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An optical network is a communication system that uses light signals, instead of electronic ones, to send information between two or more points. The points could be computers in an office, large urban centers or even nations in the global telecommunications system. These include limited range local-area networks (LAN) or wide-area networks (WAN), which cross metropolitan and regional areas as well as long-distance national, international and transoceanic networks.
The basic building blocks of these networks are Fiber-optic cables-the so-called “pipes”-which carry signals from node to node, with switches directing them to their destination. It is a form of optical communication that relies on optical amplifiers, lasers or LEDs and wave division multiplexing (WDM) to transmit large quantities of data, generally across fiber-optic cables. Because it is capable of achieving extremely high bandwidth, it is an enabling technology for the Internet and telecommunication networks that transmit the vast majority of all human and machine-to-machine information.
With the massive growth of over-the-top applications, cloud computing, mobile devices and the need for consumers and employees to have constant access to their data and applications, optical networking solutions are rapidly being adopted by businesses across multiple industries as their bandwidth and distance requirements continue to grow.
Optical networks comprise optical transmitters and receivers, fiber optic cables, optical switches and other optical components. Point-to-point networks make permanent connections among two or more points so any pair of nodes can communicate with each other; point to multipoint networks broadcast the same signals simultaneously to many different nodes; switched networks like the telephone system include switches that make temporary connections among pairs of nodes.
Energy Efficiency
With network traffic demand continuously rising, the corresponding increase in power consumption will be of real concern for future technologies. While current optical networking techniques are already reducing power consumption, more power-reduction innovation are needed to fulfill future demand. The design of every modern optical data networks should now attempt to resolve challenging physical constraints associated with energy and capacity.
Therefore, the energy consumption in optical transmission systems is an increasingly important problem within our information society. In 2010, the Internet and its data centres consumed 870 Billion kWh — equivalent to the yearly output of 10 nuclear power reactors. If energy efficiency does not improve, and Internet traffic continues to grow at its current rate, then within eight years the Internet will consume more than twice the electricity produced globally today. In general, the energy needed to transport data will increase much more than the energy needed for its computation.
“The challenge lies in meeting that inevitable demand for capacity and performance, while keeping costs at a reasonable level and minimising the environmental impacts,” says Peter Andrekson, Professor of Photonics at the Department of Microtechnology and Nanoscience at Chalmers.
Making the fibre optic communications More Energy Efficient
One of the energy efficient technology is all optical networking. The goal of having a network entirely in the optical domain is to remove the intermediate electronics, the so-called electronic bottleneck, which tend to be more costly, more power consuming, less scalable, but much simpler in electronic format. Current techniques in long-distance systems with 100 Gb/s per wavelength require intensive electronic processing by the transmitter and receivers, which consume high power. Raising capacities and speeds in these systems will increase the power consumed correspondingly. Signal regeneration and wavelength conversion, are currently accomplished with electronic circuits that consume much power; however, as bitrate per wavelength increases, performing these functions in the optical domain starts to be more attractive, but remain challenging because of the complex optical modulation and processing formats required.
Two promising techniques essential to the prevalence of all-optical networking are optical burst switching (OBS) and optical packet switching (OPS). OBS involves the preparation and management of data bursts in networks. A control packet is transmitted shortly before a large data burst to schedule the required resources at the intermediate nodes. Scheduling bursts with OBS while maintaining high network utilization efficiency remains challenging. This technique could immensely contribute to energy-efficiency because it allows dynamic functionality, which could reduce the network’s total equipment energy consumption during periods of low traffic levels by a factor of 2-100. OPS involves replacing the electronic switching process including and buffers of current switching with optical alternatives. Buffers are needed during current switches to reduce the noise spikes during circuit switching. The size and design of optical buffers pose significant difficulty in the advancement of OPS in all-optical networks.
Researchers at Chalmers recently completed a 5-year research project looking at how to make fibre optic communications systems more energy efficient. The focus of this project was the analysis of the trade-off between energy consumption, cost, and performance, based on the optical and electrical hardware, signal processing algorithms, system level design, and, most importantly, the joint optimization thereof.
The outcome of this work will be a set of essential guidelines on how to design optical links and hardware with a given performance target, while minimizing the energy consumption. In addition, it will provide an understanding of the true bottlenecks in terms of energy consumption and point to areas where breakthroughs are needed.
We address both long-haul transport systems (where performance is most significant) and short-haul interconnects (where cost is also very important, since each link is not shared among many users). The project will have an impact in academia (sharing knowledge through high-quality research publications), industry (through close industry collaboration), and society (by enabling sustainable growth of IT), and will highlight Sweden’s commitment and leadership for a sustainable future.
We will develop novel power consumption models for electronics, optics, algorithms and emerging technologies, as well as demonstrating novel energy-efficient link building blocks. These will then come together in an activity that will perform the various trade-offs required to minimize the energy consumption of the given links. We aim at experimentally verifying full links that are 5 to 10 times more power efficient than today’s state-of-the-art. Finally we will verify our models in challenging demonstrator projects, which will vertically integrate the optic, electronic and algorithmic components.
Components of an fiber-optical networking system include: Fiber. Multi-mode or single-mode; Laser or LED light source; Multiplexer/demultiplexer, also called mux/demux, filter, or prism. These can include Optical Add/Drop Multiplexer (OADM) and Reconfigurable Optical Add/Drop Multiplexer (ROADM). Optical switch, to direct light between ports without an optical-electrical-optical conversion. Optical splitter, to send a signal down different fiber paths. Circulator, to tie in other components, such as an OADM.
Optical amplifier, and Wave division multiplexer.
Optical Fibers
The main driver of optical communication networks is advancement on optical fibers and optical components. An optical signal consists of a series of pulses produced by switching a laser beam off and on. Its speed depends on how fast the beam can be switched on and off, and how much the pulses spread in length during transmission, an effect called dispersion. The amount of dispersion depends on the type of fiber, the fiber length and the nature of the optical signal. The more dispersion, the more difficult it is to distinguish between adjacent pulses. With current technology, different types of fiber can be combined to reduce dispersion effects, allowing transmission at 10 gigabits per second for a few thousand kilometers. To achieve faster transmission speeds, researchers are exploring ways to actively compensate for dispersion.
Optical systems with advent the laser and optical fiber advances are driving optical communications, which support unlimited bandwidth and very low losses for long distance. Companies are racing to introduce new fiber technologies to enahance the speed to terrabit / sec and petabits/sec. CableLabs President and CEO Phil McKinney, conducting interviews at CES 2018, says the path forward to boost fiber capacity is by borrowing techniques from long-haul networks and getting rid of unnecessary overhead to increase speed. Coherent optics utilizes the ability to transmit multiple bits of information using light, including amplitude and phase modulation, instead of a simple binary on-off approach. Adding QPSK and QAM, plus polarization means you can move a lot more data onto a single strand of fiber.
Lasers
Light sources used for fiber optic transmission need to meet several criteria: they must be the right wavelength, be able to be modulated fast enough to transmit data and be efficiently coupled into fiber. Lasers have proven to be ideal light sources because of their high bandwidth capability and narrow spectral output. There are 3 main types of lasers in the market today for fiber optic transmitters – VCSELs, FP lasers and DFB lasers.
VCSELs: The vertical-cavity surface-emitting laser, or VCSEL is a type of semiconductor laser diode which emits laser beams perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from laser cavities in the middle of a chip.
Transceivers typically use 850nm wavelength VCSELs. VCSELs are cheap to make and are quite adequate for distances of around 500 meters or less. VCSELs have the advantage of low power consumption and high coupling efficiency with optical fibers.
FP Lasers: Single spatial mode lasers which can support multiple longitudinal modes are called Fabry-Perot (FP) lasers. In this device, two parallel ends of the semiconductor are cleaved along the crystal axis, creating reflective mirrors forming a Fabry-Perot laser cavity with the semiconductor as the gain medium. The characteristics of such devices are large output power, smaller divergence angle, narrow spectrum and high modulation rate. They are suitable for longer distances than VCSELs.Because FP lasers produce discrete wavelengths, they cannot support transmission over very long distances or transmission over WDM systems.
DFB lasers: DFB lasers incorporate a grating into one of the cladding layers surrounding the active layer of a laser diode. The grating reflects only a specific wavelength back into the cavity and allow others to pass through. It is essentially feeding back the desired wavelength into the cavity. Therefore, the feedback is “distributed,” and thus the name distributed feedback laser. DFBs are more expensive than FP lasers but ideal for long distance transmission.
University of Queensland Research, laser breakthrough
University of Queensland Research Fellow Dr Martin Plöschner from the School of Information Technology and Electrical Engineering (ITEE) said the laser light pulses relayed along the glass or plastic fibres travel at different speeds and can overlap, slowing down the process.
“Imagine yelling to a friend through a long concrete pipe,” Dr Carpenter said. “Your message will distort depending on how much the pipe echoes, and you’ll also have to wait for the echoes to die down from one message before you can send the next. “It’s a similar problem in large groups of computer servers, with the amount of echo dependent on the shape and colour of the lasers being launched into the optical fibre.” Measuring the properties of lasers is vital to making improvements, but there has been no method to fully capture this complexity.
They developed a tool that measures the output of vertical-cavity surface-emitting lasers (VCSELs) and allows the examination of the large amounts of data their light carries. “The system itself is about the size of a shoebox and is simply inserted into the path of the laser beam,” Dr Plöschner said. “It can tell us how the laser beam evolves in time and changes its shape and colour.
“Our tool will make it possible to identify the beam features that contribute to ‘pulse spreading’ in the optical link, which slows down data,” Dr Plöschner said. “Laser engineers can then design lasers without these rogue features, leading to optical links with higher speed and longer distance of operation. “And any tool that can facilitate faster data transfer over longer distances is helpful.”
The research has been published in Nature Communications.
Optical Amplifiers
The clearest optical fibers can transmit signals more than 100 kilometers without amplification-much farther than copper wires. When the signal must span a longer distance, it is passed through an optical amplifier, which multiplies the strength of the optical signal.
Optical amplification plays a key role in virtually all laser-based technologies such as optical communication, used for instance in data centers to communicate between servers and between continents through trans-oceanic fiber links, to ranging applications like coherent Frequency Modulated Continuous Wave (FMCW) LiDAR – an emerging technology that can detect and track objects farther, faster, and with greater precision than ever before.
Then, optical fiber amplifiers were invented in 1989 for high expanded transmission distance while operating over the C-band, with low cost and very low noise. Electronic repeaters were replaced by optical amplifiers, allowing to be the signal in the optical domain. The fourth generation of lightwave systems using optical amplifiers and the lithium niobate external modulator, transmission distances of nearly 1000 km could be reacted at 2.5 and 10 Gbps.
The most widely used optical amplifiers are fibers doped with atoms of erbium, a rare-earth element that absorbs light energy from an external pump laser. The erbium atoms then release that energy to amplify weak optical signals across the entire band of wavelengths that the laser transmits. With careful control, a string of dozens of optical fiber amplifiers can transmit signals thousands of kilometers across the ocean.
Wavelength Division Multiplexing (WDM)
A single fiber can transmit many separate signals simultaneously at different wavelengths of light, a technique called wavelength-division multiplexing. Wavelength Division Multiplexing (WDM) is a widespread technology deployed for the internet to reduce traffic induced by increasing number of applications. Along advent a WDM in 1992, the data rates increased for network throughput, and bit rate of 10 Tbps was achieved in 2001.
WDM exploits a multiplexing technique similar to FDM in the radio communications in order to increase the network capacity. This system combines (muxing) multiple optical signals so that they can be transported as a group, over a single fiber, and separates (demuxing) the wavelength channels in the receiving end. Like the number of radio stations, the maximum number of optical channels is limited by the slice of spectrum used for each channel and the total amount of spectrum available. Devices called “demultiplexers” separate the optical channels and distribute them to separate optical receivers. Demultiplexers slice the spectrum into very narrow chunks, isolating each optical channel from adjacent ones.
There are two primary types of WDM technology: Coarse wavelength division multiplexing (CWDM), which typically supports up to eight wavelength channels transmitted through a ber at the same time. Dense wavelength division multiplexing (DWDM), which supports up to 96 (and theoretically more) simultaneous wavelengths channels, each carrying separate data streams on an individual ber
The DWDM systems from the beginning of 1995 could transmit 2-16 wavelength channels at the same time. The early DWDM transported optical signals point to point. In the late 1990’s, advances in the fiber optic technology provided the basis to increase the DWDM capacity to 40-120 wavelengths and extended the transmission distances to 1000 km. In 2017, CableLabs demonstrated the ability to deliver 256 Gbps over 80 kilometers on a single wavelength with 16 QAM “using electronics bought on eBay,” said McKinney. By multiplexing eight wavelengths, speeds of 2048 Gbps — 2 Tbps — were achieved. Multiplying the number of optical channels by the data rate on each optical channel gives total transmission capacity of a fiber.
Laboratory experiments have transmitted more than 10 trillion bits (10 terabits) per second through more than 100 kilometers of fiber. However, commercial transmission rates typically do not exceed a few hundred gigabits per second. By tweaking up encoding and modulation, 4 Tbps should be reachable in the near term while speeds of up to 50 Tbps are possible.
Achieving these high data rates and multiple channels requires sophisticated components. Semiconductor lasers-which generate the light pulses used in almost all fiber optic communications systems-must emit only a very narrow range of wavelengths to limit dispersion. Fibers also are designed to limit dispersion. In addition, advances in the optical waveguide technology made it possible to enhance the capability. Improvements in both EDFA and fiber optic technology contributed to the significant increase in the number of wavelengths that could be transported. Erbium-doped amplifiers were the most commonly used means of supporting dense wavelength division multiplexing systems. In fact, EDFAs were so prevalent that, as WDM became the technology of choice in the optical networks, the erbium amplifier became “the optical amplifier of choice for WDM applications.”
As a result of this ability to transport large traffic volumes, WDM has become the common basis of nearly every global communication network and thus, a foundation of the Internet today. Demand for bandwidth is driven primarily by Internet Protocol (IP) traffic from video services, telemedicine, social networking, mobile phone use and cloud-based computing. At the same time, machine-to-machine, IoT and scientific community traffic require support for the large-scale exchange of data files. According to the Cisco Visual Networking Index, global IP traffic will be more than 150,700 Gbits per second in 2022. Of that, video content will equal 82% of all IP traffic, all transmitted by optical networking
Optical Switches
One challenge to optical networking is how to switch light signals. When a signal arrives at its destination, it must be separated from the rest of the channels. To drop one signal at an intermediate point, an optical filter separates the proper wavelength from the rest. Equipment at that point may also add a new signal to the now unoccupied wavelength.
Optical switches may operate on a single wavelength, or on all the wavelengths transmitted through a fiber. A fixed filter, like the one described above, could be replaced by a switch that selects one of several filters to divert the desired wavelength to the intermediate point. A third kind of switch separates the wavelengths into separate beams, and a moving mirror directs one or more of the wavelengths in a different direction. Other optical switches simultaneously switch all wavelengths passing through a fiber; one example is a mirror at the fiber output that could tilt between two different positions to reroute all optical channels in case of a fiber break.
The preceding examples are called “all-optical” switches because they operate on light signals. A different class of switches convert optical signals into an electronic form which can be switched electronically; the resulting electronic signal then feeds into an optical transmitter to generate a new optical signal. These are called opto-electro-optical switches.
As the technology continues to advance, optical networks will need to convert signals from one wavelength to another. This can be done now with opto-electro-optical wavelength converters that convert the input optical signal into electronic form to drive a transmitter at the second wavelength. All-optical wavelength converters have been demonstrated in the laboratory, but are not yet used in practical systems. Laser sources that can be tuned to many different wavelengths also will be needed; several types have been demonstrated, and some are in commercial production.
The Network System Research Institute at the National Institute of Information and Communications Technology (NICT; Tokyo, Japan) has developed and demonstrated the first large-scale optical switching testbed capable of handling 1 petabit per second (Pbit/s) optical signals; this data rate is equivalent to sending 8K video to 10 million people simultaneously.
Smart, error correcting chips:
Currently, some of the most energy-intensive components are error-correction data chips, which are used in optical systems to compensate for noise and interference. The Chalmers researchers have now succeeded in designing these chips with optimised circuits.
The improved data chips were designed by Chalmers and manufactured in Grenoble in France. The Chalmers researchers subsequently verified the chips’ performance and measured the energy usage, which was just a tenth of current error-correcting chips.
At an energy transfer speed of 1 terabyte per second (1 terabyte = 1 trillion bits) the researchers demonstrated that the chip drew less energy than 2 picojoules (1 picojoule = 1 trillionth of a joule) per bit. This equates to a power consumption of 2 Watts at this data rate. Comparatively, the current energy usage at such high transfer speeds is around 50 picojoules per bit, around 50 Watts.
Twisted light technique
Research from DTU, Fujikura & NTT is notable in that the team was able to reduce the power consumption of the optics to around 5% compared with more mainstream techniques, which could lead to a new generation of very power efficient optic components.
Research conducted by the RMIT University, Melbourne, Australia, have developed a nanophotonic device that has achieved a 100 fold increase in current attainable fiber optic speeds by using a twisted-light technique. This technique carries data on light waves that have been twisted into a spiral form, to increase the optic cable capacity further, this technique is known as orbital angular momentum (OAM). The nanophotonic device uses ultra thin topological nanosheets to measure a fraction of a millimeter of twisted light, the nano-electronic device is embedded within a connector smaller than the size of a USB connector, it fits easily at the end of an optical fiber cable. The device can also be used to receive quantum information sent via twisted light, it is likely to be used in a new range of quantum communication and quantum computing research.
Network energy optimization
Energy savings can also be made through controlling fibre optic communications at the network level. By mathematically modelling the energy consumption in different network resources, data traffic can be controlled and directed so that the resources are utilised optimally. This is especially valuable if traffic varies over time, as is the case in most networks. For this, the researchers developed an optimisation algorithm which can reduce network energy consumption by up to 70%.
The recipe for these successes has been the broad approach of the project, with scientists from three different research areas collaborating to find the most energy-saving overall solution possible, without sacrificing system performance. These research breakthroughs offer great potential for making the internet of the future considerably more energy-efficient. Several scientific articles have been published in the three research disciplines of optical hardware, electronics systems and communication networks.
Standards and protocols
Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) have evolved as the most commonly used protocols for optical networks. The Optical Transport Network (OTN) protocol was developed by the International Telecommunication Union as a successor and allows interoperability across the network as described by Recommendation G.709. Both protocols allow for delivery of a variety of protocols such as Asynchronous Transfer Mode (ATM), Ethernet, TCP/IP and others.
References and resources also include:
https://www.technologyreview.com/2002/01/22/235271/overview-of-optical-networking
https://www.eurekalert.org/pub_releases/2020-10/au-ndo101320.php
https://www.photonics.com/Articles/Nanostructure_Allows_Large_Incidence_Angles_in/a66342
