Utilizing phase memory of a two-level quantum system for adaptive homodyne detection of optical phase modulation

We propose to use phase memory of an ensemble of two-level centers, irradiated by the resonantly tuned detected signal wave, for homodyne interferometric detection of fast optical phase modulation. Collinearly propagating dipole radiation of the excited two-level centers acts as a coherent reference wave (local oscillator) necessary to transform the phase modulation into the amplitude modulation behind the resonant medium. The phase of this radiation follows the average phase of the incident detected wave with the characteristic transverse relaxation time T-2 that ensures adaptive properties of this interferometric detection configuration to slowly varying environmental conditions. We present experimental demonstration with the acetylene-filled hollow-core microstructured optical fibers at a wavelength of 1530.37 nm of the acetylene P9 absorption line, which is inside the telecommunications wavelengths range. It is shown that the response is quadratic when the acetylene similar to 500 MHz inhomogeneously broadened absorption line is excited in its center, but can be essentially linearized for excitation at one of the line sides.