An interferometer configured for use in optical coherence domain (OCT) reflectometry systems is disclosed. In the preferred embodiments, efficient routing of light and a balanced detection arrangement provide a high signal to noise ratio. In a first set of embodiments, a 3.times.3 coupler is used to split light along separate sample and reference paths and also for combining light returning from those paths and supplying the interfered collected light to the detection system. In an alternate set of embodiments, a pair of cascaded 2.times.2 couplers provides a similar function. The interferometer can be used with various OCT modalities including time-domain and frequency domain approaches.