Stimulated rotational Raman scattering in nitrogen in long air paths.

We studied the growth from amplified spontaneous emission of stimulated Raman scattering in air using a 20-cm-diameter, linearly polarized, 1053-nm laser beam propagating over a 20-150-in air path. For 2.5-nsec square pulses we found about 1% conversion on the S(8) and S(10) rotational Raman lines of nitrogen at an intensity-length product of 12 TW/cm, which implies a small-signal gain coefficient of 2.5 cm/TW. For 1-nsec square pulses, 1% conversion requires an intensity-length product of about 16 TW/cm. The beam quality deteriorates severely above Raman threshold.