Turning optical fiber cable into microphone

grarpamp grarpamp at gmail.com
Thu Jun 16 18:01:27 PDT 2016


On 6/16/16, jim bell <jdb10987 at yahoo.com> wrote:
> Presumably, the light (probably actually the IR, or infrared) leaves the end
> of the fiber, and then reflects off a (very nearby, say 10 micron away)
> flat, ultra-thin membrane (perpendicular to the fiber) which has been coated
> to make it optically reflective.  The resulting reflection re-enters the
> fiber, and returns to the source.    The light source (presumably a laser)
> is then optically-mixed with the reflection IR, and this results in an
> FM-modulated signal.         Jim Bell

> The way they describe it, they send light down a cable, then watch for the
> reflection to come back to them.
> The reflection should look the same with every single pulse of light, unless
> something affects it, which in this case would be the vibrations of sound.
> So it should pick up sound along the entire length of cable, which would
> return a lot of ambient noise if you're trying to hear inside someone's
> house from all the way back at a utility company.
> I don't know how good their audio filtering abilities are, but it seems like
> wherever you take measurements can't be too far from what you want to hear.

>  Looking for stuff I found this post:
> http://blogs.wsj.com/tech-europe/2012/09/04/turning-a-fiber-optic-cable-into-a-microphone/
> Is internet cable in house a hot mic?Is it the end of the fiber, the field
> around the cable, or both?

Without searching for their mentioned acronym and tech, and since
any moron can setup what amounts to a $100 optical audio bounce
off a glass window pane, be it a tiny polished fiber end or bedroom
window...

given the frequency limit, distances, and going more exotic, I was
thinking audio / groundwave causing lateral physical stress upon
the fiber inducing lensing effects possibly detectable by insane
OTDR / DSP tech.

Then again, I'm insane.



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