Heterodyne array receiver systems for both ground based and satellite telescope
facilities are now becoming feasible for imaging in the submillimetre/terahertz
regions of the EM spectrum. Phase gratings can be usefully employed as high
efficiency passive multiplexing devices in the local oscillator (LO) injection chain of
such receivers, ensuring that each element of the array is adequately biased and that
the reflected LO power level at the array is minimised. For the wavelengths of interest
both transmission and reflection gratings can be manufactured by milling an
appropriate pattern of slots into the surface(s) of a suitable material. Thus, the
required phase modulation is produced by the resulting pattern of varying optical path
lengths suffered by the incident wave-front. We report on work we are undertaking to
develop all reflection quasi-optical multiplexing systems so as to reduce reflection
losses at the grating and minimise the number of surfaces that can contribute to
standing wave effects in the optical system. As part of this endeavour we have also
developed a quasi-optical technique for analysing the inevitable degradation due to
multiple reflections on transmission grating design. This analysis is based on the
Gaussian beam mode technique, and a further application of this technique allows one
to assess tolerance limitations on the grating
