Marcin Szpulak

Strong infrared spectral broadening in low-loss As-S chalcogenide suspended core microstructured optical fibers

We report the fabrication and characterization of the first guiding chalcogenide As(2)S(3) microstructured optical fibers (MOFs) with a suspended core. At 1.55 microm, the measured losses are approximately 0.7 dB/m or 0.35 dB/m according to the MOF core size. The fibers have been designed to present a zero dispersion wavelength (ZDW) around 2 microm. By pumping the fibers at 1.55 microm, strong spectral broadenings are obtained in both 1.8 and 45-m-long fibers by using a picosecond fiber laser.

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.

Fiber Bragg Gratings in Germanium-Doped Highly Birefringent Microstructured Optical Fibers

We present a dedicated fiber Bragg grating (FBG) inscription experiment to investigate the compatibility of a microstructured optical fiber (MOF) with conventional FBG inscription setups. For the studied MOF, the angular orientation of the fiber in the interferometric excimer laser setup was found to have no significant influence on the final reflection of the inscribed FBGs. We also show that an array of multiplexed FBGs can be inscribed in a single MOF with a repeatability and quality that match fiber sensing requirements.

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.

Effect of coupling between fundamental and cladding modes on bending losses in photonic crystal fibers.

The paper presents a fully vectorial analysis of bending losses in photonic crystal fibers employing edge/nodal hybrid elements and perfectly matched layers boundary conditions. The oscillatory character of losses vs. both the wavelength and the bending radius has been demonstrated. The shown oscillations originate from the coupling between the fundamental mode guided in the core and the gallery of cladding modes arising due to light reflection from the boundary between solid and holey part of the cladding.

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.

Chalcogenide As

A chalcogenide optical fiber of special design is proposed to convert a short-wavelength infrared radiation (around 2 mum) up to a second transparency window of atmospheric air (around 4.5 mu m) by degenerate four-wave mixing. The fiber supports a small core surrounded by three large air holes. The zero-dispersion wavelength is shifted down to 2 mum in this fiber by properly tailoring the fiber core. We demonstrate by solving the nonlinear Schrodinger equation that efficient wavelength-conversion can be obtained by pumping the fiber with a Tm:SiO2 pulsed fiber laser.

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We present a summary of the simulation exercise carried out within the EC Cost Action P11 on the rigorous modeling of photonic crystal fiber (PCF) with an elliptically deformed core and noncircular air holes with a high fill factor. The aim of the exercise is to calculate using different numerical methods and to compare several fiber characteristics, such as the spectral dependence of the phase and the group effective indices, the birefringence, the group velocity dispersion and the confinement losses. The simulations are performed using four rigorous approaches: the finite element method (FEM), the source model technique (SMT), the plane wave method (PWM), and the localized function method (LFM). Furthermore, we consider a simplified equivalent fiber method (EFM), in which the real structure of the holey fiber is replaced by an equivalent step index waveguide composed of an elliptical glass core surrounded by air cladding. All these methods are shown to converge well and to provide highly consistent estimations of the PCF characteristics. Qualitative arguments based on the general properties of the wave equation are applied to explain the physical mechanisms one can utilize to tailor the propagation characteristics of nonlinear PCFs.

S

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.

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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.