Free-space communication exploiting the transverse spatial structure dimension of light waves has also attracted more and more attention. In addition to traditional multiplexing technology, orbital angular momentum (OAM), which is also related to the spatial phase structure (spiral phase front) of an electromagnetic wave, has also shown its possible use both in free-space and fiber transmission links to improve the transmission capacity.
Jian Wang’s group, Multi-Dimensional Photonics Laboratory (MDPL), has been devoted to the research of free-space OAM communications for many years and achieved progress:
PhD candidate Jun Liu from MDPL, directed by Prof. Wang, has demonstrated a simple configuration incorporating a single polarization-sensitive phase-only liquidcrystal spatial light modulator (LC-SLM) to facilitate polarization-insensitive spatial light modulation. In addition, several polarization-insensitive optical communication subsystems employing OAM modes, e.g. polarization-insensitive single OAM mode transmission, polarization-insensitive OAM modes multicasting, and polarization-insensitive OAM modes multiplexing are demonstrated (Optics Express 24, 4258-4269, 2016). Moreover, analogical to WDM system, we need a space-selective switch working as a “WSS” in SDM system. PhD candidate Jun Liu from MDPL, directed by Prof. Wang, has proposed and demonstrated several kinds of OAM-incorporated space-selective switch functions, i.e. OAM mode switching, space switching, and joint OAM mode and space switching (Scientific Reports6, PP.37331, 2016).
Figure 1 (a)Concept of polarization-insensitive optical communications using space dimension of photons (b) Concept of N × N joint OAM mode and space switching fabric.
For a practical free-space orbital angular momentum (OAM) optical (FSO) link, a critical challenge is the atmospheric turbulence that will cause fluctuations in both intensity and phase of the received light, impairing link performance. Atmospheric turbulence can distort the helical phase fronts of OAM beams leading to the decrease of received power and introducing crosstalk between multiple channels. To overcome this limitation, compensation techniques are needed for distorted OAM communication links.
Recently, Prof. Wang Jian and PHD student Shuhui Li experimentally demonstrated an adaptive optics compensation scheme for distotred OAM multicasting links. The experimental results show that the scheme can efficiently compensate for the atmospheric turbulence induced distortions, i.e., reducing power fluctuation ofmulticasted OAM channels, suppressing inter-channel crosstalk, and improving the bit-error rate (BER) performance. This work “Compensation of a distorted N-fold orbital angular momentum multicasting link using adaptive optics” was published on Optics Letters (Vol. 41, PP. 1482-1485, 2016). In addition,PHD student Chen Shi and PHD student Shuhui Li, directedby Prof. Wang, experimentally demonstrated an adaptive optics compensation scheme for 20-Gbit/s High-Speed Bessel Beam Encoding/Decoding Link. This work “Demonstration of 20-Gbit/s High-Speed Bessel Beam Encoding/Decoding Link with Adaptive Turbulence Compensation” was published on Optics Letters (Vol. 41, PP. 4680-4683, 2016).
Figure 2 (a)Concept and principle of turbulence compensation for a distorted OAM multicasting link. (b) Concept and principle of high-speed encoding/decoding of Bessel beams through turbulence assisted by adaptive compensation.
In addition, Prof. Jian Wang was invited to review the advances in communications using optical vortices (Photonics Research 5, pp. B14-B28, 2016).A polarization vortex (vector beam) with a polarization singularity has spatially variant polarizations. A phase vortex with phase singularity or screw dislocation has a spiral phase front.In this paper, we review recent advances in optical communications using optical vortices. First, basic concepts of polarization/phase vortex modulation and multiplexing in communications and key techniques of polarization/phase vortex generation and (de)multiplexing are introduced. Second, free-space and fiber optical communications using optical vortex modulation and optical vortex multiplexing are presented. Finally, key challenges and perspectives of optical communications using optical vortices are discussed.
Figure 3 (a)Schematic illustration of field distributions of polarization vortex and phase vortex beams. (b) Schematic illustration of physical dimensions of photons and orthogonal states in modulation schemes and multiplexing techniques.
These works arepartiallysupported by the National Basic Research Program of China (973Program) (2014CB340004),the NationalNatural Science Foundation of China (NSFC) (11274131,11574001,61222502),the Program for New Century ExcellentTalents in University (NCET-11-0182),and theWuhan Science andTechnology Plan Project (2014070404010201).
Links of the published papers:
[1] Jun Liu and Jian Wang*, “Polarization-insensitive PAM-4-carrying free-space orbital angular momentum (OAM) communications,” Optics Express 24(4), 4258-4269 (2016).
https://www.osapublishing.org/oe/abstract.cfm?uri=oe-24-4-4258
[2] Jun Liu and Jian Wang*, “Demonstration of reconfigurable joint orbital angular momentum mode and space switching,” Scientific Reports 6, 37331 (2016).
http://www.nature.com/articles/srep37331
[3] Shuhui Li and Jian Wang *, “Compensation of a distorted N-fold orbital angular momentum multicasting link using adaptive optics,” Optics Letters 41(7), 1482-1485 (2016).
https://www.osapublishing.org/ol/abstract.cfm?uri=ol-41-7-1482
[4] Shi Chen, Shuhui Li, Yifan Zhao, Jun Liu, Long Zhu, Andong Wang, Jing Du, Li Shen, and Jian Wang*, “Demonstration of 20-Gbit/s High-Speed Bessel Beam Encoding/Decoding Link with Adaptive Turbulence Compensation,” Opt. Letters 41 (20), 4680-4683 (2016).
https://www.osapublishing.org/ol/abstract.cfm?uri=ol-41-20-4680
[5] Jian Wang,“Advances in communications using optical vortices,”Photonics Research 4(5), B14-B28 (2016).
https://www.osapublishing.org/prj/abstract.cfm?uri=prj-4-5-B14
