Frequency stabilization control algorithm based on longitudinal Zeeman laser

Abstract

The application provides a frequency stabilization control algorithm based on a longitudinal Zeeman laser, and the method is as follows: in a preheating stage, the initial temperature of a laser tube is measured when the laser is powered on, the required mode-slip number is calculated, the laser tube is heated to a predetermined mode at a fixed power, and when the three conditions of laser light power output, the light power difference between left-handed circularly polarized light and right-handed circularly polarized light in a frequency stabilization state, and the mode-slip time being greater than the fastest mode-slip time are met simultaneously, the frequency stabilization mode is entered, and the light power difference between the left-handed circularly polarized light and the right-handed circularly polarized light is zero by using temperature closed-loop control and light power closed-loop control in the frequency stabilization mode. The application designs a frequency stabilization control algorithm with fast locking and high stability.

Description

Technical Field

[0001] This invention relates to a frequency stabilization control algorithm based on a longitudinal Zeeman laser, which can be used to improve the frequency stability of a longitudinal Zeeman laser and shorten the frequency stabilization lock-in time, belonging to the field of laser frequency stabilization technology. Background Technology

[0002] As a core dual-frequency light source for high-end equipment such as ultra-precision interferometry, geometric metrology, nano-positioning, and semiconductor lithography, the longitudinal Zeeman helium-neon laser's frequency stability, frequency difference consistency, and long-term operational reliability are key indicators determining the measurement accuracy of the entire system. Currently, the precision metrology field generally requires lasers to achieve a long-term relative frequency stability of 10-1. −9 It has the capacity to quickly start up, reproduce the power-on frequency, and resist environmental interference.

[0003] Longitudinal Zeeman lasers rely on an external longitudinal magnetic field to split a single spectral line into two circularly polarized beams, one left-handed and one right-handed, using the power difference between the two beams as the frequency stabilization reference. Affected by multiple factors such as ambient temperature fluctuations, mechanical vibrations, power supply disturbances, magnetic field drift, and cavity thermal deformation, the laser's operating point will continuously deviate from the ideal frequency stabilization zero point. Hardware circuitry and passive temperature control alone are insufficient to offset these disturbances; therefore, a frequency stabilization algorithm is necessary to identify errors, calculate deviations, and output control signals to achieve dynamic closed-loop locking.

[0004] Early frequency stabilization solutions relied on discrete circuits to build the logic, resulting in simple control logic and fixed parameters. These solutions suffered from problems such as response lag, large overshoot, and weak anti-interference capabilities. Under complex operating conditions, they were prone to frequency drift, closed-loop lockout, and frequency jitter, only meeting the needs of low-precision applications. With the continuous upgrading of precision measurement technology, application scenarios have placed higher demands on the light source: on the one hand, the laser needs to lock quickly upon startup and have a shorter warm-up and stabilization time; on the other hand, the system needs to maintain frequency stability under external disturbances such as gradual temperature changes, electromagnetic interference, and slight vibrations, while ensuring frequency reproducibility across multiple devices and multiple startups.

[0005] This paper presents a frequency stabilization control algorithm based on a longitudinal Zeeman laser to shorten the frequency stabilization lock-in time and improve the frequency stability of the laser. Summary of the Invention

[0006] To address the requirements of longitudinal Zeeman lasers for high frequency stability and rapid frequency stabilization and locking, a frequency stabilization control algorithm for longitudinal Zeeman lasers is designed, which effectively shortens the system locking time and improves the frequency stability of the laser output.

[0007] This invention relates to a frequency stabilization control algorithm based on a longitudinal Zeeman laser, comprising the following steps:

[0008] The frequency stabilization operation of a laser consists of two stages: the preheating stage and the frequency stabilization control stage.

[0009] When the laser is powered on, a preheating stage is first performed. During the preheating stage, the initial temperature of the laser tube is detected, and then the corresponding mode number is calculated based on the mode temperature mapping relationship calibrated by the natural preheating experiment.

[0010] The controller completes the calculation of the number of steps for the laser mode and outputs a constant heating power to heat the laser tube, thereby adjusting the laser's resonant state to the predetermined mode.

[0011] Once the predetermined mode is reached, the drive voltage is dynamically adjusted.

[0012] When the laser has optical power output, the optical power difference between left-handed and right-handed circularly polarized light is in a frequency-stabilized state, and the mode-travel time is greater than the fastest mode-travel time, the laser enters the frequency-stabilized mode.

[0013] In frequency stabilization mode, temperature closed-loop control and optical power closed-loop control are used to make the optical power difference between left- and right-circularly polarized light zero.

[0014] The frequency stabilization control module introduces a fuzzy adaptive PID-Smith control algorithm to dynamically regulate the output voltage of the heating drive unit. By changing the cavity length of the laser resonator, the power difference between the left- and right-circularly polarized light is kept at zero, thus achieving stable control of the laser frequency. Attached Figure Description

[0015] Figure 1 This is a flowchart of a frequency stabilization control algorithm based on a longitudinal Zeeman laser.

[0016] Figure 2 This is a diagram of a fuzzy adaptive PID-Smith control system based on a frequency stabilization control algorithm for a longitudinal Zeeman laser.

[0017] Figure 3 This is a schematic diagram of the Zeeman mode of a frequency stabilization control algorithm based on a longitudinal Zeeman laser. Detailed Implementation

[0018] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0019] Before implementing frequency stabilization control, a natural preheating experiment must be completed: During the experiment, the heating module is not turned on to heat the laser tube. After the laser is powered on, it automatically heats up and automatically switches modes, while the temperature measuring circuit continuously records the temperature of the outer wall of the laser tube. This experiment serves two purposes: First, it establishes the relationship between the switching mode number and temperature. After the laser is powered on, the switching mode number is calculated based on the temperature difference between the initial temperature and the set temperature, combined with the mode-temperature relationship, thus completing mode matching before frequency stabilization. Second, it selects the preset temperature control value based on the highest temperature of the laser's natural preheating. If the frequency stabilization set temperature is lower than the peak temperature of the natural preheating, the tube temperature will continue to rise after frequency stabilization is completed, resulting in frequency shift and reducing the accuracy of output frequency reproduction.

[0020] like Figure 1 As shown, during the preheating stage, the laser first measures the initial temperature, and based on the relationship between mode and temperature obtained from the natural preheating experiment, calculates the total number of modes that the laser needs to operate on.

[0021] The controller completes the calculation of the number of steps for the laser mode and outputs a constant heating power to heat the laser tube, thereby adjusting the laser's resonant state to the predetermined mode.

[0022] The drive voltage is dynamically adjusted after the predetermined mode is reached.

[0023] When the laser has optical power output, the optical power difference between left-handed and right-handed circularly polarized light is in a frequency-stabilized state, and the mode-travel time is greater than the fastest mode-travel time, the laser enters the frequency-stabilized mode.

[0024] In frequency stabilization mode, temperature closed-loop control and optical power closed-loop control are used to make the optical power difference between left- and right-circularly polarized light zero.

[0025] When entering the frequency stabilization control phase, the inner loop uses temperature change values ​​for closed-loop control. First, the controller reads the return value from the temperature measurement module, records the current temperature value, and uses it as the set target temperature. Then, temperature adjustment is performed: heating starts when the temperature is below 1 / 3 of the mold running temperature, and heating stops when the temperature is above 1 / 3 of the mold running temperature.

[0026] When entering the frequency stabilization control stage, the outer loop uses closed-loop control based on the optical power difference. The method uses the difference between the optical power of left-handed circularly polarized light and the optical power of right-handed circularly polarized light. When the electrothermal drive device heats the laser tube, the resonant cavity length changes, causing a change in the optical power of both the left-handed and right-handed circularly polarized light. The ADC reads the difference between the two optical powers and inputs it to the controller. The controller performs zero-crossing detection on the difference between the two optical powers. Only when the difference between the two optical powers is zero and the optical power value of the left-handed circularly polarized light at the previous moment is less than the optical power value of the right-handed circularly polarized light, is this zero point the frequency stabilization control point.

[0027] When the frequency stabilization control point is near, if the power of the left-hand circularly polarized light is greater than that of the right-hand circularly polarized light, the electrothermal drive device does not operate and waits for the laser tube to cool down naturally. If the power of the left-hand circularly polarized light is less than that of the right-hand circularly polarized light, the electrothermal drive device operates and heats up to fine-tune the length of the laser tube cavity.

[0028] like Figure 2 As shown, the fuzzy adaptive PID algorithm uses the deviation e of the controlled variable relative to the setpoint and the rate of change of the deviation ec to build a PID parameter self-adjustment criterion. It can monitor the deviation and its evolution trend in real time during the control cycle, realize the online autonomous correction of PID parameters, and effectively improve the dynamic adjustment capability and steady-state control effect of the system.

[0029] like Figure 3 As shown, the temperature rise of the laser tube causes the resonant cavity to thermally elongate along the horizontal optical axis, and the two sets of longitudinal modes in the cavity shift to the low frequency direction accordingly. Figure 3 This shows the instantaneous state of most phases of the pattern scan cycle: initially, as... Figure 3 The optical cavity length shown in a) is 1 / 4 wavelength, which is too short, resulting in the following: Figure 3 The image shown in h) indicates that 1 / 8 of the wavelength is too long, as... Figure 3 As shown in e), this point is the frequency stabilization lock point.

Claims

1. A frequency stabilization control algorithm based on a longitudinal Zeeman laser, characterized in that... Includes the following steps: When the laser is powered on, a preheating stage is first performed. During the preheating stage, the initial temperature of the laser tube is detected, and then the corresponding mode number is calculated based on the mode temperature mapping relationship calibrated by the natural preheating experiment. The controller completes the calculation of the number of steps for the laser mode and outputs a constant heating power to heat the laser tube, thereby adjusting the laser's resonant state to the predetermined mode. Once the predetermined mode is reached, the drive voltage is dynamically adjusted. When the laser has optical power output, the optical power difference between left-handed and right-handed circularly polarized light is in a frequency-stabilized state, and the mode-travel time is greater than the fastest mode-travel time, the laser enters the frequency-stabilized mode. In frequency stabilization mode, temperature closed-loop control and optical power closed-loop control are used to make the optical power difference between left- and right-circularly polarized light zero.

2. The frequency stabilization control algorithm based on a longitudinal Zeeman laser according to claim 1, characterized in that: The number of modes is determined by the relationship between the highest temperature at which the laser tube reaches a stable frequency under natural preheating without external heating and the total number of modes.

3. The frequency stabilization control algorithm based on a longitudinal Zeeman laser according to claim 1, characterized in that: The difference in optical power between left- and right-circularly polarized light when the laser tube naturally preheats to a stable frequency without external heating and the fastest mode-shifting time are used as the criteria for determining whether the frequency has been stabilized.

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