Efficient large-scale multiplexing of fiber Bragg grating and fiber Fabry-Perot sensors for structural health monitoring applications

Fiber Bragg gratings have been demonstrated as a versatile sensor for structural health monitoring. We present an efficient and cost effective multiplexing method for fiber Bragg grating and fiber Fabry-Perot sensors based on a broadband mode-locked fiber laser source and interferometric interrogation. The broadband, pulsed laser source permits time and wavelength division multiplexing to be employed to achieve very high sensor counts. Interferometric interrogation also permits high strain resolutions over large frequency ranges to be achieved. The proposed system has the capability to interrogate several hundred fiber Bragg gratings or fiber Fabry-Perot sensors on a single fiber, whilst achieving sub-microstrain resolution over bandwidths greater than 100 kHz. Strain resolutions of 30n epsilon/Hz(1/2) and 2 n epsilon/Hz(1/2) are demonstrated with the fiber Bragg grating and fiber Fabry-Perot sensor respectively. The fiber Fabry-Perot sensor provides an increase in the strain resolution over the fiber Bragg grating sensor of greater than a factor of 10. The fiber Bragg gratings are low reflectivity and could be fabricated during the fiber draw process providing a cost effective method for array fabrication. This system would find applications in several health monitoring applications where large sensor counts are necessary, in particular acoustic emission

BADANIE PARAMETRÓW ŚWIATŁOWODOWYCH SYSTEMÓW POMIAROWYCH

At present there exist a lot of technical devices, the failure of which can be connected not only with huge financial losses, but with the treat to the environment as well. Therefore, an important problem is the effective devices conditions diagnostics, including electronic components and check of their operation. Timely faults detecting allows introducing the prevention measures and avoiding serious consequences. Fiber-optic sensors have several advantages, more important of which include the immunity to electromagnetic disturbances, little weight and possibility to be included into the structure being measured. The most perspective are the sensors based on the Bragg fiber gratings. Bragg fiber gratings have several advantages, for instance, they allow creating the distributed measuring massifs, which contain several sensors. As well, they are insensitive to the optic power source vibrations. Variety of using the fiber sensors based on the Bragg fiber gratings has led to producing the Bragg fiber gratings with different spectral characteristics. Homogeneous Bragg fiber gratings have the spectra with solid side lobes, which can influence at the temperature sensor processing characteristics. To level the side lobes there is applied the apodization method, which is one of the means to affect the spectral form. The article herein considers the issues of the Bragg fiber gratings mathematical and computer modeling using the transfer matrix method. Transfer matrix method allows defining the optical components spectral characteristics based on the bound modes theory and description of electromagnetic wave, passing through an optic fiber. In the article there have been analyzed the Bragg fiber gratings in compliance with spectral features, such as transmission and reflectance spectra. As well, there has been carried out the experiment with influence of various parameters at the Bragg fiber gratings spectral characteristics. There have been studied the Bragg fiber gratings spectral features and selected the grating optimal parameters for designing the fiber-optic sensors based on the Bragg fiber gratings.Obecnie istnieje wiele urządzeń technicznych, których awarię można powiązać nie tylko z ogromnymi stratami finansowymi, ale także z troską o środowisko. Dlatego ważnym problemem jest skuteczna diagnostyka warunków pracy urządzeń, w tym elementów elektronicznych i kontrola ich działania. Wykrywanie błędów w odpowiednim czasie umożliwia wprowadzenie środków zapobiegawczych i uniknięcie poważnych konsekwencji. Czujniki światłowodowe mają kilka zalet, z których ważniejsze to odporność na zakłócenia elektromagnetyczne, niewielka waga i możliwość włączenia do mierzonej struktury. Najbardziej perspektywiczne są czujniki oparte na siatkach Bragga (FBG). Optyczne siatki Bragga mają kilka zalet, na przykład umożliwiają tworzenie rozproszonych układów pomiarowych, które zawierają kilka czujników. Są również niewrażliwe na wibracje źródła zasilania optycznego. Różnorodność wykorzystania czujników światłowodowych opartych na siatkach Bragga doprowadziła do wytworzenia siatek Bragga o różnych charakterystykach spektralnych. Jednorodne, optyczne siatki Bragga mają widma z pełnymi płatkami bocznymi, które mogą wpływać na charakterystykę przetwarzania czujnika temperatury. Aby wyrównać płatki boczne, stosuje się metodę apodyzacji, która jest jednym ze sposobów wpływania na formę widmową. W niniejszym artykule omówiono zagadnienia światłowodowych siatek Bragga, modelowania matematycznego i komputerowego z wykorzystaniem metody macierzy transferu. Metoda macierzy transferu pozwala na określenie charakterystyki widmowej składników optycznych w oparciu o teorię modów wiązanych i opis fali elektromagnetycznej, przechodzącej przez światłowód. W artykule przeanalizowano siatki Bragga zgodnie z cechami widmowymi, takimi jak widma transmisji i odbicia. Przeprowadzono również eksperyment z wpływem różnych parametrów na charakterystyki widmowe siatek Bragga. Zbadano cechy widmowe siatek Bragga i wybrano optymalne parametry siatki do projektowania czujników światłowodowych opartych na siatkach Bragga

Active Fiber Bragg Grating Flow Sensor Powered By In-Fiber Light

Tunable fiber Bragg gratings (FBGs) are key components for optical communications and sensing applications. Current tuning mechanisms include on-fiber electric heating, the piezo-electric effect, and mechanical stretching and bending. Unfortunately, all of these tuning mechanisms rely on external electrical power supplied by non-optical means. Additional electrical cabling increases manufacturing cost and the risks of failure associated with additional on-fiber electrical contacts and fragile packaging, which are susceptible to electromagnetic interference. These limitations make current fiber components no longer suitable for use in hostile environments, such as extreme temperature, corrosive, and humid environments.The research herein presents a tunable fiber Bragg grating device without sophisticated packaging and external electrical wiring. Shown for the first time, the resonance wavelength, spectrum width, and chirp can be directly controlled by in-fiber light as well as spectral responses of metal-coated fiber Bragg gratings. In-fiber diode laser light at 910-nm was leaked from the fiber and absorbed by the surrounding metallic coating to raise the grating's temperature and to change the background refractive index distribution of the gratings. Wide tunability of the resonance wavelength and spectral width was demonstrated in both uniform and linear chirped gratings.Applications of in-fiber light-powered active grating sensors are demonstrated for dual function temperature and flow sensors based on self-heated optical hot wire anemometry. A grating flow sensor has been experimentally evaluated for different grating lengths and input laser powers. The grating flow sensor demonstrated a minimum measurable flow velocity for nitrogen gas flow of 0.35-m/s at atmosphere pressure, which is comparable to or better than most MEMS-based flow sensors. Optical fiber is not used only for optical signal delivery, but also as a multi-function cable that can deliver optical power for on-fiber self-heating. This one-fiber solution provides a new dimension to designing multifunctional fiber sensors without compromising their intrinsic advantages, which include immunity to electromagnetic fields, low cost, long lifetime, and the capability to function in harsh environments

Coupled-core fiber Bragg gratings for low-cost sensing

[EN] Sensors based on Bragg gratings inscribed in conventional single mode fibers are expensive due to the need of a sophisticated, but low-speed, interrogation system. As an alternative to overcome this issue, in this work, it is proposed and demonstrated the use of coupled-core optical fiber Bragg gratings. It was found that the relative reflectivity from such gratings changed when the coupled-core fiber was subjected to point or periodic bending. This feature makes the interrogation of such gratings simple, fast, and cost-effective. The reflectivity changes of the gratings are attributed to the properties of the supermodes supported by the coupled-core fiber. As potential applications of the referred gratings, intensity-modulated vector bending and vibration sensing are demonstrated. We believe that the results reported here can pave the way to the development of many inexpensive sensors. Besides, coupled-core fiber Bragg gratings may expand the use of grating technology in other areas.This work is part of the Projects No. PGC2018-101997-B-I00 and RTI2018-094669-B-C31 funded by the MCIN/AEI/10.13039/501100011033/and FEDER, Una manera de hacer Europa; and the scholarship PAID-01-18 Granted by the Universitat Politecnica de Valencia

Elliptical-core two mode fiber sensors and devices incorporating photoinduced refractive index gratings

Results of experiments performed using germanium-doped, elliptical core, two-mode optical fibers whose sensitivity to strain was spatially varied through the use of chirped, refractive-index gratings permanently induced into the core using Argon-ion laser light are presented. This type of distributed sensor falls into the class of eighted-fiber sensors which, through a variety of means, weight the strain sensitivity of a fiber according to a specified spatial profile. We describe results of a weighted-fiber vibration mode filter which successfully enhances the particular vibration mode whose spatial profile corresponds to the profile of the grating chirp. We report on the high temperature survivability of such grating-based sensors and discuss the possibility of multiplexing more than one sensor within a single fiber

Fiber-Optic Devices as Temperature Sensors for Temperature Measurements in AC Magnetic Fields

We report on the investigation of several fiber-optic devices as potential sensors for temperature measurements in AC magnetic fields. Common temperature sensors, such as thermocouples, thermistors or diodes, will create random and/or systematic errors when placed in a magnetic field. A DC magnetic field is susceptible to create a systematic offset to the measurement, while in an AC magnetic field of variable frequency random errors which cannot be corrected for can also be introduced. Fiber-Bragg-gratings and thin film filters have an inherent temperature dependence. Detrimental for their primary applications, the same dependence allows one to use such devices as temperature sensors. In an AC magnetic field, they present the advantage of being immune to electromagnetic interference. Moreover, for fiber-Bragg-gratings, the shape factor and small mass of the bare-fiber device make it convenient for temperature measurements on small samples. We studied several thin-film filters and fiber-Bragg-gratings and compared their temperature measurement capabilities in AC magnetic fields of 0 to 150 Gauss, 0 to 20 KHz to the results provided by off-the-shelf thermocouples and thermistor-based temperature measurement systems

Sensing with coupled-core optical fiber Bragg gratings

[EN] Sensitive bending and vibration sensors based on a coupled-core optical fiber with Bragg gratings are proposed and demonstrated. The interrogation of such sensors is cost effective without comprising the sensors performance.This work was supported by the Spanish Ministry of Science and Innovation under projects No. PGC2018-101997-B100 and RTI2018-0944669-BC31 and the Universitat Politècnica de València with the scholarship PAID-01-18.Flores-Bravo, JA.; Madrigal-Madrigal, J.; Zubia, J.; Margulis, W.; Sales Maicas, S.; Villatoro, J. (2021). Sensing with coupled-core optical fiber Bragg gratings. Optica Publishing Group. 1-2. https://doi.org/10.1364/FIO.2021.FM2C.21

Optical fiber temperature sensor based on fiber bragg grating

It is important to study optical fiber temperature sensor based on fiber Bragg Grating. The Fiber Bragg grating sensors can offer highly sensitive, cost effective solutions for optical sensing. While fiber Bragg gratings have been implemented in various sensing applications over the past few decades, recent efforts explore the limits of reflected and transmitted for FBG sensors. The FBG temperature sensor, is essential to establish the smallest distance of single mode fibre in order to reduce optical losses of the FBG system. One of the most generally used deployed optical sensors is the fiber Bragg grating, with optical circulator, ASE source and OSA, the FBG which reflects a wavelength of light that shifts in response to variations in temperature. The advantage of reflection that its can easily detect the Bragg reflected signal. The FBG is very sensitive to variations in temperature degrees over a temperature range of (30–60) °C. The variation of wavelength of an fiber Bragg grating is caused by the temperature. Moreover, change in temperature affect both the effective refractive and grating period of an FBG, which result in a shift in the reflected wavelength. Moreover, most of the existing FBG sensors systems on the market provide a limited wavelength resolution Therefore, it is the purpose of this thesis to enhance the grating sensors sensitivity to temperature. In addition, the purpose to make a small comparison between the transmitted and reflected spectrum. It is apparent that any shift in the Bragg wavelength is influenced by the temperature. Moreover, the sensitivity was calculated equal to 17.1pm°C with SNR equal to 13.7 dB based on the reflection spectrum

High temperature sensors based on hydrogen loaded fiber Bragg gratings

The different topics of this thesis include temperature sensors based on the hydrogen-loaded fiber Bragg gratings, and the thermal stability of hydrogen loaded fiber Bragg gratings in the high temperature range. The aim of this thesis is to investigate a new type of high temperature fiber Bragg grating. We have fabricated the hydrogen loaded fiber Bragg gratings for high temperature sensor applications. These gratings differ from other types of fiber Bragg gratings, because their refractive index structure is attributed to the change in the hydroxyl groups and germanium oxygen-deficient centers. An annealed hydrogen loaded fiber Bragg grating has shown to survive at temperatures in excess of 936{493}C. The gratings of the sensors retained their reflectivity up to 70%, when they were kept at 700{493}C for 90 minutes. The annealed fiber Bragg grating can be stabilized at temperatures in excess 700{493}C for sensor applications. The main results of this thesis provide a better understanding of the thermal response of the hydrogen-loaded fiber Bragg gratings and their decay behavior at elevated temperatures. It is demonstrated that temperature annealing treatment of these fiber Bragg gratings is able to enhance effectively the grating's thermal stability. Finally, photosensitivity of pure silica and germanium-doped optical fibers at the presence of hydroxyl groups was also studied

All-fiber sensors for radiation measurements in radiotherapy

The paper presents the development and investigation of distributed and a quasi-distributed fiber optic sensors for the real-time monitoring of radiations during cancer treatments. Both sensors rely on ad-hoc developed nanoparticle-doped optical fibers with enhanced sensitivity to radiation. The distributed sensor is interrogated with an OFDR-based instrument and allows the reconstruction of the spatial dose distribution along the fiber. The quasi-distributed sensor is implemented through fiber Bragg gratings inscribed with a femtosecond laser in the few-mode section of a single mode-multi mode-single mode interferometer