Volume Phase Holographic Gratings: Polarization Properties and Diffraction Efficiency

We discuss the polarization properties and first-order diffractionefficiencies of volume phase holographic (VPH) transmission gratings, which canbe exploited to improve the throughput of modern spectrographs. The wavelengthof peak efficiency can be tuned by adjustment of the incidence angle. We showthat the variation of the Kogelnik efficiency versus Bragg angle depends onlyon one parameter, given by $P_{tune} = (\Delta n d)/(n \Lambda)$, where:$\Delta n$ is semi-amplitude of the refractive index modulation; $n$ is theaverage index; $d$ is the thickness of the active layer; and $\Lambda$ is thegrating period. The efficiency has a well defined dependence on polarization.In particular, it is possible to obtain theoretical 100% diffraction efficiencywith one linear polarization at any angle or to obtain 100% efficiency withunpolarized light at specific angles. In the latter case, high efficiency isthe result of aligning the peaks of the s- and p-polarizationefficiency-versus-thickness curves. The first of these `s-p-phased gratings'for astronomy is in use with the 6dF spectrograph. Consideration ofpolarization is particularly important for high spectral resolution, whichrequires large incidence angles. We also discuss the possibility of separatingpolarization states for improved throughput along the entire optical train of aspectrograph.Comment: 9 pages, accepted by Publ. Astron. Soc. Pacifi