The effects of beamsplitter emission in a balanced Fourier transform spectrometer

A Fourier transform spectrometer (FTS), with its high throughput and free spectral range, represents the most efficient class of spectrometer and is ideally suited to applications in the energy starved THz region. For broad-band/continuum observations, the challenging dynamic range requirements of the interferogram signal near zero optical path difference can be reduced by port balancing, a technique in which a broadband spectral source is placed at the second, complementary FTS input port. Studies of the performance of the Herschel SPIRE FTS instrument have shown that beamsplitter emission may contribute significantly to the measured interferogram when port balancing is optimum. In this paper we describe an experiment that has been designed to explore this effect further, and compare the results with a theoretical model.