Remember the grade school mnenonic for the colors of the spectrum? In case you forgot, it was a fellow named Roy G. Biv. Researchers at have created a novel optical fiber that personifies the moniker. Shoot a brief pulse of a single color of laser light down the fiber and it generates every color of the rainbow--from the invisible infrared, though the entire visible spectrum of red, orange, yellow, green, blue, indigo and violet.
The fiber gets its unusual properties from a porous outer layer. Most optical fibers have a core of pure silica and a cladding layer that is doped with ions to change its refractive index. Light travelling down the fiber stays inside because it is reflected off the interface between the two layers. Instead, the Bell scientists, headed by Jinendra Ranka, created a fiber in which the outer surface is perforated by billions of tiny holes. "The cladding is essentially air," says Ranka. "It creates a much greater difference between the refractive index in the core and the outer layer."
When they injected extremely brief pulses of laser light --just 100 femtoseconds long--into the fiber, a curious phenomenon occurred. As the pulses of light interacted with the cladding so called non-linear effects created light of all wavelengths. As the pulse travels down the fiber, it generated colors deeper and deeper into the rainbow, starting with red and ending with indigo--just as a prism separates the colors of visible light. The result was a fiber that glowed with all the colors of the spectrum over a very short distance, just 75 centimeters. Generation of the continuum is made possible by the extremely small effective area, unusual dispersion characteristics and low loss of the air-silica microstructure fiber.
The new fiber will never be used to transmit telephone conversations but it is likely to become far more than a laboratory curiosity. Although the same effects are possible with very powerful lasers, the Bell Labs group accomplished the feat at very low wattages--those approaching tiny semiconductor lasers use in many computer devices. Indeed, the report caused a small sensation when the researchers presented a post-deadline paper at a recent Conference on Lasers and Electro-Optics (CLEO) in Baltimore. Other investigators were quick to see possibilities in projection displays, more detailed medical images and optical data storage.
The colors in today's displays, for example, are created by light that stimulates just three colors of phosphors, which glow red, blue and green. The interaction of these various pixels creates the other colors of the spectrum that we see on television or our computer screens. But the Bell Labs fiber can generate every specific wavelength. It is possible that one tiny laser, emitting just one color of light, could paint an entire projection display in true color.
Also, by having the entire spectrum at their fingertips, researchers have access to tremendous bandwidth for storage and other applications. After all, there is more to light than meets the eye.