  This paper describes a cryogenic optical testbed developed to characterize
u-Spec spectrometers in a dedicated dilution refrigerator (DR) system. u-Spec
is a far-infrared integrated spectrometer that is an analog to a Rowland-type
grating spectrometer. It employs a single-crystal silicon substrate with
niobium microstrip lines and aluminum kinetic inductance detectors (KIDs).
Current designs with a resolution of 512 are in fabrication for the EXCLAIM
(Experiment for Cryogenic Large Aperture Intensity Mapping) balloon mission.
The primary spectrometer performance and design parameters are efficiency, NEP,
inter-channel isolation, spectral resolution, and frequency response for each
channel. Here we present the development and design of an optical
characterization facility and preliminary validation of that facility with
earlier prototype R=64 devices. We have conducted and describe initial optical
measurements of R = 64 devices using a swept photomixer line source. We also
discuss the test plan for optical characterization of the EXCLAIM R = 512
u-Spec devices in this new testbed.
