Low-Loss Optical Waveguides and Polarization Splitters with Oxidized AlxGa1-xAs Layers

1. 1 Low-Loss Optical Waveguides and Polarization Splitters withOxidized AlxGa1-xAs Layers By Alpan Bek September 1998

2. 2 Outline Motivation Introduction Theory and Design Fabrication Characterization & Measurements Discussions & Conclusions Future Applications

3. 3 Motivation Electrical Optical Information, Laser Fiber-optics to Integrated Optics Hybrid Fabrication, LiNbO3 Monolithic Fabrication, AlGaAs Low-loss integrated optical circuit design Improve device performance

4. 4 Introduction Loss is a major problem in integrated optical circuitry. High insertion loss Optimization for minimum propagation loss. Electro-optic effect, for optical field modulation. Use of metal electrodes result in excess propagation loss.

5. 5 Optical waveguides confine light, light propagates in the waveguide. Three semiconductor layers nc=ns<nf : slab waveguide Maxwell’s equations for isotropic, lossless medium : ?x E = -µ0(?H/?t), ? x H = ?0n2(?E/?t) Scalar wave equation : ?2? + k0ni2? = 0 (? = Ei or Hi ) Waveguide Theory & Design

6. 6 E=E(x,y)ei( ? t - ? z ) , H=H(x,y)ei(? t - ? z) , ? is the prop. cons. in z Two independent sets of variables (two modes) : a) Ey , Hx , Hz ( TE mode) b) Ex , Hy , Ez ( TM mode) Boundary conditions result in an eigenvalue equation : (kh/2)2+(?h/2)2 = (k02h2/4)(nf2-nc2) k : in the middle layer ? : in the other layers Number of modes can be found by investigating the plot :

7. 7 Vertical confinement by index differences, lateral confinement by physical modification that results in an effective index Rib Waveguide We can find an effective index ( neff) such that nc<neff<nf

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11. 11 Electro-optic effect & metal electrodes

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18. 18 Fabrication MBE growth (3% growth uniformity) Sample Cleaning (ACE, METH, ISO, DI) Photolithography Reactive Ion Etching (BCl3/SiCl4) Al0.98Ga0.02As Oxidation (4000C) Metal Deposition (Ti/Pt/Au) Lift-off (ACE)

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22. 22 Characterization and Measurements Spectroscopic Ellipsometry Ellipsometry of as-grown wafer Ellipsometry of AlOx Layer RIE depth profiling Optical Propagation Loss Measurement Setup Results of Loss Measurements Characterization of Polarization Splitters

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33. 33 Conclusions First time such thick Al0.98Ga0.02As layers mechanically stable when oxidized. Significant loss reduction by oxide layer (TE, TM). Better than thick top layer, because of eliminating sidewall roughness and having shorter electrode-to-electrode separation. TM output of the polarization splitters are almost as lossless as TE polarization. High extinction ratio (12.4 dB) obtained for g=3.00 µm, extrapolated ratio 20 dB for g=3.25µm. Extinction ratio can be tuned up by further optimization.

34. 34 The achieved values fit well to the expected results obtained from simulations. Only AC modulation possible due to interface states. The oxidation byproducts (mostly elemental As, hydroxydes, ...) result in screening of electric field. Post oxidation treatments like hydrogen plasma may be applied to remove the effect of interface states. Encouraging results of utilizating oxidized AlxGa1-xAs (x=0.02). Lowest propagation loss for this system.

35. 35 Future Applications Increasing birefringence by insertion of thin oxide layers into the core layer.