Few people need to inspect an object in a glass of milk. If, however, an imaging technique can see through milk, it probably can image objects effectively through other diffusing media-such as blood or even a suspension of silica powder in a polishing workshop. It then becomes an effective inspection tool in applications as diverse as manufacturing inspection, medical imaging of living tissues, even tasks requiring undersea visibility.
Current options for imaging through diffusing media include techniques such as time-resolved holography, optical coherence tomography, and scanning confocal microscopy. Each has its benefits and limitations. With confocal techniques, for example, researchers can avoid some problems intrinsic to tomography, such as the requirement to solve an inverse problem. There are, however, issues related to limited imaging sensitivity and the complexity and cost of required equipment.
Now the Laboratoire de Spectrométrie Physique of the Université Joseph Fourier de Grenoble (Saint-Martin-d'Héres, France) has produced a method for three-dimensional (3-D) imaging in turbid media that is similar to heterodyne scanning confocal micros copy, but resolves some of the limitations just discussed. According to researcher Eric Lacot and colleagues Richard Day and Frédéric Stoeckel, the technique-which is based on laser optical feedback tomography (LOFT)-relies on the resonant sensitivity of a short-cavity laser to frequency-shifted optical feedback produced by ballistic photons retrodiffused from the media. One reported advantage of the imaging method is that the laser source is also the detector. In addition to its optical-amplification duties, it provides self-aligned spatial and temporal coherent detection (acts as both a spatial and temporal filter).
The equipment setup for the LOFT experiments includes a Nd:YAG microchip laser with an 800-µm-thick cavity. The frequency shifter is an acousto-optic modulator that gives two adjustable sidebands, with the frequency shift typically ranging from a few kilohertz to several megahertz.
To capture an image, the researchers focus the beam of a laser with a cavity damping rate higher than the population-inversion damping rate on or into the turbid medium.1 Only photons backscattered from points near the focus are reinjected into the laser by mode-matching.
Before photon reinjection, though, the equipment frequency-shifts the laser beam to modulate the net gain at that frequency. Amplification follows. After a photodetector detects the modulated intensity of the laser, the lock-in amplifier of a spectrum analyzer measures the amplitude of the modulation at the radio frequency. A translation unit moves the sample to allow researchers to obtain a one, two, or three-dimensional image. A PC records and processes signals.
Amplification is on the order of 1 x 106 for a microchip laser and 1 x 103 for a diode laser. The researchers compare the principle of optical amplification in this method to that of internal electronic amplification in a photomultiplier. With this amplification, they said, the laser quantum noise becomes more significant than the photodetector noise, producing a shot-noise-limited technique without the need for a high-performance photodetector.
To illustrate image production capabilities of the technique, the researchers produced a 2-D image of a coin immersed in milk (see figure above). The round-trip path length of the reinjected photons in the medium was approximately 12 photon mean-free paths. The pixel dimensions of the reproduced image were 120 x 120 µm. The re searchers chose the z position to obtain a maximum signal for a flat part of the coin.
The study was supported by the Department of Research-Industry at the Université. The Laboratoire d'Electronique de Technologie et d'Instrumentation/Commissariat a l'Energie Atomique provided the microchip lasers.
Paula M. Noaker
REFERENCE
- E. Lacot, R. Day, and F. Stoeckel, Opt. Lett. 24, 744 (June 1, 1999).