Difference Frequency Generation Of Mid-infrared Femtosecond Laser Pulses

Mid-infrared laser pulse has great significance in scientific research, military affairs, civilial use and so on. Difference-frequency technique, as one of the important methods, has great study value. Difference frequency technique in Super-continuum femtosecond lasers is simple, and the output pulse by this technique is self-stabilization of carrier envelope phase. This mid-infrared pulses can be used in measurement of CEP, generation of atto-second laser pulse and other preceding sicence domain.The paper introduces theory and experiment enclosing generation of mid-infrared pulse by difference frequency. All works in each chapter is summaried as follows:(1) In chapter one, we introduce progress in mid-infrared femtosecond pulse generation with difference frequency techniques, and give summary of several common technique in this research domain. The self-difference-frequency technique has most superiority while compared different difference-frequency techniques.(2) Theory of difference frequency is introduced In chapter two. Electric field laws of difference frequency pulse and translation efficiency of difference frequency process are exported from coupling-wave equation. Crystals used in difference frequency are compared and analyzed. We give particular depict and specialty analysis of Quasi-phase matching. Tune specialty of PPLN is calculated and analyzed.(3) The theory and configuration of 5fs Ti: sapphire amplifier laser system is introduced in chapter three. This system include sub-ten femtosecond Ti: sapphire oscillator, stretcher, nine-pass amplification, compressor with pairs of double-prisms, stretcher with hollow-core fiber, and repressor with chirped mirrors. Then we obtain super-continuum laser pulse, covering spectrum from 450~950nm. The shortest pulse width is 4.6fs. Single pulse energy is 400μJ.(4) The fourth part introduces experiment research on generation of mid-infrared pulse with difference frequency. We use 5fs super-continuum amplified pulse that introduced in chapter three as pump pulse, putting MgO:PPLN as difference frequency crystal. Through adjust focusing and control disperses, we obtain mid-infrared laser pulse center wavelength at 2.6μm. In the experiment, we choose preamplifier, mode-locked amplifier, monochromator and other instruments to detect feeble mid-infrared pulse, and we also find that center wavelength of infrared wavelength pulse can be affected by the focusing location of difference-frequency crystal.(5) In chapter five, we summarize all the work that I have done, and anticipate the developing work. We hope the experiment result can be optimized, amplified and used in future.