Elka Karakoleva

Single-mode fiber polished into the core as a sensor element

Abstract The light transmittance of a single-mode fiber, polished into the core, at different polishing depths is investigated experimentally. New results concerning amplitude, polarization and wavelength characteristics of such waveguide structure are presented, revealing the feasibility of this structure to act as a simple fiber-optic sensing element.

Pulsed laser deposited ZnO film on side-polished fiber as a gas sensing element

A simple sensor element consisting of a side-polished single-mode fiber and a planar metal oxide waveguide is described. The thin ZnO planar waveguide was produced on the polished fiber surface by pulsed laser deposition at optimized processing parameters. A measurement scheme for in situ control of the film thickness during the deposition process was developed and used. X-ray diffraction measurements and scanning electron microscopy were used to characterize the structure and the surface morphology of the planar waveguide, respectively. The numerical evaluation of the sensor sensitivity predicts the possibility to detect refractive index changes of less than 10(-4). Furthermore, preliminary gas sensor tests were performed by using a mixture of 1.5% butane diluted in N(2) and pure butane. A shift of the spectral position of the resonance points was observed from 3 to 5 s after gas exposure, which corresponds to refractive index changes of 3 x 10(-5) and 1.2 x 10(-3) for 1.5% butane and for pure butane, respectively.

Numerical investigation of refractometric sensor elements based on side polished fibres using the Galerkin method

The Galerkin method was applied to solve the vector wave equation in order to determine the propagation constants and the transverse electric fields of the modes propagating along side polished single-mode and two-mode optical fibres. The effective refractive indices of the modes were calculated depending on the values of the residual cladding (minimum distance between a fibre core and a polished surface) and the superstrate refractive index. The influence of the fibre parameters and working wavelength on the refractometric sensitivity was estimated in the case when a side polished fibre with inscribed in-fibre Bragg grating is used as a sensor element.

Fibre bragg grating refractometer with enhanced sensitivity and temperature reference using distinct evanescent field distributions

Evanescent field interaction in side-polished fibre Bragg gratings (FBG) provides a sensor for measuring refractive indices of liquid analytes and thin films with spectral read-out. The specific advantages of FBG sensors - networking capability, independence of variable signal transmission losses - give the potential to a higher diversity and accuracy of opto-chemical measurements than the measurement of transmission intensities through evanescent field sensitive fibre structures.

LOCALIZED FUNCTION METHOD APPLYING A SET OF SINE FUNCTIONS TO MODEL PHOTONIC CRYSTAL FIBRES

A development and an application of the localized function method based on the Galerkin method applying a set of sine functions to approximate the unknown mode fields of the localized modes propagating along the photonic crystal fibres (PCFs) is proposed. A way for considerably reducing the number of integrals in the case of symmetrical shapes of holes with respect to the axes of coordinate systems located at the centres of the holes (circular, elliptical, etc.) is also presented. The method does not require an expansion of the refractive index and thus inaccuracies of the expansion can be avoided. In the case of a circular form of the holes all integrals are solved analytically.

Highly sensitive in-line fiber-optic structure for refractometric measurements

A refractometric structure consisting of a side-polished single-mode optical fiber covered with a thin film of hydrogenated amorphous silicon (a-Si:H) has been studied. a-Si:H films have been deposited by plasma-enhanced chemical vapour decomposition of silane. The sensors elements working with zero and first order optical modes of the amorphous silicon waveguide have been demonstrated. A high sensitivity of 5300 nm per unit refractive index has been obtained for the low values of superstrate refractive indices in the range of 1.33-1.38 by using a-Si:H film with a thickness of about 14 nm, i.e. the interaction with TE0 mode of the PWG. Despite of the very low thickness of the a-Si:H film there is no need of any additional layer protecting the structure from the analyzed medium. The mechanical and optical stability of the film and its very good adhesion to fused silica eliminate the possibility of the film damaging and makes the structure reliable in use.