Data center operators use 40G switches to increase their port density. The switches allow the configuration of each 40G switch port to act as four individual 10G ports. However, although the switch port is configured as if it was virtually 4 pieces of 10G ports, it physically remains one port. This port is typically in accordance with the QSFP+ standard, for which only 40GBase-SR/LR/ER optics exist.
For multimode versions, the transceiver already features 4 duplex fiber in/outputs, so only a cable or connector adaption is required. Single mode versions are based on four different wavelengths streams which are multiplexed, respectively de-multiplexed, into one fiber output / input. Therefore the break out cable cannot simply adapt to different connectors but also must feature the matching optical multiplexer inside the cable to be able to mux and demux the four lambdas from one duplex transceiver port to 4 fiber pairs.
More detailed information and a short explainer video can be found on http://www.cubeoptics.com/breakout.
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