Abstract
The capacity of free-space optical (FSO) communication links could potentially be increased by the simultaneous transmission of multiple orbital angular momentum (OAM) beams. For such an OAM multiplexing approach, one requires the collection of adequate power as well as proportion of the phase front for a system with minimal crosstalk. Here we study the design considerations for an OAM-multiplexed free-space data link, analyzing the power loss, channel crosstalk, and power penalty of the link in the case of limited-size receiver apertures and misalignment between the transmitter and the receiver. We describe the trade-offs for different transmitted beam sizes, receiver aperture sizes, and mode spacing of the transmitted OAM beams under given lateral displacements or receiver angular errors. Through simulations and some experiments, we show that (1) a system with a larger transmitted beam size and a larger receiver aperture is more tolerant to lateral displacement but less tolerant to the receiver angular error, and (2) a system with a larger mode spacing, which uses larger OAM charges, suffers more system power loss but less channel crosstalk; thus, a system with a small mode spacing shows a lower system power penalty when system power loss dominates (e.g., a small lateral displacement or receiver angular error), whereas that with a larger mode spacing shows a lower power penalty when channel crosstalk dominates (e.g., a larger lateral displacement or receiver angular error). This work could be beneficial to the practical implementation of OAM-multiplexed FSO links.
Highlights
Free-space optical (FSO) communication links can potentially benefit from the simultaneous transmission of multiple spatially orthogonal beams through a single aperture pair, such that each beam carries an independent data stream and the total capacity is multiplied by the number of beams [1,2,3,4,5,6]
To investigate the signal power and crosstalk effects on neighboring orbital angular momentum (OAM) channels, the power distribution among the different OAM modes is analyzed through the modal decomposition approach, which corresponds to the case where the received OAM beams are de-multiplexed without power loss and the power of a desired OAM channel is completely collected by the receiver, which is infinitely large and perfectly aligned with the transmitter [12,14]
To reiterate the key points, a larger beam size at the receiver will result in two opposing effects: (i) a smaller lateral displacement-induced crosstalk because the differential phase change per unit area is smaller, and (ii) a larger tilt phase error-induced crosstalk because the phase error scales with a larger optical path delay
Summary
Free-space optical (FSO) communication links can potentially benefit from the simultaneous transmission of multiple spatially orthogonal beams through a single aperture pair, such that each beam carries an independent data stream and the total capacity is multiplied by the number of beams [1,2,3,4,5,6]. The above characteristics of an OAM beam present several important challenges when designing an FSO communication link, such as (i) sufficient signal power and phase change needs to be recovered [10], and (ii) intermodal crosstalk should be minimized [11,12,13,14]. Through studying the effects of misalignment between the transmitter and the receiver (a lateral displacement or receiver angular error) on the OAM channel crosstalk and system power penalty, proper aperture sizes and mode spacing of the transmitted OAM beams could be selected to reduce system performance degradation. Mode-multiplexed communication systems using other orthogonal modal sets will likely suffer from signal power loss and intermodal crosstalk in a relatively similar fashion as OAM modes but with different parameters governing the link; the methods in this paper can be modified and adapted to potentially be used to determine the performance of other mode-multiplexed systems
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