Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[burakkucat]

Just wondering what sort of (contiguous) length of standard single mode optical fibre, under laboratory conditions, would result in attenuations of --

• 5 dB
• 45 dB

[XGS_Is_On]

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/

[burakkucat]

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.

[XGS_Is_On]

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.

[burakkucat]

<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. 

[tonygibbs16]

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

[XGS_Is_On]

<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. 

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.

[burakkucat]

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.)