Wavelength Tunable Filter with Poly-Si/SiO2 Multi-Layer by Thermo-optic Effect)

초록

Wavelength tunable filter is the key component of Wavelength Division Multiplexing (WDM) systems. In this paper, we utilize thermo-optic effect of multi-layers whose refractive indexes are changed by temperature variation effect, to tune transmitted wavelength. Tunable filter is designed using Fabry-Perot interference effect. Distributed Bragg Reflector (DBR) mirror structure is composed of 4-pairs of high index poly-Si layer (n=3.48) and low index SiO2 layer (n=1.44). The filter has the structure of DBR layer/tunable cavity layer/DBR layer. The cavity material is poly-Si. Thickness of DBR mirror material is λ/4 and that of cavity layer is λ/2, where λ is the center wavelength of 1550nm. Poly-Si has very high thermo-optic coefficient of dn/dT=2.25 x 10-4K-1. Thus it is efficient material for thermo-optic tunable filter. Poly-Si is deposited by LPCVD at 625℃ using SiH4, and SiO2 by PECVD at 300℃ using SiH4/N2O. Backside of the wafer substrate is etched to form a membrane in order to reduce insertion loss and parasitic optical resonances. For backside etching, we use 42 weight % KOH in hot bath to maintain the fixed temperature 80oC. As the temperature increases, peak wavelength shifts to the longer wavelength. At 90oC, the peak is centered at 1563.8nm and Δλ=6.0nm. For the temperature range of 64.7oC, tuning range is as large as 9.4nm and thus the tuning efficiency is 0.144nm/K-1. In conclusion, we show the multi-layer DBR mirrors with bucking effect provides an efficient tunable filter with low insertion loss and very large tuning range.