When you bend or stress an optical fibre, even slightly, it changes the way that light propagates along it. An interesting use of this phenomenon is the concept of a fibre-optic microphone. You send a pulse of light down the fibre, and "listen" for the echo: the scattering, dispersions and reflections caused by stresses on it. With a fast enough box of electronics, you can calculate what sound waves were hitting the cable at any given point along its length.

This is of great use to the mining industry. Optical fibres are cheap (even if the box of electronics isn't) so you can lay a few kilometres of cable all over the ground before you blow it up. Apparently, knowing what your explosions sound like from a thousand different angles is quite useful.

Proposed is to take these relatively cheap optical fibres and embed them inside the ropes used for climbing and caving. In the event of an emergency, with someone trapped or stranded somewhere along the line, a rescue team could plug into the rope's optical fibre and listen for anyone crying for help. They would even get a pretty accurate idea of how far along the rope the person is located.
-- mitxela, Mar 29 2019

Fiber Bragg grating https://en.wikipedi...Fiber_Bragg_grating
Mentioned in my anno. Lets you make measurements at predetermined points along a fiber [notexactly, Mar 31 2019]

Distributed Acoustic Sensing https://en.wikipedi...ed_acoustic_sensing
[mitxela, Apr 01 2019]

A fiber optic microphone would make a nice addition to a phone to improve room-wide audio and phone video sound quality.
-- beanangel, Mar 30 2019


I thought the only information you can get about where an event is happening along a fiber optic cable is where a reflection occurred. Refraction changes chromatic and modal dispersion but I wouldn't think you'd know where those events happened, only that one light wave profile went in and another came out. You can precisely measure where a reflection event happens by timing how long it takes for the pulse of light you sent out to hit the thing, a cable splice or a break, and then bounce back.

Are you saying they blow up fiber optic cables for some reason to create a profile of the explosion or something? Do you have a link?
-- doctorremulac3, Mar 30 2019


I believe the idea is to deploy loads of microphones/seismometers all over the ground, set off an explosion, and record the resulting shockwave and its echoes at many locations simultaneously. Analysing the results lets you build a model of what's under the ground, hopefully telling you that there's something down there worth drilling for.
-- Wrongfellow, Mar 31 2019


// where a reflection occurred //

Strictly speaking I think it's only the scattering that's used for fibreoptic sensors. There is a lot of scattering in a fibre, and it's wavelength dependent. For communications they choose wavelengths where scattering and absorption are minimized.

The reconstruction for fibreoptic sensors consists of having a very fast ADC, and running a bunch of deconvolutions (possibly in matlab).

I once interviewed someone who had worked for a company that built these devices. I don't have a link but he said that their product was able to sample at acoustic bandwidth along a 2km fibre with a spatial resolution of 1 metre.

For my proposed idea, there are at least two problems I can think of so far. Will it work if the rope is tied into a knot? Most fibres leak and eventually stop working if they go through a bend radius of less than about an inch. Possibly the fibre could be coiled up inside the rope to minimize this. Secondly I don't know how sensitive the microphone can be, obviously explosions are a lot louder than a measly person's voice. Third, fall-arrest rope used by recreational climbers (but not cavers) is stretchy, because they normally don't put any weight on the rope until they fall. Coiling the fibre inside the rope could also work here.
-- mitxela, Mar 31 2019


//Strictly speaking I think it's only the scattering that's used for fibreoptic sensors.//

Well, yes but the only thing measured is the backscattered light, the part of the light pulse that comes back after hitting something. It's basically optical RADAR, at least with an OTDR. Unless you're talking about measuring dispersion. Sorry if I'm a bit foggy on what we're doing here. Not saying it won't work, I'm just unclear on what you're measuring and how you're measuring it. Are there devices on both sides of the cable or are you just using an OTDR type of device at one end?

OK, how's this? You've got a zig-zagged fiber cable, each angle beyond the bend radius of an un-occluded cable. So as the person hangs from it, it straightens it out. Light passes freely from the point where the person is hanging and as they move, the cable / fiber thingy which is springy moves up and down giving a varying reflective point.

So you'd be able to say "OK, the person is hanging 125 feet down the hole and he's stopped moving because we're getting a stable end backscatter point. Ooops, he's moving again because the backscatter point is bouncing around as he imparts movement to the rope."

Problem with that fancy approach is you can basically get the same info with a rope with footage markings on it.
-- doctorremulac3, Mar 31 2019


[+] Yep. I've been thinking in the past few months of using fiber Bragg gratings (or, if those aren't sensitive enough, a more conventional optically-read seismometer fed by a fiber) for semi-passive downhole seismometry (plus thermometry, and maybe optical heating for heat flow measurement) on Mars. I'm planning to have deployable surface stations that the balloons can carry to various places and install, and I want no electronics to go down the hole, just the stacer with a pneumatic drill on the end and optically-connected sensors all the way down. That way all the electronics can easily be kept in the warm electronics box at the surface.

…I should really post the whole mission plan sometime, so you have context for all of that.

// I thought the only information you can get about where an event is happening along a fiber optic cable is where a reflection occurred. Refraction changes chromatic and modal dispersion but I wouldn't think you'd know where those events happened, only that one light wave profile went in and another came out. You can precisely measure where a reflection event happens by timing how long it takes for the pulse of light you sent out to hit the thing, a cable splice or a break, and then bounce back. //

I don't know (well, didn't until I read the second quote after this) if you can make spatially resolved measurements without them, but if you add fiber Bragg gratings [link] to your fiber, you can set specific known points along it at which you can measure stretch and/or temperature.

// Strictly speaking I think it's only the scattering that's used for fibreoptic sensors. There is a lot of scattering in a fibre, and it's wavelength dependent. //

Interesting. Links?

// I once interviewed someone who had worked for a company that built these devices. I don't have a link but he said that their product was able to sample at acoustic bandwidth along a 2km fibre with a spatial resolution of 1 metre. //

Impressive! Do you remember what company?

// So you'd be able to say "OK, the person is hanging 125 feet down the hole and he's stopped moving because we're getting a stable end backscatter point. Ooops, he's moving again because the backscatter point is bouncing around as he imparts movement to the rope." //

I feel like the fiber length to the point at which the person is hanging would be constant, but the transmission around each bend would vary periodically.
-- notexactly, Mar 31 2019


//but if you add fiber Bragg gratings [link]//

That's one big reason I come to this site, to learn stuff. Thanks for the link.

//I feel like the fiber length to the point at which the person is hanging would be constant, but the transmission around each bend would vary periodically.//

Yea, that's right. Same length, but light passes through the straightened fiber better than through the kinked fiber. The lower down he goes, the more straightened fiber. Great idea except there's no good reason to do it.
-- doctorremulac3, Mar 31 2019


// OTDR type of device at one end // Pretty much. Device is just at one end.

//// There is a lot of scattering in a fibre, and it's wavelength dependent. //// Interesting. Links? // Mostly rayleigh scattering, which is the same mechanism that makes the sky blue, and has a 1/T^4 dependence. Absobtion graphs for fibres have a few characteristic peaks and valleys due to other effects too, the wavelengths for communications are usually chosen to fit into those valleys.

// Do you remember what company? // Sorry, no, but look at the [link] for more about it. Googling gave me at least one result which claimed similar numbers so I don't think he was making it up.
-- mitxela, Apr 01 2019


RE: link.

That's really very cool.
-- doctorremulac3, Apr 01 2019



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