The Dynamic Study On Thermal Nonlinear Effect Based On 4f Pump-probe Technology

When the laser propagates through the material, the thermal effect is always existent, and the thermal nonlinear effect was the earliest one of the optical effect researches. Previous research focused on the steady. In recent years, with the wide application of pulsed lasers, the research of the transient thermal appears especially important; it is also reported by some literatures. But there is no report about the dynamic process of the transient thermal nonlinear refraction.We use the 4f pump-probe to study the process of the transient thermal nonlinear refraction. We start from transient thermal nonlinear principle, and use the Crank-Nicholson finite difference method to solve the acoustic-wave and thermal propagation equation, simulating the change by the time. At the same time, the influence of the thermal effect is simulated by altering such parameters as the laser width, beam radius, pulse energy etc.In the experiment, we introduce a method of measure the optics nonlinear index: 4f coherent imaging system with phase object. This method is very simple and accurate, and compare to the traditional method-z scan. The process of transient thermal nonlinear effect was studied using the time-resolved pump-probe system based on 4f coherent imaging system with phase object with nanosecond laser pulse at the wavelength of 532nm. The laser source we used is a Q-switched and mode locked Nd: YAG laser (Continuum Surelite). The delay time is 5ns 8ns and 10ns. The sample is C70/toluene, we get different linear and nonlinear image at the diverse delay time by using the CCD. We choose the linear flare as incident flare to fit experimental curves, and find when the delay time increases, the heat of absorption is growing, the influence of temperature variations increase, and make the transient thermal refraction obvious enhancement. The result is good fit to our theoretical numerical simulation.