Basic Optical Properties Of The Photoelastic Modulator Part II: Residual Birefringence in the Optical Element

The photoelastic modulator (PEM) is a polarization modulator that operates at the resonant frequency of its optical element. The PEM is made of isotropic optical materials, in contrast to birefringent materials used in electro-optic modulators. These two characteristics, operation at resonance and the use of isotropic optical materials, give the PEM unique optical features, such as high modulation purity and efficiency, broad spectral range, high power handling capability, large acceptance angle, large useful aperture and high retardation stability. These features make the PEM an effective polarization modulator in a variety of applications. Sometimes it is the only choice for high sensitivity applications. In an effort to characterize the PEMs more thoroughly, we are carrying out a series of tests on the basic properties of the PEM. Residual birefringence is an important property that affects the quality of a PEM. In the second paper in a series, we focus on the measurement of residual birefringence in the optical element of a PEM and maintaining the residual birefringence at a low level in the final PEM product.