摘要： Detection (or demodulation) of the phase modulation is a critical operation in different photonic applications, in particular, in interferometric fiber sensors, material evaluation via laser induced ultrasound, coherent communications etc. Different techniques were proposed to overcome the main problem of conventional interferometers: necessity of stabilization of the quadrature operation point and compensation of the complex spatial structure of the detected wave. Here one can mention, active operation point stabilization via external opto-electronic loop [1], adaptive dynamic gratings based systems [2], self-reference (i.e. without external reference wave) confocal Fabry-Perot cavities [3], and configuration based on quantum phase memory of an ensemble of two-level atoms/molecules [4]. All these seemingly different approaches have the same common feature – they are based on utilization of a narrow absorption/gain spectral profile, artificial, as in the first two above-mentioned techniques, or natural, as in the two last cases. Here we analyse a general case of passing of the phase modulated wave through a narrow absorption/gain spectral line as a technique for its demodulation (i.e. its partial transformation into the intensity modulation wave) with the following detection by conventional photodetector.