Abstract
Context. Optical interferometers are subject to many atmospheric and instrumental artifacts that contribute to the degradation of their instrumental contrast, hence their performances. The differential birefringence is, among these effects, one of the trickiest to control, in particular for instrument using fibers, where it can be far larger than the one arising in the optical mirror trains. Several solutions have been tested in the past, ranging from polarization splitting to fiber tweaking. We adopt a new solution for the PIONIER instrument, a four-telescope (4T) combiner at the Very Large Telescope Interferometer (VLTI). Aims. We present a method to cancel the instrumental birefringence in an optical interferometer, allowing the joint detection of the fringe patterns of both polarizations, and substantial gains to be made in both signal-to-noise ratio and readout speed. Methods. A thin (2 mm) plate of birefringent material (LiNbO3) is inserted in each of the four beams. The incidence angle of each plate is adjustable. This allows us to introduce a controlled amount of birefringence in each beam and to cancel the instrumental differential birefringence. We present our derivation of the induced birefringence versus incidence angle and discuss the design choices. Results. Our proposed solution is implemented in the Pionier instrument. Before correction, the instrumental birefringence was of order 5 μm (path length). The adjustment takes about one hour, results in a birefringence of less than 0.1 μm, and is stable for at least the duration of an observing run (several days). Conclusions. We demonstrate on an operational near-infrared interferometer a novel, simple, low-cost, and effective technique to control the differential birefringence. The predictability and stability of the correction make this technique ideal for an automated correction in the VLTI second generation instruments.
Highlights
In an astronomical optical interferometer, light gathered by several telescopes is brought to a common recombination point
The design of the optical trains must be such that the polarization orientation is preserved, since orthogonal polarizations of natural light are mutually incoherent
If the birefringence in two light paths is not perfectly balanced, the interference fringe patterns in the two orthogonal polarizations suffer a relative shift, and a polarization-insensitive detection of the fringe pattern suffers a loss of contrast with respect to the separate detection of pure polarization states
Summary
In an astronomical optical interferometer, light gathered by several telescopes is brought to a common recombination point. The design of the optical trains must be such that the polarization orientation is preserved, since orthogonal polarizations of natural light are mutually incoherent. An optical device is used to introduce a differential birefringence compensation Following this idea, Lagorceix & Reynaud (1995) proposed a fibered Babinet compensator while the FLUOR instrument succesfully used mechanically stressed fiber loops (Coudé du Foresto et al 1998). This paper addresses one definite issue of polarization in optical interferometry: the loss of fringe contrast due to birefringence in the instrument. We propose a solution to the problem of instrument-induced birefringence, that allows us to restore a full fringe contrast. It is characterized by its ease of implementation and precise quantitative control.
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