in that: It includes an optical system device, a data acquisition device, and an adjustment device; The optical system device comprises a laser and an interferometer, the laser output by the laser enters the interferometer, the interferometer simultaneously outputs a strong light signal and a weak light signal, and the strong light signal is used as the output of the laser power locking system; The data acquisition device collects the weak light signal and generates a detection signal to output to the adjustment device. The adjustment device generates a feedback signal according to the detection signal and outputs it to the piezoelectric ceramic in the interferometer for adjustment to achieve power locking of the strong light signal. The data acquisition device comprises a reference voltage determination module, a light detection module and an amplifier, wherein the reference voltage determination module is connected to the positive input terminal of the amplifier, and the light detection module is connected to the negative input terminal of the amplifier; Wherein, the light detection module includes a photodiode, and when the output terminal voltage of the photodiode is the same as the output terminal voltage of the reference voltage determination module, the power of the strong light signal is locked.

2. The laser power locking system according to claim 1, characterized in that: The interferometer comprises a first high-reflection mirror, a second high-reflection mirror, a third high-reflection mirror and a fourth high-reflection mirror, wherein the first high-reflection mirror and the third high-reflection mirror are an input mirror and an output mirror respectively; A weak light signal obtained after the laser is transmitted through the first high-reflection mirror enters the second high-reflection mirror, the second high-reflection mirror reflects the weak light signal and reaches the third high-reflection mirror, the third high-reflection mirror transmits the weak light signal and outputs the interferometer; The strong light signal obtained after the laser is reflected by the first high-reflection mirror enters the fourth high-reflection mirror, the fourth high-reflection mirror reflects the strong light signal and reaches the third high-reflection mirror, and the third high-reflection mirror reflects the strong light signal and outputs it to the interferometer.

3. The laser power locking system according to claim 1, characterized in that: The regulating device includes a data processing device and a feedback control device; The data processing device comprises an analog-to-digital conversion module and a data processing module. The analog-to-digital conversion module converts the analog signal output by the data acquisition device into a digital signal, and the data processing module converts the digital signal into a digital control signal.

4. The laser power locking system according to claim 3, characterized in that: The feedback control device includes a digital-to-analog conversion module and a high-voltage amplifier. The digital-to-analog conversion module is used to convert the digital control signal into an analog control signal. The high-voltage amplifier acts on the piezoelectric ceramic after adjusting the bias voltage and gain according to the analog control signal.

5. The laser power locking system according to claim 3, characterized in that: The data processing module is a field programmable gate array.

6. The laser power locking system according to claim 3, characterized in that: The data processing device processes the analog signal through PID regulation to obtain the control signal.

7. The laser power locking system according to claim 6, characterized in that: The data processing device processes the analog signal and outputs the control signal to the feedback control device, specifically including: Converting the analog signal into a digital signal; Taking the difference between the digital signal and the expected value as the first deviation value at the current sampling moment; The first deviation value is sequentially passed through three D flip-flops triggered by the rising edge of the same clock to obtain three deviation values ​​in succession in time: a first deviation value, a second deviation value, and a third deviation value; The first deviation value, the second deviation value and the third deviation value are respectively multiplied by a proportional coefficient, an integral coefficient and a differential coefficient and then added to obtain a first control amount; The first control amount is limited to obtain a second control amount, and the second control amount is output as a control signal to the feedback control device.

8. The laser power locking system according to claim 7, characterized in that: The proportional coefficient is used to adjust the sensitivity of the controller to errors, the integral coefficient is used to eliminate long-term errors, and the differential coefficient is used to predict subsequent errors.

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