ANALYSIS & OPINION AUTOMATION
The final frontier
Duncan Ellis shares his views about the increased focus on automation from network operators, and how the physical layer has so far stubbornly resisted the move
operators are increasingly focusing on automation – typically via soſtware defined networking (SDN) or network function virtualisation (NFV) – to increase service velocity and remove cost and errors from their networks. However, one layer – the physical layer – has stubbornly resisted the move to soſtware definition or automation. When it comes to optical distribution frames
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and meet-me rooms, actions to re-architect the physical network take place in a patch room where an engineer manually interlinks the spans to create the required topology. Tis allows new connections to be added, old connections to be removed and new services to be created. Te process is manual and therefore opex-heavy, slow and out of step with the progress being made in all other areas of the network. It is also prone to error and can even impact adjacent services through the disturbance of patch cords.
Automation to date Te demand to extend automation with remotely reconfigurable physical connectivity is growing and large-scale fibre cross-connect is the key enabler to such a layer ‘0’ revolution. Many optical switching technologies have been proposed, demonstrated and commercialised, including 2D and 3D MEMS optical switches, thermo-optical switches, liquid crystal optical switches, mechanical beam steering optical switches and robotic optical switches. 2D MEMS, thermos-optical, and liquid crystal
26 FIBRE SYSTEMS Issue 20 • Summer 2018
s network speeds have exploded through 100G and 200G to beyond 400G to cope with increased bandwidth requirements, network
Automation can enable physical fibre connections to be made remotely and quickly
based optical switching each have scalability constraints with port count limited to 32x32 for reasonable switch insertion loss, but there are other technologies that can yield large port- count optical switches. For example, some operators have deployed
3D MEMS optical switches. Tese use two- dimensional MEMS mirror arrays to steer optical beams in free space and provide a level of network automation at a purely optical layer. Tere are some constraints limiting the port-count and optical loss performance of a 3D MEMS optical switch. An optimised design for a dual mirror configuration today can achieve an optical switch with a maximum port count of about 400, and a nominal optical loss of about 1.5dB. Te optical losses of different connections in a 3D MEMS optical switch are intrinsically different. Consequently, 3D MEMS optical switches usually have a relatively large insertion loss spread, and a maximum loss up to 3~4dB. Scaling up the port-count of a single 3D
MEMS optical switch is a big challenge. Te larger switch requires more MEMS mirrors, which means longer connection path lengths and consequently larger collimated beams. It also necessitates larger mirrors to avoid clipping loss, all leading to more silicon real estate.
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Although promising in many aspects, a couple of practical challenges still exist for digital silicon photonic MEMS switching
Unfortunately, the lithography reticle size sets the hard wall limit. Using the multi-stage CLOS architecture, larger port-count switches can, in fact, be built using multiple 3D MEMS optical switches. But the total optical loss quickly becomes unbearable for many applications. A further drawback of these switches is that
connections are not locked, making them unsuitable for mission-critical applications. Another optical switching technology with
some commercial success is direct collimator steering. A direct light path between the input fibre and the output fibre is set without clipping loss using reflecting mirrors with limited apertures for beam steering. With proper fibre collimator design, low insertion loss of ~0.4dB
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