A new instrumentation system for FBG interrogation is proposed. It combines the use of a VCSEL as a tunable laser and an optical frequency comb generator as a selective broadband source. Features such as auto-calibration and wave-length tracking are provided with this interrogation system. First a fast sweep searches for the FBG spectral position, its reflection profile and finds the best set of operating points for the optical comb. Then, the VCSEL is tuned with the FBG through a feedback loop to withstand large-range slow changes (strain and temperature) and, at the same time, the faster phenomena (vibrations and ultrasounds) are measured by the optical frequency comb generator. In practice, a dual comb is proposed to interrogate actively the FBG reflection in order to obtain radio frequency signals that contain the FBG dynamic-strain due to applied vibrations.
KEYWORDS: Fiber Bragg gratings, Sensors, Frequency combs, Signal detection, Radio optics, Electro optics, Signal processing, Photodetectors, Modulators, Electro optical sensors
In this document, a FBG interrogation system based on a multimode optical source and a direct read-out is proposed for measuring fast phenomena such as vibrations and ultrasounds. The system is based on an electro-optic dual optical frequency-comb. This architecture allows the configuration of the multimode optical source parameters such as the number of modes that are within the reflected spectrum (FWHM) of the FBG. Results are presented for the dual-comb operating under optimized control when mapping these optical modes onto detectable tones of multiples of 100 kHz around a centre radiofrequency tone (40 MHz). Dynamic strain is induced onto the fiber through an actuator, which generates changes in the reflected wavelength of the FBG and in turn the modes within the reflected spectrum. The electrical signals are analyzed using fast Fourier transform algorithms allowing identification of the vibrations.
The performance of a 1540 nm pigtailed VCSEL has been characterized in terms of output power, frequency range and rate of wavelength sweeping for its application in fibre Bragg grating interrogation. Results are presented for the laser operating under optimized control when used to illuminate a single FBG mounted under tension between a fixed plane and a loud speaker cone subject to a range of frequencies at arbitrary amplitudes and transients. The ultimate objective is to extend the set-up to a parallel fibre topology supporting 8 FBGs with equal wavelengths.
A setup simulating High Voltage DC (HVDC) transformers barriers was developed to demonstrate the effectiveness of an optical fiber (OF) sensor in detecting partial discharges (PD) under these peculiar conditions. Different PD detection techniques were compared: electrical methods, and acoustic methods. Standard piezoelectric sensors (R15i-AST) and the above mentioned OF sensors were used for acoustic detection. The OF sensor was able to detect PD acoustically with a sensitivity better than the other detection methods. The multichannel instrumentation system was tested in real HVDC conditions with the aim of analyzing the behavior of the insulation (mineral oil/pressboard).
A multichannel interferometric system is proposed for the ultrasonic detection of partial discharges using intrinsic fiber
optic sensors that may be immersed in oil. It is based on a heterodyne scheme which drives at least four sensor heads in
order to localize the source of the acoustic emissions. The optoelectronic implementation and the experimental tests are
presented to optimize the resolution (4 channels - 4 mrad). In addition, the results of ultrasound measurements at 150-
kHz with an optical fiber sensor immersed in water in an acoustic test bench are shown, in which a resolution better than
10 Pa was obtained.
An all-fiber interferometric acoustic emission (AE) sensor was designed for partial discharges (PD) and characterized
against typical PZT sensors (R15i). The device is based on a multi-layer fiber-optic probe and it is able to detect PD
ultrasound emission at 150 kHz. It may be placed next to the discharge source immersed in oil - or as an ultrasonic
hydrophone -, thus overcoming the difficulties of acoustic attenuation which are characteristic of the external detection
on the walls. The device provides suitable narrowband acoustic detection of the PD activity, like the PZT transducers
mounted on the exterior of the transformer tank. The results show similar sensitivity, but wider directivity of the fiberoptic
sensor in order to inspect large zones within the tank. It represents a simple and cheap alternative for detecting AE
and it is the base of a multi-channel heterodyne interferometer which will drive at least four sensors to provide
information for locating the source.
An acoustic emission sensor was designed for partial discharges (PD), constructed, calibrated, and tested. The device is
based on an interferometric fiber-optic probe and is able to detect PD ultrasound emission at 150 kHz. It may be placed
next to the discharge source, thus overcoming the difficulties of acoustic attenuation. The device works in the
appropriate bandwidth for narrowband acoustic detection of PD activity, like the PZT transducers mounted on the
exterior of the transformer tank. It represents a simple and cheap alternative for detecting acoustic emission, susceptible
of being used in a multi-channel optical configuration and able to provide information for locating the source.
The sensor is an optical fiber coil exposed to the ultrasonic waves that is interrogated with an all-fiber Mach-Zehnder
interferometer. We report first the calibration at the natural frequency of the coil (20 kHz) and at the main frequency of
the application (150 kHz), and compared with the response of PZT transducers. The sensitivity decays with the
frequency, but it is comparable with the PZT sensitivity by placing the sensor next to the source, which is possible with
the immersed approach (or embedded). A certain range of compensation is obtained at low frequencies with a feed-back
loop. A second feed-back loop with electronic resonance around 150 kHz is used in order to improve the sensitivity.
Thus, the conditioning circuit provides directly the amplified optical phase signal. Results of acoustic emission with both
frequencies simultaneously are presented.
Laser optoacoustics is based on the interaction of light with materials producing the thermoelastic effect forming acoustic waves which are characteristic of the medium in which they traverse. This technique is currently on trial for use in biomedical imaging applications and is achieving great success. The work presented here develops an innovative technique for wideband acoustic detection using a fibre optic sensor in a high sensitive multi-coil Mach-Zehnder interferometric configuration. A comparative analysis is performed using both electrical and optical detection techniques on gels which are commonly used to mimic human soft tissue. Indications of future work in this area will be presented throughout this paper.
Optical fiber technology is able to answer the demanded development of EMI immune and harsh withstanding sensors for the Electric Power Industry, particularly applied to Power Transformers, and specifically for measurements within them. Apart from temperature, the other main magnitudes of interest are vibrations and partial discharges, which are studied here on the base of the interferometric sensors that we are developing for these purposes. This work is centered on the new concept of interferometric sensing by detecting multi-fringe outputs, which is applied for the measurement of each magnitude independently or both at the same time from only one probe. A high resolution read-out of the optical phase without ambiguity is demonstrated with this approach of demodulation and with the probes designed for monitoring inside power transformers. Results of in-field trials are presented for synchronous vibrations (100 Hz - 2 kHz) of two power transformers. First results of calibration of acoustic sensing with these probes on the base of the same interferometric concept are also presented in order to apply them for the measurement of partial discharges by detecting ultrasounds (20 kHz - 200 kHz). A multi-purpose exploitation of the sensor heads installed at the core and the windings of a power transformer is explored for detecting at the same time both, vibrations and partial discharge induced ultrasonic pulses. The multi-fringe interferometric output due to vibrations is proposed as a repetitive reference for the demodulation of the higher frequency asynchronous signals.
Interferometric sensing with an optical fiber as an intrinsic transducer is presented for core vibrations and windings vibrations inside two power transformers. Different approaches to in-field measurements are compared experimentally. The design of the fiber-optic probes is of enhanced sensitivity, with compensation of external influences and is shown to withstand the harsh transformer conditions. The sensors are based on detecting dynamic deformations in-contact with the magnetic laminates of the core, or dynamic displacement between two elements of a block inside the transformer. In all cases a differential scheme is used for detecting the elongation on one or more sections of a fiber, but several configurations are considered in the detection of the optical phase. Our results show direct measurements of the vibrations of the transformers at significant regions for its predictive diagnosis.
Interferometric sensing is used for vibrations inside power transformers. A probe with enhanced sensitivity is designed, obtaining synchronous multi-fringe outputs. This approach provides very high resolution demonstrated in the application, with a simple but powerful demodulation technique.
We investigate the measurement of vibrations at the core and windings of the transformer, as well as the differential temperature between zones. We present first results of vibrations measurements at the magnetic core of an oil- filled power transformers. The sensor is based on an intrinsic transducer of the dynamic strain at the surface into changes of the optical path of a segment of fiber. The sensing length is bonded to the core, immersing in oil and inside the transformer environment. Interferometric sensing is accomplished for high sensitivity because the vibrations are due to dynamic strain up to a few (mu) (epsilon) . It allows short probe lengths to be used, though an increase of the length could be used for enhancing the sensitivity. A common accessing of the fibers of the interferometer is used for reducing the common disturbance. The spectral decomposition of the output signals is shown and it is related to the electric reference and the behavior of the transformer.
We present the results of our experiment for the characterization of mechanical vibrations and temperature gradients of a carbon fiber board. The sensor system is a multichannel interferometer with sensing optical fibers embedded onto the plate under test. We investigate the simultaneous measurement of dynamic gradients of temperature and propagation of vibrations applying a synchronous sampling at the frequency of vibrations. The signals resulting from this technique provides the information of the optical phase due to temperature changes and the relative amplitude of vibrations. We measure the optical phase difference between four separated fiber-optic channels relative to a common reference path. The results are the calibration of the sensor and the spatial propagation of vibrations and temperature gradient.
In this paper we present the results of our experiment for the characterization of mechanical vibrations, using a multichannel interferometer with sensing optical fibers embedded into a carbon fiber board. We investigate the application of this measurement system for dynamic gradients of vibrations at medium frequency (50 Hz to 1 kHz). The optical phase is biased to operate at quadrature conditions (optical phase = π/2) in order to get the maximum sensitivity. We measure the differential optical phase of four spatially separated embedded fiber-optic channels relative to a common reference path. The tests are the calibration of the sensor response to vibrations and the spatial propagation of vibrations. This scheme provides an improvement of previous characterizations with strain gages at the surface because of the high sensitivity of the sensor, better spatial resolution (separation of the sensing regions up to 5 mm), and measurements performed inside the board. The signal processing required for phase measurements is also analyzed, as well as it is presented a comparison with the electronic measurements at the surface.
In this paper we present the results of an automated instrumentation system that we have designed for temperature gradient characterization in composite materials using the optical fibers embedded in it to construct a multichannel interferometer. The objective is to develop a specific automated measurement system that is able to interrogate different interferometric channels and electronic sensors at the same time. It is also of main concern the study of the interferometric signal processing and the disturbance analysis of such technique applied to this multichannel approach. Synchronous differential optical phase measurements have been used for both characterizations of common phase errors and spatial temperature gradient. Details of the performance, the system design and the experimental results obtained are given.
In this paper we present the results that have been obtained with an automated electronic instrumentation system for monitoring of strain and vibrations in a composite material board with embedded optical fibers. The objective of this study is to provide an electronic instrumentation reference for the interferometric measurements using optical fiber sensors. The main features of our system are the simultaneous multipoint strain and vibrations measurements which allows us real time data acquisition and processing. In this work we provide results about the natural frequency and damping coefficient of the composite material board.
In this paper we present results of our interferometric system for embedded optical fiber in composite material, where a laser diode is used as light source. Optical phase changes induced by laser diode injection current modulation are characterized in order to apply an optoelectronic setup for stabilization and optical phase modulation. He-Ne source versus laser diode are compared for the system. Details on parameter characterization, specially visibility drift and environmental disturbance impact, are analyzed. Experimental design and the results observed are also given.
KEYWORDS: Sensors, Composites, Temperature metrology, Signal processing, Carbon, Data acquisition, Control systems, Temperature sensors, Calibration, Error analysis
In this paper, we present results of our automated data acquisition system for temperature and strain measurements in composite materials. The objective is to provide electronic instrumentation in a plate of carbon fiber and to automate the measurement process in order to compare and analyze the results obtained through interferometric sensors based on embedded optical fiber. The acquisition system is developed to operate in quasi-real time and connected to a PC for providing the temperature and strain maps in the material. Details on performance, system design and the results obtained are given.
In this paper we present results of our stabilization scheme for an optical fiber Mach-Zehnder interferometer with a diode laser as light source. It is developed for compensating the drift path difference produced by external parameters as the environmental temperature. The optoelectronic setup in which the interference signal is fed back to the injection current of the laser diode is investigated in order to obtain a stabilized system. Details on parameter characterization, system design and the results observed are given.
In this paper, the first large-signal circuit model for laterally coupled semiconductor laser structures is presented. The present work gives application to the study of the transient regime of two coupled semiconductor lasers. The model includes both the electrical and optical characteristics of the device. The circuits elements that compose the model can be differentiated in two very different groups: on one hand, we have obtained circuit elements that describe the lasing processes in a single emitter. This elements coincide with already developed models for the single emitter. On the other hand, new circuit elements that account for the coupling terms have appeared. Also, a new variable has been modeled, the phase difference between the emitters. We will test our model by comparing the results obtained with the circuit model with those obtained by direct integration of the rate equations with standard numerical integration methods. Further indications on how to use the same approach with more complex structures will be given.
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