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We report a detailed study of vector modulation instability (VMI) in highly birefringent fibers with circularly polarized modes in the normal dispersion regime. We show that because of suppression of coherent terms, the VMI in circularly birefringent fibers is governed by one set of coupled-mode nonlinear Schrödinger equations regardless of the fiber birefringence. In consequence, the VMI sidebands are polarized linearly and orthogonally to the pump up to the birefringence level of 10-5, similarly like in isotropic fibers. For greater birefringence the polarization states of the sidebands become elliptical with opposite handedness while the azimuth angle deviates from orthogonality to the pump. We also point on the dependence of the critical power beyond which the VMI cannot exist upon ellipticity angle θ of the eigenmodes. We show that the critical power gradually increases with the ellipticity angle and for θ > 17.6° the VMI gain is not limited, in contrast to linearly birefringent fibers. Our findings were confirmed experimentally by observation of the isotropic-like VMI in the spun side-hole fiber with nearly circularly polarized eigenmodes, in spite of relatively high birefringence of the order of 2 × 10-6.

We present a novel method for determining chromatic dispersion profile from a broadband spectral interferogram obtained by using a white-light interferometry technique. The proposed method is based on direct calculations of a second derivative of the registered spectral intensity at extremal points, which in result gives a spectral phase difference derivative further used for dispersion determining. Although the method is best suited for processing interferograms with modulation around a zero level (zero-mean value interferograms) and slowly changing envelope, the conducted numerical tests show that it is highly tolerable to different types of perturbations of the input interference signal. The proposed method can be applied both to spectral interferograms with monotonically changing phase difference as well as containing stationary phase difference points. We demonstrate the effectiveness and accuracy of the proposed method in measurements of chromatic dispersion of commercially available samples like the BK7 glass plate and the Corning SMF-28 optical fiber.

In this study, we show that transformation optics formalism can be used to rigorously model a wide range of twisted anisotropic fibers, which could only be analyzed using perturbative methods. If the material anisotropy of fibers has an intrinsic origin or is induced by axially or helically symmetric physical factors, then they can be transformed into a form usable in rigorous two-dimensional (2D) modeling. We demonstrate the effectiveness of the proposed approach in 2D modeling of the propagation characteristics of first-order eigenmodes in twisted and spun fibers with high linear birefringence. We derive the equivalent electric permittivity tensors for such fibers in the helical coordinate system and study the evolution of the first-order modes toward vortex modes with increasing twist rate. The obtained results confirm that the proposed method can reveal phenomena that cannot be predicted by analytical approaches.

We report a white-light interferometric method for broad-band measurements of chromatic dispersion of higher-order-modes (HOMs) selectively excited in an optical fiber using a spatial light modulator (SLM). To excite a specific mode we appropriately modulated a phase distribution across a supercontinuum input beam with the SLM used in reflective configuration. For this purpose, the SLM surface was divided into azimuthally and radially distributed sectors which introduce the phase shifts equal alternately to 0 or π radians, similarly as in the targeted mode. The voltage applied to respective sectors of the SLM was corrected versus wavelength to ensure broad-band dispersion measurements for the required mode. For a given voltage setting, the dispersion measurements were possible without any correction over 250 nm in the visible and over even greater range in the infrared. We demonstrate feasibility of the proposed approach in the measurements of chromatic dispersion for all modes supported by Corning SMF-28e, i.e., LP01, LP11, LP21, LP02, and LP31. The measurements were conducted in the spectral range from 450 nm up to the cut-off wavelengths of respective higher order modes and up to 1600 nm for the fundamental mode.

Helical core fibers (HCFs) suffer from low coupling efficiency and unavoidable excitation of higher order modes below the cutoff wavelength because of a core tilt with respect to the symmetry axis of the cladding. We propose an effective way of increasing the coupling efficiency to a HCF by untwisting its beginning section in a hydrogen flame. The proposed solution provides also a control over the excitation of higher order modes in HCFs and can be applied in splicing as well as in a free-space launching configuration. We experimentally demonstrate that by using the proposed method, the coupling efficiency between a SMF-28 and HCF can be increased to the level reachable for straight fibers, which is limited only by a modal fields mismatch. We also present detailed numerical and analytical studies of the coupling efficiency between a HCF and SMF versus the pitch distance in the partially untwisted input section of the HCF, which takes into account fundamental and higher-order modes.

We report on a new type of long-period-grating generated in a helical core fiber by bending. The grating arises from bend-induced modulation of an equivalent refractive index in the helical core with a period equal to the helix pitch. We experimentally demonstrate that such grating induces multiple resonant couplings between the fundamental modes guided in the central core and the helical side-core. We have also shown that by varying a direction of bending, one can generate the phase shifted grating. The experimental results are supported by numerical simulations based on the coupled mode equations.

We demonstrate for the first time a fiber-optic surface plasmon resonance sensor employing the interrogation method based on spectral interferometric measurements of the phase shift between fiber polarization modes. The sensor was fabricated on a specially designed birefringent D-shape fiber made of silica glass, which exhibits modal birefringence of the order of 10−5 and allows for direct deposition of a metal layer in the vicinity of the core. The sensor characteristics measured using the spectral interferometry method were compared with the reference results obtained using the conventional spectral transmission method. The two measuring techniques show similar sensitivity of 2700 nm/RIU for the refractive index of external medium equal to 1.386, however, the detection accuracy of the interferometric method is up to 3 times better.

We experimentally demonstrate the possibility of quasi-adiabatic conversion of L P 11 modes to vortex modes in a twisted highly birefringent fiber with a gradually increasing twist rate. Based on the value of the effective indices, the L P 11 modes are selectively converted to right- and left-handed circularly polarized vortex modes H E 21 with a total angular momentum of ± 2 and to quasi- T E 01 / T M 01 modes with a total angular momentum of 0. Since the proposed conversion method has a purely topological origin, it is broadband in nature, in contrast to the methods based on resonant effects.

We present an effective method for direct fiber coupling of a quantum dot (QD) that is deterministically incorporated into a cylindrical mesa. For precise positioning of the fiber with respect to the QD-mesa, we use a scanning procedure relying on interference of light reflected back from the fiber end-face and the top surface of the mesa, applicable for both single-mode and multi-mode fibers. The central part of the fiber end-face is etched to control the required distance between the top surface of the mesa and the fiber core. Emission around 1260 nm from a fiber-coupled InGaAs/GaAs QD is demonstrated and its stability is proven over multiple cooling cycles. Moreover, a single photon character of emission from such system for a line emitting above 1200 nm is proven experimentally by photon autocorrelation measurements with an obtained value of the second order correlation function at zero time-delay well below 0.5.

We report on experimental studies of polarimetric sensitivity to torsion and temperature in a series of spun highly birefringent side-hole fibers with spin pitches ranging from 5 to 200 mm. The polarimetric sensitivities to torsion and temperature were measured by monitoring a displacement of the spectral interference fringes arising in the output signal because of interference of ellipticaly polarized modes. The experimental results show that the sensitivity to torsion normalized to the fringe width in the spun highly birefringent fibers increases asymptotically with the twist rate to the value of 1/π rad-1 . In contrast to the torsional sensitivity, the temperature sensitivity decays asymptotically to zero with increasing fiber twist rate. Therefore, the spun fibers with short spin pitches are especially well suited for torsion measurements because the torsional sensitivity is enhanced in such fibers while at the same time the cross-sensitivity to temperature is reduced.

Publisher’s Note: This conference presentation, originally published on 14 May, 2019, was withdrawn on 19 May, 2019 per author request.

We show that the spectral interferometry method and the lateral point-force method used up to now to measure spectral dependence of the group and the phase modal birefringence in highly birefringent fibers with linearly polarized eigenmodes, can be after some modifications extended for the class of spun highly birefringent fibers with elliptically polarized modes. By combining the two methods, it is possible to determine spectral dependence of the group and phase elliptical birefringence in spun highly birefringent fibers. Moreover, if the fiber spin pitch is independently measured, the spectral dependence of ellipticity angle of polarization eigenmodes as well as the built-in linear phase and group birefringence, can be also obtained using the analytical relations between the parameters of spun and non-spun fibers. We demonstrate the effectiveness of the proposed approach in spectral measurements (700-1600 nm) of the spun side-hole and microstructured highly birefringent fibers with different birefringence dispersion and spin pitches ranging from 4.1 to 200 mm.

We propose an improvement of the interferometric method used up to now to measure the chromatic dispersion in single mode optical fibers, which enables dispersion measurements in higher-order modes over a wide spectral range. To selectively excite a specific mode, a spatial light modulator was used in the reflective configuration to generate an appropriate phase distribution across an input supercontinuum beam. We demonstrate the feasibility of the proposed approach using chromatic dispersion measurements of the six lowest order spatial modes supported by an optical fiber in the spectral range from 450 to 1600 nm. Moreover, we present the results of numerical simulations that confirm sufficient selectivity of higher-order mode excitation.

We present a novel method for determining a spectral phase derivative from a single spectral interferogram obtained by using a white-light interferometry technique, which is further used for calculation of chromatic dispersion profile. The method is based on direct calculations of a second derivative of the registered spectral intensity at extremal points. We explain theoretical background of the method, discuss its limitations and evaluate an accuracy by numerical simulations. Additionally, we validate the proposed method in dispersion measurements of the BK7 glass plate and the SMF-28 fiber.

We have studied the effect of symmetry on coupling between core and cladding modes in helical microstructured fibers, which gives rise to resonant loss peaks observed in the fiber transmission spectra. We demonstrate that the selection rules for orbital and spin angular momenta of coupled modes, proposed first for twisted conventional fibers, are universal characteristics and correctly identify the coupled cladding modes in helical microstructured optical fibers (MOFs). Moreover, we show for the first time (to our knowledge) the effect of coupling between modes of opposite polarization handedness in twisted MOFs.

We present a comprehensive study of an in-line Mach-Zehnder intermodal interferometer fabricated in a boron-doped two-mode highly birefringent microstructured fiber. We observed different interference signals at the output of the interferometer, related to the intermodal interference of the fundamental and the first order modes of the orthogonal polarizations and a beating of the polarimetric signal related to the difference in the group modal birefringence between the fundamental and the first order modes, respectively. The proposed interferometer was tested for measurements of hydrostatic pressure and temperature for different alignments of the input polarizer with no analyzer at the output. The sensitivities to hydrostatic pressure of the intermodal interference signals for x- and y-polarizations had an opposite sign and were equal to 0.229 nm/MPa and −0.179 nm/MPa, respectively, while the temperature sensitivities for both polarizations were similar and equal 0.020 nm/°C and 0.019 nm/°C. In the case of pressure, for the simultaneous excitation of both polarization modes, we observed a displacement of intermodal fringes with a sensitivity depending on the azimuth of the input polarization state, as well as on the displacement of their envelope with a sensitivity of 2.14 nm/MPa, accompanied by a change in the fringes visibility. Such properties of the proposed interferometer allow for convenient adjustments to the pressure sensitivity of the intermodal fringes and possible applications for the simultaneous interrogation of temperature and pressure.

The possibility of using polymer fiber as a refractive index sensor is presented. The sensor is based on a Fabry-Perot interferometer formed at the tip of the polymer fiber. The interference is granted due to reflections between a fiber Bragg grating and the fiber end-face. The sensor was characterized to refractive index changes at constant temperature using a fast Fourier transform analysis of the interference signal. A sensitivity of −1.94 RIU−1 was achieved with a resolution of 1 × 10−4 RIU and a cross sensitivity to temperature of 1× 10−4 RIU/T.

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 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 x MPa–1 x m–1 at 750 nm. Web of Science 13 9 11438 11424

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.

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.

The strain sensitivity of a two-mode birefringent side-hole fiber is measured in the spectral domain. In a simple experimental setup comprising a broadband source, a polarizer, a two-mode birefringent side-hole fiber under varied elongations, 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 highest visibility (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 with the equalization wavelength are processed to retrieve the phase as a function of the wavelength. From the retrieved phase functions corresponding to different elongations of a two-mode birefringent side-hole fiber under test, the spectral strain sensitivity is obtained. Using this approach, the intermodal spectral strain sensitivity was measured for both x and y polarizations. Moreover, the spectral polarimetric sensitivity to strain was measured for the fundamental mode when a birefringent delay line was used in tandem with the fiber. Its spectral dependence was also compared with that obtained from a shift of the spectral interferograms not including the equalization wavelength, and good agreement was confirmed. Web of Science 12 9 12081 12070

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%.

In this paper we present an all-fiber interferometric sensor for the simultaneous measurement of strain and temperature. It is composed of a specially fabricated twin-core fiber spliced between two pieces of a single-mode fiber. Due to the refractive index difference between the two cores in a twin-core fiber, a differential interference pattern is produced at the sensor output. The phase response of the interferometer to strain and temperature is measured in the 850–1250 nm spectral range, showing zero sensitivity to strain at 1000 nm. Due to the significant difference in sensitivities to both parameters, our interferometer is suitable for two-parameter sensing. The simultaneous response of the interferometer to strain and temperature was studied using the two-wavelength interrogation method and a novel approach based on the spectral fitting of the differential phase response. As the latter technique uses all the gathered spectral information, it is more reliable and yields the results with better accuracy.

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 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.

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.