光泵THz激光器输出频率稳定特性研究

Study on Output Frequency Stability for Optically Pumped THz Lasers

在太赫兹(THz)成像、雷达探测、相干通信等许多应用领域中,THz辐射源的频率稳定性是直接影响其应用效果的核心问题之一。基于双光子迁移效应建立了光泵THz激光器输出激光频率漂移的物理模型,推导出THz激光频率漂移的解析计算公式。以光泵甲醇(CH3OH)为例,给出了不同压强下的甲醇吸收谱线,定量分析了泵浦光频率漂移和泵浦功率对THz激光频率稳定性的影响,并讨论了THz激光腔内工作气体压强对THz激光频率漂移的影响。研究结果表明:随着泵浦光功率的增加,THz激光频率的漂移量逐渐增加;随着THz腔内工作气体压强升高,THz激光的频率漂移逐渐下降;当泵浦光频率漂移量在一定范围时,将出现THz激光的频率漂移量极值,且泵浦光的频率漂移量等于工作气体吸收谱线宽度的1/4时,THz激光输出的频率漂移达到极值。由此可见,在实际工作中,不仅需要合理选择腔内的工作条件(压强、温度),而且还需要采取措施将泵浦光的频率漂移控制在一定范围以内,以提高THz激光的输出频率稳定性。

The frequency stability of the terahertz （THz）radiation source is one of the key issues which directly affect its appli-cation regarding to THz imaging,Radar detection,Coherent communication and so on.Based on two-photon light shift effect, the physical model for the frequency shift of optically pumped THz lasers has been built up and the analytical formula for calcu-lating the frequency shift has been derived.Taking the optically pumped methanol（CH3 OH）for example,the absorption line of methanol under different pressure has been given,the influence of the bump laser frequency offset and the power of pump laser on the frequency stability of output THz laser have been analyzed quantitatively while the effect of the pressure in THz cavity on the THz laser frequency drift has also been discussed.The results show that THz laser frequency drift increases with the in-creasing of the bump power,whereas it decreases with the increasing of pressure in THz cavity.The maximum of THz laser fre-quency drift reaches under the condition that the bump laser frequency drift is equal to a quarter of the absorbed line width of the operating gas when the bump laser frequency offset locates within a certain range.In practice,in order to improve the output fre-quency stability of THz laser,not only the appropriate working conditions （pressure,temperature）should be chosen,but also the measure for controlling the bump laser frequency shift within a certain range should be taken.