Square glass rods have always been one of the best ways to create a uniform field of light from a laser beam (except for laser-speckle effects); such a field can be useful for materials-processing applications. An optical fiber with a square core can serve this purpose and transmit optical power over long distances too. Researchers at the University of Southampton (Southampton, England) have developed a jacketed microstructured air-clad fiber that creates a uniform field and, because it is made entirely from pure silica, is straightforward to fabricate and very transmissive.
The fiber preform consists of a square array of circular rods with smaller rods in the interstices, surrounded by thin-walled capillaries and contained within a jacket tube. Pressurizing some of the capillaries while drawing the fiber results in a 380 μm solid square core surrounded by a border of air holes 20 μm wide with 2 μm glass webs between them, and a solid jacket. When filled with, for example (from left to right), single-mode 633 nm light, single-mode 1060 nm light, or multimode 915 nm light, the fiber core produces a uniform field. Contact John Hayes at [email protected].