This paper describes research that demonstrated gated, femtosecond, digital holography, enabling 3D microscopic viewing inside dense, almost opaque sprays, and providing a new and powerful diagnostics capability for viewing fuel atomization processes never seen before. The method works by exploiting the extremely short coherence and pulse length (approximately 30 micrometers in this implementation) provided by a femtosecond laser combined with digital holography to eliminate multiple and wide angle scattered light from particles surrounding the injection region, which normally obscures the image of interest. Photons that follow a path that differs in length by more than 30 micrometers from a straight path through the field to the sensor do not contribute to the holographic recording of photons that travel in a near straight path (ballistic and "snake" photons). To further enhance the method, off-axis digital holography was incorporated to enhance signal to noise ratio and image processing capability in reconstructed images by separating the conjugate images, which overlap and interfere in conventional in-line holography. This also enables digital holographic interferometry. Fundamental relationships and limitations were also examined. The project is a continuing collaboration between MetroLaser and the University of California, Irvine.
Holographic studies have been performed which examine the fragmentation process during vapor explosion of a water-in-fuel (hexadecane/water) emulsion droplet. Holograms were taken at 700 to 1000 microseconds after the vapor explosion. Photographs of the reconstructed holograms reveal a wide range of fragment droplet sizes created during the explosion process. Fragment droplet diameters range from below 10 microns to over 100 microns. It is estimated that between ten thousand and a million fragment droplets can result from this extremely violent vapor explosion process. This enhanced atomization is thus expected to have a pronounced effect on vaporization processes which are present during combustion of emulsified fuels.
Holographic interferometry has received many plaudits as an extremely powerful flow diagnostic method while users have languished at the cumbersome methods available and long time required to extract even the elementary data from the hologram. Recent years have seen breakthroughs that, when combined, will address the problems of producing the usable raw data quickly (the processed hologram) and transferring it into a computer in a tractable form. Keys to the former are either thermoplastic devices or on-line film processors. Keys to the latter are electronic imaging and analyzing systems that can digitize data reconstructed from holograms and then perform complex operations on the digitized data. The authors have tested, individually, all of the components required for producing fully refined, on-line density data in wind tunnels employing holographic interferometry systems, and are currently developing the fully integrated system. This paper describes the state-of-the-art system concept, components, options, and potential capabilities for realizable systems.
Abstract A common-path Zeeman interferometry technique has been deveioped and applied for the measurements of refractive index gradients of the fluids to an accuracy of Δn=9.4 × 10−9. The Zeeman interferometry technique has over 100 times better resolution than the other interferometry techniques. A linear refractive index profile of the solution at the interface of the growing L-arginine phosphate crystal from solution was measured successfully using a scanning Zeeman interferometry technique.
Get PDF Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text J. D. Trolinger, W. M. Farmer, and R. A. Belz, "Multiple Exposure Holography of Time Varying Three-Dimensional Fields," Appl. Opt. 7, 1640-1641 (1968) Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article
This paper describes a new computer model and code that was developed as a quick-look tool for augmenting CFD and experimental aero-optical investigations. The code exploits existing empirical and theoretical knowledge about turbulent shear and boundary layers, turbulence statistics, and relevant optical properties of turbulent flow. It is designed to simulate aero-optical effects on light propagating through various types of aerodynamic flow fields, exploiting the ability of computers to easily mimic random media. This enables optical propagation experiments through relevant types of random optical media to be performed quickly in the computer. The resultant aero-optics computational test simulator enables a systems or test designer to simulate and run aero-optical tests that will be useful for systems evaluations, comparisons, test planning, instrument set up, and data analysis and interpretation. In this paper we show how simple exercises and experiments performed with the simplest variant of the code provide interesting and useful answers relevant to aero-optical effects. I. Background 1 . These wavefront effects are caused by refractive index variations that occur in aerodynamic flow fields and distort the wavefront by delaying or advancing parts of it by differing amounts. For most gases, index-of-refraction fluctuations are proportional to the fluid density fluctuations through the Gladstone-Dale constant, which strongly depends upon gas type and weakly on temperature. In this discussion we will essentially speak of density variations and refractive index variations interchangeably. Refractive index variations in a turbulent flow can be the result of temperature gradients, pressure gradients, or non-uniform mixing of different gases, depending on the scenario and type of turbulence. Many types of optical systems (i.e., designators, rangers, LIDAR, laser communicators, directed energy weapons) receive or project light through an aerodynamic flow field, which effectively inserts an unwanted, time-varying, optical element into the system leading to loss of resolution, boresight error, tracking error, reduction in signal to noise, and reduced energy density on a target. The capability of almost all of these devices is deteriorated by aero-optical effects on the optical wavefront caused by flow features of the aircraft, such as shear and boundary layers. These problems have been observed in many flight and wind tunnel tests since the early years of testing of USAF airborne lasers 2,3
Holographic metrology, unlike most other applications of holography, has always thrived and continues to thrive by continuously incorporating new supporting technologies that make it more powerful and useful. Successes, failures, lives, and deaths are examined and recognized as evolutionary steps that position the field where opportunities are as great and as many as ever. This is a story of that evolution. Comparisons and analogies with other applications of holography such as data storage, archiving, the arts, entertainment, advertising, and security and their evolution are interesting. Critical events, successes, mistakes, and coincidences represent milestones of abandonment or failure to deliver in many holography communities that followed a different evolutionary path. Events and new technical developments continue to emerge in supporting fields that can revive and expand all holography applications. New opportunities are described with encouragement to act on them and take some risks. Don't wait until all of the required technology and hardware are available, because good scientists always act before then. The paper is about "making holography great again" and your opportunity to be a part of the upcoming revolution. Although the discussion focuses on holographic metrology, the same principles should apply to other holography communities.
