Generation And Measurement Of High Contrast Ultrashort Intense Laser Pulses

In this thesis, the generation and measurement of high contrast, intense, ultrashort pulses have been studied. Various factors affecting the contrast and pulse shape of ultrashort light pulses from a chirped pulse amplification (CPA) laser system are identified. The level of contrast resulting from influence of these factors is estimated. Methods for improving and controlling the pulse shape and increasing the contrast are discussed. Ultrahigh contrast, 1-ps pulses were generated from a CPA system with no temporal structure up to eleven orders of magnitude. This is eight orders of magnitude higher contrast than the original pulse. This contrast boost was achieved using two techniques. One is the optical pulse cleaning based on the nonlinear birefringence of the chirping fiber and applied to the pulses before amplification. The other is the fast saturable absorber. The fast saturable absorber was placed after amplification and compression of the pulse. The measurements of high-contrast, ultrashort pulse with high temporal resolution and high dynamic range is a separate issue. The only known devices simultaneously possessing subpicosecond resolution and high-dynamic detection range are non-collinear beam correlators based on frequency conversion. The factors that limit the dynamic range of non-collinear autocorrelators have been analyzed. The main physical factors are the finite energy of the pulse, and surface and bulk scattering from converting crystal. A specially designed second-order autocorrelator with dynamic detection range of 1012 and temporal resolution of 50 fs was built to measure high-contrast pulses generated from the CPA system. Self-phase modulation (SPM) undergone by a chirped pulse plays an important role in determining the final shape of the compressed pulse in CPA lasers, even at relatively low peak values of SPM, which is called B-integral. The final value of the nonlinear index of refraction of the laser host materials and relay optics represents a fundamental limitation to generation of the distortion-free, structure-free ultrashort pulses. It is experimentally demonstrated in this thesis that the B-integral accumulated in a chirped-pulse-amplification (CPA) laser can be canceled using a semiconductor with a negative nonlinear index of refraction. We demonstrate this using GaAs at 1.053 μm. Optimum semiconductor sample parameters for minimizing the B-integral such as gap energy, nonlinear and linear absorption, nonlinear refraction, and thickness for different pulse durations and intensities are discussed. It is important to know the type, sign and amount of the optical nonlinearities for the optical components which are used in any system, or in particularly in a CPA system. A new method to measure the third and higher order nonlinearities of materials based on frequency domain interferometry is experimentally demonstrated. The fast and slow refractive non-linearity of semiconductors in the presence of linear and nonlinear two-photon absorption were measured in one single measurement. This was done with GaAs at 1.053 μm and with CdSe at 0.81 μm—two materials which have negative nonlinear index of refraction at given wavelength and hence can be used as B-integral compensators