Electrically Tunable Bragg Gratings in Polymer Fiber
Thermal technique shifts the gratings’ reflectivity by several nanometers.
The capability of fiber Bragg gratings to reflect only resonant wavelengths while transmitting all others has found numerous applications in telecom, optical sensing and other technologies. Because the resonant wavelengths can be shifted by heating or straining the fiber, these gratings also can be made into tunable filters. Such tunable fiber Bragg gratings in conventional silica glass fibers have proved quite useful but, until now, these gratings have not been demonstrated in polymer optical fibers. The omission is significant because polymer fibers have high temperature sensitivity and their development could lead to commercially viable, widely tunable optical filters.
Figure 1. The reflectivity of the Bragg grating in the polymer fiber shifted linearly with the heat applied to a metallic thin film surrounding the fiber. Images reprinted with permission of Optics Letters.
Now an international collaboration has demonstrated what it believes is the first tunable Bragg grating in polymer fiber that uses a thin, electrically resistive film on the fiber to induce the thermal change that tunes the resonant wavelength. The collaboration includes scientists from the Higher Technical Institute and the Frederick Institute of Technology, both in Cyprus, from Aston University and University College London, both in the UK, and from the University of New South Wales in Australia.
The investigators fabricated their tunable Bragg gratings by first inscribing them with a reflectivity peak at about 1570 nm in polymer [polymethylmethacrylate (PMMA)] fibers. Using a 325-nm HeCd laser from Kimmon Electric Co. of Tokyo, they illuminated the fibers through a phase mask whose period was 1060.85 nm. They then applied a thin lead/copper film to the outside of the fiber. The lead served as an adhesive agent to bind the metal to the fiber, and the copper served as a resistive element for electrical heating of the fiber.
But applying the metal film was a tricky procedure because heating the grating to an excessively high temperature would damage it. The scientists first exposed the polymer grating to ultraviolet lamps centered at 172 nm. The ultraviolet light penetrated only about a micrometer into the fiber, a depth sufficient to break some of the polymer bonds on the surface but not deep enough to disturb the fiber’s optical properties. The dangling polymer bonds on the fiber’s surface offered attractive binding sites for the metallic particles that chemically and physically adhered to the fiber.
Because the thermal conductivity of PMMA is much smaller than that of silica (0.17 W/mK vs. 1.38 W/mK), the researchers expected that the behavior of their PMMA grating would be different from the behavior of a similar silica-glass fiber Bragg grating. They developed the differential equation describing heat flow in the fiber and, by comparing the predicted results with their experimental measurements, concluded that the radial thermal gradient was negligible and that their fiber could be treated as a one-dimensional system.
Figure 2. The reflection spectrum of the polymer fiber Bragg grating did not change shape as it was heated, indicating that the heating was consistently spatially uniform.
They launched light from a broadband source into the fiber and measured the transmission with an optical spectrum analyzer. The results showed a shift in the grating’s reflectivity that was linear with the heating power applied to the metallic thin film that coated the grating (Figure 1). The shape of the spectrum did not change as the grating was tuned across its range, a strong indication of uniform spatial heating of the grating (Figure 2).
The dual-peak nature of the spectrum in Figure 2 would be problematic in many applications, but the scientists believe that that effect was an artifact introduced by a dual exposure in some of the early gratings they fabricated. As they worked to perfect their technique of inscribing gratings in the polymer fiber, they often overwrote new gratings on unsuccessful previous attempts. The data in Figure 2 were taken with such a grating, and the dual peaks result from the presence of a new grating inscribed on top of an older, weaker one.
They have subsequently improved the fabrication technique and have created many polymer-fiber gratings with single reflection peaks.
Optics Letters, Feb.1, 2007, pp. 214-216.
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