next previous
Up: Integrated optics for astronomical


3 Results and discussion

  As far as an astronomer is concerned, the physical quantities of interest when dealing with an interferometric instrument are the instrumental contrast, the optical stability, and the total optical throughput.

3.1 Laser-light constrasts

A 93% contrast is obtained with the He-Ne laser. The main source of contrast variations with time comes from temperature gradient and mechanical constraints on the input fibers. When special care is taken to avoid fiber bending and twists, the laser contrast variation is lower than 7$\%$ rms over a week. Using high-birefringent fibers which are far less sensitive to mechanical stresses will improve the contrast stability.

3.2 White-light contrasts

With an halogen white-light source, the contrast obtained is of the order of 7$\%$ with a potassium ion beam combiner connected with low-birefringent fibers (Fig. 2b) and 78$\%$ with a silver ion beam combiner connected with high-birefringent fibers (Fig. 2c). Two main sources of interferometric contrast drops between the two components have been identified: chromatic dispersion and polarization mismatch.

The consequence of residual differential dispersion between the two arms is to spread out the fringe envelope and decrease the contrast. Since the delay line translation is not perfectly linear, the Fourier relation between space and time is affected and an accurate estimate of the dispersion is difficult. Only the number of fringes and the shape of the interferogram gives an idea of the existing differential dispersion. The theoretical number of fringes is given by the formula $2\frac{\lambda}{\Delta \lambda}\sim 10$ and the interferogram contains about 13 fringes. Such a spread is not sufficient to explain the contrast drop between the laser- and white-light interferograms. More detailed studies of residual chromatic dispersion are in progress.

In the present case the contrast decay is mainly explained by differential birefringence. Low-birefringent fibers are known to be highly sensitive to mechanical constraints and temperature changes, leading to unpredictable birefringence. Coupling between polarisation modes can occur leading to a contrast loss which can be worse for unpolarized incident light (case of the thermal white-light source). This is confirmed by the preliminary results obtained with high-birefringent fibers and the incident light polarized along the neutral axes: the contrast reaches 78$\%$ (Fig. 2c). The apparent asymmetry of the interferogram could be due to residual differential polarization and/or dispersion. Full characterizations are in progress and will be reported in Paper III [Haguenauer et al. (1999)].

3.3 Total throughput

\includegraphics [angle=-90,width=8.8cm]{}\end{figure} Figure 3: Schematic view of the beam combiner with successives optical losses (see text for details). Number of output photons are given for 100 incoherent photons injected in each channel for potassium- and silver-exchanged waveguides

Figure 3 summarizes the photon losses in the two components. We express the losses in terms of remaining photons when 100 incoherent photons are injected at each waveguide input. For the component made from potassium ion exchange, we obtain 20 and 14 photons on each photometric channel and less than 20 photons in the interferometric channel leading to a total of 54 photons for 200 photons injected, hence a total throughput of 27%. For silver ion exchange, respectively 30, 31 and 25 photons have been measured leading to a throughput of 43%. The main difference between the two results comes from the coupling efficiency between the fiber and the waveguides and the propagation losses.

Table 1: Estimation of optical losses at different levels of fiber-connected beam combiners with respectively potassium- and silver-exchanged waveguides. The number of detected and estimated output photons are given for our 4-cm components. Last column gives an idea of what performances can be achieved in the future

 Component &K$^+$\space &Ag$^+$\space...
 ... $Y$-junction is radiated out in the substrate (see Paper~I).} \\  \end{tabular}

Table 1 summarizes estimation of losses coming from different origins. The propagation losses and the coupling losses have been measured with a straight waveguide manufactured in the same conditions. The Fresnel losses have been theoretically estimated to 4$\%$. Any function causes additional losses which cannot be evaluated separately but have been estimated to 10$\%$. One should notice that the reverse Y-junction acts as only one of the two outputs of an optical beamsplitter (see Paper I). Therefore 50$\%$ of the light is radiated outside the waveguide. The first two columns of Table 1 show that our measurements are consistent with the theoretical performances computed from the different optical losses reported in the table.

Last column of Table 1 gives an order of magnitude of expected improvement in the future. The main progress concerns the beam combination function. We should be able to retrieve the second half of the combined photons thanks to new combination schemes like X-couplers, multiaxial beam combiners or multimode interferometric (MMI) multiplexers (see Paper I) at the cost of a slight chromaticity of the function. Some components including these new functions are being manufactured and will be soon tested. The ultimate optical throughput would be around 70-80%, twice more than our current results.

next previous
Up: Integrated optics for astronomical

Copyright The European Southern Observatory (ESO)