  The Gemini Planet Imager (GPI) is a complex optical system designed to
directly detect the self-emission of young planets within two arcseconds of
their host stars. After suppressing the starlight with an advanced AO system
and apodized coronagraph, the dominant residual contamination in the focal
plane are speckles from the atmosphere and optical surfaces. Since speckles are
diffractive in nature their positions in the field are strongly wavelength
dependent, while an actual companion planet will remain at fixed separation. By
comparing multiple images at different wavelengths taken simultaneously, we can
freeze the speckle pattern and extract the planet light adding an order of
magnitude of contrast. To achieve a bandpass of 20%, sufficient to perform
speckle suppression, and to observe the entire two arcsecond field of view at
diffraction limited sampling, we designed and built an integral field
spectrograph with extremely low wavefront error and almost no chromatic
aberration. The spectrograph is fully cryogenic and operates in the wavelength
range 1 to 2.4 microns with five selectable filters. A prism is used to produce
a spectral resolution of 45 in the primary detection band and maintain high
throughput. Based on the OSIRIS spectrograph at Keck, we selected to use a
lenslet-based spectrograph to achieve an rms wavefront error of approximately
25 nm. Over 36,000 spectra are taken simultaneously and reassembled into image
cubes that have roughly 192x192 spatial elements and contain between 11 and 20
spectral channels. The primary dispersion prism can be replaced with a
Wollaston prism for dual polarization measurements. The spectrograph also has a
pupil-viewing mode for alignment and calibration.
