Engineering supermode silicon/III-V hybrid waveguides for laser oscillation

Abstract

We analyze a silicon/III-V hybrid semiconductor waveguide structure for laser oscillation. We show that, by optimally designing and controlling the resonant supermode behavior in such structures, the modal gain can be enhanced five times compared with that of the existing silicon evanescent laser, while maintaining efficient coupling to outside silicon waveguide circuits. © 2008 Optical Society of America OCIS codes: 250.3140, 130.2790, 130.3120, 140.5960. During the past two decades, there has been an everincreasing pace of activity in the area of realizing laser emission from silicon ͑Si͒ [1][2][3][4][5][6]. Recently, we proposed a novel supermode Si/ III-V hybrid waveguide system for laser oscillation, amplification, and modulation [7]. The structure is illustrated in Fig. 1. The hybrid device consists of two parallel waveguides in close proximity coupled to each other. The top waveguide is fabricated in a III-V compound semiconductor and supplies the gain, and the bottom is a waveguide fabricated in Si. The eigenmodes of this hybrid structure, the supermodes, can be controlled by suitable design of dimensions of the waveguides [7]. The supermode is thus designed such that, in the left region (main body) most of the modal energy is concentrated in the III-V waveguide so that the mode experiences maximal gain, while in the right region near the output facet, the width of the Si waveguide is widened adiabatically so that most of the mode transforms to the Si waveguide for coupling to other Si photonic circuits. The fundamental difference between this scheme and the reported evanescent lasers by Fang et al.[6] is the use of supermode coupling rather than evanescent coupling. This makes it possible to fundamentally break the trade-off between the gain available to an optical mode and the output coupling efficiency intrinsic to the hybrid evanescent lasers. Therefore we expect a laser designed with this principle to operate with higher efficiency and be far shorter.In this paper, we address several issues in engineering the supermode hybrid Si/ III-V waveguide structure for laser oscillation. By optimally designing the III-V wafer layer structure, the III-V waveguide width, the Si waveguide width, and the adiabatic taper, we will show that the modal gain in the supermode resonator can be improved by a factor of 5 compared with that in the reported evanescent lasers. Finally, we will discuss the tolerance of misalignment of the two waveguides during fabrication.First we focus on the optimal layer structure of the III-V active wafer. In the evanescent laser design by Fang et al. [6], the quantum wells (QWs) needed to be as close to the Si waveguide as possible to have more of the mode field evanescently penetrating into the QW region. In our supermode scheme, the resonant supermode can have most of the energy concentrated in the III-V waveguide without reducing the distance between the QWs and the Si waveguide. Thus, we seek to engineer the wafer to make full use of the available gain from the QWs. Since all the...

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