Investigation of light source and scattering medium related to vapor-screen flow visualization in a supersonic wind tunnel

Methods for increasing the radiant in light sheets used for vapor screen set-ups were investigated. Both high-pressure mercury arc lamps and lasers were considered. Pulsed operation of the air-cooled 1-kW lamps increased the light output but decreased reliability. An ellipsoidal mirror improved the output of the air-cooled lamps by concentrating the light but increased the complexity of the housing. Water-cooled-4-kW lamps coupled with high-aperture Fresnel lenses provided reasonable improvements over the air-cooled lamps. Fanned laser beams measurements of scattered light versus dew point made in conjunction with successful attempts to control the fluid injection. A number of smoke generators are described and test results comparing smoke and vapor screens are shown. Finally, one test included a periscope system to relay the image to a camera outside the flow

Ultrafast nonlinear optics of bulk and two-dimensional materials for infrared applications

This thesis presents the results of an experimental study into the nonlinear optical properties of novel nonlinear materials at infrared regions of the electromagnetic spectrum for the realisation of nonlinear optical devices in the near- and mid-infrared. Because of its exceptional nonlinear optical properties and its promise of implementation in a range of mid-infrared applications graphene had a prominent place in this research. Extensive investigations in the nonlinear optical properties of single and multilayer chemical vapour deposition (CVD) graphene are presented. This study revealed that graphene presents a nonlinear phase shift due to a negative, irradiance-dependent nonlinear refraction. The high peak powers available enabled the study of both saturable absorption (SA) and two-photon absorption (2PA), identifying the irradiance limits at which the contribution of two-photon absorption exceeded that of saturable absorption. Moreover, the nonlinear optical properties of graphene-polyvinyl alcohol (G-PVA) composite films were studied. The results indicate the thermal damage of the host polymer due to graphene heating and temperature transfer. Studies in the third order nonlinear optical properties of chalcogenide glasses with the perspective of integration with graphene for the development of mid-infrared devices and applications are also performed. Of all the glasses investigated, gallium lanthanum sulphide (GLS) was found to have the most interesting nonlinear optical properties. Its optical Kerr nonlinearity was found to be approximately 35 times higher than silica and the upper limit of its two-photon absorption coefficient was the lowest of all the chalcogenide glasses analysed, implying that GLS would be an excellent candidate for ultrafast all-optical switching. Subsequently GLS was chosen as the host material for optical waveguide and device fabrication via ultrafast laser inscription (ULI). Near- and mid-infrared waveguides were successfully fabricated; fundamental features such as, refractive index profiles and material dispersion were investigated. The Zero Dispersion Wavelength (ZDW) of GLS was experimentally measured for the first time; the ZDW was determined to be between 3.66-3.71 μm for the waveguides and about 3.61 μm for the bulk. Single mode directional couplers at 1550 nm were also developed and their ultrafast all-optical switching properties were investigated, leading to the assessment of the nonlinear refractive index n2 of the ULI modified area. Furthermore, waveguides in Er3+ doped GLS were successfully fabricated and the infrared transitions at 1550 and 2750 nm were detected opening the potential for GLS waveguide lasers

Shack-Hartmann and Interferometric Hybrid Wavefront Sensor

This document reports results of wave-optics simulations used to test the performance of a hybrid wavefront sensor designed to combine the self-referencing interferometer and Shack-Hartmann wavefront sensors in an optimal way. Optimal hybrid-wavefront sensor design required a thorough analysis of the noise characteristics of each wavefront sensor to produce noise models that assist in the design of an optimal phase-estimation algorithm. Feasible architectures and algorithms for combining wavefront sensors were chosen, and the noise models of the individual wavefront sensors were combined to form a model for the noise-induced error of the resulting hybrid sensor. The hybrid wavefront sensor and phase-estimation algorithm developed through this work showed improvement over a comparable stand-alone self-referencing interferometer and Shack-Hartmann wavefront sensor in open-loop wave-optics simulations

Photogenetic Retinal Prosthesis

The last few decades have witnessed an immense effort to develop working retinal implants for patients suffering from retinal degeneration diseases such as retinitis pigmentosa. However, it is becoming apparent that this approach is unable to restore levels of vision that will be sufficient to offer significant improvement in the quality of life of patients. Herein, a new type of retinal prosthesis that is based on genetic expression of microbial light sensitive ion channel, Chanelrhodopsin-2 (ChR2), and a remote light stimulation is examined. First, the dynamics of the ChR2 stimulation is characterized and it is shown that (1) the temporal resolution of ChR2-evoked spiking is limited by a continuous drop in its depolarization efficiency that is due to (a) frequency-independent desensitization process and (b) slow photocurrent shutting, which leads to a frequency-dependent post-spike depolarization and (2) the ChR2 response to light can be accurately reproduced by a four-state model consisting of two interconnected branches of open and close states. Then, a stimulation prototype is developed and its functionality is demonstrated in-vitro. The prototype uses a new micro-emissive matrix which enables generating of two-dimensional stimulation patterns with enhanced resolution compared to the conventional retinal implants. Finally, based on the micro-emitters matrix, a new technique for sub-cellular and network-level neuroscience experimentations is shown. The capacity to excite sub-cellular compartments is demonstrated and an example utility to fast map variability in dendrites conductance is shown. The outcomes of this thesis present an outline and a first proof-of-concept for a future photogenetic retinal prosthesis. In addition, they provide the emerging optogenetic technology with a detailed analysis of its temporal resolution and a tool to expand its spatial resolution, which can have immediate high impact applications in modulating the activity of sub-cellular compartments, mapping neuronal networks and studying synchrony and plasticity effects

Wave-optics Investigation of Turbulence Thermal Blooming Interaction: II. Using Time-dependent Simulations

Part II of this two-part paper uses wave-optics simulations to look at the Monte Carlo averages associated with turbulence and time-dependent thermal blooming (TDTB). The goal is to investigate turbulence thermal blooming interaction (TTBI). At wavelengths near 1 μm, TTBI increases the amount of constructive and destructive interference (i.e., scintillation) that results from high-power laser beam propagation through distributed-volume atmospheric aberrations. As a result, we use the spherical-wave Rytov number, the number of wind-clearing periods, and the distortion number to gauge the strength of the simulated turbulence and TDTB. These parameters simply greatly given propagation paths with constant atmospheric conditions. In addition, we use the log-amplitude variance and the branch-point density to quantify the effects of TTBI. These metrics result from a point-source beacon being backpropagated from the target plane to the source plane through the simulated turbulence and TDTB. Overall, the results show that the log-amplitude variance and branch-point density increase significantly due to TTBI. This outcome poses a major problem for beam-control systems that perform phase compensation

Branch Point Mitigation of Thermal Blooming Phase Compensation Instability

Thermal blooming can have a major impact on high-energy laser (HEL) beam propagation in the atmosphere. In theory, an adaptiveoptics (AO) system can mitigate the nonlinear optical effects induced by thermal blooming; however, when a single deformable mirror is used for phase-only compensation, analysis predicts the possibility of instability. This instability is appropriately termed phase compensation instability (PCI) and arises with the time-dependent development of spatial perturbations found within the HEL beam. These spatial perturbations act as local hot spots that produce negative-lens-like optical effects in the atmosphere. An AO system corrects for the hot spots by applying positive-lens-like phase compensations. In turn, this increases the strength of the thermal blooming and leads to a runaway condition, i.e. positive feedback in the AO control loop. This study uses a series of computational wave-optics experiments to explore the conditions for insipient PCI. Horizontal propagation is modeled with the effects of extinction, thermal blooming, and turbulence for a focused Gaussian beam. In addition, a nominal AO system is used for phase compensation from a point source beacon. Results show that the development of branch points under strong thermal blooming reduces the possibility of PCI. Parameters within the AO system, such as the number of actuators on the deformable mirror and the resolution of the wavefront sensor, are varied to determine the impact of branch points in the development of PCI

Wave-optics Investigation of Turbulence Thermal Blooming Interaction: I. Using Steady-state Simulations

Part I of this two-part paper uses wave-optics simulations to look at the Monte Carlo averages associated with turbulence and steady-state thermal blooming (SSTB). The goal is to investigate turbulence thermal blooming interaction (TTBI). At wavelengths near 1 μm, TTBI increases the amount of constructive and destructive interference (i.e., scintillation) that results from high-power laser beam propagation through distributed-volume atmospheric aberrations. As a result, we use the spherical-wave Rytov number and the distortion number to gauge the strength of the simulated turbulence and SSTB. These parameters simplify greatly given propagation paths with constant atmospheric conditions. In addition, we use the log-amplitude variance and the branch-point density to quantify the effects of TTBI. These metrics result from a point-source beacon being backpropagated from the target plane to the source plane through the simulated turbulence and SSTB. Overall, the results show that the log-amplitude variance and branch-point density increase significantly due to TTBI. This outcome poses a major problem for beam-control systems that perform phase compensation