Use of multi-notch filter for simultaneous recording of stokes and anti-stokes Raman signals close to the exciting line

An optical prefilter called Xtranotch, incorporating multiple holographic n otch filters (or more generally Bragg diffraction notch filters) has been d esigned to record, with a single-stage spectrograph, both Stokes and anti-S tokes regions of the Raman spectrum simultaneously, with the smallest possi ble blind region around the Rayleigh line compatible with the best performa nce of current supernotch holographic filters. Thus, with this arrangement it is possible to observe simultaneously low-wavenumber Raman lines down to ca 28 cm(-1) on each side of the Rayleigh line together with a high signal throughput along the entire spectrum. Optical alignment of this multi-notc h filter is made easier owing to the use of a mechanical system which compe nsates for the beam deviation occurring when Bragg diffraction notch filter s are rotated. Better laser line rejection can also be obtained by replacin g one of the holographic notch filters by a more efficient grating filter b ut at the expense of throughput in the low-wavenumber region, Copyright (C) 1999 John Wiley & Sons, Ltd.