Study On Controllable Visible Supercontinuum Generation And Mid-IR Supecontinuum Generation

Supercontinuum laser sources have broad spectra as traditional white light source,in addition they have high brightness and coherences, and therefore they have a widevariety of applications in fundamental science, industry, optical communication andmedicine. In the recent decade, the researches on supercontinuum source havedeveloped rapidly with the advent of photonic crystal fibers (PCF). Althoughsupercontinuum sources extend over two octaves in silica PCF, the studies oncontrolling of spectra broadening of supercontinuum, visible supercontinuum generationand mid-IR supercontinuum generation still lag behind. These issues have attractedconsiderable attention in the area of supercontinuum generation. In this thesiscontrollable visible supercontinuum generation, mid-infrared supercontinuumgeneration and supercontinuum generation in fiber amplifiers are investigated in theoryand experiment in thus thesis. The primary contents are presented as follows:1. The low loss all-solid photonic bandgap fiber (AS-PBGF) whose bandgapcovers the whole visible light region is designed and applied in controllable visiblesupercontinuum generation. The AS-PBGF is designed and its first bandgap covers thewhole visible light region. The bandgaps and the group velocity dispersion of thefabricated AS-PBGF are studied through numerical simulation and experiment. The firstbandgap covers from around0.45to0.9μm, and the zero-dispersion wavelength (ZDW)is0.816μm, therefore, supercontinuum generation in this fiber pumped by a tunablefemtosecond laser operating around800nm is studied experimentally and numerically.The output spectrum covers the range from0.68to0.9μm and the long wavelengthedge is suppressed by the photonic bangap. The supercontinuum from0.53to0.9μm isobtained using sub-nanosecond pulses from532nm microchip laser and its longwavelength reaches the edge of the bandgap. The generated visible supercontinuum isrestricted in the bandgap.2. Fluoride, tellurite and chalcogenide glass fibers are designed, and thesupercontinuum generation in these fibers are investigated by numerically. Thefeasibilities of extending the spectrum to around5μm are analyzed. The opticalparameters of single-mode ZBLAN fiber are calculated, including Raman responsefunction. Numerical calculation and experimental measurment confirm that the ZDW ofthe single-mode ZBLAN is1.49μm, and therefore the lasers operating at1.55μm and2μm are used to pump the ZBLAN fiber in the simulations. The supercontinuumgeneration is numerically simulated with different peak power and pulse duration. Fromthe results, it can be concluded that the longer pulse can relax the requirement of peakpower and readily broaden the spectra to5μm. Suspended core tellurite microstructuredoptical fiber (MOF) and endlessly single-mode tellurite PCF are both designed and the ZDWs are below1.55μm and2.0μm respectively. These fibers can be used for mid-IRsupercontinuum generation pumped by fiber lasers operating at1.55μm and2.0μmrespectively. Numerical studies demonstrate that the supercontinuum readily extends tonear5μm in suspended core MOF. And the endlessly single-mode fiber is appropriatefor high power mid-IR supercontinuum generation ranged from700nm to5000nm andthe endlessly single-mode property can ensure high beam quality. Suspended core MOFmade of As2S3with higher nonlinear coefficient is designed to have a ZDW below2.0μm. The numerical results indicate that500W peak power is enough to achievemid-infrared supercontinuum covering800~5000nm through15cm As2S3MOFpumped by picosecond pulse at2.0μm. The high nonlinear coefficient of As2S3MOFreduces the demand of pump laser, and this kind of fiber can be applicable to compactlow-threshold mid-IR supercontinuum source.3. An8W nanosecond pulsed laser without distortions in spectrum is achieved inall-fiber Er/Yb co-doped double-clad fiber amplifier (EYDFA). A1550nm passivemode-locked fiber laser based on semiconductor saturable absorber mirror is built andthe output pulse width is around1ns. The nanosecond pulse is amplified in the hybridEr doped fiber amplifier (EDFA) and EYDFA successionally. The band pass filters areused to suppress the noise in input signal in each stage of the amplifier, thus thethreshold of modulation instability is increased. The nanosecond pulse at1550nm withaverage power of up to8W is achieved, and the feature of MI and any other significantnonlinearity distortions are not observed in the spectra. The side-mode suppression ratiois up to40dB.4. In the process of amplification of the seeded pulse in EYDFA, thesupercontinuum is generated directly from the fiber amplifier and extended tomid-infrared region by Tm/Ho co-doped fiber amplifier (THFA) in succession.Nanosecond pulse fission occurs initiated by modulation instability when it is amplifiedin EYDFA, and the spectrum is broadened due to nonlinear effects and extended from1530to above1700nm covering the total C-band to U-band in communication. AnEYDFA is used to amplify a nanosecond seeded pulse with higher peak power fromelectric modulated laser diode, and the output spectrum extends beyond2μm. A pieceof cascaded THDF is employed to gain flat mid-infrared supercontinuum from1.75μmto2.6μm.