Research On The Nonlinear Wavelength Extension Of Mid-infrared Fiber Laser Based On 2 μm Pulse Pumping

In view of the demands on the wavelength diversity of mid-infrared pulse laser in the fields of frontier research,gas detection,material processing and so on,as well as the Occident’s curbing on the development of≥2.1μm mid-infrared laser technology,the efficient ways to obtain long-wavelength mid-infrared pulse fiber lasers are mainly focused in this dissertation.Using different pulse seeds at 2μm waveband,fiber amplifier’s function on the power raising,and the wavelength conversion capability of the fibers,the main research contents and results are listed as follows:Firstly,considering that the repetition rate of electronically gain-modulated and Q-switched pulse is lower,a Ho-doped fiber gain-modulated laser with high repetition rate and watt-level power based on h-shaped mode-locked pulse seed is built.The h-shaped nanosecond seed at 1.985μm is obtained,using the producing mechanism of wave-breaking-free pulse in nonlinear polarization rotation（NPR）mode-locked cavity.With the help of one-stage Tm-doped fiber amplifier,its average power and energy are enhanced to 3.92 W and 2.71μJ respectively.On this basis,the Ho-doped fiber gain-modulated laser is investigated,by means of the wave-breaking-free and high repetition rate of h-shaped mode-locked pulse.Because the in-band pumping scheme promotes the energy conversion,this gain-modulated pulse fiber laser possesses a work efficiency of up to 75.4%corresponding to the 1.985μm pump absorption.The shortest duration,maximum average output power and pulse energy of gain-modulated pulse are 10.8 ns,1.13 W and 0.79μJ respectively.Its repetition rate is consistent with h-shaped pulse seed,which is 1.435 MHz.To our knowledge,this is the first gain-modulated fiber laser with MHz-level repetition rate at 2.1μm waveband.Subsequently,to increase the conversion efficiency of single-wavelength Raman pulse,a 2.18μm all-fiber Raman laser seeded by the rectangular mode-locked pulse source is constructed.By enhancing the nonlinear phase-shift accumulation to excite completely peak power clamping effect,a dissipative soliton resonance（DSR）rectangular seed without wave breaking and with the pulse duration tunable from 2.34 ns to 14.08 ns is generated in the NPR mode-locked Tm-doped fiber laser.Using fiber amplifier to regulate DSR’s pulse energy and peak power,the pulse spectral evolution is investigated after the Raman-induced frequency shift ocurrs in UNHA7 fiber.Based on steep leading edge of DSR pulse,the Raman threshold in single-pass structure is further reduced.With the help of DSR’s high energy ratio,the Raman gain accumulation is promoted.Finally,a pump-to-Raman conversion efficiency of up to 67.4%is obtained,using the low peak intensity of 10.25 ns DSR pulse to suppress cascaded Raman shift.At the repetition rate near MHz level,a 2.18μm high-energy nanosecond pulse with a spectral purity of 96.8%and an energy of 1.03μJ is achieved.Its conversion efficiency and spectral purity are the highest among the reports on mid-infrared nanosecond Raman pulse fiber lasers till now.To further extend the wavelengths of nanosecond Raman pulse,an efficient Raman fiber laser at 2.4μm waveband is demonstrated,by using high energy h-shaped pulse to pump new silica fiber and soft glass fiber.In the three-stage Tm-doped fiber amplifiers seeded by 2μm electraonically modulated rectangular pulse,the modulation instability is suppressed effectively,through the architecture with one forward and two backward pumps,as well as the filtering function of fiber grating.This avoids pulse splitting and spectral distortion well.When seed parameters set as 100 k Hz and 20 ns,a h-shaped pulse with the average power of up to 2.36 W is obtained,under the gain-reshaping effect of amplifier.Using high-intensity spike of 2μm h-shaped pulse,a second-order cascaded Raman shift at 2.19μm and 2.42μm is excited in a commercially new fiber named SM2000D.The relationship between the performance of second-order Raman pulse and the length of SM2000D fiber is also studied.When fiber length is 7 m,it is found that maximum output power of the first-order Raman signal can reach 0.785 W,corresponding to a conversion efficiency of 49.1%.In contrast to the reports on similar cascaded Raman pulses,the conversion efficiency at 2.2μm waveband is increased by more than 1.5 times.At the maximum pump,the Raman efficiency and average power of 2.42μm signal are21.3%and 0.428 W,which both are the highest among the reports on all-fiber nanosecond Raman pulse lasers at 2.4μm waveband till now.When TBY glass fiber is used as the Raman medium,the high nonlinear efftec is suppressed well,by increasing seed pulse duration to 30 ns.A 2.38μm first-order Raman nanosecond pulse with the conversion efficiency of 17.6%and the average power of 0.306 W is realized,relying on high Raman gain and large frequency-shift amount of TBY fiber.This is the first soft glass fiber nanosecond Raman laser where average power is broken to hundreds of milliwatts level.In addition to the wavelength extension of nanosecond pulse,the femtosecond lasers with tunable wavelength are also investigated through SSFS effect in optical fibers.Based on thulium-doped fiber mode-locked conventional soliton sources,the limitation of Kelly sideband on the frequency shift performance of femtosecond pulses in the amplifier is revealed.In a hybrid mode-locked Tm-doped fiber laser where PD-ISO and AFLF are used as the polarizer respectively,a conventional soliton with strong Kelly sideband and a sideband-suppressed conventional soliton are obtained at 1.95μm.Their pulse duration,repetition rate and spectrum main lobe are identical,excpet for sidebands’intensity difference.When using them as the seed of a Tm-doped fiber amplifier,it is found that in strong sideband soliton system,Kelly sideband could fleetly occupy the amplification energy,resulting in the amplitudes,wavelength and output ratio of Raman soliton to be greatly reduced.However,at the same pump condition,maximum wavelength difference of monochromatic Raman solitons between the sideband-suppressed and unsuppressed systems is close to 210 nm.Moreover,the output ratio of Raman soliton is increased by more than 26%after the Kelly sideband is suppressed.These indicate that the tuning range and the output ratio of Raman solitons can be greatly enhanced in the amplifier when the sideband-suppressed soliton is used as the seed.Finally,based on soliton source without the Kelly sidebands,a three-stage fiber cascaded frequency shift model comibining with a Er-doped ZBLAN fiber amplifier is designed to realize high-efficiency femtosecond laser with a wide tuning range.Through numerical simulation,the relationships between the soliton output characteristics and the incident energy are explored in 94 mol.%highly Ge-doped fiber and Er-doped ZBLAN fiber amplifier.In virtue of amplifier’s energy regeneration,the femtosecond pulses with widely tunable range of 2～4.4μm and 2～5μm are theoretically realized when In F₃ fiber and TBZN fiber are used as the third-stage medium respectively.Corresponding to 2.8μm signal launched into In F₃ fiber,4.4μm soliton has 45.9%conversion efficiency,234fs pulse duration and 7.8 n J pulse energy.Comparing to the frequency shift results in cascaded TBZN fiber and single-stage In F₃ fiber,we found that when In F₃ fiber acts as the third cascaded medium in the cascaded structure,it is more helpful to obtain mid-infrared broadband tunable femtosecond pulse with high efficiency,large energy and good spectral quality.