Nonlinear Sagnac interferometer for temporal pulse cleaning of high peak power femtosecond laser

Summary form only given. The development of laser chains based on the chirped pulse amplification leads now to peak powers higher than 100 TW. The focused intensity can reach 10/sup 21/ W/cm/sup 2/. To keep the laser matter interaction in the femtosecond regime, the pulse has to exhibit a high temporal contrast. The most important problem consists in the presence of an amplified spontaneous emission (ASE) background coming from the amplification process. At the output of the laser the ratio between the femtosecond pulse intensity and the ASE level is about 10/sup 6/ leading to an ASE intensity as high as 10/sup 15/ W/cm/sup 2/, far above the ionization threshold of most materials and susceptible to strongly modify the interaction process. We propose to rely on an unbalanced nonlinear Sagnac interferometer which is implemented after the first CPA system amplifies pulses to the millijoule level.