Rainbow stars: a multifrequency laser for generation of ultrashort optical pulses (Invited Paper)

Numerous equally-spaced emission lines can be generated by stimulated Raman scattering and subsequent four-wave Raman mixing. The spectral domain extends from the near-infrared to the deep-ultraviolet, and then an ultimately-short optical pulse can be generated by phase locking in the process of four-wave Raman mixing. In fact, an intense 17-fs optical pulse has been generated using this technique. A comb of 1.6-fs optical pulses, i.e., the shortest optical pulses, has also been generated by other researchers. On the other hand, it is possible to generate highly-repetitive pulses using a continuous wave (CW) laser as a pump source and molecular hydrogen as a Raman medium in a high-finesse cavity. Coherent superposition of the two-color beam comprising of the fundamental beam and the Stokes beam provides a sinusoidal wave modulated at 17 THz. It is also possible to generate more than three emission lines simultaneously, which should generate an impulsive wave. It is noted that such Raman emission can be generated in a hollow-core fiber, making the device extremely simple. Such a device may have a potential for use in basic science and technology, e.g., the generation of a three-primary-color laser for display.