Posted by Nicholas Dorogy
Reconstructing acousto-elastic signals using Laser Doppler Vibrometry (LDV) measurements capitalizes on the precise, non-invasive nature of laser remote sensing to study sensitive or isolated targets. An LDV operates by continuously emitting a laser at an object of interest. A portion of the light directed at the object is naturally reflected back to the system. This reflected light is referred to as the “reflected signal.” Inside the LDV system, the reflected signal is directed to a photo detector.
The LDV system also partitions a fraction of the original waveform emitted by the laser and directs this “reference beam” to the same photo detector as the reflected signal. Inside the photo detector, the reference beam and the reflected signal interfere. If the target is not vibrating, the interference pattern of the two signals is constant. However, when the target vibrates, the interference pattern will change.
This change is a natural result of a phenomenon known as Doppler shifting. You might be familiar with the frequency increase when a siren approaches you and the subsequent frequency decrease as it moves away. Likewise, if the surface of an object is moving toward the laser, the reflected signal will become “compressed” as it meets and reflects the laser more quickly. This is the frequency increase that we hear as the siren moves towards us. Likewise, if the target moves away from the laser, the time interval between reflections will become larger, thus causing a frequency decrease.
As discovered by Christian Doppler in 1842, the subsequent frequency shifts are directly proportional to the velocity of the target and the receiver. Thus, by recording these frequency shifts in the LDV, we are able to measure the velocity of the surface of the target. This measurement is exactly analogous to the operation of a conventional geophone. This technique is easily extensible to more advanced investigation. Instead of using a geophone to measure seismic activity or generic vibrations, we might instead deploy an LDV system to recover these signals from a remote platform.
As a demonstration of this concept, I recorded controlled-source vibrations with an LDV system and attempted to reconstruct the source waveform solely using these recordings. To do this, I use a speaker to generate acoustic waves. However, instead of directly measuring the vibration of the speaker, I record the vibrations of a tissue placed several meters in front of the speaker. The speaker was played at a conversational volume level and the LDV recorded the reflections off the tissue at a sampling rate of 25 kHz. The signal was then filtered to select 400–4,000 Hz, normalized, and uniformly amplitude gained. The spectrogram below displays the frequency vs time of a well-known classical song composed by Carl Phillips Emanuel Bach. The audio file was reconstructed using only the raw data measured by the LDV. Listen to the song and leave a comment if you think you recognize it!
Beyond this trivial demonstration, LDV technology is particularly powerful when attempting to conduct seismic studies in space. Landing seismometers on the surface of small planetary bodies is hampered by microgravity conditions, mission engineering complexity, and the challenges associated with seismic coupling in loosely coalesced materials. Therefore, a spacecraft equipped with an LDV would enable an alternative remote sensing approach that is unaffected by the issues of gravitational and seismic coupling. Furthermore, if a second LDV is introduced, passive seismic surveys may now be conducted without the added risk and limitations associated with an active source. Moreover, because the LDVs are hosted in an orbiting spacecraft, the target can be continuously monitored to provide global tomographic coverage.