Femtosecond Continuum Generation

Abstract

We obtain white light continuum pulses with durations as short as 80 fsec. The broad spectral range (0.19μ–1.6μ) minimal chirp, stable repetitive character, and availability of powers extending to the gigawatt range suggest these pulses will provide a powerful new tool for femtosecond spectroscopy. We obtain these pulses by using short (65 fsec) pulses from our colliding pulse mode locked laser [1] which have been amplified to gigawatt powers in an amplifier designed specifically for short pulse amplification [2]. The short duration and high intensity of these amplified pulses permit us to generate continuum pulses in a short length of nonlinear medium with the consequence that pulse distortion by group velocity dispersion is minimal. We thus generate pulses which are temporally short and which also have almost negligible chirp over broad spectral regions. Such a feature is essential for femtosecond time resolution in most practical applications. This approach is also of special interest in that the temporal distribution of the continuum is determined primarily by the generation mechanism rather than by group velocity dispersion in the generating or analyzing media. We can thus present convincing evidence, e.g., that self phase modulation plays a prominent role in the continuum generation process.