  This paper presents the opto-mechanical integration and alignment, functional
and optical performance verification of the NIR arm of Son Of X-Shooter (SOXS)
instrument. SOXS will be a single object spectroscopic facility for the ESO-NTT
3.6-m telescope, made by two arms high efficiency spectrographs, able to cover
the spectral range 350 2050 nm with a mean resolving power R~4500. In
particular the NIR arm is a cryogenic echelle cross-dispersed spectrograph
spanning the 780-2050 nm range. We describe the integration and alignment
method performed to assemble the different opto-mechanical elements and their
installation on the NIR vacuum vessel, which mostly relies on mechanical
characterization. The tests done to assess the image quality, linear dispersion
and orders trace in laboratory conditions are summarized. The full optical
performance verification, namely echellogram format, image quality and
resulting spectral resolving power in the whole NIR arm (optical path and
science detector) is detailed. Such verification is one of the most relevant
prerequisites for the subsequent full instrument assembly and provisional
acceptance in Europe milestone, foreseen in 2024.
