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



From: Blake Hadley <moosehadley@gmail.com>
To: This 1s <this1s@mailbox.org>
Cc: CypherPunks <cypherpunks@cpunks.org>
Sent: Thursday, June 16, 2016 12:36 PM
Subject: Re: Turning optical fiber cable into microphone

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.

On Thu, Jun 16, 2016 at 1:56 PM, This 1s <this1s@mailbox.org> wrote:
Looking for stuff I found this post:

Is internet cable in house a hot mic?
Is it the end of the fiber, the field around the cable, or both?

Thanks in advance :D