Time gate, optical layout, and wavelength effects on ballistic imaging

UMR 6614-CORIA, CNRS, Université et INSA de Rouen, BP12, Avenue de l'Université, 76801 Saint-Etienne du Rouvray Cedex, France.
Journal of the Optical Society of America A (Impact Factor: 1.56). 10/2009; 26(9):1995-2004. DOI: 10.1364/JOSAA.26.001995
Source: PubMed


A method to distinguish a hidden object from a perturbing environment is to use an ultrashort femtosecond pulse of light and a time-resolved detection. To separate ballistic light containing information on a hidden object from multiscattered light coming from the surrounding environment that scrambles the signal, an optical Kerr gate can be used. It consists of a carbon disulfide (CS(2)) cell in which birefringence is optically induced. An imaging beam passes through the studied medium while a pump pulse is used to open the gate. The time-delayed scattered light is excluded from measurements by the gate, and the multiple-scattering scrambling effect is reduced. In previous works, the two beams had the same wavelength. We propose a new two-color experimental setup for ballistic imaging in which a second harmonic is generated and used for the image, while the fundamental is used for gate switching. This setup allows one to obtain better resolution by using a spectral filtering to eliminate noise from the pump pulse, instead of a spatial filtering. This new setup is suitable for use in ballistic imaging of dense sprays, multidiffusive, and large enough to show scattered light time delays greater than the gate duration (tau=1.3 ps).

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    • "A major difference in this setup compared to other existing setups in literature [7] [8] [9] is that here the pump and probe beams are incident at the Kerr medium in a collinear fashion, which resolves most of the major issues with optical Kerr gating. Here, a β-barium borate (BBO) crystal is introduced in the path of the probe beam before it illuminates the object (O) to change its wavelength (from λ "
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    ABSTRACT: A novel setup is presented for ballistic imaging using an efficient ultrafast Kerr effect based optical time-gate with gating times of the order of ~0.8 picoseconds. At first, the major drawbacks of the classical non-collinear optical setup are discussed. Then, the new collinear arrangement is proposed, which overcomes these issues and improves the achievable imaging spatial resolution and gate timings. Few preliminary results for ballistic imaging of liquid sprays/jets are presented for this arrangement. It is shown that using a solid state Kerr medium (GGG crystal), instead of the classical liquid CS$_2$, allows reduction in the opening time of the optical gate.
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    • "More in-depth discussions of the quantitative relation of these parameters can be found in [62, 65, 66, 67, 68, 69, 70]. A special case of the temporal modulation methods is the so-called early photon (also known as ballistic photon) imaging method, which uses ultrafast laser and detector time-gating to reduce the TPSF of the imaging system, thereby improving the spatial resolution [71, 72, 73]. "
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    ABSTRACT: Diffuse optical imaging is highly versatile and has a very broad range of applications in biology and medicine. It covers diffuse optical tomography, fluorescence diffuse optical tomography, bioluminescence, and a number of other new imaging methods. These methods of diffuse optical imaging have diversified instrument configurations but share the same core physical principle - light propagation in highly diffusive media, i.e., the biological tissue. In this review, the author summarizes the latest development in instrumentation and methodology available to diffuse optical imaging in terms of system architecture, light source, photo-detection, spectral separation, signal modulation, and lastly imaging contrast.
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    • "Although the detection system's depth of sharpness range is limited (80 µm) still multiple scattering is an issue and the disturbing stray light makes the optical access of the inner spray structures difficult. Therefore, further spray analyses also with enhanced optical techniques such as ballistic imaging [31] [32] or Xray techniques [33] are necessary. "
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