Numerical Investigation On The Coherence Properties Of1-5μm Supercontinuum Spectra In A Single Mode Fluoride Fiber

Supercontinuum (SC) light source has emerged as a fascinating topic in thesphere of nonlinear optics、high power laser and even the design and fabrication ofphotonic crystal fiber due to its varied applications in microscopy, frequencymetrology, optical Lidar as well as defense. The technique of SC spectrum covering0.3to~2.5μm in microstructure silica fiber has been perfected and the output powerreached was as high as hundreds watts of order of magnitude. However, an SC sourcewith wavelength longer than2.5μm is vital for real applications. In fluoride fiber, anSC spectrum with a spectral range from~1.9to~3.8μm and power of21.8W wasdemonstrated. Despite the high output power and the broad spectrum bandwidth, theSC coherence in the Mid-infrared regime is seldom reported and rarely investigated indetail. For real applications, it is desirable to gain broadband SC radiation covering1-5μm and high coherence simultaneously. Our aim in this dissertation is to study onthe acquisition of the light source numerically. One conventional approach is tooptimize the pulse duration、peak power of the pump as well as the fiber length.However, optimized parameters lack flexibility. An alternative method, thus, isproposed to enhance the flexibility in the dissertation.For the popular method, we computationally examined the SC generation in thesingle mode fluoride fiber pumped both by1.56μm and1.97μm fs-ps fiber laser tostudy the underlying mechanism of the spectral expansion and optimized thecoherence properties in1-5μm Mid-infrared region through modification amongpump pulse duration,peak power as well as fiber length. Concerned with the1.97μmpump case, modulation instability associated spectrum broadening is responsible for the deterioration of the coherence properties. On one side, a pulse of short pulseduration and low peak power is expected to maintain a superior coherence. One theother hand, a pulse with enough peak power should be satisfied to initiate dramaticspectrum broadening. Therefore, a pulse of ultra-short duration and high peak poweris ideal for1-5μm SC light source generation with unity coherence. Whenmodulation instability takes place, the fiber length should be chose near the length atwhich extreme spectrum expansion is excited to guarantee a broadband SC sourcewith high coherence. For pump wavelength at1.56μm, a newly formed pump in theanomalous dispersion region, but not far away from the zero dispersion wavelength,spawn the considerable spectral broadening. To suppress modulation instability, thepulse duration of the formed pump should be as short as possible and thecorresponding peak power should be high enough to generate1-5μm SC spectrum.Such formed pulse can be obtained through pumping with a mode-locked laser ofhigh peak power and ultra-short pulse duration.A new approach to improve the coherence proposed in the dissertation is the SCpulse synthesis via Michelson-interferometer. The simulation results demonstratedthat the synthesis method can enhance the coherence properties and maintain thebandwidth at the expanse of the power of the SC light source.