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.

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

We designed waveguide crossings using the wavefront matching method to reduce the excess loss of a silica-based optical 8 × 8 matrix switch. We fabricated the switches using silica-based planar lightwave circuit (PLC) technology and confirmed that the excess loss of our designed waveguide crossings was lower than that of conventional crossings by 0.3dB. The average insertion loss of the fabricated switch including fiber-coupling loss was 1.7dB, which is the lowest value yet reported for a PLC-based switch. The designed waveguide crossings are practically useful for improving the performance of PLC switches with a large number of crossings.

Loss reduction of silica-based 8 * 8 optical matrix switch by optimizing waveguide crossings using WFM method

A novel cascaded Mach-Zehnder interferometer configuration is proposed for an adaptive delay DPSK demodulator. We demonstrate that the 2-cascaded MZI configuration can realize three delay settings substantially, and can suppress the penalty caused by bandwidth narrowing at cascaded ROADM filters.

Silica-based adaptive-delay DPSK demodulator with a cascaded Mach-Zehnder interferometer configuration

We have successfully demonstrated an AWG with a wide and rectangular passband by introducing a new concept using 0th and 1st order modes. An interference circuit consisting of a tandem MZI and a mode converter at the entrance of the 1st slab waveguide perturbs the input light field, thus attaining a 0.5-dB bandwidth of 69% relative to its channel spacing.

Wide passband tandem MZI-synchronized AWG employing mode converter and multimode waveguide

We propose a waveguide frontend with integrated polarization diversity optics for a wavelength selective switch (WSS) array with a liquid crystal on silicon switching engine to simplify the free space optics configuration and the alignment process in optical modules. The polarization diversity function is realized by the integration of a waveguide-type polarization beam splitter and a polarization rotating half-wave plate in a beam launcher using silica-based planar lightwave circuit technology. We confirmed experimentally the feasibility of using our proposed waveguide frontend in a two-in-one 1 × 20 WSS. The experimental results show that the fabricated waveguide frontend provides a polarization diversity function without any degradation in optical performance.

Wavelength selective switch array employing silica-based waveguide frontend with integrated polarization diversity optics.

PROBLEM TO BE SOLVED: To provide an optical phase monitor device etc., that achieve phase control essentially causing no phase error regardless of configurations of an optical filter and a demodulator. SOLUTION: The optical phase monitor device monitors the phase of an optical signal from an optical modulating light source having an occupation band A(Hz) in an optical frequency range, and has a Mach-Zehnder interferometer wherein a free spectrum range FSR on which the optical signal from the optical modulating light source is incident is at B(Hz). Hereat, B(Hz) satisfies B>A or B=mA (m: an integer of ≥2), and the phase of the optical signal from the optical modulating light source is monitored with the light intensity ratio of two optical output ports of the Mach-Zehnder interferometer. The optical phase monitor device is connected to an optical filter having a phase adjusting mechanism or an output-side branch of an optical demodulator, and the phase adjusting mechanism is controlled so that a monitor result has a constant target value. A multi-value light polarization device can be actualized by which output signal light is rarely deteriorated. COPYRIGHT: (C)2010,JPO&INPIT

Yohei Sakamaki

Papers

Loss reduction of silica-based 8 * 8 optical matrix switch by optimizing waveguide crossings using WFM method

Silica-based adaptive-delay DPSK demodulator with a cascaded Mach-Zehnder interferometer configuration

Wide passband tandem MZI-synchronized AWG employing mode converter and multimode waveguide

Wavelength selective switch array employing silica-based waveguide frontend with integrated polarization diversity optics.

Optical phase monitor device, optical filter with optical phase monitor, and optical filter phase control method