Silicon waveguides have strong light confinement properties due to a very high index core (silicon, refractive index ~3.5) material surrounded by a much lower index glass (silica, refractive index ~1.5) cladding. Due to this property, silicon waveguides can turn light paths through sharp bends without suffering any appreciable loss. This enables miniaturization of functional optical components and enhances dense integration of devices on waveguide chips.
A large mismatch between the common single-mode optical fiber dimension (~ 9 (m in diameter) and that of the silicon waveguide (~1 μm × 1 μm) makes it difficult to couple light in and out of the chip. Incorporating a two-dimensional (2D) waveguide section that is tapered vertically as well as laterally between the fiber and the waveguide effectively improves on the coupling situation (Figure 1). This tapered section acts as a classic adiabatic modal transformer that transforms the input fundamental mode shape to that of the waveguide mode.
Theoretical predictions of coupling loss between optical fibers and silicon waveguides, with and without tapers, are first performed using a commercial software package called FimmWave (a product of Photon Design, Ltd., Oxford, UK). Two types of tapered waveguide sections are considered, one with a lateral (1D), and the other with a combination of lateral and vertical (2D) tapers. The calculated coupling loss values are shown in the table below. The results point to a significant predicted coupling loss improvement through the use of a 2D waveguide taper design.
The starting material for fabricating 1D and 2D tapered waveguides is a 3-μmthick layer of crystalline silicon on Silicon on Insulator (SOI) substrate. The insulator is a 2-μm-thick layer of thermally grown SiO2 on silicon substrates. The 1D waveguides are first defined by photolithography and a dry etching process. Subsequently, nine step etches are performed to create the 2D tapers along the wafer thickness in the 1D flare area (Figure 2). The coupling loss between the HNA fiber and the waveguides is estimated by measuring the propagation loss and the total loss in each type of waveguide. It is determined that the loss per facet for a 2D taper is ~ 2 dB, and for a 1D taper it is ~ 4 dB. These values compare favorably to the 8-dB coupling loss observed with a non-tapered waveguide, and are in excellent agreement with the simulated predictions as shown in Table I.
This article was written by A. N. M. M. Choudhury, T. R. Stanczyk, and D. Richardson of the Optics Division of Molex Inc., and A. Donval, R. Oron, and M. Oron of KiloLambda Technologies Ltd. For more information, visit https://info.ims.ca/5654-202.