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

There is provided a demodulation circuit for multi-value differential phase modulation signal that resolves a polarization dependency due to birefringence caused by a waveguide structure and polarization-coupled light, and a polarization dependency due to dynamic birefringence generated during driving of a fine adjustment function unit. The demodulation circuit is constituted by an optical delay interference circuit of a waveguide interference type. It is possible to improve the S/N ratio of a demodulation signal of the demodulation circuit constituted by the optical delay interference circuit. It is also possible to reduce the polarization dependency and the temperature dependency of the optical delay interference circuit at a time. Disposing a polarization converter and a channel for filling a temperature compensation material makes it possible to provide a circuit configuration suitable for eliminating the polarization dependency and the temperature dependency of the optical delay interference circuit. Further, highly axisymmetrical circuit layout is possible, so that a PDf caused by a polarization-coupled light can be effectively eliminated.

Optical delay interference circuit

PROBLEM TO BE SOLVED: To attain the miniaturization of an optical circuit having desired transmitting characteristics by applying a holographic wave transmitting medium SOLUTION: In the optical waveguide composed of a clad layer and a core part which is embedded in the clad layer and has a higher refractive index than the clad layer, the core part is provided with a localized refractive index modulation part of continuous gradation The refractive index modulation part is formed by applying the holographic wave transmitting medium which holographically transmits a wave by a two-dimensional refractive index distribution The index modulation quantity of the refractive index modulation part is made up of a plurality of aperiodic peaks COPYRIGHT: (C)2006,JPO&NCIPI

Optical waveguide and its manufacturing method

In this paper, we describe the procedure for designing low-loss Y-branch waveguides by the wavefront matching (WFM) method, and report experimental results as proof of concept. The designed Y-branches were fabricated using silica-based planar lightwave circuit (PLC) technology. A Y-branch fabricated with a 0.45%- waveguide exhibited a low excess loss of less than 0.2 dB over a wide wavelength range of 1250 to 1650 nm. In addition, we demonstrate that the WFM-designed Y-branches enable us to provide compact 1 32 splitters with an average insertion loss of 16.0 dB at a wavelength of 1550 nm. In addition, we present some experimental results obtained using samples with different values, and show that our design method is more efficient for a higher waveguide suitable for functional PLC devices.

Low-Loss Y-Branch Waveguides Designed by

PROBLEM TO BE SOLVED: To provide a multi-port optical switch suitable for two-way optical communication. SOLUTION: The multi-port optical switch has N ports (N: an integer of ≥3) and is so constituted that the ports can be connected to all other ports. Consequently, two-way communication between respective ports becomes possible. In an embodiment, a multi-port optical switch has N one-input, M-output optical switches (M: an integer of ≥N-1), which are interconnected through N-1 output ports. In another embodiment, a one-input, M-output optical switch comprises a plurality of two-input, two-output optical switches, and respective one-input, M-output optical switches are interconnected through cross-side output ports of the two-input, two-output optical switches. Consequently, an optical signal between ports passes through a cross port twice without fail to provide the one-input, M-output optical switch having a superior extinction ratio. COPYRIGHT: (C)2006,JPO&NCIPI

Multi-port optical switch

The present invention provides an optical functional circuit where a holographic wave propagation medium is applied and a circuit property is excellent such as small transmission loss and crosstalk. The optical functional circuit where a plurality of circuit elements are formed on a substrate includes the wave propagation medium for converting an optical path of a leakage light so that the leakage light that is not emitted from a predetermined output port of the circuit element is not coupled to a different circuit element. This wave propagation medium is constituted by an optical waveguide that is provided with a clad layer formed on the substrate and a core embedded in the clad layer, and a part of the optical waveguide is formed in accordance with a refractive index distribution which is multiple scattered.

Yohei Sakamaki

Papers

Optical delay interference circuit

Optical waveguide and its manufacturing method

Low-Loss Y-Branch Waveguides Designed by

Multi-port optical switch

Optical functional circuit