Application of the phase memory of acetylene (C2H2) vibrational-rotational transitions in the 1520-1540 nm wavelength range for a self-referencing homodyne detection of a sub-nanosecond optical phase modulation is reported. In the proposed configuration the collinearly propagating coherent dipole radiation of the excited molecule acts like a phase-locked reference wave (local oscillator) that enables transformation of the initial phase modulation into the intensity one. This technique does not need high light intensity and can operate in a linear range of the medium optical absorption. The linear optical phase demodulation (i.e., transformation of the phase-to-amplitude light modulation) is interpreted as an introduction of an additional phase shift to the carrier frequency component of the modulated signal in the maximum of the dispersion curve of a narrow optical absorption peak. It has been experimentally demonstrated with the bulk 10 cm long cuvette filled with low pressure (similar to 2 Torr) acetylene at room temperature. Effective demodulation of the milliwatt-scale incident laser wave of a single-and multi-mode structure is shown. As expected, the response to the fast (< 1 ns) phase modulation was quadratic when the acetylene inhomogeneous Doppler-broadened (similar to 500 MHz) absorption line is excited in its center and was linearized by tuning at one of the absorption line sides. It is of a differentiating (high-pass) type with the cutoff frequency determined by the total spectral width of the utilized absorption line. Expected detection resolution is determined by the photon noise of the incident light intensity. (C) 2019 Optical Society of America
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