Jan Wojcik

Microstructured low and high birefringence four core fibers for sensing applications

Optical fiber strain or bend sensors have been demonstrated for complex monitoring various structures especially in aerospace, marine civil engineering and especially in optical security systems. Optical fiber sensors based on Fabry-Perot interferometers and fiber Bragg gratings are useful for point or slow-multiplied sensor systems. Distributed optical fiber system based on Sagnac interferometers used of low or high-birefringence optical fibers as strain/bend sensing elements. Special optical fibers as four core single mode fibers in Mach-Zehnder interferometer system have been used. In all described cases the high influence of temperature on sensor characteristics is very important disadvantage at measurement systems. In this paper we describe fabrication and characterization of the new four core PCF index guided fiber designed for interferometric bend measurements.

Nonlinear light propagation in photonic crystal fibers filled with nematic liquid crystals

In this work nonlinear light propagation in a photonic crystal fiber (PCF) infiltrated with a nematic liquid crystal (NLC) is presented. Such a photonic structure, called the photonic liquid crystal fiber (PLCF), combines the passive PCF and the active NLC guest mixture. The analyzed configuration with a periodic modulation of spatial refractive index distribution corresponds to the matrix of waveguides. This kind of structure can be controlled by optical power and additionally by temperature and it allows for studying variety of discrete optical phenomena. For properly chosen parameters of the analyzed fiber, discrete diffraction in the linear case and generation of the discrete spatial soliton in nonlinear regime can be obtained. In this paper a possibility of the transverse light localization and delocalization due to both focusing and defocusing Kerr-type nonlinearity was analyzed. In the case of the positive nonlinearity the refractive index increases as a function of light intensity in such a way that the stronger guiding of the light within NLC cores is obtained. Light modifies the refractive index distribution inducing a defect in the periodic structure. That can lead to the situation in which light becomes self-localized and its diffractive broadening is eliminated. Eventually the discrete soliton can be created. In the case of negative nonlinearity, the difference between NLC waveguides and glass refractive indices decreases and the beam guidance becomes weaker for higher light intensities. In such a case the generation of the bright soliton is possible only in the regime of negative discrete diffraction. However, in the case of defocusing nonlinearity a decrease of refractive index with the optical power can lead to the bandgap shifting. The incident beam with a frequency initially within a bandgap is then turned outside the bandgap resulting in changing of the propagation mechanism to the modified total internal reflection.

Characterization of elliptical-core side-hole fibers for interferometric pressure sensing

Results of preliminary testing of two newly developed highly birefringent fibers, especially designed for hydrostatic pressure sensing employing a white-light interferometric technique are presented. In particular, measurements of pressure and temperature sensitivities, temperature-pressure cross-sensitivity, and mode coupling effect induced by pressure and temperature are reported.

Multiplexed polarimetric sensors with highly birefringent optical fibers for smart structures

Results of initial studies of multiplexed polarimetric fiber optic sensing systems for smart structures applications are presented. The prepared polarimetric smart structures based on Fibercore bow-tie fibers were subjected to deformation effects such as those induced by hydrostatic pressure and temperature, whereas polarization properties of the transmitted optical signal have been investigated. The presence of the smart structure modifies the output characteristics of the highly birefringent fiber due to elastic properties of the structure. The applied experimental procedure had an objective to compare the phenomena occurring in both: the embedded fibers and the separated highly birefringent fibers influenced by the same deformation effects.

Liquid crystalline optical fibers for pressure monitoring

Initial results of experimental studies of light propagation by optical fibers with liquid crystalline cores under hydrostatic pressure conditions are reported. Specially drawn hollow-core fibers (capillary tubes of radii 15 microns) were filled with a liquid crystal mixture. The whole system composed of the fiber and the liquid crystal has been placed in a high pressure chamber designed to sustain pressures up to 100 MPa. The liquid crystalline-core optical fiber acts as an optically anisotropic medium characterized by an index ellipsoid, and can serve as a fiber with easily controlled birefringence. Since hydrostatic pressure generate stress effects occurring in the system, a new class of fiber-optic pressure sensors can be introduced. The paper presents preliminary characteristics of the pressure sensor utilizing liquid crystalline-core fibers. Envisaged areas of applications include pipe-lines, mining instrumentation, process control, and environmental protection.

Pressure sensitivity of the birefringent photonic crystal fiber with triple defect

We investigated theoretically and experimentally an impact of hydrostatic pressure on phase modal birefringence in birefringent photonic crystal holey fiber of new construction. The birefringence in this fiber is induced by highly elliptical shape of the core, which consists of triple defect in the hexagonal structure. Using finite element method, we first calculated the stress components and deformations induced by hydrostatic pressure in the fiber cross-section. In the second step, the distribution of the stress-related corrections of refractive index were determined. Finally, we calculated the sensitivity of the phase modal birefringence (dB/dp) to hydrostatic pressure versus wavelength. The contribution of the geometrical effects related only to deformation of the holey structure as well as the stress-related contribution to the overall pressure sensitivities were analyzed separately. Our results show that these two factors decrease the phase modal birefringence, which results in negative sign of dB/dp. We also measured the pressure sensitivity for several wavelengths using polarimetric technique. The experimental and theoretical values of dB/dp show very good agreement.

Analysis of sensitivity of side-hole optical fibers to pressure and temperature by the finite element method

In our laboratory there was elaborated the mode of preparation of side-hole optical fibers characterized by high sensitivity to pressure/sensitivity to temperature ratio. This ratio is two magnitude order higher than this obtained for normal SMPM optical fibers. This work presents the quantitative explanation of this advantageous phenomenon. It has presented the mode and the results of calculations of selectivity to pressure in relation to temperature for defined class of side-hole optical fibers. It has resulted that there exist numerous structures of side-hole fibers for which the sensitivity to pressure/sensitivity to temperature ratio is not lower than 1000. Described mode of calculations of selectivity of birefringent optical fibers to pressure in relation to temperature will permit to optimization of these sensor fibers for technological purposes.

Rocking filter in microstructured birefringent fiber for hydrostatic pressure measurements

We present sensing characteristics of higher order rocking filters fabricated in highly birefringent microstructured fiber which resonantly couple polarization modes at several wavelengths. First rocking filter (RF1) shows tree resonances arising at 855, 1271, and 1623 nm, while in the second filter (RF2) the resonances arise at 908, 1145, 1354 and 1548 nm. We measured sensitivity to temperature in both filters and to hydrostatic pressure in the RF1. Our results show that both filters have very low response to temperature ranging from 1.38 to 3.03 pm/K depending on the resonance order. Simultaneously, the sensitivity to hydrostatic pressure is very high and reaches 6.14 and 3.30 nm/MPa, respectively for the first and the second resonance in the RF1. These unique sensitivity characteristics make the filters an excellent device for hydrostatic pressure measurement with no need for temperature compensation.

Photonic liquid crystal fibers for electric field and hydrostatic pressure sensing

Photonic Crystal Fibers, optical fibers with regular structure of micro-holes running along the axial direction, have ability to change their optical properties through inserting different materials into their holes. The paper presents our latest experimental results of the influence of external electric field and hydrostatic pressure on propagation properties of the photonic crystal fibers infiltrated with liquid crystals clearly indicating great potential for electric field and hydrostatic pressure sensing applications. Operating range of both electric field and hydrostatic pressure sensors can be tailored by different combination of a host photonic crystal fiber and a liquid crystal used for infiltration. Moreover, by changing the operating wavelength different sensor responses can be obtained.

Measurement of modal birefringence and temperature sensitivity of birefringent holey fibers

We present the results of measurements of modal birefringence and temperature sensitivity of birefringent holey fibers fabricated by Fiber Optic Group, University of Marie Curie-Sklodowska (UMCS) in Lublin, Poland. The birefringence measurements were carried out in a wide spectral range of 0.63 - 1.57 μm in two fibers with different hole diameters and pitch distances. Our results show that absolute value of birefringence increases against wavelength and is one order of magnitude greater than in conventional highly birefringent fibers. The measurements of temperature sensitivity carried out for bare fibers show that zero sensitivity can be achieved at certain wavelength.