A frequency tuning method based on longitudinal mode power difference

Abstract

The application discloses a frequency tuning method based on longitudinal mode power difference, which comprises the following steps: installing a polarizer on the anode of a laser tube to perform frequency tuning on single polarized light; using tail light detection, the tail light output by the laser tube is divided into two beams of vertical polarized light through a Wollaston polarizing prism, a two-quadrant photoelectric receiver detects the power of the two beams of polarized light and converts the optical power signals into electrical signals and outputs; voltage signals of the two beams of polarized light are acquired and the power difference of the two beams of polarized light is controlled through setting longitudinal mode voltage difference; a control amount is calculated according to the power difference; the control amount is transmitted to a heating film through a pulse width modulation system, and the cavity length of a resonant cavity is controlled through the heating film. The application converts the power of two beams of mutually vertical longitudinal mode polarized light of a thermal frequency stabilization He-Ne laser into electrical signal output through a photoelectric receiver, adjusts and controls the voltage difference of the double longitudinal mode receiving voltage through a digital controller algorithm to change the output power of the laser, and then the control of the output frequency of the laser is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of thermal frequency stabilization laser frequency tuning, and particularly relates to a frequency tuning method based on longitudinal mode power difference. BACKGROUND

[0002] The full-cavity double-longitudinal-mode He-Ne thermal frequency stabilization laser has the characteristics of large frequency difference, simple structure and good frequency reproducibility, and is widely used in high-precision measurement systems such as laser interferometer, spectrum analyzer and high-resolution imaging system. The commonly used interference displacement measurement methods mainly include single-frequency and double-frequency laser interference. The double-longitudinal-mode thermal frequency stabilization laser can be used as a light source for single-frequency and double-frequency laser interferometers. In the application of single-frequency laser interferometer, the double-longitudinal-mode thermal frequency stabilization laser adopts equal power locking, and a polaroid is installed to screen specific polarized light to realize single-frequency output, but this method not only reduces the light power by half, but also the output frequency of the laser is constant, which cannot meet the accurate tuning requirement of the output frequency in specific occasions. Therefore, a frequency tuning method based on longitudinal mode power difference is needed. SUMMARY

[0003] In view of the existing technical defects, the purpose of the present application is to provide a frequency tuning method based on longitudinal mode power difference.

[0004] In order to achieve the above purpose, the present application adopts the following technical scheme:

[0005] The present application comprises the following steps:

[0006] A polaroid is installed on the anode of the laser tube to tune the frequency by polarized light;

[0007] Tail light detection is used, and the tail light output by the laser tube is divided into two perpendicular polarized lights by a Wollaston polarizing prism, and a two-quadrant photoelectric receiver detects the power of the two polarized lights and converts the light power signal into an electrical signal output;

[0008] The voltage signals of the two polarized lights are obtained, and the power difference of the two polarized lights is controlled by setting the longitudinal mode voltage difference;

[0009] The control quantity is calculated according to the power difference;

[0010] The pulse width modulation system transmits the control quantity to the heating film, the cavity length of the resonant cavity is controlled by the heating film, the two polarized lights are stabilized to different voltage difference values, the control quantity uniformly changes the voltage difference value in the form of increment, so as to change the longitudinal mode power to realize frequency tuning, and the fitting formula of the double-longitudinal-mode voltage difference and the single-mode frequency is:

[0011] y=(0.03851±0.00214)x+(636.784±0.1)

[0012] Wherein x is the double longitudinal mode voltage difference; y is the average power of single mode output.

[0013] Further, the algorithm for calculating the control amount according to the power difference adopts the PID control formula as follows:

[0014]

[0015] The PID formula is discretized control, and after discretization, the following formula is obtained:

[0016]

[0017] Wherein, u(t) is the output of the controller at time t, e(t) is the input signal of the controller, T i is the integral time, T d is the differential time, K p is the proportional coefficient (or proportional gain), K i is the integral coefficient, and K d is the differential coefficient.

[0018] Further, the laser tube is a 633nm all-in-cavity He-Ne laser tube.

[0019] In another aspect, a frequency tuning device based on longitudinal mode power difference is used to perform the frequency tuning method based on longitudinal mode power difference, which comprises a polarizer, a laser tube, a heating film, a Wollaston polarizing prism, a two-quadrant photoelectric receiver, an AD collector, a digital controller, a pulse width modulator and a controllable laser frequency output, the anode of the laser tube is provided with a polarizer, the surface of the laser tube is provided with a heating film, the tail light output end of the laser tube is connected with the light path incident end of the Wollaston polarizing prism, the light path output end of the Wollaston polarizing prism is connected with the input end of the two-quadrant photoelectric receiver, the output end of the two-quadrant photoelectric receiver is connected with the input end of the AD collector, the output end of the AD collector is connected with the input end of the digital controller, the digital controller is connected with the input end of the pulse width modulator, and the output end of the pulse width modulator is connected with the input end of the heating film.

[0020] Compared with the prior art, the present application has the following beneficial effects:

[0021] 1. The present application realizes the precise tuning of the output frequency by controlling the power between the longitudinal modes of the 633nm double longitudinal mode He-Ne laser, overcomes the limitation that the single frequency output frequency of the double longitudinal mode laser cannot be precisely adjusted, and meets the demand of high-precision measurement on laser frequency tuning.

[0022] 2. The present application adopts an intelligent digital control system, generates a linear relationship between the longitudinal mode power difference and the frequency tuning through software algorithm calculation, precisely controls the cavity length by using the pulse width modulation technology, realizes the precise control of the laser frequency, and meets the strict requirements of scientific research experiments and high-precision measurement.

[0023] 3. The application ensures that the resolution of the single-mode output frequency of the 633 nm dual longitudinal mode He-Ne laser reaches 0.038 MHz / mv, realizes laser frequency tuning on the order of 10 kHz, and guarantees the reliability and accuracy of laser frequency control. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 A flowchart of the frequency tuning method based on longitudinal mode power difference of the application.

[0025] Figure 2 A flowchart of the algorithm of the application.

[0026] Figure 3 A fitting curve diagram of the average frequency of the laser and the voltage difference of the application

[0027] Wherein 1 is a polarizer; 2 is a 633 nm all-in-cavity He-Ne laser tube; 3 is a heating film; 4 is a Wollaston polarizing prism; 5 is a two-quadrant photoelectric receiver; 6 is an AD collector; 7 is a digital controller; 8 is a pulse width modulation; 9 is a controllable laser frequency output; DETAILED DESCRIPTION

[0028] The application will be described in detail below with reference to the embodiments and drawings, but it should be understood that the embodiments and drawings are only used to exemplarily describe the application, and cannot constitute any limitation on the protection scope of the application.

[0029] As shown in Figure 1 A frequency tuning device based on longitudinal mode power difference is used to perform the frequency tuning method based on longitudinal mode power difference, which includes a polarizer, a laser tube, a heating film, a Wollaston polarizing prism, a two-quadrant photoelectric receiver, an AD collector, a digital controller, a pulse width modulator and a controllable laser frequency output. The anode of the laser tube is provided with a polarizer, the inner cavity surface of the laser tube is provided with a heating film, the tail light output end of the laser tube is connected with the light path incident end of the Wollaston polarizing prism, the light path output end of the Wollaston polarizing prism is connected with the input end of the two-quadrant photoelectric receiver, the output end of the two-quadrant photoelectric receiver is connected with the input end of the AD collector, the output end of the AD collector is connected with the input end of the digital controller, the digital controller is connected with the input end of the pulse width modulator, and the output end of the pulse width modulator is connected with the input end of the heating film.

[0030] In the technical solution, a 633nm full-in-cavity He-Ne laser tube is adopted, a polarizer is installed at the anode of the laser tube, so that one polarized light is filtered out, and the other polarized light is left for frequency tuning. In order to not affect the output of the anode, tail light detection is used, the tail light output by the laser tube is divided into two perpendicular polarized lights through a Wollaston polarizing prism, a two-quadrant photoelectric receiver detects the power of the two polarized lights and converts the optical power signals into electrical signals. An AD acquisition system acquires the voltage signals of the two polarized lights and uploads them to a digital control system, a digital controller sets the longitudinal mode pressure difference to control the power difference of the two polarized lights, a software algorithm is used to calculate the control amount according to the power difference, and a pulse width modulation system transmits the control amount to a heating film, so that the resonant cavity length is controlled through the heating film, the two polarized lights are stabilized to different voltage difference values, and the frequency of the single longitudinal mode output light is accurately controlled. The software algorithm uniformly changes the voltage difference value in the form of increment, so as to change the longitudinal mode power, and realize the linear change of the output frequency of the double longitudinal mode He-Ne laser.

[0031] As shown in Figure 2 , the present application comprises the following steps:

[0032] A polarizer is installed at the anode of the laser tube to perform frequency tuning through polarized light;

[0033] Tail light detection is used, the tail light output by the laser tube is divided into two perpendicular polarized lights through a Wollaston polarizing prism, a two-quadrant photoelectric receiver detects the power of the two polarized lights and converts the optical power signals into electrical signals.

[0034] The voltage signals of the two polarized lights are acquired and the longitudinal mode pressure difference is set to control the power difference of the two polarized lights;

[0035] The control amount is calculated according to the power difference;

[0036] The pulse width modulation system transmits the control amount to the heating film, so that the resonant cavity length is controlled through the heating film, the two polarized lights are stabilized to different voltage difference values, and the control amount uniformly changes the voltage difference value in the form of increment, so as to change the longitudinal mode power, wherein the fitting formula of the double longitudinal mode pressure difference and the single mode frequency is:

[0037] =(0.03851±0.00214)x+(636.784±0.1)

[0038] Wherein x is the voltage difference value of the double longitudinal mode; y is the average power of the single mode output.

[0039] In the embodiment, the algorithm for calculating the control amount according to the power difference adopts the following PID control formula:

[0040]

[0041] The PID formula is discretized for control, and after discretization, the following formula is obtained:

[0042]

[0043] Where u(t) is the controller output at time t, e(t) is the controller input signal, Ti is the integral time, Td is the derivative time, Kp is the proportional coefficient (or proportional gain), Ki is the integral coefficient, and Kd is the derivative coefficient.

[0044] In this embodiment, the laser tube is a 633nm full-cavity He-Ne laser tube.

[0045] Specific implementation example: A 633nm dual-longitudinal-mode He-Ne laser with an output power of 0.5mW, utilizing the above... Figure 1 The system uses a polarizer mounted on the output anode of the laser tube to filter out one beam of polarized light, achieving single-frequency output. The tail beam is split in two by a Wollaston prism, and the optical signal is converted into an electrical signal by a photodetector. An AD acquisition system collects the dual longitudinal mode voltage values ​​and uploads them to a digital control system. The longitudinal mode power is controlled by controlling the change in the voltage difference between the two longitudinal modes. An algorithm calculates the control quantity based on the power difference, and pulse width modulation controls the change in the resonant cavity length, thereby controlling the single-frequency laser output frequency.

[0046] Figure 3 The straight line in the graph is the fitted linear regression line, and the gray dots are the actual measured data points. As can be seen from the graph, the fitted line is very close to the data points, indicating a strong linear relationship between the laser's average frequency and the voltage difference, and that the model fits the data very well.

[0047] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A frequency tuning method based on longitudinal mode power difference, characterized in that, It comprises the following steps: A polarizer is installed on the anode of the laser tube to filter out a polarized light, and the single polarized light is frequency tuned; Tail light detection is used, and the tail light output by the laser tube is divided into two perpendicular polarized lights by a Wollaston polarizing prism, and two quadrant photoreceivers detect the power of the two polarized lights and convert the optical power signals into electrical signals; The voltage signals of the two polarized lights are obtained, and the power difference of the two polarized lights is controlled by setting the longitudinal mode voltage difference; The control amount is calculated according to the power difference; The control amount is transmitted to the heating film by the pulse width modulator, the cavity length of the resonant cavity is controlled by the heating film, the two polarized lights are stabilized to different voltage difference values, and the control amount changes the voltage difference value in an incremental form to change the longitudinal mode power, wherein the fitting formula of the double longitudinal mode voltage difference and the single mode frequency is: ， Where x is the double longitudinal mode voltage difference; y is the average frequency of the single mode output; The algorithm for calculating the control amount according to the power difference adopts the PID control formula as follows: The PID formula is discretized for control, and after discretization, the following formula is obtained: where, is the output of the controller at time t, is the input signal to the controller, is the integration time, is the integration coefficient, is the derivative time, is the proportional coefficient, is the derivative coefficient.

2. The longitudinal mode power difference based frequency tuning method of claim 1, wherein, The laser tube is a 633nm all-in-cavity He-Ne laser tube.

3. A longitudinal mode power difference based frequency tuning apparatus for performing the longitudinal mode power difference based frequency tuning method of any one of claims 1-2, characterized by, It comprises a polarizer, a laser tube, a heating film, a Wollaston polarizing prism, a two-quadrant photoreceiver, an AD collector, a digital controller, a pulse width modulator and a controllable laser frequency output, the polarizer is installed on the anode of the laser tube, the surface of the laser tube is provided with the heating film, the tail light output end of the laser tube is connected with the light path incident end of the Wollaston polarizing prism, the light path output end of the Wollaston polarizing prism is connected with the input end of the two-quadrant photoreceiver, the output end of the two-quadrant photoreceiver is connected with the input end of the AD collector, the output end of the AD collector is connected with the input end of the digital controller, the digital controller is connected with the input end of the pulse width modulator, and the output end of the pulse width modulator is connected with the input end of the heating film.

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