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ABSTRACT: The effective indices of the cladding modes of optical fibers depend on the refractive index of the medium surrounding the fiber. We show experimentally and theoretically that while cladding modes with similar effective indices normally have similar refractometric sensitivities, the addition of a 50 nm thick gold sheath enhances the sensitivity of some EH modes by more than one order of magnitude while nearly completely suppressing the sensitivity of neighbouring HE modes (by three orders of magnitude, down to insignificant levels). A differential sensitivity of $1000 nm/ (refractive index unit) is experimentally reported between adjacent EH and HE grating resonances. V C 2011 American Institute of Physics. [doi:10.1063/1.3615712] There are many ways to use optical fibers to measure the refractive index of materials. When the evanescent field of a propagating mode extends outside of the fiber, the effec-tive index of the mode (n eff) depends on the refractive index of the surrounding medium (SRI ¼ "surrounding refractive index"). 1 The sensitivity of mode effective indices depends on the overlap of the field with the medium to be measured, and most fiber optic based refractometers have increasing sensitivity for SRI approaching the index of glass or 1.45. In order to achieve high sensitivity for lower SRI, such as water-based environments, high order cladding modes (hav-ing lower values of n eff) must be used near their cut-off wavelengths (to maximize their evanescent field penetra-tion). 2–4 In all cases, the intrinsic sensitivity of fiber cladding modes S I (Dn eff /DSRI) peaks near 0.02 for wavelengths where the modes reach cut-off. If a grating is used to mea-sure n eff , the intrinsic sensitivity yields a spectral shift sensi-tivity S k ¼ Dk/DSRI ¼ S I Â Dk/Dn eff , where Dk/Dn eff is equal to the grating period K, because of the phase matching condition for modes coupled by a grating, 4 k i clad
Applied Physics Letters 07/2011; 99:041118. · 3.84 Impact Factor
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ABSTRACT: Published by the American Institute of Physics. Related Articles Simultaneous measurement of refractive index and temperature by integrating an external Fabry-Perot cavity with a fiber Bragg grating Rev. Sci. Instrum. 83, 053113 (2012) Multiplex and simultaneous measurement of displacement and temperature using tapered fiber and fiber Bragg grating Rev. Sci. Instrum. 83, 053109 (2012) Fiber-optic liquid level sensor based on coupling optical path length variation Rev. Sci. Instrum. 83, 055006 (2012) Fiber optic displacement sensor with a large extendable measurement range while maintaining equally high sensitivity, linearity, and accuracy Rev. Sci. Instrum. 83, 045002 (2012) Transmission and temperature sensing characteristics of a selectively liquid-filled photonic-bandgap-fiber-based Sagnac interferometer
Applied Physics Letters 01/2011; 99212(141104). · 3.84 Impact Factor
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ABSTRACT: In this letter, a novel infrared (IR) radiation detector is proposed for early fire detection. It consists of two matched apodized fiber Bragg gratings (FBGs) cascaded in a single optical fiber and separated by a few centimeters. One of the gratings is covered with an IR-absorbing layer that converts the radiation into heat while the second FBG provides a temperature reference. To obtain a fast and cost-effective detector, the demodulation technique consists in measuring the total reflected power, which monotonically evolves in a range of flux that depends on the FBGs spectral characteristics. With the 24-mm-long Gaussian apodized FBGs used in this work, we report a mean sensitivity of 7 nW/(W/m<sup>2</sup>) in the range 0-3000 W/m<sup>2</sup>.
IEEE Photonics Technology Letters 01/2011; · 2.19 Impact Factor
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ABSTRACT: We have studied the gamma-radiation induced effect on the Bragg peak position in fiber gratings. For the first time, to the best of our knowledge, a short-wavelength shift was observed for the Type I gratings written in an intrinsically photosensitive fiber. Based on the kinetics of the peak shift observed under irradiation and during post-irradiation annealing, we propose that two different types of defects are generated by ionizing radiation. One type is responsible for the refractive index increase during irradiation, while the creation of the defects of another type results in the index decrease. The concurrence of the two types of defects can be a way to obtain high stability of the gratings under irradiation.
IEEE Transactions on Nuclear Science 01/2011; · 1.45 Impact Factor
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ABSTRACT: A novel interrogation technique for tilted fiber Bragg gratings (TFBGs) sensors based on single wavelength time delay measurements is presented. To this aim, the group delay of a weakly TFBG has been characterized by direct time-domain measurement. The experimental characterization shows the capability of TFBGs to speed up and slow down the propagation velocity of an optical pulse in optical fiber. These sharp group delay features of the TFBGs are exploited for refractometric applications to detect the surrounding refractive index (SRI) variations by single wavelength time delay measurements. The obtained results demonstrate the possibility to detect SRI changes by means of single wavelength time delay measurements with enhanced sensitivity in the range 1.33-1.40 with respect to previously reported interrogation techniques.
IEEE Photonics Technology Letters 01/2010; · 2.19 Impact Factor
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ABSTRACT: By now, fiber Bragg gratings (FBGs) represent a well assessed technology in sensing field. Unfortunately, to make FBGs sensitive
to surrounding refractive index, hosting fiber structuring is needed. In last years, also tilted FBGs (TFBGs) have been proposed
as promising technological platform for sensing applications. However, complex spectral features limit the practical exploitation
of this technology. It would be extremely useful to merge the peculiar spectral characteristics of both grating types. To
address this issue, here, we propose a hybrid cavity involving two unbalanced uniform FBGs written at both sides of a TFBG.
The proposed configuration provides a wavelength gated reflection signal. Such a structure preserves uniform FBG advantages
in terms of interrogation methods and allows the possibility to contemporarily measure multiple parameters.
12/2009: pages 121-125;
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ABSTRACT: We report the possibility to reduce the polarization-dependent loss (PDL) and the differential group delay (DGD) in fiber Bragg gratings (FBGs) by inducing polarization-mode coupling (PMC). To induce the PMC, the fiber is twisted before the inscription and then relaxed once the FBG is written. The theoretical concept as well as the simulation method used to study FBGs exhibiting PMC is detailed. The obtained results point out a significant decrease of the gratings' PDL and DGD values. In addition, experimental results, in good agreement with the theory, demonstrate the possibility to implement this method in practice.
IEEE Photonics Technology Letters 12/2009; · 2.19 Impact Factor
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ABSTRACT: In this paper, we describe a quasi-distributed optical fiber sensor combining two well-known technologies: a concatenation of low reflective identical fiber Bragg gratings (FBGs) sensors is interrogated by means of a commercial optical time domain reflectometer (OTDR). To realize an efficient demodulation technique with a cost-effective sensing system, the OTDR pulses are reshaped by a wavelength-tunable apodized FBG filter and directed to the cascade of FBGs sensors. An accuracy of about 20 pm in the reconstruction of the Bragg wavelength is reported, yielding to an accuracy in the determination of the axial strain value of the order of 15 microstrains.
01/2009: pages 103-110;
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ABSTRACT: In this letter, a novel demodulation technique based on the monitoring of the polarization-dependent loss in a 1-nm wavelength range is proposed to measure the surrounding refractive index by means of weakly tilted fiber Bragg gratings. A 110<sup>-3</sup> refractive index resolution as well as a temperature-insensitive behaviour are reported.
IEEE Photonics Technology Letters 01/2009; · 2.19 Impact Factor
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ABSTRACT: In this note, we present a quasi-distributed temperature monitoring system based on the concatenation of identical low-reflective fiber Bragg gratings (FBGs) and interrogated by means of an optical time-domain reflectometer (OTDR). An original wavelength-sensitive system placed before the OTDR detector is used to analyze the reflected signal. This system allows the height of the FBG reflection peaks in the OTDR trace to depend on their resonance wavelength, and therefore to the local temperature. In addition, a simple but original reference method is proposed. The configuration of the whole interrogating device is kept very basic, as a standard OTDR and some passive components are used. The cost of the overall system is therefore very limited. In this note, the wavelength-sensitive system is studied in details, as well as the reference method. Experimental results are reported.
Measurement Science and Technology 12/2008; 20(2):027001. · 1.49 Impact Factor
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ABSTRACT: We experimentally report the possibility to reduce PDL and DGD in fibre Bragg gratings by inducing polarization mode coupling. This is obtained by using a twisted-fibre for the inscription.
Optical Communication, 2008. ECOC 2008. 34th European Conference on; 10/2008
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ABSTRACT: In this paper, we present an experimental analysis of the effects induced in the transmitted spectrum by depositing a uniform high refractive index (HRI) thin coating on weakly tilted fiber Bragg gratings (TFBGs). Experiments were conducted on two weakly TFBGs presenting different tilt angles (5deg and 7deg) covered by two different coating thicknesses using the dip-coating technique. The transmission spectra of the obtained structures were collected as the surrounding refractive index (SRI) changed in the range 1-1.47. The coated gratings present some interesting differences in their spectral evolution in comparison to bare gratings. An important effect of HRI coatings is indeed to enhance the SRI sensitivity of the spectral position of the dips corresponding to cladding modes couplings, leading to an improvement of the efficiency of previous demodulation techniques exploited in the case of bare TFBGs.
IEEE Sensors Journal 08/2008; · 1.52 Impact Factor
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ABSTRACT: Weakly tilted fiber Bragg gratings with gratings planes tilted at small angles with respect to the fiber axis couple light to backward going core mode and cladding modes. Their transmitted spectrum is characterized by narrow resonance dips below the Bragg wavelength corresponding to the core mode coupling. The amplitude spectral evolutions of weakly tilted fiber Bragg gratings in response to diverse physical perturbations such as temperature, mechanical strains, bending and surrounding refractive index changes are presented. Different techniques allowing to efficiently correlate the spectral evolution with the information to be measured are reported. We demonstrate that a selective monitoring of one cladding mode shift with respect to the Bragg wavelength gives temperature-insensitive strain measurements whereas a global monitoring of the cladding modes spectrum offers temperature-insensitive surrounding refractive index measurements. We also point out the possibility of using this global monitoring for bending and transverse strain sensing purposes. Finally, we present the effect of coating (the grating is covered by a polymer) on the sensitivity of weakly tilted fiber Bragg grating to surrounding refractive index changes. For every application, the performances of weakly tilted fiber Bragg gratings sensors are discussed.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
04/2008;
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ABSTRACT: The explosion risk linked to the use of hydrogen (H2) as combustible requires low-cost and efficient sensors. We present a multipoint in-fiber sensor capable of H2 leak detection as low as 1% concentration in air with a response time smaller than a few seconds. Our solution utilizes uniform fiber Bragg gratings (FBGs) surrounded by a catalytic sensitive layer made of a ceramic doped with a noble metal. In the presence of H2 in air, the sensitive layer undergoes an exothermic reaction and elevates the temperature around the FBGs. The sensor interrogation technique is based on the monitoring of the resonant wavelength shift. In this paper, the performances of the sensor are also discussed.© (2008) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.
04/2008;
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ABSTRACT: The presence of weak birefringence in fiber gratings slightly affects their amplitude response. In this case, we report that the wavelength dependencies of chromatic dispersion and differential group delay (DGD) are linked by the birefringence value. This relationship is used to analyze the grating DGD and exploited to get the grating birefringence. Experimental results are finally reported in good agreement with the theory.
IEEE Photonics Technology Letters 04/2008; · 2.19 Impact Factor
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ABSTRACT: The explosion risk linked to the use of hydrogen as fuel requires low-cost and efficient sensors. We present here a multipoint in-fiber sensor capable of hydrogen leak detection in air as low as 1% concentration with a response time smaller than a few seconds. Our solution makes use of fiber Bragg gratings (FBGs) covered by a catalytic sensitive layer made of a ceramic doped with noble metal which, in turn, induces a temperature elevation around the FBGs in the presence of hydrogen in air.
IEEE Photonics Technology Letters 02/2008; · 2.19 Impact Factor
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ABSTRACT: This work reports on the experimental investigation of the spectral effects induced by uniform high refractive index nano-coatings deposited on weakly tilted fiber Bragg gratings (TFBGs). To this aim, weakly TFBGs with different tilt angles were fabricated and then coated with thin overlays of syndiotactic polystyrene. Automated dip coating technique was used to obtain ring-shaped coatings with thickness ranging in hundreds of nanometers. The transmission spectra of the obtained devices were collected in order to outline the effects of thickness, tilt angle, and surrounding refractive index. Experimental results reveal interesting differences with respect to the uncoated case, both in terms of spectral envelope and wavelength shifts related to cladding modes coupling.
IEEE Photonics Technology Letters 01/2008; · 2.19 Impact Factor
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ABSTRACT: We experimentally demonstrate the reduction of Differential Group Delay and Polarization Dependent Loss in Superimposed Fiber Bragg Gratings when the fiber is rotated after each inscription, which is of great importance in WDM applications.
Optical Communication (ECOC), 2007 33rd European Conference and Ehxibition of; 10/2007
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ABSTRACT: We model the relationship that exists between the chromatic dispersion and differential group delay in LPG. Our theory, confirmed by experimental results, gives a deeper knowledge of birefringent LPG properties which are complex to model.
Optical Communication (ECOC), 2007 33rd European Conference and Ehxibition of; 10/2007
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ABSTRACT: In this paper, we present the sensitivity characteristics of core and cladding modes in weakly tilted fibre Bragg grating sensors with different cladding thicknesses. Both experimental and analysis results are presented. The results show that the strain sensitivity of cladding mode resonances in tilted fibre Bragg gratings does not depend on the cladding diameter but on the wavelength separation from the core resonance over the range of the cladding diameters studied.
Measurement Science and Technology 09/2007; 18(10):3117. · 1.49 Impact Factor
