Tadeusz Martynkien

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.

Dispersion effects in elliptical-core highly birefringent fibers

Modal birefringence and its sensitivity to temperature and hydrostatic pressure were measured versus wavelength in three elliptical-core fibers and one fiber with stress-induced birefringence. We carried out the measurements in the spectral range from 633 to 843 nm by using interferometric methods. In fibers with elliptical cores all the measured parameters showed high chromatic dependence, whereas in fibers with stress-induced birefringence this dependence was weak. We modeled the dispersion characteristics of two elliptical-core fibers by using the modified perturbation approach first proposed by Kumar. The modification consists of introducing into the expression for the normalized propagation constants an additional perturbation term that contains information about stress-induced birefringence. The results of modeling show that the temperature and pressure sensitivity of elliptical-core fiber are associated primarily with variations in stress induced by these parameters. The agreement between measured and calculated values of sensitivity in the worst case was equal to 20% for modal birefringence and temperature sensitivity and 50% for pressure sensitivity. Lower agreement between measured and calculated values of pressure sensitivity is most probably associated with uncertainties in the material constants used in modeling.

Effects of hydrostatic pressure on phase and group modal birefringence in microstructured holey fibers

We calculated the sensitivity of phase (dB/dp) and group (dG/dp) modal birefringence to hydrostatic pressure versus wavelength in two birefringent holey fibers of different construction, where B is the phase modal birefringence, G is the group modal birefringence, and p is the pressure applied to the fiber. The contributions of the geometrical effects that were related only to deformation of the holey structure and the stress-related contribution to the overall pressure sensitivities were analyzed separately. Our results show that these two factors decrease the phase modal birefringence in both structures, which results in negative signs of dB/dp and dG/dp. Furthermore, we demonstrate that the geometrical effects are much weaker than the stress-related effects and contribute only a few percent to the overall pressure sensitivity.

Dispersion of group and phase modal birefringence in elliptical-core fiber measured by white-light spectral interferometry

We present a white-light spectral interferometric technique employing a low-resolution spectrometer for measurement of the dispersion of the group and phase modal birefringence in an elliptical-core optical fi ber over a wide spectral range. The technique utilizes a tandem con fi guration of a Michelson interferometer and the optical fi ber to record a series of spectral interferograms and to measure the equalization wavelengths as a function of the optical path difference in the Michelson interferometer, or equivalently, the wavelength dependence of the group modal birefringence in the optical fi ber. Applying a polynomial fi t to the measured data, the wavelength dependence of the phase modal birefringence can also be determined.

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.

Optical refractometer based on a birefringent Bragg grating written in an H-shaped fiber

An optical refractometer based on a birefringent (Hi-Bi) fiber Bragg grating (FBG) written in a new H-shaped fiber structure is proposed. This structure is formed by opening the two holes of a side-hole fiber using chemical etching. When the Hi-Bi FBG is immersed in different liquids, different responses of the slow and fast wavelengths are obtained. The refractometer is also capable of simultaneous measurement of refractive index and temperature.

Modeling and measurement of temperature sensitivity in birefringent photonic crystal holey fibers.

We analyzed theoretically the spectral dependence of polarimetric sensitivity to temperature (KT) and the susceptibility of phase modal birefringence to temperature (dB/dT) in several birefringent photonic crystal holey fibers of different construction. Contributions to dB/dT related to thermal expansion of the fiber dimensions and that related to temperature-induced changes in glass and air refractive indices were calculated separately. Our results showed that dB/dT depends strongly on the material used for manufacturing the fiber and on the fibers geometry. We demonstrate that, by properly designing the birefringent holey fiber, it is possible to reduce its temperature sensitivity significantly and even to ensure a null response to temperature at a specific wavelength. Furthermore, we show that the temperature sensitivity in a fiber with arbitrary geometry can be significantly reduced by proper choice of the glass used in the fibers manufacture. We also measured the polarimetric sensitivity to temperature and identified its sign in two silica-air fibers. The experimental values are in good agreement with the results of modeling.

Measurements of polarimetric sensitivity to hydrostatic pressure, strain and temperature in birefringent dual-core microstructured polymer fiber

We experimentally characterized a birefringent microstructured polymer fiber of specific construction, which allows for single mode propagation in two cores separated by a pair of large holes. The fiber exhibits high birefringence in each of the cores as well as relatively weak coupling between the cores. Spectral dependence of the group and the phase modal birefringence was measured using an interferometric method. We have also measured the sensing characteristics of the fiber such as polarimetric sensitivity to hydrostatic pressure, strain and temperature. Moreover, we have studied the effect of hydrostatic pressure and strain on coupling between the cores.

Nonlinear femtosecond pulse propagation in an all-solid photonic bandgap fiber

Nonlinear femtosecond pulse propagation in all-solid photonic bandgap fiber is investigated experimentally and numerically for both the photonic bandgap guiding in the central silica core and the total internal reflection in germanium doped inclusions.

Experimental investigations of bending loss oscillations in large mode area photonic crystal fibers

We demonstrate experimentally that bending loss in large mode area photonic crystal fibers oscillates with wavelength. To do so we carried out loss measurements for different fiber bend radii and for different angular orientations. These results confirm the oscillatory behavior of bending loss vs. wavelength as predicted recently by numerical analysis [J. Olszewski et al., Opt. Express 13, 6015 (2005)]. We also found good agreement between our measurement results and our simulations relying on a finite element method with perfectly matched layers and an equivalent index model.