Study of Longitudinal Stimulated Brillouin Scattering by Numerical Solutions and Experiments
|School||National University of Defense Science and Technology|
|Keywords||longitudinal stimulated Brillouin scattering(LSBS) coherent damage three- dimensional solution parallel program parameters of pump laser parameters of focusing parameters of material threshold damage threshold gain|
Intensely coherent acoustic field could bring damage when its intensity exceeds the maximum pull stress that optics-brittle material could endure.The damage is called coherent damage.Infuences to damage threshold and damage characteristics of parameters in LSBS system such as pump laser(including wavelength,intensity,pulseduration,ineraction time and Stokesratio),intensity of seeded laser,material(length and category) and focusing(position of focusing and focus length) were all investigated in this paper by transient one-dimensional numerical solution. Every LSBS physical model was considered.Infleuences of parameters in LSBS system to threshold and pulse compression were investigated at the same time.All of the one-dimensional solutions to LSBS were supplied for elementary cognitions to characteristics of LSBS and would be complementary to the following three-dimensional solutions.Most of the scholors take up with experiments, engineering and one-dimensional solution for LSBS in our country.For overseas scholors, three-dimensional solutions are abounding.But all of the solutions including semi-analysis and numerical simulation have been dealt with a lot of approximation and ignorer.We got the fullest-scale three-dimensional coupled wave equations for LSBS up to now and simulated the characteristics such as Spatial and temeral distribution of Stokes field ,fidelity of phase conjugation and energy reflectivity et.al..It would spend long time to complete one computation for three-dimensional numerical simulation with serial Fortran program.To improve efficiency of computation, parallel computation was introduced through LAM in Linux system and Mpirun in Windows system and all of the three-dimensional solutions were performed by parallel computing.Spatial distribution of laser’s phase including pump laser and Stokes laser were given firstime and it showed the wavefront reversal of Stokes laser compared to pump laser.Through reasonable setup and Fresnel diffraction integral, the theory of aberration correction by LSBS was proved.The aberration of wavefront was introduced by Zernike polynomial.LSBS generation model for distributed noise is used generally at present, but it is reasonable too that spontaneous Brillouin scattering supplies noise before stimulated Brillouin scattering arising.We compared these two LSBS generation model through three-dimensional simulation.It is recognized that gain of LSBS depends on bandwidth of pump laser.Electic modulation to pump laser was introduced in this paper to modulate bandwidth of pump laser.Infuences of bandwidth to characteristics of LSBS such as gain,distribution of Stokes field and acoustic field were studied and we got a way to suppress LSBS in fiber communication.LSBS amplifier was simulated for the fact that Stokes seeded laser could improve quality of phase conjugation. Influences of intensity of Stokes seeded laser on characteristics of LSBS such as gain,distribution of Stokes field ,fidelity of phase conjugation and energy reflectivity of Stokes field et. al. was studied.Influences of parameters such as pump laser (including beam radius, wavelength, intensity, energy, pulseduration,interaction time et. al.),focusing (focus length,position of focus) and material (length and kind) were studied to get high fidelity of phase conjugation and energy reflectivity.It was investigated in two states:intensely transient for short interaction time and quasi-stable state for long interaction time which was longer than five times of decay time of acoustic wave.Rules of influences on spatial and temeral distribution of Stokes field,fidelity of phase conjugation and energy reflectivity were all given out.Experimental investigation was carried through, too. The solid optical glasses(K9 glass and fused silica made in our country) were used to substitute traditional LSBS medium of liquid and gas which are poisonous and need to be encapsulated and cooled. Infueneces of pulse duration,pulse repetition,pump energy ,focus length,length of material,category of material and with or without transmission film were all investigated.We got energy reflectivity of 90% for Stokes laser in both K9 glass and fused silica under the condition of worse quality of laser beam. It was proved that K9 glass could be a new kind of LSBS medium.Experimental investigation for pump laser with multi-longitudinal mode were firstly carried through in solid and transparent medium elementarily.It was proved that LSBS could happen for pump laser of multi-longitudinal mode although optical damage might happen at the same time and optical damage was not sure to prevent happening of LSBS.LSBS of double cell was investigated,too.It was concluded that system of double cell was not better than system of single cell for energy reflectivity.