A topical review of numerical and experimental studies of supercontinuum generation in photonic crystal fiber is presented over the full range of experimentally reported parameters, from the femtosecond to the continuous-wave regime. Results from numerical simulations are used to discuss the temporal and spectral characteristics of the supercontinuum, and to interpret the physics of the underlying spectral broadening processes. Particular attention is given to the case of supercontinuum generation seeded by femtosecond pulses in the anomalous group velocity dispersion regime of photonic crystal fiber, where the processes of soliton fission, stimulated Raman scattering, and dispersive wave generation are reviewed in detail. The corresponding intensity and phase stability properties of the supercontinuum spectra generated under different conditions are also discussed.

Supercontinuum generation in photonic crystal fiber

Supercontinuum generation, photonic crystal fiber

Silica nanowires provide strong mode confinement in a cylindrical silica-core/air-cladding geometry and serve a model system for studying nonlinear propagation of short optical pulses inside fibers. We report on the fiber diameter dependence of the supercontinuum generated by femtosecond laser pulses in silica fiber tapers with average diameters in the range of 200 nm to 1200 nm. We observe a strong diameter-dependence of the spectral broadening, which can be attributed to the fiber's diameter-dependent dispersion and nonlinearity. The short interaction length (less than 20 mm) and the low energy threshold for supercontinuum generation (about 1 nJ) make tapered fibers with diameters between 400 nm and 800 nm an ideal source of coherent white-light source in nanophotonics.

Supercontinuum generation in submicrometer diameter silica fibers.

We report the fabrication of a novel high nonlinear fiber made of Ge-Sb-Se chalcogenide glasses with high numerical aperture (∼1.0), where the core and the cladding glasses consist of Ge15Sb25Se60 and Ge15Sb20Se65 (mol. %), respectively. The nonlinear refractive index (n2) of the core glass is 19×10-18  m2/W at 1.55 μm, and its laser-induced damage threshold under irradiation of 3.0 μm fs laser is approximately 3674  GW/cm2. By pumping a 20-cm-long fiber with a core diameter of 23 μm using 150 fs pulses at 6.0 μm, supercontinuum spanning from ∼1.8 to ∼14  μm was generated.

Ultrabroad supercontinuum generated from a highly nonlinear Ge-Sb-Se fiber.

Chalcogenide glasses have the advantages of a wide transparency window (over 20 μm) and high optical nonlinearity (up to a thousand times greater than that of silica glasses), making them good candidates for mid-infrared supercontinuum generation. In this review, we describe both the history and recent developments in mid-infrared supercontinuum generation from chalcogenide fibers according to three kinds of fiber structures: step-index, microstructured and tapered fibers. We also review the coherence properties of mid-infrared supercontinuum generation and all-fiber supercontinuum sources based on chalcogenide fibers.

A Review of Mid-Infrared Supercontinuum Generation in Chalcogenide Glass Fibers

Soliton induced supercontinuum (SC) generation using an index-guiding triangular photonic crystal fiber (PCF) is experimentally studied by pumping with 140-fs pulses at several wavelengths in the positive slope anomalous dispersion region. The focused beam incident upon the PCF core is first confirmed by in situ observation based on the reflection method to make sure accurate input coupling conditions. The influence of pumping wavelength on the spectral profile of SC is examined as a function of input peak power P/sub 0/. Processes initiating SC generation are governed by the fission of higher-order solitons, which lead to the soliton self-frequency shift (SSFS) by intrapulse Raman scattering associated with blue-shifted Cherenkov type phase matched radiation. The fission of higher-order solitons is clearly observed in the input P/sub 0/ dependent spectrum and a plot of the square root of frequency shift versus input P/sub 0/. Further increase of input P/sub 0/ leads to a new SSFS accompanied by an additional blue-shifted peak located at shorter wavelength than that of the previous one. These processes continue to develop successive blue-shifted and SSFS peaks as increasing the input P/sub 0/, which determine the spectral width of SC. Spectral profile of SC is also dominated by these initial and growth processes. SC spectra obtained in the vicinity of zero dispersion wavelength shows smooth but relatively narrow, while SC generation at deep anomalous dispersion site exhibits broad bandwidth at relatively low P/sub 0/. The input P/sub 0/ required for approaching a saturated SC which is almost the same feature within the tuning pump wavelengths is decreased as increasing the degree of anomalous dispersion.

K. Sakamaki

Papers

Soliton induced supercontinuum generation in photonic crystal fiber