A decade or more ago, the term "specialty fibers" would often bring to mind the hard-polymer-clad and metal-clad silica fibers used as light pipes in harsh environments. But, owing to the subsequent growth of fiber-sensing applications, the development of fiber lasers and amplifiers, the rise (and subsequent downward readjustment) of fiberoptic telecommunications, and the advent of the microstructured ("holey") fiber, specialty-fiber types are proliferating.
The specialty fiber grabbing the most news these days is of the holey sort. Microstructured fibers, which confine light through either a photonic bandgap or through refractive-index guiding, have some extraordinary properties. Some photonic-bandgap versions channel light through a hollow air-filled core surrounded by a higher-index holey cladding, while others, when pumped with ultrafast IR pulses, produce a supercontinuum spanning the visible spectrum. In the first article in this Optoelectronics World, Jason Eichenholz outlines these and other versions of this fast-developing technology.
Transmitting the 10.6-µm carbon dioxide laser line fiberoptically with low loss is important for medical and industrial applications. Hollow-core glass fibers with dielectric or dielectric and metal inner coatings serve this purpose well and are being continually improved. These waveguides can be designed for other laser wavelengths, too, such as the 2.94-µm erbium:YAG wavelength, as well as for incoherent light for broadband spectroscopic and radiometric applications. In the second article, James Harrington discusses different types of hollow-core IR fibers, including versions with omnidirectional multilayer inner coatings that exhibit complete photonic bandgaps.
One particular type of specialty fiber that has been around for a while is the polarization-maintaining fiber—a product of the telecommunications industry. But uses of this fiber have expanded into applications such as fiber gyroscopes and lasers. Improvements in the fiber itself are what have enabled this expansion. In the last article, Adrian Carter and Bryce Samson describe these improvements, including a polarization-maintaining fiber that has a loss in one axis, effectively making it a polarizing optical element.