Tapered standard fiber produces supercontinuum

Ultrafast pulses propagating through properly constructed photonic-crystal optical fibers can generate supercontinuum light (see Laser Focus World, July 1999, p. 17). Such an output can be useful for optical coherence tomography, optical frequency metrology, and pulse compression. However, photonic-crystal fibers must be custom-made and so are not widely available. Researchers at the University of Bath (Bath, England) have produced supercontinuum light from a standard single-mode telecommunications fiber, potentially easing access to such a light source. The one added fabrication step, tapering a section of the fiber, can be done simply by heating the section in a flame and stretching it. The researchers tapered a 90-mm section of fiber to a 2-mm diameter, with the taper waist connected at both ends to untapered fiber by taper transitions. Pulses at 850 nm with energy of 3.9 nJ and 200 to 500 fs duration from a Ti:sapphire laser were launched into the fiber. The untapered fiber had a numerical aperture of 0.1 and a cutoff wavelength of 1250 nm; within the taper, the light spread to fill the entire taper diameter. The output bandwidth reached from 370 to 1545 nm at the 20-dB level. Contact Timothy Birks at[email protected].