VPH transmission grating design considerations for spectrograph designers

Volume phase holographic gratings (VPHGs) have gained popularity for use in astronomical spectrographs due to their high first order diffraction efficiency, angle tuning capabilities, low scatter, durability, and ability to be manufactured in large formats; gratings up to 30 cm across are routinely achievable. The nature of the VPHG manufacturing process offers optical and instrument designers a tremendous amount of flexibility to optimize performance for a specific application or need. In this talk, we will explore VPHGs from a grating designer’s perspective, aiming to give insight to spectrograph designers as they develop grating requirements for new spectrographs. We will address considerations and limitations with respect to grating size, angle of incidence (AOI), dispersion, wavelength of operation, and bandwidth. We will look at how AOI affects grating design and efficiency, stepping through several AOI regimes to explore the diffraction efficiency and spectral profiles which can be achieved, as well as the physics behind them – from required film thickness to the relative efficiency of s- and p- polarization as index modulation is varied in each regime. We will also touch upon the advantages offered by use of a grism configuration (GRating + prISM), and provide examples of both routine and unusual examples of grism implementation.