  The Prime Focus Spectrograph (PFS) is a major instrument under development
for the 8.2 m Subaru telescope. Four identical spectrograph modules are located
in a room above one Nasmyth focus. A 55~m fiber optic cable feeds light to the
spectrographs from a robotic positioner at the prime focus, behind the
wide-field corrector developed for Hyper Suprime-Cam. The positioner contains
2400 fibers and covers a 1.3~degree hexagonal field of view.
  The spectrograph optical design consists of a Schmidt collimator, two
dichroic beamsplitters to split the light into three channels, and for each
channel a volume phase holographic (VPH) grating and a dual-corrector, modified
Schmidt reimaging camera. This design provides a 275~mm collimated beam
diameter, wide simultaneous wavelength coverage from 380~nm to 1.26~\textmu m,
and good imaging performance at the fast f/1.05 focal ratio required from the
cameras to avoid oversampling the fibers. The three channels are designated as
the blue, red, and near-infrared (NIR), and cover the bandpasses 380--650~nm
(blue), 630--970~nm (red), and 0.94--1.26~\textmu m (NIR). A mosaic of two
Hamamatsu 2k$\times$4k, 15~\textmu m pixel CCDs records the spectra in the blue
and red channels, while the NIR channel employs a 4k$\times$4k,
substrate-removed HAWAII-4RG array from Teledyne, with 15~\textmu m pixels and
a 1.7~\textmu m wavelength cutoff.
  VPH gratings were an obvious choice for PFS and a set of three prototype VPH
gratings (one each of the blue, red, and NIR designs) was ordered and has been
recently delivered. In this paper we present the design and specifications for
the PFS gratings, the plan and setups used for testing both the prototype and
final gratings, and results from recent optical testing of the prototype
grating set.
