Orbital angular momentum (OAM), which describes the “phase twist” (helical phase pattern) of light beams, has recently gained interest due to its potential applications in many diverse areas. Particularly promising is the use of OAM for optical communications since: (i) coaxially propagating OAM beams with different azimuthal OAM states are mutually orthogonal, (ii) inter-beam crosstalk can be minimized, and (iii) the beams can be efficiently multiplexed and demultiplexed. As a result, multiple OAM states could be used as different carriers for multiplexing and transmitting multiple data streams, thereby potentially increasing the system capacity. In this paper, we review recent progress in OAM beam generation/detection, multiplexing/demultiplexing, and its potential applications in different scenarios including free-space optical communications, fiber-optic communications, and RF communications. Technical challenges and perspectives of OAM beams are also discussed.

Optical communications using orbital angular momentum beams

We demonstrate a 3x1 fiber-based photonic lantern spatial-multiplexer with mode-selectivity greater than 6 dB and transmission loss of less than 0.3 dB. The total insertion loss of the mode-selective multiplexers when coupled to a graded-index few-mode fiber was < 2 dB. These mode multiplexers showed mode-dependent loss below 0.5 dB. To our knowledge these are the lowest insertion and mode-dependent loss devices, which are also fully compatible with conventional few-mode fiber technology and broadband operation.

Mode-selective photonic lanterns for space-division multiplexing

Multicore fibers (MCFs) are expected as a good candidate for overcoming the capacity limit of a current optical communication system. This paper describes the recent progress on the MCFs for space-division multiplexing to be utilized in future large capacity long-distance transmission systems. Tradeoff issue between low crosstalk and high core density in MCFs is presented and prospect of large-space multiplicity of MCFs is discussed.

Multicore Fiber Technology

We present the first in-band diode-pumped TDFAs operating in the 2µm wavelength region and test their suitability as high performance amplifiers in potential future telecommunication networks. We demonstrate amplification over a 240nm wide window in the range 1810 - 2050nm with up to 36dB gain and noise figure as low as 4.5dB.

Diode-pumped wideband thulium-doped fiber amplifiers for optical communications in the 1800 - 2050 nm window.

• Due to the limitation of the outer cladding size of MCFs related to their mechanical reliability, the number of cores as well as the core arrangement have to be carefully determined based on the required modulation format and transmission distance. • By introducing heterogeneous core arrangement, further decrement of core-to-core distance is possible with keeping lower crosstalk level compared with the homogeneous MCFs. • The combination of MCF and FMF, which is FM-MCF, is a very promising approach to realize space multiplicity over 50. • Further development on related devices such as Fan-in/Fan-out and amplifier for FM-MCF transmission is highly expected. • Investigation towards cost saving by using MCF transmission system is underway and further development on devise integration is expected for deploying MCFs in the real network.

Multicore fiber technology

The feature of a multicore fiber with one-ring structure is theoretically analyzed and experimentally demonstrated. The one-ring structure overcomes the issues of the hexagonal close-pack structure. The possibility of 10-core fiber with Aeff of 110 μm2 and 12-core fiber with Aeff of 80 μm2 is theoretically presented. The fabricated 12-core fibers based on the simulation results realized Aeff of 80 μm2 and crosstalk less than −40 dB at 1550 nm after 100-km propagation. The MCF with the number of core larger than seven and the small crosstalk was demonstrated for the first time.

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Masanori Kosihba

Papers

12-core fiber with one ring structure for extremely large capacity transmission