A new single-mode optical fiber made from sapphire rather than the usual silica can withstand temperatures of over 2000°C as well as high levels of radiation. Although the fiber’s length is currently limited to 1 cm, the technique used to construct it could be extended up to several meters, making it useful for remote sensing in ultra-extreme environments.
Rather than having a core and cladding, sapphire fibers consist of a solid single-crystal material. The interface between the sapphire and the surrounding air guides light. The crystal’s relatively large (between 60 and 425 microns) diameter means that sapphire fibers are highly multimodal: a typical 75-micron fiber may contain 20 000 modes. Light in each mode propagates at a different speed, resulting in the distortion of optical signals.
The researchers found a way to make a single-mode sapphire fiber by writing a channel along the length of a 425- micron -diameter fiber such that the light is contained within a tiny cross-section less than 10 microns in diameter. The researchers chose a point within the sapphire fiber cross-section as the “core.” They then exposed the sapphire around this region to extremely short (170 femtoseconds) pulses of laser light, effectively forming a cladding by lowering the local refractive index.