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Abstract: Spatial reuse of optical frequencies has been shown to be possible through a novel spatial domain multiplexing (SDM) technique that uses spatial multiplexer at the input end to launch multiple channels of the same wavelength inside a single strand of carrier fiber and then employs spatial filtering methods to de-multiplex the different optical channels at the output end. The individual SDM channels are confined to dedicated spatial locations inside the fiber while traversing through it owing to helical propagation of light. This presents attenuation and dispersion models of such a system. Experimentally obtained beam profile and resultant crosstalk of two such spatially multiplexed co-propagating SDM channels of the same wavelength over standard step index multimode optical fibers are also presented. [ABSTRACT FROM AUTHOR]

• Multiwavelength Tm-doped fiber laser based on the nonlinear polarization rotation effect is reported. • The switching of wavelength difference is realized in dual-wavelength operation. • The mechanism of multiwavelength output and wavelength difference switching is clarified. In this paper, we report on the characteristics of multiwavelength output and wavelength difference switching in a Tm-doped fiber laser based on the nonlinear polarization rotation effect. The multiwavelength output and wavelength difference switching were realized through joint action of nonuniform intensity-dependent loss caused by the nonlinear polarization rotation effect and the polarization controller intertwined with Tm-doped fiber. Under dual-wavelength state, one wavelength could be switched within wavelength intervals of 44.7, 58.0, 68.2, and 73.1 nm, and the position of the other wavelength nearly remained unchanged in the switching process. The fiber laser with switchable wavelength difference has a broad application prospect in laser space communication and optical fiber sensing. [ABSTRACT FROM AUTHOR]