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Harvesting Polarization for Optical Signal-Processing Applications Saif A. Al Graiti & Drew N. Maywar, Rochester Institute of Technology The global market demand for higher-bandwidth communication networks is increasing exponentially. While opticalfiber networks provide low-loss, high-bandwidth transmission, the processing of optical signals often requires bottleneck-inducing conversion to & from the electrical domain, where the processing is performed on electronic signals. This bottleneck may be alleviated by expanding signal-processing capabilities directly in the optical domain itself. To this end, research performed around the globe has demonstrated optical-signal processing gates such as optical flip-flops for optical memory, optical AND gates, optical XOR gates, optical half-adders, and optical datawavelength converters. We have improved the performance of many of these gates by harvesting the dynamics of optical polarization, while also demonstrating a new kind of memory behavior that expands the fundamental understanding of a bistable optical signal produced by a nonlinear photonic resonator.
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Demonstrated new concept for optical memory, whereby the state of polarization exhibits hysteresis.
The common concept of optical memory produced by a nonliear photonic resonator is that the optical power exhibits a hysteresis curve, as shown in the left-most graph of Figure 1. For example, points "A" and "B" in the hysteresis curve exhibit different output optical powers for the same input power, with their occurance depending on the history (memory) of the input optical power. The power ratio between points A and B is quantified as the contrast C, where C is only 3.4 dB for the optical-power hysteresis curve shown Figure 1. Our research reveals that, under the correct conditions, the optical signal may also exhibit hysteresis curves in its state of polarization (SOP). Such action is shown in the right-most graphs of Figure 1, using the Poincare sphere (bottom) and the three individual Stoke parameters (top). The Stokes-parameter graphs clearly reveal the bistable SOP. Each point along the optical signal has a specific SOP; for example, SOPA of the blue branch is different than SOPB of the red branch. This bistable action is a significant departure from the traditional optical-power hysteresis curves and we refer to the associated behavior as bistable polarization rotation. An optimized 30% rotation between SOPA and SOPB is measured in the lab using an optical polarimeter. To obtain this new memory behavior, the nonlinear photonic resonator is selected that exhibits gain anisotropy & birefringence, and the input optical power is injected into both polarization modes of the resonator (which are aligned along the x an y axes shown in Figure 1). This memory behavior is made possible by the simultaneous occurrence of two underlying physical processes — dispersive optical bistability & nonlinear polarization rotation. The ability to generate a bistable polarization-rotating signal opens up new capabilities for optical-signal processing. 30 | The ROCHESTER ENGINEER MARCH 2021
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