| Chirped Pulse Amplification |
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CPA was invented by Gérard Mourou at the University Of Rochester in the mid 1980s . Before then, the peak Power of laser pulses was limited due to the fact that a laser pulse at Intensities of Gigawatt s per Square Centimeter causes serious damage to the Gain Medium through Nonlinear Processes such as Self-focusing . For example, some of the most powerful compressed CPA laser beams, even in an unfocused large aperture (after exiting the compression grating) can exceed intensities of 700 gigawatts/cm2, which if allowed to propagate in Air would instantly self focus and form a Plasma or cause Filament Propagation , both of which would ruin the original beam's desirable qualities and could even cause back-reflection potentially damaging the laser's components. In order to keep the intensity of laser pulses below the Threshold of the nonlinear effects, the laser systems had to be large and expensive, and the peak power of laser pulses was limited to the high gigawatt level or terawatt level for very large multi beam facilities. In CPA, on the other hand, an ultrashort laser pulse is stretched out in time prior to introducing it to the gain medium using a pair of Gratings that are arranged so that the low-frequency component of the laser pulse travels a shorter path than the high-frequency component does. After going through the grating pair, the laser pulse becomes positively Chirp ed, that is, the high-frequency component lags behind the low-frequency component, and has longer Pulse Duration than the original by a factor of 103 to 105. Then the stretched pulse, whose intensity is sufficiently low compared with the intensity limit of gigawatts per square centimeter, is safely introduced to the gain medium and amplified by a factor of approximately 1010. Finally, the amplified laser pulse is recompressed back to the original pulse width through the reversal process of stretching, achieving orders of magnitude higher peak power than laser systems could generate before the invention of CPA. In addition to the higher peak power, CPA makes it possible to miniaturize laser systems (the compressor being the biggest part). A compact high-power laser, known as a tabletop terawatt laser (T3 laser), can be created based on the CPA technique. COMPRESSOR AND STRETCHER There are many ways to construct compressors and stretchers. The Material Dispersion can be used. Depending on which frequency is higher, that of the light or that of the middle of the bandgap of the material, this results in a stretcher or compressor. In the middle is net dispersion und just below the middle is the Soliton region. Glas fibres can have their dispersion tailored to meet the needs. Typically 200 mm glass are used. A in 4-f configuration and mirroring the second prism/grating eliminates this effect. A smaller telescope leads to a compressor and a bigger telescope leads to a stretcher. Prisms and gratings are sometimes combined to correct higher order chirps. Typically a 10 m large prism compressor is used. One or multiple reflections between a pair of Chirped Mirrors or similar device allow any form of chirp. This is often used in conjunction with the other techniques to correct for higher orders. Typically 10 reflections are needed, since 0.3 mm thick dielectric stacks are not manufactureable. In the dazzler light is going through birefringent material. A shear noise going through the crystal induces additional stress birefringence, which rotates the polarization. Typically 50 mm of birefringent material is needed. In the spatial light modulator the light is analyzed sent through a LCD and composed back together. The bandwidht of a single element of the LCD is typically 1 nm so that the pulse time width can get 1 mm (at 1 µm central wavelength). SEE ALSO |
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