Mariusz Makara

Highly birefringent microstructured fibers with enhanced sensitivity to hydrostatic pressure

We designed, manufactured and characterized two birefringent microstructured fibers that feature a 5-fold increase in polarimetric sensitivity to hydrostatic pressure compared to the earlier reported values for microstructured fibers. We demonstrate a good agreement between the finite element simulations and the experimental values for the polarimetric sensitivity to pressure and to temperature. The sensitivity to hydrostatic pressure has a negative sign and exceeds -43 rad/MPa x m at 1.55 microm for both fibers. In combination with the very low sensitivity to temperature, this makes our fibers the candidates of choice for the development of microstructured fiber based hydrostatic pressure measurement systems.

Experimental and theoretical investigations of birefringent holey fibers with a triple defect.

We have manufactured and characterized a birefringent holey fiber of a new construction. The birefringence in this fiber is induced by the highly elliptical shape of the core, which consists of a triple defect in a hexagonal structure. Using a hybrid edge-nodal finite-element method, we calculated the spectral dependence of phase and group modal birefringence for spatial modes E11 and E21 in idealized and in real fiber, whose geometry we determined by using a scanning-electron microscope. Results of our calculations show that technological imperfections significantly affect the fibers birefringence. Normalized cutoff wavelengths for higher-order modes relative to the filling factor were also determined for the idealized structure. We observed a significant disagreement between theoretical and experimental values of cutoff wavelengths, which was attributed to high confinement losses near the cutoff condition. We also measured the spectral dependence of the phase and the group modal birefringence for spatial modes E11 and E21. The measured parameters showed good agreement with the results of modeling.

Measurements of polarimetric sensitivity to temperature in birefringent holey fibres

We measured spectral dependence of the polarimetric sensitivity to temperature in three birefringent holey fibres with different geometries. Our results show that thermal properties of the birefringent photonic crystal fibres depend very much on the air hole arrangement. Using the procedure which allows us to determine the sign of temperature sensitivity, we demonstrated that one of the investigated fibres is insensitive to temperature at a certain wavelength. We also measured the temperature sensitivities for the fibres with and without polymer coating and showed that the coating changes significantly the fibre response to temperature. Furthermore, we demonstrated experimentally that the spectral dependence of the polarimetric sensitivity to temperature in birefringent holey fibres obeys a scaling law.

Control Over the Pressure Sensitivity of Bragg Grating-Based Sensors in Highly Birefringent Microstructured Optical Fibers

We present fiber Bragg grating (FBG)-based hydrostatic pressure sensing with highly birefringent microstructured optical fibers. Since small deformations of the microstructure can have a large influence on the material birefringence and pressure sensitivity of the fiber, we have evaluated two microstructured fibers that were made from comparable fiber preforms, but fabricated using different temperature and pressure conditions. The magnitude and sign of the pressure sensitivity are found to be different for both fibers. We have simulated the corresponding change of the Bragg peak separation with finite-element models and experimentally verified our results. We achieve very high experimental sensitivities of -15 and 33 pm/MPa for both sensors. To our knowledge, these are the highest sensitivities ever reported for birefringent FBG-based hydrostatic pressure sensing.

Photonic crystal fibers: new opportunities for sensing

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.

Highly birefringent dual-mode microstructured fiber with enhanced polarimetric strain sensitivity of the second order mode

We present the results of theoretical and experimental characterization of a designed and manufactured dual-mode highly birefringent microstructured fiber. We also demonstrate the measured values of polarimetric temperature and strain sensitivity of both the fundamental and second order modes. As the mode field of the second order mode has a strong interaction with the fiber air holes, we observed a significant (over two orders of magnitude) increase in the polarimetric strain sensitivity of this mode in comparison to the fundamental mode. The enhanced strain sensitivity together with the low temperature sensitivity makes our fiber very attractive for application as extremely sensitive temperature independent strain transducers.

Spectral-domain measurements of birefringence and sensing characteristics of a side-hole microstructured fiber

We experimentally characterized a birefringent side-hole microstructured fiber in the visible wavelength region. The spectral dependence of the group and phase modal birefringence was measured using the methods of spectral interferometry. The phase modal birefringence of the investigated fiber increases with wavelength, but its positive sign is opposite to the sign of the group modal birefringence. We also measured the sensing characteristics of the fiber using a method of tandem spectral interferometry. Spectral interferograms corresponding to different values of a physical parameter were processed to retrieve the spectral phase functions and to determine the spectral dependence of polarimetric sensitivity to strain, temperature and hydrostatic pressure. A negative sign of the polarimetric sensitivity was deduced from the simulation results utilizing the known modal birefringence dispersion of the fiber. Our experimental results show that the investigated fiber has a very high polarimetric sensitivity to hydrostatic pressure, reaching −200 rad × MPa−1× m−1 at 750 nm.

Sensing and transmission characteristics of a rocking filter fabricated in a side-hole fiber with zero group birefringence

We report on sensing and transmission characteristics of rocking filters fabricated in a silica side-hole fiber with group birefringence changing its sign at certain wavelength (λ(G = 0)), which corresponds to parabolic-like spectral dependence of beat length. Unusual birefringence dispersion of the side-hole fiber is induced by an elliptical germanium doped core located in a narrow glass bridge between two holes. Rocking filters fabricated in such a fiber have two resonances of the same order located on both sides of λ(G = 0). The sensitivity of both resonances has an opposite sign, which makes it possible to double the response of the rocking filter by applying the differential interrogation scheme. We demonstrate that in this way a pressure sensitivity of the rocking filter can be enlarged to 132 nm/MPa. We also show that by fabricating the rocking filter with a period close to maximum beat length a coupling between polarization modes can be obtained in a broad band reaching 240 nm.

Sensing characteristics of the rocking filters in microstructured fibers optimized for hydrostatic pressure measurements

We report on the sensing characteristics of rocking filters fabricated in two microstructured fibers with enhanced polarimetric sensitivity to hydrostatic pressure. The filter fabricated in the first fiber shows a very high sensitivity to pressure ranging from 16.2 to 43.4 nm/MPa, depending on the resonance order and features an extremely low cross-sensitivity between pressure and temperature 28 ÷ 89 × 10(3) K/MPa. The filter fabricated in the second fiber has an extreme sensitivity to pressure ranging from -72.6 to -177 nm/MPa, but a less favorable cross-sensitivity between pressure and temperature of 1.05 ÷ 3.50 × 10(3) K/MPa. These characteristics allow using the rocking filters for pressure measurements with mbar resolution.

Influence of Fiber Orientation on Femtosecond Bragg Grating Inscription in Pure Silica Microstructured Optical Fibers

We studied the influence of the fiber orientation on the growth of fiber Bragg gratings (FBGs) in pure silica microstructured optical fibers (MOFs) during femtosecond UV laser inscription. To do so we simulated the transverse coupling efficiency as a function of the relative angle between the inscribing laser beam and the internal microstructure for hexagonal lattice and highly birefringent MOFs by a finite-difference time-domain (FDTD) method. The orientation was predicted to play a far more important role in the highly birefringent MOF. We confirmed these simulation results with the fabrication of wavelength-multiplexed FBG arrays in pure silica core MOFs under different fiber orientations with 266-nm femtosecond laser pulses and a Talbot interferometer configuration.