In this paper, we demonstrate the use of single-mode Cobalt-doped fibers for active distributed temperature sensing with optical heating. A high-power EDFA at 1550nm is used for optically heating the fiber, while a Brillouin Optical FrequencyDomain Analysis (BOFDA) set-up operating at 850nm is employed to monitor the corresponding temperature changes. Owing to the use of a dual wavelength scheme, longer sensing distances can be reached even with relatively high doping concentrations. As a proof of concept, we demonstrate the possibility to distinguish two 1-m fiber sections, one immersed into water and another one surrounded by air.
KEYWORDS: Sensors, Spatial resolution, Modulation, Time metrology, Refractive index, Acoustics, Biological and chemical sensing, Polarization, Scattering, Temperature metrology
Brillouin fiber sensors permit to perform distributed temperature and strain measurements along an optical fiber through the changes of the Brillouin Frequency Shift (BFS). In conventional Brillouin schemes operating in the time-domain, the BFS is detected by analyzing the interaction between a pulsed pump wave, and a counter-propagating probe wave. The duration of the pump pulse determines the spatial resolution, which however is limited to about 1m due to phonon lifetime. Alternatively, Brillouin optical frequency-domain analysis (BOFDA) and Brillouin optical correlation-domain analysis (BOCDA) configurations provide cm-scale, or even mm-scale distributed sensing capabilities. In particular, BOFDA sensors make use of a continuous wave (CW) pump wave with superimposed a small-signal modulation. The analysis consists in determining, by use of a vector network analyzer (VNA), the amplitude and phase of the corresponding modulation induced on the probe wave intensity, over a discrete number of modulation frequencies. In BOFDA sensors, spatial resolution can be improved down to the cm-range or mm-range, thanks to the pre-activation of the acoustic wave involved in the scattering process. In this work, we show that mm-scale spatial resolution can be exploited, in a BOFDA configuration, to perform both physical (temperature) and chemical (refractive index) sensing. For the latter, a sidepolished fiber was used in order to make the BFS sensitive to the surrounding refractive index (SRI). A sensitivity of the BFS to the SRI as large as 293 MHz/RIU at nsm = 1.40 is demonstrated experimentally and validated numerically.
To measure the strain and/or temperature along an optical fiber with one-end access, a Brillouin optical frequency-domain reflectometry (BOFDR) technique is presented in this paper. It is based on detecting the spontaneous Brillouin scattering from a sinusoidally modulated pump light. Compared to the Brillouin optical frequency-domain analysis (BOFDA), this new BOFDR sensor approach presents the advantage that the measurements are free from the distorting components related to acoustic wave modulation, thus simplifying the associated data processing.
A new scheme for fast distributed sensing based on Brillouin Optical Time-Domain Analysis (BOTDA) is proposed and demonstrated, based on the use of a frequency swept microwave source for the generation of the probe wave. The entire BOTDA measurement is taken within the duration of the frequency sweep, at a frequency granularity depending on the duration of the sweep, the repetition rate of the pump pulses and the number of averages.
Brillouin scattering properties of a bend-insensitive fiber were experimentally investigated at 850 nm and 1550 nm wavelength using Brillouin optical time-domain analysis (BOTDA). A diversity scheme has been adopted, in which the BFS distribution along the same fiber is acquired at the two operating wavelengths. Based on the different wavelength scaling factor of temperature and strain coefficients, simultaneous measurement of strain and temperature at 1-m spatial resolution has been obtained in a proof-of-concept experiment.
This paper presents a numerical study of the phase noise from the laser in Brillouin Optical Time-Domain analysis
(BOTDA) sensors. Due to laser phase noise, the phase shift cumulated by pump and probe beams during interaction in a
generic fiber position is a stochastic variable, with zero mean and variance increasing with pulse duration. For negligibly
small pulse leakage, the induced noise is independent of fiber length; otherwise, it increases with fiber length as long as
the laser coherence length is longer than fiber.
By analyzing the operation of the slope assisted Brillouin Optical Time-Domain Analysis (BOTDA) method, it comes
out that the acquisition rate is practically limited by two fundamental factors: the polarization scrambling frequency and
the phase noise from the laser. As regards polarization scrambling, we show experimentally that the scrambling
frequency poses a limit on the maximum acquisition rate for a given averaging factor. As regards phase noise, we show
numerically and experimentally that the slope assisted method is particularly sensitive to the laser phase noise, due to the
specific positioning of the pump-probe frequency shift on the Brillouin Gain Spectrum (BGS).
We report distributed temperature measurements in a perfluorinated graded-index polymer optical fiber (POF) with 50 μm core diameter, carried out by stimulated Brillouin scattering. Brillouin optical frequency-domain analysis (BOFDA) has been performed to extract the temperature profile along a 20-m POF fiber sample, at a nominal spatial resolution of 4 m. The results indicate that POFs are potentially useful for distributed temperature measurements based on stimulated Brillouin scattering.
We propose the application of a distributed optical fiber sensor based on stimulated Brillouin scattering, as an integrated system for safety monitoring of railway infrastructures. The strain distribution was measured dynamically along a 60 meters length of rail track, as well as along a 3-m stone arch bridge. The results indicate that distributed sensing technology is able to provide useful information in railway traffic and safety monitoring.
We report an experimental study on a cantilever beam, aimed to verify the feasibility of modal analysis by distributed
Brillouin sensing for structural damage identification. Damage identification was carried out for three defect cases,
analyzing the changes of the natural frequencies and mode shapes of the first two bending modes. Comparison with finite element method (FEM) analysis shows that the damage can be detected and localized, within the limitation dictated by the spatial resolution (30 cm) of our sensor.
We present the application of a Brillouin distributed sensor for railway traffic monitoring. The field test is performed on the Italian regional line San Severo - Peschici. A single-mode optical fiber was attached along a rail length of about 60 meters. The strain associated to train passage was acquired along the monitored rail length, at 31 Hz acquisition rate and 1-m spatial resolution. The sensor is capable to retrieve useful information in railway traffic monitoring, such as train identification, axle counting, speed detection, and dynamic load calculation.
Ferrule-top (FT) cantilevers are a new generation of all optical micromechanical sensors obtained by carving
microstructures on the top of ferrule terminated fibers. In this paper, we will demonstrate how this plug and play design
can be used for the development of a new generation of sensors and actuators for harsh environments, where
commercially available devices would be prone to failure. Ferrule-top sensors can work in two main modes - static and
dynamic. The static mode is based on recording elastic deflection of the cantilever; the dynamic mode relies on tracking
changes in its mechanical properties (resonance frequency, quality factor). Depending on the application, one can choose
which mode is most suitable or combine both to achieve best performance. We will illustrate the relation between
specific measured quantity (humidity, flow) and the behavior of the sensor. Further, we will show the setup in which the
sensor can be actuated using light, giving the possibility to excite the cantilever without any electronics on the sensing
head. This technique might by use for the development of fully optical beamsteering microdevices.
We demonstrate both numerically and experimentally the possibility to bring the spatial resolution of Brillouin optical
frequency-domain analysis (BOFDA) distributed sensors in the cm-range. The numerical analysis is aimed to understand
the effects of acoustic wave modulation on the accuracy of BOFDA systems when operating with submeter spatial
resolution. We demonstrate that these effects lead to an error in strain/temperature estimation, which depends on the
amount of strain/temperature change, and on the peak searching algorithm applied to the acquired spectra. Experimental
measurements of BOFDA traces up to 3.5 GHz, equivalent to 3-cm spatial resolution, are presented and discussed.
We report the results of a load test performed on a road-bridge. In particular, the tests were performed by a portable
prototype based on stimulated Brillouin scattering in optical fibers. The optical fiber sensor was able to provide the strain
profile along the beam, with a spatial resolution of three meters and a strain accuracy of ± 20 εμ. Comparison with finite-elements-method simulations, as well as with data collected by vibrating wire strain gauges, permitted to confirm the
validity of the SBS-based approach in monitoring the deformation of large structures.
In this work, we extend the use of the gain-loss technique in order to enhance the long-range capabilities of Brillouinbased
distributed sensors. In particular, we take advantage of the use of an unbalanced pair of Stokes and anti-Stokes
lines, in order to better compensate for SBS spectra distortion resulting from pump depletion. Numerical results
confirmed the validity of the approach.
We present a novel technique for dynamic strain measurements in optical fibers, based on the stimulated Brillouin
scattering interaction between two counter-propagating optical pulses. The technique permits to address dynamically and
randomly the position at which vibration is measured. Moreover, in the proposed method the sampling rate is only
limited by the time-of-flight of the pulses along the sensing fiber. Preliminary experimental results, carried out with a
perturbation frequency up to 98 Hz, demonstrate the validity of the proposed technique.
We present a numerical and experimental analysis of a technical solution, capable of alleviating the problem of pump
depletion in a long-range Brillouin distributed fiber sensor. This solution takes advantage of the presence of two
sidebands in the probe wave to generate a dual gain-loss Brillouin interaction, giving rise to reduced pump depletion.
Experimental results, carried out by using both a Brillouin optical frequency-domain analysis (BOFDA) configuration
and a Brillouin optical time-domain analysis (BOTDA) configuration, permitted to evaluate the advantages and
limitations of the gain-loss technique. An extensive experimental and numerical analysis has been carried out, in order to
understand the differences on the effectiveness of the technique, between BOTDA and BOFDA set-ups.
In this work we show that integrated silicon hollow core AntiResonant Reflecting Optical Waveguide (ARROW) can be
used as a basic tool for the realization of optical sensors. ARROW waveguides, with hollow core, permit to confine the
light in a low refractive index liquid core, by means of two cladding layers designed to form a high reflectivity Fabry-
Perot antiresonant cavity. This arrangement allows to realize microchannels that can simultaneously act as microfluidic
networks and optical waveguides with a strong advantage in the integration and with an increased interaction efficiency
between the light and the liquid substance that can be very useful in sensing applications (fluorescence, absorption
spectroscopy, etc.). Another ARROW waveguides advantage is the ability to tailor the wavelength response of the
device. In fact, the waveguide propagation losses strongly depend on the change of the resonant condition inside the
interference cladding.
In this paper we report three sensing applications of hollow core ARROW waveguide. A long path absorbance cell for
colorimetric protein assay, a high sensitivity integrated refractometer and a micro flow cytometer for cell/particles
analysis. The proposed devices have been realized in standard silicon technology by using two silicon wafers bonded
together.
A reconstruction algorithm employing an accurate modeling of the stimulated Brillouin scattering (SBS) interaction in a single-mode optical fiber, is experimentally demonstrated. Distributed temperature measureme nts are carried out by using a Brillouin optical frequency-domain analysis configuration. Reconstructions are carried out by fully taking into account the influence of the modulation of the acoustic wave involved in the SBS interaction. The sensor shows good accuracy in determining any temperature disturbances as narrow as 40 centimeters placed along the fiber.
A novel waveguide geometry for an integrated optics bio-sensor suitable for fluorescence detection is presented. In particular, we propose a polymeric waveguide realized on a glass substrate. This new geometry is aimed to an efficient evanescent-wave excitation of the fluorophores and subsequent collection of the fluorescence emission with no need of optical filters. The absence of any optical filters simplifies the device operation and permits to avoid the losses resulting from the use of the filter itself.
In this work, the response of fiber Bragg gratings (FBG) subjected to longitudinal ultrasonic (US) field has been theoretically and numerically investigated. US field interaction has been modeled taking into account the direct deformation of the grating pitch combined with changes in local refractive index due to elasto-optic effect. Numerical results show that both broadband and narrowband FBG interrogation schemes can be used to detect US presence. In both schemes, sensitivity of the FBG response decreases when reducing the ultrasound wavelength below the grating length. US excitation has been also found to induce a FBG optical spectrum shape distortion for high power ultrasounds and US wavelengths comparable to the grating length. The results of this analysis provide useful tools for the design of grating based ultrasound sensors meeting specific requirements in terms of field intensity and frequency.
The experimental validation of a numerical technique for temperature/strain sensing, based on frequency domain fiber-optic Brillouin measurements, is presented. In this technique both measurements and the signal processing are performed in the frequency domain. The algorithm is based on a harmonic expansion of the unknown profile, whose coefficients are determined by means of a multidimensional minimization. Preliminary results prove the validity of the proposed technique.
The reconstruction of temperature or strain profiles along an optical fiber in Brillouin scattering-based systems, is usually made by directly reading the Brillouin power spectra extracted for each fiber location. Such approach suffers from systematic errors due mainly to nonlocal effects. In this paper, we propose a novel numerical technique for temperature/strain profile reconstructions based on Brillouin optical-fiber time-domain analysis (BOTDA) sensors. In this approach we search directly for the temperature/strain profile along the fiber that matches the measured data. The algorithm is based on an harmonic expansion of the unknown profile, whose coefficients are determined by means of a multidimensional minimization. Several numerical simulations, even in presence of noise, have proved the capability of the proposed algorithm to compensate for systematic errors suffered by classical approaches.
The spectral response changing of a Fiber Bragg grating due to the interactions with an ultrasound wave have been numerically analyzed by a transfer matrix approach has been used, taking into account the geometrical and elasto-optic effects. Numerical analysis show that it's possible to characterize the ultrasound wave, only if the ratio between the ultrasonic wavelength and the length of the used grating exceeds an established value, which depends on both the characteristics of the Bragg grating and the ultrasound amplitude itself.
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