Simulation and calibration of a compact millimeter-wavelength Fourier transform spectrometer

This paper presents the simulation and calibration of a Fourier transform spectrometer (FTS) developed to measure the spectrum of radiation sources between 50 GHz and 330 GHz, such as the cosmic microwave background. The recorded signal is modified from the ideal by properties of the interferometer and the detection system. We have developed a ray-trace-based simulation with which we can model these effects. The model can be verified with measurements and used to understand the instrument's systematic effects and to design new optimized configurations. The optimization comprises parameters of the design, such as large étendu, maximal spectral resolution, compact size, operational simplicity, and light weight, that conflict and need to be balanced. The numerical simulation consists of two parts: time-stream signal analysis and a ray-trace-based simulation that includes polarization and path length calculations and can account for the effects of beam loss and change of focus as the delay-generating mirror travels on its path. The simulation can study the coherence level and frequency resolution of the FTS instrument. While not exercised in this study, the simulation also can be used to study the effect of mirror figure and polarizer non-idealities, walk-off rays in the beam due to the large étendu, as well as misalignment of optical elements. We then present the comparison between simulations of a spectrally unresolved source and measurements by the FTS.