5 Discussion

  Integrated optics is extremely attractive in astronomical interferometry for combining two or more beams and for various functions [Kern et al. (1996)]. In this section we discuss some intrinsic properties of integrated optics components. This analysis leads us to list some specific advantages and applications for this approach.

5.1 Optical losses

We have to distinguish several optical losses:

1.

Fresnel losses. At the air/waveguide or air/coupling fiber interfaces, Fresnel losses occur. They equal about 4% but can be reduced by anti-reflection coatings deposited at the inputs and the outputs of the waveguides.

2.

Coupling losses.

The light injection in the waveguide can be either direct or, more usually, via an optical fiber. According to the chosen solution, coupling losses exist at the air/waveguide interface or air/coupling fiber and at the fiber/waveguide interface. For an efficient coupling, the incident energy has to match as much as possible with the propagating mode (numerical apertures, fiber core and waveguide sizes, waveguide profile shape).

All these conditions cannot be easily satisfied. In etching technology, the process provides channels with non spherical sections, leading to coupling losses of about 0.33 dB or 7$\%$ excluding Fresnel losses). With ion exchange technology, the coupling efficiency clearly depends on the diffusion process and more specifically on the channel depth. The losses are of the order of 2-3% if the waveguide is embedded inside the substrate.

3.

Propagation losses.

Standard glasses provide low propagation losses for wavelengths less than $2.5~ \ensuremath{\mu\mbox{m}}$. With ion exchange technology, the propagation losses depend upon the diffused ions. For the more used ions ($\mbox{K}^+$, $\mbox{Tl}^+$ or $\mbox{Ag}^+$) these losses remain less than 0.2 dB/cm (a 1 cm-long component has a throughput of 94.5$\%$). Silicon etching technology exhibits propagation losses of 5 dB/m. Therefore integrated optics cannot be used to realize lengthy components aimed at transportation.

4.

Losses intrinsic to the integrated optics structure.

Depending on the integrated optics design, light can be partially lost because of uncontrolled radiated modes like in the calssical reverse Y-junction. When used with two incident beams in opposite phases, the flux is radiated inside the substrate. This point is critical in astronomical interferometry where we wish to maximize the optical throughput. For this specific application, optimization and simulation of various components are in progress [Schanen-Duport et al. (1998)].

5.2 Spectral behavior

5.2.1 Available spectral ranges

The off-the-shelves components are generally designed for telecom spectral bands (0.8 $\ensuremath{\mu\mbox{m}}$, 1.3 $\ensuremath{\mu\mbox{m}}$ and 1.5 $\ensuremath{\mu\mbox{m}}$). They can directly be used to manufacture astronomical components for the I, J and H bands of the atmosphere. Standard glasses have an optical throughput higher than 90$\%$ in the visible and the near-infrared domain (up to $2.5~ \ensuremath{\mu\mbox{m}}$, see [Schanen-Duport et al. (1996)]). Ion exchange technology provide integrated components for the K atmospheric bands ($2.2~\ensuremath{\mu\mbox{m}}$). For higher wavelengths ($5 ~\ensuremath{\mu\mbox{m}}$ or $10~\ensuremath{\mu\mbox{m}}$), different technologies are under study.

Optical waveguides remain single-mode over a given spectral range (an octave in wavelength). This range is wide enough to cover a single atmospheric band but not for several bands. However the compactness of integrated optics components allow to use one optimized component for each band without increasing the overall size of the instrument.

Y-junctions provides achromatic power division and beam combination which make them attractive despite the loss of 50% of the information in the latter function. The other functions should be studied and optimized in order to limit the chromatic dependence over the spectral range. Finally we recommend to calibrate the device with spectral gain tables like in standard astronomical imaging in order to suppress any device-dependent effects.

5.2.2 Chromatic dispersion

Like fiber optics, integrated optics components have intrinsic chromatic dispersion which could lead to a visibility loss over typical spectral bandwidths of $0.2-0.4~\ensuremath{\mu\mbox{m}}$ from the atmospheric bands. However the losses are greatly reduced since:

• the mask has been designed to provide symmetrical interferometric arms with identical lengths, curvature, etc.;
• the optical path difference between two arms is directly proportional to the length of the device. For a typical length of a few centimeters, the optical length difference cannot exceed 100 $\ensuremath{\mu\mbox{m}}$ essentially due to machining defects (cutting and polishing);
• the process for both technologies (ion exchange and etching) provides a good homogeneity for the index difference inside the waveguides.

Therefore even if we cannot preclude any contrast losses due to chromatic dispersion, we think that this effect will remain small.

However since the integrated optics component is part of an instrument, special care must be taken to avoid other sources of chromatic dispersion. In particular, optical fibers if used to inject stellar light into the device have to be optimized accurately [Reynaud & Lagorceix (1996)].

5.2.3 Dispersion capabilities

Within the context of spectral interferometric measurements (Sect.  2.7) the waveguide output is equivalent to the input slit of a spectrograph and is able to directly feed a spectrograph grating avoiding the cylindrical optics used to compress the Airy pattern in the direction perpendicular to the fringes [Petrov et al. (1998)].

5.3 Polarization behaviour

  Both integrated optics technologies control the orientation of the neutral axes and thus provide components with intrinsic maintain of polarization properties. Provided that the design is symmetrical, the component does not introduce differential polarization, which is a crucial advantage for astronomical interferometry. Note that the coupling with polarization maintaining optical fibers has to be done with a great accuracy.

5.4 Thermal background

Because of their small size, integrated optics components can easily be integrated in a single camera dewar. Therefore no relay optics are needed between the component and the detector, reducing the photon losses. Moreover the waveguide can be cooled and put close to the detector and the dewar can be blind, which reduces the thermal background.