Large-Ratio Stretcher For Ultra-Short Pulses Using Photonic Crystal Fiber

All types of ultra-high peak-power laser systems are emerging, because they have more and more important applications in the fields of national economy, military and scientific research. The Large-ratio stretching of the ultra-short pulse seed laser and recompression technology are the key techniques for chirped pulse amplification (CPA) or optical parametric chirped pulse amplification (OPCPA) of ultrahigh peak-power laser systems. In this paper, we use photonic crystal fiber (PCF) to design large-ratio stretcher for ultra-short pulses. Because of the PCF's design with flexibility of changing the radius and lattice pitch of air-holes, dielectric materials and arrangement of air holes, it can easily get different dispersion values, nonlinear coefficients, birefringence and effective mode areas, etc. Therefore, PCF provides a new design approach for many optical devices.In the paper, we briefly introduce the research situation of PCF and pulse stretcher at home and abroad. PCF have been shown desirable characteristics such as endlessly single mode, adjustable dispersion, high birefringence and adjustable effective mode field area, etc. The cladding effective index of PCF can be calculated through vector effective index method. Through theoretical analysis and calculation, we obtain the characteristic equation of cladding space-filling mode and the formula of second-order and third-order dispersion. The effective refractive index of cladding can be obtained by the characteristic equation of cladding space-filling mode. And then use numerical aperture to calculate nonlinear coefficient as step-index fibers. The relationship of nonlinear coefficient of PCF with the radius and lattice pitch of air holes is also analyzed.In this paper, we propose a PCF structure that can be used as the ultra-short pulse stretcher with stretching ratio of more then 10,000. From analysis of the stretcher's requirements, PCF should have high and flattened second-order dispersion value at the wavelength of 800nm. Through theoretical analysis and simulation, with the air hole radius of 0.265μm and lattice pitch of 1.27μm, the PCF can provide more than 300-nm working wavelength range with flattened dispersion suitable for most ultra-short pulse laser sources around 800nm. And the second-order dispersion is 35.72 ps2/km, third-order dispersion 0.0002 ps3/km and nonlinear coefficient 36.4W-1.km-1. Results indicate that more than 10,000 stretching ratio is achievable for ultra-short pulses with pulse width less than 100fs. Such approach may be more practical in the near future due to its all-fiber nature.