A promising polarization-insensitive thermo-optic switch based on multimode polymeric waveguides is reported. This device has a packing density of 40 channels/cm. Simulation result shows that an extinction ratio of greater than 20 dB can be achieved with the device-electrode interaction length of 30 mm. The thermo-optic switch operating at wavelengths of 632.8 nm and 1.3 m has been demonstrated experimentally with extinction ratios of 21 and 22 dB, respectively. Such a device has an intrinsic

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... de optical bandwidth due to the large dynamic range of the phase-matching condition implied by the multimode waveguides. The material employed provides us with a switching speed of 100 s. Integrated optics plays an important role in several areas of optical communication, such as advanced information processing, optoelectronic interconnections, and fiber-optic communications. 1,2 High performance waveguide modulators and switches are important devices to fully utilize the opportunities provided by integrated optics. 3-5 All major optical modulators and switches used in telecommunication applications use single mode to cover the long distance requirements. From the packaging point of view, a few defects in waveguides and slight misalignments during device fabrication will degrade device performance significantly. Moreover, devices based on single-mode waveguides can only work within a narrow wavelength range. For example, a Mach-Zehnder interferometer designed to work at a certain wavelength will have problems when it is used at some other wavelengths. Switches and modulators based on multimode waveguides, because of their large dimension, can be made under relatively easier fabrication conditions and packaging reliability. Furthermore, coupling light between multimode waveguides and multimode optical fibers is usually easy and efficient. Therefore, multimode waveguides devices are compatible with data communication applications for short interconnection distance. In this letter, we present a thermo-optic switch based on multimode polymeric waveguides. This device has such advantages as large fabrication tolerance, high device packing density of 40 channels/cm, and an ultrawide operating wavelength range. The schematic and real device structures of the multimode thermo-optic switch are shown in Figs. 1͑a͒, and 1͑b͒, respectively. The device consists of a multimode guiding channel, a pair of heating electrodes, and a planar dumping waveguide under the guiding channel. The planar dumping layer is designed to be highly lossy ͑ϳ26 dB/cm͒ so that optic energy coupled from the guiding channel can be efficiently dumped in the dumping layer. As a result, optical energy can only be coupled from the guiding channel to the dumping layer, while the coupling from the dumping layer to the guiding channel is minimized. Thus, the unidirectional coupling is achieved. 6 The electrodes were implemented to thermo-optically control the phase matching condition 7 of the coupling between the guiding channel and the dumping layer. Because there is only one channel per switch in this structure, the device packing density ͑number of switches per unit area͒ can be twice that of devices with a coplanar configuration, 6 where guiding channel and dumpling channel are implemented in the same layer. Based on the unidirectional coupling mechanism, 6 we simulated the dumping efficiency from the guiding channel to the planar dumping layer. Figure 2 shows the simulation result of the sum of the dumping efficiencies to all modes of the dumping layer. Note that the dumping efficiency of more than 20 dB can be achieved with the channel length of 30 mm. This shows that the planar dumping layer can effectively improve the dumping efficiency. To control the phase matching condition thermooptically, a pair of heating electrodes was implemented on top of the guiding channel. By applying current induced heat through the electrodes, the temperature difference between the guiding channel and the other areas of the device (⌬T) can be changed continuously. Beam propagation software ͑BeamProp®͒ was used to simulate the device operations at two switching states (⌬Tϭ0 and 30°C͒ at 632.8 nm and 1.3 m, respectively. The simulation result of 632.8 nm is a͒ Electronic mail: chen@ece.utexas.edu FIG. 1. Structure of the unidirectional thermo-optical switch. ͑a͒ Schematic view of the device structure. ͑b͒ SEM view of the device structure.