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18 Jul 2006
Researchers in the US have exploited polymer photodetecting fibers to demonstrate a simpler way to measure optical fields.
Optical fields are typically measured using complicated arrays of point detectors combined with arrangements of lenses, filters and beam splitters. Now, however, researchers at MIT's Department of Materials' Science and Engineering have shown how flexible arrays of photodetecting fibres offer an easier and quicker way to make optical field measurements (Nature Materials 5 532).
The photosensitive fiber is manufactured using a special process developed at MIT. It consists of a photoconductive amorphous AST chalcogenide glass core in contact with a metal electrode running along its length, which is surrounded by a transparent polymer cladding.
This design generates a current when light impinges on the fibers' surface. The current flows into an external circuit and can be measured on a computer to form a picture of the incident intensity distribution. One key advantage of this approach is that the fiber can measure the field in all directions, while traditional detectors are restricted in facing one-half of the available space. Multiple fibers can also be used to measure the angle of beam incidence.
The team developed three fiber arrangements: spherical; planar; and dual planar. The mechanically self-supporting, spherical arrangement - which consists of a spherical array of photodetecting fibres with a longitudinal and latitudinal geometry (20 x 22 fibres) on a globe - provides the simplest method for making measurements from any direction. In this case, incident light beams are partially absorbed on entering the sphere and again partially absorbed on exit.
However, to produce images of arbitrary optical light-intensity distributions, such as diffraction fields from light incident upon objects, the researchers built a planar fiber web. This web consisted of a rows and columns (32 x 32) of fibers, which produced a parallel projection of the incident optical intensity distribution in each direction. By combining two of these webs, the researchers were also able to determine the amplitude and phase information of the incident pattern.
According to the team, the devices have a wide range of potential applications in optical imaging and sensing. In addition, by changing the composition of the semiconducting glass fibre, it could be possible to detect other regions of the electromagnetic spectrum and physical quantities such as sound, heat and chemical contaminants.
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