NEW FREQUENCY-DOMAIN ELECTRIC BIREFRINGENCE SPECTROMETER USING AN ADVANCED DIGITAL LOCK-IN TECHNIQUE

We have developed a frequency-domain electric birefringence spectrometer to measure stationary birefringence DELTA-n(dc) and alternating one DELTA-n2-omega induced by an applied sinusoidal electric field with an angular frequency omega. The new spectrometer is based on (1) the optimization of birefringence detection S/N ratio by taking into account the effects of optical noise, shot noise, optical leakage, and parasitic birefringence, (2) the electric field modulation technique, and (3) the advanced digital lock-in technique for further S/N enhancement. The electric field amplitude modulation at an angular frequency OMEGA shifts DELTA-n(dc) and DELTA-n2-omega signals to locate OMEGA and 2-omega +/- OMEGA, which are demodulated with sinusoidal reference waves at OMEGA and 2-omega and retrieved by digital data accumulation, i.e., the advanced digital lock-in technique. These techniques ensure the signal detection with extremely narrow bandwidth by removing the adverse effects of odd harmonic noises, drift, and 1/f noise in the measurement system. The detection sensitivity of 10(-9) rad in a wide range of omega is accessible.