Ultrafast Fiber Lasers And Their Dynamics

Ultrashort fiber lasers can be used in wide range of applicationsincluding communications systems for time-division multiplexing (TDM)or wavelength-division multiplexing (WDM), electro-optic samplingsystems, optical frequency combs, biology and so on. New applicationareas are explored involving ultrashort fiber lasers. They have developedextensively in recent years, many topics on ultrashort fiber lasers areworth of study.Our research focus on passively mode-locked erbium-doped fiberlasers, including dynamics of ultrashort fiber lasers and generation ofhigh-energy ultrashort pulses. The main achievements are as follows:1. Transitions between different types of pulses solutions of theGinzburg-Landau equation (GLE) have been studied experimentally in amode-locked fiber laser. It is demonstrated that the different pulsescorresponding to different solutions of the GLE can be generated in asingle mode-locked laser. Dispersion-managed solitons, dissipativesolitons, and similaritons can be emitted depending on the parameterssuch as the pulse intensity, the linear cavity phase-delay bias, and thebirefringence effect in the cavity. The three nonlinear waves show different features, especially the spectral shapes and dynamicsaccompanying pump power scaling. We also study this numerically, andthe numerical results agree with the experimental results well.2. Modulation instability (MI) in passively mode-locked dissipativesolitons lasers has been studied. The factors that affect MI, including theintensity of the nonlinear wave and the linear phase delay of the cavity,have been experimentally studied. It is found that MI induces sidebandsin the spectrum of dissipative solitons. The sidebands can cause thepedestal on the pulse in time domain and thus limit the pulse duration.Additionally, a simple method to eliminate the sidebands is proposed andnearly pedestal free pulses are generated correspondingly. Finally, basedon MI, a method to measure the cavity net dispersion is proposed andapplied to two dissipative soliton lasers with different net dispersion. It isshown that the positions of the adjacent spectral sidebands can determinethe intracavity net dispersion, and the measurement error is limited by theaccuracy of the optical spectrum analyzer. This indicates it is a simpleand precise method to measure the intracavity net dispersion.3. We have demonstrated the direct generation of4.6nJ,78.9-fsultrashort pulses with58kW peak power in an all-fibernet-normal-dispersion oscillator. Such high peak power is a record in anall-fiber oscillator and is comparable to chirped-pulse amplification (CPA)systems. The corresponding spectrum has steep spectral edges, which is the typical shape of dissipative solitons (DS). Also, we find that thespectral width is tunable from40nm to58nm by adjusting polarizationcontrollers (PC), and the pulse duration can be tuned from1.7-ps to120-fs through increasing the pump power, which is attractive toapplications. Besides, we demonstrate experimentally the relationshipbetween spectral width and the pump power of DS laser for the first time.4. We observed bound states of dissipative solitons in a netpositive-dispersion fiber laser for the first time, in addition to harmonicmode-locking of these kinds of pulses. Bound states of two to five pulsesare observed. The separation between the pulses depends on the pumppower as well as the settings of polarization controllers.2ndto10th-orderharmonic mode-locking are also found. The repetition rate of10th-orderharmonic mode-locking is232MHz, which is the highest one indissipative soliton fiber lasers. The pulse can be compressed to153fswith energy of0.5nJ at the repetion rate of232MHz.5. We experimentally demonstrated direct generation of51nJ,284-fs ultrashort pulses in an all-fiber cavity-optimized erbium-dopedlaser with a low repetition rate of2.68MHz. The conversion efficiency ofthe laser is about10%. The laser is mode-locked by nonlinearpolarization rotation. Such high energy and ultrashort duration benefitsfrom using large output ratio (90%), elongating the cavity length,dispersion management and a bidirectional high-power pumping configuration in the laser. Further decreasing the net dispersion of thelaser by using dispersion-compensated fiber can reduce the pulse durationto160fs. In addition, variation of spectral width due to an invisible filterin the laser is experimentally demonstrated.6. We have proposed an ultrafast fiber laser using dual mode-lockingmechanisms. The laser can generate128-fs pulses, which is the shortestone in the anomalous-dispersion regime. Benefiting from its ultrashortduration, Kelly sidebands are eliminated in the pulse spectrum. Thecoexistence of dual mode-locking mechanisms can decrease the cavitylength to12m with a repetition rate of16MHz, the cavity of which is theshortest in figure-8fiber lasers.7. We present an all-fiber self-starting high-energy soliton laserbased on a chirped fiber Bragg grating (CFBG). Pulse energy can beincreased by enlongating cavity length, which not only reduces therepetition rate of pulses but also degrades the stability of lasers. Thelength of CFBG is in mm range, but the dispersion is very large, whichcan increase the pulse energy. Comparing to increasing pulse energy byenlongating cavity length, employing CFBG in mode-locked fiber laserscan increase the repetition rate and stability of the fiber lasers. The singlepulse energy is4.8nJ, and the pulse duration is4.4ps with a repetitionrate of16.6MHz. The laser can be self-started benefiting from itsall-fiber construction. 8. For the first time, we use similaritons as the seed pulse of achirped-pulse amplification system. By optimizing the compressor, oursystem can generate4.1nJ,75.6-fs pedestal free pulses with54kW peakpower. The corresponding spectrum width is51nm, and has no spectralspikes. Such high-quality ultrashort pulses benefits from the linear chirpof similaritons. Furthermore, we experimentally demonstrated thevariation of the similariton spectral width and compressed pulse durationto the amplifier gain, which is consistent with the theoretical study.9. We designed a passively mode-locked fiber laser based on solitonself-frequency shift (SSFS). The laser is mode-locked by nonlinearpolarization rotation. The pulse generated by mode-locking exhibits SSFSas a result of passing through Raman-enhaced fiber. The wavelength ofSSFS pulse is beyond the central wavelength of gain fiber, which showsimportant applications in designing new wavelength ultrashort fiberlasers.10. We observed optical domain walls in a normal-dispersion fiberlaser. The optical domain walls are generated by interaction between twowavelengths. It was found that saturable absorber plays an important rolein generating optical domain walls. The optical domain walls can not beobserved without saturalble absorbers. Furthermore, we observe boundstates of optical domain walls by disturbing the fiber for the first time. The dissertation studied the dynamics of ultrafast fiber laserssystematically, and grasped the principles and techologies of ultrafastfiber lasers, which promotes the development of ultrafast fiber lasers.