Optical pulses propagating through a multimode fiber with random mode mixing experience temporal broadening and distortion. Principal modes have been proposed to overcome modal dispersion. They are the eigenstates of the time delay operator and the associated eigenvalues are the delay times. Principal modes retain the spatial profiles of output fields to the first order of frequency variation. In the weak mode coupling regime, principal modes are superpositions of fiber eigenmodes with similar propagation constants. In the strong mode coupling regime, a principal mode is composed of all fiber modes with very different propagation constant, yet it has a well-defined delay time due to multipath interference, which can be controlled by adjusting the spatial profile of incident field.
The spectral bandwidth of principal modes determines the temporal width of optical pulses that can be transmitted through the multimode fiber without distortion. In the weak mode coupling regime, principal modes with short and long delay times have broader bandwidths, while in the strong mode coupling regime, the principal modes with intermediate delay times have the broadest bandwidths. The opposite behaviors reveal two distinct mechanisms that are responsible for the principal mode bandwidth in the weak and strong mode coupling regimes. We further investigate how the mode-dependent loss modifies the principal modes. Our study provides physical understanding of spatiotemporal dynamics in a multimode fiber with varying degree of mode mixing, which is important for controlling pulse propagation through a multimode fiber.
Wen Xiong, Philipp Ambichl, Yaron Bromberg, Brandon Redding, Stefan Rotter, and Hui Cao, "Spatiotemporal control of light transmission through a multimode fiber (Conference Presentation)," Proc. SPIE 10073, Adaptive Optics and Wavefront Control for Biological Systems III, 1007312 (Presented at SPIE BiOS: January 29, 2017; Published: 24 April 2017); https://doi.org/10.1117/12.2251420.5380600107001.
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