On Fri, 16 Oct 2020 15:02:36 +0100, The Natural Philosopher wrote:

... but I think Openreach are installing GPON (Gigabit Passive
Optical Network). This takes the fibre from the head end "line
card"
and optically splits it 32 (or even 64) ways. This reduces the
range
but it's still in the order of 10 km or more providing up to 1
Gbps
symetrical (if they wanted to).

yes, and no. You have described what the Open Reach engineer said
happened (optical splitting), but he said the fibre went 10 + miles to a
big exchange, and could do more than that.

Pre or post the optical split? The unsplit headend feed is quite
"pokey" and can do considerable distances without amplification, 40
km from here to Hexham for example.

It is really probably about the amount of *power* that can be sent down
it - the splitter is a different sort of loss from a long fibre, in that
its probably a flat loss across the band, not a load of frequency
dependent phase shifts.

The limit of multimode ...

Are Openreach using multimode? Singlemode has a far greater range and
doesn't mess up the signal(s). I don't think DWDM would work over
multimode. I think GPON only uses two "carriers", (up/down), within a
single fibre and uses TDM to share those two carriers between
customers. That might work over multimode but for how far I don't
know.

As you say the range of singlemode is really down to how bright (ie
power) you can make the source laser. Powers are such that caution
needs to be applied to protect your eyes when working on such sytems.
You can't see the IR laser light.

--
Cheers
Dave.