High performance microwave photonic response of superstructured fibre Bragg grating with designed apodisation function

The microwave photonic responses based on the superstructure fibre Bragg gratings with designed apodisation profile are investigated. The rejection level of more than 60 dB for a bandpass filtering response is demonstrated.

Fabrication and characterisation of ultra-long-period fibre gratings

We report here for the first time the fabrication and characterisation of long-period fibre gratings (LPFGs) with period size up to several millimetres. The resonant loss peaks of these ultra-long-period gratings are generated from the coupling of the forward propagating core mode to the cladding modes of fundamental and harmonic orders. The dependence of temperature sensitivity of the coupled cladding modes on the diffraction order has been investigated. The possibility of utilising resonant peaks of different diffraction orders to perform simultaneous multi-parameter measurement has been explored. © 2002 Elsevier Science B.V. All rights reserved.

A simple method for the fabrication of intrinsically apodized chirped fibre Bragg gratings

A new and simple fabrication technique is reported for the UV inscription of intrinsically apodized chirped fibre gratings at an arbitrary Bragg wavelength employing a single chirped phase-mask in a scanning Talbot interferometer set-up. Chirped gratings have been successfully produced over a large wavelength range and with bandwidths up to 5 nm. These gratings exhibit the time-delay response of a small ripple effect. In the present paper a comparison with previously reported fabrication methods is given, showing the advantages and disadvantages of the different methods.

Quadratic behaviour of fibre Bragg grating temperature coefficients

We describe the characterization of the temperature and strain responses of fiber Bragg grating sensors by use of an interferometric interrogation technique to provide an absolute measurement of the grating wavelength. The fiber Bragg grating temperature response was found to be nonlinear over the temperature range -70 °C to 80 °C. The nonlinearity was observed to be a quadratic function of temperature, arising from the linear dependence on temperature of the thermo-optic coefficient of silica glass over this range, and is in good agreement with a theoretical model.

Improved WDM performance of a fibre optical parametric amplifier using Raman-assisted pumping

We experimentally demonstrate a Raman-Assisted Fibre Optical Parametric Amplifier (RA-FOPA) with 20dB net gain using wavelength division multiplexed signals. We report amplification of 10×58Gb/s 100GHz-spaced QPSK signals and show that by appropriate tuning of the parametric pump power and frequency, gain improvement of up to 5dB can be achieved for the RA-FOPA compared with combined individual contributions from the parametric and Raman pumps. We compare the RAFOPA with an equivalent-gain conventional FOPA and find that four-wave mixing crosstalk is substantially reduced by up to 5.8 ± 0.4dB using the RA-FOPA. Worst-case performance penalty of the RA-FOPA is found to be only 1.0 ± 0.2dB over all measured OSNRs, frequencies and input powers, making it an attractive proposal for future communications systems.

Analysis of single and multiple, non-permanent, tunable, birefringent spectral holes in a fibre-Bragg grating stop-band produced via uniaxial pressure

We present an experimental and numerical study of transversely loaded uniform fibre-Bragg gratings. A novel loading configuration is described, producing pressure-induced spectral holes in an initially strong uniform grating. The birefringence properties of these gratings are analysed. It is shown that the frequency splitting of the two spectral holes, corresponding to two orthogonal polarisation states, can be adjusted precisely using this loading configuration. We finally demonstrate a new and simple scheme to induce multiple spectral holes in the stop-band. © 2003 Elsevier Science B.V. All rights reserved.

Revealing statistical properties of quasi-CW fibre lasers in bandwidth-limited measurements

We introduce a general technique how to reveal in experiments of limited electrical bandwidth which is lower than the optical bandwidth of the optical signal under study, whether the statistical properties of the light source obey Gaussian distribution or mode correlations do exist. To do that one needs to perform measurements by decreasing the measurement bandwidth. We develop a simple model of bandwidth-limited measurements and predict universal laws how intensity probability density function and intensity auto-correlation function of ideal completely stochastic source of Gaussian statistics depend on limited measurement bandwidth and measurement noise level. Results of experimental investigation are in good agreement with model predictions. In particular, we reveal partial mode correlations in the radiation of quasi-CW Raman fibre laser.

High extinction ratio in-fibre polarisers based on 45° tilted fibre Bragg gratings

We report a near-ideal in-fiber polarizer implemented by use of 45° tilted fiber Bragg grating structures that are UV inscribed in hydrogenated Ge-doped fiber. We demonstrate a polarization-extinction ratio of 33 dB over a 100-nm operation range near 1550 nm. We further show an achievement of 99.5% degree of polarization for unpolarized light with these gratings. We also theoretically investigate tilted grating structures based on the Green's function calculation, therein revealing the unique polarization characteristics, which are in excellent agreement with experimental data.

Liquid level sensor based on an excessively tilted fibre grating

We propose and demonstrate an optical liquid level sensor based on the surrounding medium refractive index (SRI) sensing using an excessively tilted fibre grating (ETFG). When the ETFG submerged in water, two sets of cladding modes are coupled, corresponding to air- and water-surrounded grating structures, respectively. The coupling strengths of the two sets of cladding modes evolve with the submerging length of the grating, providing a mechanism to measure the liquid level. Comparing with long-period fibre grating based liquid level sensor, the ETFG sensor has a much higher SRI responsivity for liquids with refractive index around 1.33 and a lower thermal cross sensitivity. © 2013 Published by Elsevier B.V. All rights reserved.

A technique for mitigating the effect of the writing-beam profile on fibre Bragg grating fabrication

We propose and demonstrate a pre-compensation mechanism to account for the writing-beam profile which when applied to the design of advanced fibre Bragg gratings helps to achieve a desired design spectral response. We use the example of a complex multi-channel grating as an example to demonstrate the improvement achievable using the pre- compensation and find good agreement between experimental results and numerical calculations.

High stability multiplexed fibre interferometer and its application on absolute displacement measurement and on-line surface metrology

We propose a self-reference multiplexed fibre interferometer (MFI) by using a tunable laser and fibre Bragg grating (FBG). The optical measurement system multiplexes two Michelson fibre interferometers with shared optical path in the main part of optical system. One fibre optic interferometer is used as a reference interferometer to monitor and control the high accuracy of the measurement system under environmental perturbations. The other is used as a measurement interferometer to obtain information from the target. An active phase tracking homodyne (APTH) technique is applied for signal processing to achieve high resolution. MFI can be utilised for high precision absolute displacement measurement with different combination of wavelengths from the tuneable laser. By means of Wavelength-Division-Multiplexing (WDM) technique, MFI is also capable of realising on-line surface measurement, in which traditional stylus scanning is replaced by spatial light-wave scanning so as to greatly improve the measurement speed and robustness. © 2004 Optical Society of America.