3 ms·

Duplex fiber where a separate strand for TX/RX can use the same frequency. Simplex fiber, that is point to point, may in fact be able to use the same frequency

Duplex fiber where a separate strand for TX/RX can use the same frequency. Simplex fiber, that is point to point, may in fact be able to use the same frequency, but it's not done in practice. Possibly due to the difficulty in directing the outgoing and incoming signals to different pieces of hardware(receiver and transmitter) GPON networks(typically used by Fios, etc...) have multiple passive splitters on the strand. For example, a common PON splitter would have 1 strand in from the OLT(optical line terminal, essentially the head-end), 32 strands out. The light received from the OLT is split x32 and sent to each downstream port. The devices on the downstream ports(ONU/ONT, Optical Network Terminal) receive this signal and also transmit it on the same strand. That transmitted signal must be coupled back into the single upstream port, from every attached ONT. I imagine there is significant difficulty getting the light to traverse the splitter and exit only via the upstream port. So a different wavelength is used so that it doesn't matter if the light is received by the other units, and also it can be more easily filtered (by wavelength instead of direction). My knowledge of optics is not sufficient to fully explain why it's easier to filter by wavelength instead of direction.

amluto 7y ago
> My knowledge of optics is not sufficient to fully explain why it's easier to filter by wavelength instead of direction. Reciprocity. For most optical devices, if x% of light coming in port A comes out port B, then x% of light coming in port B comes out port A. Non-reciprocal materials exist, but they’re unusual and may be rather inefficient and complicated to work with. Wavelength splitters are reciprocal and are relatively easy to construct. In fact they’re cheap enough that people use them for art. (The usual splitter is a piece of dichroic glass.) The Wikipedia article is so-so: https://en.m.wikipedia.org/wiki/Reciprocity_(electromagnetism) https://en.m.wikipedia.org/wiki/Reciprocity_(electromagnetis...
dfox 7y ago
The reason why it is easier to filter by wavelength is that almost any optical path reflects significant amount of light back, primarily from various discontinuities along the fiber (connectors, splices, and also the actual receiver on the other end) but also from imperfections in the fiber itself. On copper interconnects there are similar issues, but for reasonably controlled cabling (eg. 1000-base-T) they can be minimized enough, that what remains is either insignificant or can be compensated for by DSP magic.

amluto 7y ago

> My knowledge of optics is not sufficient to fully explain why it's easier to filter by wavelength instead of direction. Reciprocity. For most optical devices, if x% of light coming in port A comes out port B, then x% of light coming in port B comes out port A. Non-reciprocal materials exist, but they’re unusual and may be rather inefficient and complicated to work with. Wavelength splitters are reciprocal and are relatively easy to construct. In fact they’re cheap enough that people use them for art. (The usual splitter is a piece of dichroic glass.) The Wikipedia article is so-so: https://en.m.wikipedia.org/wiki/Reciprocity_(electromagnetism) https://en.m.wikipedia.org/wiki/Reciprocity_(electromagnetis...

dfox 7y ago

The reason why it is easier to filter by wavelength is that almost any optical path reflects significant amount of light back, primarily from various discontinuities along the fiber (connectors, splices, and also the actual receiver on the other end) but also from imperfections in the fiber itself. On copper interconnects there are similar issues, but for reasonably controlled cabling (eg. 1000-base-T) they can be minimized enough, that what remains is either insignificant or can be compensated for by DSP magic.