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So I'm a hardware engineer learning about networks, and one interesting subject that my book glassed over was how hybrid fiber-coax networks work. One thing that was throwing me off was that the analog signals on the coax were being converted to light at fiber nodes then sent to the local head end--I had been under the impression that fiber was always digital.

The relevant wikipedia article mentions the use of broadband optical transmitters, but doesn't elaborate on their function. The lasers and transistor detectors that make up digital links make sense to me, but analog doesn't seem obvious. Lasers/LEDs cant be frequency modulated, so instead are they intensity modulated? How do the channels get muxed in and out of the fiber? If they don't get muxed before the detector, how does each photodiode tuned to detect a given channel?

  • One thing to think about is that most coax cables now only use digital. Even television signals on coax cable are digital, while network communication over coax has been digital for a long time. – Ron Maupin Jul 19 '17 at 13:31
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    Both coax and optical transmission systems are best thought of as analogue systems carrying digital signals. The channels are muxed in and out of the fibre using optical filters which allow a small set of wavelengths to pass (see Bragg grate and thin film filters). We try not to 'tune' receivers but to filter at the mux, so the Rx only ever sees the frequency it should. Even so we have hundreds of part numbers of SFPs for DWDM systems with filters on their Tx. – vk5tu Aug 1 '17 at 18:25
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 18 '17 at 13:24
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I just stumbled across https://www.nanog.org/sites/default/files/08-Noll.pdf which details the architecture: the RF signal is in fact amplitude modulated onto the fiber.

Conversion of RF Signals to/from Intensity Modulated Optical Signals for long-­distance transmission

This is possible since the optical wave frequency is much higher than the frequencies carried on coax - 1310 nm translate to 229 THz, while the coax runs less than 1 GHz.

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A fiber optic transmitter is a device which includes a LED or laser source and signal conditioning electronics that is used to inject a signal into fiber.

Fiber optic receivers capture the light from a fiber optic cable, decode the binary data it is sending and then convert into an electrical signal.

Information is sent from a source to a transmitter by means of an electrical signal. The transmitter then takes that binary data and transfers it to a light signal. The light signal is passed through fiber optic cables and connectors until it reaches the receiver. The receiver then takes that light signal, translates it back to an electrical signal allowing the binary data to be read by the user.

A transceiver is a device which combines the functions of both the transmitter and receiver.

Loopbacks are used for testing and some other applications. They feed data back to the same unit so that the original transmitted data can be compared to that received, thus identifiying transmission errors.

Hope this would help!

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  • This doesn't answer the question - it was specifically about how the analog coax signal is transmitted optically. – Zac67 Sep 17 '17 at 9:02

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