Investigating Of Femtosecond Laser Pulse Propagating In Hollow Core Photonic Band Gap Fibers

A new type of fiber, known as microstructure fiber, has emerged in the past several years. These fibers are characterized by wavelength-scale microstructures distributing regularly or randomly in the cladding region running along the entire fiber lengths, which have resulted in some unusual properties unattainable for conventional optical fibers, such as endless single mode, flexible dispersion, high nonlinear and photonic band gap. Therefore, microstructure fibers have great potential in the field of information communication, biomedicine, transducer and ultrashort pulse laser technology. Hollow core photonic band gap microstructure fiber is one kind of microstructure fibers which guide light by photonic band gap. However, this fiber has preponderance on delivery ultrashort pulse laser. which enable it in the field of biomedicine. The propagating properties of microstructure fibers are investigated theoretically and experimentally in the present dissertation.Hollow core photonic band gap microstructure fiber is simulated by using of theoretical method. Firstly, the photonic band gaps of microstructure fiber are calculated by using of full-vectorial plane-wave expansion method. The photonic band gaps of different crystal lattice constant and difference between hollow core and all solid PBG fibers are analysed. Secondly, we used the finite element method to simulate the effective mode index and dispersion properties of hollow-core photonic band gap microstructure fibers.Propagating of femtosecond laser pulse in a new structure hollow-core microstructure fiber is investigated. The transmission spectra and loss of microstructure fiber are measured on experiment. We researched the relations between the spectrum of output pulses through fiber core and laser central wavelength, laser power and fiber length. It is demonstrated that nanojoule femtosecond laser pulses coupled into the air core propagate with nearly preserved spectral profiles through the shorter length fiber. Combining to the property of dispersion, we give analysis as follows: The spectrum of output pulses is mainly affected by Four-wave Mixing; the role of self-steepening and higher order dispersion is more highlighted as the increment of average power. The secondary cores between air holes in the cladding of this fiber can be served as waveguide channels. These channels allow multiple frequency conversion for sub-nanojoule femtosecond laser pulses. Supercontinuum spectra with specific wavelength bands can be produced in some channels. A broadband continuum spectra extending with from 450nm to 1000nm generated with subnaojoule femtosecond laser pulses coupled into a secondary core with zero GVD wavelength of 690nm. A four wave mixing process with anti-Stokes emission around 540-560nm took place in one channel with zero GVD wavelength of 630nm. The results show that the microstructure fiber has potential applications in some domains where both frequency conversion and delivery of femtosecond laser pulses are required.