Abstract

Recently, there has been much attention given to the spectral broadening of femtosecond laser pulses in microstructured and tapered fibers for applications including optical coherence tomography (OCT)¹ and frequency metrology.² In these fiber structures, the interplay of enhanced mode confinement and decreased group-velocity dispersion (GVD) results in efficient self-phase modulation and the generation of an ultra-broad continuum.^3,4 Operation around the zero-dispersion wavelength (ZDW) is clearly important, however, the enhanced waveguide dispersion can only shift the overall zero to wavelengths shorter than the material ZDW. Great results were obtained with Ti:sapphire around 800 nm but there is much motivation for similar effects at important wavelengths such as 1.3 μm and 1.55 μm. Yanovksy et al. showed that spectral splitting occurs for pulses propagating at the ZDW in ordinary single-mode fiber, but substantial spectral broadening could not be obtained.⁵ Calculations show that for decreasing taper diameter, a second zero in GVD approaches the material ZDW from the longer wavelength side (Figure 1). It has been predicted that the shape of the continuum should depend on the dispersion-slope, giving yet additional motivation for the study of the second ZDW.⁶ Here we report the generation of ultrabroadband spectra by exploitation of that second zero-dispersion wavelength.

© 2002 Optical Society of America