Search

For the first time to our knowledge, we demonstrate a coherent supercontinuum in silica fibers reaching 2.2 µm in a long wavelength range. The process of supercontinuum generation was studied experimentally and numerically in two microstructured fibers with a germanium doped core, having flat all-normal chromatic dispersion optimized for pumping at 1.55 µm. The fibers were pumped with two pulse lasers operating at 1.56 µm with different pulse duration times equal respectively to 23 fs and 460 fs. The experimental results are in a good agreement with the simulations conducted by solving the generalized nonlinear Schrodinger equation with the split-step Fourier method. The simulations also confirmed high coherence of the generated spectra and revealed that their long wavelength edge (2.2 µm) is related to OH contamination. Therefore, improving the fibers purity will result in further up-shift of the long wavelength spectra limit.

The use of a polymer fiber as a refractive index sensor is proposed. A fiber Bragg grating is inscribed near the fiber tip and the fiber is cut shorter thus creating a Fabry-Perot cavity. The reflections between the fiber Bragg grating and the fiber end-face create a Fabry-Perot interferometer. The sensor was characterized to refractive index changes at constant temperature and to temperature at constant refractive index using a fast Fourier transform analysis of the interference signal. The sensor revealed a sensitivity of − 1 . 94 RIU − 1 with a resolution of 1 × 10 − 3 RIU and low sensitivity to temperature, with a cross sensitivity to temperature of 3 . 6 × 10 − 4 RIU / °C .

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 present a PMMA birefringent fiber with enhanced polarimetric sensitivity to hydrostatic pressure, obtained by enlarging selected holes in the microstructured cladding. The fiber shows a linear response to pressure with small hysteresis in the pressure range up to 8.5 MPa and the sensitivity of 48 ${\rm rad}/{\rm MPa}\cdot{\rm m}$ at 600 nm. The average temperature sensitivity measured in the range 22–40 $^{\circ}{\rm C}$ is 1 ${\rm rad}/{\rm K}\cdot{\rm m}$ at 600 nm. The fiber, however, shows significant hysteresis increasing with the temperature range. We also present the results of numerical simulations of the birefringence and the sensitivity to pressure. The contribution of stress and deformation effects to the overall pressure sensitivity in the investigated PMMA fiber and the silica fiber of the same geometry is compared. This allows us to explain different signs of pressure sensitivity observed in silica and the PMMA fibers.

We report on sensing characteristics of a long period grating fabricated in polymer microstructured fiber using transverse periodic loading combined with fiber heating. The grating is characterized for strain and hydrostatic pressure sensitivity and shows linear response to both parameters with sensitivity coefficients equal, respectively, -1.39nnm/mstrain and 2.29 nm/MPa. We also measure the variation of the resonance depth induced by both parameters and study the temperature response of the grating.

Using transformation optics formalism, we numerically analyzed the effect of resonant couplings between core and cladding modes in a helical core fiber with large core offset. Our study revealed much richer spectrum of resonant couplings than predicted by perturbative methods in fibers with small core offsets. Moreover, we demonstrated that the resonant couplings to lower order cladding modes predicted by perturbative methods are in fact prohibited for larger core offsets. Finally, we demonstrated the existence of spectrally broad polarization sensitive couplings to the cladding modes suggesting that helical core fibers with large core offsets may be used as broadband circular polarizers.

In this work, we demonstrate the possibility of fabricating short-length long-period gratings and rocking filters in highly birefringent Photonic Crystal Fiber using a CO 2 laser. In our experiments both kinds of gratings were made in the same Boron doped highly birefringent PCF using similar exposure parameters. We also present the sensing capabilities of both fabricated gratings to temperature, strain and hydrostatic pressure by interrogation of the wavelength shifts at different resonances.

We report on low-loss patch cords composed of a highly reproducible low-loss fusion splicing of photonic crystal fibers (PCFs) with a standard single mode fiber (SMF). Distinct from other papers in this area we report on the results for different types of PCFs, including LMA fibers with similar to a SMF core size and a mode field diameter (MFD), as well as polarization maintaining (PM) Bow-Tie PCF with an elliptical GeO(2) doped core and also a suspended-core (SC) PCF with a tiny core. We show that all studied splices between SMF and PCFs exhibit dispersive and non-reciprocal, in view of a light propagation direction, transmission losses. Defined as a larger decrease of a transmitted optical power comparing both propagation directions, achieved splicing losses, are equal to 0.46 +/- 0.03 dB, 0.34 +/- 0.01 dB, 0.67 +/- 0.17 dB and 3.09 +/- 0.366 dB at 1550 nm for LMA8, LMA10, Bow-Tie PCF and SC PCFs, respectively. Moreover, additionally to developed low-loss splicing, we report on low-loss patch cords for all mentioned above PCFs spliced on both ends with SMF pigtails ended with FC/APC connectors achieving the total losses at 1550 nm equal to 1.07 +/- 0.07 dB for LMA8, 0.72 +/- 0.20 dB for LMA10, 1.30 +/- 0.06 dB for Bow-Tie PCF, and 9.36 +/- 0.20 dB for SC PCF.

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 present KrF excimer laser-induced dynamics of Bragg grating growths in GeO2 doped microstructured optical fibers. The studied fibers all have 6 rings of airholes in a hexagonal lattice and a GeO2 doped region in the center of the microstructure. We compare the growth rates of fiber Bragg gratings in the different microstructured fibers with UV grating inscription. The influence of the doping level, the airhole filling factor, the airhole pitch distance and the fiber orientation are investigated. We expand the range of microstructured optical fibers in which Bragg gratings can be inscribed, achieving reflection strengths that are useable for FBG-based sensing applications, even for doped regions with GeO2 concentrations as low as 1.36 mol% and 0.45 mol%.

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.

We report on experimental studies of polarimetric sensitivity to hydrostatic pressure and temperature in two highly birefringent index guided photonic crystal fibers, in which birefringence is induced by one row of the cladding holes with diameters smaller than the other cladding holes. The sensitivity measurements were carried out in the spectral range from 0.6 μm to 1.6 μm. Our results show that absolute value of the polarimetric sensitivity to hydrostatic pressure can reach 23 rad/MPa × m, which is almost one order of magnitude higher than in conventional fibers with elliptical core. Simultaneously, polarimetric sensitivity to temperature is at least two orders of magnitude lower than in conventional highly birefringent fibers. Moreover, we proved experimentally that one of the investigated fibers is completely insensitive to temperature at certain wavelength.

We present two polymer birefringent fibers with enhanced polarimetric sensitivity to hydrostatic pressure. In the first fiber, with birefringence induced by the arrangement of holes in the microstructured cladding, an increased sensitivity to pressure was obtained by enlarging selected holes in the cladding. The second is a side-hole 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. The transmission and sensing characteristics of both fibers are compared, including spectral losses, birefringence, polarization cross-talk and polarimetric sensitivity to hydrostatic pressure.

We show that diffusion of azobenzene from the solution in methanol into a cladding of a polymer fiber facilitates fabrication of long period gratings by the use of a He–Cd focused laser beam. We have measured a diffusion rate into PMMA cladding of the microstructured fibers annealed in advance at different temperatures and showed that the diffusion rate is strongly affected by temperature treatment of the fiber. We have also investigated an impact of the azobenzene diffusion on fiber spectral loss and cladding surface quality. Furthermore, we have examined a temporal stability of the fabricated long period gratings and their response to temperature and tensile strain.

International audience; In this article, we study the possibility to use a pair of specifically designed microstructured optical fiber Bragg gratings (MOFBGs) as a multi-component strain sensor when embedded within composite materials. The dependence on the orientation of the transverse sensitivity of the MOFBGs is exploited to build a sensing device able to measure the strain field along the three principal mechanical directions of a laminate composite. We developed an analytical and numerical model of such a sensor and benchmarked it with experiments performed on laminated composite coupons equipped with this sensor. We report on a theoretical strain resolution of about 5 μ in the transverse directions of the composite material, which is a six-fold improvement over results reported in literature. © 2015 IOP Publishing Ltd.

The pressure sensitivity of two photonic crystal fibers (PCFs) was measured. A PCF pressure sensor was then successfully developed with PCF PM-1550-01. The measurement results of the pressure sensor at three different temperatures are presented, and in the working region the maximum deviation is within 1% of the dynamic range of the sensor

Employing several interferometric methods, we measured in a broad spectral range the wavelength dependences of the phase modal birefringence and the polarization mode dispersion for the LP01 and even LP11 spatial modes supported by a birefringent holey fibre. We also determined the wavelength dependence of the intermodal dispersion between the X- and Y-polarized LP01 and even LP11 spatial modes. Furthermore, using a full-vector finite-element method, we modelled all the measured dispersion characteristics and demonstrated good agreement between experimental and theoretical results.

We investigated polarization properties of the photonic bandgap hollow core fiber operating in the first communication window. Using specially developed interferometric method, we measured dependence of phase and group modal birefringence upon wavelength in a wide spectral range. Our results show that the phase birefringence (Δ n ) is of the order of 10 −5 , depends parabolic like upon wavelength, and reaches minimum value in the vicinity of the bandgap center. The group modal birefringence (Δ N ) decreases monotonically vs. wavelength from positive to negative value and crosses zero near the bandgap center. Due to high chromatic dispersion of the phase modal birefringence, the absolute value of Δ N is one order of magnitude higher than Δ n at the bandgap boundaries. We also determined the susceptibility of the phase and the group modal birefringence to strain and hydrostatic pressure in the whole analyzed spectral range.

This paper presents a dynamic pressure calibration of the fiber-optic interferometric sensor based on highly birefringent side-hole fibers. Earlier, we tested other types of fiber-optic sensors based on the same principle for measurements of static and quasi-static pressure. To apply the sensor for measurements of fast pressure changes, the dynamic analysis is crucial due to the occurrence of resonance phenomena and due to the possibility of false pressure readings. We applied a static calibration procedure to initially determine the pressure sensitivity and temperature stability of the sensor. Next, we compared the characteristics of the fiber-optic sensor with the responses of a calibrated piezoelectric dynamic pressure sensor at an operating range of 110 bar with a sampling rate equal to 200 kHz. The characteristics of the fiber-optic sensor are in good agreement with those of the reference piezoelectric sensor for the linear and exponential functions and for the half-sine when the pulse is wider than 400 ms. However, for half-sine pulses narrower than 250 ms, resonance oscillations occur only in the reference sensor. Construction of the fiber-optic sensor reduces the undesirable oscillations, which thereby makes it possible to operate on frequencies higher by about one order than in the case of the piezoelectric sensor. It clearly shows that the highly birefringent fiber-optic sensors can be successfully applied for measurements of rapid pressure changes.

We present a white-light spectral interferometric technique employing a low-resolution spectrometer for a direct measurement of the dispersion of the group birefringence of a calcite crystal over the wavelength range approximately from 490 to 780 nm. The technique utilizes a tandem configuration of a Michelson interferometer and a calcite crystal of known thickness to record a series of spectral interferograms and to measure the equalization wavelength as a function of the optical path difference (OPD) in the Michelson interferometer, or equivalently, the wavelength dependence of the group birefringence of the calcite crystal. We confirm that the measured group birefringence dispersion agrees well with that described by the dispersion equation proposed by Ghosh. Furthermore, we determine precisely the thickness of the calcite crystal from the slope of linear dependence of the measured OPD on the group birefringence given by the dispersion equation.

We experimentally characterized a holey fiber, in which birefringence is induced by two large holes adjacent to the fiber core. Employing a lateral force method, we measured the spectral dependence of the phase modal birefringence for E 11 and E 21 spatial modes. The group modal birefringence was also measured for both spatial modes using scanning wavelength method. Finally, we determined the fiber polarimetric sensitivity to hydrostatic pressure and to strain for E 11 and E 21 modes in a wide spectral range. Using special procedures, we also identified the sign of all measured parameters.

We show that holey fibers of specific construction can be used as very wide-band fiber-optic polarizers. Similarly to traditional polarizing fibers, the operation principle of the holey polarizers is related to the difference in cut-off wavelengths of the two orthogonally polarized fundamental modes. Two fiber structures were analyzed and optimized for highest possible polarization bandwidth. In the first structure having the polarization bandwidth up to 480 nm, the dissimilarity in the cut-off wavelengths is induced by introducing a pair of large air holes adjacent to the fiber core. In the second structure, the polarization bandwidth is further enlarged up to 1300 nm by adding a small hole in the center of the fiber core.

We present two spectral-domain interferometric techniques for measuring the wavelength dependence of the group optical path difference (OPD) between the LP y 01 and LP y 11 modes of a bow–tie optical fibre. One technique utilizes a set-up comprising a white-light source, a tandem configuration of a Michelson interferometer with a two-mode optical fibre under test and a low-resolution spectrometer. In the set-up we record a series of the spectral interferograms for measuring directly the dispersion of the intermodal group OPD. The other technique utilizes a set-up comprising a superluminescent diode (SLD), a two-mode optical fibre under test and a high-resolution spectrometer. In the set-up we record a single spectral interferogram for determining the dispersion of both the overall phase with 2π ambiguity and the intermodal group OPD. The experimental results obtained by both measurement techniques are compared each other confirming good agreement.

Using a specially developed spectral interferometric method, we measured the sensitivity to temperature, elongation and hydrostatic pressure of silica holey fibre with hexagonal symmetry, operating in a single-mode regime. The susceptibility of the group refractive index to those parameters determined with our measurement method is respectively equal to dN/dT = +1.28 × 10−5 K−1, dN/de = −0.40 1/strain and dN/dp = −1.0 × 10−5 MPa−1.

A software-based temperature-compensated fiber-optic white-light interferometric (WLI) strain sensor is presented. Measurement error of this sensor is studied theoretically and experimentally. The results quantitatively indicate how the CCD camera pixel resolution, interference fringe pattern center determination errors, strain sensing sensitivity, sensing fiber and compensating fiber lengths affect the measurement errors. Measurement accuracy limitation for the individual strain and temperature sensors with specified highly birefringent fibers and WLI decoding system is also investigated. The main sources of the central fringe position measurement error as well as solutions for further improvements are particularly discussed. Critical factors for sensor design and potential for civil and composite structure monitoring are highlighted by comparing sensitivities, fiber length limitation, power budget and dynamic ranges with those of other fiber interferometers. The analysis provides a convenient way for sensor design to satisfy applications with different accuracy requirements.

In this communication, we present a universal read-out system, which can be used to decode polarimetric fiber-optic sensors based on highly birefringent fibers. All such sensors use the same sensing principle, relying upon the dependence of modal birefringence on different physical parameters. To register the measurand-induced phase changes between polarization modes, we use the coherence-addressing principle. This requires that the interrogated sensor be powered by a broadband source (superluminescent diode) and that the total optical path delay introduced by the sensor be balanced in the decoding interferometer. The system performance in decoding temperature, elongation and hydrostatic pressure sensor is demonstrated.

Summary A spectral-domain white-light interferometric technique is used in measurement of the dispersion of the group modal birefringences for the LP01 and LP11 spatial modes in an elliptical-core optical fibre. The dispersion of the intermodal group optical path differences (OPDs) between the LP01 and LP11 modes of each polarization is also measured. The technique, which employs a low-resolution spectrometer at the output of a tandem configuration of a Michelson interferometer and a two-mode optical fibre, utilizes the resolving of so-called equalization wavelengths at which the OPD in the interferometer is the same as the differential or intermodal group OPD. The measured data are fitted to polynomials to obtain the dispersion of both the phase modal birefringences and the differences between propagation constants of two LP modes. The results obtained are compared with the results of an adequate theoretical analysis.

A spectral-domain white-light interferometric technique employing a low-resolution spectrometer is used in measurement of the intermodal dispersion in optical fibres. The technique utilizes a tandem configuration of a compensated Michelson interferometer and a few-mode optical fibre and the resolving of so-called equalization wavelengths at which the optical path difference (OPD) in the interferometer is the same as the intermodal group OPDs. The intermodal dispersion in three different optical fibres was measured and the results for two of them were compared with the results of an adequate theoretical analysis using a model of a weakly guiding, step-index optical fibre.

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.

Using earlier developed perturbation method, which takes into account geometrical and stress-induced effects, we have numerically analyzed how core ellipticity and difference in refractive index between core and cladding influence modal birefringence and its sensitivity to temperature and hydrostatic pressure in SiO 2 /GeO 2 elliptical-core fibers. The calculations were carried out for the set of fibers having the same cut-off wavelength ( λ c =700 nm). Our analysis shows that the dependence of modal birefringence and its sensitivity to temperature and hydrostatic pressure upon core elipticity saturates for ellipticities greater than 4. Furthermore, we show that sensitivity to temperature and hydrostatic pressure increases with the difference in refractive index between the core and cladding in a manner slightly weaker than linear. These results may be helpful when designing elliptical-core fibers with required sensing characteristics.

A digital demodulation method for read-out of phase changes induced in coherence-multiplexed sensors based on highly birefringent fibers is described. The method employs the fringe counting principle and enables registration of the phase shifts simultaneously induced in two multiplexed sensors with a maximum frequency of 10 kHz and resolution of 1/4 of the interference fringe. The performance of three multiplexed systems interrogated using the proposed detection method is investigated. The first system is composed of two serial multiplexed sensors serving for measurements of pressure and temperature changes in the same location, while the two other systems are composed of two parallel or serial multiplexed temperature-compensated sensors serving for pressure measurements at different locations.

Summary We present the results of measurements and modeling of spectral behavior of phase and group modal birefringence and their sensitivities to temperature and pressure in one of the most popular ellipticalcore fiber available on the market, which is Corning PMF-38. The measurements were carried out in the spectral range from 630 nm to 850 nm using white-light and coherent interferometry. The modeling was fulfilled using the modified perturbation approach first proposed by Kumar. The modification consists in introducing the first order correction term associated with stress induced changes of refractive index and makes it possible to model the fiber response to different physical parameters. Data concerning the fiber geometry and composition, which are necessary in modeling, were obtained using an electron scanning microscope. Although the perturbation approach is a relatively simple method, the theoretical and experimental results show good agreement in the investigated spectral range.

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 present dedicated shear stress optical fiber sensors for in situ disbond monitoring of adhesive bonds. The shear stress sensitivity of these sensors is about 60 pm MPa−1, which corresponds to a shear strain sensing resolution of 50 μϵ. By integrating a combination of three such sensors in the adhesive bond line of a single lap joint, we can assess the internal shear stress distribution when the joint is tensile loaded. Disbonding of this joint was initiated by cyclic tensile loading, and the sensor responses were monitored during this process. Our results show that this sensing system can detect disbonds as small as 100 μm.

We present the use of microstructured optical fiber Bragg grating-based sensors for strain and temperature monitoring inside the concrete buffer of the Belgian supercontainer concept, demonstrated in a half-scale test in 2013. This test incorporated several optical fiber sensors inside the concrete buffer for production and condition monitoring. The optical fiber sensors presented here consist of small carbon-reinforced composite plates in which highly birefringent Butterfly microstructured optical fibers, equipped with fiber Bragg gratings, were embedded. The double reflection spectrum of these MOFGBs allows to simultaneously monitor strain and temperature, as confirmed by comparison with data obtained from thermocouples and vibrating-wire sensors installed near the MOFBGs.

We present shear stress sensing with a Bragg grating sensor fabricated in a highly birefringent microstructured optical fiber. This sensor has a shear strain sensing resolution of 0.04 pm/mu epsilon when embedded in a shear loaded adhesive bond. We achieve discrete shear stress mapping in an adhesive bond by embedding a multitude of these sensors at different locations in the bond line. Experiments and numerical modeling show the limited influence of angular misalignment of the sensor on its shear stress response. Finally, we discuss the cross-sensitivity of this sensor to shear strain and temperature.

The application of shear stress sensors in structural health monitoring remains limited because current sensors are either difficult to implement, they feature a low measurement resolution or the interrogation of the output signal is complex. We propose to use fiber Bragg grating-based sensors fabricated in dedicated highly birefringent microstructured optical fibers. When embedded in a host material, the orientation angle of the fiber should be chosen such that their polarization axes are aligned parallel with the direction of maximum shear stress when the host is mechanically loaded. We present experimental results of sensors embedded in the adhesive layer of single lap and double lap structural joints. These tests demonstrate that when the joints are tension loaded, the embedded sensors have a shear stress sensitivity of around 60 pm/MPa. We study the influence of the adhesive material on the sensor response, as well as the influence of sensor orientation and location in the bond line Finally, we demonstrate the minimal thermal cross-sensitivity of the shear stress sensitivity of this sensor.

A method for measuring the sensitivity of Bragg gratings to external parameters for different spatial and polarization modes is presented together with the results of measurements of sensitivity to temperature and elongation for LPx01, LPy01, LPx11 and LPy11 modes. The difference in response according to the type of mode shows the possibility of performing simultaneous temperature and elongation measurements using two-mode Bragg gratings.

A new type of fiber-optic pressure sensor based on a specially developed side-hole fiber is presented. It allows for unambiguous and fast phase-shift measurements in the range from -/spl pi//2 to +/spl pi//2 with a sampling rate of 5 kHz and resolution of about 1% of full scale (2 /spl middot/ 10/sup -3/ atm).

Fiber Bragg gratings are very useful in many applications. This is why a higher control of their properties is desirable. Therefore, a grating preprocessing technique, based on the dependence of the index guided photonic crystal fiber (IG-PCF) properties on their structural parameters, is proposed allowing a higher control of the Bragg wavelength. Using the proper combination of thermo-mechanical IG-PCFs tapering and ultraviolet photoinscription methods, it is possible to develop new optical fiber devices. Their effectiveness is demonstrated by comparison with the results on single-mode fiber. Finally, to demonstrate the feasibility of the technique, a wideband chirped Bragg grating is fabricated.

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.

A white-light fiber-optic interferometric system composed of sensing and receiving interferometers separated with a dichroic-type linking interferometer was analyzed in order to determine the minimum group imbalance necessary to separate signal and noise patterns. For this purpose the analytical formulas for contrasts of the cross-interference patterns as well as the patterns produced by the sensing and receiving interferometers were derived. The contrast behavior was analyzed as a function of alignment at all coupling points and the dichroism of the linking interferometer. It is shown that the minimum group imbalance, which is the parameter responsible for temperature stability of the sensor, may be decreased to half for certain alignments.

A method is presented for measuring modal birefringence and polarization mode dispersion in highly birefringent fibers. The method employs the principle of white-light interferometry for dispersion measurement, and classical coherent interferometry for birefringence measurement. The key parts of the measuring system are a Wollaston prism made of crystalline quartz used as a shearing type interferometer and a set of quartz delay plates for compensation of retardations up to 1000/spl lambda/ introduced by the tested fibers. The interferometer is combined with a CCD camera and with a processing system allowing for convenient measurement data acquisition and handling.

We studied vector frequency conversion in externally tuned microstructured fibers for applications as a novel, nonlinear fiber-optic sensor. We investigated both experimentally and numerically a possibility of shifting vector and scalar modulation instability gain bands by pressure-induced changes in the linear properties of a microstructured fiber. Our results show that polarization-dependent vector nonlinear processes sensitive to variation of fiber group velocity difference (group birefringence) exhibit a clear advantage for pressure-sensing applications compared with scalar nonlinear processes only sensitive to group velocity dispersion changes. Analytical predictions and numerical simulations confirm our measurement results.

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

This paper presents the sensor characteristics of highly birefringent polymer side-hole optical fiber. The fiber core with greater refractive index was made of copolymer PMMA-PS, while the cladding of pure PMMA. The fabricated fiber showed relatively low losses of the order of 6 dB/m in the visible range. We measured several sensing characteristics in the fabricated fiber, including birefringence and polarimetric sensitivity to pressure and temperature. The fiber showed high polarimetric sensitivity to pressure, which is directly related to the presence of two large holes transferring symmetrical load applied to the cladding into nonsymmetric stress distribution in the core region. This in turn changes modal birefringence of the investigated fiber and increases the sensitivity to pressure.

The intermodal sensitivity of a two-mode birefringent holey fiber to strain, temperature and hydrostatic pressure is measured in the spectral domain. In an experimental setup comprising a broadband source, a polarizer, a twomode birefringent holey fiber under a variable physical parameter, an analyzer and a compact spectrometer, the spectral interferograms are resolved. These are characterized by the equalization wavelength at which spectral interference fringes have the largest period due to the zero group optical path difference between the fundamental, the LP01 mode and the higher-order, the LP11 mode. The spectral interferograms are processed to retrieve the phase as a function of the wavelength. From the retrieved phase functions corresponding to different values of the physical parameter, the intermodal sensitivity as a function of wavelength is obtained. Using this approach, the intermodal sensitivity to strain, temperature and hydrostatic pressure is measured for two orthogonal (x and y) polarizations.

We present the experimental investigations of the Bragg gratings writing process in the microstructured polymer fiber using UV beam of 325 nm wavelength and standard phase mask technique. The good quality higher order peaks placed at λ B /2=782 nm and 2 λ B /3=1040 nm for the grating with primary peak at λ B =1555 nm were experimentally demonstrated for the first time. The growth dynamics for all the peaks was investigated.

Polarimetric sensitivity of a birefringent holey fiber to strain, temperature and hydrostatic pressure is measured in the spectral domain. In an experimental setup comprising a broadband source, a polarizer, a birefringent holey fiber under a variable physical parameter, a birefringent delay line, an analyzer and a compact spectrometer, the spectral interferograms are resolved. These are characterized by the equalization wavelength at which spectral interference fringes have the largest period due to the zero overall group modal birefringence. The spectral interferograms are processed to retrieve the phase as a function of the wavelength. From the retrieved phase functions corresponding to different values of the physical parameter, the polarimetric sensitivity as a function of wavelength is obtained. Using this approach, the polarimetric sensitivity to strain, temperature and hydrostatic pressure is measured.