On Tue, 22 Sep 2020 17:47:03 -0700, John Larkin <jlarkin@highland_atwork_technology.com> wrote:>On Tue, 22 Sep 2020 15:14:18 -0700 (PDT), jrwalliker@gmail.com wrote: > >>On Tuesday, 22 September 2020 20:47:28 UTC+1, John Larkin wrote: >>> On Tue, 22 Sep 2020 11:52:01 -0700, Don Y >>> <blockedofcourse@foo.invalid> wrote: >>> >>> >Any COMPELLING reason to opt for single multimode fiber, instead? >>> >I'm only looking at a few hundred feet run. >>> > >>> >(the dual fiber solution is "free" for me) >>> >>> I'm about to get my first articles of a duplex fiber link, ECL in and >>> out at maybe 1M to 2 GHz. It's for testing missiles or something. >>> >>> I'm using a Cisco 10G 850nm multimode SFP module, which is probably >>> good for a few hundred meters at my speed. It will run through several >>> gymbal joints that have "fiberoptic slip rings" >>> >>> What's strange is that Amazon wants about $20 for the SFP. Digikey >>> wants $94. The list price is $1200. >>> >>> How fast do you plan to go? Need DC coupling? >> >>I bought four of those a few years ago on eBay for about $10 in total. >>Hardly anyone uses 850nm multi-mode anymore which is why they are so >>cheap. >>Single mode fibre is now usually cheaper than multimode. >> >>John > >I think the customer's optical slip ring things only work multimode. > >It's sort of mind bogling to make a duplex fiberoptic swivel joint, >and pipe light through 3 axes of twirling gymbals.See page six: .<https://www.schleifring.de/wp-content/uploads/2019/09/SCHLEIFRING_Surveillance.pdf> Schleifring uses the Dove Prism approach. See US Patent 5,157,745 to Gregory Ames of the US Navy. Also see Speer and Koch, "The diversity of fiber optic rotary connectors", SPIE Vol 839, Components for Fiber Optic Applications II, 1987, pages 122-129. Joe Gwinn
Single mode, dual fiber optical link downsides
Started by ●September 22, 2020
Reply by ●September 23, 20202020-09-23
Reply by ●September 23, 20202020-09-23
On Wednesday, September 23, 2020 at 6:44:11 AM UTC-7, Don Y wrote:> On 9/22/2020 5:09 PM, Grant Taylor wrote:> > If you have dual single mode fiber, you also have two single strand single mode > > fiber too. ;-) > I don't see what that buys me. IIRC, I can use multimode fiber for single mode > operation but not the reverse. So, I'm still stuck with two unidirectional > lanes (?)Oh, no; you cannot use multimode fiber for single-mode operation, because it will add dispersion and alter polarization... all the benefits of single-mode fiber are lost. Single-direction operation, yes. Single-mode operation, no. Two unidirectional lanes isn't 'stuck', it's 'well-equipped' (though having a spare would be nice, too).
Reply by ●September 23, 20202020-09-23
On 9/23/20 7:44 AM, Don Y wrote:> I can use multimode fiber for single mode operation but not the > reverse.I have a friend and colleague that tried to run single-mode optics over a multi-mode cable. It didn't go well. It would just barely work well enough for the SAN equipment to talk to each other. But as soon as he tried putting any real traffic (not even full load) over it, it failed miserably and lost link. That was 2 or 4 Gbps Fibre Channel. Anything 10 Gbps or above uses signaling more complex than simple on-off keying. So you start running into a LOT more problems.> So, I'm still stuck with two unidirectional lanes (?)Yes, that's how fiber links traditionally work. You have to use special Bi-Directional optics to put both directions on a single fiber. They work by using different wavelengths so that they won't (destructively) interfere with each other. -- Grant. . . . unix || die
Reply by ●September 23, 20202020-09-23
On 9/23/2020 12:50 PM, whit3rd wrote:> On Wednesday, September 23, 2020 at 6:44:11 AM UTC-7, Don Y wrote: >> On 9/22/2020 5:09 PM, Grant Taylor wrote: > >>> If you have dual single mode fiber, you also have two single strand single mode >>> fiber too. ;-) >> I don't see what that buys me. IIRC, I can use multimode fiber for single mode >> operation but not the reverse. So, I'm still stuck with two unidirectional >> lanes (?) > > Oh, no; you cannot use multimode fiber for single-mode operation, because it will > add dispersion and alter polarization... all the benefits of single-mode fiber > are lost. Single-direction operation, yes. Single-mode operation, no.You add a mode conditioning patch cable. You don't magically get all the benefits of single mode but the cable isn't "wasted" or a multimode converter needed.> Two unidirectional lanes isn't 'stuck', it's 'well-equipped' (though having a spare would > be nice, too).
Reply by ●September 23, 20202020-09-23
On 2020-09-22 20:47, John Larkin wrote:> On Tue, 22 Sep 2020 15:14:18 -0700 (PDT), jrwalliker@gmail.com wrote: > >> On Tuesday, 22 September 2020 20:47:28 UTC+1, John Larkin wrote: >>> On Tue, 22 Sep 2020 11:52:01 -0700, Don Y >>> <blockedofcourse@foo.invalid> wrote: >>> >>>> Any COMPELLING reason to opt for single multimode fiber, instead? >>>> I'm only looking at a few hundred feet run. >>>> >>>> (the dual fiber solution is "free" for me) >>> >>> I'm about to get my first articles of a duplex fiber link, ECL in and >>> out at maybe 1M to 2 GHz. It's for testing missiles or something. >>> >>> I'm using a Cisco 10G 850nm multimode SFP module, which is probably >>> good for a few hundred meters at my speed. It will run through several >>> gymbal joints that have "fiberoptic slip rings" >>> >>> What's strange is that Amazon wants about $20 for the SFP. Digikey >>> wants $94. The list price is $1200. >>> >>> How fast do you plan to go? Need DC coupling? >> >> I bought four of those a few years ago on eBay for about $10 in total. >> Hardly anyone uses 850nm multi-mode anymore which is why they are so >> cheap. >> Single mode fibre is now usually cheaper than multimode. >> >> John > > I think the customer's optical slip ring things only work multimode. > > It's sort of mind bogling to make a duplex fiberoptic swivel joint, > and pipe light through 3 axes of twirling gymbals. >Depending on the application, you can do continuous rotation using the Indian dancer's trick. Try it with a can of soda: hold it by the bottom, up near your ear. Rotate it towards you as you pass it under your armpit and back up next to your ear. Unlimited continuous rotation without a rotating joint. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 23, 20202020-09-23
On 2020-09-22 22:46, Grant Taylor wrote:> On 9/22/20 6:47 PM, John Larkin wrote: >> I think the customer's optical slip ring things only work multimode. >> >> It's sort of mind bogling to make a duplex fiberoptic swivel joint, >> and pipe light through 3 axes of twirling gymbals. > > The only slip ring that I've seen was for a single fiber. Hence why we > /had/ /to/ use Bi-Di optics over a single fiber. > > The device we were using was single-mode. But I (naively) assume that > there are multi-mode counterparts. > > >If you have a hollow shaft and really good bearings, you can do that for a single channel--it just has to stay within a micron or two of the rotation axis. MM allows a lot more slop. Some fibre slip rings intended for solid shafts use one or two MM fibres on the stationary side, illuminating a split fibre bundle that fans out into a ring around the outside of the rotating side, so that at least one Rx fibre is illuminated at all times. That requires a larger detector, which is slower. Alternatively you can do it the other way round, which is faster but wastes beaucoup laser power. It's easier at smaller radii. One could think of weirder things, e.g. a circular optical waveguide whose core is exposed on one side, and a sliding tap using some coupling fluid and frustrated TIR. Dunno if anybody has done that. It would produce some entertaining data synchronization problems at higher speeds, but probably not as bad as a sliding metal contact. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 23, 20202020-09-23
On 2020-09-23 15:50, whit3rd wrote:> On Wednesday, September 23, 2020 at 6:44:11 AM UTC-7, Don Y wrote: >> On 9/22/2020 5:09 PM, Grant Taylor wrote: > >>> If you have dual single mode fiber, you also have two single strand single mode >>> fiber too. ;-) >> I don't see what that buys me. IIRC, I can use multimode fiber for single mode >> operation but not the reverse. So, I'm still stuck with two unidirectional >> lanes (?) > > Oh, no; you cannot use multimode fiber for single-mode operation, because it will > add dispersion and alter polarization... all the benefits of single-mode fiber > are lost. Single-direction operation, yes. Single-mode operation, no. > > Two unidirectional lanes isn't 'stuck', it's 'well-equipped' (though having a spare would > be nice, too). >That's not the issue. Fibre modes are orthogonal, and your average piece of MMF has about 100 of them. Light gets spread out through the mode volume and loses phase coherence pretty fast, so at the other end you can couple at most 1% of the light from MMF into SMF, compared with 100% in the other direction. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 23, 20202020-09-23
On 24/09/2020 07:37, Phil Hobbs wrote:> On 2020-09-22 22:46, Grant Taylor wrote: >> On 9/22/20 6:47 PM, John Larkin wrote: >>> I think the customer's optical slip ring things only work multimode. >>> >>> It's sort of mind bogling to make a duplex fiberoptic swivel joint, >>> and pipe light through 3 axes of twirling gymbals. >> >> The only slip ring that I've seen was for a single fiber. Hence why >> we /had/ /to/ use Bi-Di optics over a single fiber. >> >> The device we were using was single-mode. But I (naively) assume that >> there are multi-mode counterparts. >> >> >> > > If you have a hollow shaft and really good bearings, you can do that for > a single channel--it just has to stay within a micron or two of the > rotation axis. MM allows a lot more slop.The designs I have seen usually put a collimating lens on the source fibre and pass an expanded beam across the gap between the rotating parts, and then focus it onto the receiving fibre with another lens. That allows a lot more translational slop, though only limited angular slop (which is probably fine if there is more than one rows of balls / rollers in the bearing races on the axis). Some of the multi-channel ones with geared dove prisms look quite mechanically complicated and expensive, and are probably better avoided by using some some sort of WDM or digital multiplexing. Still, there seem to be a lot of fibre optic slip rings available, with up to 16 channels from several suppliers, in a choice of single or multi-mode versions.
Reply by ●September 23, 20202020-09-23
On 2020-09-23 20:18, Chris Jones wrote:> On 24/09/2020 07:37, Phil Hobbs wrote: >> On 2020-09-22 22:46, Grant Taylor wrote: >>> On 9/22/20 6:47 PM, John Larkin wrote: >>>> I think the customer's optical slip ring things only work multimode. >>>> >>>> It's sort of mind bogling to make a duplex fiberoptic swivel joint, >>>> and pipe light through 3 axes of twirling gymbals. >>> >>> The only slip ring that I've seen was for a single fiber. Hence why >>> we /had/ /to/ use Bi-Di optics over a single fiber. >>> >>> The device we were using was single-mode. But I (naively) assume >>> that there are multi-mode counterparts. >>> >>> >>> >> >> If you have a hollow shaft and really good bearings, you can do that >> for a single channel--it just has to stay within a micron or two of >> the rotation axis. MM allows a lot more slop. > > The designs I have seen usually put a collimating lens on the source > fibre and pass an expanded beam across the gap between the rotating > parts, and then focus it onto the receiving fibre with another lens.BITD expanded-beam connectors were fairly common, but IIRC Amphenol is the only company making them these days.> That allows a lot more translational slop, though only limited angular > slop (which is probably fine if there is more than one rows of balls / > rollers in the bearing races on the axis). > > Some of the multi-channel ones with geared dove prisms look quite > mechanically complicated and expensive, and are probably better avoided > by using some some sort of WDM or digital multiplexing. Still, there > seem to be a lot of fibre optic slip rings available, with up to 16 > channels from several suppliers, in a choice of single or multi-mode > versions.A Dove prism rotates the image at twice its own speed, interestingly without doing a great lot to the polarization. (It will go mildly elliptical, but a single TIR bounce off an air/low index glass causes way less retardation than a Fresnel rhomb, which is a pretty good achromatic quarter-wave plate. Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
Reply by ●September 26, 20202020-09-26
Don Y <blockedofcourse@foo.invalid> writes:>That was, essentially, the question: IS there any reason to >avoid the dual single-mode solution? Yeah, it "looks" a bit >more dated...The cost of connectors and terminating doubles. It may be that is trivial in your case, but if deploying over a large campus, etc.....>> Single fiber usually means that you need Bi-Directional >> optics which are more rare and more expensive.A friend who deploys systems as I mentioned said sources from fs.com, amid others & said: A simplex, dual color (bidi sfp) gigabit SFP module is $12, a duplex (dual fiber) is $9. So for $6 a link, you can get by with 1 strand. $6 doesn't even buy the connectors for the ends of the cable. -- A host is a host from coast to coast.................wb8foz@nrk.com & no one will talk to a host that's close.......................... Unless the host (that isn't close).........................pob 1433 is busy, hung or dead....................................20915-1433
