We present the results of numerical analysis showing that high geometrical birefringence can be obtained in large mode area photonic crystal fibers. Simulations were carried out using Finite Element Method with Perfectly Matched Layer boundary conditions. To induce possibly high phase modal birefringence, we introduced a few small holes into the central region of the core, which breaks its hexagonal symmetry. Such modification of the fiber geometry is additionally responsible for increasing the effective area of the guided mode. For the optimized fiber design with a pitch distance Λ = 8.8 μm, the phase birefringence reaches B = 1.0×10<sup>-4</sup> at wavelength λ = 1.3 μm and B = 1.5×10<sup>-4</sup> at λ = 1.5 μm. In addition to high modal birefringence, an important advantage of the proposed fiber design is a single mode operation for wavelength greater than 1.3 μm.
We review exceptional properties of the photonic crystal fibres enabling sensing applications of this new class of fibres.
First, the sensing capabilities of highly birefringent index guided fibres are discussed. This includes dispersion
characteristics of phase and group modal birefringence in different fibre structures, and sensitivity of these parameters to
hydrostatic pressures and temperature. We demonstrate that index guided and photonic bandgap holey fibres of specific
construction can be used as wide-band fibre-optic polarizer. We also show that combining of geometrical and stress
effects makes it possible to design the holey fibres with either zero phase or group modal birefringence at virtually any
given wavelength. Finally, different designs and performance of PCFs suitable for gas sensing are overviewed.
We investigated theoretically and experimentally the wavelength dependences of phase and group modal birefringence
for the fundamental (E<sub>11</sub>) and the higher order mode (E<sub>31</sub>) supported by index guiding highly birefringent photonic
crystal fiber. The birefringence in the investigated structure was induced by asymmetrical cladding consisting of one row
of cladding holes with a diameter lower than the other cladding holes. The numerical simulations carried out with use of
the full-vector finite elements method show that the birefringence of the E<sub>31</sub> mode can be about 30% higher than of the
fundamental mode. Additionally, we measured the modal birefringence of the both modes using scanning wavelength
method. A comparatively good agreement between the calculation and experimental results was obtained confirming the validity of the theoretical analysis.
We studied both experimentally and numerically the spectral behavior of modal birefringence in channel waveguides
inscribed in PMMA by DUV illumination. The measurements of birefringence were carried out using spectral
interference method for different waveguide widths, respectively 2, 3, 4, 5, 6 μm which were inscribed using 3 J/cm<sup>2</sup>.
We have also modeled the spectral dependence of birefringence related only to waveguide geometry using FEM method.
High discrepancy reaching one order of magnitude was observed between measured and calculated birefringence. This is
related to the fact that numerical analysis accounts only for so called geometrical birefringence induced by the
asymmetry of the waveguide, while in measurements the overall modal birefringence relates to both the geometrical and
the material effect. Our measurement results indicate that the contribution of the material birefringence in the analyzed
waveguides may reach up to 3×10<sup>-4</sup> in a short wavelength range.
Fast, frequent, accurate and reliable measurements of physical factors such as temperature, stress or strain play a key role when it comes to ensuring the smooth operation of processes in many domestic, commercial and industrial constructions or devices. For example, most fabrication devices and production process rely on temperature and stress measurements to operate; and most large buildings depend on a series of temperature sensors to control the heating or cooling to maintain the temperature.
Photonic crystal fibres (PCF), constitute a class of optical fibres, which has a large potential for number of novel applications either in the telecom or in the sensing domain. Analysis of sensing characteristics of different photonic crystal fibre structures, including effective index and mode field distribution, photonic bandgap, chromatic dispersion, phase and group modal birefringence, confinement and bending losses, sensitivity to temperature, hydrostatic pressure, and other physical parameters are revealed.
The benefits of PCF allow fabrication of different types of specialty microstructured fibres such as endlessly single mode, double clad, germanium or rare earth doped, highly birefringent, and many other microstructured fibres as sensor components. The developed characterization techniques of specialty microstructured fibres are reviewed as well. Finally, the new microstructured fibres and fibre component for sensing applications which were designed, fabricated and characterized will be presented. One of the demonstrated components is the effective Bragg grating written in highly birefringent and single mode photonic crystal fibre.
In this communicate, we present a numerical approach allowing to model propagation characteristics of the large core birefringent holey fibers with stress applying elements. The main advantage of the proposed method is that it takes into account simultaneously both geometry of the holey region as well as material birefringence induced by stress applying elements. Using this approach, we calculated the spectral dependence of phase and group modal birefringence for different geometry of the analyzed fiber. Furthermore, the spectral dependence of polarimetric sensitivity to temperature was determined. The calculation results were compared with experimental data published earlier.