We present experimental advances in comparative studies of optical parametric amplification (OPA) in microstructured fused silica solid-core fibers and hollow-core fibers filled with acetylene (C2H2). Both media exhibit third-order nonlinearity, enabling the OPA process in collinear configurations with a high spatial concentration of light power. In the former, non-resonant case, we investigated the parametric amplification via four-wave mixing (FWM) with a degenerate pump by picosecond laser pulses centered at a wavelength of 737 nm. This process ensured the generation of the correlated signal/idler photon pairs that could be parametrically amplified in a similar nonlinear micro-structured fiber. For the resonant acetylene-filled fibers, we present an experimental evaluation of the OPA gain in a degenerate collinear FWM at 1530 nm near the P9 acetylene absorption line. We specifically studied the transformation of amplitude modulation in the quasi-continuous W-scale input pump wave to output phase modulation and vice versa. Our research compares OPA efficiencies and the potential to generate squeezed and entangled light states in resonant and non- resonant fiber-based media.
We present experimental results on polarization ellipse self-rotation (PSR) and polarization hole burning (PHB) in the acetylene-filled hole-core photonic crystal fiber for the P5, P7, and P9 vibrational-rotational Doppler broadened (~500MHz) lines at the gas pressures 0.4-4Torr and W-scale power. The experimental values of PHB and PSR effects are in good agreement between them and with predictions of the developed semi-classic model for the acetylene molecules as the saturable rotating dipoles (gyrators) with random orientations. The maximum experimental PSR value normalized to the gas initial absorbance proved to be significantly lower than that observed in alkali-metal vapors earlier..
We present results of the experimental investigations of noise characteristics of the self-referencing optical phase demodulator based on a bulk low-pressure acetylene cell at the wavelength 1530.37nm of P9 acetylene absorption line. The configuration is simple, does not need phase locking, and can be operated with high-etendue waves. While the intensity noise of the utilized semiconductor laser at frequency >250MHz was below the photonic noise, the demodulator resolution (i.e., the minimum detectable phase modulation amplitude) was significantly reduced by an excessive laser phase noise. To reduce the output noise, we propose and analyze different balanced detection configurations that increase the resolution to the standard quantum level.
Analysis of influence of the Maxwell distribution of the transverse thermal velocities and of the flight-time-determined characteristic relaxation rates (i.e. the inverse relaxation times T1,2) of the acetylene (C2H2) molecules in the hollow-core photonic crystal fiber on nonlinear optical effects are presented. The theoretical predictions are compared with the experimental data obtained in the ~0.4Torr acetylene-filled fiber cell at the wavelength 1530.37nm of the most effective P9 vibrational-rotational transition of 12C2H2. At room temperature and the fiber mode field diameter of 7.5 μm, the average transverse thermal velocity of ~390m/s ensured relaxation times T1,2 ~8-10ns. These are in good agreement with the corresponding values experimentally measured using delayed optical nutation and two-photon echo techniques. The experimentally observed nonlinear effect of the polarization ellipse self-rotation proves to be at least two orders of magnitude less efficient comparing with that reported earlier for the alkali metals vapors.
Detection of fast optical phase modulation is a critical procedure in different areas of modern optical technology. Conventional homodyne detection technique needs mixing with the local oscillator, or with the reference wave, phase-locked with the detected wave with the quadrature phase difference (+/-)Pi/2 for linear demodulation. Additional complications appear from the necessity to ensure similarity of polarizations of these two light-waves and their complicated wave-fronts in case of detection of the light reflected from a rough inspected surface. All these problems can be solved in the self-reference configuration based on confocal Fabri-Perot cavity, but it is complicated and needs frequency-locking with the detected wave. We propose utilization of phase memory of an ensemble of acetylene molecules (C2H2) vibration-rotational transitions for a self-reference homodyne detection of sub-ns optical phase modulation in 1520-1540nm wavelength range. In the reported configuration, the collinearly propagating dipole radiation of the excited by the incident light two-level centers acts like the coherent properly phased reference wave necessary to transform the phase modulation into the intensity one. It is experimentally demonstrated in the optical fiber compatible hollow-core photonic crystal fiber cell filled with the 0.4Torr gas at 1530nm wavelength of the acetylene P9 absorption line at the sub-mW scale cw power. The response to the detected phase modulation was quadratic when the acetylene inhomogeneous absorption line was excited in its center but was linearized by tuning to one side of the absorption line. Similar self-reference detection of the multimode wavefronts is also possible in the bulk gas cells.
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 T2 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.
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 (12C2H2) 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 × 10−32 Cm).
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.
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