Scott R. Harris

Quantitative Measurements of Internal Circulation in Droplets Using Flow Tagging Velocimetry

We demonstrate the use of the photoactivated nonintrusive tracking of molecular motion flow tagging technique to obtain quantitative velocity measurements in free falling water droplets and in electrohydrodynamic Taylor cones. A simple ray tracing procedure is outlined to remove the optical distortion caused by the droplet surface curvature. This correction is applied to free falling droplet images, and the vertical component of velocity is measured across the droplet. Maximum vertical velocities in the droplet are 15.9 ± 3.3 mm/s. Without ray tracing, the optical distortion is shown to cause errors in the sign of velocity as well as errors of over 100% in velocity magnitude. Preliminary velocity measurements in a Taylor cone are also presented. Centerline velocities in the Taylor cone are approximately 4 μ/s.

Flow tagging velocimetry using caged dye photo-activated fluorophores

The use of caged dye photo-activated fluorophore velocimetry is described and representative examples are presented. After a brief survey of recently reported measurements, a more detailed example of the flow produced in a cylinder with a single rotating end wall is presented. Simultaneous stereoscopic image sets in the (r,z) and (r,θ) planes have been obtained over a Reynolds number range of roughly 102-105. At low Reynolds numbers (0-2000), the steady, axisymmetrical flow is found to quantitatively agree with predictions from a numerical flow solver. At higher Reynolds numbers (from 5×103 to 105), the flow develops considerable turbulent three-dimensional structure.

Characterization and application of a liquid crystal beam steering device

Optical Phased Array technology promises to reduce the size, weight, and power consumption of optical pointing and steering systems by replacing complex mechanically gimballed mirrors with small, lightweight, nonmechanical devices. This paper develops and describes several diagnostic techniques that can be used to evaluate the performance of Optical Phased Arrays (OPAs) and demonstrates their use by applying them to the evaluation of a commercially available liquid crystal device. Finally, the operation of this device is demonstrated by integrating it into a steered imaging system.

Numerical optimization of the performance of nematic liquid crystal optical phased arrays

In a one-dimensional liquid crystal optical phased array (LCOPA), a liquid crystal layer is electrically addressed by an array of long, narrow electrodes. A spatially periodic voltage profile can be applied to the electrodes in order to induce a sawtooth-shaped index of refraction variation in the liquid crystal layer that will steer an optical beam in a fashion analogous to that of a blazed diffraction grating. Because of non-ideal device behavior, measured phase vs. voltage data cannot be used to predict the control voltages necessary to achieve efficient steering. This paper presents a simple application of optimization to determine the appropriate voltages for every electrode in order to optimize the steering efficiency. Experimental results show that this approach can quickly determine optimal voltages for a desired far field diffraction pattern. Steering efficiency improvements of over 100 percent are obtained as compared to open loop device calibration.

Simultaneous MHz Rate Flow Visualization and Wavefront Sensing for Aero-Optics

Abstract : Newly developed optical diagnostic techniques are used to examine the temporal evolution of wavefront aberrations imposed upon an optical beam as it passes through a rectangular compressible turbulent jet. A recently developed MHz rate system based on a pulse burst laser and ultra highspeed CCD camera is used for flow visualization. A newly developed two-dimensional MHz rate Shack-Hartman wavefront sensor is used to measure the wavefront distortion. The wavefront sensor consists of a HeNe laser, a micro-lens array and an ultra high-speed camera. The MHz rate flow visualization and wavefront sensing are used simultaneously to investigate the aero-optic effects of an ideally expanded, high Reynolds number, Mach 1.3 rectangular jet. Preliminary results are presented that show the potential of the technique to acquire time-resolved two-dimensional wavefront data with detailed aero-optic effects. A number of improvements to the technique that will be used in future work are also discussed.

Polarization effects in nematic liquid crystal optical phased arrays

In a one-dimensional liquid crystal optical phased array (LCOPA), a liquid crystal layer is electrically addressed by an array of long, narrow electrodes. A spatially periodic voltage profile can be applied to the liquid crystals in order to induce a sawtooth-shaped index of refraction variation in the liquid crystal layer that will steer an optical beam in a fashion analogous to that of a blazed diffraction grating. In reality, an LCOPA is a thick, periodic, anisotropic structure with complicated polarization properties. The changes in polarization as a beam passes through an LCOPA can have negative practical effects, particularly in optical systems where LCOPAs and other polarization-sensitive optical elements are cascaded. This paper presents experimental measurements of the polarization state of the light diffracted by an LCOPA as well as a discussion of the origin of these effects.

Simultaneous High-resolution Optical Wavefront and Flow Diagnostics for High-speed Flows

Abstract : Simultaneous high spatial-resolution flow visualization and wavefront sensing are used to investigate the optical aberrations that occur due to a compressible shear layer. A preliminary model is developed to relate flow visualization images with wavefronts measured using a Shack-Hartmann wavefront sensor. Initial results are quite encouraging as a comparison between the Shack-Hartmann measured wavefronts and wavefronts produced by applying the model to flow visualization images produces correlation levels well above 0.7. Future work will incorporate a more realistic geometry to further develop the model and investigate the effects of individual large-scale structures on wavefront distortion with more detail.

Broadband image steering using a liquid crystal device

Here we investigate a novel approach to steering broadband imagery with a Liquid Crystal Optical Phased Array (LCOPA). Our approach overcomes the deleterious blurring and echoing effects inherent in the use of such a device. We develop a model for the LCOPA and formulate a method in which a steered, graybody scene may be restored through the application of a Wiener filter. We also show this approach may be extended to scenes that are not strictly composed of graybodies but instead are only spectrally smooth over an appropriate bandwidth. Experimental results are presented that demonstrate the effectiveness of this approach.

PHANTOMM flow tagging measurements in complex 3D flows

Using the PHANTOMM flow tagging technique we obtain quantitative measurements of velocity, as well as visualizations similar to those obtained with standard laser sheet fluorescence techniques, of the flow in a cylinder driven by a rotating end wall. In addition, the axisymmetric Navier-Stokes equations are solved numerically using a vorticity stream function formulation. The computed flow is compared to measurements in order to evaluate and validate the PHANTOMM technique so that one can be confident of measurements in flow regimes where accurate computations are not available. (Author)

Visualizing aero-optic interactions about a nose-mounted turret

One aspect of the propagation-physics challenge associated with airborne, free-space, optical communications (FSOC), for example, is the characterization and mitigation of link losses due to aero-optic interactions. That is, air-density gradients due to compressibility effects in turbulent boundary layers, separated flows, and freeshear flows can disturb the wavefront in the near field of the transceiver. To better understand these aero-optical mechanisms, a model of a nose-mounted, FSOC transceiver recently was placed in a compressible-flow wind tunnel, and the resulting wavefront degradations, as a function of flow scenario, were recorded. High-speed, time-resolved movies of the aero-optic disturbances have been realized, using a Schlieren-imaging technique, and a very-highframe-rate camera. Discrete, vortical structures (amid otherwise-irregular shedding) were seen to emerge and convect past the clear aperture. The frequencies of these disturbances have been estimated from the movies, and these have been compared with high-speed, time-resolved wavefront reconstructions. Losses of -3.5 dB (for the case of Mach - 0.45 at 10 kft, side view, and λ - 1.55 μm, for example), and disturbance frequencies of - 1200 Hz (and higher) were observed. The system-level impact of the resulting wavefront degradations will be discussed.