Estimating the phase in groundbased interferometry: performance comparison between singlemode and multimode schemes

Aims. We compare the performance of multi and singlemode interferometry for estimating the phase of the complex visibility. Methods. We provide a theoretical description of the interferometric signal that enables us to derive the phase error with detector, photon, and atmospheric noises, for both multi and singlemode cases. Results. We show that despite the loss of flux which occurs when the light is injected in the singlemode component (i.e. singlemode fibers, integrated optics), the spatial filtering properties of these singlemode devices often enable a better performance than multimode concepts. In the high-flux regime, which is speckle-noise dominated, singlemode interferometry is always more efficient, and its performance is significantly better when the correction provided by adaptive optics becomes poor, by a factor of 2 and more when the Strehl ratio is lower than 10%. In low-light level cases (detector noise regime), multimode interferometry reaches better performance, yet the gain never exceeds ~20%, which corresponds to the percentage of photon loss caused by the injection in the guides. Besides, we demonstrate that singlemode interferometry is also more robust to the turbulence for both fringe tracking and phase referencing, with the exception of narrow fields of view (≲ 1′′). Conclusions. Our conclusion is therefore that from a theoretical point of view and contrarily to a widespread opinion, fringe trackers built with singlemode optics should be considered as a both practical and competitive solution.