Cascaded long-period gratings with nanostructured coatings

The response of the transmission spectrum of cascaded long period gratings (LPGs) to the deposition of nanostructured coatings by use of the Langmuir-Blodgett technique is investigated. The phase of the interference fringes within the LPGs' attenuation bands is shown to be highly sensitive to the optical thickness of the coating, for thicknesses of the order of 100 nm.     

Optical fiber long-period gratings with Langmuir-Blodgett thin-film overlays

An overlay material was deposited by the Langmuir-Blodgett technique onto a single-mode optical fiber containing a long-period grating. The long-period grating exhibits characteristic attenuation bands in its transmission spectrum whose central wavelengths were observed to depend on the optical thickness of the overlay material, even for materials that have a refractive index higher than that of silica.     

Shadow Moiré method for the determination of the source position in three-dimensional shearography

Shearography is a full-field non-contact optical technique used for characterisation of surface strain. In a multi-component system, the displacement derivative components are measured using a number of illumination positions. These components are then transformed into a three-dimensional coordinate system, using a knowledge of the source positions. This process is highly sensitive to errors in the knowledge of the source position. Shadow Moiré, with either linear or circular gratings, can be used to measure angle of illumination, with the measurement sensitivity and accuracy variable by changing the grating pitch. Circular gratings have a measurement range determined by multiple fringe analysis and linear gratings have a different measurement range determined by sub-fringe analysis. In this paper vertical linear, horizontal linear and circular gratings are combined to extend the measurement range and the accuracy of the measurement of the source position in two directions. Using this method the source position was measured to an accuracy of ±3%.     

Magnetoelectric effect in cobalt ferrite-barium titanate composites and their electrical properties

CoFe₂O₄-BaTiO₃ composites were prepared using conventional ceramic double sintering process with various compositions. Presence of two phases in the composites was confirmed using X-ray diffraction. The dc resistivity and thermoemf as a function of temperature in the temperature range 300 K to 600 K were measured. Variation of dielectric constant (ɛ′) with frequency in the range 100 Hz to 1 MHz and also with temperature at a fixed frequency of 1 kHz was studied. The ac conductivity was derived from dielectric constant (ɛ′) and loss tangent (tan δ). The nature of conduction is discussed on the basis of small polaron hopping model. The static value of magnetoelectric conversion factor has been studied as a function of magnetic field.     

Self-mixing interference effects in tunable diode laser absorption spectroscopy

We report the effects of self-mixing interference on gas detection using tunable diode laser spectroscopy. For very weak feedback, the laser diode output intensity gains a sinusoidal modulation analogous to that caused by low finesse etalons in the optical path. Our experiments show that self-mixing interference can arise from both specular reflections (e.g. cell windows) and diffuse reflections (e.g. Spectralon^™ and retroreflective tape), potentially in a wider range of circumstances than etalon-induced interference. The form and magnitude of the modulation is shown to agree with theory. We have quantified the effect of these spurious signals on methane detection using wavelength modulation spectroscopy and discuss the implications for real gas detectors.     

Development of extended field Doppler velocimetry for turbomachinery applications

The development of a portable, single component Doppler global velocimetry (DGV) head, based around a wavelength-stabilised argonion laser and a fast digital image-processing system, is described. The normalised two-dimensional DGV image, in which intensities are linearly related to velocities, can be displayed and updated at the 25 Hz camera frame rate, greatly easing the problem of system alignment. The effect of each individual system component upon the velocity resolution achieved for the system as a whole is discussed, and correction factors are calculated to account for the finite aperture and field of view of real systems and for divergence of the illuminating light sheet. Axial velocities of up to 100 m/s in a straight duct flow have been measured, demonstrating an rms velocity resolution of 2.5 m/s. The potential of the technique for gas turbine applications has been demonstrated by measuring the position of a shock in a transonic flow. At a Mach number of 2.3 and mass flow rate of 0.79 kg/s the velocity change across the shock was measured to be approximately 130 m/s.     

Shape and slope measurement by source displacement in shearography

Shearography is a full field non-contact optical technique usually used to measure the gradient of the displacement of a surface subjected to thermal or mechanical loading. This paper describes the use of shearography for surface slope and shape measurement. Interferometric speckle patterns obtained before and after displacement of the optical source are correlated to yield correlation fringes which are, in general, a mixture of slope fringes and carrier fringes. This paper contains a full treatment of the sensitivity of slope fringes to the parameters; the illumination and the imaging geometry and the magnitude and the direction of the source displacement. The slope fringes are corrected for distortion, which is due to the necessary off-axis illumination, and are scaled using parameters calculated using a mathematical model. Experimentally generated phase-stepped slope fringes are unwrapped and integrated to recover the object shape.     

Second-harmonic generation in Langmuir Blodgett waveguide overlays on single-mode optical fiber

Second-harmonic radiation has been obtained from Langmuir-Blodgett films of E-N -octadecyl-4-[2-(4-dibutylaminophenyl)ethenyl]quinolinium octadecylsulfate, deposited as a waveguide overlay upon optical fiber that is single mode at the pump wavelength (lambda=1064 nm) . A quadratic relationship between the pump power and second-harmonic intensity was observed.