A switchable optical add-drop multiplexer using ion-exchange waveguides and a POLICRYPS grating overlayer

A novel add-drop multiplexer using POLICRYPS [1] gratings and ion-exchange optical waveguides has been investigated for operation in the fiber optic C-band (1530-1565 nm). The add-drop is a key component used in optical networks and it is composed of a symmetrical structure with three regions. A first input region is made of two antisymmetrical single-mode waveguides which merge together in a second region to form a single two-mode waveguide. A POLICRYPS grating, tilted with respect to the light propagation, is placed on top of the two-mode section acting as a switchable overlayer. A third output region has a specular shape of the first region with two single-mode antisymmetrical waveguides. Supposing that a multiwavelength light beam comes into the device through the narrower input waveguide, a resonant wavelength is reflected by the grating, since it breaks the orthogonality of the two modes, back out the wider “drop port”. The rest of the channels continue on the output narrower port. Additionally, a signal at the dropped wavelength can be added through the wider port. In the design a compromise must be made to choose the depth of the waveguides in the substrate. The device has been designed considering the y-junctions and the grating region using basically the coupled wave theory [2]. Different geometry structures have been designed in particular in order to minimize structural losses. The optimized structure is composed of double-ion-exchange waveguides, K+-Na+ and Ag+-Na+, with an index change at the surface of 0.05, width of 10 um, thickness of 3.5 um and buried with a diffusion depth of 1 um. The grating thickness has to be less than 1 um and a polymer with refractive index of 1.51 or 1.52 must be used. The device shows a -3 dB bandwidth of 1 nm, losses of -0.73 dB, crosstalk of -18 dB for a spacing channel of 1.6 nm, and crosstalk of -24 dB for a spacing channel of 3.2 nm. The reflected power at the input port is below -20 dB. These results are obtained considering a grating index modulation of just 3x10-3, provided by an applied electric field. [1] R. Caputo, L. De Sio, A. Veltri, C. Umeton, A. V. Sukhov, Opt. Lett., vol. 29., no. 11, pp. 1261-1263, 2004. [2] D. Wiesmann, R. Germann and G. L. Bona, OSA J. Opt. Soc. Am., vol. 20, no. 3, pp. 417-423, 2003.