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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.

The behavior of a chain of sensors based on highly birefringent fibers and operating in two multiplexing configurations was studied as a function of alignment. The analytical relations between alignment at each splice in the chain and the visibility of the interference pattern associated with each sensor were found. These relations make it possible to determine alignment that ensures the desired visibility distributions for sensors along the cascade. In the experimental part of this work, the derived formulas were verified for a few specific chains composed of a limited number of multiplexed sensors.

Two measuring systems are presented enabling determination of the refractive index profile and its anisotropy as well as principal stress components in optical fibers and preforms, respectively. The system for optical fibers is a scanning-type, differentiating interferometer used to measure directly the wavefront derivative, from which, after the inverse Abel transformation, the index profile is obtained. The required high sensitivity of measurement is achieved by applying the sinusoidal modulation of the input beam ellipticity and the homodyne detection of the first harmonic component of the output intensity. After removing the Wollaston prism, the system can be used to measure the retardation function that is related to the fiber residual birefringence. The dynamic spatial-filtering technique, used until now to measure the ray deflection function, has been modified for testing the preforms. An optionally applied linear modulator of ellipticity of the input beam was added, to enable the measurement of the retardation function also. The system can be easily switched from the measurement of the ray deflection function to the measurement of the retardation function by moving only a single element.

The method of holographic simulation of birefringent elements is presented. It is shown that the simulation of a birefringent wedge is particularly easy and can be useful for the detection of small birefringences of the order of λ/10.

The problem of incoherent imaging of extended objects by optical systems which change the state of light polarization is discussed in detail. The optical transfer function of the system has been determined for two cases, i.e. for objects which emit similarly and differently polarized light. For the second case it was necessary to introduce the vectorial optical transfer function (OTF). Numerical calculations of the OTF have been performed for a perfect system with a simple polarizing mask in the exit pupil.

We measured the spectral dependence of the polarimetric sensitivity to temperature in three HB PCFs of different construction. The birefringence in the investigated fibers is induced respectively by elliptical shape of the core, by a pair of large cladding holes adjacent to the core or by one row of the cladding holes with diameters smaller than the other cladding holes. Our results show that the temperature sensitivity in the birefringent PCFs can vary in the range from 0 to 0.1 rad/K×m, is highly dispersive, and highly dependent upon fiber geometry. We also demonstrate that for one of the investigated fibers, the temperature sensitivity changes its sign at λ=1.44 μm, which makes this fiber completely insensitive to temperature at this wavelength

We present a numerical investigation of fundamental and higher order modes propagating in doped core birefringent holey fiber. The conditions for the co-existence of two competing light guiding mechanisms, their consequences on the mode propagation and the potentialities for Bragg grating applications with this are discussed.

The design flexibility of photonic crystal fibers has accelerated the development of specialty optical fibers for a wide variety of applications. Optical fiber sensor applications for instance can benefit from this fiber technology. Fiber Bragg grating inscriptions in photonic crystal fibers have been reported with inscription setups that go from continuous-wave to femtosecond pulsed laser sources. However, the compatibility of the microstructures in these fibers with conventional ultraviolet inscription techniques was never before investigated in a broad range of (Germanium doped) fibers. We present UV laser induced dynamics of Bragg gratings growths in photonic crystal fibers with a hexagonal arrangement of 6 rings of airholes around a Germanium doped core region. The average refractive index increase and the refractive index modulation by the grating inscription process are compared for microstructures with several doping levels, airhole filling factors, airhole pitch distance and fiber orientation. We show how the parameters of the microstructure can influence the Bragg grating inscriptions. In addition we expand the range of fibers in which Bragg gratings, with reflection strengths that are useable for sensing purposes, can be inscribed to fibers with Germanium doping concentrations as low as 1.36 and 0.45 mol%.

We report on the sensing characteristics of rocking filters fabricated in two microstructured fibers with enhanced polarimetric sensitivity to hydrostatic pressure. The filter fabricated in the first fiber shows a very high sensitivity to pressure ranging from 16.2 to 43.4 nm/MPa, depending on the resonance order and features an extremely low cross-sensitivity between pressure and temperature 28 divided by 89 x 10(3) K/MPa. The filter fabricated in the second fiber has an extreme sensitivity to pressure ranging from -72.6 to -177 nm/MPa, but a less favorable cross-sensitivity between pressure and temperature of 1.05 divided by 3.50 x 10(3) K/MPa. These characteristics allow using the rocking filters for pressure measurements with mbar resolution. (C) 2012 Optical Society of America

The combination of the functionalities of Fiber Bragg Gratings (FBGs) and Photonic Crystal Fibers (PCFs) has unveiled new potential for FBG based sensors. The fabrication of FBGs in PCFs has been reported in literature. However, using dedicated PCFs to improve the sensitivity of FBG-based sensors has received only limited attention. In this report we therefore show how to eliminate some of the drawbacks of FBGs in conventional step-index fibers for sensor applications by exploiting the design flexibility of PCFs. The added value of PCFs stems from the ability to design an optical fiber in which an FBG acts as a sensor with a selective sensitivity, e.g. a sensor that is sensitive to strain but not to temperature. For this purpose we use a PCF with a birefringence on the order of 10 -3 , which is one order of magnitude larger than for conventional birefringent fibers. The two FBG reflection peaks are therefore significantly separated from each other, e.g. 2 nm, which makes these FBGs suited for sensing purposes since both peaks can be unambiguously and accurately identified. As a conclusion we summarize the advantages and disadvantages of our approach to design and fabricate selective FBG-based sensors.

We designed, fabricated, and characterized birefringent holey fibers with zero polarimetric sensitivity to temperature. The sensitivity measurements were carried out in a wide spectral range of 0.68-1.55`µm in fibers with different hole and pitch values and with birefringence induced by a pair of large holes adjacent to the core. Our results show that zero sensitivity to temperature can be obtained at certain wavelengths for the bare fibers with properly adjusted geometrical parameters. Moreover, the spectral measurements of the sensitivity to temperature are in good agreement with the modeling results for all the investigated fibers.

Microstructured fibers (MOF), also called photonic crystal fibers (PCF), constitute a class of optical fibers, which has a large potential for number of novel applications either in the telecom or in the sensing domain. However, some of the applications require the use of specialty fibers with a doped core. We have made a preliminary exploration of PCF with doped regions and with inscribed Bragg gratings. The extensive study of the fiber cross-section structure in respect to possibilities of writing the Bragg gratings and the sensitivities of PCF Bragg gratings was our main concern. Selective measurement of strain without temperature compensation is achieved with fiber Bragg grating (FBG) in highly birefringent (HB) PCF, since such grating is characterized by two reflection bands corresponding to the two polarization modes generated due to the fiber birefringence. The measurement range of such FBG in HB fiber sensor depends on how strong is the separation of the polarization modes, which is expressed as phase birefringence. In next step, we have modeled, designed and fabricated specialty PCF with Ge doped core in such way that after writing the Bragg grating into the fiber we have obtained a sensors exhibiting low sensitivity to any temperature drifts. Traditional optical fiber sensors are not able to make such a distinction between stress and temperatures and require complex temperature compensation mechanisms.

A novel method based on distributed-feedback laser diode (DFB-LD) continuous wavelength-scanning for acquiring precise spectra of phase-shift fiber gratings is presented. Compared to the traditional method, the spectral resolution retrieved by this method is only limited by the optical line-width of the light source, which can reach up to the order of femtometer and is much higher than that of high-resolution optical spectrum analyzer (generally on the order of picometer). In addition, a Signal-to-Noise Ratio (SNR) advantage can be provided owing to a much higher spectral density of DFB-LD than amplified spontaneous emission (ASE) source. Precise spectra of three phase-shift fiber grating samples have been obtained at a resolution of 23.5 femtometer. (C) 2013 Optical Society of America

We show that index guided and photonic bandgap holey fibers of specific construction can be used as a wide-band fiber-optic polarizers. Similarly to traditional polarizing fibers, the operation principle of the index guided PCF polarizers is related to the difference in cut-off wavelengths of the two orthogonally polarized fundamental modes. Several fiber structures were analyzed and optimized for possibly highest polarization bandwidth. Furthermore, we investigated polarizing properties of four PBG PCFs with elliptical core. Our results show that after optimization of the cladding geometry, the polarization dependent loss in the analyzed PBG structures are so high that they can be used as fiber-optic polarizers in the full bandgap range.

A new spectral-domain white-light interferometric technique employing a low-resolution spectrometer is used to measure dispersion characteristics of two-mode optical fibers. The technique utilizes the fact that the spectral interference fringes are resolved at the output of a tandem configuration of the compensated (non-dispersive) Michelson interferometer and a two-mode optical fiber only in the vicinities of so-called equalization wavelengths at which the optical path difference (OPD) in the interferometer is the same as the intermodal group OPDs. The white-light spectral interferometric technique is used to measure the spectral dependences of both the differences between propagation constants of modes and the intermodal group OPD for elliptical-core (highly birefringent) optical fibers. The measured dispersion characteristics of the optical fibers are compared with those corresponding to the results of an adequate theoretical analysis using the known parameters of the optical fibers.

We report on the first inscription of fibre Bragg gratings in microstructure optical fibre using a NIR femtosecond laser system, inscribed using the point-by-point method, and without the use of any oil immersion techniques to remove the effects of fibre curvature or the role of the holey microstructure. The Bragg gratings are second order and recorded in a microstructure optical fibre that is highly birefringent, single-mode at 1550nm, photosensitive and compatible with FBG inscription technology due to the minimized number of air holes surrounding the fibre core. Both axial strain and temperature sensitivity are measured.

We report what we believe to be the first highly symmetric first-order IR femtosecond laser fiber Bragg gratings within the telecommunications C band in free-standing optical fiber, fabricated with a relatively low NA lens and without use of oil immersion techniques. This grating features the smallest dimensions for a point-by-point fiber grating reported so far (to our knowledge). This achievement paves the way to rapid mass manufacturing of highly efficient and stable Bragg gratings using ultrafast lasers in any type of fiber. Mastering this femtosecond grating inscription technique also allowed the fabrication of the first Bragg gratings with direct near-IR femtosecond inscription in photonic crystal fibers, and without the use of techniques that rely on the compensation of the holey structure. (C) 2010 Optical Society of America

We demonstrate that adding a boron-doped inclusion in the core of a polarizing photonic crystal fiber significantly increases its polarization bandwidth. We present the characteristics of two fabricated fibers, including spectral dependence of loss in both polarization modes and polarization-dependent loss in straight as well as bent fibers. Our experiments prove that bending significantly increases the polarization band of the boron-doped fibers on the short wavelength edge. For a properly adjusted bending radius a single polarization guidance can be obtained in a wide spectral range. Moreover, we present the results of loss simulations carried out for the actual geometry of the fabricated fibers, which are in agreement with the measurement results.

We investigated a particular design of a highly birefringent PCF with attractive features for pressure sensing applications. A plane-wave method together with the finite element method were used to numerically calculate phase and group modal birefringence, pressure and temperature sensitivities of our fiber. The simulation results together with the experiments demonstrate a considerable difference between a very high phase birefringence (B ∼ 10−3) and a very low negative group birefringence (G −10−3). Our fiber exhibits a low and positive temperature sensitivity (KT < 0.1 rad/(K⋅m)), and relatively high and negative mechanical (pressure) sensitivity (Kp ≤ −10 rad/(MPa⋅m)), which supports its possible use as a mechanical sensor that does not require any temperature compensation.

We have manufactured and characterized a birefringent holey fiber of a new construction. The birefringence in this fiber is induced by the highly elliptical shape of the core, which consists of a triple defect in a hexagonal structure. Using a hybrid edge–nodal finite-element method, we calculated the spectral dependence of phase and group modal birefringence for spatial modes E11 and E21 in idealized and in real fiber, whose geometry we determined by using a scanning-electron microscope. Results of our calculations show that technological imperfections significantly affect the fiber's birefringence. Normalized cutoff wavelengths for higher-order modes relative to the filling factor were also determined for the idealized structure. We observed a significant disagreement between theoretical and experimental values of cutoff wavelengths, which was attributed to high confinement losses near the cutoff condition. We also measured the spectral dependence of the phase and the group modal birefringence for spatial modes E11 and E21. The measured parameters showed good agreement with the results of modeling.

We measured and calculated the spectral dependence of the polarimetric sensitivity to temperature (dB/dT) in a photonic crystal holey fiber. The birefringence in this fiber is induced by the elliptical shape of the core, which consists of a triple defect in the hexagonal structure. Comparison of the temperature sensitivity measured for the bare fiber and for the fiber with polymer coating shows that the stress induced by the thermal expansion of the polymer contributes significantly to the overall temperature response. Measurements carried out for the bare fiber demonstrate that the temperature sensitivity has a positive sign and strongly increases vs. wavelength. Additionally, we present the results of calculations of the spectral dependence of dB/dT obtained using a finite element method. The calculated and measured values of dB/dT show relatively good agreement.

We proposed and theoretically analyzed birefringent holey fibers of new construction. The birefringence in these fibers is induced by a highly asymmetrical cladding, which is composed of only two rows of large holes separated by one row of small holes The fiber cores have the form of single defects made of pure silica or containing GeO 2 doped circular inclusion The geometries of both fibers were preliminarily optimized in order to assure minimum number of structural elements, while keeping the confinement losses of the fundamental mode below 1 dB/km. We used an edge finite element method to calculate the spectral dependence of the confinement losses and the phase birefringence. Due to small number of the cladding holes, the proposed fiber construction with GeO 2 doped inclusion in the core region may be especially useful for inscription the Bragg gratings.

We designed, manufactured and characterized two birefringent microstructured fibers that feature a 5-fold increase in polarimetric sensitivity to hydrostatic pressure compared to the earlier reported values for microstructured fibers. We demonstrate a good agreement between the finite element simulations and the experimental values for the polarimetric sensitivity to pressure and to temperature. The sensitivity to hydrostatic pressure has a negative sign and exceeds -43 rad/MPa x m at 1.55 mu m for both fibers. In combination with the very low sensitivity to temperature, this makes our fibers the candidates of choice for the development of microstructured fiber based hydrostatic pressure measurement systems. (C) 2010 Optical Society of America

We present fiber Bragg grating based transversal load sensing with a highly birefringent microstructured optical fiber. For the bare fiber, the change of the Bragg peak separation under a transverse line load was simulated with a finite-element model and experimentally verified. We also show that microstructured optical fibers with fiber Bragg gratings can be successfully embedded in a carbon fiber reinforced composite material. The linear dependence of the Bragg peak separation to a transversal stress in the composite sample was measured to be 15.3 pm/MPa.

A rocking filter is a type of long period grating that rotates azimuth of a linearly polarized light launched into one polarization mode of a birefringent fiber. If the grating period equals a beat length of the birefringent fiber (l=L B ), a resonant coupling between the modes of orthogonal polarization arises [1]. In this paper, we demonstrate that in microstructured fibers (MSFs) the phase matching condition between polarization modes can be obtained simultaneously at several wavelengths, because the phase birefringence in MSFs is very dispersive and increases against wavelength [2]. In Fig. 1b, we show the transmission characteristic for the excited polarization mode registered for the rocking filter fabricated in highly birefringent MSF consisting of 13 coupling points with a period of Λ=8 mm. The measured positions of respective resonances arising at 855, 1271 and 1623 nm are in good agreement with the calculated values obtained using Jones matrix formalism.

We have systematically measured the differential stress-optic coefficient, DeltaC, in a number of poly(methyl methacrylate) (PMMA) fibers drawn with different stress, ranging from 2 up to 27 MPa. DeltaC was determined in transverse illumination by measuring the dependence of birefringence on additional axial stress applied to the fiber. Our results show that DeltaC in PMMA fibers has a negative sign and ranges from -4.5 to -4.5x10(-12) Pa(-1), depending on the drawing stress. Increase of the drawing stress results in greater initial fiber birefringence and lower DeltaC.

We developed a highly birefringent micro-structured optical fiber that, in combination with a fiber Bragg grating sensor, allows measuring transverse strains in reinforced composites. The first generation of this dedicated fiber sensor featured a hydrostatic pressure sensitivity of −15 pm/MPa and yielded a transverse strain sensitivity of −0.16 pm/µe when embedded in a carbon fiber reinforced polymer. The second generation of this sensor has now been fabricated and hydrostatic pressure experiments and FEM simulations show that this generation returns a sensitivity of more than twice that of the first generation. FEM simulations additionally show an increased sensitivity when this sensor is embedded in a reinforced composite, achieving an unprecedented transverse strain sensitivity of 0.29 pm/µe. We explain how the optimized micro-structure yields this record-high sensitivity. In addition we demonstrate the selectivity of the bare fiber sensor, which remains insensitive to temperature changes or axial strain. This sensor can therefore play an important role in the domain of structural health monitoring.

Femtosecond pumping in an anomalous chromatic dispersion results in the generation of ultra-broad supercontinuum (SC) spectrum [1, 2]. However, such SC is associated with many pulses in the time domain and it is not flat in the spectral domain. Heidt et al. have shown that SC generated by pumping all-normal dispersion (ANDi) fiber is single pulse, has high coherence and spectral flatness [3-5]. Such all-normal dispersion supercontinuum is well suited for applications in an ultrafast spectroscopy. Mid-infrared SC is of particular interest since strong absorption lines of numerous chemical compounds are located in this spectral range. Unfortunately, long wavelength range of ANDi SC generated in silica microstructured fibers was limited so far to only 1.5 μm [4]. It means that full transparency window of silica glass was not covered. ANDi SC in mid-infrared was generated in more exotic glasses like soft-glass [6], ZBLAN-chalcogenide [7] and chalcogenide [8].

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.