Triple wavelength SOA-based fiber ring laser for use in wavelength-division multiplexed FBG vibration sensor array

A novel and simple multi-wavelength fiber ring laser has been developed to implement an optical source of a fiber Bragg grating (FBG) sensor array in which respective wavelength component of the oscillation output provides an optical source of one of the wavelength-division multiplexed (WDM) FBG vibration sensors. The laser consists of a semiconductor optical amplifier (SOA) as a gain medium and of FBGs as wavelength selection elements. Since an SOA is an inhomogeneous gain medium, stable multi-wavelength oscillation can be realized to serve an optical source for the FBG vibration sensor array. In the experiment, a dual-wavelength fiber ring laser has been fabricated and tested, and simultaneous two-point vibration measurement has been successfully demonstrated.

Laboratory core flooding experiments in reservoir sandstone under different sequestration pressures using multichannel fiber Bragg grating sensor arrays

Studying the temperature field of photovoltaic modules is important for improving their power generation efficiency. To solve the problem of traditional sensors being unsuitable for measuring the spatial temperature field, we designed a real-time detection scheme of the photovoltaic module temperature field based on a fiber Bragg grating (FBG) sensor array. In this scheme, wavelength division multiplexing and space division multiplexing technologies were applied. The multi-channel FBG sensor strings were arranged on the surface and in the near field of the photovoltaic module. Different FBG strings were selected through optical switches, and the wavelength of the FBG string was addressed and demodulated using the tunable laser method and a peak-seeking algorithm. A measurement experiment of the photovoltaic module temperature field was carried out in an outdoor environment. The experimental results showed that the fluctuation law of the photovoltaic module surface and near-field temperature is basically consistent with that of solar radiation power. The temperature of the photovoltaic module decayed from the surface to space. Within 6 mm of the photovoltaic module surface, the temperature sharply dropped, and then the downward trend became flat. The lower the solar radiation power and the higher the wind speed, the faster the temperature decay. This method provides technical support for measuring the temperature field of a photovoltaic module and other heat source equipment.

This paper proposes multipoint optical fiber sensing system based on fiber Bragg grating (FBG) sensor array. The low reflectivity FBG sensors having the same Bragg wavelength are connected cascaded in the same fiber line as the sensing elements of optical fiber sensing system. This can increase the monitoring points and reduce manufacturing costs of FBG sensor. To verify the effectiveness, the system simulator based on equivalent circuit model by microwave technology was developed. Simulation result shows that though multi-reflections can be caused by FBG sensors with the same Bragg wavelength connected cascaded in the same line, the effect may become weak as reflectivity of FBG sensors are reduced to 0.02.

We report on the use of a frequency-rnodulated continuous wave (FMCW) technique for multiplexing fiber Bragg grating (FBG) sensors. This technique is based on the modulation of light intensity from a broadband source by a linear swept-frequency RF carrier. Signals from the FBG sensors located at different positions in an array are separated in frequency-domainand demodulated using a tunable optical filter. The potential and limitation of the technique are discussed. A 3-sensor FMCWmultiplexed FBG array of parallel topology and a 6-sensor hybrid FMCWIWDM system were experimentally demonstratedwith -30 dB crosstalk between sensors and 2 tc resolution in terms ofroot-mean-square strain value.Keywords: optical fiber sensors, fiber Bragg grating sensors, frequency-modulated continuous wave, strain measurements. 1. Introduction Fiber Bragg gratings (FBGs) have been identified as an enabling technology for a variety of applications from aerospace, marine, to civil engineering structural monitoring, from medical, nuclear power industry, to electrical power industry[1]. In many applications such as condition monitoring of civil structures such as highways, bridges and buildings. FBG sensorsneed to be multiplexed in order to achieve quasi-distributed measurement and to reduce cost per sensing point to compete withconventional electrical or optical sensors.The most popular technique for multiplexing FBG sensors is the wavelength division multiplexing (WDM) technique[2,3]. The maximum sensor number that can be multiplexed using the WDM technique is determined by the ratio ofthe sourcespectral width over the spacing between the Bragg wavelengths of the FBGs. For applications requiring a larger multiplexinggain, a combination ofthe WDM and time-division multiplexing (TDM) techniques [3] may be used.A TDM system employs a pulsed or gated CW source to illuminate gratings in sequence as the pulse travels along thefiber, the reflected signals from the gratings are separated in time domain [4]. The physical separation between the gratings isdetermined by the input pulse width. A fast time-gated photodetector can be used to select the signal from an individual grating

Fiber Bragg grating (FBG) sensors were used to measure the torsional strain distribution in a second-generation (rare-earth) Ba2Cu3Oy high-temperature superconducting sample tape (SST) at 77 K. Two SST samples were prepared. A slit was fabricated in each sample to allow an optical fiber to be embedded. In the first sample, a 10-mm-long single grating element was inserted into the slit. In the second sample, three grating elements of 10-mm length each were put into the slit. These grating elements were fabricated along a single-mode fiber and could be addressed individually using the wavelength division multiplexing (WDM) technique. Finally, the samples were recoated with a primary and secondary coating to form an embedded sensing system. When the high-temperature superconducting (HTS) tape is subjected to torsion, there is a strain induced in the FBG sensor, which varies the grating period accordingly. It was interrogated using a tunable laser in a WDM scheme. Based on the wavelength shift and the photoelastic properties of the FBG sensor embedded to the HTS tape, the torsional deformation of the HTS tape can be obtained. In this paper, the design and technology requirements to embed the single and WDM FBG sensors into the coated conductors are presented. Initial experiments to measure the torsional strain distribution at 77 K are reported.

Focus on sensors

In this study, a down-scaled wind turbine blade was designed and fabricated using glass and carbon fiber materials for the skin and stiffener, respectively. In the course of its fabrication, an array of FBG (fiber Bragg grating) sensors was embedded in the composite laminates. The embedded FBG sensor array was used to measure the residual strain in the stiffener before and after the curing process, and after fabrication of the blade, the FBG array was used to monitor the structural conditions, including structural dynamic behavior during the structural testing of the blade. The results of the tests showed that the FBG sensor array effectively measured the residual strain distribution of the blade stiffener before and after the curing process. And the measured natural frequencies and mode shapes by the FBG array matched the results obtained from the FE analysis and conventional accelerometers.

In this paper, the development of loop mirror multi-wavelength Brillouin fiber laser utilizing semiconductor optical amplifier and fiber Bragg grating is successfully demonstrated. A multi-wavelength BFL structure employs a single mode fiber that acts as the gain medium, while the fiber Bragg grating act to reflect the signals and semiconductor optical amplifier is used to amplify the signal are employed to produce multiple channels. The implementation of difference parameter for semiconductor optical amplifier driven current, Brillouin pump power, Brillouin pump wavelength and length of single mode fiber, plays an important role in producing the multiple channels. The highest number of channels which is 15 achieved once 9 km of SMF length, semiconductor optical amplifier current at 800mA and low Brillouin pump power at -12 dBm are utilized in the laser structure. Furthermore, the implementation of semiconductor optical amplifier and reflectivity of fiber Bragg grating in Brillouin fiber laser structure gives a better performance in producing the multiple channels.

We propose and demonstrate experimentally a multipoint fiber Bragg grating (FBG) sensor system, based on fiber Bragg gratings (FBGs), using a semiconductor optical amplifier (SOA) as a gain medium. Multiple wavelength operation is based on the inhomogeneous broadening effect of the SOA of the sensing system. In addition, the behavior of the proposed sensor configuration is also studied.

Fiber Bragg grating (FBG) sensors are promising candidates for strain measurements in smart structures. The wavelength-encoded nature of the reflected signals from FBGs allows for absolute strain measurement and for multi-point operation based on wavelength division multiplexing (WDM). To measure strain variation with good accuracy, detection of the small shift in the Bragg wavelength is essential. In this paper, we report the use of a digital matched filter (DMF) for improving the wavelength detection accuracy. The DMF technique has been used extensively in image processing and pattern recognition for the detection of weak signals in a noisy environment. We apply it here for the detection of Bragg wavelength in FBG sensors when the SNR is low.

In this paper, one large Fiber Bragg Grating (FBG) sensor sensing network for bridge local health monitoring was studied, designed and implemented to Harbin Sifangtai Bridge, China. According to the hot point principle and finite element analysis results, FBG sensors installation scheme was made to monitor the local strain of key elements, respectively. 60 FBG sensors had been installed in monitoring points along the height of I-shape girders distributed in three main monitoring sections along the bridge. Monitoring results in load testing period was obtained, the static and dynamic results show that FBG sensors can monitoring the local strain precisely, and the local strain response is in the safe range under test vehicle load. Monitoring results in service period was obtained, 9 months after load test. Results show that maximum local strain responses in three main monitoring sections are in the safe range, under the dynamic vehicle load. Comparison of the monitoring results from FBG sensor sensing network are used to unveil the local health condition of Sifangtai Bridge reflected by local strain response.

Embedded fiber Bragg grating (FBG) sensors are attractive for in-situ structural monitoring, especially in fiber reinforced composites. Their implementation in metallic structures is hindered by the thermal limit of the protective coating, typically a polymer material. The purpose of this study is to demonstrate the embedding of FBG sensors into metals with the ultimate objective of using FBG sensors for structural health monitoring of metallic structures. To that end, ultrasonic additive manufacturing (UAM) is utilized. UAM is a solid-state manufacturing process based on ultrasonic metal welding that allows for layered addition of metallic foils without melting. Embedding FBGs through UAM is shown to result in total cross-sectional encapsulation of the sensors within the metal matrix, which encourages uniform strain transfer. Since the UAM process takes place at essentially room temperature, the industry standard acrylate protective coating can be used rather than requiring a new coating applied to the FBGs prior to embedment. Measurements presented in this paper show that UAM-embedded FBG sensors accurately track strain at temperatures higher than 400 °C. The data reveals the conditions under which detrimental wavelength hopping takes place due to non-uniformity of the load transferred to the FBG. Further, optical cross-sectioning of the test specimens shows inhibition of the thermal degradation of the protective coating. It is hypothesized that the lack of an atmosphere around the fully-encapsulated FBGs makes it possible to operate the sensors at temperatures well above what has been possible until now. Embedded FBGs were shown to retain their coatings when subjected to a thermal loading that would result in over 50 percent degradation (by volume and mass) in atmospherically exposed fiber.

The performance of a fiber grating interrogation system using an acousto-optic tunable filter (AOTF) is analyzed. An equivalent linear circuit is presented to describe the system, which tracks the wavelength of a fiber Bragg grating (FBG) sensor in a FBG sensor array. From the model, the tracking resolution of a shot-noise-limited system has been determined for both transmissive and reflective interrogation of FBGs. An optimum ratio between the bandwidths of the FBG and the AOTF has been derived. Experimental results of a tracking system are presented to validate the theoretical predictions. The theory should also be suitable for interrogation systems using other forms of tunable bandpass filters for tracking the FBG wavelength.

Based on the principle of the wavelength division multiplex (WDM), a data processing method which can increase the number of fiber Bragg grating (FBG) sensors in series is given. In this method, genetic algorithm (GA) is used to detect the Bragg wavelength of FBG. Simulations and experiments demonstrated that the Bragg wavelength can be detected when a crosstalk exists, even if the spectrums of two FBGs are fully overlapped.