In an in situ interrogation system for multiple wavelength interferometers a fringe spectrum that includes non-quadrature-spaced radiation-intensity samples is analyzed to obtain a high resolution relative phase measurement of the optical path length difference associated with the fringe spectrum. The fringe spectrum can be analyzed to obtain a fringe number and a quadrant as well, which can be combined with the relative phase measurement to obtain a high precision measurement of the absolute optical path length difference. An environmental condition corresponding to the absolute optical path length difference can be measured using the measurement of the absolute optical path length difference including salinity, pressure, density, and refractive index of a medium.