A. Witkowska

Wavelength-independent all-fiber mode converters.

We have used two different photonic crystal fiber (PCF) techniques to make all-fiber mode converters. An LP(01) to LP(11) mode converter was made by the ferrule technique on a drawing tower, and an LP(01) to LP(02) mode converter was made by controlled hole inflation of an existing PCF on a tapering rig. Both devices rely on adiabatic propagation rather than resonant coupling; so high extinction was achieved across a wide wavelength range.

All-fiber anamorphic core-shape transitions

We demonstrate low-loss anamorphic transitions between circular and rectangular fiber cores with aspect ratios up to 5:1, and hence improved coupling from a diode laser by using only a spherical lens. Differential hole pressurization and localized heating of a stock photonic crystal fiber inflates the holes at different rates. Some holes are plugged in the fiber end face where pressure is applied, so they remain at ambient pressure. Alternatively, holes of different sizes expand at different rates because the effective pressure due to surface tension differs.

All-fiber LP11 mode convertors.

We describe two all-fiber devices for converting light from the fundamental mode to the second-order set of modes in optical fibers. The first is made by controlled hole collapse in a photonic crystal fiber, and the second is a twisted fused coupler made from few-moded conventional fiber. As well as having applications within fiber optics, the devices can be used to generate azimuthally polarized free-space beams.

Enhanced visible continuum generation from a microchip 1064nm laser.

We demonstrate a cascaded nonlinear process using pump conversion to 742 nm by four-wave mixing in the normal dispersion regime then continuum generation by modulation instability to generate bright single-mode visible continuum with an average power up to -20 dBm/nm, from a compact 1064 nm infrared source in a monolithic single-mode photonic crystal fibre with a tapered section in one end.

Characterization of a photonic crystal fiber mode converter using low coherence interferometry

The relative group delay of the different modes present in an all-fiber LP11 mode converter at a central wavelength of 750 nm is observed using low coherence interferometric imaging. We have simultaneously measured the relative group delay and computed the intensity and the phase distribution of the modes emitted from the mode converter end face using a Fourier technique, providing unequivocal identification of the modes involved.

Visible continuum generation from a microchip 1062 nm laser source

We demonstrate a cascaded nonlinear process in an all-fibre monolithic device. Four wave mixing followed by modulation instability generates bright single-mode visible supercontinuum from an infrared source in a single-mode photonic crystal fibre.

LP 01 to TE 01 , fibre mode convertor

Mode convertors couple light from one fibre mode to another. They have applications in dispersion compensation, nonlinear optics and atom guiding for example. We demonstrate coupling from the fundamental LP01 mode to the circularly symmetric TE01 mode, which has applications for atom guidance.

Fibre core shape transitions for optical interfacing

Transitions in core shape are formed in photonic crystal fibres by differential pressurisation of the holes and localised heating. Low-loss anamorphic transitions to rectangular and annular cores are formed, and improved coupling from diode lasers to fibres thereby demonstrated.

Post-processing and tapering of PCFs

New methods have been developed in post-processing and tapering of PCFs to achieve a variety of low-loss optical devices. Interfacing between waveguides with very different properties and highly efficient mode convertors are presented.

Broadband Mode Convertors in Photonic Crystal Fibres

We have used tapering of photonic crystal fibres to make all-fibre mode converters for the LP11 and LP02 modes. The devices rely on adiabatic propagation rather than resonant coupling, allowing high extinction across a wide wavelength range.