Fig. 3
Quadrature interferometer used in the 1500-m laser strainmeter (left). A quarter-wave plate inserted in the interferometer produces a 90° phase shift between the horizontal and vertical polarizations. As a result, the intensities of the interference beams for both polarizations separated by the polarizing beam splitter complementarily change as sine and cosine (bottom right). By normalizing the observed elliptic Lissajous curve (top right) into a circular one, mirror motion can be determined from the phase angle