Development of a Fourier transform spectrometer for the calibration of THz on-chip spectrometers

Astrophysical measurements at far-infrared (FIR) wavelengths have many applications, including probing the fuel for cosmic star formation and understanding the role of dust in the interstellar medium. We are currently developing an on-chip spectrometer operable in the 100-200μm [50-100 cm-1 ] band with sub-micron spectral resolution coupled to a low-noise kinetic inductance detector (KID) array with planned noise equivalent power (NEP) < 10-19 WHz1/2. Ultimately we will need to evaluate the spectral response of these chips at very low photon backgrounds. While the spectral performance can be simulated, it is crucial to make measurements of the as-built properties of the chip with a calibrator. To this end, we have designed and fabricated a cryogenic Fourier Transform Spectrometer (FTS). Since self-emission from the FTS at room temperature would dominate a cold black body source, the FTS optics are cooled to a temperature of 5K, which greatly reduces the photon background and simulates space-like conditions. The FTS is based on the Michelson design, with 6 fixed Aucoated Al mirrors, back-To-back moving mirrors that increase the effective optical path length by a factor of 2, and a kapton film beamsplitter. Light is coupled into the FTS using HDPE collimating lenses protected by bandpass filters, and the output beam is coupled directly into the device under test through a shared vacuum space. In this paper, we report on the FTS design and discuss its fabrication and testing plan.