Vibration Measurement: Laser-speckle monitoring could allow remote listening

Laser-based sensing of mechanical vibrations has become a relatively common high-end technique for characterizing mechanical structures—it can also be used to remotely and covertly capture conversations. Existing sensing approaches include interferometry, which senses displacement, and laser Doppler vibrometry, which uses a frequency-shifting Bragg cell and interferometry to measure velocity. In addition to complexity and sensitivity to air turbulence and such, both these techniques are limited to distances less than the lasers’ coherence length.

Silvio Bianchi, a researcher at the Università di Roma Sapienza (Rome, Italy), has developed a simple alternative, where a single-pixel detector (or camera) monitors the intensity of a small portion of the scattered light from a remote vibrating surface; the resulting fluctuations produced by shifts in the laser-speckle pattern result in a measurement of the surface’s vibration.¹

In the experimental setup, a 532-nm-emitting laser and associated optics illuminate a test membrane a distance away. Scattered light creates a speckle field, a small portion of which is captured by an apertured detector (see figure). An audio speaker near the membrane is used to set up test vibrations. The single-pixel detector is simulated by a digital camera, which also allows the capture of extra experimental info.

The simplest case, in which a local tilt on the membrane simply causes a lateral shift of the speckle pattern across the detector, was assumed first. The camera was positioned 240 mm from the target membrane, acquiring frames at an 800 Hz frame rate for 2 s while the membrane vibrated at 150 Hz.

Experiments were performed for camera apertures of varying sizes; because the speckle size was about 100 μm in the particular setup, the aperture sizes that produced the most change as the speckle pattern moved had diameters of between 60 and 90 μm. When the diameter was made more than 500 μm in diameter, the changes in intensity were under 10%.

Single-pixel version

A single-pixel photodiode with a 1-mm-diameter aperture was then set up 5 m away from the target membrane and sampled at 8 kHz while the experimenter’s voice drove the membrane vibration. The output as a function of time clearly showed the imprint of the voice. (Such a setup could easily be modified to collect the same portion of scattered light at a distance of 250 m by using a 2-in.-diameter collection lens.)

Next, the more-complex case of a target surface that does not remain in the same orientation as it vibrates, but moves around and/or changes shape (as real objects might) was considered. For experiment, the target membrane was set to vibrate at 100 Hz while simultaneously being translated at speeds between 2 and 4 cm/s—the digital camera was again used for detection.

The result was a lowering and broadening of the 100 Hz detected output. Using the digital camera to capture 200 × 200 pixel images and a computer for calculation allowed unwanted harmonics and signal degradation due to speckle decorrelation to be filtered out, permitting measurements to be taken, though at a lower rate (less than 100 Hz) limited by the 13 ms capture and analysis time.

REFERENCE
1 S. Bianchi, Appl. Opt., 53, 5, 931 (February 10, 2014).