Long-length Fiber Bragg Grating Research Articles

Flight demonstration of aircraft wing monitoring using optical fiber distributed sensing system

We conduct flight demonstrations of aircraft monitoring by using optical fiber distributed sensing technique. We monitor a main wing of a flying test bed that is a middle-sized passenger jet. We use optical frequency domain reflectometry and long-length fiber Bragg gratings (FBGs). The sensing system measures the strain distribution profile within the FBGs with a mm order spatial resolution in real-time. Thanks to the high spatial resolution, we could observe local strain distributions due to the ribs of the wing. We also monitored strain distribution variations corresponding to various maneuvers during flight. We discussed the correlation with flight maneuvers and the interpretation of the monitoring data.

Flight demonstration of aircraft fuselage and bulkhead monitoring using optical fiber distributed sensing system

We have developed an optical fiber distributed sensing system based on optical frequency domain reflectometry (OFDR) that uses long-length fiber Bragg gratings (FBGs). This technique obtains strain data not as a point data from an FBG but as a distributed profile within the FBG. This system can measure the strain distribution profile with an adjustable high spatial resolution of the mm or sub-mm order in real-time. In this study, we applied this OFDR-FBG technique to a flying test bed that is a mid-sized jet passenger aircraft. We conducted flight tests and monitored the structural responses of a fuselage stringer and the bulkhead of the flying test bed during flights. The strain distribution variations were successfully monitored for various events including taxiing, takeoff, landing and several other maneuvers. The monitoring was effective not only for measuring the strain amplitude applied to the individual structural parts but also for understanding the characteristics of the structural responses in accordance with the flight maneuvers. We studied the correlations between various maneuvers and strains to explore the relationship between the operation and condition of aircraft.

Vibration monitoring of a helicopter blade model using the optical fiber distributed strain sensing technique.

We demonstrate a dynamic distributed monitoring technique using a long-length fiber Bragg grating (FBG) interrogated by optical frequency domain reflectometry (OFDR) that measures strain at a speed of 150 Hz, spatial resolution of 1 mm, and measurement range of 20 m. A 5 m FBG is bonded to a 5.5 m helicopter blade model, and vibration is applied by the step relaxation method. The time domain responses of the strain distributions are measured, and the blade deflections are calculated based on the strain distributions. Frequency response functions are obtained using the time domain responses of the calculated deflection induced by the preload release, and the modal parameters are retrieved. Experimental results demonstrated the dynamic monitoring performances and the applicability to the modal analysis of the OFDR-FBG technique.

An optical fiber sensing technique for temperature distribution measurements in microwave heating

We introduce an optical fiber sensing technique that can measure the temperature distributions along a fiber during microwave heating. We used a long-length fiber Bragg grating (FBG) as an electromagnetic-immune sensor and interrogated temperature distributions along the FBG by an optical frequency domain reflectometry. Water in a glass tube with a length of 820 mm was heated in a microwave oven, and its temperature distribution along the glass tube was measured using the sensing system. The temperature distribution was obtained in 5 mm intervals. Infrared radiometry was also used to compare the temperature measurement results. Time and spatial variations of the temperature distribution profiles were monitored for several microwave input powers. The results clearly depict inhomogeneous temperature profiles. The applicability and effectiveness of the optical fiber distributed measurement technique in microwave heating are demonstrated.

Signal processing method based on group delay calculation for distributed Bragg wavelength shift in optical frequency domain reflectometry

A signal processing method based on group delay calculations is introduced for distributed measurements of long-length fiber Bragg gratings (FBGs) based on optical frequency domain reflectometry (OFDR). Bragg wavelength shifts in interfered signals of OFDR are regarded as group delay. By calculating group delay, the distribution of Bragg wavelength shifts is obtained with high computational efficiency. We introduce weighted averaging process for noise reduction. This method required only 3.5% of signal processing time which was necessary for conventional equivalent signal processing based on short-time Fourier transform. The method also showed high sensitivity to experimental signals where non-uniform strain distributions existed in a long-length FBG.

Lateral Load Measurements Based on a Distributed Sensing System of Optical Frequency-Domain Reflectometry Using Long-Length Fiber Bragg Gratings

In this paper, we demonstrate the measurements of lateral loads in a distributed manner using long-length fiber Bragg gratings (FBGs) based on optical frequency-domain reflectometry (OFDR). Birefringence is induced at specific areas of the long-length FBGs by lateral compression. The distributions of the Bragg peaks were monitored by the OFDR sensing system, from which we calculated the applied lateral loads. Theoretical calculation results confirmed the validity of the experimental results. In addition, position-wise beat signals of the power of the obtained spectra were observed with smaller lateral loads that were not sufficient to provide two distinguished Bragg peaks. This unique phenomenon for the frequency-domain reflectometry scheme is explained theoretically. We proposed a method for utilizing the beat cycle to calculate smaller lateral loads.

Multiplexing of Long-Length Fiber Bragg Grating Distributed Sensors Based on Synthesis of Optical Coherence Function

Long-length fiber Bragg grating (FBG) distributed sensors are multiplexed, and measurement of Bragg wavelength distribution along the gratings are demonstrated. This sensing system adopts synthesis of optical coherence function (SOCF) as a reflectometry technique to measure the distribution of the local reflection spectra inside the long-length FBGs. Simulations have been carried out to determine appropriate FBG reflectivity for the multiplexing. In the proposed system, which is a heterodyne interferometer, heterodyne beat frequency for each FBG gets different from those for the others, when the measurement speed becomes higher. By introducing a way for compensating the difference, three long-length distributed FBG sensors, which are originally uniform and have the same Bragg wavelength, are successfully multiplexed with 1 kHz sampling rate.

Strain monitoring of a single-lap joint with embedded fiber-optic distributed sensors

We have developed a fiber-optic distributed sensor which can measure strain distributions along fiber Bragg grating (FBG) with the high spatial resolution. This sensing system is based on optical frequency domain reflectometry and a long-length FBG whose length is about 100 mm can be used. We can identify the longitudinal strain at an arbitrary position along the FBG using signal processing technology. In this study, long-length FBGs were embedded into the adhesive layers of the two single-lap joints and we could successfully measure the strain distributions inside the adhesives. In one single-lap joint, the adherends were carbon fiber reinforced plastics and in another one, they were aluminum. Theadhesive was epoxy in both cases. The measured results were compared with the calculated ones by nonlinear finite element (FE) analysis in which the large displacement and the elasto–plastic response of the adherend or adhesive material were account for. We found that in most of the applied loads, the agreement between the measured results and the calculated ones obtained from an intact FE model is excellent. While the measured strain distributions inside the adhesive layer of the aluminum single-lap joint were varied at the end of the overlap in the higher applied loads and they were much different from those of the intact model, an FE model with debonding was made and it could represent such variations. We could also monitor the strain distributions inside the adhesive during the manufacturing process and we observed the perturbation in residual strain distributions after curing. Consequently, we can say that the fiber-optic distributed sensor with the high spatial resolution is very useful not only to assess the structural integrity of adhesive joints but also to improve numerical analysis techniques and manufacturing processes for them.

J044021 分布型光ファイバセンサを用いた接着継手内部のひずみ計測([J04402]知的材料・構造システム(2))

We have developed the distributed strain measurement system based on optical frequency domain reflectometry (OFDR) system. It can measure a strain distribution with the high spatial resolution at an arbitrary position along long-length fiber Bragg grating (FBG) sensors. In this study, we embedded the long-length FBG sensors into the adhesive of a single-lap joint. We could successfully measure the strain distribution at the interface between the adhesive and the adherend in the tensile test.

Proposal and experimental verification of Bragg wavelength distribution measurement within a long-length FBG by synthesis of optical coherence function

In this paper, a sensor system for measuring continuous Bragg wavelength distribution in a long-length fiber Bragg grating is newly proposed, using synthesis of optical coherence function (SOCF), which is one of the spatial resolving techniques used for reflectometry. Experimental results are also reported. In the process of synthesizing optical coherence function, it is found that an apodization scheme is necessary to obtain the reflection spectrum of local section in a long-length FBG around the coherence peak. As a verification of this method, the detection of local Bragg wavelength shift due to temperature change within a short section in a long-length FBG is demonstrated experimentally.