A multi-component shearography instrument is used to investigate the in-plane surface strain fiel... more A multi-component shearography instrument is used to investigate the in-plane surface strain fields in titanium 10-2-3 alloy specimens subjected to tensile loads. Titanium 10-2-3 alloy samples containing fatigue cracks with lengths from 0.7 mm to 5 mm were investigated using the shearography instrumentation. In-plane and out-of-plane measurements of surface strain are presented, along with comparative data from resistance strain gauges.
A fibre-optic refractive index sensor based on a long period grating (LPG) with a nanoassembled m... more A fibre-optic refractive index sensor based on a long period grating (LPG) with a nanoassembled mesoporous coating of alternate layers of poly(allylamine hydrochloride) (PAH) and SiO2 nanospheres was demonstrated. PAH/SiO2 coatings of different thicknesses were deposited onto an LPG operating near its phase matching turning point in order to study the effect of the film thickness and porosity on sensor performance. Three aqueous solutions of ethanol, glucose and sucrose of different concentrations were used to allow characterization of the refractive index (RI) over a wide RI range (1.3330-1.4906). The device showed a high sensitivity (1927 nm/refractive index unit) to RI changes with a response time less than 2 s. In particular, the current study suggested that the low RI of the mesoporous film, 1.20 at 633 nm, facilitates the measurement of external indices higher than that of the cladding, extending the range of operation of LPG-based RI sensors. The ability of this device to monitor, in real time, RI changes during a dilution process is demonstrated and discussed.
Previous measurement by Qu et al, of laser-induced fluorescence in bronchus tissue are simulated ... more Previous measurement by Qu et al, of laser-induced fluorescence in bronchus tissue are simulated using an electromagnetic (EM) scattering model. The EM parameters derived indicate that the changes in the epithelium thickness with carcinoma in situ can be detected with an optical radar process, hence providing an early detection of cancer. Further simulation using the EM model indicates various practical methods of extracting the thickness changes. These include frequency modulated continuous wave optical radar, multilaser continuous wave radar and low coherence interferometry. Initial optical experiments and calculations clarify which method is likely to lead to the development of endoscope instrumentation, but the availability of suitable optical components is a major consideration. Some early interferometry results indicate that a resolution of at least 40 micrometers can be readily reached for histological imaging. For 10 micrometers cell resolution, additional techniques are required.
Optical heterodyne carriers have been generated by mixing the stimulated Brillouin scattered ligh... more Optical heterodyne carriers have been generated by mixing the stimulated Brillouin scattered light generated from optical fiber ring resonators. The first technique mixes the SBS generated from two separate resonators to produce a carrier in the 200 - 400 MHz region. For this system a temperature coefficient of the carrier frequency of approximately 5 MHzK-1 was obtained. The second technique mixes the two SBS signals produced from the orthogonal polarization eigenmodes of a ring resonator constructed from high birefringence fiber. A carrier frequency of 11.68 MHz was obtained with a temperature coefficient of 6.7 kHz K-1.
Much research has been carried out on fibre Bragg grating (FBG) external cavity semiconductor las... more Much research has been carried out on fibre Bragg grating (FBG) external cavity semiconductor lasers for application as stabilised carrier wave and modulated laser sources and as tuneable lasers. In this paper two novel FBG external cavity lasers are experimentally demonstrated. Both use the light reflected from the FBGs to force the lasers to operate at the Bragg wavelengths. They offer tuneable beat frequencies and are able to switch the polarisation and wavelength of the laser.
Single headed 3D laser Doppler velocimetry (LDV) geometries generally rely upon the use of 3 Dopp... more Single headed 3D laser Doppler velocimetry (LDV) geometries generally rely upon the use of 3 Doppler difference channels, inclined at differing angles with respect to the mechanical axes of the probe. The transformation between the non-orthogonal measurement coordinate system and the Cartesian system can result in large errors in the calculated velocities. A theoretical analysis of the geometrically induced uncertainties in measurements produced by four single headed 3D LDV configurations is presented. These considerations have lead to the development of a single headed LDV probe based around the use of two Doppler difference channels to directly measure the transverse velocity channels, and a reference beam channel to measure the on axis velocity component. The probe may be operated in two regimes using cw radiation and wavelength division multiplexing to distinguish the three channels, or using a pulsed source and time division multiplexing.
A volume holographic spectral filter bank formed in photorefractive BaTiO3 is proposed and demons... more A volume holographic spectral filter bank formed in photorefractive BaTiO3 is proposed and demonstrated as a demodulation scheme for in-fiber Bragg grating sensors. A strain range of 2500 microsecond(s) train, with minimum detectable strain of 4 microsecond(s) train/(root)Hz is reported. Extension of the technique to allow intensity referencing and parallel demodulation of an array of sensors is discussed.
An intensity-based interrogation technique for arrays of fiber bragg grating (FBG) sensors is rep... more An intensity-based interrogation technique for arrays of fiber bragg grating (FBG) sensors is reported. The technique is based upon each FBG forming one mirror of a Michelson interferometer. Source wavelength modulation is combined with an unbalanced interferometer to produce a carrier signal. Carrier frequencies are characteristic of the optical path length imbalance and hence grating position within the array. The intensity of the carrier signal is directly related to the optical power reflected from the grating and hence the strain applied to the grating. Strain resolution of approximately 3 micrometers /m is demonstrated with an approximately 350micrometers /m sensor range. Multiplexing is demonstrated and techniques to extend the range are discussed.
A cascaded long period grating Mach-Zehnder interferometer is used to monitor the change in refra... more A cascaded long period grating Mach-Zehnder interferometer is used to monitor the change in refractive index of a UV cured epoxy resin over a cure cycle. Fourier techniques are used to calculate the phase shift and frequency spectral amplitude of the associated fringe pattern during the cure. The results are compared with the refractive index change during cure calculated using a Fresnel reflection based technique.
... Enhanced sensitivity fibre Bragg grating (FBG) load sensor Ricardo Correia, Edmon Chehura, Ji... more ... Enhanced sensitivity fibre Bragg grating (FBG) load sensor Ricardo Correia, Edmon Chehura, Jin Li, Stephen W James and Ralph P Tatam Engineering Photonics Group, School of Engineering, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK ...
Laser diode emission wavelength modulation is demonstrated as a technique for measuring the phase... more Laser diode emission wavelength modulation is demonstrated as a technique for measuring the phase and gradient of displacement amplitude of a vibrating object using continuous wave illumination in a path length imbalanced speckle shearing interferometer. The unwanted mismatch in the magnification of the two images in the shearing interferometer and sensitivity to radial shear introduced by a path length imbalance is overcome by the use of a transparent block of high refractive index in one path in the interferometer.
A multi-component shearography instrument is used to investigate the in-plane surface strain fiel... more A multi-component shearography instrument is used to investigate the in-plane surface strain fields in titanium 10-2-3 alloy specimens subjected to tensile loads. Titanium 10-2-3 alloy samples containing fatigue cracks with lengths from 0.7 mm to 5 mm were investigated using the shearography instrumentation. In-plane and out-of-plane measurements of surface strain are presented, along with comparative data from resistance strain gauges.
A fibre-optic refractive index sensor based on a long period grating (LPG) with a nanoassembled m... more A fibre-optic refractive index sensor based on a long period grating (LPG) with a nanoassembled mesoporous coating of alternate layers of poly(allylamine hydrochloride) (PAH) and SiO2 nanospheres was demonstrated. PAH/SiO2 coatings of different thicknesses were deposited onto an LPG operating near its phase matching turning point in order to study the effect of the film thickness and porosity on sensor performance. Three aqueous solutions of ethanol, glucose and sucrose of different concentrations were used to allow characterization of the refractive index (RI) over a wide RI range (1.3330-1.4906). The device showed a high sensitivity (1927 nm/refractive index unit) to RI changes with a response time less than 2 s. In particular, the current study suggested that the low RI of the mesoporous film, 1.20 at 633 nm, facilitates the measurement of external indices higher than that of the cladding, extending the range of operation of LPG-based RI sensors. The ability of this device to monitor, in real time, RI changes during a dilution process is demonstrated and discussed.
Previous measurement by Qu et al, of laser-induced fluorescence in bronchus tissue are simulated ... more Previous measurement by Qu et al, of laser-induced fluorescence in bronchus tissue are simulated using an electromagnetic (EM) scattering model. The EM parameters derived indicate that the changes in the epithelium thickness with carcinoma in situ can be detected with an optical radar process, hence providing an early detection of cancer. Further simulation using the EM model indicates various practical methods of extracting the thickness changes. These include frequency modulated continuous wave optical radar, multilaser continuous wave radar and low coherence interferometry. Initial optical experiments and calculations clarify which method is likely to lead to the development of endoscope instrumentation, but the availability of suitable optical components is a major consideration. Some early interferometry results indicate that a resolution of at least 40 micrometers can be readily reached for histological imaging. For 10 micrometers cell resolution, additional techniques are required.
Optical heterodyne carriers have been generated by mixing the stimulated Brillouin scattered ligh... more Optical heterodyne carriers have been generated by mixing the stimulated Brillouin scattered light generated from optical fiber ring resonators. The first technique mixes the SBS generated from two separate resonators to produce a carrier in the 200 - 400 MHz region. For this system a temperature coefficient of the carrier frequency of approximately 5 MHzK-1 was obtained. The second technique mixes the two SBS signals produced from the orthogonal polarization eigenmodes of a ring resonator constructed from high birefringence fiber. A carrier frequency of 11.68 MHz was obtained with a temperature coefficient of 6.7 kHz K-1.
Much research has been carried out on fibre Bragg grating (FBG) external cavity semiconductor las... more Much research has been carried out on fibre Bragg grating (FBG) external cavity semiconductor lasers for application as stabilised carrier wave and modulated laser sources and as tuneable lasers. In this paper two novel FBG external cavity lasers are experimentally demonstrated. Both use the light reflected from the FBGs to force the lasers to operate at the Bragg wavelengths. They offer tuneable beat frequencies and are able to switch the polarisation and wavelength of the laser.
Single headed 3D laser Doppler velocimetry (LDV) geometries generally rely upon the use of 3 Dopp... more Single headed 3D laser Doppler velocimetry (LDV) geometries generally rely upon the use of 3 Doppler difference channels, inclined at differing angles with respect to the mechanical axes of the probe. The transformation between the non-orthogonal measurement coordinate system and the Cartesian system can result in large errors in the calculated velocities. A theoretical analysis of the geometrically induced uncertainties in measurements produced by four single headed 3D LDV configurations is presented. These considerations have lead to the development of a single headed LDV probe based around the use of two Doppler difference channels to directly measure the transverse velocity channels, and a reference beam channel to measure the on axis velocity component. The probe may be operated in two regimes using cw radiation and wavelength division multiplexing to distinguish the three channels, or using a pulsed source and time division multiplexing.
A volume holographic spectral filter bank formed in photorefractive BaTiO3 is proposed and demons... more A volume holographic spectral filter bank formed in photorefractive BaTiO3 is proposed and demonstrated as a demodulation scheme for in-fiber Bragg grating sensors. A strain range of 2500 microsecond(s) train, with minimum detectable strain of 4 microsecond(s) train/(root)Hz is reported. Extension of the technique to allow intensity referencing and parallel demodulation of an array of sensors is discussed.
An intensity-based interrogation technique for arrays of fiber bragg grating (FBG) sensors is rep... more An intensity-based interrogation technique for arrays of fiber bragg grating (FBG) sensors is reported. The technique is based upon each FBG forming one mirror of a Michelson interferometer. Source wavelength modulation is combined with an unbalanced interferometer to produce a carrier signal. Carrier frequencies are characteristic of the optical path length imbalance and hence grating position within the array. The intensity of the carrier signal is directly related to the optical power reflected from the grating and hence the strain applied to the grating. Strain resolution of approximately 3 micrometers /m is demonstrated with an approximately 350micrometers /m sensor range. Multiplexing is demonstrated and techniques to extend the range are discussed.
A cascaded long period grating Mach-Zehnder interferometer is used to monitor the change in refra... more A cascaded long period grating Mach-Zehnder interferometer is used to monitor the change in refractive index of a UV cured epoxy resin over a cure cycle. Fourier techniques are used to calculate the phase shift and frequency spectral amplitude of the associated fringe pattern during the cure. The results are compared with the refractive index change during cure calculated using a Fresnel reflection based technique.
... Enhanced sensitivity fibre Bragg grating (FBG) load sensor Ricardo Correia, Edmon Chehura, Ji... more ... Enhanced sensitivity fibre Bragg grating (FBG) load sensor Ricardo Correia, Edmon Chehura, Jin Li, Stephen W James and Ralph P Tatam Engineering Photonics Group, School of Engineering, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK ...
Laser diode emission wavelength modulation is demonstrated as a technique for measuring the phase... more Laser diode emission wavelength modulation is demonstrated as a technique for measuring the phase and gradient of displacement amplitude of a vibrating object using continuous wave illumination in a path length imbalanced speckle shearing interferometer. The unwanted mismatch in the magnification of the two images in the shearing interferometer and sensitivity to radial shear introduced by a path length imbalance is overcome by the use of a transparent block of high refractive index in one path in the interferometer.
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