633nm内腔式He-Ne激光器热-频率特性研究

Research on Thermal-Frequency Characteristics of 633 nm Internal-Mirror He-Ne Laser

研究了在-20~40℃环境温度下633nm内腔式He-Ne激光管的自由运转特性,设计了激光器系统的预热和稳频控制方案,实现了633nm内腔式He-Ne激光器的频率稳定。当室温约为24℃时,锁定后的热稳频激光器与高精度碘稳定激光器的拍频结果显示,3h内频率的相对标准不确定度为u=6.4×10^(-9),阿伦方差为7.0×10^(-9)(采样时间τ=1s),3个月内的频率复现性优于4.6×10^(-11)。研究了在-20~40℃范围内不同环境温度下锁定后输出激光的频率漂移规律,实验结果显示,稳频后激光输出频率随环境温度的漂移量约为293kHz/℃,与采用压力估算模型计算得到的漂移值268kHz/℃相一致。因此,当激光器工作在一个较大的温度范围内时,可以通过插值校准来获得更准确的参考输出频率。

Objective In length measurement,the laser interferometer is used as the length-based standard at all levels,and the measured length value is calculated according to the emission wavelength of its light source.The stable and reliable output laser frequency(or wavelength)characteristics of various frequency stabilized lasers as the light source of the interferometer are the basic working conditions to ensure the normal operation of the interferometer.The 633nm internal-mirror He-Ne laser with the double longitudinal mode power balance frequency stabilization principle is controlled based on the central frequency of the laser medium gain curve.However,the central frequency of the gain curve is affected by the air pressure in the tube.Therefore,the laser frequency after frequency stabilization is related to the gas pressure in the tube.For the full-cavity He-Ne laser tube,the change of the pressure in the cavity is mainly caused by the temperature change,and the pressure in the cavity increases linearly with the increase of temperature.Therefore,the frequency of the laser output is closely related to the temperature.As the frequency stabilized He-Ne laser is used more and more widely,there are higher requirements for the environmental adaptability of the frequency stabilized lasers.For example,the frequency stabilized He-Ne laser used in the laser interferometric absolute gravimeter needs to work at different ambient temperatures.Therefore,the frequency reproducibility of the current frequency stabilized He-Ne laser at extreme temperatures is greatly restricted.Methods In this paper,the free-running characteristics of the 633 nm internal-mirror He-Ne laser tube at-20--40 ℃are studied.According to these characteristics,apreheating and frequency stabilization control scheme of the laser system is designed,and the frequency stability of the 633nm internal-mirror He-Ne laser is realized.When the room temperature is about 24 ℃,the locked thermally stabilized laser beats with the high-precision iodine stabilized laser,and the relative standard uncertainty and Allen variance of the thermally stabilized laser are obtained.In addition,6sets of frequency reproducibility experiments are carried out in 3months.The ambient temperature is T=(24±2)℃,and the measurement time is more than 1heach time.The frequency reproducibility of the thermal frequency stabilization control system of the 633nm internal-mirror He-Ne laser is investigated,and the influence of ambient temperature on the frequency stabilization results is studied.Different ambient temperatures for frequency stabilization experiments are set up.The ambient temperature is in-20--40 ℃ with an interval of about 20 ℃.The temperature and beat values of the laser tube wall after frequency stabilization are measured at different ambient temperatures.At the same time,the pressure estimation model is used to estimate the frequency drift with ambient temperature after frequency stabilization,which is compared and verified with the experimental results.Results and Discussions The free-running process of the laser tube at the ambient temperature of-20--40 ℃is studied.The experimental results show that when the laser tube reaches the approximate thermal equilibrium state at different ambient temperatures,the voltage difference of the two longitudinal modes changes for 40cycles(Fig.5),and the thermal expansion rate of the cavity length is also the same,which shows that the ratio of heating to heat dissipation of the laser system is constant in the thermal equilibrium state.In addition,there is a linear relationship between the wall temperature of the laser tube and the initial ambient temperature after the system reaches the thermal equilibrium(Fig.6),which shows that even if the laser is at different ambient temperatures,the heat exchange capacity between the laser system and the external environment is approximately unchanged,so the difference between the temperature of the wall of the laser tube and the initial ambient temperature is fixed when the system finally reaches the thermal equilibrium.In addition,the influence of ambient temperature on the frequency stabilization results is investigated.The experimental results show that the temperature of the laser tube wall changes with the ambient temperature after frequency stabilization,and the relationship between them is linear(Fig.11).At the same time,the drift of the laser output frequency with the ambient temperature after frequency stabilization is about 293kHz/℃,which is consistent with the drift value of 268kHz/℃ calculated by the pressure estimation model.After linear interpolation calibration,the difference between the actual frequency value and the reference output frequency value is less than 1.81 MHz(Fig.12).Conclusions In this paper,the free-running characteristics of the 633nm internal-mirror He-Ne laser tube at the ambient temperature of-20--40 ℃ are studied.The experimental results show that when the laser tube reaches the thermal equilibrium at different ambient temperatures,the difference between the laser tube temperature and the ambient temperature is fixed,the expansion rate is also the same,and the voltage difference of the double longitudinal mode changes for the same period.According to these characteristics,the preheating and frequency stabilization control scheme of the laser system is designed,and the frequency stabilization of the 633nm internal-mirror He-Ne laser is realized.When the room temperature is about 24℃,the beat frequency results of the locked thermally stabilized laser and the high-precision iodine stabilized laser show that the relative standard uncertainty of frequency within 3his u=6.4×10;.When the sampling timeτ=1s,the corresponding Allen variance is 7.0×10;.When 0.1s≤τ≤2000s,the Allan variance is better than 4.3×10;,and the frequency reproducibility within 3months is better than 4.6×10;.This paper also studies the frequency drift law of the output laser after locking at the ambient temperature of-20--40 ℃.The experimental results show that the temperature of the laser tube wall changes linearly with the ambient temperature after frequency stabilization.At the same time,the drift of the laser output frequency with the ambient temperature after frequency stabilization is about 293kHz/℃,which is consistent with the drift value of 268kHz/℃calculated by the pressure estimation model.In this regard,when the laser is working in a large temperature range(such as-20--40 ℃),interpolation calibration can be used to obtain a more accurate reference output frequency.