In pursuit of a more economical fiber for evanescent field sensors, researchers in Australia have determined that annealing standard commercial telecommunication fiber yields a suitable refractive-index profile for such applications. The researchers, from La Trobe University in Bundoora, the University of Melbourne in Parkville, Australian National University in Canberra and the Optical Fibre Technology Centre in Eveleigh, presented their findings in the Feb. 1 issue of Applied Optics.To demonstrate the process, they used sulfuric acid to strip the polyimide coating from step-index multimode fiber and a 10-cm-long furnace to heat fiber lengths at 670 or 970 °C for up to 145.5 hours. Annealing causes fluorine in the cladding of the fiber to diffuse both toward the core and out of the fiber, producing an annular region of higher refractive index at the surface of the fiber.