MEMS based Widely Tunable 1-D Photonic Band Gap Filter

The Project was successfully completed by Mr. Renilkumar and Dr. Prita Nair (PI) from Physics department, SSN College of Engineering, Tamilnadu, India.

The project aims to Fabricate a MEMS based widely tunable optical channel drop filter for WDM networks and to study mechanical characterization including imaging of the widely tunable 1-D PBG filter.The filter consists of a pair of high contrast Si/Air grating layers acts as broadband reflectors which forms a tunable resonant cavity at the center. Both the grating layers are integrated with electrostatic comb drive actuators and freely hanging on folded springs. The fabrication is a three layer process,Layer 1 : Metallization for ohmic contact. (Ti/Au) . Layer 2 : Etching of critical parts of the tunable filter such as PBG mirrors (grating layers), support beams and electrostatic actuators. Layer 3 : Fabrication of fiber groves and releasing of movable structures (grating layers and associated actuator parts).

Photonic band gap structures or Photonic crystals (PCs) are engineered materials having periodically varying refractive indices or dielectric constants, whose minimum features are on the order of wavelengths and are able to microscopically, manipulate the propagation of electromagnetic waves. The feature that makes the material important is the presence of a band gap, i.e. the material does not support electromagnetic eigen modes of certain frequency range. This stop band depends on the periodicity, index contrasts and filling factor of the respective PC. This periodic perturbation of refractive index can be created along one, two and three directions, accordingly they are known as 1-D, 2-D and 3-D photonic crystals.

In a wavelength agile wavelength division multiplexed (WDM) network, single as well as multiple wavelength channels needs to be added and dropped at certain functional nodes. This can be addressed with tunable optical filter assisted reconfigurable optical add/drop multiplexers (ROADM), capable of catering to the bandwidth-on-demand triple-play services. In the ROADMs deployed in dense WDM core networks, arrayed waveguide gratings (AWG) are used for wavelength demultiplexing the input followed by array of MEMS mirrors for colourless wavelength routing to the required number of output ports [1]. These systems also combine capabilities of power monitoring and equalization, and have, in general, high operational expenses from the perspective of cost as well as optical power loss. The metro and local networks, on the other hand, handles lesser number of wavelengths and do not need more than 2 degree ROADMs. The main issues in this case, are the tighter control of operational expenses since the number density of such components will be more. The emerging field of photonic crystals (PC’s) coupled with well established MEMS technology can be suitably exploited for the design and fabrication of such tunable optical devices. This work aims to develop a MEMS based wavelength selective multi-wavelength Photonic band gap (PBG) filter with rapid and wide tunability, which can be used as a multi-wavelength tunable add/drop port in edge ROADMs in the metro CWDM networks.

A process has been developed to transfer sub micron features on SiO2 and Si using Cr as hard mask in RIE. The etch resistivity of Cr in RIE has been studied and its etch rate and relative etch rate with SiO2 has also been estimated.The Fabrication of MEMS based widely tunable optical channel drop filter has been successfully completed.