Compact spectrograph: design and implementation

Abstract

We have designed and fabricated a compact (3 cm × 3 cm × 2 mm) spectrograph for use as a wavelength-division demultiplexer. The design is based on an alteration to the classic Rowland mounting. Using a ray-tracing model that allows us to predict the linewidth, we have optimized the spectrograph spectral resolution by cancelling the coma term in the aberrations. We fabricated gratings holographically by using photoresist to coat a convex substrate that is illuminated by converging beams. The resulting gratings achieved 20-30% efficiencies, and the linewidths agreed with our model predictions. The gratings are made on a thin substrate (several hundred microns thick) that acts like a waveguide in the nondispersive direction, which makes it for compatible with fiber optic inputs. This maximizes optical efficiency but limits the spectral resolution because of due to multimoding effects. We have also developed an algorithm for scribing the grating with a computer-controlled ruling engine, and gratings replicated from a master that is ruled according to this algorithm have linewidths comparable to holographic gratings and efficiencies greater than 50%.