Asymmetric Beam Combination for Optical Interferometry

Optical interferometers increasingly use single-mode fibers as spatialfilters to convert varying wavefront distortion into intensity fluctuationswhich can be monitored for accurate calibration of fringe amplitudes. Here Ipropose using an asymmetric coupler to allow the photometric intensities ofeach telescope beam to be measured at the same time as the fringe visibility,but without the need for dedicated photometric outputs, which reduce the lightthroughput in the interferometric channels. In the read-noise limited caseoften encountered in the infrared, I show that a 53% improvement insignal-to-noise ratio for the visibility amplitude measurement is achievable,when compared to a balanced coupler setup with 50% photometric taps (e.g., theFLUOR experiment). In the Poisson-noise limit appropriate for visible light,the improvement is reduced to only ~8%. This scheme also reduces the cost andcomplexity of the beam combination since fewer components and detectors arerequired, and can be extended to more than two telescopes for "all-in-one" orpair-wise beam combination. Asymmetric beam combination can also be employedfor monitoring scintillation and throughput variations in systems withoutspatial filtering.Comment: 13 pages, accepted by Publications of the Astronomical Society of the Pacifi