Using beams carrying orbital angular momentum for communications and remote sensing

2018-01-31An electromagnetic (EM) wave can carry orbital angular momentum (OAM) if the wave has a helical transverse phase structure of exp(jlφ), in which φ is the transverse azimuthal angle and l is an unbounded integer (the OAM state number). OAM beams with different l values are mutually orthogonal to each other, allowing them (i) to be multiplexed together along the same beam axis and de-multiplexed with low crosstalk to establish a channel-multiplexed line-of-sight communications link (e.g., spaced division multiplexing or SDM), (ii) to form an orthogonal mode basis such that other beams could be decomposed over this basis to provide potential advantages for a remote sensing system. ❧ OAM based SDM links have been demonstrated in both the optical domain and the radio frequency (RF) domain. Specifically, a 100-Tbit/s communication link combining SDM, wavelength division multiplexing (WDM), and polarization division multiplexing (PDM) was demonstrated at the wavelength ∼1550 nm in 2013. In addition, a 32-Gbit/s communication link combining the SDM and PDM at a carrier frequency of 28 GHz was demonstrated in 2014. However, these proof-of-concept demonstrations did not take the potential system design challenges into consideration. In this thesis, the challenges and potential solutions for the OAM based free-space SDM system are discussed, including: (i) the atmospheric turbulence effects and the turbulence compensation approaches, (ii) system-level design guidelines for long-distance OAM multiplexed links, (iii) beam steering for OAM multiplexing based communication systems, and (iv) the use of Laguerre-Gaussian beams with different radial indices for channel multiplexing. ❧ Besides, OAM beams’ unique intensity and phase structures could potentially provide a powerful tool for remote sensing other than communications. As an example, this thesis also explores the measurements of an object’s opening angle and the opening’s orientation using the information provided by OAM complex spectrum