J.A.R. Williams

Optical bend sensor based on measurement of resonance mode splitting of long-period fiber grating

We report an effective new method of realizing optical bend sensing based on the measurement of bending-curvature encoded resonance mode splitting of long-period fiber grating. The bending induced mode splitting exhibits a near-linear response and the bending sensitivity achieved by this method is nearly four times higher than the previously reported wavelength shift detection method. The evolution of the transmission loss under bending appears dependent on the initial mode coupling strength.

UV-written in-fibre Bragg gratings

The techniques for fabrication and the properties of periodic and aperiodic fibre Bragg gratings produced by UV exposure in photosensitive optical fibres are reviwwed with an emphasis on applications.

Bragg grating fast tunable filter for wavelength division multiplexing

A Bragg grating fast tunable filter prototype working over a linear tuning range of 45 nm with a maximum tuning speed of 21 nm/ms has been realized. The tunable filter system is based on two piezoelectric stack actuators moving a mechanical device thus compressing an apodized fiber Bragg grating. The filter allows both traction and compression and can work in transmission and in reflection. It is designed to work with a channel spacing of 100 GHz according to the ITU specifications for wavelength division multiplexing systems.

Wide-band Fabry-Perot-like filters in optical fiber

We report the fabrication and characterization of wide-band Fabry-Perot-like resonators in optical fiber. Using Bragg gratings with variable detuning, resonators that operate ever a wavelength span in excess of 140 nm have been produced, with free spectral ranges between 0.09 nm and 11.27 nm. Numerical and experimental results for two typical resonators are presented, showing good agreement between the two.<<ETX>>

All-fibre optical sensing system: Bragg grating sensor interrogated by a long-period grating

We report here a simple, low-cost all-fibre grating strain sensing interrogation technique offering high static-strain resolution and large dynamic range . A key interrogating component of this technique is a long-period grating which acts as an edge filter converting strain-induced wavelength variation into optical power measurement. The transmission profile of the long-period grating is shown to be nearly linear over a sufficiently wide range, yielding a linear relationship between the resultant intensity and applied strain, and a high resolution achieved by standard lock-in techniques.

Phase shifted and cascaded long-period fiber gratings

We have obtained useful analytical expressions for the spectra of phase shifted and cascaded long-period gratings and have demonstrated experimental results which are in good agreement. We discuss how the introduction of a phase shift allows the tailoring of the transmission spectrum of a long-period fiber grating and how cascading two long-period gratings gives multiple transmission peaks in the band of the grating spectrum.

High-performance microwave transversal filter using fiber Bragg grating arrays

Measured results from a microwave photonic traversal filter exploiting fiber Bragg grating (FBG) arrays and achieving a high-performance tunable bandpass microwave response are presented. Tuning is affected by changing the optical carrier wavelength to select the operating gratings, and the high-quality frequency response is obtained by weighting the reflections of the gratings. This experiment demonstrates that sophisticated passive signal processing functionality can be achieved practically using FBGs.

Polarization synthesized optical transversal filter employing high birefringence fiber gratings

Polarization synthesizing is introduced into a Bragg grating based optical fiber transversal filter to realize incoherent operation. High birefringence fiber Bragg gratings are used to produce an optical transversal filter with tunable microwave frequency response and large free spectral range immune to optical coherent noise.

Fiber Bragg grating fabrication for dispersion slope compensation

Group-delay dispersion slope causes signal deterioration in long-distance high-data-rate communication systems. We report the fabrication of high-quality apodized and chirped fiber gratings with dispersion slopes as high as -1800 ps/nm/sup -2/ over a 0.7-mn bandwidth-sufficient to compensate for the dispersion slope of >25000-km length of standard fiber at a wavelength of 1550 nm. We believe this is the first time that fiber Bragg gratings have been specifically designed and fabricated for dispersion slope compensation. The fabrication technique uses a standard unchirped phase mask and two scans of the inscribing UV beam.

Fabrication and characterization of bandpass filters based on concatenated chirped fiber gratings

Practical, low-loss ( 30 dB out-of-band rejection over /spl sim/25 mm spectral widths. The approach adopted is based on the concatenation of two or more tailored broadband chirped gratings fabricated with low short-wavelength loss to maximize the rejection and stopband width in a filter with /spl sim/1 nm passband. The effect of thermal aging for similar complex structures is considered in the context of three different fiber types: standard telecommunication, high germania (Ge)-doped and boron/germania (B/Ge)-codoped. It is shown that while hydrogenated B/Ge-codoped fiber exhibits excellent photosensitivity, the resulting decrease in guiding strength and thermal stability may make germania-doped fiber the more attractive choice.