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11 Sep 2006
They may look unconventional but the photosensitive fibre structures being developed at MIT promise a new way to measure the amplitude and phase of an optical signal. Rob van den Berg talks to the team to find out more.
Using a combination of lenses, filters, beam splitters and detectors to measure an optical field could be a thing of the past thanks to Yoel Fink and his group at Massachusetts Institute of Technology, US. The team has developed a geometric approach to obtaining both the amplitude and phase of an optical field using tough photosensitive fibres woven into lightweight two- and three-dimensional structures (Nature Materials 25 June).
Fink and colleagues say that these fibres do not suffer from the same constraints as their classic glass counterparts and enable access to optical information on unprecedented length and volume scales. What's more, the group has shown that by changing the chemical composition of the fibres, they can be made to detect heat, vibrations and even specific chemical components.
Ayman Abouraddy is a research scientist in Fink's group that has been pioneering the smart fibre work over the last few years. "The basic principle behind these fibres is very simple," Abouraddy told OLE. "The core consists of a light-sensitive semiconductor chalcogenide glass. Along the full length and in intimate contact with this semiconductor material are four thin strips of metal, usually tin. When light or heat impinges on the fibre, photons are absorbed and electron-hole pairs may be generated. These are collected by the electrodes producing an electrical response."
In order to protect the fibres from the environment, a resilient polymer insulator, such as polyethersulphone, covers the semiconductor core and metal electrodes. Combining these three different materials – a semiconductor, a metal and a polymer – is not as difficult as it sounds. Just like their standard glass counterparts, these "smart" fibres are produced from a macroscopic preform, approximately 30 cm long and 3–4 cm in diameter.
Smart fibre fabrication
The fabrication process begins by preparing cylindrical rods of the glassy semiconductor material. A cylindrical shell of polymer having an inner diameter equal to that of the glass rod is prepared with four slits removed from the walls for the metal electrodes.
The glass rod is inserted into the polymer shell and a polymer sheet is then rolled around the resulting cylinder to provide a protective cladding. This is then consolidated into an integrated structure by heating under vacuum. Finally, the cylinder is put in a standard drawing tower producing hundreds of metres of fibre. This maintains the geometry and structure of the macroscopic preform and contacts are formed at the glass/metal interfaces.
"It is very important that the thermo-mechanical properties, such as the melting temperatures of the different materials, are properly matched otherwise the fibre drawing process fails," explained Abouraddy. "People have assumed that it would be impossible to integrate materials with highly different electrical and optical properties into the same fibre because they would have different thermo-mechanical properties. We have shown that this is not necessarily the case."
The fibres are mechanically tough, yet flexible, lightweight and protected (both electrically and chemically) from environmental effects. Arranging them into a closed-surface sphere creates an omnidirectional light-detection system capable of discerning the direction of illumination over 4πsr.
A more sophisticated detection scheme results from using two-dimensional arrays or webs. With a single fibre web, Abouraddy explains that it is possible to reconstruct the intensity distribution of an arbitrary optical field using an algorithm similar to that used in computerized axial tomography (CAT) scans.
"We illuminate a 32 × 32 fibre web with a simple image using a white-light lamp and each fibre records the total intensity of the light along its entire length," said Abouraddy. "In order to reconstruct an estimate of the optical intensity distribution that impinges on the web, we record a set of rotated projections and use a back-projection algorithm."
In the case of fibre webs, these projections can be obtained by rotating the web or alternatively, rotating the object that is being imaged. The image reconstruction improves as more projections are taken into account.
A unique advantage of this detector is the fact that no lens is needed because of the large dimensions used (relative to the wavelength of the light). And, as Abouraddy explains, with two parallel fibre webs it even becomes possible to reconstruct both the amplitude and the phase distributions of an incoming field. "Once the amplitude of a field is known in two different planes, the phase can be obtained using an iterative algorithm," he said.
Abouraddy is convinced that this approach will eventually lead to non-interferometric, lensless imaging, when a larger number of fibres are included in the web to form images of objects in more detail. "The system has an infinite depth of focus," he said. "An image of the object is formed regardless of the distance of the object from the webs, provided that the diffracted field at the locations of the two webs is intercepted."
According to Abouraddy, the image reproduces the object with its physical dimensions and also determines its physical distance from the webs. "Instead of choosing and positioning lenses and detector arrays to perform an optical field measurement, you now only have to design the proper geometrical constructions of polymeric, light-sensitive fibres," he added.
Changing the chemical composition
Abouraddy is keen to point out that the method is by no means limited to measurements in the optical domain. Changing the chemical composition allows the team to tune the electronic bandgap of the semiconducting material. For example, by including germanium, the material becomes sensitive to slight changes in temperature.
The team believes that there are already numerous potential applications for the thermally sensitive fibres. By weaving them into large arrays, for example, he says that thermal information over areas as large as tens of square metres can be obtained with cm2 resolution.
Spatially resolved thermal sensing enables failure detection in systems where the failure mechanism is linked to a change in temperature, such as chemical reactors or car tyres. An intriguing application involves the thermal monitoring of the body of large aircraft or measuring the skin temperature of the space shuttle beneath its thermal tiles.
The method could also be used for thermal monitoring of battlefield soldiers by medical staff. "By weaving these fibres into the clothing of soldiers we can allow them to thermally sense both the environment and their own body," said Abouraddy. "Our optically sensitive fibres may detect the tiny dots of laser light used by snipers for aiming. If a soldier is hit by a bullet, blood will rush to the wound leading to a local increase in temperature, which can be monitored."
Fibres used for infrared laser beam delivery, regardless of the guiding mechanism or materials used, must transport significant power densities through their core. This leads to another important application: self-monitoring of the fibres' condition.
Defects in fibres tend to be highly localized but even a small defect within such a high-power optical transmission line can result in an unintentional energy release with potentially catastrophic consequences. High-power infrared light travelling through the fibre will accumulate at the defect site, heating up the region and eventually leading to failure. The research team has demonstrated that it is possible to localize these defects with high precision (Nature Materials November 2005).
The Fink group was fast in coming up with a promising application of such a local temperature probe. In 2002, the group unveiled a photonic bandgap (PBG) fibre to efficiently guide high-power infrared radiation at 10.6 µm from a CO2 laser (Nature 12 December 2002). Today, these have been incorporated into a device (recently approved by the FDA for use in patients) that enables surgeons to efficiently remove cancerous tissue from the lungs using infrared laser light.
Structural perturbations such as fibre bends also tend to increase the overall losses through coupling to both higher-order propagating modes and to localized defects. "This may happen, for instance, when the fibre enters the throat," explained Abouraddy. "My colleague Mehmet Bayindir came up with the idea to surround these PBG fibres with extra layers just like those used for thermal sensing. This allows us to sense the escape of light via the heat generated as soon as it occurs and switch off the treatment laser immediately."
These applications highlight the value of combining various functionalities into a single smart fibre. And there are yet more promising prospects by going beyond the optical and thermal regimes.
"This is a very flexible process. We have found many more combinations of materials that are compatible and can be drawn into fibres," concluded Abouraddy. "You could think of adding completely different functionalities to the fibres, such as pressure sensitivity, or the ability to detect specific chemicals, just by tuning the chemical composition of the chalcogenide glass and the polymer and designing a suitable fibre structure."
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