A simple multiple beam lo coupler for sis heterodyne arrays

The construction of SIS heterodyne imaging arrays for submillimetre wavelengths requires multiple coupling of the local oscillator signal. The quasi-optical analogue of a multiple cross-guide coupler, employing 45-degrees beam-splitters successively stacked along the local oscillator beam, allows for individual adjustment of local oscillator power to each channel. We analyse the coupling as a function of the focal ratio of the incident beams and the number of beam widths off-axis through which the coupler is extended, and describe a simple construction method to realise a compact and effective design.