Continuous wave terahertz common path phase shifting interference using a liquid crystal phase shifter

A continuous wave terahertz common path phase shifting interference (PSI) method using a liquid crystal (LC) phase shifter was developed in this work. Using an LC phase shifter is a good candidate for common path PSI, however, it is difficult to achieve pure phase measurement in the terahertz region owing to the strong dichroism of LC materials. In this study, hydrogen-bonded LC, which exhibits no dichroism at 2.5 THz, was introduced to an LC phase shifter and the common path PSI was demonstrated using an optically pumped gas laser at 2.5 THz. Our experimental and calculation results indicate that the phase difference between the ordinary and extraordinary rays of x-cut quartz substrates was successfully obtained by using the four-step phase shifting algorithm. Furthermore, the bias noise such as the influence of reflection at the sample surface was eliminated during data processing based on the four-step phase shifting algorithm.