Self-Q switched fiber lasers utilizing the mechanism of stimulated Brillouin scattering(SBS)can increase the output power by several magnitudes and the scale of high output power makes them to be very attractive laser sources for applications in many technological areas

such as telecommunication

material proces-sing

laser surgical

range finding and remote sensing.In the presented work

the generation of pulse laser exploiting SBS phase conjugation in fiber was investigated theoretically and experimentally

and how the length of SBS medium affected the attribute of laser output was paid prior attention in our study.Numerical simulation was performed to study the waveform and the sequence of laser pulse.The waveform obtained by numerical computation was coincident with the curve described the relationship between reflectivity of SBS phase conjugation mirror and the time.The results show that changing Q value in resonant cavity by utilizing SBS phase conjugation is feasible.To investigate the attribute of laser output at different length of SBS medium

an all-fiber Er-doped Q-switched laser was designed

in which a 974 nm laser diode was used as pump source

Er-doped fiber(EDF)with length of 3 m was used as gain medium

a fiber Bragg grating(FBG)was used as fixed cavity mirror and standard single-mode fiber(SMF)G.652 with length of 2 m

1.5 m and 1 m were used as SBS pool individually.At the same time

an optical spectrum analyzer(ANDO6319)

a digital oscilloscope with 5 Gs/s(Tektronix TDS3052)and a optical power meter(ANDO AQ2140)were used to monitor the optical spectrum

temporal pulse shape and average power simultaneously.The high-speed photodetector designed by ourselves broadened the pulse duration by 20 times so as to reduce the damage to the apparatus caused by the high peak power of the laser pulses.When the length of SMF was 1.5 m

a train of pulse laser was generated with pulse duration of 2.6 ns

pulse period of 58.23 ns

average output power of 7.35 mW at pump power of 45 mW.Decreasing the length of SMF to 1 m

remarkable pulse laser with dual-peaks was obtained and the pulse period of it was 26.47 ns.Increasing the length of SMF to 2 m

only irregular stochastic pulsations were seen.Experiment data indicate that the length of SBS medium is vital to the formation of phase conjugation mirror.If it is too long

pulse laser can't be generated for the lack of fixed conjugation relation between Stokes lines

on the contrary

dual-peaks laser pulses occur for the change of photon lifetime in orthogonal polarized modes.Once the laser pulses are generated

the characteristics of laser pulses are only correlated with the stochastic characteristics of SBS and the pulse form maintain changelessness as the increase of the pump power.By means of these obtained experimental results

we can select proper length of SBS medium to yield perfect output pulse shapes

which enable this kind of self Q-switched fiber lasers to be more practical.In addition

enhanced stabilization of the pulsation can be achieved by modulating the pump power to be in resonance with the repetition rate.