Lü Zhi-Wei

Optical limiting effect based on stimulated Brillouin scattering in CCl4

The optical limiting effect based on stimulated Brillouin scattering (SBS) in a nonlinear medium was investigated. We numerically treated the nonlinear propagation process with a theoretical model, which includes the spontaneous nature of the initiation of SBS and obtained optical limiting effect in the process. Energy limiting, pulse reshaping and stabilization have been demonstrated on SBS mechanism with the nonlinear medium CCl4. The input optical signals were Nd:YAG nanosecond laser pulses with width varying from 16ns to 7ns then to 2ns, the relationship between the transmitted signal and launched pump signal was shown. In the experimental regime, the most stable pulse and a superior energy stabilization of the transmitted pulse were obtained when the laser pulse-width became as short as 2ns. For the energy variation of laser pulses in a wide range of 14-88mJ, the output energy was limited in a quite narrow range 4.5-5.5mJ.

Spectrum evolutions of spontaneous and pump-depleted stimulated Brillouin scatterings in liquid media

A theoretical model for calculating spontaneous and stimulated Brillouin scattering(SBS) spectra is described. An empirical formula for the Stokes output spectral linewidth, a function of spontaneous Brillouin linewidth and the exponential gain coefficient, is obtained by the calculated data fitting. The formula holds true for two cases involving pump undepletion and depletion. The lineshape change from spontaneous to highly pump-depleted SBS spectra is also investigated. The result shows that for the pump power below the SBS threshold, the Stokes output spectral lineshape evolves from Lorentzian to approximately Gaussian as the pump power increases. For the pump power near or beyond the threshold, the SBS spectrum is in the form of a steady Gaussian profile, and the spectral linewidth comes to a certain value about 7 times narrower than the spontaneous one. The theoretical results are experimentally demonstrated by using several common liquid media.

High pulse-shape fidelity realized in stimulated Brillouin scattering generator with Stokes seed injection

In order to realize high pulse-shape fidelity of stimulated Brillouin scattering, a generator with Stokes seed injection is suggested and investigated. Theoretical and experimental researches show that the SBS duration becomes longer with the increase of the seed intensity and the best pulse-shape fidelity of above 90% is experimentally obtained.

Pulse Compression Based on Laser-Induced Optical Breakdown in Suspension

Pulse compression based on laser-induced optical breakdown in suspension is investigated. The physical mechanism behind it is analyzed theoretically and validated in the Q-switched Nd:YAG laser system. A 12-ns pump pulse is suppressed to 5 ns with good fidelity in the front edge and sharp steepness in the trailing edge. The HT-270, which has a small gain coefficient and absorption coefficient, is used as a solvent, and therefore the disturbance induced by stimulated Brillouin scattering and absorption are minimized and the transmittivity is enhanced.

Investigation of stimulated rotational Raman scattering of the high-power broadband laser with applied angular dispersion

Smoothing by spectral dispersion (SSD) leads to considerable improvement on laser-irradiation uniformity in far field for fusion lasers. Phase modulation in time and spectral angular dispersion (SAD) across the beam introduced by SSD will affect the stimulated rotational Raman scattering (SRRS) gain in the near field. This paper focuses on the influence of SAD on SRRS gain under different laser conditions. Results show that the SAD will aggravate the generation of SRRS when the laser initial additional phase is constant. On the contrary, the SAD can reduce the SRRS gain if appropriate SSD parameters are adopted when the laser initial additional phase is variable. SSD has a certain application prospect in SRRS suppression.

A laser precise pointing method using deformable mirrors in target area

This paper presents a configuration and its corresponding control algorithm for using large aperture deformable mirrors in the laser precise pointing system of inertial confinement fusion (ICF) target area. Simulation results show that this configuration can greatly enhance the focus energy concentricity and the pointing accuracy of the laser beams.

Investigation on improving characteristics of two-cell SBS system with CCl4/C2H5OH liquid mixture

In order to improve the performance of the two-cell stimulated Brillouin scattering (SBS) system, this paper proposes the methods of using mixtures, which require amplifier media to have small absorption rate, and generator media to have high optical breakdown threshold and Brillouin frequency shift equal to that of the amplification media. The characteristics of the two-cell SBS system are studied experimentally by using CCl4 as amplifier medium and CCl4, C2H5OH and CCl4/C2H5OH liquid mixture as generator medium pumped by Nd:YAG Q-switched laser. The obtained results show that liquid mixture in generator cell improves the power load ability, phase conjugation fidelity, energy reflectivity (ER) and ER stability.

Study on the beam propagation characteristics of wedged lens focusing system

In this paper, we built up the physical model of the wedged lens focusing system, simulated and analyzed the beam propagation characteristics using the ray tracing method, and proposed a beam pointing method based on this system.

A method to measure the Brillouin frequency shift of the medium through Brillouin amplification ratio

In order to measure the Brillouin frequency shift (BFS) of the medium, this paper proposes a method using mixtures in a two-cell stimulated Brillouin scattering system, which uses a medium to be measured as amplifier medium and a mixture medium as generator medium. The seed light from the generator gains effective amplification in the amplifier and the amplification ratio changes with the mixing fraction. Only when the BFS of the mixture medium is equal to that of the medium in the amplifier does the seed light obtain the maximum amplification ratio. The method has the advantage of independence of the wavelength of the incident light.

The damage of the optical components induced by the stimulated Brillouin scattering

A theory of excitation of ultrasonic waves in the stimulated Brillouin scattering (SBS) process is presented in this paper. By using several reasonable approximations, a numerical calculation of the transient longitudinal SBS shows that large amplitude of acoustic waves can be built up by the nanosecond pulse of high-power laser, which may result in the damage of optical glasses. The maximal density change and the maximal acoustic wave intensity in optical glasses of 5 cm in thickness are calculated by using different parameters of the high-energy laser, such as the intensity, the pulse width, and the wave length. The damage threshold of the optical glasses is about 80 GW/cm2 when using a 1064 nm laser. The dynamic mechanism of SBS is the electrostriction effect of the components coupling with the high-power laser.