Dyanamics Of All-Fiber Ultra-Fast Mode-locked Lasers

All-fiber mode-locked lasers as ideal light sources to generate high performance femtosecond optical pulses have the advantages of high efficiency,cost-effective,good stability,small volume and easy to be integrated,so they have always been the hot researches in the field of mode-locked lasers.They have been fully applied in various practical applications including ultrafast optical spectroscopy,laser-controlled chemical reaction,biomedical imaging,frequency measurement,optical communication and material processing and others.With the rapid development of mode-locked lasers,the performances of all-fiber mode-locked lasers have been greatly improved.Shorter pulse,higher pulse energy and higher pulse peak power from all-fiber mode-locked lasers remains a priority issue requiring renewed attention by researches.The main work in this thesis revolve around all-fiber ultrafast mode-locked lasers,fosuing on the influences of disepersion,nonlinearity,gain and loss on pulse dynamics and laser peromances of mode-locked fiber lasers,aiming to construct higher performance all-fiber mode-locked lasers with short pulse,high pulse energy and peak power.The chief achievements of this thesis are as following:1.For the first time,we systematically investigate the passive harmonic mode locking(HML)of bound states of two solitons in a fiber ring laser mode-locked by the nonlinear polarization rotation(NPR).In our experiment,the stable state of HML of bound solitons(BSs)with a fixed and discrete separation is obtained.The repetition rate can be turned from the fundamental mode locking up to 9th-order HML when the pump power is increased from 168 m W to 476.1 m W.The pulse separation in the bound states is constant(1.5 ps in our experiment)and is not dependent on the pump power and polarization states.The dynamic is similar to the HML of single pulse operation.Experimental results confirm that the BS is another intrinsic feature of the laser except for single soliton and the bound soliton fiber laser with tunable pulse separation may have the great potential in applications in laser processing and biomedical imaging.2.We report that anhydrous alcohol can be used as saturable absorber(SA)for the first time,and successfully apply this kind of SA for mode-locking and Q-switching in fiber lasers.The SA used in mode-locked lasers is an optical ferrule with one alcoholic end-facet sealed by a polyethylene(PE)film.The thickness of alcohol is estimated to be 256 ?m and its modulation depth is measured to be 5.6 %.Based on such a SA,a self-starting mode-locked fiber laser has been demonstrated to generate 972 fs pulses at 1594.6 nm.The single pulse energy is up to 1.8 n J with the repetition rate of 20.97 MHz,and the signal-to-noise ratio(SNR)is higher than 50 d B.Besides,another SA is conducted through sealing anhydrous alcohol in the gap between the end-facets of two optical patch cords with a cannula.The nonlinear response of the SA with 820 ?m alcohol layer has been measured,as well with a larger modulation depth of 18%.Based on the SA,Qswitching operation in the Er-doped fiber laser has been achieved.The average output power of Q-switched fiber laser is 30.3 m W with the pulse repetition rate of 81.9 k Hz and the pulse energy up to 348.6 n J.Experimental results show that the liquid SAs are with the advantages of wide working wavelength range,tunable modulation depth,good thermal diffusivity and high damage threshold and therefore are an ideal choice as SAs to realize higher pulse energy femtosecond fiber lasers.3.Based on the generalized non-linear Schrodinger equation(NSLE),we have numerically studied the impact of spectral filtering of frequency-chirped pulses on the laser characteristics in net-normal dispersion mode-locked fiber lasers(NDFLs).We show that dissipative solitons(DSs)and amplifier similaritons(ASs)can coexist in NDFLs and can switch to each other via solely altering the filter bandwidth in the cavity.The laser favors DSs operation for large filter bandwidth and it jumps to ASs operation along with reducing the filter bandwidth.The experiments are also presented to provide an evidence of the coexistence and the transition of these two soliton solutions in a NDFL mode-locked with nonlinear polarization rotation technology technology,indicating that the spectral filtering play an important role on different soliton formations in NDFLs.The experimental results are well consistent with the theoretical results.The results put forwards directional ideas on realizing switchable DS-AS fiber lasers and optimizing the performances in the NDFLs.4.We construct a L-band dissipative soliton fiber laser mode-locked by nonlinear polarization rotation technology.By optimize the gain and dispersion in the cavity,the 1600 nm laser emission is obtained.The laser is working in L band,with the pulse duration of 3.1 ps and the frequency repetition rate of 38.75 MHz.By using single-mode fiber as a compressor outside the cavity,the chirped pulse can be dechirped to 87.5 fs with the pulse energy of 1.47 n J.This is by far the shortest pulse with highest pulse energy from L-band dissipative soliton fiber lasers.5.Futher optimizing the dispersion and nonlinearity,we have achieved sub-60 fs Lband similariton fiber laser oscillator.The maximum output power is 47 m W and the repetition rate is 36 MHz,corresponding to the pulse energy of 1.3 n J.The spectrum is with parabolic shape and the spectral width is exponential boarding with the the pump power increasing.The pulse phase is with quadratic phase.All these features provide the evidence of the similariton generation.The laser delivers the chirped pulses with the duration of 2.4 ps,which can be dechirped to 57.6 fs by using standard single mode fibers(SMFs)outside the cavity.To the best of our knowledge,it is the first report that an ultrafast Er-doped all-fiber similariton laser operates at 1.6 ?m with the pulse duration less than 60 fs.6.An active mode-locked fiber laser with ultra-narrow spectrum and tunable frequency repetition rate based on SBS-induced refractive index changing effect has been proposed and experimentally demonstrated for the first time.According to the K-K relationship,the change in gain and loss results in the chang in the refractive index,so as to influence the effective cavity length,causing the frequency repetition rate of modelocked pulses change.Both 100 m and 50 m single-mode fiber has been exploited as the SBS gain medium,respectively.Thronging altering the pump power,we get the 20 k Hz and 100 k Hz tunable range of the frequency repetition rate in the long and short laser cavity respectively.Experimental results show that the pulse frequency repetition rate is proportional to the pump power,indicating the refractive index is increasing during this process.Therefore,the proposed laser is indeed a fast light fiber laser.The amazing phenomena is that the changed trend of the group refractive index of stokes pulse in experiments is in contrast to the prediction by the K-K relationship,which indicates that the group refractive index of stokes pulse should be growing larger with the increase in the pump power.The physical mechanism of the fast light fiber laser is needed to be further discussed.The very narrow bandwidth,tunable repetition rate,and fast light of our laser makes it have potential for applications in spectral imaging,quantum optics,optical sensoring and gravitate wave detection.