Study On Fiber-Pumped 2-5?m Supercontinuum Laser Source

Supercontinuum?SC?laser sources have become the ultimate“white-light”sources in a variety of applications where they can play a decisive role,including base science research,environmental monitoring,biomedicine,defense and security,etc.In particular,SC sources which locate at the 2-5?m atmospheric window benefit from low propagation loss and can be applied into long-range atmospheric applications.These applications include active infrared illumination,hyper-spectral imaging,broad-spectrum LIDAR and electro-optical countermeasure,where a broadband SC laser source with a high-brightness is the prerequisite.Besides the 2-5?m spectral band also covers a part of the important molecular fingerprint region,thus SC lasers could be applied into infrared spectroscopy.With the characteristic of broadband-spectra,high spectral intensities,all-fiber structured SC lasers share all the virtues of fiber lasers,meanwhile they avoid the expensive optical parametric oscillators and also the adverse effects brought by discrete components.To build a fiber-pumped high bright and highly-stable 2-5?m SC laser source has important implications for practical applications.Thus,all theoretical and experimental works in this dissertation are focusing on this subject.The main contents include,In relation to SC generation,fiber dispersions and nonlinearities are introduced in details.Based on the generalized nonlinear Schr?dinger equation?GNLSE?solved in a fixed coordinate system which describes laser pulses propagation in passive fibers and rare-earth doped gain fibers,the SC generation process in thulium-doped fiber amplifiers?TDFAs?,fluoride ZBLAN fibers and chalcogenide fibers are numerical simulated.The physical process of ultrafast seed pulse amplification,soliton cluster generation,and 2-2.5?m SC generation in TDFAs are unfolded for the first time.The nonlinearity of Raman soliton frequency shifting is founded to dominate the spectral broadening of SC laser toward infrared wavelengths.2D new materials of Bi₂Te₃ nanoplates are incorporated to 2?m soliton mode-locked fiber lasers as saturable absorber.Besides,experimental generations of 2?m ultrashort laser pulses from nonlinear polarization rotation mode-locking and soliton cluster generation from“1.5?2?m”frequency shifting are investigated.All these methods enrich the routeline for 2?m soliton laser pulses greatly.With soliton cluster generation from“1.5?2?m”frequency shifting,the duty cycle of 2?m seed lasers for TDFAs could be greatly adjusted,which paves the way for flexible high-power 2-5?m SC laser sources.Double clad single-mode and few-mode TDFAs are experimentally implemented to obtain spectrally-flat 2-2.5?m SC sources.It is found that seed pulses with high duty cycles could increase the TDFA efficiency,guaranteeing a SC laser source with a very high brightness.By seeding a double clad single-mode TDFA with a stretched soliton pulse,a 2-2.5?m SC source with a maximum average power of 41.6 W is realized.By seeding double clad few-mode TDFAs with 2 MHz and 4 MHz soliton cluster pulses which are frequency shifted from 1.5 to 2?m,spectrally-flat 2-2.5?m SC sources are obtained with hundred watts output power.A 3 dB spectral bandwidth of 550 nm ranging from 1987 to 2537 nm is achieved,while the maximum output power of 203.4 W is the highest power value ever reported.Post-processing method of soft glass fibers are studied.Robust and low-loss fusion splicings between silica fiber and soft-glass fiber,between soft-glass fiber and soft-glass fiber are realized.An all-fiber structured fluoride-based 2-4.5?m laser SC source pumped by a TDFA is built for the first time.The effects of fiber length,splicing efficiency,pump pulse energy on the spectral and power evolution of the ZBLAN fiber based SC source are investigated.When the ZBLAN fiber core diameter is 7?m,a watt-level spectrally-flat 2-4.5?m SC is proved,while the power ratio for spectral components with wavelengths>3.8?m is as high as 21.1%and the output power fluctuation is less than0.6%.For power scaling of mid-infrared SC laser,a ZBLAN fiber with core diameter of9?m is pumped by double clad few-mode TDFA,and a fluoride fiber based SC laser with the maximum output power of 15.2 w is obtained,which is the highest power for all-fiber mid-infrared SC source and also the power record for mid-infrared SC laser source with spectrum beyond 4?m.With the configuration of cascaded soft glass fibers,the SC generations in chalcogenide As₂S₃ and As₂Se₃ fibers pumped by a ZBLAN fiber based 2-4.2?m SC source are invesitgated in details.With the free space coupling method,a 67.1 mW 2-5?m SC source with spectrum extended to 5.5?m is obtained by pumping an As₂S₃ fiber with a 50 kHz ZBLAN SC source.While dreasing the repeatition rate to 10 kHz and adopting an As₂Se₃ fiber as the nonlinear medium,a 2-5.8?m SC source with an average power of 9.28 mW and a 10 dB bandwidth of 3220 nm ranging from 2230 to 5450 nm is obtained.With the butt-coupling method,an all-fiber spectrally-flat 2-5?m SC source with an average power of 97.1 mW,a 10 dB bandwidth of 2840 nm ranging from 2140to 4980 nm is obtained,which is the highest power ever reported for all-fiber 2-5?m SC laser sources.