Search

International audience; We analyzed for the first time the effect of variations in the number of air hole rings and the filling factor of twisted microstructured optical fibers on the resonant couplings between fundamental and cladding modes. Rigorous numerical simulations show that these parameters can be used to control the spectral width of the resonance peaks, resonance loss, and relative strength of polarization effects. Furthermore, the number of air hole rings has a decisive impact on the number of twist-induced resonances and their wavelength range.

We show that in twisted microstructured optical fibers (MOFs) the coupling between the core and cladding modes can be obtained for helix pitch much greater than previously considered. We provide an analytical model describing scaling properties of the twisted MOFs, which relates coupling conditions to dimensionless ratios between the wavelength, the lattice pitch and the helix pitch of the twisted fiber. Furthermore, we verify our model using a rigorous numerical method based on the transformation optics formalism and study its limitations. The obtained results show that for appropriately designed twisted MOFs, distinct, high loss resonance peaks can be obtained in a broad wavelength range already for the fiber with 9 mm helix pitch, thus allowing for fabrication of coupling based devices using a less demanding method involving preform spinning.

We demonstrate a polarized all-normal dispersion supercontinuum generated in a birefringent silica microstructured fiber spanning beyond 2.5 µm. To our knowledge, this is the spectra reaching the furthest in mid-infrared ever generated in normal dispersion silica fibers. The generation process was studied experimentally and numerically with 70 fs pump pulses operating at different wavelengths on short propagation distances of 48 mm and 122 mm. The all-normal operation was limited by the zero-dispersion wavelength at 2.56 µm and spectral broadening was stopped by OH absorption peak at 2.72 µm. We identified the asymmetry between propagation in both polarization axes and showed that pumping along a slow fiber axis is beneficial for a higher degree of polarization. Numerical simulations of the generation process conducted by solving the generalized nonlinear Schrödinger equation (NLSE) and coupled NLSEs system showed good agreement with experimental spectra.

We demonstrate shear stress sensing with a Bragg grating-based microstructured optical fiber sensor embedded in a single lap adhesive joint. We achieved an unprecedented shear stress sensitivity of 59.8 pm/MPa when the joint is loaded in tension. This corresponds to a shear strain sensitivity of 0.01 pm/mu epsilon. We verified these results with 2D and 3D finite element modeling. A comparative FEM study with conventional highly birefringent side-hole and bow-tie fibers shows that our dedicated fiber design yields a fourfold sensitivity improvement. (C) 2013 Optical Society of America

Using a fully vectorial numerical method based on the transformation optics formalism, we analyzed the effect of resonant coupling between the core and cladding modes in twisted, elliptical core fibers. Our rigorous simulations revealed the existence of a much richer resonance spectrum than that predicted by simplified perturbation approaches. This effect is caused by the appearance of even harmonics in the angular field distribution of higher-order cladding modes due to their interaction with the fiber core with two-fold symmetry. We also analyzed the dependence of the polarization state of the core modes upon the twist rate and wavelength. We demonstrated the effect of the resonant-polarization perturbation of the core modes, which is especially visible for the couplings between circularly polarized core modes and cladding modes of opposite handedness.

We report on the fabrication of a birefringent side-hole polymer optical fiber with an elliptical core made of polymethyl metacrylate-polystyrene (PMMA/PS) copolymer and pure PMMA cladding. The fiber core is located in a narrow PMMA bridge separating the holes. Two fibers with different bridge thickness were fabricated and characterized. We demonstrate, experimentally and numerically, that, by narrowing the bridge between the holes, one can increase simultaneously the fiber birefringence and the polarimetric sensitivity to hydrostatic pressure. In the fiber with the bridge as narrow as 5 μm, we achieved a record-high polarimetric sensitivity to hydrostatic pressure ranging between 175 and 140 rad/MPa/m in the spectral range of 600-830 nm. The phase modal birefringence in this fiber is also high and exceeds 3×10(-5) at 600 nm, which results in small polarization cross talk.

We present the possibility of mechanical inducement of a rocking filter in a birefringent microstructured polymer fiber, which resonantly couples polarization modes. A birefringence in the fiber used for rocking filter fabrication is induced by two large holes adjacent to the core. Because of the small pitch distance of the microstructured cladding, the phase and the group modal birefringence in this fiber are relatively high and equal, respectively, 1.2×10sup-4/supand -2×10sup-4/supat λ=800 nm, while the fiber loss is 5 dB/m at λ=850 nm. We demonstrate transmission characteristics of rocking filters mechanically induced in this fiber. A 22 dB deep first-order resonance located in the visible spectral range was observed, accompanied by a second-order resonance in the near-infrared. We also show that by changing the filter period and load applied to the fiber, one can tune the resonance position and depth.

We demonstrate for the first time in optical fibers the “dispersion of the axes” phenomenon, which relies upon spectral dependence of polarization planes of the guided modes. This feature was achieved in a specially designed side-hole fiber by combining linear birefringence induced by a pair of holes located in the cladding and the elliptical core tilted with respect to the symmetry plane of the holes. We performed numerical simulations of this design properties. The side-hole fiber with a tilted core was fabricated and experimentally studied. The orientation of the principal polarization planes of the fundamental modes varied by 14.5° in the analyzed spectral range. This value is in a good agreement with the simulation results.

We report for the first time on rigorous numerical simulations of a helical-core fiber by using a full vectorial method based on the transformation optics formalism. We modeled the dependence of circular birefringence of the fundamental mode on the helix pitch and analyzed the effect of a birefringence increase caused by the mode displacement induced by a core twist. Furthermore, we analyzed the complex field evolution versus the helix pitch in the first order modes, including polarization and intensity distribution. Finally, we show that the use of the rigorous vectorial method allows to better predict the confinement loss of the guided modes compared to approximate methods based on equivalent in-plane bending models.

We report on the fabrication of a birefringent microstructured PMMA fiber with polystyrene stress-applying elements located in the solid part of the cladding. A microstructured part of the cladding composed of three rings of holes was made of a technical-grade PMMA by a drilling method. The fiber shows a relatively high birefringence of the order of 4×10(-5), which weakly depends upon wavelength in the investigated spectral range from 0.6 to 1 μm. The cross talk between polarization modes is lower than -20 dB for a 1 m long fiber, while the fiber loss is about 8 dB/m at 0.83 μm. We also studied the fiber response to temperature in the range from 20°C to 60°C. The temperature induced birefringence change is negative and shows a significant hysteresis in the first cycle, which gradually disappears in successive cycles.

We present a microstructured polymer fiber dedicated to long period grating (LPGs) inscription using a focused UV laser beam. The core and the microstructured cladding of the developed fiber are made of pure poly(methyl methacrylate) (PMMA). The external layer of the solid part of the cladding has increased UV absorption due to doping with trans-4-stilbenemethanol, which shows an absorption band at around 310 nm related to trans-cis photoisomerization. We present transmission characteristics of LPGs fabricated in this fiber using the point-by-point inscription technique with a He-Cd laser beam of 325 nm wavelength. We also demonstrate that in the proposed fiber, the fabrication process is shortened six times compared to pure PMMA fibers. Moreover, we report on temperature response and long-term stability of the fabricated gratings.

We studied the transmission characteristics of higher-order rocking filters induced mechanically in birefringent microstructured fibers and standard elliptical core fibers with varying spectral dependence of phase modal birefringence. We demonstrated the effect of birefringence dispersion on polarization mode coupling induced by a point-like force. We also investigated the spectral dependence of the resonance depth and force-induced resonance wavelength shift in mechanical rocking filters. The observed phenomena were explained by a numerical model linking the spectral dependence of the polarization mode coupling coefficient with the dispersion of intrinsic fiber birefringence and applied force.

We analyzed the effect of fiber bending on spectral position and strength of the surface plasmon resonance arising due to the interaction of the fundamental mode guided in the core of the microstructured fiber with a metal layer. Fully vectorial simulations were performed using the finite element method with perfectly matched layers boundary conditions. To conduct the simulations, we adopted the concept of an equivalent bent fiber developed recently on the ground of transformation optics formalism. In this approach, the bent fiber with a metal layer is replaced by an equivalent fiber with appropriate spatial distributions of electric permittivity and magnetic permeability tensors. The obtained results explain the mechanisms responsible for the change in the SPR spectrum induced by bending and by the geometry of the microstructured fiber. By modifying the holes layout in the microstructured cladding, we designed the fiber, in which the depth of the surface plasmon resonance is in a high degree tunable by bending.

We have studied the effect of waveguide alignment on the reflection spectrum of Bragg gratings fabricated using a multiple order phase mask. We have demonstrated that splitting of certain Bragg peaks observed in earlier experiments reported in literature is caused by formation of the gratings with different periodicities in the waveguide tilted with respect to the phase mask plane due to the interference of non-symmetrical diffraction orders. Analytical expressions for spectral separation of the split peaks have been derived and verified against the experimental data recently presented in literature. The analytical predictions were also confirmed by numerical simulations of intensity distributions behind the multiple order diffraction grating and its projection on the tilted waveguide.

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.

We demonstrate that quasi-phase-matching of the third-harmonic generation process can be obtained for a pulse pump in the photonic crystal fiber with a refractive-index grating. Conversion efficiency is calculated numerically using a system of coupled generalized nonlinear Schrodinger equations. We propose a special design of the microstructured fiber for the third-harmonic generation and analyze different phenomena limiting the maximum efficiency for short (femtosecond) and long (picosecond) pump pulses. Moreover, we show that a certain level of a group-velocity mismatch between the pump and the third harmonic can increase the maximum efficiency in the long pulse regime.

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.

We demonstrate that higher order rocking filters coupling energy between polarization modes at several wavelengths can be fabricated in a birefringent photonic crystal fiber using a fusion arc splicer. Three resonant couplings were identified, respectively at 855, 1271, and 1623 nm for the filter with a pitch distance of 8 mm characterized in this work. We also measured the filter response to temperature, elongation and hydrostatic pressure at the first and the second resonance. Our results show that the fabricated filter has very low sensitivity to temperature 1.77 and 1.38 pm/K, moderate sensitivity to elongation 1.35 and 1.12 nm/mstrain, and extremely high sensitivity to hydrostatic pressure 6.14 and 3.30 nm/MPa, respectively at the first and the second resonance.

We present a new method for measuring the group dispersion of the fundamental mode of a holey fiber over a wide wavelength range by white-light interferometry employing a low-resolution spectrometer. The method utilizes an unbalanced Mach-Zehnder interferometer with a fiber under test placed in one arm and the other arm with adjustable path length. A series of spectral signals are recorded to measure the equalization wavelength as a function of the path length, or equivalently the group dispersion. We reveal that some of the spectral signals are due to the fundamental mode supported by the fiber and some are due to light guided by the outer cladding of the fiber. Knowing the group dispersion of the cladding made of pure silica, we measure the wavelength dependence of the group effective index of the fundamental mode of the holey fiber. Furthermore, using a full-vector finite element method, we model the group dispersion and demonstrate good agreement between experiment and theory.

Polarization analysis reveals the vectorial nature of nonlinear-optical spectral transformations of femtosecond Cr:forsterite laser pulses in a highly birefringent photonic-crystal fiber. The birefringence in this fiber is induced by elliptical shape of the core of the size of 2.0 μm by 5.7 μm. We demonstrate the possibility of polarization control of the blueshifted radiation at the fiber output originating from the Cherenkov emission of dispersive waves induced by solitons that are generated by input 1.25⁢ μm laser pulses. Polarization-sensitive, two-color nonlinear-optical processes in such a fiber, pumped by the fundamental-frequency radiation of the Cr:forsterite laser and its second harmonic, are shown to allow the generation of a broadband spectrum covering the range from 430 to 660 nm.

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.

The sensing characteristics of a Bragg grating imprinted in a specially developed highly birefringent side-hole fiber were investigated in detail. We showed that such a grating has almost identical sensitivities to temperature for both linearly polarized modes LP01(x) and LP01(y) (approximately 5.9 pm/K) and significantly different sensitivities to hydrostatic pressure (-1.93 pm/MPa for LP01(x) and +5.37 pm/MPa for LP01(y) mode). The sensitivity differences versus mode polarization are so high that this effect can be employed for simultaneous sensing of temperature and pressure by interrogation of the wavelength shifts at LP01(x) and LP01(y) modes. Applying interferometric methods, we also measured the sensitivity of the host side-hole fiber to temperature and hydrostatic pressure for each polarization mode. Our results show that there is good agreement between the normalized sensitivities of the host side-hole fiber and those of the grating for the same polarization modes.

A new type of demodulation system for low-coherence interferometric sensors based on highly birefringent fibers is presented. The optical path delay introduced by the sensor is compensated in four detection channels by quartz crystalline plates of appropriate thickness. The system can be used to decode a single-point sensor with a resolution of 2.5 x 10(-3) or two serially multiplexed sensors with decreased resolution. In a multiplexed configuration each sensor is served by two detection channels. By tilting the quartz plates, we can tune the initial phase shift between interference signals in successive channels to differ by pi/8 or pi/4, respectively, for a single-point or a multiplexed configuration. We transferred the sinusoidal intensity changes into digital pulses by appropriate electronic processing, which eventually allows for an unambiguous phase-shift measurement with a resolution of 1/8 or 1/4 of an interference fringe. The system performance for the measurement of hydrostatic pressure changes and simultaneous changes of hydrostatic pressure and temperature is demonstrated. The pressure sensors are based on side-hole fiber to ensure high sensitivity and an operation range of 2.4 MPa. A new configuration for temperature compensation of hydrostatic pressure sensors is proposed, which is better suited for dynamic pressure measurements. In this configuration the sensing and compensating fibers are located in the same compartment of the sensor housing.

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.

A fiber-optic polarimetric strain sensor of l(S)=10-cm sensing length with three-wavelength passive quadrature digital phase demodulation is investigated. The demodulation unit uses a superluminescent diode light source with narrow-band interference filters in front of the photodiodes and real-time processing of the interference intensities by an arctan-phase-stepping algorithm. Quasi-static strain sensing is performed during slow periodic compression of a composite reinforced plastic rod with a sensor glued to its surface. The measured displacement sensitivity of delta?/deltal=125 mrad/mum, with a resistive strain gauge as a reference, agrees well with the value of 119 mrad/mum previously determined by fringe-distance measurement [Bock et al., Pure Appl. Opt. 5, 125 (1996)]. Despite a coherence-limited fringe contrast of only a few percent, a linearity of the phase-strain characteristic of the order of 1% and a strain resolution of 2.5muepsilon are demonstrated.

The behavior of a system of fiber-optic white-light interferometric sensors is analyzed. Analytical expressions for contrasts of all the interference patterns that may occur at the setup's output are derived as a function of coupling coefficients between successive sensors. Two cases of exemplary systems consisting of highly birefringent optical fibers, one with nondichroic the other with some polarizing fibers, are analyzed.

We determine the cross-sensitivity coefficient that represents the interaction between temperature and hydrostatic pressure in Corning elliptical-core, highly birefringent (HB) fiber. The measurement method we propose is especially useful to determine the cross-sensitivity effects in weakly sensitive HB fibers. The method involves registration of the residual temperature drift of thermally compensated polarimetric sensors at specially chosen values of pressure applied to the sensing fibers.

A method for measuring modal birefringence and polarization mode dispersion in highly birefringent fibers is presented. It employs a white-light interference phenomenon arising between polarization modes as a result of mode coupling induced by a pointlike lateral force applied in approximately one half the length of the tested fiber. This permits the use of a Wollaston prism without a delay line as an analyzing interferometer. Results of measurements of modal birefringence and polarization mode dispersion at λ(0)= 826 nm are reported for four commercially available fibers: the York Bow-Tie, the Fujikura Panda, the Andrew E-type, and the Andrew D-type.

A shearing type scanning interferometer is presented. The polarization modulation technique and homo-dyne detection of the first harmonic component of intensity carrying information about a phase shift between interfering beams were used. The interferometer has a variable measuring range, max +/- lambda/4, and sensitivity of the order of lambda/10,000. The modulator is based on the Faraday cell making possible a simple calibration of the interferometer. Measurement results of the refractive-index profile in optical fibers are presented. A detailed analysis of the systematic errors of the interferometer is also performed.