Why do they put those long loops of fiber inside fiber optic boards? - Page 1
Forum section: electronics / general

author: Beamin
author_rank: Super Contributor
timestamp: May 18, 2018, 09:27:39 pm
content: I have noticed when they have a fiber optic cable go into a board, think its called SDI OE, the fiber goes to a connecter on the back then it will go through a 4" loops of a couple of turns. Why do they do this? The light doesn't care how long the cable is like in RF. Almost every fiber to electrical signal board I have seen has this. Looks almost like strain relief but that doesn't make sense.

author: sokoloff
author_rank: Super Contributor
timestamp: May 18, 2018, 09:48:06 pm
content: A service loop or loops allows the tech to make the field-termination with the fiber terminator tool not jammed right up inside the enclosure.I haven't seen that in pre-made fiber patch cables as much.

author: helius
author_rank: Super Contributor
timestamp: May 18, 2018, 10:04:57 pm
content: Additionally, many optical fibers are terminated or spliced using trays in which the cables must lay flat, providing alignment. If an optical cable is not long enough to be placed into the tray it can't be spliced and can only be junked and replaced.

author: Beamin
author_rank: Super Contributor
timestamp: May 18, 2018, 10:12:29 pm
content: https://youtu.be/2quzAl9zn6oThat kind of makes sense but why have the cable 20" long in things where you would replace the whole thing like in this video?

author: dmills
author_rank: Super Contributor
timestamp: May 19, 2018, 12:04:54 am
content: Two reasons really:Fibre connectors tend to be somewhat delicate so leaving enough to make re-termination trivial is a good thing, and the biggie: bend radius, that shit don't like being bent too tightly so you are going to have to bend it around in a big loop anyway, in which case why not make for plenty of length?These days of course we would probably just stick an SFP+ cage on and have done with it.Regards, Dan.

author: Beamin
author_rank: Super Contributor
timestamp: May 19, 2018, 10:28:42 am
content: So in the video I posted at around the time stamp you see a large loop (board with yellow cable) with 6+ turns of cable. I can see splicing and not bending but that would be 4 or 5 times more then you would need to not bend the cable. These companies don't generally waste materials so I still don't see why you would want it so long unless there is some other reason and I don't see why you would splice on these particular boards if there was damage you would just replace.

author: sokoloff
author_rank: Super Contributor
timestamp: May 19, 2018, 10:34:41 am
content: Quote from: Beamin on May 19, 2018, 10:28:42 amAroundwhattime stamp?

author: dmills
author_rank: Super Contributor
timestamp: May 19, 2018, 12:42:29 pm
content: Sometimes it is very much more production friendly to buy the laser or photodiode with the fibre pigtail pre terminated to it, and even the connector pre fitted on the other end (Saves you from needing a clean room and someone with the skills to do fibre termination).These come in various lengths but when you are only building a few tens of boards a month it is hardly worth getting a custom one, so you buy an assembly that is too long and just coil the excess up.Regards, Dan.

author: TerraHertz
author_rank: Super Contributor
timestamp: May 20, 2018, 07:24:57 am
content: Maybe so the fiber is long enough to be brought out and plugged into a portable fiber communications analyzer?

author: filssavi
author_rank: Frequent Contributor
timestamp: May 20, 2018, 07:54:27 am
content: It could also be that the fiber there is not just ordinary fiber but it is doped and/or designed in such a way to “condition” the optical signal. For example double clad fibers can be used to compensate (and thus cancel) dispersion (pulse broadening due to different travel phase velocity at different frequencies).I think this is a more likely explanations since optical fiber is expensive and leaving 2/3 meters spare for terminations doesn’t seem reasonable

author: sokoloff
author_rank: Super Contributor
timestamp: May 20, 2018, 09:19:57 am
content: Quote from: filssavi on May 20, 2018, 07:54:27 amOptical fiber raw material is quite cheap. Labor and connectors are expensive, but the fiber itself is well under $1/meter.

author: TerraHertz
author_rank: Super Contributor
timestamp: May 20, 2018, 12:37:25 pm
content: Especially when dealing with fiber cables that go kilometers underground, in sheaths containing dozens of fibers, you want to be very very sure you never run out of spare length at the end on any single fiber, to re-terminate if the connectors go bad.

author: Beamin
author_rank: Super Contributor
timestamp: June 03, 2018, 05:47:57 pm
content: Quote from: sokoloff on May 19, 2018, 10:34:41 am6:32 I thought I started the link at 6:32. I see the having under ground liks and not wanting to mess with that but in the video this is video broadcast equipment and doesn't deal with that.The idea that it is special cable to cut distortion sounds most reasonable but they do that in every fiber device even the cheaper ones?I'm sure there is some really simple explanation.

author: madires
author_rank: Super Contributor
timestamp: June 03, 2018, 06:14:25 pm
content: The launching fiber is quite useful for an OTDR check. With a too short fiber you couldn't check the connectors of the line card. The extra fiber adds sufficient delay between the connectors and the laser/photo diode to make the OTDR see the reflection of light at the connectors.

author: Yansi
author_rank: Super Contributor
timestamp: June 03, 2018, 06:18:55 pm
content: Quote from: dmills on May 19, 2018, 12:42:29 pmYou absolutely do not need a clean room to do splices on optical fibers. It is done using small almost handheld portable battery operated machines, even on the streets near the electrical panels, in cellars or wherever needed. The machines for optical fiber splicing are completely automated nowdays: fiber positioning, checking, splicing, final measurement, etc. Usually operated just with a few button presses. The only thing usually done outside the machine is fiber breaking: the fiber must be cut using a tool that breaks the fiber almost dead perpendicular.Doing fiber splices is a job for a trained monkey these days.

author: Beamin
author_rank: Super Contributor
timestamp: June 03, 2018, 07:38:08 pm
content: Quote from: Yansi on June 03, 2018, 06:18:55 pmDo they pay the monkeys in bananas? What's this tool called I imagine it's expensive have you got to play with one?

author: Yansi
author_rank: Super Contributor
timestamp: June 03, 2018, 07:45:17 pm
content: Yes I have, my dad is certified to use one. He is a member of the local networking association and took the course to learn how to operate these things.No they do not pay in bananas, but we get here in central EU unlimited fiber internet connection for a price of a few bananas instead.It is called a fusion splicer. And yes these are expensive toys for the big boys.

author: coppice
author_rank: Super Contributor
timestamp: June 03, 2018, 07:53:39 pm
content: Quote from: Yansi on June 03, 2018, 06:18:55 pmThe original strategy of building a clean room over the trench every time they got to the end of one reel of fibre, and needed to splice on the start of the next reel, proved somewhat impractical.

author: dmills
author_rank: Super Contributor
timestamp: June 03, 2018, 08:35:46 pm
content: Those are NOT modern broadcast boards, I would say late 90s probably, and I don't believe the process was just cleave and stuff into a hand held automated splicer back then. You don't need a clean room to cleave and splice fibre, but aligning it with the diode face and associated fast axis lens and then gluing it into place is best done in a reasonably clean environment....Besides, even today a fusion splicer is NOT a cheap bit of kit, and for small volume you would likely buy the laser assembly pre terminated (Actually, today you just stick an SFP cage on there and pop a module in, job done, cheaper and more flexible).Regards, Dan.

author: Beamin
author_rank: Super Contributor
timestamp: June 07, 2018, 04:28:57 am
content: Quote from: coppice on June 03, 2018, 07:53:39 pmHow can you say you need a clean room then say it's done on the streets or in cellars?A dusty pit on the side of the road is the opposite of a clean room. Maybe if the pit is muddy the air would be cleaner but still not clean, definitely not clean room clean.From the video it seems the splicer seals off the outside environment when you shut the door.