  Optical interferometers increasingly use single-mode fibers as spatial
filters to convert varying wavefront distortion into intensity fluctuations
which can be monitored for accurate calibration of fringe amplitudes. Here I
propose using an asymmetric coupler to allow the photometric intensities of
each telescope beam to be measured at the same time as the fringe visibility,
but without the need for dedicated photometric outputs, which reduce the light
throughput in the interferometric channels. In the read-noise limited case
often encountered in the infrared, I show that a 53% improvement in
signal-to-noise ratio for the visibility amplitude measurement is achievable,
when compared to a balanced coupler setup with 50% photometric taps (e.g., the
FLUOR experiment). In the Poisson-noise limit appropriate for visible light,
the improvement is reduced to only ~8%. This scheme also reduces the cost and
complexity of the beam combination since fewer components and detectors are
required, and can be extended to more than two telescopes for "all-in-one" or
pair-wise beam combination. Asymmetric beam combination can also be employed
for monitoring scintillation and throughput variations in systems without
spatial filtering.
