A volume pinhole for mode-filtering of the femtosecond laser for use in the generation of high-order vibrational and rotational emissions by four-wave Raman mixing

The beam quality of a femtosecond dye/KrF excimer laser was improved using a pinhole, prepared from an acrylic plate. The pinhole acts as a volume lig ht absorber that discriminates high-order transverse modes of the laser bea m. A breakdown at the surface of the pinhole, which degrades the quality of the transmitted beam, can be avoided using this spatial filter. This appro ach was useful in minimizing self-phase modulation, which exhausts the limi ted laser energy and reduces the Raman output. A conversion efficiency of s ome vibrational and rotational emissions was improved, even though the lase r pulse energy was substantially reduced by mode filtering.