Adjusting Methods of the Off-Axis Parabolic Mirror in Terahertz Time Domain System

According to the geometric optics, off-axis parabolic mirrors are used in terahertz time domain system for focusing the terahertz pulse. Theoretically, the focus of the parabolic reflector to eliminate the effects of spherical aberration, but a very high sensitivity to its optical axis misalignment, subtle misalignment will reduce the focal spot quality and therefore the need for precision optical axis adjustment is important. This article discusses terahertz time-domain spectroscopy system adjustment method in four off-axis parabolic mirror, after many experiments have proved feasible. In this work, two optical path adjustment methods were analyzed. The first approach is based on adjustment of spot shape from collimation and focus. In the second method, depend on the principle that the incident light path is different from reflected light path in parabolic cross section. Through testing this two methods have been proved feasible.