Simultaneous absolute measurements of principal angle and phase retardation with a new common-path heterodyne interferometer

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Abstract
This study demonstrates a new method for simultaneously measuring both the angle of the principal axis and the phase retardation of the linear birefringence in optical materials. We used a circular common-path interferometer (polariscope) as the basic structure modulated by an electro-optic (EO) modulator. An algorithm was developed to simultaneously measure the principal axis and the phase retardation of a λ/4 or λ/8 plate as a sample. In the case of a λ/4 plate, the average absolute error of the principal axis is approximately 3.77°, and that of the phase retardation is approximately 1.03° (1.09%). The retardation error is within the 5% uncertainty range of a commercial wave plate. Fortunately, the nonlinear error caused by the reflection phase retardation of the beam splitter dose not appear in the new system. Therefore the error could be attributed to misalignment and defects in the EO modulator or the other optical components. As for the repeatability of this new common-path heterodyne interferometer, the average deviation for the principal axis is 0.186° and the phase retardation is 0.356°. For the stability, the average deviation for the principal axis is 0.405° and the phase retardation is 0.635°. The resolution of this new system is estimated to be ∼0.5°, and the principal axis and phase retardation could be measured up to π and 2π, respectively, without ambiguity.