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Raman shifting in the absence of multiple Stokes orders with a 1.3-um Nd:YAG laser in hydrogen

A Q-switched Nd:YAG laser generating 160 mJ in 60 ns at 1.3 micrometers has been shifted to 2.9 micrometers by Raman conversion in hydrogen. The energy of the 2.9 micrometers photons is insufficient to undergo further Stokes shifts. Thus the first Stokes line is the terminal wavelength for this process. While terminal Stokes components have been reached in previous Raman shifting studies, these components have always been the result of multiple Stokes shifts. In a single pass configuration photon conversion of up to 0.37 was realized. Conversion into anti-Stokes lines of up to third order was observed, albeit at insignificant energies. With the addition of an unoptimized resonator the photon conversion rose to 0.49. The pressure dependence of Stokes conversion was also investigated. The data show a smooth increase in output followed by saturation. This is in accordance with conversion into a single terminal Stokes lines. However, in contradiction with theory, the pump linewidth was observed to matter a great deal. Amplified elastic or near elastic scattering is suggested as an explanation for this result. Overall, the success of this work bodes well for forthcoming attempts to obtain tunable mid-IR radiation by means of a terminal first Stokes shift.