Contrast improvement at petawatt-class lasers using ultrafast optical-parametric amplification

One main source of contrast degradation in ultra-high-intensity laser systems is due to noise in the amplifiers. Depending on the amplification technique this can be amplified spontaneous emission (ASE) or parametric fluorescence. Since the noise cannot fall below the quantum-noise level, the approach presented here aims for increasing the signal to improve the contrast. This is accomplished with an ultrashort optical parametric amplifier (uOPA). Since the only source of noise is parametric fluorescence which is confined to the pump pulse duration using a short pump-laser in the order of 1 ps prevents contrast degradation outside this temporal window. Therefore a main challenge of the uOPA is up to the generation of a pump-laser. The pump pulses are generated in a compact chirped pulse amplifier using a single chirped volume Bragg grating for stretching and compression and a diode-pumped Ytterbium-doped amplifier which has enough bandwidth to support sub-picosecond laser pulses. A gain level exceeding 104 was reached in the uOPA which allows for reducing the gain in the consecutive amplifiers of the laser system to end up with the same output level. Furthermore it is believed that this is the first unambiguous confirmation that the ASE level in a CPA system linearly depends on the seeding energy.