Fibre light for power - Page 1
Forum section: electronics / beginners

author: msknight
author_rank: Regular Contributor
timestamp: December 14, 2021, 11:53:02 am
content: This is going to be a really stupid question, but I'm going to ask it anyway.One of the problems with fibre telephone systems is that it relies on the remote end having power. Current UK phone systems (mostly) are powered by the telephone exchange over copper. We've had the situation where Storm Arwen took out telecoms for many homes and as there wasn't a mobile phone signal, they were left without means of telephoning anyone, not even emergency services.At the receiving end, what's to stop some of the light transmission being collected by solar cells and used to power the receiving unit as well as carrying the signal?If the light was kept "on" rather than "dark" when there was no active signal being transmitted, it could also be used to charge a battery at the receiving end to cope with short term larger current draws? This would also presume a stronger light could be transmitted by the telephone exchange and the remote station could lower the strength of its light (and thus power consumption) to a level where the telephone exchange finds acceptable.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 14, 2021, 12:27:17 pm
content: Yes I can be done. Kinda I had a video isolator that used a laser diode to power a circuit via a fibe optic link.But that link was a few cm of bundled fibres. Losses are at least 40% per km. Put another way, after 5km you only have 1% of the laser light. 0.01% at 10km. The fibre cable from the exchange would need to be very thick and requires very high power lasers at the exchange.Very expensive and very dangerous.

author: Steffalompen
author_rank: Contributor
timestamp: December 14, 2021, 05:22:14 pm
content: Aren't fiber comms dependent on repeater stations? I presume your local switching box or whatever it's called would be powerless too, in an outage.

author: nfmax
author_rank: Super Contributor
timestamp: December 14, 2021, 05:39:53 pm
content: Remotely-pumped optical amplifiers are a thing, but in that case there is no conversion to electric power involved. Amplifiers can be a long way from the pump laser - at least multiple 10s of kilometres. But the pump powers involved are most definitely not safe.As far as I know the local fibre to the home network is all passive, and operates at (multiple) wavelengths, all beyond the band gap of silicon. In theory, you could add a power wavelength at say 980nm, but as the optical power is split 16, 32, or more ways in the local network and the losses are high at such a short wavelength, there wouldn’t be much left at the NTE.

author: ve7xen
author_rank: Super Contributor
timestamp: December 14, 2021, 08:46:11 pm
content: In theory I suppose it is possible, but it would be totally impractical. Typical GPON maxes out at around 6dBm (4mW) of transmit power, passively split 32 or 64x, so the ONT receives only maybe -10dBm, which is 10-100s of microwatts. Even if you could harvest 100% of that optical power (you won't come close), it's not enough to be useful.If you wanted to throw more power for this purpose alone, not only would you need relatively expensive filtering / splitting optics at the receiver to protect it, you would run into eye safety issues. Anything running into a layman's home is going to have to be intrinsically eye-safe (Class 1), so the maximum power is quite low, maybe 1mW. Still not very useful, even ignoring the insertion loss of the cabling system and the cost/safety problems of the many-watt laser you will need at the head-end for each 32 customers.And you can expect that anywhere you are going to run this is going to have local power almost all of the time anyway, so this would only be used very rarely. It makes far more sense to just provision batteries at the ONT, and when fibre services are replacing copper, at least around here, that is typically part of the default installation to provide a few hours of standby power for the phone lines.

author: nfmax
author_rank: Super Contributor
timestamp: December 14, 2021, 08:53:56 pm
content: Quote from: ve7xen on December 14, 2021, 08:46:11 pmUnfortunately, not in BT OpenReach landI had to provide my own UPS, which sustains not only the ONT but also the router, PoE switches, WiFi base station, etc.

author: Ranayna
author_rank: Super Contributor
timestamp: December 15, 2021, 11:11:47 am
content: There is stuff like that available:https://www.fiberopticlink.com/product/fiber-optic-isolation-systems/power-solutions-for-fiber-optic-isolation-systems/power-over-fiber-system-pof/#featuresBut they won't tell you their price, so expect it to be expensive as hell. Also 1 Watt is not all that much electrical power. They don't mention the laser power of the transmitter anywhere. I hope they have a mechanism to detect if someone unplugs the transmitter though, i expect that laser to be nasty.

author: voltsandjolts
author_rank: Unknown
timestamp: December 15, 2021, 12:19:06 pm
content: For backup purposes, I think it would be much simpler to have a wind-up phone charger or such like, rather than reinventing the fibre phone network.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 15, 2021, 05:53:51 pm
content: Or a battery

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 15, 2021, 06:11:15 pm
content: Alf Traeger and his pedal wirelesshttps://nla.gov.au/nla.obj-142495257/view

author: Gregg
author_rank: Super Contributor
timestamp: December 15, 2021, 10:41:04 pm
content: Almost all fiber repeaters and utility controlled end terminals that I know of in the US have at least battery backup. These are usually 48V nominal sealed lead acid and have around 4 to 8 hour design backup capability. Also fiber providers have portable generators and hopefully will dispatch technicians before the batteries fail. The major repeater sites have built in generators capable of days to weeks of backup operation.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: December 15, 2021, 11:43:24 pm
content: It is my understanding that most telecom fiber sources are IR lasers at around 1300nm to minimize absorption in the fiber. I'm not familiar with a good way to convert that wavelength to electrical potential.

author: james_s
author_rank: Super Contributor
timestamp: December 15, 2021, 11:56:01 pm
content: There's really no way I can think of around the fact that a typical fiber ONT draws perhaps ~10 watts, and the optical power going into the fiber is I would guess a few tens of milliwatts. They are also bidirectional over a single fiber as far as I know, so you need a laser in the end that's in your house too. A battery is much more practical.

author: mc172
author_rank: Frequent Contributor
timestamp: December 16, 2021, 12:02:45 am
content: Quote from: Ranayna on December 15, 2021, 11:11:47 amI love the DANGER sign on the side suggesting that you avoid exposure to the beam of light that you can't see.

author: ve7xen
author_rank: Super Contributor
timestamp: December 16, 2021, 01:35:40 am
content: Quote from: Ranayna on December 15, 2021, 11:11:47 amIt does say it consumes 75W of input power, and requires 2 strands of fibre (plus one for the safety interlock) to deliver 1W (electrical) to the load at 32'. Gives some idea of the efficiency. My uneducated guess would be that they are using 2x5W (optical) laser diodes, but that does assume some reasonably efficient way to turn the optical power back to electrical. Loss in the fibre system at such a short distance should only be 1dB or less, so 8W gets to the receiver, if you can recover 12% of it then you get your 1W.It seems to claim it's a Class 1 laser product which is frankly ridiculous. They do have a feedback mechanism to shut off the laser in the event of a fault, but it's on a separate fibre strand, with a separate connector, so it'd be absolutely trivial to expose yourself to one of the power beams. IMO this doesn't actually qualify as a Class 1 device, since it emits Class 4 laser radiation that is reasonably accessible. You could engineer your way around this though with a better safety interlock (but this requires fancier and efficiency killing optics), and maybe get away with it. Still don't think telcos would want the liability (and definitely not the cost) of installing something like this at a residence.It's kind of crazy that it's worth doing ever, but I guess I can imagine some isolated high voltage scenarios.QuoteWe are talking about the ONT in a subscriber's home, not the backhaul infrastructure or even the OLT.

author: james_s
author_rank: Super Contributor
timestamp: December 16, 2021, 03:38:31 am
content: Quote from: mc172 on December 16, 2021, 12:02:45 am1W of coherent optical power is extremely dangerous, especially if it's a near-IR wavelength that is nearly invisible but can still be focused onto your retina. I believe that warning is required even if you can't be exposed without disassembling something.

author: msknight
author_rank: Regular Contributor
timestamp: December 16, 2021, 11:21:32 am
content: We do have a bit of a problem in the UK. The fibre cabinets do have batteries in them, four hours I believe, but they are a target for thieves. The domestic home has its own problems as people don't seem to be aware of power backup, a number of our new homes are so small that you can put a double bed in the master bedroom, you can sit on the end and your knees can touch the wall... so having somewhere to put a UPS is likely the lowest of the low on people's priorities. The system itself needs to have a solution to the problem, IMHO.We had Storn Arwen which proved the point when areas lost power for something like ten days and the VoIP failed completely. With no mobile phone coverage, that was out of the question as well, and even then a mobile phone would have been out of the loop after about a days worth of charge.Lack of planning and investment (and some nimbyism) mean that there was no way for those people to cope. It's taken people a bit by surprise that in the UK, mains power could be out for so many people, for such a long time (and the inevitable heating and cooking issues especially in winter) ......and the only solution I can see is to be able to somehow pass power from the main telephone exchange to the domestic premises (as we do now) but with the removal of copper, there is no easy solution around.

author: mc172
author_rank: Frequent Contributor
timestamp: December 16, 2021, 02:30:25 pm
content: Quote from: james_s on December 16, 2021, 03:38:31 amI'm not saying that it's not dangerous, the concept of actively avoiding something not visible just amused me.

author: Ranayna
author_rank: Super Contributor
timestamp: December 16, 2021, 02:44:37 pm
content: They are saying to use 830nm Laser, that should be visible. At least I can see the 850 nm of standard multimode SFP modules. And I curse every time I see them, that my stupid colleagues did not fit the rubber bung to unused ports, or unplug the module.

author: james_s
author_rank: Super Contributor
timestamp: December 16, 2021, 06:10:52 pm
content: Quote from: mc172 on December 16, 2021, 02:30:25 pmElectricity is invisible too under most circumstances, it's still perfectly sensible to warn somebody about the presence of dangerous voltages so they don't go poking around unaware. Avoid exposure, ie don't poke around inside this thing while it is powered up.

author: james_s
author_rank: Super Contributor
timestamp: December 16, 2021, 06:13:10 pm
content: Quote from: Ranayna on December 16, 2021, 02:44:37 pmYou can see it in the sense that you can see a bit of light, but it is *vastly* brighter than it appears. The eye doesn't have a hard cutoff where light goes from visible to invisible at a specific wavelength, it gradually tapers off beyond the peak.

author: LaserSteve
author_rank: Super Contributor
timestamp: December 17, 2021, 04:00:05 am
content: A good rule of thumb is that of you can detect anything over 670 nm coming out of a fiber, then you are probably at risk. 808 nm lasers often radiate considerable incoherent red light, and that is often what people see. The red spontaneous emission is a tiny fraction of the laser's actual power. Be careful with 800 nm sources. 800 nm band is strongly absorbed by nerve tissue and blood veins, often goes right through skin till it hits more critical tissue. It is a whopper of a retinal hazard as well.Power over fiber or power over free space is usually a bad idea except for very short distances. Solar is generally a better idea then POF. Conversion to energy at the far end of the fiber is generally Very Poor at best.LaserSteve sends.

author: Ranayna
author_rank: Super Contributor
timestamp: December 17, 2021, 08:58:27 am
content: @LaserSteve: I'm sure that you are right about that. That's why i'm always a bit pissed when i see that my esteemed colleagues never bother plugging unused fibre ports. There is one room in particular, where the switch is nicely on eye height as soon as you go through the door. And of course half the ports are unplugged, but openBack in the day, i have to admit, i routinely checked the light presence of short range 850 nm fibres by looking at them. And while it might be my imagination, i always had a bad feeling in my eyes after i did that with a couple of modules. I stopped doing that now.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 17, 2021, 01:10:33 pm
content: Somewhere around 850 nm would be the point that if you can see the light, you would be above the level where it could also so damage to the eye. Chances there is a bit of unintended red light (e.g. 700 nm range) with some source.One can see some light from some IR remoted, but this are the short wave tail, not the main wavelength.SI photodiodes are quite efficient in the NIR range (e.g. 800 nm) and could reach >50% efficiency there. Some 0.7 V voltage is possible for a relatively small area diode and the quantum efficiency is commonly better than 90% in that wavelength range.Power via fiber is more like a thing for remote sensors, not practical for the phone. If needed get an UPS for the phone system.

author: ve7xen
author_rank: Super Contributor
timestamp: December 17, 2021, 06:31:01 pm
content: Quote from: Ranayna on December 17, 2021, 08:58:27 amNormal SFPs are Class 1 devices with some single-digit mW of output at most, so should be eye-safe under all conditions. The light is also not collimated, so will diverge quickly away from the aperture. Unless you're working with long-haul or DWDM transport, nothing you encounter in a normal data centre would be a risk. I wouldn't be worried about incidental exposure, but I wouldn't be staring down the ferrules of random fibres, either.