Article

Cross-correlated (C-2) imaging of fiber and waveguide modes

Photonics Center, Department of Electrical and Computer Engineering, Boston University, Boston, MA 02215, USA.
Optics Express (Impact Factor: 3.49). 07/2011; 19(14):13008-19. DOI: 10.1364/OE.19.013008
Source: PubMed

ABSTRACT We demonstrate a method that enables reconstruction of waveguide or fiber modes without assuming any optical properties of the test waveguide. The optical low-coherence interferometric technique accounts for the impact of dispersion on the cross-correlation signal. This approach reveals modal content even at small intermodal delays, thus providing a universally applicable method for determining the modal weights, profiles, relative group-delays and dispersion of all guided or quasi-guided (leaky) modes. Our current implementation allows us to measure delays on a femtosecond time-scale, mode discrimination down to about - 30 dB, and dispersion values as high as 500 ps/nm/km. We expect this technique to be especially useful in testing fundamental mode operation of multi-mode structures, prevalent in high-power fiber lasers.

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Available from: Damian N Schimpf, Aug 24, 2015
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    • "Up to this date, several mode analysis techniques have been demonstrated . Among the most employed one, we can refer to spatially and spectrally resolved imaging (S 2 imaging) [10], low coherence interferometry [11], correlation filter technique (CFT) [12], cross-correlated imaging [13] and high-speed modal decomposition [14]. One technique is preferred according to the type of beam to be decomposed (single-mode (SM) or multi-mode (MM)) and depending on the modal information to access. "
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    ABSTRACT: Spatially and spectrally resolved imaging (S^2 imaging) and correlation filter technique (CFT) are two very different, widespread fiber mode analysis techniques. Both techniques have been successfully employed to decompose few-modes and multimode beams respectively. In this study, we present a novel experimental tool combining S^2 imaging and CFT mode analyses in a unique system. We demonstrate that both methods are complementary with the ability to fully resolve scalar and vector-valued transverse modal fields. Using results from the combined experiment, mode powers (rho^2) evaluated from CFT analysis and S^2 imaging are directly compared for a wide range of fiber beams (from single- to multi-mode). As a result, we experimentally identify the mode detection limit of each mode analysis and prove that S^2 imaging accuracy range can be considerably increased employing an analytical mode evaluation method. The conclusion contains a table summarizing the expertise of each mode analysis.
    Journal of Lightwave Technology 03/2014; 32(6-6):1068-1074. DOI:10.1109/JLT.2013.2297337 · 2.86 Impact Factor
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    • ") imaging [22] [23], relies on the modal interference directly in the time domain, so that the requirements on optical bandwidth are less restrictive than they are in S 2 imaging, which makes C 2 imaging an interesting candidate for modal characterization of relatively short LMA fibers. Since C 2 imaging relies on the interference between an external reference beam and each of the modes propagating in the test fiber, one can measure the modal content as well as dispersion and polarization of all the test modes. "
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    ABSTRACT: We analyze the modal properties of an 85μm core distributed mode filtering rod fiber using cross-correlated (C2) imaging. We evaluate suppression of higher-order modes (HOMs) under severely misaligned mode excitation and identify a single-mode regime where HOMs are suppressed by more than 20dB.
    Proceedings of SPIE - The International Society for Optical Engineering 02/2013; DOI:10.1117/12.2002808 · 0.20 Impact Factor
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    • "We not only observe spatial separation of the modes, as predicted by the phase matching condition, but find that modes with different azimuthal symmetry exhibit different radiation patterns and have polarization sensitivity even at shallow blaze angles, a result which is confirmed by volume current method (VCM) modeling [4]. In order to gain more detailed insight to the mode partitioning, the fiber is placed in an optical low coherence cross-correlation interferometer (C 2 ) [5] which allows us to precisely measure the power partitioning (as "
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    ABSTRACT: We demonstrate a sidetap modal channel monitor based on a tilted Bragg grating, where different modes radiate at different angles. We qualitatively correlate the observed modal power partitioning with more accurate interferometer-based measurements.
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