MEMS based optical cross connects for fiber optical communication

MEMS based optical cross connects experience a growing market demand. They are used in optical fiber networks as well as optical measurement systems where they add functionality or increase the performance of the systems. 2x2 MEMS optical cross connects proved excellent performance and large optical cross connects with over 100 input and output channels are used to route the worldwide data traffic. However, large optical cross connects have a high cost. Medium sized optical cross connects having four to sixteen input and output ports and moderate price clearly cover an industry need. Therefore, the subject of this thesis is the development of a matrix type optical cross connect with four or eight input and output ports. The pitch of the individual mirrors was set to 250 µm to enable a simplified assembly to fiber ribbons. A novel two-sided etching process of 110 µm thin silicon wafers enabled to fabricate silicon structures with a high accuracy on both sides of the wafer. The optical mirrors are situated vertically on very compact actuators with a footprint of 250 x 250 µm2. The actuators consist of a fixed electrode and a counterelectrode suspended by torsion beams with a diameter of 0.8 µm. The simulation, fabrication and characterisation of microfabricated Graded Index lenses are presented. Theses lenses are needed to collimate the light exiting from the single-mode fibers, which is necessary to reduce losses originating from diffraction. The lenses are fabricated from a selected multi-mode fiber with a parabolic refractive index distribution in the fiber core. Since the diameter of the lenses equals the diameter of the signal carrying fibers they can be passively aligned in U-grooves that are etched into the silicon chip. The lenses yielded optical losses of less than 2 dB for a coupling length of 2 mm. The measurements of complete optical cross connects yielded encouraging results. The maximal operating frequency was measured to be larger than 200 Hz and the optical losses of some channels were close to the losses caused by the lenses, which means that the quality of the mirrors is good. However, some mirrors yielded losses that exceeded largely the targeted 3 dB. The reason was found to be the fragility of the silicon chip causing alignment problems after the assembly. A modification of the fabrication process allowing to fabricate more rigid mirror matrices is suggested.

Thèse de doctorat : Université de Neuchâtel, 2006 ; 1860