We demonstrated the inscription of arc-induced long-period fiber gratings (LPFGs) in the B/Ge co-doped fiber and in the SMF28 fiber by using grating periods shorter than 150 μm and 200 μm, respectively. This achievement was a result of the development of a high voltage power supply that allows for a constant and stable electric current ranging from 10.5 mA up to 21 mA. The fabricated LPFGs were characterized as a function of the external refractive index from 1.333 up to 1.420 and a refractive index sensitivity of the order of 1000 nm/RIU was obtained without further optimization.
In this paper, we report the development of a reduced temperature sensitivity optical fiber sensor for refractive index measurement based on Superimposed Long-Period Gratings (SLPG) inscribed by the electric arc technique in standard fiber. The reduced sensitivity to temperature is achieved by calculation of the difference between resonance wavelengths of two guided cladding modes.
In this paper it is presented an all-fiber implementation of the hot-wire needle probe concept, widely used to measure the thermal properties of materials, particularly the thermal conductivity. It is based on the heating of a metal thin film deposited on the surface of the fiber induced by the coupling of laser light into the cladding via a long period grating, and determination, using a fiber Bragg grating, of the time dependence of the temperature of the surrounding medium at a fixed distance of the fiber. The medium considered in this research was the air and the results obtained indicate the feasibility of this approach and point out future developments.
In this work, we propose a compact sensor head to perform cryogenic temperature measurements based on a long-period fiber grating. The presented configuration enables the sensor to be interrogated in reflection since a phase-shifted is produced by Fresnel reflection on the end-face of the fiber, cleaved at a quarter-period separation distance from the end of the grating.
This paper presents a comparative study of the behaviour of different kinds of optical fibre sensors in response to high
temperatures. It compares the performance of regenerated fibre Bragg gratings (FBGs) written in hydrogen-loaded and
non-loaded fibres with long period gratings (LPGs) written through the two different processes of ultraviolet (UV)
irradiation and electrical arc discharges. This work shows the importance of the use of hydrogen-loaded fibres to achieve
regenerated FBGs capable of withstanding high temperatures as high as 955°C. In addition, the results demonstrated that
LPGs recorded by electric arc discharges have higher thermal resistance than LPGs written by UV radiation.
In this work an all optical hot-wire flowmeter based on a silver coated fibre incorporating a long period grating and a
Bragg grating is demonstrated. Optical energy at 1480 nm propagating down the fibre is coupled by the long period
grating into the fibre cladding and absorbed by the metallic coating deposited on the fibre surface over the Bragg grating
position. This absorption acts like a hot-wire raising locally the fibre temperature, which is effectively detected by the
FBG resonance shift. The temperature raise depends on the flow speed of the surrounding air that has the effect to cool
the fibre. In this way, the FBG Bragg wavelength shift can be related with the flow speed. Results obtained demonstrate
the working principle and a flow speed resolution of 0.08 m/s is demonstrated.
A long-period grating written in the SMF-28 fiber was heat treated at 1000 °C for 15 days. The spectrum of the grating
shifted to longer wavelengths and the amplitude of the cladding mode resonances decreased as a result of structural
relaxation. The background loss increased considerably for time longer than 200 h, and this loss is caused by
devitrification of the fiber.
In this work we describe the characterization of high Q optical microresonators using an all fiber based system. Silica
microspheres fabricated on a fiber tip by electric arc discharge are characterized using a simple interrogation system
based on an adiabatic fiber taper coupler and on the collection of scattered radiation by a multimode fiber.
In this work the behavior of an optical fiber Long Period Grating (LPG) refractometer with the variations of the
surrounding refractive index is discussed. The objective is to characterize optical fiber refractometers sensitive to
surrounding refractive index, higher and lower than the cladding. For values of surrounding refractive index higher than
the cladding, the LPG does not show enough sensitivity. For this reason, a nanolayer of an organic material was coated
onto the fiber, using the Langmuir-Blodgett technique. We characterized LPG covered with different nanolayers
thickness (110 and 120 nm) relatively to changes in surrounding refractive index.
Optical coherence tomography (OCT) imaging at 1060 nm region proved to be a successful alternative in ophthalmology
not only for resolving intraretinal layers, but also for enabling sufficient penetration to monitor the sub-retinal
vasculature in the choroids when compared to most commonly used OCT imaging systems at 800 nm region. To
encourage further clinical research at this particular wavelength, we have developed a compact fiber optic source based
on amplified spontaneous emission (ASE) centered at ~1060 nm with ~70 nm spectral bandwidth at full-width half
maximum (FWHM) and output power >20 mW. Our approach is based on a combination of slightly shifted ASE
emission spectra from a combination of two rare-earth doped fibers (Ytterbium and Neodymium). Spectral shaping and
power optimization have been achieved using in-fiber filtering solutions. We have tested the performances of the source
in an OCT system optimized for this wavelength.
In this work, an LPG-based Mach-Zehnder modal interferometer with a tapered fiber section between the two LPGs is
studied as a sensing structure for measuring environmental refractive index. To interrogate this sensing device, coherence
addressing and pseudo-heterodyne processing were used. A fiber taper was made between the two LPGs to improve and
tailor the sensitivity of the sensor. Experimental results show that the sensitivity to external refractive index increases
with the length of the taper and that the sensitivity enhancement is stronger for lower order cladding modes.
In this work, interferometric sensors based on antiresonance reflecting waveguide (ARROW) fibers were developed and
their performances were characterized for measurement of physical parameters, particularly strain and temperature. Two
types of ARROW fibers were considered and signal demodulation was achieved using the white light interferometric
technique. Application issues of sensing heads based on these fibers are also addressed.
In this work, sensitivity to strain, temperature and curvature of a sensor relying on modal interferometry in hollow-core
photonic crystal fibre is studied. The sensing structure is simply a piece of hollow-core fibre connected in both ends to
standard single mode fibre. An interference pattern that is associated to the interference of the light that propagates in
the hollow core fundamental mode with light that propagates in other modes is observed. The phase of this interference
pattern changes with the measurand interaction, which is the basis for considering this structure for sensing. The phase
recovery is performed using a white light interferometric technique.
In this work, an LPG-based Mach-Zehnder interferometer was evaluated as a sensing structure for environmental
refractive index measurement. To interrogate this sensing device, coherence addressing and pseudo-heterodyne
processing were used. The impact of stretching, bending and twisting the interferometer on the sensitivity to refractive
index changes r was studied. It is shown that, due to the antisymmetric nature of cladding modes in arc-induced LPGs, it
is possible to tune the system sensitivity by simple mechanical action.
It is described a fibre optic sensor for liquid refractive index measurement based on the visibility variations of a Fabry-Perot interferometer with interfering waves generated in a short Bragg grating and in the fibre tip (Fresnel reflection) in
contact with the liquid. The sensor was characterized emerging the fibre tip in distilled water with different
concentrations of ethylene glycol. A linear relation was obtained, with a readout resolution of ≈ 10-3. It was also
observed that the temperature direct cross sensitivity is residual.
A fiber optic sensor for liquid refractive index measurement based on a Fabry-Pérot interferometer is described. The interferometer is achieved between the reflection of a short fiber Bragg grating and the Fresnel reflection from the cleaved fiber end. This fiber end is then in contact with the liquid sample to provide refractive index measurements. The sensor is characterized by immersing the fiber tip in distilled water with different concentrations of ethylene glycol. A linear relation of the interferometer fringe visibility with refractive index variation is observed, and a resolution of ~10−3 is obtained. It is also shown that the sensor operation is independent of temperature effects, other than the one related to temperature-induced change of the liquid refractive index.
A long-period-grating-based fiber optic Michelson modal interferometer with coherence addressing and heterodyne interrogation is studied as a sensing structure for measuring environmental refractive index, temperature, and liquid level. The effects of several system parameters on the measurements are investigated. Experimental results show that the sensitivity to the external refractive index increases with the order of cladding mode and with a reduction of the fiber diameter. The decrease of the fiber diameter from 125 µm down to 70 µm enhances the sensitivity to the external index by a factor of 2.7. It is also shown that the use of a silica-core fiber increases the sensitivity to the external index by a factor of 1.4 and reduces the thermal sensitivity by a factor of 2.5 compared to a standard fiber.
A novel Mach-Zehnder interferometer based on a fibre multimode interference structure combined with a long period
fibre grating is proposed. The multimode interference is achieved through the use of a multimode fibre section spliced
between two single-mode fibres, with a length adjusted to couple a fraction of light into the cladding modes. A LPG
placed after the multimode fibre couples light back into the fibre core completing the Mach-Zehnder interferometer. This
novel configuration was demonstrated as a bending sensor.
In this work, the LPG-assisted fibre Michelson modal interferometer is studied as a sensing structure for
environmental refractive index, temperature and liquid level when coherence addressing and heterodyne interrogation are
considered. The effects on measurand sensitivity of the order of the cladding mode excited by the LPG, of the degree of
etching of the sensing fibre and of the fibre type used are investigated.
We study the origin of antisymmetric perturbation of the fiber in arc-induced long-period gratings that couple the core
mode into the antisymmetric cladding modes. We demonstrate that this perturbation is caused by a temperature gradient
in the fiber, which is induced, in turn, by a temperature gradient in the arc discharge.
In this work it is presented a novel in-fibre modal interferometer based on a non-adiabatic biconical fused taper that
couples light between the cladding and the core, combined with the Fresnel reflection at the fibre end. It is observed
that the returned light from this fibre structure shows a channelled spectrum similar to that of a two-wave Michelson
interferometer. The application of this device as a fibre optic flowmeter sensor is demonstrated.
The suitability of semiconductor nanoparticles to provide a reference signal in luminescence based chemical sensors is addressed. A CdSe-ZnS nanocrystal, with emission peak at 520 nm is used to provide a reference signal. Measurements of oxygen concentration, which are based on the dynamic quenching of the luminescence of a Ruthenium complex, are performed. Both dye and the nanocrystal are immobilized in a sol-gel matrix and are excited by a blue LED. Results are presented showing that the ratio between the reference and the sensor signals is highly insensitive to fluctuations of the excitation optical power. Preliminary results show that nanocrystals could be used to measure temperature and provide a reference signal.
The use of semiconductor nano-particles as temperature probes in luminescence chemical sensing applications is addressed. Temperature changes the intensity, the peak wavelength and the spectral width of the quantum dots luminescent emission in a linear and reversible way. Results are presented that show the feasibility of implementing a self-referenced intensity based sensor to perform temperature measurements independent of the optical power level in the sensing system. Additionally, it is demonstrated that self-referenced temperature measurements in multiple points could be performed using reflection or transmission based optical fiber configurations.
Semiconductor nano-particles, or quantum dots, with their relatively high quantum yields, narrow luminescence spectrum, outstanding photostability and the ability to tune their optical properties, are ideal for biological tagging applications and a very powerful tool for chemical sensors. In this paper an overview of this rapidly expanding area of research is presented. Additionally, some results are shown, in the framework of optical oxygen sensors, which establish quantum dots as suitable temperature and intensity references for application in luminescence based chemical sensors.
An optical fiber sensor for the measurement of oxygen in gaseous environments, which is based on the quenching of the fluorescence of a ruthenium complex, is presented. The sensing chemistry is immobilized in a sol-gel based solid matrix that is coated on a tapered optical fiber probe. Oxygen measurement is performed both by phae and fluorescence intensity spectroscopy. Experimental results show that the fluorescence intensity and the lifetime depend both on oxygen and temperature. A scheme for simultaneous determination of the temperature and the oxygen concentration is proposed. Temperature measurement is performed using the excitation radiation and an absorption long pass filter. Preliminary results are presented which show a temperature measurement independent of oxygen and of optical power level.
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