Abstract

Spatial light modulator (SLM) is a powerful tool for the generation and application of ultrashort laser pulses. We have developed a novel SLM consisting of fused silica plates for the use of high-intensity femtosecond laser pulses over a wide range of spectrum. Figure 1 shows the experimental apparatus. A linear array of 48 fused silica plates with a thickness of 1 mm is set near to the Fourier plane of 4-f configuration composed of a grating and a concave mirror. The angle of each plate to the incident beam is finely changed by bimorphous piezo actuators, giving the phase shift expressed as 2πd/λ {(n²-sin²θ)^I/2-cosθ} where n is the refractive index, d the thickness of the plates, and θ the angle of incidence to the plate. The typical phase retardation at wavelengths around 800 nm is 6π radians for Δθ =1 degree at θ=10 degrees, which was also verified using an interferometer. For feedback control of the femtosecond laser pulses, SHG autocorrelation signals are sent to a personal computer which regulates the applied voltage to the actuators based on the simulated annealing algorithm. We have tried two modes of operation with regard to the initial status of the SLM, i.e., with and without initial alignment of the fused silica plates so as to give zero dispersion to the laser pulse. In both cases, 20-fs pulses from a Ti:sapphire laser are stretched by inserting a fused silica block into the optical path. Then, the system starts to compress the pulse compensating dispersions induced by the block and the SLM itself. Figure 2 shows the result in the case that the plates are aligned at random, SHG intensity as a function of the number of iteration, autocorrelation traces before (left inset), and after (right inset) compression. The pulses were successfully compressed to 20 fs within a few minutes using the system.

© 2000 IEEE