Studies On Nonlinear Frequency Conversion Technologies Based On Photonic Crystal Fiber Femtosecond Laser System

In this dissertation, nonlinear optical frequency conversion technologies based on a Yb³⁺-doped large-mode-area photonic crystal fiber (PCF) femtosecond laser system are employed to produce new laser wavelength, which cannot be generated directly from PCF fundamental source. The frequency conversion technologies can overcome the limitation of the emission spectrum band of gain medium, and extend the fundamental wavelength to shorter wavelength and longer wavelength. At a high repetition, the fundamental pulses at near-infrared spectrum band can be converted to the visible and the ultraviolet ultrashort pulses. On the other hand, optical parametric amplifier, optical parametric oscillator and intra-pulse stimulated Raman scattering effecting can generate continuously tunable femtosecond pulse cover range from the visible to the near-infrared based on the 2-order nonlinear of pieces of crystal or 3-order of silica fibers. The main contents detail as follows:1. Factors which limit the crystal length for frequency doubling of ultrashort pulses and effects of the group velocity mismatches on the conversion efficiency and the pulse duration are investigated theoretically. Causes of the elliptic transverse mode of the second harmonic, and effects of the focal length on the transverse mode are analyzed and illustrated experimentally at the condition of type-â… phase matching in BBO crystal. At the repetition rate of 50 MHz, sequential frequency doubling and tripling of the fundamental pulses in nonlinear crystals enables the generation of green-light pulses at 520 nm with a maximum average power of 10.5 W and 4.7-W ultraviolet laser pulses at 347 nm. Tunable fourth harmonic UV pulses with a maximum average power of 2.14 W have been produced within the wavelength range from 257 to 263 nm. Pulse duration of the fourth harmonic pulses is determined through difference-frequency-generation cross-correlation method. 120-fs and 410-fs UV pulses are produced, respectively, through fourth-harmonic generation in a 0.18-mm-thick BBO crystal and a 0.8-mm-thick BBO crystal. According to the endlessly single mode properties, single-mode delivery of PCF for UV light at the range of 260 nm is also demonstrated in this part.2. The BBO- type-â… phase matched noncolinear femtosecond optical parametric amplifier pumped by the second harmonic (520 nm) of the PCF laser is investigated theoretically and experimentally. The conditions for broadband phase matching are derived and the corresponding optimal phase matching angle and nonlinear angle are calculated theoretically. Techniques for compensation of spatial walk-ff, group velocity mismatch are also discussed. The roles of intra-pulse stimulated Raman scattering effecting are analyzed. Soliton self-frequency shift phenomenon is observed at a low power, and 16 fs soliton pulses are obtained with a piece of PCF. Seeded by the broadened fundamental spectrum in fibers, a high repetition rate (1 MHz) nonlinear femtosecond optical parametric amplifier system is experimentally demonstrated, and continuously tunable signal light pulses idle light pulses cover the wavelength region from 830 nm to 1430 nm are produced, with the shortest pulse of 62 fs.3. Four-wave-mixing (FWM) principle based on the 3-order nonlinear effecting of PCF and the phase matching methods in fibers are studied theoretically. Dispersion flatten double cladding PCF with the zero-dispersion-wavelength in the vicinity of the output wavelength of the Yb³⁺-doped PCF femtosecond laser is designed through numerical calculation. Phase matching and the parametric gain of FWM pumped in the anomalous-dispersion regime and normal-dispersion regime are numerically simulation, and the simulation results show that the designed PCF are competent for degenerate FWM with broad tunable range and large gain bandwidth. Numerical simulation in this part can supply a theoretical instruction for PCF optical parametric amplifiers and optical parametric oscillators.