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


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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    • "Based on the consistency between the theoretical and experimental results, we demonstrate a TFBG-based side-tap modal monitor for few mode fibers. We use cross-correlated (C 2 ) imaging [15], [16], an interferometric mode measurement technique which provides accurate modal information, but cannot do so in real time, to measure the relative modal power, and correlate the results with those of our side-tap imaging method. We find that the degree of correlation (in measured mode content via the two distinct techniques) varies somewhat depending on mode order, but on average, the TFBG-based single-shot measurement correlates well with the reference C 2 technique, providing confidence for the use of these devices in MDM networks or for monitoring the performance of fiber lasers. "
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    ABSTRACT: We demonstrate a radiation mode coupler based on a tilted fiber Bragg grating that spatially demultiplexes different modes in a few-mode fiber. The observed radiation patterns match well with those theoretically predicted using the volume current method. The correlations in measured relative modal weights obtained from this device and an accurate interferometric measurement used as a reference, suggest that this technique has potential applications in providing real-time, nondestructive modal power measurement for mode division-multiplexed communications systems or monitoring the modal content in fiber lasers.
    Full-text · Article · Jul 2015 · Journal of Lightwave Technology
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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.
    Full-text · Article · Mar 2014 · Journal of Lightwave Technology
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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.
    Full-text · Article · Feb 2013 · Proceedings of SPIE - The International Society for Optical Engineering
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