| Ultrashort pulse fiber laser is quite attractive for its characteristics of short pulse duration,high output power and high peak power,and has been widely applied in plenty of fields such as industrial processing,medical diagnosis and treatment,optoelectronic countermeasures,and strong field physics research so on.Compared with solid-state laser,ultrashort pulse fiber laser has the advantages of compactness,flexibility,high electro-optical conversion efficiency,good environmental stability,and maintenancefree,and it plays an important role on the research and exploration in laser-related applications.As the main technology to obtain ultrashort pulse,passive mode-locking technology consists of real saturable absorber mode-locking and artificial saturable absorber mode-locking.In order to guarrentee the all-fiber structure of lasers and overcome the disadvantange of real saturable absorbers,such as low damage threlod,slow relaxing time,and potential degrading over time,artificial absorber mode-locking technology such as nonlinear amplifying loop mirror(NALM),nonlinear polarization evolution(NPE)using polarization-maintaining(PM)fiber become more promising and attractive.Nowadays,to meet the needs of industrial processing and scientific research,the research on ultrashort fiber laser is much beyond the pulse generation and power amplification,but also the accurate modulation on the pulse both on temporal and spectral.This dissertation focuses on the generation,amplification and pulse modulation of all-fiber virtual saturable absorber mode-locked ultrashort pulsed lasers.The main work includes:1.The theoretical and experimental study on the all-fiber mode-locked laser based on artificial saturable absorbersTheoretically,the dynamics of ultrashort pulse propagating through the fiber along with the interaction of gain,loss,dispersion and nonlinearity is analyzed.Based on the principle of all polarization-maintaining NALM and NPE mode-locking,the influence of different parameters,such as light intensity splitting ratio(crossing splicing angle),fiber length,gain and other parameters on the saturable absorption curve,is discussed.The effect of non-reciprocal phase bias on the laser saturable absorption curve is also analyzed.Experimentally,mode-locked fiber lasers with different cavity structures and different mode-locking mechanisms are mainly studied.1)With the implementation of figure-of-“8” cavity structure,all-PM NALM modelocked fiber lasers with different cavity length operating at all normal dispersion were established.Mode-locking pulses with different repetition rate varying from 21 MHz to100 k Hz were obtained by adjusting the passive fiber length and position of cavity,and the pulse energy and pulse duration increased as the decreasing of pulse repetition rate.The largest pulse energy of 104 n J and pulse duration of 300 ps was obtained at the repetition rate of 100 k Hz,and the broadest spectrum bandwidth of 30 nm was slao obtained at the repetition rate of 5.92 MHz.All the output ultrashort pulses were characterized with the features of broad spectral bandwidth,high pulse energy and compressible to sub-picosecond level.2)With the implementation of figure-of-“9” cavity structure,all-PM dispersionmanaged NALM mode-locked fiber laser operating at different net cavity dispersion were established.The stable mode-locked pulse with a 3 d B spectral bandwidth of 20 nm centered at 1031.5 nm,pulse duration of 1.86 ps is obtained when the net cavity dispersion is very close to zero,and the pulse can be de-chirped to 175 fs by a pair of grating.When the cavity is in all normal dispersion,the dissipative soliton pulse and noise-like pulse are obtained at different pump power,and the pulse energy are 0.44 n J and 2.1 n J,respectively.3)With the implementation of linear cavity structure,all-PM NPE mode-locked fiber laser was establised,and the light intensity in PM fiber was distributed by crossing splicing,and the group delay-induced walking-off was compensated by a Faraday mirror,the laser delivered an output with average power of 2 m W and spectral bandwidth of 21.69 nm at the repetition rate of 5.95 MHz,and the pulse duration can be compressed from 9.5 ps to 179 fs by a segment of hollow core fiber with anomalous dispersion.2.Research on all-fiber chirped pulse amplification technologyThe theory of CPA technology is introduced,including the requirements of frontend seed for ultrashort pulse CPA system,the choice of stretcher and compressor,and how to suppress the accumulation of nonlinearity in the main amplifier.Experimentally,an all-fiber ultrashort pulse CPA laser system was established,employing the figure-of-“9” mode-locked fiber laser as front-end seed source,and the ultrashort pulse was stretched by 500 m passive fiber and then amplified to 18.62 W in 20/130μm large mode area amplifier at the repetition rate of 1 MHz.The output pulse was compressed to 706 fs with a pulse energy of 11.52μJ,corresponding to the peak power of 11.09 MW.3.Research on time domain modulation of ultrashort pulses and a novel method to generation of all-fiber tunable burst mode pulseBased on the fast and short period of transmission of NALM,a novel method to generation of tunable burst mode pulse was proposed.A single pulse was injected to a polarization-maintaining NALM and can be modulated to a burst with many sub-pulses in time domain.The number and duration of sub-pulse can be tunable by adjusting the parameters of NALM such as the gain,loop length,effective nonlinear coefficient of the fiber,coupler splitting ratio and the power of input pulse,which allowed to control the bursts flexibly.Then a proof-of-principle experiment was carried out,and tunable burst mode pulse was obtained by adjusting the gain of NALM.4.Research on frequency domain modulation of ultrashort pulses and a novel allfiber tunable spectral shaper for pre-compensating gain-narrowingBased on the principle of time-to-frequency-domain mapping,a new theory of spectral modulation technology was proposed.The modulation did not directly act on the frequency domain of the pulse,but through the time-to-frequency domain mapping of the linear frequency chirp,the modulation in the frequency domain was characterized with the identical intensity profile as that in time domain.According to the influence of the NALM inverse saturable absorption curve on the pulse time domain,it was predicted that a saddle-shaped spectral pulse with a sinking in the middle can be generated with the NALM spectral shaper.The generation of a continuously tunable saddle-shaped spectral pulse was verified from theoretical simulations and experiments,and it was applied to pre-compensate the gain-narrowing of ultrashort pulse during the amplification.It was shown that the output pulse spectral bandwidth increases by 9 nm with the NALM shaper,which indicated that the gain-narrowing was suppressed significantly,and the compressed pulse width was reduced from 669 fs to 342 fs,and the peak power of the pulse was improved. |
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