Micromachined tunable Fabry-Perot interferometers based on compound semiconductors have earlier been proposed for fiber optic communications employing wavelength division multiplexing (WDM) for wavelengths around 1.55 µm. The cavity length in micromachined interferometers is varied by displacing one of the two distributed Bragg reflector (DBR) mirrors by electrostatic actuation of supporting beams. The filter's optical response for varying cavity lengths is simulated by a transfer matrix method, and the optical tuning efficiency of the filter is 0.53. We investigate three conventional filter designs using the finite element method (FEM) and compare it with a new proposed filter design. Using a mathematical model, deflection is analytically calculated and compared with finite element analysis results. Due to the way in which the mirror is integrated with a suspending framework of beams, bending within the mirror during actuation cannot be averted. The filter's optical performance demands that the mirror remain so flat that the maximum bending deflection is 1 nm for the mirror of given dimensions. Using a criterion based on mechanical and optical considerations, the dimensions of the beams suspending the mirror are optimized for each filter design under investigation. Combining the optical and mechanical simulations by FEM, wavelength tuning characteristics for each filter design are determined.
High reflectivity Distributed Bragg Reflector (DBR) mirrors comprising of quarter wavelength stacks of epitaxial or dielectric layers are essential in various photonic active and passive devices. Our focus has been on optimizing plasma enhanced chemical vapor deposited (PECVD) Si<sub>3</sub>N<sub>4</sub>/SiO<sub>2</sub> (170nm/255nm ± 20nm) based DBR stacks for high reflectivity to be incorporated in a novel structure comprising of all dielectric based mirrors for tunable resonant cavity devices in a MEMS configuration. Simulated results based on Macleod program predicts a high reflectivity of 99.7% for 10.5 pairs of quarter wavelength layers of Si<sub>3</sub>N<sub>4</sub> and SiO<sub>2</sub> centered at the telecom wavelength of 1550 nm. Si<sub>3</sub>N<sub>4</sub>/SiO<sub>2</sub> DBR pairs of 5.5, 7.5 and 10.5 are deposited by PECVD on InP substrate and reflectance spectra observed on spectrophotometer. A flat pass band of 354nm bandwidth and 99.8% peak reflectivity for 10.5 pairs of DBR stack is observed. Characterization of DBR layers is done using ToF-SIMS for interface abruptness. XPS and Ellispometry is used to ascertain stoichiometry and refractive index respectively. CHF<sub>3</sub> chemistry is used to etch the DBR mirror stacks to achieve highly anisotropic walls by ICP technique, as observed under SEM. We are able to demonstrate filtering of a single wavelength in the C-band region, for different cavity spacing of SiO<sub>2</sub> layer, sandwiched between two highly reflecting mirrors.
The use of micromachined tunable Fabry-Perot interferometers based on compound semiconductors with an integrated photodetector have earlier been proposed [1-3] for fiber optic communications employing Wavelength Division Multiplexing (WDM) for wavelengths around 1.55 μm. The variation of the Fabry-Perot cavity length in these devices is achieved by electrostatic actuation induced movements of a Distributed Bragg Reflector (DBR) mirror suspended by beams. A new filter design has been proposed and its electrostatic actuation induced deflection has been investigated along with three different filter designs using Finite Element Analysis (FEA). Using a mathematical model, deflection is analytically calculated and compared with FEA results. Due to the way in which the mirror is integrated with suspending framework of beams, bending within the mirror during actuation cannot be averted. The filter’s optical performance demands that mirror remain so flat that the maximum bending deflection is 1 nm for the mirror of given dimensions. Using a criterion based on mechanical and optical considerations the length of the beams suspending the mirror has been optimized for each of filter designs under investigation.