Fiber grating laser sensors have been attracting great interest because of their high signal-to-noise ratio and narrow linewidth that permit high resolution sensing. According to the working ...principle, fiber grating laser sensors can be classified into two types: wavelength encoding sensor and polarimetric heterodyning sensor. The former responds to external perturbations in terms of shift in the operation wavelength of the fiber laser, which is similar to that of fiber grating sensor. The latter converts measurand into change in beat frequency between the two orthogonal polarization modes from the fiber laser. The polarimetric fiber grating laser sensor not only has almost all advantages of passive fiber grating sensors, but also has a distinctive advantage of ease of interrogation. This is because the beat frequency is in the RF domain, which avoids the employment of expensive wavelength measurement devices. This type of sensor has been demonstrated for measurement of temperature, axial strain, lateral force, hydrostatic pressure, bending, displacement, acceleration, electric current, and acoustic and ultrasonic signal. In this paper, we review the principle, fabrication, characterization, and implementation of the polarimetric heterodyning fiber grating laser sensors, and the sensor multiplexing in the RF domain.
'Smart' structural health monitoring of composite materials with optical fiber sensors is becoming more and more important, especially in the aviation industry. This paper presents an overview of ...hybrid fiber-optic sensing systems based on scattering techniques, fiber Bragg gratings, interferometric techniques, and polarimetric methods in structural health monitoring. The main purpose of this manuscript is to analyze the possibilities of using hybrid sensors based on fiber optics to monitor composite structures, with a particular emphasis on aircraft structures. Since it is difficult to indicate the most comprehensive approach due to different parameters of the described sensors, the review contains a detailed description of available solutions. We hope that this work will allow for a better and faster selection of the right solution for the problem at hand.
•Available Faraday mirrors cannot be used in fiber-optics current sensors for ITER diagnosis.•A compensation formula allows fulfilling the ITER requirements if the mirror detuning is known.•Existing ...mirror detuning measurement methods provide a sufficient accuracy for ITER application.
Plasma current measurements in ITER are safety-related and must therefore satisfy a very demanding specification. In this paper, the use of the Fiber Optics Current Sensor (FOCS) operating in the reflection mode with a Faraday mirror to perform plasma current measurements is analyzed. Based on the Jones matrix formalism, we performed numerical simulations to investigate the impact of the Faraday mirror detuning on the measurement accuracy. We show that the use of standard commercial components does not allow to satisfy the ITER requirements for the whole plasma current range. A simple solution to the problem is proposed, which consists in taking into account a mirror calibration in the current estimator. We show that the achievable mirror calibration accuracy is sufficient to fulfill the ITER requirements.
The paper presents the experimental results of the investigations of optical fibre current sensors with an external transformation. The head of the sensor is made of the glass which has a high value ...of the Verdet constant. The sensor was built on the glass bar (with high value of the Verdet coefficient), polarisers, plastic optical fibres, a light source and a detector module. The paper deals with magnetic field measurements of two types of electrical current conductors: a typical conductor with circular cross-section and a busduct. Output signals from the optical current sensor were displayed on the digital oscilloscope and memorized in μComp for a further analysis. This type of sensor was investigated on a laboratory stand and on an electrical power line which included conductor where the high intensity electric current flow. The constructed optoelectronic current sensor was tested for detection of current with intensity of the order of 200 A, with an accuracy of a few percent. Practically, the upper border of the current detection by means of this sensor is much higher.
Fiber optic sensors have a lot to offer in the field of structural health monitoring. The most widely investigated and implemented fiber optic sensors for structural health monitoring are fiber Bragg ...grating and fiber optic polarimetric sensor. Fiber Bragg grating sensors provide localized strain data, thereby providing local damage information, while fiber optic polarimetric sensors are known for their capabilities of global damage monitoring for both static and dynamic loadings. However, each sensor has to be used with its own instrumentation and processing system. In this article, it is shown that an fiber Bragg grating written on a polarizing maintaining fiber can discern information from both fiber Bragg grating and fiber optic polarimetric sensors using only one decoding system. This reduces costs and complexities. Furthermore, by proper multiplexing the polarizing maintaining-fiber Bragg grating sensor, it is possible to predict the damage location in plates. The results demonstrate that the damage site can be located in two-dimensional structures using this multiplexed sensing array.
The technique of nonreciprocal phase modulation, known from Sagnac-type interferometers, is adopted to interrogate the differential phase shift of the orthogonal light waves in a birefringent fiber ...sensor. The sensing section of the novel interferometer consists of a series of two birefringent fibers with balanced differential group delay. A key element is a 45°-Faraday rotator which swaps the polarization states of the orthogonal waves after their round trip through the two fiber sections. An integrated-optic birefringent phase modulator introduces the nonreciprocal phase modulation at the detector end of the sensor. The interferometer is applied to voltage sensing.
The technique of non-reciprocal optical phase modulation known from fiber gyroscopes is adapted to interrogate a polarimetric optical voltage sensor. The piezoelectric deformation of a transducer by ...an applied ac voltage is transmitted to a birefringent fiber resulting in a periodic phase shift of the two orthogonal polarizations of the fundamental fiber mode. A modulator at the detector end of the sensor introduces a nonreciprocal phase modulation of the light waves. The voltage is recovered from the first and second harmonics of the modulator frequency and side bands in the detected signal.
The spectral and thermooptical sensitivity responses of highly birefringent photonic crystal fibers (PCFs) with elliptically elongated veins integrated in their profile are being investigated using a ...novel sensitivity analysis based on an accurate semivectorial modal solver combined with temperature-dependent Sellmeier equations for pure silica and dry air. We demonstrate that by an appropriate selection of the design parameters it is possible to optimize the sensitivity of the modal birefringence or even to obtain null response at specific wavelengths. Thus, our investigation adds evidence to the potential use of highly birefringent PCFs as polarimetric sensors. Optimized overall temperature sensitivity could be obtained up to 7.5middot10 -7 K -1 at operational wavelength of 1.55 mu m
A new approach for multiplexing interferometric fiber optic sensors is described and studied. The approach is based on a lattice of unbalanced Mach-Zehnder interferometers. This configuration is ...inherently low-loss and can be implemented and deployed in a simple manner. The signals from the cascaded sensors are extracted from the measured data via a layer peeling algorithm. A fifth-order polarimetric implementation of the proposed multiplexing technique has been constructed and characterized. The third sensor was buried in a sandbox while the rest of the sensors were spooled on a bobbin. The system successfully separated and monitored the pressure variations in the different sensors with minor crosstalk.
Optical-fiber sensors are emerging as a superior non-destructive means for evaluating the condition of concrete structures. In contrast to existing non-destructive evaluation techniques, optical ...fibers are able to detect minute variations in structural conditions through remote measurements. Structures fully integrated with optical fibers will be able to monitor the initiation and progress of various mechanical or environmentally-induced degradations in concrete elements. Recent advances in fiber-optic sensor technology and the possibility of their use in concrete structures have instigated the development of a number of research activities. Owing to inherent interdisciplinary nature of the field of fiber optics, the expertise of the researchers active in the research and development of fiber-optic sensors covers a wide spectrum of disciplines including concrete engineering, as well as opto-electronics and physics. This article is intended for rapid dissemination of the current state-of-the-art in this emerging technology. However, because of the interdisciplinary nature of the subject, a brief discussion on the physical nature of optical fibers is also presented.
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