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The reason for using CO2 lasers for this application over the more common Nd:YAG is because of it's longer wavelength. The longer wavelength makes better use of the pondermotive potential that is related to the square of the wavelength. The difficulty lies in achieving a short enough pulselengths at 10um. This is accomplished by using a CO2 seed laser and a high pressure amplifier. The seed laser is used to generate the low energy ultrashort pulse, <10ps. This pulse is then amplified by passing it through a series of 10atm high pressure CO2 amplifiers. At 10atm, collisional broadening results in the overlap of the adjacent CO2 rotational lines. This creates a 1THz wide quasi-continuous gain spectrum and pulses as short as 1ps can be amplified without any distortion. At 1atm the strongly modulated rotational structure of CO2 modifies the frequency spectrum of 1ps pulses.
Due to it's experience with building tunable high pressure CO2 lasers for laser isotope separation, the photonics group of SDI is uniquely suited for the development and manufacturing of the high pressure CO2 amplifiers needed for these systems. Using the lasers that the photonics group has developed and manufactured, a picosecond laser pulse can be focussed down to a 10um spot, and an electric field in the region of 30GV/cm can be generated reliably. This is substantially larger than the fields generated by conventional particle accelerators. |
