273 research outputs found
Multi-mode fiber imaging with selective mode control
Single multi-mode fibers are attractive for endoscopy due to their small footprint, high number of degrees of freedom and flexible design. We present an endoscopy system in which the working principle involves calibration of the fiber transmission matrix, calculation of scanning spots in the desired location of the object plane, fluorescence excitation and collection back through the same fiber. Many approaches to multimode fiber imaging have been reported, but a common limitation of all existing methods is the sensitivity of the fiber to environmental perturbations. While, some degree of robustness has been shown in [1] a more methodical control over perturbation resilience is desirable. An analysis of the perturbation effects in a multimode fiber reveals a direct relation to intermodal coupling [2], which suggests that control over the fiber modes can potentially improve fiber robustness. In this presentation we demonstrate a mathematical approach to controlling the fiber modes excited at the distal tip of a multimode fiber.
Towards this end, the desired field in the image plane that defines the location of the focal spot, is decomposed in the fiber modes basis and the new mode coefficients, corresponding to the selected set of fiber modes to be excited, are computed by solving a least squares problem. The estimated mode coefficients allow calculation of the optimal phase mask required at the input of the fiber. Selectively exciting fiber modes to reduce intermodal coupling is promising towards improving robustness of multimode fiber endoscopes.
References:
[1] A. M. Caravaca-Aguirre, E. Niv, D. B. Conkey, and R. Piestun, “Real-time resilient focusing through a
bending multimode fiber,” Opt. Express 21(10), 12881–12887 (2013).
[2] Antonio M. Caravaca-Aguirre and Rafael Piestun, “Single multimode fiber endoscope,” Opt. Express 25,
1656-1665 (2017).
[3] Shay Ohayon, Antonio Miguel Caravaca-Aguirre, Rafael Piestun, James J. DiCarlo, “Deep brain
fluorescence imaging with minimally invasive ultra-thin optical fibers”, arXiv:1703.07633(2017
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Lock-in detection of photoacoustic feedback signal for focusing through scattering media using wave-front shaping.
Wave-front shaping techniques enable focusing and imaging through scattering media. Unfortunately, most approaches require invasive feedback inside or behind the sample, or use of spatial correlations (memory effect) limiting the application to specific types of samples. Recent approaches overcome these limitations by taking advantage of acoustic waves via the photoacoustic (PA) effect or via photon tagging. We present a fully analog signal processing lock-in scheme for PA detection to improve focusing through scattering media and to efficiently extract nonlinear photoacoustic signals towards wave-front optimization. Our implementation improves PA feedback performance in terms of SNR, speed, and resolution
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Single multimode fiber endoscope.
Multimode fibers can guide thousands of modes capable of delivering spatial information. Unfortunately, mode dispersion and coupling have so far prevented their use in endoscopic applications. To address this long-lasting challenge, we present a robust scanning fluorescence endoscope. A spatial light modulator shapes the input excitation wavefront to focus light on the distal tip of the fiber and to rapidly scan the focus over the region of interest. A detector array collects the fluorescence emission propagated back from the sample to the proximal tip of the fiber. We demonstrate that proper selection of the multimode fiber is critical for a robust calibration and for high signal-to-background ratio performance. We compare different types of multimode fibers and experimentally show that a focus created through a graded-index fiber can withstand a few millimeters of fiber distal tip translation. The resulting scanning endoscopic microscope images fluorescent samples over a field of view of 80µm with a resolution of 2µm
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Material anisotropy as a degree of freedom in optical design.
We present an approach for the design of refractive optical elements using materials degrees of freedom that are accessible via engineered materials. Starting from first principles and an unconstrained general material, we specify homogeneous refractive lenses that focus light with diffraction-limited resolution resulting from a tailored anisotropic refractive index. We analyze the performance, physical feasibility, and advantages over isotropic lenses. Materials degrees of freedom enable new flexibility for imaging system designs with lower complexity expanding the existing aspheric and graded index paradigms
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