Proceedings PaperShort Laser Pulses ; Optical Applications
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Optical processing of optical signals involves beam interactions through materials : mixing of signals Si and S., by the material non-linear susceptibility may generate new waves S3, of amplitude proportional to the product of the interfering beams amplitudes. Combination of beam multiplications with usual linear beam superposition (addition) achieved by conventional interferometers is the general basis of all-optical operations. Unfortunately optical materials often exhibit very weak non-linear susceptibilities : generation of the S3 component frequently taluirT5 that the product of the interaction length by the intensity of the co-propagating waves lies in the range 10 -10 W.m. It is clearly not possible to work at such levels of continuous laser power. Thus very active research is presently devoted to new highly non-linear materials where wave interactions may occur at moderate c.w. power. But stronger non-linearities appear generally connected with slower responses to field excitations. SHORT LASER PUL-SES, on the subnano- to subpicosecond time scales, offer another effective alternative to high-speed "opto-optical" signal processing : indeed one picosecond pulse ofi5nanojole energy focused onto the Airy disk of minimum area (one squared wavelength) shines the focus by a 10 W.m illumination. Then, despite low pulse energy, the above "interaction product" reaches the threshold of non-linear susceptibilities after transmission through less than one millimeter of conventional optical material. For instance such pulses will be produced at 100 MHz repetition rat! in the "mode-locked" emission of 1.5 m long Laser cavity, with moderate average power (1 W) and linewidth (10 Hz, i.e. 3 nm around 1000 nm average wavelength).