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Mode-division multiplexing has shown its ability to significantly increase the capacity of free-space optical communications. An accurate alignment is crucial to enable such links due to possible performance degradation induced by mode crosstalk and narrow beam divergence. Conventionally, a beacon beam is necessary for system alignment due to multiple local maximums in the mode-division multiplexed beam profile. However, the beacon beam introduces excess system complexity, power consumption, and alignment errors. Here we demonstrate a beaconless system with significantly higher alignment accuracy and faster acquisition. This system also excludes excess complexity, power consumption, and alignment errors, facilitating simplified system calibration and supporting a record-high 5.14 Tbit/s line rate in a single-wavelength free-space optical link. We anticipate our paper to be a starting point for more sophisticated alignment scenarios in future multi-Terabit mode-division multiplexing free-space optical communications for long-distance applications with a generalised mode basis.

We experimentally demonstrate the enhanced atmospheric turbulence resiliency in a 137.8 Gbit/s/mode mode-division multiplexing free-space optical communication link through the application of a successive interference cancellation digital signal processing algorithm. The turbulence resiliency is further enhanced through redundant receive channels in the mode-division multiplexing link. The proof of concept demonstration is performed using commercially available mode-selective photonic lanterns, a commercial transponder, and a spatial light modulator based turbulence emulator. In this link, 5 spatial modes with each mode carrying 34.46 GBaud dual-polarization quadrature phase shift keying signals are successfully transmitted with an average bit error rate lower than the hard-decision forward error correction limit. As a result, we achieved a record-high mode- and polarization-division multiplexing channel number of 10, a record-high line rate of 689.23 Gbit/s, and a record-high net spectral efficiency of 13.9 bit/s/Hz in emulated turbulent links in a mode-division multiplexing free-space optical system.

Lasers can be identified by their relatively long coherence lengths using interferometry. A Mach Zehnder interferometer incorporating liquid crystal polarization modulators is demonstrated as a means of low cost, robust laser detection. Temporal modulations, as a signature of coherence can be induced by modulating polarization changes in liquid crystal modulators, using low voltages. Sensitivities of less than 10nW can be achieved. The suitability as a means of laser detection is discussed.

An unconventional encoding scheme called concurrent coding, has recently been demonstrated and shown to offer interesting features and benefits in comparison to conventional techniques, e.g. robustness against burst errors and improved efficiency of transmitted power. This concept has been demonstrated for the first time with optical communications where standard light emitting diodes (LEDs) have been used to transmit information encoded with concurrent coding. The technique successfully transmits and decodes data despite unpredictable interruptions to the transmission causing significant drop-outs to the detected signal. The technique also shows how it is possible to send a single block of data in isolation with no pre-synchronisation required between transmitter and receiver, and no specific synchronisation sequence appended to the transmission. This work also demonstrates for the first time the successful use of multithreaded (overlaid) concurrent codes

The detection of laser radiation originating from space is a positive indicator of Extra Terrestrial Intelligence (ETI). Thus far the optical search for ETI (OSETI) has looked for enhanced brightness in the form of either narrow-band spectral emission or time correlated photons from laser pulses. In this paper it is proposed to look for coherence properties of incoming light in a manner that can distinguish against atomic emission lines. The use of photon sensitive detectors and a modulating asymmetric interferometer has been modelled. The results suggest that the sensitivity of detection for continuous laser sources would be better than current spectroscopic approaches, with detection thresholds of $<$1 photon s$^{-1}$m$^{-2}$ against a background with a spectral bandwidth of 0.1nm over an observation time of 750s., Comment: 17 pages, 7 figures

Concurrent coding is an unconventional encoding technique that simultaneously provides protection against noise, burst errors and interference. This simple-to-understand concept is investigated by distinguishing 2 types of code, open and closed, with the majority of the investigation concentrating on closed codes. Concurrent coding is shown to possess an inherent method of synchronisation thus requiring no additional synchronisation signals to be added. This enables an isolated codeword transmission to be synchronised and decoded in the presence of noise and burst errors. Comparisons are made with the spread spectrum technique CDMA. With a like-for-like comparison concurrent coding performs comparably against random noise effects, performs better against burst errors and is far superior in terms of transmitted energy efficiency, Comment: 17 pages including 12 figures

Three dimensional laser beam steering has been demonstrated using a single optical device -a DMD micro-mirror array. Laser beam focus position is controlled using dynamically adjustable zone plates. These zone plates take the form of elliptical Fresnel zone plates or other variations such as binary Gabor zone plates. Active beam control can be realised without the need for a pair of galvanometer mirrors and a focusing lens. Writing multiple zone plate patterns to the DMD enables multiple focussed spots to be generated and controlled independently