Design of a 4 x 4 optical re-routing switch

Optical 4x4 switches have been designed and processed. These switches are used in optical telecommunication networks to alter the path of signals carried by light. A 4x4 switch thus has 4 optical input ports and 4 optical output ports and couples an optical signal from any of the input ports to any of the output ports in such a way that no two input signals from separate input ports are coupled to the same output port. The basic element in our 4x4 switch is the 2x2 thermo-optic switch. This switch is based on a symmetric Mach-Zender interferometer equipped with a thermo-optic phase shifter in one arm and was successfully demonstrated earlier by Dan Zauner and Lars-Ulrik Andersen at MIC. The thermo-optic switch is slow with a response time around 0.5 msec, and each 2x2 swith consumes typically around 0.5 Watt of electrical power. The general functionality requirement for a 4x4 switch results in 24 possible switching combinations. To achieve that number and maintain channel uniformity, 6 2x2 thermo-optic switches arranged in a 2x3 matrix are required. The design work involved primarily 3 dB couplers and crossing waveguides. Measurements on processed wafers so far show that it is possible to achieve switches with low insertion losses (a few dB). Future work work will concentrate on reducing the crosstalk from the crossing waveguides and on developing more process tolerant designs for the 3-dB couplers.