Tunable Thin-Film Filters Based on Thermo-Optic Semiconductor Films

Adam Payne

Tunable filters are a key enabling technology for the development of widely deployed wavelength division multiplexed fiber optic networks. Utilizing the thermo-optic effect in thin film semiconductors is one of the best means for achieving this goal. We have designed and fabricated tunable optical devices using hydrogenated amorphous silicon as the optically active layer. By using the large thermo-optic effect (dn/dt ?10-4/K) in this material, a Fabry-Perot filter was made with a total index modulation of ?n/n?0.03. At the technologically relevant wavelength of 1550nm, this modulation translates into >30nm of tunability. We discuss the use of plasma enhanced chemical vapor deposition to fabricate these filters. The large index contrast between an alternative dielectric (SiNx, n ?1.8) and the semiconductor (a-Si:H, n ?3.5) enables highly reflective mirrors to be produced with fewer quarter wave optical thickness layers than with the standard Ta2O5/SiO2 system. Other advantages of this thin film approach are discussed and reference is made to future potential devices which can be made with this material system.