Pulsed Erbium-doped Fiber Laser Based On Graphene Saturable Absorber

Passively mode-locked and Q-switched fiber laser,with the advantages of high beam quality,stability,compact system and simple structure,have a wide range of applications in optical communications,bio-diagnostics,materials processing,defense and so on.Saturable absorber is the critical device in passively mode-locked and Q-switched fiber laser.Currently,the semiconductor saturable absorber mirror(SESAM)has been most widely used in fiber laser.Hower,there is some disadvantages,such as,expensive price,complex production process and narrow operating bandwidth.With the development of new materials,carbon nanotubes saturable absorber attracted widespread attention.Carbon nanotubes are the direct band gap material.The band gap size is determined by diameter and chiral decision.Recently,graphene has become a scientific research focus.In addition to its excellent electrical,thermal and mechanical properties,graphene has the unique broadband optical characteristics because of its linear bands.Otherwise,the graphene saturable absorber also has high thermal damage threshold,low saturation intensity,short recovery time,a large modulation depth and the tunable operating wavelength,which favor the high-power,long-time stable operation of the laser.Based on graphene saturable absorber,this thesis deeply study the nonlinear optical properties of graphene and its application in a pulsed fiber laser,and reveal the pulse dynamic mechanism of pulsed fiber laser,which is important and valuable for the development of broadband,stable,compact pulsed laser source.Specific research results are summarized as follows:First,a tunable dual-wavelength synchronously Q-switched fiber laser based on graphene saturable absorber has been proposed.A tunable dual-wavelength synchronously Q-switched fiber laser has been built by using graphene saturable absorber.By adjusting the polarization controller,dual-wavelength Q-switched pulses with wavelength spacing of 25.6 nm and wavelength interval of 5.7 nm have been obtained.Using a 1-nm band pass filter,the single wavelength pulses of the two dual-wavelength Q.-switched pulses are synchronous.Furthermore,adjusting the polarization controller,the synchronously Q-switched pulses are also achieved regardless of the two wavelengths changing in the relative position or absolute position(the location in the spectrum).In order to exclude the impact of the filter,the repetition rates of dual-wavelength synchronously Q-switched pulse are similar with changing the cavity birefringent filter into the Mach-Zehnder filter.The graphene saturable absorber may be still useful in a duel-wavelength fiber lasers,and its absorption coefficient is the same for the two wavelengths,independent on filter.Second,the Q-switching mode-locked operation is obtained in the fiber lasers by using multilayer(22 layers)graphene saturable absorber.By Nonlinear measurements,the saturable absorber modulation depth of the multilayer graphene saturable absorber is up to 40%.The Q-switched mode-locked pulse train with the minimum pulse envelope width of 13.84 ?s and the maximum pulse peak power of 35.89 W is achieved.The operating characteristics of mode-locked fiber laser have also been theoretically analyzed.Third,the Q-switched pulse has been obtained in fiber laser mode-locked by the substrate-free graphene saturable absorber reflector.The substrate is necessary for the traditional graphene saturable absorber working on the laser.By nonlinear measurements,modulation depth of 0.5%and the saturation intensity density of 351 W/cmcm of graphene saturable absorber reflector fabricated by CVD method are obtained.After the reflector is fused into the erbium-doped fiber laser,the Q-switched pulses with repetition rate of 22 kHz,pulse width of 11 ?s,single pulse energy of 23 nJ and center wavelength of 1530 nm have been obtained.The results not only enrich the nonlinear optical properties of graphene and expand the application of the graphene saturable absorber in pulsed fiber lasersFourth,the low repetition rate noise-like pulse have been achieved in mode-locked fiber laser based on graphene saturable absorber.Mode-locked operations,including the single-pulse noise-like mode-locked state,the two-pulse noise-like mode-locked state and pump hysteresis,have been achieved in a long-cavity fiber laser based on the graphene saturable absorber.Pulse energy is up to 5.9 nJ in the single noise-like pulse operation,which is larger than that generated in the abnormal-dispersion cavity laser.Compared with the nonlinear polarization rotation technology,the long-cavity fiber laser based on graphene saturable absorber could be more easily self-starting.