Compact graphene mode-locked wavelength-tunable erbium-doped fiber lasers: from all anomalous dispersion towards all normal dispersion

Soliton operation and soliton wavelength tuning of erbium-doped fiber lasers mode locked with atomic layer graphene was experimentally investigated under various cavity dispersion conditions. It was shown that not only wide range soliton wavelength tuning but also soltion pulse width variation could be obtained in the fiber lasers. Our results show that the graphene mode locked erbium-doped fiber lasers provide a compact, user friendly and low cost wavelength tunable ultrahsort pulse source

The use of a CFBG fibre optical sensor to detect disbond development in composite/composite and metal/composite adhesively bonded joints

This paper describes a novel technique to monitor disbond initiation and propagation in both composite-composite and metal-composite adhesively bonded joints using chirped fibre Bragg grating (CFBG) sensors embedded within fibre-reinforced composite adherends. Characteristic changes in the reflected spectra from the sensors indicate both (a) disbond initiation (due, for example, to fatigue loading) and (b) the current position of the disbond front (to within about 2 mm). For composite-composite bonded joints, the reflected spectra are recorded with the joint under a small load (in practice, this could be the self-weight of the structure). In the case of a metal-composite joint, the difference in the coefficient of thermal expansion between the adherends is sufficient to enable disbond propagation to be monitored when the joint is unloaded. The experimental results have been modelled using a combination of finite-element analysis and commercial software for predicting FBG spectra, with very good agreement between the experimental observations of the reflected spectra and the predicted spectra, for disbonds of different lengths

Multimode fiber optic laser Doppler anemometer with side-projected catheter

Optical Engineering3761677-1683OPEG

NDE detection of manufacturing defects in composite bonded joints using CFBG sensors

This paper reports the use of chirped fibre Bragg grating (CFBG) sensors to detect defects introduced during the bonding of composite joints. The single-lap joints were made from transparent GFRP cross-ply composites which had a CFBG sensor embedded within one of the two adherends during the manufacturing stage. Joints with defects simulated using PTFE or introduced real air-gaps have been investigated. The sensors were interrogated with no load on the joints and also under a moderate quasi-static load. For the joints with PTFE “defects”, the perturbations observed in the reflected spectra provided a clear indication of the location of the defects and were in very good agreement with modelling predictions. Changes in the reflected spectra for the air-gap defects also provided an indication of the location of the defects and the spectrum changes were in reasonable agreement with modelling predictions. However, further work is required to establish the limitations of the technique for air-gap defects

Structural health monitoring of bonded composite joints using embedded chirped fibre Bragg gratings

Chirped fibre Bragg gratings (CFBGs) have been used to monitor disbond initiation and disbond growth in composite bonded joints. The CFBGs have been embedded within, but near the surface of, a transparent GFRP composite adherend that has been bonded to a second transparent adherend. The low wavelength end of the CFBGs has been arranged to be adjacent to the end of the first adherend. Disbond initiation is readily detected as a modification to the reflected spectrum of the CFBG, consisting of the low wavelength part of the reflected spectrum being shifted to lower wavelengths; this is due to the unloading of the adherend resulting from the disbond. Disbond growth is detected by the movement of a dip in the reflected spectrum of the CFBG (as a consequence of the load redistribution at the disbond front); this dip moves to higher wavelengths as the disbond propagates. The relationship between the shift of the dip in the reflected spectrum with the position of the disbond front has been determined directly through the use of transparent GFRP joints

Prediction of the reflected spectra from chirped fibre Bragg gratings embedded within cracked crossply laminates

Matrix cracking damage is a generic type of damage that develops under load in the off-axis plies of laminated composites and is generally the precursor of more serious damage mechanisms, particularly delamination. Hence, it is important to identify and if possible locate this type of damage. Chirped fibre Bragg grating sensors have been embedded in a transparent glass fibre reinforced plastic crossply laminate and changes to the reflected spectra as a consequence of crack development have been studied. An approximately sinusoidal variation of the intensity of the reflected spectrum occurs at the position of the crack, enabling both crack development and crack position to be identified. A simulation of a reflected spectrum, incorporating a stress transfer model to predict the strains and an optical model to predict the reflected spectrum, is in reasonable agreement with the experimental results

The use of an embedded chirped fibre Bragg grating sensor to monitor disbond initiation and growth in adhesively bonded composite/metal single lap joints

Chirped fibre Bragg grating (CFBG) sensors embedded within glass fibre reinforced plastic (GFRP) adherends have been used to monitor disbond initiation and growth in an adhesively bonded GFRP/aluminium alloy single lap joint. The elevated temperature curing of the adhesive used in the manufacture of the joint leads to thermal strains being generated within the GFRP and aluminium adherends. Disbond initiation and growth between the adherends during fatigue cycling causes a relaxation of the residual thermal strains within the composite adherend and perturbations (peaks or dips) in the reflection spectra from the CFBG sensor. These perturbations can be used, when the joint is unloaded, to monitor both the initiation of a disbond in the joint and to monitor the growth of the disbond with fatigue cycling

Reflected spectra prediction for chirped fibre Bragg gratings used for disbond detection in composite/composite bonded joints

In this paper it is shown that a chirped fibre Bragg grating sensor embedded within a composite adherend can be used to monitor disbond initiation and propagation in an adhesively bonded single lapjoint. Characteristic changes in the reflected spectra from the sensor indicate both disbond initiation and the current position of the disbond front to within about 2 mm (a distance which depends on adherend material and sensor position in relation to the adhesive bondline). When the sensor extends the full overlap length, disbond initiation from either end of the overlap can be monitored. The results have been modelled using a combination of finite-element analysis and commercial software for predicting FBG spectra; the predicted spectra are in very good agreement with experiment. The CFBG sensor technique could provide the basis for monitoring a wide range of bonded joints and structures where one adherend is a composite material

Detection of defects in as manufactured GFRP-GFRP and CFRP-CFRP composite bonded joints using chirped fibre Bragg grating sensors

Chirped fibre Bragg grating (CFBG) sensors have been embedded in composite coupons which have been used to form one half of single lap bonded joints. The bonded joints have been made with deliberately included defects, consisting of either a PTFE insert or an air gap, and the sensors have been used to detect the presence and location of the defects. The experimental results, and the modelling, show that defects in both GFRP-GFRP joints and CFRP-CFRP joints can be detected by the embedded CFBG sensors, though it is easier to detect defects in the lower stiffness (GFRP) joints. <br/