Chirped pulse four-wave Raman mixing

In order to generate a 1-fs optical pulse, it is necessary to expand the frequency domain of the laser from the infrared to the ultraviolet region. One of the approaches for this is to use four-wave Raman mixing of molecular hydrogen. A two-color beam consisting of a fundamental beam of a Ti: sapphire laser (800 nm, 35 fs) and an output beam of an optical parametric amplifier (1200 nm, 50 fs) pumped by the Ti: sapphire laser has been utilized for the enhancement of efficiency. In this approach, the frequencies of the laser emissions were adjusted at 3ω and 2ω (carrier envelope offset frequencies), respectively, where ω is the vibrational Raman shift frequency (4155 cm¹) of molecular hydrogen. In fact, numerous emissions extending from 13ω (183 nm) to 2ω (1200 nm) covering the entire optical region have been generated [1]. The efficiency would be further enhanced using a laser with larger pulse energy. It becomes, however, difficult to avoid other nonlinear optical effects such as self phase modulation and optical breakdown in the hydrogen gas. In order to overcome this problem, we studied the effect of chirp in the pump beams to efficiently generate anti-Stokes Raman emission based on four-wave Raman mixing by reducing the laser peak power and then by avoiding the above undesirable effects.