Digital spectropolarimeter for the measurement of optical polarization

A computerized setup for optical polarization measurements is described. The heart of this setup is a photoelastic modulator (PEM) whose retardation phase can be kept constant when the wavelength of modulated light is scanned in a wide range, and a dual-channel gated photon counter which has the capability of compensating the variation of excitation intensity with time and wavelength. Unlike conventional analog setups, where the difference of two perpendicularly polarized light components (I+ − I−) and the average of these components 1/2(I+ + I−) are extracted electronically and the ratio 2(I+ − I−)/(I+ + I−) is recorded as degree of polarization, this spectropolarimeter directly records the I+ and I− components (for circular polarization) or I∥ and I⊥ components (for linear polarization) of the analyzed light signal. Once the spectra of the two components are stored as a numerical file in the computer, they can be processed to extract the degree of polarization as a function of wavelength, temperature, etc. The curvature and asymmetry of the modulator response are taken into account exactly in data processing. The spectropolarimeter has proved to be very accurate due to the capability of compensating the fluctuation in excitation source and of electronically tracking the modulation wavelength.