Research On 2-3μm Flat SC Based On Silica Highly Nonlinear Fiber

In recent years,supercontinuum(SC)lasers have attracted much attention in optical coherence tomography,spectral-based pollution monitoring,chemical isolation detection,and many other applications due to their spectral wideband coverage and excellent beam quality.Improving the flatness of the SC laser can reduce the difficulty of power balance in applications.The 2-3 μm band covers many gas absorption peaks and the flat 2-3 μm SC laser has important application value in atmospheric monitoring.In this thesis,the flat2-3 μm SC based on cascaded fiber amplifiers and silica-based highly nonlinear fibers is studied.The research content is as follows:(1)Based on the generalized nonlinear Schrodinger equation,this thesis describes the propagation process of optical pulses in optical fibers.Secondly,the dispersion and nonlinear effects(self-phase modulation,modulation instability,stimulated Raman scattering and soliton self-frequency shift,dispersive wave,etc.)in optical fiber are described.Then,the mechanism of supercontinuum generation under different dispersion intervals is discussed.Finally,the feasibility of producing a 2 μm SC spectrum by quartz fiber is verified by numerical simulation.(2)A 2-2.6 μm flat SC laser based on a 1550 nm pulsed laser is constructed.Firstly,an Erbium-ytterbium co-doped amplifier(EYDFA)was used to amplify the power of the1550 nm pulsed laser,and the single-mode fiber SM28 e was used to accumulate nonlinear effects for achieving frequency conversion from 1.5 to 2 μm.A Tm-doped fiber amplifier(TDFA)is cascaded to absorb spectra below 1.9 μm to produce a flat SC laser of 2-2.6μm.Meanwhile,the effects of different fiber lengths,pulse width,and repetition frequency of seed source on the power and spectral bandwidth of the output spectrum are investigated.(3)Based on the above 2-2.6 μm SC laser,different lengths and types of silica-based highly nonlinear fibers(NL1550 and UHNA7)are cascaded to further expand the wavelength.Based on NL1550,a 1.5-3.2 μm SC source is achieved.The spectral range of 3 d B is from 1855 nm to 2877 nm,and the bandwidth is up to 1022 nm.The spectral range of 1 d B is from 1959 nm to 2837 nm,and the bandwidth is 878 nm.On this basis,the output power is 4.12 W and the optical conversion efficiency is up to 68%.For the SC generated by UHNA7 fiber,the 3 d B bandwidth of 889 nm,the corresponding wavelength range of 1960-2849 nm,the output of 2.99 W,and the corresponding optical conversion efficiency is 48%.The results show that the 2-3 SC generated by silica-based highly nonlinear fiber has good application potential.