A detailed investigation of the nonlinear multimodal interference in a short graded-index multimode optical fiber is presented. The analysis is performed for a specific device geometry, where the light is coupled in and out of the multimode fiber via single-mode fibers. The same device geometry was recently used to obtain ultra-low-loss coupling between two single-mode optical fibers with very different mode-field diameters.Our results indicate the potential application of this simple geometry for nonlinear devices, such as in nonlinear switching, optical signal processing, or as saturable absorbers in mode-locked fiber lasers. Saturable absorption in this all-fiber configuration is discussed and it is shown that it provides attractive properties that can potentially be used in high pulse energy mode-locked fiber lasers.

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A short piece of commercial-grade SMF-28 optical fiber is pumped with a 680 ps high-peak power green laser. Red Stokes and blue anti-Stokes beams are generated spontaneously from vacuum noise in different modes in the fiber via intermodal four-wave mixing. Detailed experimental and theoretical analyses are performed and are in reasonable agreement. The large spectral shifts from the pump protect the Stokes and anti-Stokes from contamination by spontaneous Raman scattering noise. This work highlights the predictive power and limitations of a theoretical model to explain the experimental results for a process that relies on the amplification of quantum vacuum energy over more than 11 orders of magnitude.

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We present a novel double-clad step-index few-mode fiber that operates as a five-sampled tunable true-time delay line. The unique feature of this design lies in its particular modal chromatic dispersion behavior, which varies in constant incremental steps among adjacent groups of modes. This property, which to the best of our knowledge has not been reported in any other few-mode fiber to date, is the key to tunable operation of radiofrequency signal processing functionalities implemented in few-mode fibers. The performance of the designed true-time delay line is theoretically evaluated for two different microwave photonics applications, namely tunable signal filtering and optical beamforming networks for phased array antennas. In the 35-nm optical wavelength tuning range of the C-band, the free spectral range of the microwave filter and the beam-pointing angle in the phased array antenna can be continuously tuned from 12.4 up to 57 GHz and 12.6° up to 90°, respectively.

nanophotonic structures [1] avoid absorption losses which drastically enhances the overall performance, [3][4][5][6][7][8][9] especially in the transmission regime. Recently, all-dielectric metasurfaces have reached remarkable efficiencies, often matching or outperforming conventional optical elements. [1] Here, we suggest to broaden the application range of metasurfaces and employ them for mode multiplexing and dynamic control in fiber optics, offering near lossless transmission within the telecom C-and L-bands, [4] which will make metasurfaces a promising technology for mode multiplexers and other applications in optical communications.Optical fiber communications is a key enabling technology in our modern information-centric society. In the past decades, there has been a significant increase in the amount of data transmitted through a fiber, [10] which has been possible by multiplexing light across the physical dimensions of time, wavelength, polarization, and quadratures (amplitude and phase). However, the data rates of existing technologies are rapidly approaching the Shannon limit. [11] To surpass this limit, considerable attention has been given to multiplexing data over the last physical dimension: space. [11][12][13][14] This technology, known as space-division multiplexing, can increase the throughput by transmitting data over individual cores of multicore fibers, [15] and/or orthogonal spatial modes of few-mode or multimode fibers (MMFs). [16,17] With these techniques, a two-order increase in the throughput is achievable compared to a single-mode fiber (SMF). [18] Among the space-division multiplexing approaches, multiplexing data onto orthogonal spatial modes, known as mode-division multiplexing (MDM), has the biggest potential for further scaling down cost-per-bit and energy-per-bit of information transfer. Meanwhile, implementation of MDM requires efficient mode multiplexers and demultiplexers. So far, holographic excitation with spatial light modulators (SLMs), [19] photonic lanterns, [20] integrated devices, [21] and free-space multiplane light conversion, [22] are among the demonstrated techniques for mode multiplexing/demultiplexing. However, there exists a common issue with the currently developed techniques: it is not possible to simultaneously convert each of the two orthogonal polarizations of an input beam to two individual higher-order modes without using some polarization diversity setup, which increases losses and sensitivity to alignment tolerances. Independent conversion of two polarization-multiplexed fundamental modes A subwavelength-thick spatial-mode multiplexer based on a highly transparent all-dielectric Mie-resonant metasurface is demonstrated with a broadband response covering major optical communication wavelength bands. The metasurface is employed to convert simultaneously each orthogonal polarization of LP 01 inputs into individual higher-order TM 01 and TE 01 vectorial modes, without the need of a polarization diversity setup. This is not feasible using current mode multipl...

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The main differences in nonlinear switching behavior between multicore versus multimode waveguide couplers are highlighted. By gradually decreasing the separation between the two cores of a dual-core waveguide and interpolating from a multicore to a multimode scenario, the role of the linear coupling, self-phase modulation, cross-phase modulation, and four-wave mixing terms are explored, and the key reasons are identified behind higher switching power requirements and lower switching quality in multimode nonlinear couplers.