26 February 2013 Distinguishing dispersion from distributed scattering in S2 fiber mode analysis
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Abstract
We introduce a new spectrogram approach for analyzing spatially- and spectrally-resolved interferometry (S2) data that overcomes the previously overlooked ambiguity between dispersion and distributed scattering. Traditionally, S2 yields a one-dimensional spectrum of inter-modal group delays between higher-order-modes (HOMs) and the dominant fundamental mode. According to this interpretation, inter-modal group delay broadening is considered to be a location signature of HOM scattering; for example, a narrow peak in the spectrum is interpreted to be a discrete scattering event whereas a broad feature is interpreted to be distributed scattering along the fiber. Since the inter-modal dispersion is high for weakly guided HOMs, discrete scattering events will also manifest as broadened features. For the first time, we demonstrate a spectrogram approach to S2 analysis in which the spectral interference data is analyzed over small staggered wavelength windows and the inter-modal group delay is plotted as a function of wavelength. In this new two-dimensional map the wavelength dependence of the inter-modal group delay produces an inclination of streaks traversing the spectrogram. This new perspective resolves the ambiguity as to whether group delay broadening is caused by fiber dispersion or distributed scattering that is inherent to the previous one-dimensional S2 mapping. The spectrogram is a more accurate map of mode conversion along the fiber length and is essential for evaluating high power fibers and devices. Results for standard telecom single mode fiber and a large-mode-area fiber are presented.
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J. Jasapara, A. D. Yablon, "Distinguishing dispersion from distributed scattering in S2 fiber mode analysis", Proc. SPIE 8601, Fiber Lasers X: Technology, Systems, and Applications, 86011W (26 February 2013); doi: 10.1117/12.2000385; https://doi.org/10.1117/12.2000385
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