an apparatus is described which detects the presence or absence of coherent light and provides an estimate of the coherent light &# 39 ; s wavelength . the apparatus employs a common - path &# 34 ; polarization interferometer &# 34 ; in which the two linear polarization paths act as interferometer legs . electro - optic modulation is used to effect periodic differential path length changes in the interferometer . the apparatus performs synchronous time - integrating detection on the light emerging from the interferometer to measure coherent contributions in the presence of obscuring incoherent light .