CLEO '99 Postdeadline: Airy Fiber Performs Frequency Conversion

Air/Silica Microstructure Fiber Generates Single Transverse Mode Continuum Extending Over 1000 nm

By: Yvonne Carts-Powell

BALTIMORE, MD—Using 75-cm length of silica microstructure fiber, Bell Laboratories (Murray Hill, NJ) researchers have converted 0.8 nJ pulses from a femtosecond laser into a single-transverse-mode output that spans the spectrum from the near-infrared to violet (see Figure 1). Jinendra Ranka and colleagues reported on the work during the postdeadline paper sessions at the Conference on Lasers and Electro-Optics (CLEO '99; May 23-28).

According to Ranka, applications for the technology include optical displays and data storage. Ultrafast researchers were immediately struck by the potential of the fiber for creating few-femtosecond pulses, and descended in force on Ranka after his presentation. Other potential applications that could take advantage of the wide wavelength range include spectroscopy and medical diagnostics.

Fiber design
The microstructure fiber incorporates several air holes, running the length of the fiber, arranged in a circle around the core, which changes some of the basic characteristics of the fiber, including it's zero dispersion point and it's light-guiding capabilities. The work Ranka reported on used six holes surrounding the core, but other numbers of holes and sizes of holes work similarly.

Conventional fiber exhibits a characteristic zero-dispersion wavelength of 1.28 µ, so dispersion shifted fibers can only be designed to move the zero-dispersion wavelength to greater than 1.28 mm. These silica microstructure fibers, on the other hand, feature a large waveguide component in their dispersion characteristics, and can thus be designed for zero group-velocity dispersion at wavelengths of about 770 nm.

In addition, the use of the air holes strongly confines the light to the core. "Typical fibers are index-guided with small changes of refractive index—about a 4% difference," explains Ranka, "but with air, there is a huge difference in the refractive index." These characteristics lead to a number of interesting nonlinear effects such as those demonstrated above.

The researchers demonstrated efficiently generating a visible continuum with only 100 pJ of energy in a few-femtosecond-long pulse from a Ti:sapphire laser. Before, such a continuum would have required megawatt peak powers from large and expensive solid-state lasers. "Maybe we could create a battery-powered continuum," says Ranka.

References
1. Jinendra Ranka et. al., "Efficient visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nanometers," paper #CPD8, Proceedings of the Conference on Lasers and Electro-Optics (CLEO '99), May 23-28, Baltimore, MD.

About the author…
Yvonne Carts-Powell is a freelance science writer based in Belmont, MA.