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.