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Waveguide gratings: new solutions for the design of filters

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Abstract

Theoretical studies of reflection and transmission curves of waveguide gratings have shown that these structures permit one to realize filtering functions[1]. More recent studies have shown that some of these functions are difficult to obtain with classical multilayers[2]. For example, it is possible to realize narrowband "notch" (inverse) filters with only a modulated layer (grating) lying on a silica substrate. For such structures, the bandpass depends on the thickness of the modulated region: a smaller thickness leads to a narrower bandpass. Hence it is possible to realize narrowband (ΔZ < 1 nm) filters with very simple structures. Yet these components have shown to be highly sensitive to the angle of incidence[3]. making them difficult to use for industrial applications. To solve this problem, we have considered structures with two gratings (see Figure 1). In this case the bandpass of the filter depends on height hg, dielectric constant εg and width ag of the top grating of period 2d. The dielectric contrast of the modulated waveguide modify the dispersion of the waveguide, making (under particular conditions) the resonant structure weakly sensitive to the angle of incidence[4,5]. Theoretical calculations have shown that all the parameters can be chosen in order to increase the angular tolerance of grating waveguide filters. An improvement by a factor 10 is obtained with respect to classical grating waveguide filters (see Figure 2). Moreover, the possibility of realizing these structures by classical photolithography makes them conceivable for industrial applications. Experimental results will be given.

© 1998 Optical Society of America

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