Dispersive Imaging Spectrograph Designs For Raman Spectroscopy And Their Impact On Spectral And Spatial Resolution

By Jay Zakrzewski and Kevin Didona, Headwall Photonics, Inc.

Raman spectroscopy is experiencing a rapid increase in commercial and military application integration. There has been a large amount of commercial discussion regarding relatively low cost, low resolution single point (single fiber) spectrometers for handheld transportable applications, although only limited advancement of higher performance designs. Applications which require maximum signal collection and dynamic range, including those which require multichannel or other input aperture imaging capability will benefit if one can fill the entire height of the tallest spectroscopy CCD with minimal distortion of the input image.

Most commercially available dispersive spectrometer optical designs have difficulty imaging all field points of the full Raman spectral bandwidth onto a flat focal plane simultaneously. Czerny-Turner designs are typically throughput limited to approximately f/4 or slower, and exhibit relatively high image distortions as one moves further away from the central input axis (spatially over the slit height) as well as along the spectral axis. Torroidal mirrors are often employed to minimize this effect, although this provides optimized correction for only one point on the focal plane. Others, which are based on axial-transmissive designs, suffer at short focal lengths from chromatic aberrations, smile, and keystone. For best results, these often require the use of curved input apertures, non-standard refractive lens material selection, and wavelength optimized anti-reflective coatings on each lens surface to compensate for chromatic aberrations, reflection losses, and potential ghost images. In addition, common axial-transmissive designs suffer from throughput vignetting at both the short and long wavelength regions.

This paper describes the performance of Headwalls’ Raman Explorer™, a novel aberration corrected high reciprocal dispersion retro-reflective concentric imaging spectrograph design with multiple inputs and high efficiency through resonance, which produces minimal image blur over the full CCD focal plane with exceptionally high signal to noise efficiency and f/2.4 throughput. This design images 1:1 straight slit or stacked linear arrays of optical fibers covering the full height of spectroscopy CCD’s with optimal channel separation and no image curvature.