The guiding properties of fabricated air-silica Bragg fibers with different geometric
characteristics have been numerically investigated through a modal solver based on the finite element method.
The method has been used to compute the dispersion curves, the loss spectra and the field distribution of the modes
sustained by the Bragg fibers under investigation.
In particular, the silica bridge influence on the fundamental mode has been analyzed,
by considering structures with different cross sections, that is an ideal Bragg fiber, without the silica nonosupports,
a squared air-hole one and, finally,
a rounded air-hole one, which better describes the real fiber transverse section.
Results have shown the presence of anti-crossing points in the effective index curves
associated with the transition of the guided mode to a surface mode.
Moreover, it has been verified that these surface modes are responsible of the loss peaks in the fiber transmission spectra,
also experimentally measured.
Surface modes are mainly localized in the regions of the cladding where
the bridge supports join the cladding rings, forming silica islands where the field can focuses.
In order to realize an efficient absorption measurement based evanescent-wave sensor, a long interaction length and a strong penetration of the optical field into the sample space is required. For an optical fiber based device, with a solid silica core immersed into a liquid sample, the strength of the evanescent field increases with decreasing core radius. When the core diameter is comparable to the wavelength of the light, a large fraction of the light propagates in the evanescent field. We demonstrate evanescent-wave sensing on aqueous solutions of fluorophore labeled biomolecules positioned in the air holes of a hollow-core photonic crystal fiber (PCF). The aqueous solutions can be positioned in close proximity to light guided in small cores without removing the coating and cladding, thus ensuring a very robust device. In order to make selective DNA detection, we coated the inside of the hollow-core PCF with a sensing layer, which by hybridization selectively immobilize specific molecules. A fluorescence measurement method, where a line-shaped laser beam expose the fiber from the side and excites the fluorophore molecules, was realized. The emitted fluorescence tunnels via the evanescent field into the fiber core(s) and is analyzed by a spectrometer at the fiber end.
Photonic crystal materials and waveguides have since their appearance in 1987 attracted very much attention from the scientific community. From being a more academia discipline, new components and functionalities have emerged, and photonic crystals have today started to enter the field of commercial devices. Especially the photonic crystal fiber (PCF) with its lattice of air holes running along the length of the fiber has matured, and the technology provides a large variety of novel optical properties and improvements compared to standard optical fibers. With respect to optical sensors, the photonic crystal structures have several important properties. First of all the wavelength-scale periodically-arranged material structures provide completely new means of fabricating tailored optical properties either using modified total internal reflection or the photonic bandgap effect. Secondly, the new materials with numerous micro- or even nano-scale structures and voids allow for superior mode control, use of polarization properties, and even more a the potential of close interaction between optical field and the material under test. The present paper will be using the example of the relatively mature photonic crystal fiber to discuss the fundamental optical properties of the photonic crystals, and recent examples of their use as optical sensors will be reviewed.