Three-guide optical couplers in GaAs

Abstract

Three-guide optical couplers consisting of three 4.75-μm-wide slab-coupled rib guides separated by 4.25μm have been fabricated in GaAs. The performance of these couplers at 1.28 μm is in close agreement with that predicted using a modified effective-index method to obtain an approximate analytical solution for this type of coupler. The coupling length needed to symmetrically transfer power from the center guide to the two outside guides was 3.2 mm. At this length, less than 1 percent of the power remained in the center guide. The length needed to transfer power from one outside guide to the other outside guide was ≈ 6.4 mm, which is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\approx \sqrt{2}</tex> - times that of a similar two-guide coupler and twice that required to couple power from the center guide to the two outside guides. The power transfer efficiency in this case is not as good as when power was inputted into the center guide. Three-guide couplers of this type should prove useful as power dividers and combiners, especially in cases where waveguide bend losses preclude the use of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">"Y"</tex> - junctions. They may also prove useful as replacements for two-guide couplers where either sharper transfer characteristics are desired or where losses due to waveguide bends are again unacceptable.