3:45pm zyxwvutsrqpon - 4:OOpm wcc2 Low Loss Optical Waveguides and Polarization SplittersUtilizing Oxidized AI,Gal.,As Layers zyx A. Bek*, A. Aydinli*, J. G. Champlain, R. Naone, and N. Dagli Electrical and Computer Engineering Department University of California SantaBarbara, CA, 93106 *On leave from Physics Department, Bilkent University, Ankara, Turkey Utilization of oxidized Al,Gal-,As layers in electronic and optoelectronic devices has led to significant improvements in device performance. An example is the lower threshold currents in vertical cavity emitting lasers with oxidized Al,Gal-,As apertures El]. In this abstract we report on another application of oxidized Al,Gal_,As in guided wave optics. We utilize low index of refraction of oxidized Al,Gal-,As for stronger optical confinement to reduce electrode related losses and to increase birefringence in polarization splitting directional couplers. It is long known that metal electrodes in close proximity to optical waveguides lead to excess optical propagation loss in particular for TM mode due to efficient coupling between zyx TM polarized light and surface plasmons in a metal [2]. While polarization dependent optical loss due to metal electrodes has been used to fabricate various types of polarization dependent optical devices [3], it has also hindered progress in polarization independent devices. One solution to the problem is to increase the semiconductor cladding layer thickness under the metal electrode. However, this may increase the upper layer cladding thickness significantly requiring excessive epitaxial growth time, excess applied voltage and planarization problems during fabrication. Another possibility is to introduce a low index material under the metal electrode to effectively attenuate the field before it reaches the electrode. In this work we report on the introduction of an oxidized Al,Gal-,As layer into Al,Gal-,As/GaAs optical waveguides with metal electrodes. This approach reduces the loss and at the same time increases the birefringence. We also apply such low loss waveguides in directional couplers for polarization splitting. A schematic of the waveguides used in this work is given in Fig. 1. All epitaxial layers are undoped and grown by MBE on SI GaAs substrates. On these layers rib waveguides are fabricated using RE etching in SiC&:BC13. Samples with Al.gsGao2As top most cladding are then oxidized at 400 OC for 15 min. in H2O atmosphere with N2 as the carrier gas. Finally, Ti/Pt/Au (20 nm/50 nm/500 nm) electrodes are e-beam deposited on the ribs using liftoff. Propagation loss and facet reflectivity of the rib waveguides are measured using the Fabry-Perot resonance and sequential cleaving method. The results are shown in Table 1. The facet reflectivities for all structures are in agreement within 3% with both existing experimentaldata and theoretical predictions. The loss results clearly show that 0.4 pm Al.98Ga02As layer is not thick enough to reduce the field amplitude to insignificant levels at the metal electrode interface. As a result WG1 loss coefficient is 3.7 dB/cm and 6.0 dB/cm for TE and TM modes. This is especially high for TM mode as expected. On the other hand waveguide with oxidized A1.98Ga 02As top most cladding (WG2) has dramatically lower propagation loss due to the fact that lower index of refraction of oxidized Alg8Gao2As (-1.6) quenches the evanescent field of the propagating light on the metal electrode. This low loss coefficient also demonstrates the high optical quality of oxidized Al.98Ga02As interface. It should be noted that oxidized A1.98Gao2As thickness of 0.4 pm used in this work is significantlyhigher than that of in previous studies. Such thick layers of oxidized Ale98Gao2Asare found to be crack free and resulted in very high quality waveguides as long as 1 cm. Using a thick cladding layer under the metal as in WG3 yields propagation losses of 1.5 dB/cm and 2.1 dB/cm for TE and TM modes respectively. TM loss coefficient is still higher indicating incomplete suppression of the field at the metal electrode interface. While some of this loss can be attributed to RE etched sidewalls, it is clear from oxidized samples that this is quite small. To reduce the propagation loss further one needs to increase the cladding thickness of WG3 even further. Another effect of lower oxidized Al.98Gao2As index is the increased waveguide birefringence. This allows one to make a directional coupler to separate the TE and TM parts of an incoming light as shown in Fig. 2. Such couplers of various coupling lengths are also fabricated. TE or TM polarized light is launched from either one of the input ports and polarized light intensity at both outputs are measured. For both polarizations the ratio of the light intensity at one of the ports to the total light intensity from both ports is plotted in Fig. 3 as a function of the straight coupling section length. The power transfer functions based on coupled mode theory are least squares fitted to the data. In this case the highest TEDM extinction ratio is 7.5 dB. This can be increased significantly by optimizing the length or the gap 384 0-7803-4947-4/98/$10.00 01998 IEEE