technology. This has been necessitated by the need to access hostile or difficult environments in situ and in vivo. strategies to date have included the use of specialist fibres and miniaturized scanning systems accompanied by ingenious microfabricated lenses. Here we present a novel approach for this field by utilizing disordered light within a standard multimode optical fibre for lensless microscopy and optical mode conversion. We demonstrate the modalities of bright- and dark-field imaging and scanning fluorescence microscopy at acquisition rates that allow observation of dynamic processes such as Brownian motion of mesoscopic particles. Furthermore, we show how such control can realize a new form of mode converter and generate various types of advanced light fields such as propagation-invariant beams and optical vortices. These may be useful for future fibre-based implementations of super-resolution or light-sheet microscopy.

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Exploiting multimode waveguides for pure fibre-based imaging

The prediction of light propagation up to hundreds of millimetres within straight or even deformed segments of multimode fibres is demonstrated. The concept is applied in an endoscope and exceptional resolution and footprint are obtained.

Seeing through chaos in multimode fibres

Single-mode excitation of step-index multimode fibers with light sources with short temporal coherence lengths is demonstrated. Multimode fiber designs with reduced microbending-induced mode coupling are described that allow the propagation of the fundamental mode over long lengths with negligible mode coupling even in the presence of tight fiber bends. At a wavelength of 1.56microm a fiber with a core diameter of 45microm can preserve the fundamental mode for a propagation length of ~20m . Such fibers allow coiling with a coil diameter as small as 7cm.

Single-mode excitation of multimode fibers with ultrashort pulses.

A new generation of compact and robust ultrashort pulse lasers is currently emerging based on rare-earth doped fiber gain media. This paper reviews the development of high-power fiber technology, which recently has led to millijoule energies and /spl sim/10 W average powers from various femtosecond fiber systems. These results indicate that fiber technology has a significant potential to replace conventional solid-state lasers and promises important advantages both for practical use and for achieving high powers and energies. Chirped pulse amplification and different mode-size scaling techniques compose the foundation of this ultrashort-pulse fiber technology. Mode-size scaling can be achieved either by using multimode core fibers, which can produce a diffraction-limited beam at the fiber amplifier output, or by "mode-cleaning" of multimode core fiber output through saturated optical parametric amplification.

Mode-scalable fiber-based chirped pulse amplification systems

Glass fibers are in general not thick enough to withstand external forces on their own without suffering axial distortion, mode coupling, and loss. Thus, plastic jackets must be carefully designed to provide effective protection. We evaluate jacket designs ranging from the mere use of soft materials to the application of multiple plastic coatings and graphite reinforcement. We compute the distortion loss as a function of dimensional variations and of lateral forces considered typical for cable packaging. The protective quality of a jacket is found to depend on a combination of stiffness and compressibility and on the fiber characteristics.

Optical-fiber packaging and its influence on fiber straightness and loss

A simple method has been devised for the experimental determination of mode conversion coefficients in multimode fibers and involves only the observation of the far-field output as the angle of incidence of a collimated input beam is changed. The normalized mode coupling coefficient in a liquid-core fiber is D = 3 x 10-6 rad2 / m and increases by as much as a factor of 10 when transverse pressure is applied. Values some 2 orders of magnitude larger are found in glass-core fibers. There is good agreement between the theory presented and experiment.

H. Matsumura

Papers

Mode conversion coefficients in optical fibers.

Gigahertz bandwidths in multimode, liquid-core, optical fibre waveguide