A phase-modulated interferometer for high-precision spectroscopy

We present a novel spectroscopic method based on a phase-modulated interfer ometer which is suitable for the high-precision measurement of absorption a nd index of refraction profiles. A comparison with competing methods, that is with interferometry and mil spectroscopy, is given. The combination of t hese two methods, the phase-modulated interferometer, is shown to be best s uited for experiments aiming at the realization of novel optical media, for example media exhibiting an ultra-large index of refraction or strong disp ersion without absorption. The theory of operation and a theoretical and ex perimental signal-to-noise analysis are presented. Current detection limits for optical phase shift and relative absorption are 1.1 x 10(-5) rad/root Hz and 2x10(-5)/root Hz, respectively. We demonstrate the effectiveness of this novel technique by investigating the absorption and index of refractio n profiles of the 4s(2) S-1(0) --> 4s4p P-1(1) resonance transition at 423 nm in calcium.