Direct measurement of transverse force has been investigated in this work by using two specialist single mode
Polarisation-Maintaining (PM) side-hole(s) fibres. Variations in the pressure sensitivity of side-hole fibres with different
rotation angles and magnitudes of the applied external force have been investigated both theoretically and experimentally
and it was confirmed that they are dependent upon the force direction, with reference to the fast or slow axis of the
fibres. The experimental results obtained have shown good agreement with theoretical predictions when appropriate
cross-comparisons were made. Thus these fibres have shown potential to be used as optical fibre sensors for the
measurement of pressure, force and mass of an object, applied in different directions, over a wide range and in real time.
In this paper Tm/Yb doped microspheres have been successfully created through melting the ends of fibre cores etched
from single mode Tm/Yb fibres designed and fabricated specifically for the laser emission centred at mid-infrared region.
When coupled with a fibre taper, a microsphere fibre laser cavity can thus be configured creating a system when pumped
by a ~1600nm laser source using a monochromator to monitor the spectral characteristics of the laser output produced.
The performance and characteristics of the Tm/Yb microsphere lasers are discussed in detail in the paper. The laser has
shown low threshold in terms of the pump power making it well suited to potential optical fibre sensor applications.
A distributed-feedback (DFB) grating was written in twin-hole fibres with internal electrodes. Due to the intrinsic
birefringence, the grating has two ultra-narrow peaks (~0.41 pm and ~0.27 pm) corresponding to x- and y-polarization.
The separation between them can vary from 40 pm to 104 pm when the temperature increases from room temperature to
96°C. The dominant contributions of the Bragg wavelength shift as the increasing temperature are the change in
refracitive index of the fibre and the expansion of the substrate (largest). Under the current pulses excitation, full on-off
switching with response time ~2.5 ns has been achieved for x-polarization of the DFB grating.
This persentation gives an overveiw of the field of microwave photonics with an emphasis on new fiber based devices which we belive have a real practical potential. Microwave photonics can be considered as the fruitful meeting point bewteen optics and microwave engineering, where optoelectronic devices and systems are used both for processing at microwave rates and for signal handling in microwave systems. The use of specialty fibers, glass poling and naturally fiber Bragg gratings opens new perspectives for the realization of low-cost devices with appropriate functionality. The application field for optical microwave transmission and processing spans from radar technology to cable TV and mobile communications systems. Over the last few years very much attention has been directed towards radio-over-fiber systems for the next-generation mobile communications infrastructure as well as hybrid fiber radio for picocell systems at 60 GHz or above. As a matter of fact, the higher the microwave frequencies, the greater are the similarities with the optical carrier and the more there is to be gained by processing the microwave signal in the optical domain. Other important application examples are beamforming networks for phased array antennas and subcarrier processing for routing in optical networks.
Design and fabrication of superimposed fibre Bragg gratings are considered. Firstly, an accurate numerical model for computing the reflection and transmission spectra of the superimposed gratings is presented. The model employs the transfer matrix method and takes into account the photosensitivity profile of the fibre, which is understood as the dependence of the refractive index change versus UV light exposure dose. It is shown that the model accurately reproduces the impact of saturation of the photosensitivity profile on the reflection and transmission spectra of the superimposed grating.
Secondly, by varying the initial phase of each elementary grating comprising the superimposed one, the latter is optimised such that the total maximum refractive index change required is minimised. The optimisation is carried out by means of the genetic algorithm, a global optimisation technique. A method for an almost optimal use of the fibre photosensitivity is proposed. The method overcomes the problems related to the non-infinitesimal length of the interferometric pattern in the grating writing setup. A successful fabrication of optimised 8- and 16-channel filters for a 2.5 Gb/s DWDM system with 50 GHz channel separation is reported. It is shown that the optimised superimposed gratings have a smaller insertion loss than similar non-optimised ones.
Optical low coherence reflectometry (OLCR) is applied to the precise characterization of Bragg gratings in optical fibers. Grating with lengths below two millimeters have been fabricated in standard telecommunication fibers. Their OLCR spectra can be used to determine the induced index changes, which range from 0.64*10<SUP>-3</SUP> to 1.16*10<SUP>-3</SUP>, with an accuracy of a few percent. These changes are obtained by adjusting the position of the oscillation minima in the OLCR spectra.