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What is the purpose of a fibre optic splitter?

One easily finds purely optical devices which split a given fibre like this:

     X
     |
  +-+++-+
  | | | |
  A B C D       number of outputs from 2 to 32 or more

My understanding is that light entering at X exits at A to D, at one quarter of the power out of each. Conversely, light entering at A exits only at X, also at a quarter of the input power.

They are advertised, as an example, "1x4 PLC Splitter, Mini Plug-in Type, SMF, SC/APC. Mounted in the wall mount FTTH box for fiber optic signal distribution." They vary in number of splits, mounting, connectors, single- and multi-mode, and their various technologies for implementation. Some have uneven power splits, presumably for uneven lengths or further splitting.

  1. What kind of equipment is at each end of this?
  2. What kind of framing would be typical?
  3. Do I understand the optics correctly?
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Optical splitters can be used in a number of situations:

  • PON - A passive optical network is split (1:16 or more) to serve multiple endpoints through a single fiber. Directions are multiplexed using different wavelengths (wavelength-division duplex). The downlink simply goes to all endpoints and is selective filtered there. The uplinks are merged by the splitter. Only one uplink (ONU) can send at any time using a "grant" from the root OLT (similar to a token).
  • link multiplexing - You can run several independent point-to-point links using different wavelengths on a single fiber pair (wavelength-division multiplex). A simple splitter can be used instead of a more sophisticated wavelength-dependent multiplexer when the power budget allows.
  • tapping - A 1:2 optical splitter can be used to listen in on an optical link.
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I've seen passive fiber optic splitters being used a number of times in situation where you need to have a copy of all traffic on a port, but are unable to use 1:1 flow sampling or port mirroring. This can be useful for traffic analysis or lawful interception.

In that case, typical equipment would be a router and some server able to collect a copy of all traffic, and you'd use an uneven power split to make sure the link on the router would never suffer from optical budget loss.

There was a talk on this topic at one of the previous NLNOG events. Slides are in Dutch but the diagrams (and a little bit of Google translate) may be of some use. You can find them here and a video of the presentation (also in Dutch) here.

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