Optical angular momentum in air core fibers
Gregg, Patrick Clayton
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As data consumption continues to grow, the backbone of the internet, comprising single mode fiber (SMF)-based infrastructure, is fundamentally limited by nonlinear optical effects. One strategy to address this bottleneck, space division multiplexing (SDM), utilizes multiple modes in a single fiber as independent data channels. Orbital Angular Momentum (OAM) carrying modes, which have twisting phase fronts tracing out helices as the beams propagate, have recently received tremendous attention as a means of achieving low-crosstalk, digital signal processing (DSP)-free transmission with enhanced capacity. Terabit-scale transmission using 4 OAM modes over 1.1km has been demonstrated, but questions remain – how many OAM modes can fibers support, and how stable is propagation over longer lengths? In this thesis, we investigate angular momentum carrying modes in a novel class of fibers featuring an air core. We find that high-order OAM states, although arising in degenerate pairs, counterintuitively resist mode coupling due to OAM conservation, pointing to a unique stability inherent to OAM modes in fibers. We achieve OAM propagation up to 13.4km lengths, and achieve mode purities greater than 15dB at data-center length-scales. We use these fibers to transmit wavelength-division multiplexed data with 25 GHz channel spacing, 10 GBaud rates and quadrature-phase-shift keyed modulation formats in 12 modes simultaneously, over 1.2km, and over a large number of wavelengths across the C-band (1530-1565nm). However, transmission over every mode in every channel of the C-band was prevented by the accidental degeneracy of OAM states with undesired modes. To achieve a larger ensemble of stable modes over a larger wavelength range, we study new fiber designs that avoid this accidental degeneracy problem. We find that the most scalable modal eigenbasis is a set of states that carry non-integer amounts of average OAM, also called spin-orbit coupled modes in analogy with similar effects observed in atomic physics. We demonstrate excitation and transmission of 24 such modes over device lengths (10m). The achievement of a record number of uncoupled modes in fibers confirms the viability of angular momentum states as data carriers, and potential applications include links in data centers, high capacity optical amplifiers, and quantum communications links.
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