Optical fiber array for the delivery of high peak-power laser pulses for fluid flow measurements

Department of Physics, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh, UK.
Applied Optics (Impact Factor: 1.78). 07/2007; 46(17):3432-8. DOI: 10.1364/AO.46.003432
Source: PubMed


Fiber delivery of 64.7 mJ laser pulses (approximately 6 ns duration) from a Q-switched Nd:YAG laser operating at 532 nm is demonstrated. A custom diffractive optical element was used to shape the laser beam and facilitate coupling into a linear fiber array. This launch arrangement achieves an improvement in launch efficiency compared with a circular fiber bundle evaluated in previous work and the delivery of higher pulse energies is demonstrated. The bundle is capable of delivering light of sufficient pulse energy and, importantly, with suitable focusability, to generate a thin light sheet for the fluid flow measurement technique of particle image velocimetry (PIV). Fiber delivery offers an advantage, in terms of optical access, for the application of PIV to enclosed measurement volumes, such as the cylinder of a combustion engine.

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    ABSTRACT: We describe a technique to package 15-mu m-diam single-mode fibers on a silicon substrate that can be incorporated into an endoscopic probe tip. The single-mode fibers in the array can be used in coherent imaging applications such as optical coherence tomography. Fiber-to-substrate and fiber-to-fiber coupling effects are studied using beam propagation techniques to determine the different design characteristics and the maximum length of the reduced diameter fiber that can be packaged in the probe tip. Single-mode fibers are etched to reduce the cladding diameter from 125 to 15 mu m. A 2-mu m-thick silica layer is grown in the silicon substrate to minimize the fiber-substrate coupling. Reduced diameter fibers are placed into a 5-mm by 150-mu m trench etched in a silicon-silica substrate and fixed with UV curable cement. Active alignment is used to ensure the correct alignment of fibers. The fiber array is experimentally evaluated to test fiber placement accuracy, throughput, and cross talk. Optical coherence tomography images are also obtained with the array. (c) 2008 Society of Photo-Optical Instrumentation Engineers.
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    ABSTRACT: Particle image velocimetry (PIV) is a successful flow mapping technique which can optically quantify large portions of a flow regime. This enables the method to be completely non-intrusive. The ability to be non-intrusive to any flow has allowed PIV to be used in a large range of industrial sectors for many applications. However, a fundamental disadvantage of the conventional PIV technique is that it cannot easily be used with flows which have no or limited optical access. Flows which have limited optical access for PIV measurement have been addressed using endoscopic PIV techniques. This system uses two separate probes which relay a light sheet and imaging optics to a planar position within the desired flow regime. This system is effective in medical and engineering applications. The present study has been involved in the development of a new endoscopic PIV system which integrates the illumination and imaging optics into one rigid probe. This paper focuses on the validation of the images taken from the novel single stem endoscopic PIV system. The probe is used within atomized spray flow and is compared with conventional PIV measurement and also pitot-static data. The endoscopic PIV system provides images which create localized velocity maps that are comparable with the global measurement of the conventional PIV system. The velocity information for both systems clearly show similar results for the spray characterization and are also validated using the pitot-static data.
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    ABSTRACT: We demonstrate the design and implementation of a fiber-optic beam-delivery system using a large-aperture, tapered step-index fiber for high-speed PIV in turbulent combustions flows. The tapered fiber in conjunction with a diffractive-optical-element (DOE) fiber-optic coupler significantly increases the damage threshold of the fiber, enabling fiber-optic beam delivery of sufficient nanosecond, 532-nm, laser pulse energy for high-speed PIV measurements. The fiber successfully transmits 1-kHz and 10-kHz laser pulses with energies of ~5.3 mJ and ~2 mJ, respectively, for more than 25 min without any indication of damage. It is experimentally demonstrated that the tapered fiber possesses the high coupling efficiency (~80%) and moderate beam quality for PIV. Additionally, the nearly uniform output-beam profile exiting the fiber is ideal for PIV applications. Comparative PIV measurements are made using a conventionally (bulk-optic) delivered light sheet, and a similar order of measurement accuracy is obtained with and without fiber coupling. Effective use of fiber-coupled, 10-kHz PIV is demonstrated for instantaneous 2D velocity-field measurements in turbulent reacting flows. We have also developed and implemented a fiber-coupled, high-speed PIV and PLIF system for measuring hydroxyl radical (OH) concentration and velocity in a realistic 4-MW combustion rig. Simultaneous OH-PLIF and PIV imaging at a data-acquisition rate of 10 kHz is demonstrated in turbulent premixed flames behind a bluff body. Our results show significant promise for the performance of fiber-coupled high-speed PIV and OH-PLIF in harsh laser-diagnostic environments such as those encountered in gas-turbine test beds and the cylinder of a combustion engine. © 2013 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.
    No preview · Conference Paper · Jan 2013
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