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single mode (SMF) and multimode fibre (MMF). However, high-order off-axis modes of MMF cannot be collimated as perfectly as the fundamental mode. Consequently, the loss of MMF switches is typically higher, and the switch port- count is smaller than with SMF switches. Silicon photonics technology, as illustrated in Figure 2, is creating new opportunities for fast optical circuit switching - silicon photonic switches have been demonstrated with microsecond and 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 PROGRESS THROUGH SILICON PHOTONICS
connect fabric exist, such technologies fail to meet the requirements of emerging IT applications because of limited scalability resulting from fabrication limits, performance issues, insertion loss, and switching time.
BETTER AUTOMATION THROUGH ROBOTICS
Given the limitations of existing optical switching technologies, robotic optical networking is an innovative way to physically change the connectivity of a network without undue impairment of its optical characteristics. Robotic fibre switches (see Figure 3) have been developed that can make large cross-connect fabrics with low insertion losses and robust connections. The matrix design of the robotic fibre switch with cross-bar switching completely prevents fibre from entangling. The robotic fibre switch can pick up the connector pair and make a connection within 20 seconds or less. Because every connection in a robotic switch is made using LC connector- like mating, low optical loss is achieved independent of the switching path. The 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 supports both SMF and MMF. The robots are shared among hundreds of fibre ports; hence the cost of the switch can
Figure 3: Illustration of a robotic matrix optical switch design
be relatively low. 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 robotic based optical cross-connect provides significantly improved optical performance, is easily integrated into an SDN Controller and research demonstrates that an agile physical layer can improve the cost savings of multi- layer optimisation from a 19 percent to a 34 percent cost saving. With the deployment of optical fibre ever-increasing in response to today’s digital demands, effective automation of the physical layer is crucial. Robotic fibre switching is a cost- effective, technically compliant solution and may be the answer to a 20-year- old problem – how to automate the physical connectivity in fibre networks without undue impairment of its optical characteristics.
Figure 2: Schematic of a silicon photonic MEMS matrix switch
switch can be made that overcomes the cumulative losses. Switches with a size larger than 100×100 are theoretically possible and a 50×50 silicon photonic switch, monolithically integrated on a 7.6×7.6mm2 chip, already has been successfully demonstrated with an on- chip insertion loss of 8.5dB. Although promising, challenges still exist for digital silicon photonic MEMS switching. Due to 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,
especially where 10s or even 100s of fibres are involved. In addition, silicon photonic MEMS switches with polarisation diversity are yet to be demonstrated. Consequently, while various optical switching
technologies for large size photonic cross-
ISSUE 15 | Q4 2018
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