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

A novel technology for simultaneous WDM signal monitoring is presented based on ultrafast field sampling. Its flexibility regarding channel allocation and/or channel bandwidth is unique and attractive, as is its ultrafast nature. Two- and four-channel WDM signals, with the total bandwidth of 800 GHz, are successfully discriminated and observed with the proposed scheme.

Channel-allocation-adaptive WDM signal observation based on sequential ultrafast field sampling

PROBLEM TO BE SOLVED: To provide 90° optical hybrid circuit that attains wavelength non-dependence.SOLUTION: The 90° optical hybrid circuit includes: a first arm waveguide and a second arm waveguide connected to a first optical splitter; a third arm waveguide and a fourth arm waveguide connected to a second optical splitter; a first optical coupler connected to the first arm waveguide and the third arm waveguide; a second optical coupler connected to the second arm waveguide and the fourth arm waveguide; and a phase shift mechanism configured, so that the absolute value of the sum θ of a phase difference of light transmitted from the first arm waveguide and the second arm waveguide and the phase difference in the light transmitted from the third arm waveguide and the fourth arm waveguide becomes 90+360m degrees, if m is an integer for wavelength λ=λlying within a wavelength band to be used, so that the absolute value of dθ/dλ in λ=λbecomes a minimum.

90° optical hybrid circuit

We review the configuration and performance of two optical modulators that employ the silica-PLC and LN phase modulator assembly technique. One is a 43-Gbit/s FSK modulator, and the other is an 86-Gbit/s DQPSK modulator.

High-speed optical functional modulators using hybrid assembly technique with silica-based planar lightwave circuits and LiNbO3 devices

We report experimental results for small-angle waveguide crossings designed with our recently proposed wavefront matching method. We successfully obtained lower loss and crosstalk values than conventional straight waveguide crossings. (2 pages)

http://ieeexplore.ieee.org/iel5/10509/33301/01576180.pdf

Low loss and low crosstalk waveguide crossings with small angles designed by wavefront matching method

This paper describes a signal quality monitoring technique for wavelength division multiplexing (WDM) signals that uses an ultrafast field sampling approach, where the observed signal is parallelly sampled with N channel coherent mixers that include a delay of nT , where n is the channel number and T is the unit delay. Previously, we demonstrated that the technique was able to monitor ON-OFF Keying WDM signals with the aid of a digital finite impulse response filter. In this paper we apply the technique to a dual-channel sampling configuration to monitor differential phase-shift keying (DPSK) WDM signals. We show that the dual 16-branch serial sampling of the total field allows us to observe an 8-ch 25 Gb/s DPSK signal with a total data rate of 200 Gb/s. In addition, the measurement result is comparable to the result obtained by using an optical filter. We also confirm that the technique is capable of monitoring the optical signal to noise ratio (OSNR) up to over 20 dB. Moreover, by adaptively changing the filter profile, we simulate the dependence of the OSNR on the filter profile. In this paper, the technique uses an equivalent sampling approach with a sampling pulse that has a repetition frequency (sampling rate) as low as 10 MHz, so the use of the technology is limited to monitoring the signal. The technique can be used to receive an actual data transmission if the sampling rate is 1/NT, where N is the number of channels.

Simultaneous Dense Differential Phase-Shift Keying Wavelength Division Multiplexing Signal Quality Observation Based on Sequential Ultrafast Field Sampling

Yohei Sakamaki

Papers

Channel-allocation-adaptive WDM signal observation based on sequential ultrafast field sampling

90° optical hybrid circuit

High-speed optical functional modulators using hybrid assembly technique with silica-based planar lightwave circuits and LiNbO3 devices

Low loss and low crosstalk waveguide crossings with small angles designed by wavefront matching method

Simultaneous Dense Differential Phase-Shift Keying Wavelength Division Multiplexing Signal Quality Observation Based on Sequential Ultrafast Field Sampling