Philip St. John Russell

Supercontinuum generation by stimulated Raman scattering and parametric four-wave mixing in photonic crystal fibers

Supercontinuum generation is investigated experimentally and numerically in a highly nonlinear index-guiding photonic crystal optical fiber in a regime in which self-phase modulation of the pump wave makes a negligible contribution to spectral broadening. An ultrabroadband octave-spanning white-light continuum is generated with 60-ps pump pulses of subkilowatt peak power. The primary mechanism of spectral broadening is identified as the combined action of stimulated Raman scattering and parametric four-wave mixing. The observation of a strong anti-Stokes Raman component reveals the importance of the coupling between stimulated Raman scattering and parametric four-wave mixing in highly nonlinear photonic crystal fibers and also indicates that non-phase-matched processes contribute to the continuum. Additionally, the pump input polarization affects the generated continuum through the influence of polarization modulational instability. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate the importance of index-guiding photonic crystal fibers for the design of picosecond and nanosecond supercontinuum light sources.

White-Light Supercontinuum Generation with 60-ps Pump Pulses in a Photonic Crystal Fiber

The generation of a spatially single-mode white-light supercontinuum has been observed in a photonic crystal fiber pumped with 60-ps pulses of subkilowatt peak power. The spectral broadening is identified as being due to the combined action of stimulated Raman scattering and parametric four-wave-mixing generation, with a negligible contribution from the self-phase modulation of the pump pulses. The experimental results are in good agreement with detailed numerical simulations. These findings demonstrate that ultrafast femtosecond pulses are not needed for efficient supercontinuum generation in photonic crystal fibers.

Scalar modulation instability in the normal dispersion regime by use of a photonic crystal fiber

Modulation instability at high frequencies has been demonstrated in the normal dispersion regime by use of a photonic crystal fiber. This fiber-optic parametric generator provides efficient conversion of red pump light into blue and near-infrared light.

Simultaneous generation of spectrally distinct third harmonics in a photonic crystal fiber

By coupling femtosecond pulses at lambda - 1.55mum in a short length (Z - 95 cm) of photonic crystal fiber, we observe the simultaneous generation of two visible radiation components. Frequency-resolved optical gating experiments combined with analysis and modal simulations suggest that the mechanism for their generation is third-harmonic conversion of the fundamental pulse and its split Raman self-shifted component.

Carbon dioxide laser fabrication of fused-fiber couplers and tapers

We report the development of a fiber taper and fused-fiber coupler fabrication rig that uses a scanning, focused, CO(2) laser beam as the heat source. As a result of the pointlike heat source and the versatility associated with scanning, tapers of any transition shape and uniform taper waist can be produced. Tapers with both a linear shape and an exponential transition shape were measured. The taper waist uniformity was measured and shown to be better than +/-1.2%. The rig was also used to make fused-fiber couplers. Couplers with excess loss below -0.1 dB were routinely produced.

Highly increased photonic band gaps in silica/air structures

We explore the possibilities of achieving larger out-of-plane band gaps in two-dimensional silica/air photonic crystals with hexagonal symmetry. By modification of the basic hexagonal unit-cell, we demonstrate a new photonic crystal structure, for which the size of the band gaps is increased several times compared to those of simple hexagonal structures. The new structure allows design of silica/air photonic crystals exhibiting complete out-of-plane two-dimensional photonic band gaps for realistic fabrication parameters. We propose a new design of photonic crystal fibers based on the modified hexagonal structure. The advantages of the new design over recently fabricated photonic crystal fibers are discussed, and a calculation of a guided mode, localized at a low-index core region, is presented.

Polarization dependent harmonic generation in microstructured fibers

We report on the control of visible harmonic generation in microstructured fiber through the polarization state of the fundamental radiation. By coupling ë=1.55 ìm femtosecond pulses that have the same peak power into a short length (Z=20 cm) of high- microstructured fiber, we observe the generation of distinct visible spectral components in the visible at the output of the fiber in dependence of the input pulses polarization state.

Silica/Air Photonic Crystal Fibres

We describe the fabrication, characterisation and applications of silica/air photonic crystal fibres with microscopic arrays of air capillaries running along their length.

Photonic crystal fiber devices

Photonic crystal fibers (PCFs) have been receiving increasing attention over the past few years. They are single material fibers that use an array of air holes in the cladding to confine light to a core, instead of the more usual refractive index step within the solid material of a conventional fiber. As PCFs become more well-understood mainstream structures, the need arises to develop techniques to process them post-fabrication to form all-fiber devices. We have chosen to study heat-treatment processes analogous to the tapering of conventional fibers, except that in PCFs there is a second degree of freedom to exploit. Not only can the fiber be stretched to locally reduce its cross-sectional area, the air holes can be changed in size by heating alone under the effect of surface tension.

Development of a double-clad photonic-crystal-fiber-based scanning microscope

Despite the fact that laser scanning confocal microscopy (LSCM) has become an important tool in modern biological laboratories, it is bulky, inflexible and has limited field of view, thus limiting its applications. To overcome these drawbacks, we report the development of a compact dual-clad photonic-crystal-fiber (DCPCF) based multiphoton scanning microscope. In this novel microscope, beam-scanning is achieved by directly scanning an optical fiber, in contrast to conventional beam scanning achieved by varying the incident angle of a laser beam at an objective entrance pupil. The fiber delivers femtosecond laser pulses for two-photon excitation and collects fluorescence back through the same fiber. Conventional fibers, either single-mode fiber (SMF) or multimode fiber (MMF), are not suitable for this detection configuration because of the low collection efficiency for a SMF and low excitation rate for a MMF. Our newly invented DCPCF allows one to optimize collection and excitation efficiency at the same time. In addition, when a gradient-index (GRIN) lens is used to focus the fiber output to a tight spot, the fluorescence signal collected back through the GRIN lens forms a large spot at the fiber tip because of the chromatic aberrations of the GRIN lens. This problem prevents a standard fiber from being applicable, but is completely overcome by the DCPCF. We demonstrate that this next generation scanning confocal microscope has an extremely simple structure and a number of unique features owing to its fundamentally different scanning mechanism: high flexibility, arbitrarily large scan range, aberration-free imaging, and low cost.