Experimental results on the transient optical nutation effect inside an acetylene-filled hollow-core photonic crystal fiber (HC-PCF) are reported. The experiments used 15 ns optical pulses with peak powers up to 5 W. The light wavelength was centered at 1530.37 nm, which corresponds to the P9 acetylene (<sup>12</sup>C<sub>2</sub>H<sub>2</sub>) vibrational-rotational absorption line. The gas pressure inside the PCF, with hollow core diameter of ∼10.3 μm, was kept around 0.12 Torr. Comparison of the experimental data with numerical simulations using the Maxwell-Bloch equations allowed us to evaluate the characteristic longitudinal and transverse relaxation times around 10 ns, as well as the transition dipole moment (1.36 × 10<sup>−32</sup> Cm).
We present a new concept of the homodyne interferometric adaptive detection of optical phase modulation. To ensure adaptivity, i.e. stabilization of the interferometer operation point, we utilize the phase memory of a two-level quantum system, resonantly illuminated with the information bearing signal wave. Phase modulation of the transmitted signal wave transforms into the intensity modulation via interference with the collinearly propagating dipole radiation of the excited two-level system. The latter acts like a reference wave since it has a phase corresponding to that of the signal wave but averaged over the transverse relaxation time T<sub>2</sub> of the quantum system. Experimental demonstration with the acetylene-filled hollow-core micro-structured optical fiber at the communication wavelength of 1530nm of the acetylene P9 absorption line is presented. It is shown that the response to the introduced phase modulation is quadratic when the acetylene inhomogeneously broadened absorption line is excited in its center and is a linear one if it is excited at one of the absorption line sides.
Low-pressure acetylene in the hollow-core photonic crystal structure fibers is an excellent medium for the room-temperature investigation of the coherent quantum effects in communication wavelength region. Pulsed excitation enables observation of new coherent phenomena like optical nutation or photon echo and evaluation of important temporal characteristics of the light-molecule interactions. We also report original experimental results on the pulsed excitation of the electromagnetically induced transparency in co- and counter-propagation configurations.