We have used the frequency comb generated by a femtosecond mode-locked laser and broadened to more than an optical octave in a photonic crystal fiber to realize a frequency chain that links a 10 MHz radio frequency reference phase-coherently in one step to the optical region. By comparison with a similar frequency chain we set an upper limit for the uncertainty of this new approach to 5. 1x10(-16). This opens the door for measurement and synthesis of virtually any optical frequency and is ready to revolutionize frequency metrology.

Supercontinuum light with a spectrum more than two octaves broad (370-1545 nm at the 20-dB level) was generated in a standard telecommunications fiber by femtosecond pulses from an unamplified Ti:sapphire laser. The fiber had been tapered to a diameter of :2mum over a 90-mm length. The pulse energy was 3.9 nJ (average power, 300 mW). This source of high-intensity single-mode white light should find widespread applications in frequency metrology and spectroscopy, especially since no unconventional fibers are needed.

We describe a silica hollow-core fiber for mid-infrared transmission with a minimum attenuation of 34 dB/km at 3050 nm wavelength. The design is based on the use of a negative curvature core wall. Similar fiber designed for longer wavelengths has a transmission band extending beyond 4 µm.

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We report on an experimental study of supercontinuum generation in photonic crystal fibers with low-intensity femtosecond pulses, which provides evidence for a novel spectral broadening mechanism. The observed results agree with our theoretical calculations carried out without making the slowly varying envelope approximation. Peculiarities of the measured spectra and their theoretical explanation demonstrate that the reason for the white-light generation in photonic crystal fibers is fission of higher-order solitons into redshifted fundamental solitons and blueshifted nonsolitonic radiation.