Nonlinear Optical Effects In Photonic Crystal Fibers

Phontonic crystal fibers (PCFs) can have excelent properties shuc as single-mode propagation over broad wavelength ranges, enhanced modal confinement and the ability to engineer theirgroup velocity dispersion. Therefore, they were used generally in optical communication, sepectrscopy and sensors. Supercontinuum generation in PCFs has subsequently been widely applied in interdisciplinary fields such as optical coherence tomography and optical frequency metrology. Other nonlinear frequency conversion processes such as four-wave-mixing have applications in generation of correlated photon pairs, optical parametric amplifiers and oscillators. Another interesting class of nonlinearity reported in experiments involves multimode phase-matched harmonic generation. This has been applied to the case of UV generation. In addition, understanding the physics under some nonlinear optical process in PCFs will undoubtedly present many opportunities to discover even more surprises.Experimental investigations of femtosecond laser pulses propagation in PCFs were performed. Nonlinear optical effectes such as surpercontinuum spectrum generation, parametric four-wave mixing process, and blueshifted nonsolitonic radiation of femtosecond laser pulses in multiple PCFs were studied. The potential applications were discussed. The main contents, research conclusions and contributions to innovation are summarized as following:(1) A all-solid Bragg fiber with compound cores having high- refractive index was designed theoretically. The zero dispersion wavelengthλ₀ near 1.55μm can be obtained in the Bragg fiber, and the nonlinear coefficientγis about 40 W^-1km^-1.(2) Super-continuum generation based high-order solitons fission was performend in a in a bi-refringent photonic crystal fiber in experiments. Pulse trapping condition explains well the relations between the solitons and dispersive waves in the experimental results.(3) Multiplex frequency conversion of sub-nanojoule Ti: sapphire femtosecondlaser pulses were performed in the trefoil and quatrefoil secondary cores in a holey fiber with high air fration. This suggests the holey fiber with random structure and large air-filling fraction can serve as an efficient multiplex frequency converter and sensor.(4) Efficient blue light with central wavelength near 0.4μm was obtained by Ti: Saphire femtosecond laser pules in a high-delta microstructure fiber. The blue light can take 15% of output power. The mechanisms for its generation is analyzed to be a degenerate parametric process, where frequency of the signal light is near twice the pump frequency and frequency of the idler light can lies in terahertz region. Our experiments may give the first demonstration of terahertz-wave generation in optical fibervia a FWM proces.(5) Three scenes of polarization-dependent blue-shifted emissions of femtosecond Ti:sapphire laser pulses were carried out in secondary cores of photonic crystal fibers. The blue-shifted emissions can be narrower than 10 nm or can lie in a hollow beam, and have potential use in design of tunable optical sources and optical tweezers.