An axial transmissive Raman spectrograph is described. The instrument employs volume-phase holograms as the laser line rejection filter and transmission grating. The on-axis configuration and the use of a high-spatial-frequency transmission grating allows nearly diffraction-limited imaging and pixel-limited resolution at f/1.8. The performance of the instrument is demonstrated for fiber-optic probe and microprobe Raman spectroscopy.

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In this paper we report the continuing development of a novel fiber-optic coupled confocal Raman microscope which can offer spectral resolutions up to 0.6 cm−1 per pixel, spatial resolutions of 1 micron or less with axial resolutions of 2-3 microns. The system is based around a compact base unit which comprises a compact solid-state laser, a proprietary f/1.8 imaging spectrograph, and a TE-cooled CCD detector operating at −70°C.In Figure 1 a schematic of the fiber coupled microscope is shown. The microscope includes a integrated holographic filter module to prevent silica Raman generated in the excitation fiber from reaching the sample and to reject radiation at the excitation wavelength from being returned to the spectrograph via the collection fiber(s). The architecture has been designed to allow both fluorescence imaging and Raman microspectroscopy to be accomplished without removing the sample. The figure also depicts the positions of linear polarizers which can be used to collect polarized Raman spectra from small particles or thin polymer laminate samples.

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