  We present magneto-optical Kerr effect (MOKE) spectrometer based on a
modified Martin-Puplett interferometer, utilizing continuous wave sub-THz
low-power radiation in broad frequency range. This spectrometer is capable of
measuring the frequency dependence of the MOKE response function, both the Kerr
rotation and ellipticity, simultaneously with sub-milliradian accuracy without
the need for reference measurement. The instrument's versatility allow it to be
coupled to a cryostat with optical windows enabling studies of a variety of
quantum materials such as unconventional superconductors, two-dimensional
electron gas systems, quantum magnets, and other system showing optical Hall
response, at sub-Kelvin temperatures and in high magnetic fields. We
demonstrate the functionality of the MOKE spectrometer using an undoped InSb
wafer as a test sample.
