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Chat => Tech Chat => Topic started by: burakkucat on November 28, 2022, 05:18:08 PM
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Just wondering what sort of (contiguous) length of standard single mode optical fibre, under laboratory conditions, would result in attenuations of --
:hmm:
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Depends on the wavelength of light.
When you buy a spool of fibre cable the manufacturer will inform you of the loss at 1310 nm and 1550 nm.
https://kingfisherfiber.com/application-notes/optical-loss-testing-overview/
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Depends on the wavelength of light.
D'oh, I had forgotten about that factor. (b*cat plods off to look at the link.) Thank you.
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Welcome!
PONs are dominated by splitter loss. 15 dB of it for 1:32 split, 18 dB for 1:64, 21 dB for 1:128. Add that to perhaps 1.5 dB for splitter insertion loss, connectors and splicing and you've the equivalent of a very long fibre run before the light has travelled a metre.
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<nods>
I am surprised by the extent of the lossiness of *PON splitters . . . two of 1:128, back to back, and its getting close to the higher (45 dB) attenuation that I was pondering in my hypothetical experiment. :o
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https://www.thefoa.org/tech/loss-est.htm#:~:text=For%20singlemode%20fiber%2C%20the%20loss,per%20km%20for%201550%20nm. suggests 0.5dB per kilometre at 1310nm wavelength and 0.4dB per kilometre at 1550nm wavelength.
When I worked for BT, a rough order of magnitude was that a fibre splice was equivalent to 1 km of single mode fibre loss.
So to answer B*cat's question, it would be 10km for 5dB at 1310nm and 12.5km at 1550nm.
Kind regards,
Tony
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<nods>
I am surprised by the extent of the lossiness of *PON splitters . . . two of 1:128, back to back, and its getting close to the higher (45 dB) attenuation that I was pondering in my hypothetical experiment. :o
It's inevitable and unavoidable. The light is being split each time so photon count per port is halved, power is photon count so is halved. Attenuation on optics is a function of leakage / absorption of photons which is how it can be reduced by reducing leakage from the fibre core and why multimode has much lower range: larger fibre core, light bounces more and at more acute angles within the core, more photons lost per metre. As far as splits go 1:2 = 3 dB, 1:4 = 2 * 1:2 splits per port = 6 dB, 1:8 = 3 * 1:2 splits = 9 dB, 1:16 = 4 * 1:2 splits = 12 dB, etc. To avoid this would need amplifiers making the network no longer passive.
Can work out the loss easily enough by using the split ratio, use that larger number, take it log 2 and multiply by 3. 1:128 = 128 log 2 = 7. 7 * 3 = 21 dB.
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Thank you, both, for your recent posts. I've now got an understanding on the topic. (As for how long it will remain in my mind . . . who knows.)