High efficiency fiber-to-waveguide grating coupler structures

Research Area: Photonics packaging

Main Researcher: Gunther Roelkens

The use of a grating coupler has the advantage of resulting in a very compact coupling structure and not requiring a polished facet for coupling, paving the way to wafer-scale testing of integrated optical components. The coupling efficiencies of fabricated devices presented in literature is limited however (typically 20 to 30 percent fiber coupling efficiency is experimentally obtained), which can be sufficient for testing but is not sufficient in practical applications. This low coupling efficiency is intrinsic to the grating structure used in these cases, as an important fraction of the optical power is diffracted towards the silicon substrate and is therefore lost. Here we propose a different grating layout based on the deposition of a silicon layer prior to etching of the grating coupler, as shown below. The fiber is slightly tilted off vertical to avoid a strong second order reflection.

Proposed grating structure layout

This approach allows to obtain much higher grating directionality than in a conventional grating coupler design.Simulations show that an efficiency in the range of 60-80% is feasible, using standard CMOS technology for fabrication. A prototype of the optimized fiber coupling structure was fabricated, by local deposition of a 150nm thick amorphous silicon layer on an SOI waveguide substrate with a silicon waveguide layer thickness of 220nm. After deposition of the silicon layer, the grating structure was defined using focused ion beam etching (FIB), etching 220nm deep into the layer stack. An improvement of 2dB in fiber coupling efficiency was observed compared to the standard grating structures,although the efficiency is still substantially lower than the theoretical predictions. This discrepancy is probably related to the large optical absorption of the amorphous silicon, the damage induced into the material by the focused ion beam etching and the incorporation of gallium ions in the silicon host, substantially increasing the optical loss and can be avoided by using standard CMOS technology for the fabrication of the structures.

SEM view of a fabricated prototype structure and measured coupling spectrum

An addtional advantage of the fiber-to-waveguide grating coupler structure is that the diffractive nature of the coupling mechanism allows to integrate even more functionality in the coupling structure: the concept of using the diffractive grating coupler (which occupies an area of only 100µm2) as a structure to duplex two (sufficiently spaced) wavelength channels, can find important applications, i.e. in the fabrication of SOI based integrated transceivers for the Fiber-to-the-Home market.

Diffractive grating structure as wavelength duplexer

Other people involved:

• Thijs Spuesens
• Ananth Subramanian
• Diedrik Vermeulen

Related Research Projects

• ICT: HELIOS (pHotonics ELectronics functional Integration on CMOS)