An efficient Raman shifter for the generation of tunable infrared radiation in the range 3.0–3.5 μm

Abstract

Tunable infrared radiation with pulse energies of more than 1 mJ has been generated in the spectral range between 3.0 to 3.5 μm via third order stimulated Raman scattering of visible dye laser light (⩽ 75 mJ pulse energy) in an extended cell filled with H 2-gas without using a waveguide. A systematic investigation of the extended cell design provided the following interesting features: (i) the energy of the infrared light pulse ( E s3) rises nearly exponential with the focal length of the entrance lens and the length of the cell, (ii) E s3 depends very strongly on the H 2-pressure within the cell and exhibits a sharp maximum at a surprisingly low value of around 5 bar, (iii) the S3-radiation is emitted in a cone centered around the axis of the pump beam with virtually no axial intensity and (iv) the apex angle of the cone increases with the cell pressure but is independent of the pump energy. Results (ii) to (iv) can be rationalized on the basis of a four-wave mixing process involving the pump beam and the three possible orders of Stokes waves. The reasons for result (i) are probably more involved and unknown as yet.