利用法布里-珀罗腔开展激光模式的实验教学研究

Teaching Physics on the Understanding of Laser Modes From a Fabry-Perot Cavity

将激光束耦合进入法布里-珀罗(FP)腔,这一物理过程相当于对传输光场施加周期性边界条件,导致FP腔的输出光束为一系列具有离散分布特点的激光模式。一般来说,激光模式可以分为横模与纵模(也称作轴向模)。为使本科生更加直观地理解光束在FP谐振腔内传输这一边值条件问题,本文详尽地描述了基于FP腔开展激光模式教学的实验课程。该课程由三个小实验组成,分别是:(1)测量基模光斑的束腰半径来研究激光的横模;(2)测量FP腔的自由光谱区表征激光的纵模;(3)通过探测FP腔的透射信号与反射Pound-Drever-Hall(PDH)信号,并基于两种独立的方法测量FP腔的腔线宽和腔精细度进一步表征纵模。作为一门针对高年级本科生的创新物理实验课程,通过本实验的训练有助于理解光学、激光原理、光电子学等相关物理课程内容,也有助于启发学生掌握基于FP腔实现激光稳频的相关技术。

In physics,the process of coupling laser beams into a Fabry-Perot(FP)cavity is described as imposing boundary condition on the traveling optical field.Thereby,the output optical beams become a series of discrete laser modes from the FP resonator.In general,laser modes are categorized into transverse and longitudinal(or axial)eigenmodes.In order to provide a more intuitive understanding of the boundary-value problem about the laser beam propagation inside the FP resonator,we describe in detail a physics lab course for the study of laser modes from a FP cavity for undergraduates at the School of Physics,Huazhong University of Science and Technology.To fully characterize the laser modes from a FP resonator,three experiments as a series are designed,including:(1)a study of transverse mode from the FP cavity through the measurement of beam waist of the fundamental mode;(2)the measurement of free spectral range to characterize the longitudinal modes;and(3)two independent methods to measure the FP cavity linewidth and cavity finesse for the longitudinal mode characterization using the transmitted signal and the reflected PDH signal.The intended audience includes both undergraduates who have studied Optics,Lasers,Optoelectronics and other related courses,and researchers pursing a preliminary understanding of frequency stabilization technology based on a referenced FP cavity.