Researchers demonstrate the ability to multiplex and transfer data between twisted beams of light with different amounts of orbital angular momentum — a development that provides new opportunities for increasing the data capacity of free-space optical communications links.

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Terabit free-space data transmission employing orbital angular momentum multiplexing

Optical transmission systems with terabit per second (Tbit s-1) single-channel line rates no longer seem to be too far-fetched. New services such as cloud computing, three-dimensional high-definition television and virtual-reality applications require unprecedented optical channel bandwidths. These high-capacity optical channels, however, are fed from lower-bitrate signals. The question then is whether the lower-bitrate tributary information can viably, energy-efficiently and effortlessly be encoded to and extracted from terabit per second data streams. We demonstrate an optical fast Fourier transform scheme that provides the necessary computing power to encode lower-bitrate tributaries into 10.8 and 26.0 Tbit s-1 line-rate orthogonal frequency division multiplexing (OFDM) data streams and to decode them from fibre-transmitted OFDM data streams. Experiments show the feasibility and ease of handling terabit per second data with low energy consumption. To the best of our knowledge, this is the largest line rate ever encoded onto a single light source.

26 Tbit s-1 line-rate super-channel transmission utilizing all-optical fast Fourier transform processing

Since the first deployments of fiber-optic com- munication systems three decades ago, the capacity carried by a single-mode optical fiber has increased by a staggering 10 000 times. Most of the growth occurred in the first two decades with growth slowing to ten times in the last decade. Over the same three decades, network traffic has increased by a much smaller factor of 100, but with most of the growth occurring in the last few years, when data started dominating network traffic. At the current growth rate, the next factor of 100 in network traffic growth will occur within a decade. The large difference in growth rates between the delivered fiber capacity and the traffic demand is expected to create a capacity shortage within a decade. The first part of the paper recounts the history of traffic and capacity growth and extrapolations for the future. The second part looks into the technological chal- lenges of growing the capacity of single-mode fibers by pre- senting a capacity limit estimate of standard and advanced single-mode optical fibers. The third part presents elementary capacity considerations for transmission over multiple trans- mission modes and how it compares to a single-mode trans- mission. Finally, the last part of the paper discusses fibers supporting multiple spatial modes, including multimode and multicore fibers, and the role of digital processing techniques. Spatial multiplexing in fibers is expected to enable system capacity growth to match traffic growth in the next decades.

https://www.icg.isy.liu.se/courses/optical/Capacity_Trends.pdf

Capacity Trends and Limits of Optical Communication Networks Discussed in this paper are: optical communication network traffic evolution trends, required capacity and challenges to reaching it, and basic limits to capacity.

Since the first deployments of fiber-optic communication systems three decades ago, the capacity carried by a single-mode optical fiber has increased by a staggering 10 000 times. Most of the growth occurred in the first two decades with growth slowing to ten times in the last decade. Over the same three decades, network traffic has increased by a much smaller factor of 100, but with most of the growth occurring in the last few years, when data started dominating network traffic. At the current growth rate, the next factor of 100 in network traffic growth will occur within a decade. The large difference in growth rates between the delivered fiber capacity and the traffic demand is expected to create a capacity shortage within a decade. The first part of the paper recounts the history of traffic and capacity growth and extrapolations for the future. The second part looks into the technological challenges of growing the capacity of single-mode fibers by presenting a capacity limit estimate of standard and advanced single-mode optical fibers. The third part presents elementary capacity considerations for transmission over multiple transmission modes and how it compares to a single-mode transmission. Finally, the last part of the paper discusses fibers supporting multiple spatial modes, including multimode and multicore fibers, and the role of digital processing techniques. Spatial multiplexing in fibers is expected to enable system capacity growth to match traffic growth in the next decades.

Capacity Trends and Limits of Optical Communication Networks

Data transmission rates in optical communication systems are approaching the limits of conventional multiplexing methods. Orbital angular momentum (OAM) in optical vortex beams offers a new degree of freedom and the potential to increase the capacity of free-space optical communication systems, with OAM beams acting as information carriers for OAM division multiplexing (OAM-DM). We demonstrate independent collinear OAM channel generation, transmission and simultaneous detection using Dammann optical vortex gratings (DOVGs). We achieve 80/160 Tbit s−1 capacity with uniform power distributions along all channels, with 1600 individually modulated quadrature phase-shift keying (QPSK)/16-QAM data channels multiplexed by 10 OAM states, 80 wavelengths and two polarizations. DOVG-enabled OAM multiplexing technology removes the bottleneck of massive OAM state parallel detection and offers an opportunity to raise optical communication systems capacity to Pbit s−1 level. Dammann gratings are used to realize multiplexing based on the generation, transmission and detection of optical angular momentum (OAM). The OAM of optical vortex beams offers a new degree of freedom for multiplexing and hence the promise of higher data communication rates, but massive parallel detection of OAM states has proved challenging. Now, researchers in China, Australia and Singapore have used Dammann optical vortex gratings (DOVGs) to realize multiplexing of massive OAM channels with individual modulation and simultaneous detection capabilities. They achieved a data capacity of 80 Tbit s−1 by multiplexing 1600 channels using ten OAM states, 80 wavelengths and two polarizations. This DOVG-enabled OAM multiplexing technology removes the bottleneck of massive parallel detection of OAM states and has the potential to increase optical communication capacities to the Pbit s−1 level.

/pdf/massive-individual-orbital-angular-momentum-channels-for-29tya8t7bu.pdf

Massive individual orbital angular momentum channels for multiplexing enabled by Dammann gratings

We demonstrate the record total capacity of 69.1 Tb/s with a spectral efficiency of 6.4 b/s/Hz by employing 21.4-Gbaud 16-QAM modulation, blind digital coherent detection, and 10.8-THz ultra-wideband amplification in the C- and extended L-bands.

69.1-Tb/s (432 × 171-Gb/s) C- and extended L-band transmission over 240 km Using PDM-16-QAM modulation and digital coherent detection

We have studied the wavefront matching (WFM) method, which synthesizes optimum waveguide patterns from the desired characteristics, and demonstrated its feasibility. In this paper, we expand this method to create optimum waveguide shapes with low-loss and low-wavelength dependence in waveguide lenses, Y-branches, and waveguide crossings. We describe the design procedures for each waveguide element and report the experimental results as proof of concept. The measured results agree well with our design requirements, and the waveguide patterns designed by the WFM method exhibit better characteristics than the reference patterns.

New Optical Waveguide Design Based on Wavefront Matching Method

This paper describes ultrahigh capacity transmission based on spectrally-efficient multi-level modulation and wideband optical amplification techniques. 21.4-Gbaud polarization-division multiplexed (PDM) 16-ary quadrature amplitude modulation (QAM) signals are generated by utilizing an optical synthesis technique, wavelength-multiplexed with 25-GHz spacing by optical pre-filtering, and received by an intradyne coherent receiver based on digital signal processing (DSP) with pilotless algorithms. These techniques realize a spectral efficiency (SE) of 6.4 b/s/Hz. Furthermore, a hybrid amplification technique that combines distributed Raman and dual-band erbium-doped amplifiers (EDFAs) realizes 10.8-THz signal bandwidth in C- and extended L-bands. By using these techniques, we successfully demonstrate 69.1 Tb/s transmission over 240 km of low loss pure silica core fibers.

Ultra-High Capacity WDM Transmission Using Spectrally-Efficient PDM 16-QAM Modulation and C- and Extended L-Band Wideband Optical Amplification

http://ieeexplore.ieee.org/iel5/5760047/5760048/05760056.pdf

30 x 100-Gb/s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes

We demonstrate an optical 64QAM modulator operating at 60 Gb/s (with single polarization). Asymmetric couplers and an array of 12 high-speed phase modulators are integrated with a hybrid configuration of silica PLCs and LiNbO 3 phase modulators.

http://ieeexplore.ieee.org/iel5/5273784/5286960/05287031.pdf

Yohei Sakamaki

Papers

69.1-Tb/s (432 × 171-Gb/s) C- and extended L-band transmission over 240 km Using PDM-16-QAM modulation and digital coherent detection

New Optical Waveguide Design Based on Wavefront Matching Method

Ultra-High Capacity WDM Transmission Using Spectrally-Efficient PDM 16-QAM Modulation and C- and Extended L-Band Wideband Optical Amplification

30 x 100-Gb/s all-optical OFDM transmission over 1300 km SMF with 10 ROADM nodes

64QAM modulator with a hybrid configuration of silica PLCs and LiNbO 3 phase modulators for 100-Gb/s applications