Self-referencing mW-scale detection of sub-ns optical phase modulation in acetylene at 1530  nm

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 (∼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 (∼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.