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ANALYSIS & OPINION AUTOMATION


Scaling up the port- count of a single 3D MEMS optical switch is a big challenge


Robotic optical switches are being used and developed to make large cross-connect fabrics with low insertion losses and robust connections


large mode size mismatch between the fibre and the silicon waveguides, low loss coupling between the fibres and the silicon chip is still a problem area, especially where tens or even hundreds of fibres are involved. In addition, silicon photonic MEMS switches with polarisation diversity are yet to be demonstrated.


Robotic optical networking Tis networking is a way to physically change the connectivity of a network, without undue impairment of its optical characteristics. Robotic optical switches are being used and


developed to make large cross-connect fabrics with low insertion losses and robust connections. Te key to their effectiveness has been the development of a matrix design with cross-bar switching that avoids fibre entangling. Te robots can pick up the connector pair and make a connection within 20 seconds. As every connection in a robotic switch is


The ability to automate physical connectivity in fibre networks could prevent patching from holding up on-demand services


can be achieved. Similar to 3D MEMS optical switches, however, the insertion loss variation still exists across different connections. Non-uniformity of the fibre collimators also widens the loss spread and commercial switches using direct beam steering are specified with a 1.2dB typical loss and a 2.2dB maximum loss. Switching can be relatively fast at ~20ms.


Similar to MEMS optical switches, there is no locking mechanism for the established connections, hence this approach is not disaster-proof. Fibre-to-fibre coupling using direct beam steering can support both single and multimode fibre in theory. However, the loss of MMF switches is typically higher, and the switch port-count is smaller than for SMF switches.


Silicon photonics Looking to the future, recent progress in silicon photonics technology is creating new opportunities for fast optical circuit switching.


Fast silicon photonic switches have been


demonstrated with microsecond, even nanosecond switching times. Unfortunately, accumulated losses limit the port count of these switches (≤8×8). Combining MEMS actuated vertical adiabatic couplers as switching elements, in conjunction with a 2D silicon waveguide matrix, a scalable silicon photonic waveguide optical switch can be made that overcomes the accumulative losses. Switches larger than 100×100 are theoretically possible and a 50×50 silicon photonic switch, monolithically integrated on a 7.6×7.6mm2 chip has been successfully demonstrated with an on-chip insertion loss of 8.5dB. As the ultra-compact MEMS actuator only needs to make a digital movement of less than 1um in range, an impressive sub-microsecond switching time was achieved. Although promising in many aspects, a couple of practical challenges still exist for digital silicon photonic MEMS switching. Due to


made using LC connector-like mating, low optical loss is achieved independently of the switching path. Te lowest optical insertion loss recorded so far was an average of 0.17dB, with a standard deviation of only 0.07dB. In addition, it is possible to scale up the switches to support 16K duplex ports with less than 1dB optical loss. A fibre cloud with half a million duplex ports can be supported with <1.5dB optical loss. With LC ferrules, the same design works for


both single and multimode fibres. Robots are shared among hundreds of fibre ports, hence the cost of the switch can be relatively low and once a connection is made by the robots, the connectors are locked on the metal matrix passively, resulting in extremely secure and reliable optical connections, even under extreme conditions. A major driver in a modern network is to


increase service velocity; it addresses the on-demand nature of many requested services and accelerates time to revenue and increases customer satisfaction. Te ability to automate physical connectivity in fibre networks could not only prevent patching from holding up on-demand services, but also allow on-demand services to be delivered more cost effectively.l


Duncan Ellis is director, EMEA at Wave2Wave Solution


Issue 20 • Summer 2018 FIBRE SYSTEMS 27


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