Transition in homogeneously heated inclined plane parallel shear flows

The transition of internally heated inclined plane parallel shear flows is examined numerically for the case of finite values of the Prandtl number Pr. We show that as the strength of the homogeneously distributed heat source is increased the basic flow loses stability to two-dimensional perturbations of the transverse roll type in a Hopf bifurcation for the vertical orientation of the fluid layer, whereas perturbations of the longitudinal roll type are most dangerous for a wide range of the value of the angle of inclination. In the case of the horizontal inclination transverse roll and longitudinal roll perturbations share the responsibility for the prime instability. Following the linear stability analysis for the general inclination of the fluid layer our attention is focused on a numerical study of the finite amplitude secondary travelling-wave solutions (TW) that develop from the perturbations of the transverse roll type for the vertical inclination of the fluid layer. The stability of the secondary TW against three-dimensional perturbations is also examined and our study shows that for Pr=0.71 the secondary instability sets in as a quasi-periodic mode, while for Pr=7 it is phase-locked to the secondary TW. The present study complements and extends the recent study by Nagata and Generalis (2002) in the case of vertical inclination for Pr=0.

Optical fiber cavity ring down measurement of refractive index with a microchannel drilled by femtosecond laser

A microchannel was inscribed in the fiber of a ring cavity which was constructed using two 0.1%:99.9% couplers and a 10-m fiber loop. Cavity ring down spectroscopy was used to measure the refractive index (RI) of gels infused into the microchannel. The ring down time discloses a nonlinear increase with respect to RI of the gel and sensitivity up to 300 µs/RI unit and an index resolution of 1.4 × 10 was obtained. © 2009 IEEE.

Biochemical sensor based on a novel all-fibre cavity ring down spectroscopy technique incorporating a tilted fibre Bragg grating

A novel all-fibre cavity ring down spectroscopy technique is proposed where a tilt fibre Bragg grating (TFBG) or long-period grating (LPG) in the cavity provides sensitivity to surrounding medium. Such configuration with an LPG as the representative was theoretically analyzed. Two spectral bands were identified employable for sensing of surrounding refractive index for a weak LPG while only one band existed for a strong LPG. A TFBG, with enhanced sensitivity compared to usual LPGs, was used in a ring down cavity of 1 m constructed with 2 fibre Bragg gratings as the reflectors and the decay time changed from 220 to 450 ns when the TFBG was immersed into water from air.

Resonant cavity Fibre Bragg grating sensor interrogation

This thesis presents a novel high-performance approach to time-division-multiplexing (TDM) fibre Bragg grating (FBG) optical sensors, known as the resonant cavity architecture. A background theory of FBG optical sensing includes several techniques for multiplexing sensors. The limitations of current wavelength-division-multiplexing (WDM) schemes are contrasted against the technological and commercial advantage of TDM. The author’s hypothesis that ‘it should be possible to achieve TDM FBG sensor interrogation using an electrically switched semiconductor optical amplifier (SOA)’ is then explained. Research and development of a commercially viable optical sensor interrogator based on the resonant cavity architecture forms the remainder of this thesis. A fully programmable SOA drive system allows interrogation of sensor arrays 10km long with a spatial resolution of 8cm and a variable gain system provides dynamic compensation for fluctuating system losses. Ratiometric filter- and diffractive-element spectrometer-based wavelength measurement systems are developed and analysed for different commercial applications. The ratiometric design provides a low-cost solution that has picometre resolution and low noise using 4% reflective sensors, but is less tolerant to variation in system loss. The spectrometer design is more expensive, but delivers exceptional performance with picometre resolution, low noise and tolerance to 13dB system loss variation. Finally, this thesis details the interrogator’s peripheral components, its compliance for operation in harsh industrial environments and several examples of commercial applications where it has been deployed. Applications include laboratory instruments, temperature monitoring systems for oil production, dynamic control for wind-energy and battery powered, self-contained sub-sea strain monitoring.

Transition in inclined internally heated fluid layers

Non-linear solutions and studies of their stability are presented for flows in a homogeneously heated fluid layer under the influence of a constant pressure gradient or when the mass flux across any lateral cross-section of the channel is required to vanish. The critical Grashof number is determined by a linear stability analysis of the basic state which depends only on the z-coordinate perpendicular to the boundary. Bifurcating longitudinal rolls as well as secondary solutions depending on the streamwise x-coordinate are investigated and their amplitudes are determined as functions of the supercritical Grashof number for various Prandtl numbers and angles of inclination of the layer. Solutions that emerge from a Hopf bifurcation assume the form of propagating waves and can thus be considered as steady flows relative to an appropriately moving frame of reference. The stability of these solutions with respect to three-dimensional disturbances is also analyzed in order to identify possible bifurcation points for evolving tertiary flows.

Pattern competition in homogeneously heated fluid layers

Simulations examining pattern competition have been performed on a horizontal homogeneously heated layer that is bounded by an isothermal plane above an adiabatic plane. Several different circulation patterns arose as the heating regime applied to the horizontal layer was modified. The sequence of the patterns formed as the Grashof number was increased had the following order: laminar, z-axis rolls, squares, hexagons and pentagons, pentagons and then two different square modes of differing orientations. Fourier analysis was used to determine how the key modes interact in the presence of different patterns.

Gain switching of an external cavity grating-coupled surface emitting laser with wide tunability

The gain-switched, single frequency operation of an external cavity grating-coupled surface emitting laser with a wavelength tuning range of 100 nm was presented. The light in the grating section was coupled out of the laser at a specific angle to the surface of the device. Analysis showed that within the driving current range, lasing in the device only occurred when the external cavity was properly aligned.

Direct real-time cavity mode assessment and optimization technique for fiber lasers

A simple technique for direct real-time assessment of a fiber laser cavity-mode condition during operation is demonstrated. Mode stabilization and optimization with this cavity-mode monitoring and conditioning feedback scheme shows significant improvements to the output performance.

Direct cavity assessment and mode optimisation technique for DFB fibre laser

A technique for direct real-time assessment of a distributed feedback fibre laser cavity conditions during operation is demonstrated and used to provide a cavity mode conditioning feedback mechanism to optimise output performance. Negligible wavelength drift is demonstrated over a 52 mW pump power range.

Transition in convective flows heated internally

The stability of internally heated convective flows in a vertical channel under the influence of a pressure gradient and in the limit of small Prandtl number is examined numerically. In each of the cases studied the basic flow, which can have two inflection points, loses stability at the critical point identified by the corresponding linear analysis to two-dimensional states in a Hopf bifurcation. These marginal points determine the linear stability curve that identifies the minimum Grashof number (based on the strength of the homogeneous heat source), at which the two-dimensional periodic flow can bifurcate. The range of stability of the finite amplitude secondary flow is determined by its (linear) stability against three-dimensional infinitesimal disturbances. By first examining the behavior of the eigenvalues as functions of the Floquet parameters in the streamwise and spanwise directions we show that the secondary flow loses stability also in a Hopf bifurcation as the Grashof number increases, indicating that the tertiary flow is quasi-periodic. Secondly the Eckhaus marginal stability curve, that bounds the domain of stable transverse vortices towards smaller and larger wavenumbers, but does not cause a transition as the Grashof number increases, is also given for the cases studied in this work.

Refractometer based on fiber Bragg grating Fabry-Pérot cavity embedded with a narrow microchannel

We report on inscription of microchannels of different widths in optical fiber using femtosecond (fs) laser inscription assisted chemical etching and the narrowest channel has been created with a width down to only 1.2µm. Microchannels with 5µm and 35µm widths were fabricated together with Fabry-Pérot (FP) cavities formed by UV laser written fiber Bragg gratings (FBGs), creating high function and linear response refractometers. The device with a 5µm microchannel has exhibited a refractive index (RI) detection range up to 1.7, significantly higher than all fiber grating RI sensors. In addition, the microchannel FBG FP structures have been theoretically simulated showing excellent agreement with experimental measured characteristics.