Optical Circuit Switching over Wavelength Division Multiplexing Based on the article: “ Transparent Optical Switches: Technology Issues and Challenges ” by G. Ellinas, J. Walker, S. Chaudry, L. Lin, E. Goldstein, K. Bala

Node architectures for a Core Optical Network ● Opaque – The optical signal carrying traffic undergoes Optical to Electronic to Optical (OEO) conversions ● Transparent – The optical signal carrying traffic stays optical at all times from entry to exit

4 different architectures 1a) Fixed patch panel between WDM systems with transponders. 1b) Electrical switch fabric between WDM systems with transponders. 1c) Transparent switch between WDM systems with transponders, complemented by a OEO switch for drop traffic. 1d) Transparent switch on a transparent network. The signal stays optical until it exits the network.

Advantages of transparent network architecture ● Bit rate and data format independent, thus more scalable than electronic switches. ● Silicon micro-mirrors: – Small and low in power-consumption – Prices expected to drop with mass production

Disadvantages of transparent network architecture ● Physical impairments accumulate through optical path – (No 3R) (re-generation, re-shaping, and re-timing). ● No wavelength conversion – Inflexible wavelength utilization – Dedicated protection of the lightpaths ● Interfaces are Analogue not digital – What is the difference? ● WDM interface not standardized – Proprietary

Optical switch fabric is bit-rate independent Most lightpaths bypass the OEO switch. Drop side ports are connected to an OEO switch Providing SONET/SDH line termination through its opaque ports. 1c) Transparent switch between WDM systems with transponders, complemented by a OEO switch for control and management functions

Optical switching Optical switch may operate by mechanical means, such as physically shifting an optical fiber to drive one or more alternative fibers, or by electro-optic effects, magneto-optic effects, or other methods. Micro-electromechanical systems (MEMS) ● most promising method for high-port-count switch fabrics ● Needed for core network cross-connects.

Micro-electromechanical systems (MEMS) 3D is advanced!

Challenges associated with MEMS switch architecture ● Optical fiber bundles ● Lenslet arrays ● MEMS mirror chip ● MEMS reliability ● Mirror control ● Packaging

Optical fiber bundles ● The position of each fiber must be accurate in five dimensions, and both axes of tilt. The required accuracy is measured in micrometers (µm - one millionth of a meter), and the fiber tilt in milliradian.

Lenslet arrays ● The lenslets collimates the optical signals to lower the losses through the switch fabric. (Collimated light is light whose rays are parallel and thus has a plane wavefront.) ● Producing arrays with large numbers of lenslets is complicated and expensive

Conclusion ● The use of transparent switches is growing and deployments is emerging. – ROADM – Wavelength Selective Switches (WSS) ● Traffic keeps growing and bit rates increase substantially ● Opaque switches expected to remain together with transparent switches – Providing grooming and multiplexing functions – Control and management functions.