4 A coin-size complete interferometer

  Many functions required by interferometry (see Fig. 1) can be implemented on a single integrated optics component made from a tiny glass plate. Based on the listed available functions, one can design a beam combiner for a multi-telescope interferometer.

4.1 Beam combination

 

  

Figure 7: Different types of integrated optics beam combiners

Figure 7 displays various types of integrated optics beam combiners for two telescopes. They can easily be upgraded to the combination of a larger number of beams. We have classified these beam combiners with the same terminology as in Sect. 2.6.

A co-axial beam combiner is made of waveguide junctions. Reverse Y-junctions allow to collect only the constructive part of the interferometric signal while X-crossing junctions with small angles get the whole interferometric signal provided that asymmetric waveguides are used for the two arms. Note that directional couplers can also be used despite the narrow bandpass.

A multi-axial beam combiner is formed by individual single mode waveguides assembled by a taper that feed a planar waveguide. The light propagates freely in the horizontal direction and the beams interfere at the output of the device whereas light remains confined in the vertical direction. The fringes can be sampled on a detector.

The multiplexer has no analogs in classical optics. The light from a given input beam is mixed with the light from other input beams thanks to directional couplers. The output beams are a linear combination of the input beams whose ratios highly depend on the wavelength.

4.2 Optical Path Difference modulators

Small excursions are possible with integrated optics technologies. The phase can be modulated up to 100 $\ensuremath{\mu\mbox{m}}$ with on-the-chip electro-optics, thermo-optics or magneto-optics actuators [Alferness (1982)]. Such excursions are long enough to modulate the optical path difference around the zero-OPD location to scan the fringes.

4.3 Wavelength selection

Thin-film technology can be used to deposit any spectral filter at the output of waveguides [Richier (1996)]. A particular application of the thin-film coatings is the dichroic filters. Such components are usually integrated in telecom devices and are attractive for astronomical interferometry in order to perform various calibrations or controls.

4.4 Photometric calibration

Thanks to direct Y-junctions or direction couplers light can be partially extracted to achieve real-time photometric derivations.

4.5 Polarization control

The control of waveguide shapes permit to build polarizing components such as linear polarizers, polarization rotators or phase shifter [Lang (1997)], which can be used to compensate residual instrumental polarization. In the future, integrated optics components could eventually be coupled with crystals (such as Lithium Niobate) which induces polarization thanks to Kerr or Pockels effects.

4.6 Detection

The size of waveguides (1 to $10~\ensuremath{\mu\mbox{m}}$) is similar to the size of pixels in infrared arrays. Therefore, direct matching of the planar optics component with an infrared detector would lead to a completely integrated instrument with no relay optics between the beam combiner and the detector. Furthermore recent developments of Supra-conducting Tunnel Junctions (STJ, Feautrier et al. 1998) show that one may build pixel size detectors with photon counting capabilities over a large spectral range (from ultra-violet to near-infrared) with a very high quantum efficiency. Given its natural spectral resolution (R=50) a STJ combined with an integrated interferometer allows multichannel interferogram detection as well as fringe tracking capabilities. Since STJ are manufactured with the same etching technology as some integrated optics component, one foresees a complete integrated interferometer including one of the most sensitive detector.

In the future, detection techniques using parametric conversion [Reynaud & Lagorceix (1996)] could be implemented with optical waveguides.

4.7 Switches

Optical integrated switches [Ollier & Mottier (1996)] already exists and can be coupled with an integrated interferometer to ensure the delay line function.