"Laser listener" bugging works on skin, clothing

This is an ancient technique. The speckle pattern moves at twice the speed of the objects--it's been used to measure the motion of paper in hand-held scanners such as the HP Capshare 920 (circa 1996), and is the basis of electronic speckle pattern interferometry (ESPI, more commonly called TV interferometry).

You do have to be close enough to actually get a speckle pattern before it will work, though. The speckles get larger as you go further away, and that plus the inverse square law will make the technique less sensitive pretty fast.

Of course, if you're wearing your aluminum foil beanie with the shiny side out, it's a nice specular reflector and won't make a lot of speckle. ;)

Cheers

Phil Hobbs

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Cripes! Now you've given those foil hat types a rational excuse to use in avoiding capture by the straight jacket types. ;)

Jon

Laser bounced off window pane is older than god, curtains came right after. The Russkies did a pretty with a microwave resonater by Leon Theremin in the Moscow US Embassy.

And use it for aiming and distancing the sniper, too. Laser Ruby Ridge for closure.

So how come were getting our asses kicked in Afghanistan? Hell, everywhere.

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Could it be the N $ A has all their eyes and ears focused inside the US borders?

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Hi Phil! Bugging devices using laser speckle are ancient?

You sure about that? Got a ref or two? (And someone should tell the OSA that they're announcing embarrassingly old techniques.)

So, then who first realized that we can use it for remote listening?

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty, chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph206-762-3818

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om/uncleal/qz4.htm

May I plug the book "Spycraft.". Lots of stories about such gear in there. I've seen the paperback version in "remainders" at this point. About $5.

I just got a paperback to give away at Half Price books. Absolutely worth the read.

The Russians bugged the American Embassy in this manner back in the

1980s.

People publish old techniques all the time. I've seen papers that reproduced things I did 15 years before, e.g. the invention of solid immersion microscopy.

I don't know about the bugging application, but the speckle business has been done to death since about the 1970s.

No idea. But speckle+TV as a sensitive measuring device is as old as the hills.

Cheers

Phil Hobbs

e

I've only read about it, but a moving speckle pattern makes a nice noise source.

Say, is there any limit to the frequency that can be obtained with a moving speckle pattern?

George H.

In principle it's limited only by the bandwidth of the light. If you have an interference pattern from two identical beams of powers P1 & P2 (each measured with the other beam blocked), the detected signal will be

I_sig = R(P1 + P2 + 2 sqrt(P1 P2) cos theta),

where R is the detector responsivity and theta is the phase angle between the beams. This form is simplified, of course--you really have to do an integral of the fields across the detector. However, it shows the structure of the result--the total detected signal equals the sum of the signals from each beam alone, plus the interference term, which is what gives rise to fringes as theta varies.

Fringes, including speckle, start showing reduced contrast as the time delay between different components starts to approach the coherence length of the source--theta becomes a random variable whose variance increases without limit, and whose variations occur in a bandwidth of

2*BW_3dB of the laser.

However, that doesn't mean that the interference goes away--once you're beyond the coherence length, its *time average* goes to 0, but the full interference term is still there, converted into noise that spreads out over twice the laser bandwidth. That's often the dominant noise source in fibre measurements, for instance.

In broad-beam applications, you get some spatial averaging that tends to make theta vary with position, and so the integrated interference term is not as big a problem.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

Yeah, I saw papers back then as an undergrad. But never a peep about remote listening.

Sure. And I've heard of bugging via conventional interferometry (specular target, basically same as laser window reflections, but more sensitive.) But I've never heard of scattering-based laser bugs; listening remotely to people's vibrating clothing or faces from several hundred feet. The paper gives examples of voices picked up from skin at 100m, and from cellphone case at 60m, using

64x64 pixels.

Unless the authors are hiding some problems, it means that we could fairly easily build a binoculars-device which picks up voices from nonspecular targets at at least a few hundred feet range.

Go watch dealextreme.com to see how quickly such products appear?

((((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty, chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph206-762-3818

The origional 'laser listener' barely works, so it attracts little hobbyist interest. But this is something different. Check out that paper. It uses scattered reflection, and so can listen to the vibrating curtains. No need to reject window vibrations or even find the bounced beam, instead we take our audio from a laser spot aimed at your shirt from a few hundred ft.

Perhaps a hobbyist version could use a line-array from a fax machine?

Simple optical setup detects speech remotely Laser Focus World, Jan 2010,

(((((((((((((((((((((( ( ( (o) ) ) ))))))))))))))))))))))) William J. Beaty Research Engineer beaty, chem washington edu UW Chem Dept, Bagley Hall RM74 billb a eskimo com Box 351700, Seattle, WA 98195-1700 ph206-762-3818