Frequency tuning method based on longitudinal mode power difference

Abstract

The invention discloses a longitudinal mode power difference-based frequency tuning method, which comprises the following steps of: installing a polaroid on an anode of a laser tube, and tuning the frequency of a single beam of polarized light; tail light detection is used, tail light output by a laser tube is divided into two beams of vertical polarized light through a Wollaston polarizing prism, and a two-quadrant photoelectric receiver detects the power of the two beams of polarized light and converts optical power signals into electric signals to be output; voltage signals of the two beams of polarized light are obtained, and the power difference of the two beams of polarized light is controlled by setting the longitudinal die pressure difference; calculating a control quantity according to the power difference; the pulse width modulation system transmits the control quantity to the heating film, and the cavity length of the resonant cavity is controlled through the heating film. According to the invention, the photoelectric receiver is used for converting the power of two vertical longitudinal mode polarized light beams of the thermal frequency stabilization He-Ne laser into electric signals for output, and the digital controller algorithm is used for regulating and controlling the difference value of the double longitudinal mode receiving voltage to change the output power of the laser, so that the control of the output frequency of the laser is realized.

Description

Technical Field

[0001] The invention relates to the technical field of thermally stabilized laser frequency tuning, and in particular to a frequency tuning method based on longitudinal mode power difference. Background Art

[0002] The fully intracavity dual longitudinal mode He-Ne thermally stabilized laser has the characteristics of large frequency difference, simple structure, and good frequency reproducibility. It is widely used in high-precision measurement systems, such as laser interferometers, spectrometers, and high-resolution imaging systems. Commonly used interference displacement measurement methods are mainly single-frequency and dual-frequency laser interference. The dual longitudinal mode thermally stabilized laser can be used as the light source of single-frequency and dual-frequency laser interferometers. In the application of single-frequency laser interferometers, the dual longitudinal mode thermally stabilized laser adopts equal power locking and selects specific polarized light by installing polarizers to achieve single-frequency output. However, this method not only causes the optical power to be halved, but also the laser output frequency is constant, which cannot meet the precise tuning requirements of the output frequency in specific occasions. Therefore, a frequency tuning method based on longitudinal mode power difference is needed. Summary of the invention

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

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] The present invention comprises the following steps:

[0006] Install a polarizer on the anode of the laser tube to tune the frequency through polarized light;

[0007] Using tail light detection, the tail light output by the laser tube is divided into two beams of vertically 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 signal into an electrical signal for output;

[0008] Obtaining voltage signals of two polarized light beams and controlling the power difference of the two polarized light beams by setting a longitudinal mode voltage difference;

[0009] Calculate the control quantity based on the power difference;

[0010] The pulse width modulation system transmits the control amount to the heating film, controls the cavity length of the resonant cavity through the heating film, stabilizes the two polarized light beams to different voltage differences, and uniformly changes the voltage difference in an incremental form to change the longitudinal mode power to achieve frequency tuning. The fitting formula of the dual longitudinal mode pressure difference and the single mode frequency is:

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

[0012] Where x is the voltage difference between the two longitudinal modes; y is the average power of the single-mode output.

[0013] Furthermore, the PID control formula used by the algorithm for calculating the control quantity based on the power difference is as follows:

[0014]

[0015] The PID formula is discretized and controlled, and the discretized formula is as follows:

[0016]

[0017] Where u(t) is the output of the controller at time t, e(t) is the input signal of the controller, 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.

[0018] Furthermore, the laser tube is a 633nm full-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, including 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, wherein a polarizer is installed on the anode of the laser tube, a heating film is provided on the surface of the laser tube, a tail light output end of the laser tube is connected to the optical path incident end of the Wollaston polarizing prism, an optical path output end of the Wollaston polarizing prism is connected to the input end of the two-quadrant photoelectric receiver, an output end of the two-quadrant photoelectric receiver is connected to the input end of the AD collector, an output end of the AD collector is connected to the input end of the digital controller, the digital controller is connected to the input end of the pulse width modulator, and an output end of the pulse width modulator is connected to the input end of the heating film.

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

[0021] 1. The present invention achieves precise tuning of the output frequency by controlling the power between the longitudinal modes of a 633nm dual-longitudinal-mode He-Ne laser, thereby overcoming the limitation that the single-frequency output frequency of a dual-longitudinal-mode laser cannot be precisely adjusted, and meeting the demand for laser frequency tuning for high-precision measurement.

[0022] 2. The present invention adopts an intelligent digital control system, which generates a linear relationship between the longitudinal mode power difference and frequency tuning through software algorithm calculation, and uses pulse width modulation technology to accurately control the cavity length, thereby achieving precise control of the laser frequency and meeting the strict requirements of scientific research experiments and high-precision measurements.

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

[0024] Figure 1 The present invention is a flowchart of a frequency tuning method based on longitudinal mode power difference.

[0025] Figure 2 It is the algorithm flow chart of the present invention.

[0026] Figure 3 The fitting curve diagram of the average frequency and voltage difference of the laser of the present invention is

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

[0028] The present invention is described in detail below in conjunction with the embodiments and accompanying drawings. However, it should be understood that the embodiments and drawings are only used to exemplify the present invention and do not constitute any limitation on the protection scope of the present invention.

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

[0030] In this technical solution, a 633nm full-cavity He-Ne laser tube is used, and a polarizer is installed on the anode of the laser tube to filter out one beam of polarized light, leaving another beam of polarized light for frequency tuning. In order not to affect the anode output, tail light detection is used. The tail light output by the laser tube is divided into two beams of vertical polarized light through a Wollaston polarizing prism. The two-quadrant photoelectric receiver detects the power of the two beams of polarized light and converts the optical power signal into an electrical signal for output. The AD acquisition system collects the voltage signals of the two beams of polarized light and uploads them to the digital control system. The digital controller sets the longitudinal mode voltage difference to control the power difference of the two beams of polarized light. The control amount is calculated based on the power difference using the software algorithm. The pulse width modulation system transmits the control amount to the heating film, and the cavity length of the resonant cavity is controlled by the heating film, so that the two beams of polarized light are stabilized to different voltage differences, thereby realizing the precise control of the frequency of the single longitudinal mode output light. The software algorithm uniformly changes the voltage difference in an incremental form to change the longitudinal mode power, thereby realizing the linear change of the output frequency of the dual longitudinal mode He-Ne laser.

[0031] like Figure 2 As shown, the present invention comprises the following steps:

[0032] Install a polarizer on the anode of the laser tube to tune the frequency through polarized light;

[0033] Using tail light detection, the tail light output by the laser tube is divided into two beams of vertically 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 signal into an electrical signal for output;

[0034] Obtaining voltage signals of two polarized light beams and controlling the power difference of the two polarized light beams by setting a longitudinal mode voltage difference;

[0035] Calculate the control quantity based on the power difference;

[0036] The pulse width modulation system transmits the control amount to the heating film, and controls the cavity length of the resonant cavity through the heating film, so that the two polarized light beams are stabilized to different voltage differences. The control amount uniformly changes the voltage difference in an increasing form, thereby changing the longitudinal mode power. The fitting formula of the dual longitudinal mode pressure difference and the single mode frequency is:

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

[0038] Where x is the voltage difference between the two longitudinal modes; y is the average power of the single-mode output.

[0039] In this implementation example, the PID control formula used by the algorithm for calculating the control amount based on the power difference is as follows:

[0040]

[0041] The PID formula is discretized and controlled, and the discretized formula is as follows:

[0042]

[0043] Where u(t) is the output of the controller at time t, e(t) is the input signal of the controller, 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-inner cavity He-Ne laser tube.

[0045] Specific implementation example: 633nm dual longitudinal mode He-Ne laser with an output power of 0.5mW, using the above Figure 1 The system is equipped with a polarizer at the output anode of the laser tube to filter out a beam of polarized light to achieve single-frequency output. The tail light is split into two by a Wollaston prism, and the optical signal is converted into an electrical signal by a photoelectric receiver. The dual longitudinal mode voltage value is collected by an AD acquisition system and uploaded to the digital control system. The change of the dual longitudinal mode voltage difference is controlled to control the longitudinal mode power. The algorithm is used to calculate the control amount according to the power difference, and the pulse width modulation is used to control the change of the resonant cavity length to achieve the control of the single-frequency laser output frequency.

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

[0047] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A frequency tuning method based on longitudinal mode power difference, characterized in that: The following steps are involved: Install a polarizer on the anode of the laser tube to filter out a beam of polarized light and perform frequency tuning on the single beam of polarized light; Using tail light detection, the tail light output by the laser tube is divided into two beams of vertically 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 signal into an electrical signal for output; Obtaining voltage signals of two polarized light beams and controlling the power difference of the two polarized light beams by setting a longitudinal mode voltage difference; Calculate the control quantity based on the power difference; The pulse width modulation system transmits the control amount to the heating film, and controls the cavity length of the resonant cavity through the heating film, so that the two polarized light beams are stabilized to different voltage differences. The control amount uniformly changes the voltage difference in an increasing form, thereby changing the longitudinal mode power. The fitting formula of the dual longitudinal mode pressure difference and the single mode frequency is: y=(0.03851±0.00214)x+(636.784±0.1), Where x is the voltage difference between the two longitudinal modes; y is the average power of the single-mode output.

2. The frequency tuning method based on longitudinal mode power difference according to claim 1, characterized in that: The PID control formula used by the algorithm to calculate the control quantity based on the power difference is as follows: The PID formula is discretized and controlled, and the discretized formula is as follows: Where u(t) is the output of the controller at time t, e(t) is the input signal of the controller, 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.

3. The frequency tuning method based on longitudinal mode power difference according to claim 1, characterized in that: The laser tube is a 633nm full-inner cavity He-Ne laser tube.

4. A frequency tuning device based on longitudinal mode power difference, used to perform the frequency tuning method based on longitudinal mode power difference according to any one of claims 1 to 3, characterized in that: The invention comprises a polarizing plate, 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 polarizing plate is installed on the anode of the laser tube, the surface of the laser tube is provided with a heating film, the tail light output end of the laser tube is connected to the light path incident end of the Wollaston polarizing prism, the light path output end of the Wollaston polarizing prism is connected to the input end of the two-quadrant photoelectric receiver, the output end of the two-quadrant photoelectric receiver is connected to the input end of the AD collector, the output end of the AD collector is connected to the input end of the digital controller, the digital controller is connected to the input end of the pulse width modulator, and the output end of the pulse width modulator is connected to the input end of the heating film.

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