Optimization for liquid crystal variable retarder-based spectroscopic polarization measurements.

We present an approach for improving liquid crystal variable retarder (LCVR)-based spectroscopic polarization measurements. As deduced mathematically, the transfer coefficients from the random intensity noise to the signal noise are functions of modulation parameters of the LCVR, i.e., modulation range (MR) and initial retardation. Simulations allow more details about the roles of two parameters. A broad MR reduces effectively the values of the coefficients and leads to a better signal quality. However, as the MR narrows, initial retardation begins to influence the signal quality. To obtain a high-quality spectrum, a recommended solution is to settle the MR more than π at each wavelength. This treatment has two advantages: non-sinusoidal modulation becomes possible and the modulations do not average to zero. Moreover, it weakens the interference of non-uniform intensity distribution in wavelengths of the signal spectrum. These conclusions are proven in experiments. Further, this approach is valid for other polarimeters and ellipsometers based on LCVRs.