Tunable Fabry-Pérot Filters
Narrowband optical filters are widely used for the detection of certain wavelengths in signals. The main application areas are optical data communications and spectroscoptic methods. Implementation of those devices on the scale of MEMS technology offers the advantages of miniaturisation, i.e. good feasibility of electrostatic tuning, high intergration density of single filter elements or arrays into existing systems and insignificant influence of mechanical forces and wear.
The Fabry-Pérot filters include two mirrors and a cavity which determines the filter wavelength. To achieve a high quality of the filters, distributed bragg reflectors (DBR) with very high reflectivity are used. These consist of an alternating layer stack of materials with high and low refractive index. Cavity and DBRs are implemented as air-gap structures, providing maximum index contrast and the possibility to actuate the cavity length. The chosen material system is InP, which is the appropiate base in the near IR for further photonic device integration like detectors or emitters.
Filters fabricated at INA achieved record values concerning tuning range (221 nm at 1550 nm) and low actuation voltages (3.2V at 142 nm tuning). Current research is aimed towards the integration of filters with existing optoelectronic devices and the implementation of photonic crystal structure on the membranes of the DBRs.