A temperature independent microdisplacement sensor based on microbending-induced core-cladding mode coupling loss
via a polarization maintaining photonic-crystal-fiber (PMPCF) is presented. By core-offsetting one splice joint between
the single mode fiber (SMF) and PMPCF in an SMF-PMPCF-SMF structure, one core-cladding modal interferometer
can be constructed. By packaging the interferometer in a carbon-fiber-composite based simple-supported beam,
temperature independent microdisplacement measurement can be achieved by monitoring the extinction ratio variation
of the interference spectrum. This type of sensor exhibits the advantages of temperature independent, high sensitivity and
simple structure.
The design of a portable FBG Interrogation System is presented in this paper. By using volume holographic phase
grating as the dispersion spectral element, the interrogator achieves 5 KHz scanning frequency with a readout
wavelength resolution of 1 pm. The total power consumption is less than 25 W.
In this paper, an armored-cable-based optical fiber Bragg grating (FBG) sensor array, for perimeter fence intrusion
detection, is demonstrated and some of the field trial results are reported. The field trial was conducted at a critical local
installation in Singapore in December 2010. The sensor array was put through a series of both simulated and live
intrusion scenarios to test the stability and suitability of operation in the local environmental conditions and to determine
its capabilities in detecting and reporting these intrusions accurately to the control station. Such a sensor array can
provide perimeter intrusion detection with fine granularity and preset pin-pointing accuracy. The various types of
intrusions included aided or unaided climbs, tampering and cutting of the fence, etc. The unique sensor packaging
structure provides high sensitivity, crush resistance and protection against rodents. It is also capable of resolving
nuisance events such as rain, birds sitting on the fence or seismic vibrations. These sensors are extremely sensitive with a
response time of a few seconds. They can be customized for a desired spatial resolution and pre-determined sensitivity.
Furthermore, it is easy to cascade a series of such sensors to monitor and detect intrusion events over a long stretch of
fence line. Such sensors can be applied to real-time intrusion detection for perimeter security, pipeline security and
communications link security.
We report a liquid-level variation sensor based on a fiber Bragg grating (FBG) inscribed in a 6.5-μmdiameter
microfiber. The proposed microfiber Bragg grating (MFBG) in air has two separated reflection
peaks, which are caused by the fundamental mode reflection and the higher-order mode reflection. Each of
the two peaks will split into another two adjacent peaks when a fraction of the MFBG sensor immerses into
liquid. By measuring the reflectivity difference between the two original peaks and their respective adjacent
liquid-induced peaks, the liquid-level variation can be determined.
KEYWORDS: Raman spectroscopy, Sensors, Signal processing, Fiber optics sensors, Temperature metrology, Sensing systems, Complex systems, Spatial resolution, Signal to noise ratio, Energy transfer
Brillouin or Raman-based Distributed Temperature and Strain Sensor (DTSS) provide a chance to distributed measure
the temperature and strain over tens of kilometer with higher resolution. Lots of efforts, for example, Raman amplifier,
Signal processing methods et al, have been used to extend their sensing distance and increase their sensitivity and signal noise
ratio. A sensing fiber, which has a gradually increasing Brillouin gain-coefficient gB/Aeff or Raman gain coefficient gR/Aeff
along its length and which can keep the received Brillouin or Raman Stoke's power constant in a
Brillouin or Raman DTSS, is proposed in this paper. This novel fiber and practical fiber link with such nonlinear
coefficient profile can further double or triple extend the sensing distance of the distributed Brillouin or Raman Fiber-optic
Sensor system.
Distributed sensors, based on Brillouin effect in the optical fiber, provide an excellent method for measuring temperature
and strain over long distances. There are two types of such sensors. The first type is based on spontaneous Brillouin
scattering, and is called Brillouin optical time domain reflectometer (BOTDR). It measures the Brillouin frequency shift
or Brillouin power or Brillouin gain bandwidth to get the temperature and strain information. The second type of sensor
is based on stimulated Brillouin amplification. It is called Brillouin optical time domain analyzer(BOTDA). Normally, it
uses one laser at each fiber ends, one as pump and the other as a probe light. The probe light will experience Brillouin
amplification. Through the analysis of Brillouin gain spectrum (BGS), we can get the temperature and strain information.
Both the two types of sensors are attracting attention all over the world, and temperature resolution of less than 1 degree
and strain resolution up to 5 με was reported. The fiber distances of up to 150km was presented while other papers
reported a spatial resolution of the order of 1cm with frequency domain techniques or correlation techniques. We
proposed and analyzed our design, it is an improvement of BOTDA with a single end laser, which make it easy to
implement in field. Through simulations, optimized launch power has been found for a certain design.
We study the modal birefringence behaviour of a near-elliptic core index-guiding photonic crystal fiber (PCF) intended
for high birefringence applications. The analysis is based on a semi-vectorial field convergence algorithm combined
with the perturbation correction technique of mode calculation. The approach is accurate, simple to implement, and
much faster than any of the existing methods. The efficacy of our design analysis has been verified by comparing the
results with otherwise known solutions. Using this analysis as a design recipe, we have realized PCF of reasonably high
birefringence characteristics.
The paper describes the design features and characteristics of an embedded fiber Bragg grating (FBG) pressure sensor that has been developed to measure forces applied in the direction perpendicular to the fiber axis. This FBG-based pressure sensor provides an absolute measurement in terms of wavelength shift, which avoids problems of signal intensity fluctuation often encountered by conventional intensity-modulated or phase-modulated pressure sensors. The initial outcome of our study indicates that direct sensing using bare-FBG is infeasible for real applications due to its non-linearity and limited range of force sensing. However, when the FBG is embedded into some composite material, the sensor exhibits excellent linearity, high stability and reliability over a wider force-sensing range, making it a potentially viable sensing element. When an FBG is embedded into some fiber-reinforced composites, a level of sensing force of up to 60N with a sensitivity of 3.96 pm/N and resolution of 1pm (equivalent to 0.8με) is achievable without causing permanent deformation to the FBG.
In this paper, we present our work on the fiber Bragg grating (FBG) sensors for structural health monitoring in 5m long concrete structures. Two sets of sensors were securely fastened onto the surfaces of the top and bottom reinforced bars respectively before concrete was poured in. Another set of the sensors was mounted onto the slab surface. These sensors were then monitored to observe the strain experienced at different locations within concrete slab. Loading and unloading cycle tests and failure test were performed on the completed structure. From the results obtained using the FBG sensors, we were able to correlate t he load-strain behavior of the slabs to the failure state as observed on the slab surface. These data are useful in determining the maximum allowable load before failure sets in. At the same time, we made comparisons of the data obtained using our FBG sensors with those obtained with electrical strain gauges. The two sets of data show a similar trend during the loading and unloading tests as well as during the failure tests.
A Fiber Optic Pressure Sensor utilizing a Fiber Bragg Grating (FBG) as a sensing element has been demonstrated. In the experiment, a broad band LED source with a wavelength range of 1520 nm to 1570 nm was launched into a single mode optical fiber within which the FBG was written. With no applied force, the fiber grating reflects light strongly at a wavelength of 1549.92nm. When the FBG was subjected to a range of forces (from 0.0N to 18.0N), applied perpendicularly to its axis, the reflected light from the FBG was found to be wavelength shifted in proportion to the applied force, but in a non- linear manner. The shift in wavelength is detected using fiber Fabry-Perot interferometric technique. With the FBG embedded in carbon fiber material, the test results showed an excellent linear relationship between the wavelength shift and the applied force (pressure). In addition, force sensing range of up to 60N was obtained with the pressure sensor still remaining in good condition.
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