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Schematic diagram of collinear low coherence interferometry sensing system consisting of the following components: MO: microscope objective, BS: beamsplitter, VA: variable attenuator, DM: dichroic mirror, BE: beam expansion optics, CL: collimating lens, FC: fiber coupler , SLD: superluminescence diode (, , and theoretical axial resolution of ), CCD camera, fiber illuminator, and diode laser. ( and ). Laser power fluctuation is less than 1% during measurements. A weak optical trap is formed using a 0.4 NA objective lens (Edmund Optics), Edmund Optics Singapore Pte. Ltd., 1093 Lower Delta Road #05–10 Tiong Bahru Industrial Estate Singapore 169204. The trapping beam is expanded (telecentric) to ensure that the back aperture of the objective lens is overfilled. A spectrometer (Ocean Optics, HR4000), 830 Doublas Ave., Dundein, FL 34698, USA with 3648 pixels covering a wavelength range of 759–961 nm gave an imaging depth of 3.3 mm in air. A custom-made cavity of 1 mm in depth is filled with water and polystyrene microspheres (PolySciences, Inc.), 400 Valley Road, Warrington, PA 18976 of 10 and diameters.
(a) Low coherence image of a levitating microsphere (the dashed line shows the position of the optical center OC). The insets show CCD images of the microspheres at different axial positions, defocusing effects are clearly seen, (b) depth resolved intensity profile at time point corresponding to the middle CCD image, inset shows the optical setup. The dashed arrows marked the position of the coverslip (i) and reflecting surface of the microspheres (ii) and (iii).
(a) Schematic diagram of the 10 and microspheres in a weak optical trap. (b) Depth-resolved intensity profile of the two trapped microspheres. The dotted arrows marked the optical centers of the microspheres, (i), (ii), (iii), and (iv) indicates the reflecting surfaces.
Optical potential profiles of a microsphere measured with (a) objective lenses of 0.4 (×, ) and 0.7 (○, ) NA and (b) with (○, ) and without (×, ), a microsphere in the optical trap. The corresponding fitted optical potential profiles (lines) are also shown in the figure. The fitting accuracy as quantified by R-square is at least 0.82 in the experiments.
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