| Due to 1.6 ?m "eye safe" waveband high power lasers possessing high atmospheric transmittance and locating in the absorption band of some greenhouse gases(such as methane),they have important application value and prospect in ranging,remote sensing,lidar,and generating mid-infrared lasers.However,the 1.6?m laser emission locates at a special waveband,which lacks of suitable modulators and laser performance control technologies.Therefore,the development of new type modulated components and optical modulation schemes for 1.6 ?m lasers is significant.Normally,pulsed lasers around 1.6 ?m can be generated by passively or actively Q-switched methods.The passively Q-switched methods show the advantages of simple and compact laser system construction as well as largely tunable laser parameters.But for 1.6 ?m waveband,the passively Q-switched components are very deficient.With the development of new materials,two-dimensional Dirac materials such as graphene and topological insulator have been found with excellent electrical,optical and thermal properties.Moreover,they have excellent nonlinear optical properties(such as broadband saturable absorption,large modulation depth)and can withstand high optical power.These together provide the possibility of obtaining Q-switched lasers with high output energy.Compared with passively Q-switched technology,actively Q-switched technology is the major way to obtain the 1.6 ?m lasers with narrow pulse and high energy.But all the reported results are based on the voltage OFF type Q-switching.The voltage ON type Q-switching can eliminate the photoelastic effect,compensate thermal depolarization,and extend the life-time of Q-swither,and is expected to obtain better output characteristics of the lasers.Through Q-switched technology,we can only obtain nanosecond or microsecond pulses,but hard to obtain picosecond or femtosecond ultrashort pulses.Limited by the narrow gain band of Er:YAG crystal,it is difficult to realize the mode-locked ultrashort pulses in 1.6 ?m waveband.Recently,through amplitude modulated continuous wave(CW)pumped fiber to obtain ultrashort pulses with high repetition rate has attracted great interest,which is expected to obtain ultrashort pulses in 1.6?m wavelengthband.This paper mainly aims at the requirements of 1.6 ?m waveband pulse laser to study the passively and actively Q-switched Er:YAG lasers and the generation of ultrashort pulses in 1.6 ?m waveband:(1)We have relized the graphene saturable absorber based passively Q-switched Er:YAG laser with high output energy.To the best of our knowledge,the obtained pulse energy is about an order of magnitude higher than the reported work on graphene based passively Q-switched Er:YAG laser.Firstly,we prepared the high quality graphene saturable absorber mirror.Secondly,we studied its saturable absorption properties and acquired the saturable absorption parameters.Finally,we realized the passively Q-switched Er:YAG laser by using graphene saturable absorber mirror as a saturable absorber with the pulse width,pulse repetition rate and pulse energy of 28 ?s,12.5 kHz and 97 ?J,respectively.(2)For the first time,we have realized passive Q-switching laser with topological insulator.We first experimentally prepared the topological insulator:Bi2Te3 saturable absorber mirror.And then we studied its saturable absorption properties and acquired the saturable absorption parameters.Finally,we realized the passively Q-switched Er:YAG laser by using graphene saturable absorber mirror as a saturable absorber with the pulse width,pulse repetition rate and pulse energy of 6.3 ?s,40.7 kHz and 5.3 vJ,respectively.(3)By using voltage ON type RTP Q-switcher,we have obtained the actively Q-switched Er:YAG laser.Combined with the laser theory,the output characteristics of the laser are analyzed in detail.When the 13%output coupler was used,the laser operated in 1645 nm wavelength,with the output pulse width and pulse energy of 65 ns and 7.5 mJ respectively.While using a 24%output coupler,the laser favoured to lase at 1617 nm.Generally,the output performances of the laser operating in 1617 nm is worse than it operating in 1645 nm.We also theoretically analyze the relationship of energy storage efficiency and repetition rates of the Er:YAG laser.When the repetition rate is less than 500 Hz,the energy storage efficiency of the Er:YAG declines sharply,which is consistent with the experimental results.(4)By using single F-P etalon and VBG as wavelength selector,we have obtained the narrow-linewidth Q-switched Er:YAG laser.Using F-P etalon,we realized the 0.13 nm narrow-linewidth laser output in the graphene based passively Q-swithed Er:YAG laser,with the output power of 474 mW,corresponding to an optical to optical conversion efficiency of 2.3%.By using the VBG as a wavelength selector and a pump input mirror simultaneously for the first time,we realized the laser with narrow-linewidth output less than 0.08 nm in voltage ON type RTP actively Q-switched Er:YAG laser.The corresponding output pulse energy and pulse width is 1.36 mJ and 185 ns respectively.Our work may provide an inroad for developing more miniaturized space-based integrated path differential absorption lidar(IDPA)transmitter.(5)We proposed and theoretically researched the technical route of generating 1.6?m waveband ultrashort pulses through amplitude modulated continuous wave(CW)pumped fiber,and obtained the influence rule of the modulation and nonlinear process on its generation.Firstly,based on maxwell's equations,we derived the nonlinear schrodinger equation in amplitude modulated CW pumped fiber by reasonable approximation.Secondly,the Gaussian white noise as an additive noise is added to the initial input field,we derived the optimal length and signal to noise ratio(SNR)expression.Finally,based on nonlinear schrodinger equation,we simulated the generation of ultrashort pulses in f=0.1 THz amplitude modulated CW pumped fiber with different modulation depth(a0 =-40dB,a0 =-30dB,a0 =-20dB,a0 =-10dB).From the numerical simulation results we can see,when the modulation depth is-40 dB,the noise influence on the generation of ultrashort pulses is large,both the time domain and frequency domain are in disorder.While when the modulation depth increased to-10 dB,the obtained pulses and frequencies are quite stable,with SNR of up to 68 dB and pulse width of 268 fs.Furthermore,we analyzed the controllability of pulse repetition rate and how to improve the SNR,and concluded that changing modulation frequency can tune the pulse repetition rate,and choosing an optical fiber with a larger group velocity dispersion coefficient and smaller nonlinear coefficient should allow a higher SNR. |
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