Brian Ashton

Spun elliptically birefringent photonic crystal fibre

Elliptically birefringent fibre has been fabricated by spinning the preform of a highly linearly birefringent photonic crystal fibre (PCF) during the drawing process. The resulting Spun Highly Birefringent (SHi-Bi) PCF offers intrinsic sensitivity to magnetic fields through the Faraday effect without the high inherent temperature sensitivities suffered by conventional spun stress birefringence fibres. The ellipticity of the birefringence has been measured and temperature independence has been demonstrated.

Long-period-grating-assisted optical add-drop filter based on mismatched twin-core photosensitive-cladding fiber

A noninterferometric optical fiber add-drop channel filter based on a mismatched twin-core photosensitive-cladding fiber and long-period fiber grating is demonstrated. With assistance from the fiber grating, co-directional spectrally selective cross coupling between the two cores can be realized with an efficiency of 90%.

Characterisation of phase-shifts in gratings fabricated by over-dithering and simple displacement

Abstract Several different gratings utilizing phase-shifts in their designs are fabricated and characterised: a uniform grating with a single phase shift in the centre, a sinc-profile grating (with intrinsic phase shifts at the zeros), and a stretched cosine apodised grating with multiple and arbitrary phase shifts. The gratings are fabricated by either over-dithering the phase mask or a simple displacement of the phase-mask. The gratings are then characterised by measuring their spectral and group delay characteristics and using a grating reconstruction algorithm to determine the apodisation and phase profiles of the grating. The relative susceptibility of the different fabrication methods to errors is discussed along with the role of grating reconstruction in characterisation of gratings.

Spun elliptically birefringent photonic crystal fibre for current sensing

Spun elliptically birefringent fibre has been fabricated by spinning the preform of a highly linearly birefringent photonic crystal fibre (PCF) during the drawing process. The resulting spun highly birefringent (SHi-Bi) PCF offers sensitivity to magnetic fields for current measurements with greatly reduced temperature dependence in comparison with conventional spun stress birefringence fibres. The ellipticity of the birefringence has been measured and temperature independence has been demonstrated.

Two-point source interferometric grating writing

A novel grating-writing interferometer is demonstrated. This interferometer is compact, stable, and tunable well over 1000 nm and does not suffer from the inherent path-length differences that are present in other methods. It allows for high-quality gratings to be inscribed by preventing light from unwanted orders from affecting the fringe pattern. This technique is used to introduce a novel method of apodization based on removing the Fresnel end reflections in a grating. The experimental evidence proves the concept and shows that the technique is potentially useful for Bragg-grating inscription.

Interactive design and fabrication of complex FBGs

Gratings with a single phase shift or several phase shifts are fabricated by over-dithering the phase mask. The gratings are then characterised by measuring their spectral and group delay characteristics and using a reconstruction algorithm.

Raman oscillation on a new vibrational mode setup in phosphosilicate binary glass systems

We demonstrate Raman oscillation at a wavelength of 1158 nm using a new vibrational mode in a phosphosilicate-glass system when pumped at a 1060-nm wavelength. The lower energy P-O vibration located at 640 cm-1 associated with pure phosphate glass system is comparatively weaker and is shifted to higher energy at 800 cm-1 in the phosphosilicate binary glass. Despite the relative weakness of this vibrational mode, we obtained an efficient Raman fiber laser with the use of fiber Bragg gratings used to select laser oscillation using this mode. The measured slope efficiency with respect to the launched pump power was 60.4% and a maximum laser power of 1.8 W was produced.

FBG-based optical interface to support a multisector antenna in a spectrally efficient fiber radio system

We propose and demonstrate a fiber Bragg grating (FBG)-based optical interface for use in a spectrally efficient fiber-radio network with multisector antennas. The system has the novel feature of being specifically developed for use in existing wavelength-division-multiplexed network infrastructures. The proposed scheme supports transport of a remote local oscillator (LO) and three subcarrier multiplexed data channels, destined for different antenna sectors, using a single wavelength. The composite signal was contained within a 25-GHz band, selected via a 25-GHz dispersion-flattened FBG. Recovery of the LO and data channels is performed via optical filtering, using either a novel single grating incorporating multiple phase shifts or multiple narrow bandwidth gratings. Our measurements show that all channels within the 25-GHz band are successfully recovered with less than 2-dB optical power penalty between channels. The use of the 25-GHz grating exhibits an improvement in sensitivity of 3 dB for all data channels.

Solar hypersensitization of optical fibers

Hydrogen-loaded fibers were exposed to solar radiation for 20 days, in which time the hydrogen was allowed to out-diffuse. Gratings written in these fibers show an increased photosensitivity compared with pristine fibers. Results show the solar radiation has a similar effect on both stripped and unstripped fibers. This work agrees with the fundamental process of using low fluence, long UV sources to photosensitize hydrogen-loaded, germanium-doped fibers.

Tensile Behavior of Photonic Crystal Fibers

The mechanical strength and failure behavior of two photonic crystal silica optical fibers with different diameters were investigated using tensile test. The effect of polymer coating on the failure behavior was also studied. The results indicated that all fibers failed in a brittle manner and the failure normally initiated from fiber surfaces. The failure loads observed in the coated fibers are higher than that in bare fibers and the reason is explained by the apparent delamination between the fiber and the polymer coating when loaded on the fiber surfaces. The relationship between a characteristic parameter measured on the fracture surfaces and the failure stress was examined.