A compact circular heterodyne interferometer for simultaneous measurements of variation in the magnitude of phase retardation and principal axis angle

This paper presents a compact configuration for simultaneous measurements of variation in the magnitude of phase retardation and principal axis angle in optical materials. The configuration comprises a circular heterodyne common-path interferometer, in which a sawtooth voltage waveform is used to regulate an electro-optic (EO) modulator. Changes in phase retardation and in the angle of the principal axis are demodulated by a lock-in amplifier via a simple signal-processing algorithm. Compared to the conventional configurations, the proposed interferometer offers an easier setup, has only one detection channel, and employs a simpler signal-processing operation. The validity of the proposed measurement system is demonstrated using a quarter-wave plate as a sample. The results indicate that the average absolute errors of the phase retardation and principal axis angle of quarter-wave plate are 4.88% and 0.94°, respectively. Furthermore, the root-mean-square resolutions of the magnitude of phase retardation and the principal axis angle are shown to be 1.59° and 0.10°, respectively. The dynamic range of phase retardation measurement is up to 90° and that of the principal axis angle measurement is up to 180°.