Study On Suppression Techniques Of Stimulated Brillouin Scattering In Polarization-maintaining Narrow-linewidth Fiber Amplifers Based On Phase/intensity Modulation

High power narrow-linewidth polarization-maintaining fiber amplifiers have wide applications in gravitational wave detect,nonlinear frequency conversion,beam combination and so on,due to excellent beam quality,stability,good polarization characteristics.However,in narrow-linewidth fiber amplifiers,especially in narrow-linewidth polarization-maintaining fiber amplifiers,their output power levels are largely confined by stimulated Brillouin scattering(SBS)effect.At present,there are a great many methods to suppress SBS.In frequency domain,phase modulation can broaden the Brillouin gain spectrum of the laser,so as to suppress SBS.In time domain,we can use intensity modulation to compress the width of the laser pulses.Both methods can suppress SBS in a better degree,besides,the cost is low and the system integration performs well.Accordingly,we present a new method for SBS suppression,which is based on employing simultaneous phase and intensity modulation.We theoretically and experimentally study the SBS suppression effect of this method in narrow-linewidth polarization-maintaining fiber amplifiers,which contains the following contents:The applications and current research status of the narrow-linewidth fiber amplifiers is introduced firstly,indicating SBS is the limiting factor of power scaling for narrow-linewidth polarization-maintaining fiber amplifiers,especially for single-frequency polarization-maintaining fiber amplifier.Besides,current research status of various SBS suppression methods is introduced.The basic theory of SBS suppression in terms of phase/intensity modulation in fiber amplifiers is studied.As for phase modulation,we analyse the influence of the modulation amplitude and modulation frequency on SBS threshold.As for intensity modulation,we analyse the influence of the pulse width and repetition rate on SBS threshold.The corresponding physical mechanism is clarified and the method of simultaneous phase and intensity modulation to suppress SBS is brought forward.This method can suppress SBS in both frequency and time domain,which has great potential to scale narrow-linewidth polarization-maintaining fiber amplifier to higher power.Then,based on the SBS three coupled amplitude equations and the rate equations,together with the consideration of stimulated Raman scattering,we build the SBS theoretical model of phase/intensity modulation.Based on this theoretical model,we simulate to analyse the SBS threshold changing trend when adjusting some key modulation parameters.Those key parameters contains phase modulation frequency,phase modulation amplitude,the distribution of the modulated spectrum,intensity modulation pulse width,intensity modulation repetition rate,passive fiber length and so on.Further,we simulate the SBS threshold when phase and intensity modulations are employed simultaneously,including the SBS suppression effect when different phase and intensity modulation parameters are combined.Theoretical results show that simultaneous phase and intensity modulation can improve the SBS threshold of narrow-linewidth polarization-maintaining fiber amplifier effectively.Based on the theories and simulations afore-mentioned,we experimentally study the SBS suppression effect of phase/intensity modulation in high power polarization-maintaining fiber amplifier.Based on the acoustic-optical driven source and polarization-maintaining Yb-doped active fiber with core/inner cladding diameter of 25/400 ?m,560 W continuous laser is obtained through phase modulation alone and the output power is confined by mode instability effect.Futher,in order to avoid mode instability,we use an arbitrarily function generator as the signal source and use the polarization-maintaining Yb-doped with the core/inner cladding diameter of 20/400 ?m as the active fiber.Based on this,the change of the SBS threshold is studied with the change of phase/intensity modulation parameters.The single-frequency SBS threshold,which means there is no modulation signal employed on the input laser,is 23.1 W.Then when employing phase modulation alone,the SBS threshold is enhanced from 23.1 W to 166 W at 80 MHz phase modulation frequency and 10 V phase modulation voltages.When employing intensity modulation alone,the SBS threshold is boosted to an average/peak power of 136 W/8.5kW at repetition rate of 5 MHz and pulse width of 4ns.When employing simultaneous 100 MHz 5 V phase modulation and 20 MHz 4 ns intensity modulation,the SBS threshold achieves an average/peak power of 293W/3.9kW,with the output linewidth of 4.5 GHz,and the SBS threshold is enhanced to 13.7 times of the single-frequency SBS threshold.In this paper,we theoretically analyse and experimentally prove the effectiveness of the simultaneous phase and intensity modulation on SBS suppression.Results show that compared with employing phase or intensity modulation alone,simultaneous phase and intensity modulation can suppress SBS of polarization-maintaining fiber amplifier in a better degree.If the bandwidth of the phase modulator and the signal source can be improved,and the phase modulation amplitude and frequency can go further to increase,higher power narrow-linewidth polarization-maintaining fiber amplifier can be achieved.Those theoretical and experimental researches have some guide influences on the power scaling of the narrow-linewidth polarization-maintaining fiber amplifier.