Fiber Optic study:      What about cross-talk to adjacent fibers, in ocean ? - Page 1
Forum section: electronics / beginners

author: RJSV
author_rank: Super Contributor
timestamp: February 01, 2023, 08:07:58 pm
content: Hello.Trying to figure, if cross-talk mechanism helps for bleeding some optical signal deliberately, to another fiber, how do they avoid that, in cross-ocean cabling ?My first guess is, they cannot. The method to evade cross-talk, maybe they just assign separate wavelengths for keeping distinct channels ?Interesting yt VIDEO, (search on photonics IC). Some European research group, 'UGENT' Lab. They mention using a light activated and modulated RADIO system.

author: rstofer
author_rank: Super Contributor
timestamp: February 01, 2023, 08:21:20 pm
content: Cladding or mode of cable?https://en.wikipedia.org/wiki/Cladding_(fiber_optics)https://en.wikipedia.org/wiki/Single-mode_optical_fiberhttps://www.m2optics.com/blog/understanding-submarine-optical-fibers

author: Benta
author_rank: Super Contributor
timestamp: February 01, 2023, 08:21:41 pm
content: Long distane fiber optics use single-mode fibers and are not prone to cross talk.You might have the issue with cheap plastic optics (TOSLINK), but that's consumer stuff over a few metres.

author: Marco
author_rank: Super Contributor
timestamp: February 01, 2023, 08:52:53 pm
content: Apart from the cladding, the fibers are simply coated in opaque plastic. Not much getting through.

author: TomWinTejas
author_rank: Regular Contributor
timestamp: February 02, 2023, 03:52:38 am
content: From my experience there aren't issues among strands within a cable. It's going to depend on how the cable was manufactured, but the subsea cables I am aware of have a jacket around each strand which are color coded to help distinguish the strands when splicing. The big challenge in the subsea space used to be chromatic dispersion. Now that the industry has shifted to coherent optics for DWDM transponders that's no longer a big issue.The biggest challenge now that I am aware of surrounds spectrum sharing. Subsea cables are expensive and often times they're built by consortiums where there are multiple owners... and some owners will take their spectrum and sell IRUs to sublease for economic viability. Subsea amplifiers use constant power output so it gets tricky... the best practice is to use ASE line loading to ensure the entire spectrum is lit. The line system operator uses line loading as a fail-safe, but each network with partial spectrum is typically responsible for line loading as well... and given the potential impacts to everyone else on that fiber, any outages and upgrades need to be coordinated. It's a very different world than with terrestrial fiber where networks don't have to contend with any other networks on the same fiber strands.Nearly all of my experience is with terrestrial fiber and the subsea that I've worked with is typically newer cable systems... so it's possible there were issues in the past with previous cable constructions that I'm unaware of.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: February 02, 2023, 04:05:18 am
content: All of the answers above are correct.Another way to think of it that may grab better with some is that the extremely low loss of the single mode fibers used (typically a dB/km or less) implies equally low coupling into the fibers from the sides. You can pump a lot of power crosswise across these fibers and virtually none of it will propagate down the fiber. The energy has to be launched with some fairly restricted conditions from the ends or it doesn't follow the fiber.

author: TomWinTejas
author_rank: Regular Contributor
timestamp: February 02, 2023, 04:10:16 am
content: Quote from: CatalinaWOW on February 02, 2023, 04:05:18 amExactly, the loss is typically very low. Reference level for singlemode is 0.17 dB/km and in the field the threshold for certification testing is typically around 0.25 - 0.35 dB/km which includes allowances for splices along the path.

author: ejeffrey
author_rank: Super Contributor
timestamp: February 02, 2023, 04:38:50 am
content: I'm not familiar with how undersea cables are packed, but crosstalk between single mode fibers is very small to begin with even without additional jacketing. The mode volume of the core is so small compared to the overall size that it's very unlikely for stray light to scatter into the guided mode, and of course scattering is very low in this kind of fiber. Standard single mode fibers have an outer polymer coating that damps cladding modes quickly and keeps the power in the cladding from building up over long distances. The traditional SMF construction is a 9 micron core, 125 micron cladding, and 250 micron coating, although fibers with smaller cladding and coating diameters are common, both for greater flexibility and if you want to cram more fibers into a tube.In any case, it would be trivial to add an extra layer of absorptive material between fibers if that were a problem. I don't know if that ever becomes necessary, but if so it would be an easy problem to solve.