Space-division multiplexing (SDM) has recently attracted great attention as a promising technology to further improve the transmission capacity and spectral efficiency. Very recently, SDM employing twisted lights, also known as orbital angular momentum (OAM) carrying lights, provides an alternative approach to increasing the transmission capacity and spectral efficiency of optical communications.
Scientists have discovered that certain beams, traveling through space with a spiraling pattern suggestive of a corkscrew, carry a form of momentum called orbital angular momentum (OAM). The phenomenon of OAM has been exploited in enhancing the resolving power of microscope, high-capacity data transmission and to increase the efficiency of quantum cryptography systems.
Orbital angular momentum (OAM)
Spin angular momentum is a familiar property of light, being manifest in the polarization of the light. But In 1992, physicist Les Allen, working with and colleagues at Leiden University, in the Netherlands, pointed out that a certain spiraling beam carries another form of angular momentum—orbital angular momentum. If light with spin angular momentum is like a spinning planet, the physical analogue of OAM light could be a planet orbiting the sun.
This results in a twisting of a beam’s wavefront around its propagation axis so that the light takes on a spiral shape with zero intensity at its core. A beam can in principle have any amount of twistedness, with greater twist meaning that the wavefront rotates more quickly. The electric field spirals around like a corkscrew; hence, twisted light. The quantum number describes how sharp the spiral is, while the sign reveals the direction of the spiral.
The orbital angular momentum is the wavefront of a beam of light that’s coiling around its propagation axis. Generally the different parts of a light beam all have the same phase. However, in the case of a helical wave, the sort that carries OAM, the miniwaves in the cross section of the beam aren’t uniform. Instead, the phase of each miniwave depends on its angular location around the center of the beam.
Scientists can ‘twist’ photons – individual particles of light – by passing them through a special type of hologram, similar to that on a credit card, giving the photons a twist known as optical angular momentum. While conventional digital communications use photons as ones and zeroes to carry information, the number of intertwined twists in the photons allows them to carry additional data – something akin to adding letters alongside the ones and zeroes.
Physicists are using OAM to develop a range of new technologies. Researchers at the University of Rochester and their collaborators have developed a way to transfer 2.05 bits per photon by using “twisted light.” This remarkable achievement is possible because the researchers used the orbital angular momentum of the photons to encode information, rather than the more commonly used polarization of light. The new approach doubles the 1 bit per photon that is possible with current systems that rely on light polarization and could help increase the efficiency of quantum cryptography systems.
Hardware that can transmit and receive even a few such OAM beams could dramatically boost the capacity of optical and radio transmissions without placing any more demands on the crowded electromagnetic spectrum than we do today. Researchers from University of Southern California, in Los Angeles, and others have performed experiments to test this idea, and they worked just as the theory predicted.
OAM waves with different “twists” don’t interfere with one another. That means they can be overlaid one on top of another to carry a theoretically unlimited number of different data streams at the same time. The ability of twisted photons to carry additional information means that optical angular momentum (OAM) has the potential to create much higher-bandwidth communications technology.
The number of data channels in a fibre-optic cable can be boosted by sending down light with multiple values of OAM, just as it can be enhanced via greater frequency bandwidth. OAM light can a “twist” that, depending on where the beam hits, can cause a small object to rotate or move in an orbit around the center of the beam. Beams with OAM can also increase the resolution of microscopy and be used to manipulate microscopic objects such as nanoparticles, quantum dots and even living cells.
OAM transmission really is a novel and powerful technology, one that could allow us to transmit much more information along wireless connections and dramatically speed up parts of the networks that underpin the Internet. The technological challenge is finding good ways to harness OAM, writes Alan E. Willner, Researcher from USC in Spectrum.
Twist in time
However, until now all light beams have a had a constant twist. In the latest work, scientists at the University of Salamanca and the Institute of Photonic Sciences (ICFO) in Barcelona working with colleagues at JILA in Colorado have shown it is possible to vary the twist in time – either speeding up or slowing down the rotation of the wavefront – by creating a light pulse from high-frequency harmonics. The “self-torque” generates this variation in momentum.
“With mechanical systems you need external forces,” says Salamanca team leader Carlos Hernández-García. “But in these light beams we have the torque without the presence of external forces.” High-frequency harmonics can be created by firing intense infrared laser pulses into a gas. This ionizes the gas and free electrons stimulate ultraviolet emissions from the gas after being accelerated by the laser’s strong electric field. The emissions occur over a wide range of harmonics, resulting in light with frequencies hundreds or even thousands of times higher than that of the laser pulses.
Drawing up new models of harmonic generation, Hernández-García, Laura Rego and colleagues in Salamanca realized it should be possible to use this technique to generate self-torque. What was needed, they reasoned, were two infrared pulses with different amounts of OAM (or different values of their topological charge l) separated by a very short time interval. They reckoned that by superimposing these pulses and passing them through a gas, the l values of the resulting harmonics should vary in time.
The scheme relies on the fact that high-frequency harmonics of pulses with OAM will have proportionally higher l values than the pulses themselves (given that higher frequencies force the wavefront to twist more quickly). Because the 17th harmonic, say, of an l=1 and l=2 pulse will have l values of 17 and 34 respectively, that harmonic will have its l value vary stepwise from 17 to 34 when an l=1 pulse is merged with a trailing (and as such partially overlapping) l=2 pulse.
“As the ratio between the two pulses changes in time,” explains Hernández-García, “the OAM value of the harmonic, following conservation of momentum, also changes in time”. These ideas were then realized in the lab by Kevin Dorney and colleagues at JILA, who focused superimposed l=1 and l=2 infrared pulses, each only around 50 fs long, on to a jet of argon. One of the biggest experimental challenges was being able to observe the effects of the self-torque, given that no available technique can resolve variations in OAM over femtosecond timescales. To do that they looked for, and found, a tell-tale sign in the spatial profile of the ultraviolet laser pulse that had been predicted by the Salamanca group – a continuous change in frequency, or “chirp”, across the doughnut-shaped profile.
“The agreement with our theoretical predictions was really excellent, and that allowed us to say that the beams were generated with self-torque,” says Hernández-García. As to how the work could be applied practically, Hernández-García is reluctant to say too much – stating only that it might benefit applications that “require the ultrafast recording of information”. But Dorney is more specific, suggesting the research might enhance studies of “chiral molecules at the nanoscale” or improve understanding of the processes taking place inside the materials used to make smart phones and hard
Ben McMorran of the University of Oregon praises the researchers for their “mastery of several advanced technologies in optics”, reckoning that their work could help “control and probe the movements and orientations of electrons in materials”. But he questions the term “self-torque”, arguing that the laser pulses do not themselves generate torque but instead pass it on. “Think of these pulses not as a motor but a driveshaft,” he says.
Photonic integration and component ecosystem
Back in the 1980s, many WDM optical communications experiments were performed on large optical tables using expensive devices that were often either not meant for communications or one-off, custom-built components. The development of cost-effective, integrated devices was deemed important for WDM to be deployed widely.
The same could be said about OAM-multiplexed optical communications. Many systems experiments were performed on large optical tables using devices that were not originally meant for MDM optical communications. For the future of mode-multiplexing to thrive, R&D in integrated devices would seem to be of signaficants. Researchers have advocated of photonic integrated circuits for OAM-based optical communications, :
Nature Photonics, Interview 2012 : “Schemes for the generation, multiplexing and demultiplexing of OAM beams using superior SLMs or integrated devices would help to improve the maximum number of available OAM beams.”
“As was the case for many previous advances in optical communications, the future of OAM deployment would greatly benefit from advances in the enabling devices and subsystems (e.g., transmitters, (de)multiplexers, and receivers). Particularly with regard to integration, this represents significant opportunity to reduce cost and size and to also increase performance.”
System development would benefit from a full ecosystem of devices, including the above-mentioned components as well as: (i) amplifiers that uniformly provide gain to different modes, and (ii) waveguides that efficiently guide OAM modes with little modal coupling.
Key desirable features for these integrated devices include: low insertion loss, high amplifier gain, uniform performance for different modes, high modal purity, low modal coupling and intermodal crosstalk, high efficiency for mode conversion, high dynamic range, small size, large wavelength range, and accommodation of high numbers of modes. Other functions that could be advantageous include: (i) fast tunability and reconfigurability covering a range of OAM modes, and (ii) integration of an OAM communication system-on-a-chip that incorporates a full transceiver.
Finally, experiments commonly use SLMs to tailor the beam structure, but commercially available SLMs are generally bulky, expensive, and slow. Our favorite wishlist device would be the creation of a “super” SLM that has low cost, small footprint, large dynamic range in amplitude and phase, wide spectral range, high modal purity, fast tunability (the faster the better, to even encode data bits), and high resolution.
Different frequency ranges
Separate from using optical beams, free-space communication links can take advantage of mode multiplexing in many other carrier-wave-frequency ranges to increase system capacity. For example, OAM can be manifest in many types of electromagnetic and mechanical waves, and interesting reports have explored the use of OAM in millimeter, acoustic, and THz waves
From a system designer’s perspective, there tends to be a trade-off in different frequency ranges:
Divergence: Lower frequencies have much higher beam divergence, exacerbating the problem of collecting enough of the beam to recover the data channels.
Interaction with Matter: Lower frequencies tend to have much lower interaction with matter, such that radio waves are less affected by atmospheric-turbulence-induced modal coupling than optical waves.
There are exciting developments in the millimeter-wave application space, for which industrial labs are increasingly engaging in R&D in order to significantly increase the potential capacity of fronthaul and backhaul links. Advances in this area include the use of RF antenna arrays that are fabricated on printed-circuit boards. For example, a multiantenna element ring can emit a millimeter-wave OAM beam by selectively exciting different antenna elements with a differential phase delay . Moreover, multiple concentric rings can be fabricated, thereby emitting a larger number of multiplexed OAM beams
This world-first nanophotonic device, unveiled in Oct 2018 in Nature Communications, encodes more data and processes it much faster than conventional fiber optics by using a special form of ‘twisted’ light. Dr Haoran Ren from RMIT’s School of Science, who was co-lead author of the paper, said the tiny nanophotonic device they have built for reading twisted light is the missing key required to unlock super-fast, ultra-broadband communications.
“Our miniature OAM nano-electronic detector is designed to separate different OAM light states in a continuous order and to decode the information carried by twisted light,” Ren said. “To do this previously would require a machine the size of a table, which is completely impractical for telecommunications. By using ultrathin topological nanosheets measuring a fraction of a millimeter, our invention does this job better and fits on the end of an optical fiber.” LAIN Director and Associate Deputy Vice-Chancellor for Research Innovation and Entrepreneurship at RMIT, Professor Min Gu, said the materials used in the device were compatible with silicon-based materials use in most technology, making it easy to scale up for industry applications.
“It fits the scale of existing fiber technology and could be applied to increase the bandwidth, or potentially the processing speed, of that fiber by over 100 times within the next couple of years. This easy scalability and the massive impact it will have on telecommunications is what’s so exciting.”
Gu said the detector can also be used to receive quantum information sent via twisting light, meaning it could have applications in a whole range of cutting edge quantum communications and quantum computing research. “Our nano-electronic device will unlock the full potential of twisted light for future optical and quantum communications,” Gu said.
Metasurface laser produces super-twisted light
A new metasurface laser can produce light in any desired angular momentum state, including highly chiral or “twisted” light capable of manipulating physical objects. According to its developers at the University of the Witwatersrand (Wits) in South Africa and Harvard University in the US, this tunable, high-angular momentum light source could also be used to encode information in optical communications.
The angular momentum of light is the sum of two independent components: spin angular momentum (SAM) and orbital angular momentum (OAM). SAM is associated with circularly polarized light and arises when the electric and magnetic field vectors of light rotate over the course of a wavelength. Because SAM can have only two values – right or left circular polarization – its applications are relatively limited. OAM, on the other hand, results from the rotation of a light wave’s phase, and can take on any value. This variability makes OAM useful for a wider range of applications, including “optical spanners” – devices that trap and rotate tiny particles using light – and transferring data through optical fibres without crosstalk (multiplexing), to name but two examples.
Challenges of producing OAM states
The flexible nature of OAM means that a beam of light can, in principle, carry an unlimited amount of angular momentum. In practice, however, dialling up a desired OAM state is far from easy, explains study co-leader Andrew Forbes from Wits’ School of Physics. While various techniques exist, their efficiency – the proportion of light converted into the desired state – is limited. Alternatively, a device known as a q-plate can transform SAM into OAM with up to 100% efficiency, but it only works with pure right or left circularly polarized light. Because real light beams often have intermediate (elliptical) polarization, this is a significant drawback, since adding a fixed amount of OAM to one spin state and an equal and opposite amount to the other produces a net angular momentum of zero.
New forms of chiral light and the highest AM
The new device overcomes these obstacles by incorporating a metasurface – an artificially engineered nanostructure that interacts with light in unusual ways – into the laser cavity. The design of this metasurface builds on the Harvard group’s previous work and consists of rectangular amorphous pillars of TiO2 just 600 nm high. These nanopillars are separated by distances shorter than the wavelength of light being modulated, and they act like optical antennas – introducing spatially varying phase delays in the light rays that pass through them and moulding the light beam according to the desired profile. “In our experiment, we pass light through the metasurface many times, giving it a new twist in its phase each time we do so, while controlling the polarization of the light at the same time,” Forbes explains.
The result is a device that produces two output beams with OAM values that differ by as much as 90 units, resulting in a large non-zero total angular momentum. According to Forbes, this is the first laser that can produce such highly chiral light in any desired angular momentum state. “One of our demonstrations was a laser beam with OAMs of 10 and 100 in the same beam (with horizontal and vertical polarizations respectively),” he tells Physics World. “The prior record was just +10 and -10 (and therefore zero total AM).”
According to Federico Capasso, the study’s other co-leader and a professor of applied physics at Harvard, the use of metasurfaces was “the determining factor” in achieving a record-high optical angular momentum, L, of 100. “Alternative technologies such as q-plates and spatial light modulators (SLMs) have not even come close to these values of L,” he says. “What is more, the design limitations and fabrication constraints of those technologies can’t give the arbitrary wavefront control provided by metasurfaces, of which the non-symmetric vector vortex beams described in this paper are an outstanding example.”
Dramatically reducing light losses
According to Harvard’s Yao-Wei Huang, who constructed the metasurface used in the laser, the new design “demonstrates the highly effective coupling between arbitrary spin (a linearly, circularly, or any elliptically polarized state) and orbit (symmetric or non-symmetric helicity) of light in a compact planar structure”. Forbes adds that the device can couple non-symmetric OAM to linearly polarized states, rather than being limited to symmetric OAM and circularly polarized states, as q-plates are. “This may seem like a minor technical detail, but it means we can halve the number of elements inside the laser, so dramatically reducing light losses and allowing us to reach OAM values of 100 (a x10 advance over the prior state-of-the-art from such lasers),” he explains.
According to Forbes, another interesting feature is that the beams carrying 10 and 100 units of angular momentum are significantly different in size when they come out of the laser. When they travel around the cavity, however, they converge to a similar shape and size, where they experience optical gain. This allows for a coherent mode – a tell-tale sign of lasing – even though the actual beams appear spatially separated.
“We can use this type of light to optically drive gears in situations where physical mechanical systems would not work, such as in microfluidic systems to drive flow,” he explains. “Such systems could be used to make miniature lab-on-a-chip devices in which medicine would be performed on a single chip rather than in large experimental apparatus in the lab.” The laser, which is described in Nature Photonics, could also be made bigger by increasing the size of the metasurface and the gain volume to produce a high-power bulk device. “In both these cases, the lasing mode wouldn’t require any intra-cavity elements other than the metasurface itself,” Forbes says.
Mitigation of modal coupling and channel crosstalk
A key issue in almost any MDM communication system is dealing with intermodal power coupling and deleterious inter-data-channel crosstalk. There are many causes of modal coupling and crosstalk, including the following for free-space OAM-multiplexed optical communication links:
Turbulence: Atmospheric turbulence can cause a phase differential at different cross-sectional locations of a propagating beam. Given this phase change distribution in a changing environment, power can couple from the intended mode into other modes dynamically (e.g., perhaps on the order of milliseconds).
Misalignment: Misalignment between the transmitter and receiver means that the receiver aperture is not coaxial with the incoming OAM beams. In order to operate an OAM-multiplexed link, one needs to know the mode that is being transmitted. A receiver aperture that captures power around the center of the beam will recover the full azimuthal phase change and know which l mode was transmitted. However, a limited-size receiver aperture that is off-axis will not recover the full phase change and inadvertently “think” that some power resides in other modes.
Divergence: Free-space beams of higher OAM orders diverge faster than lower-order beams, thus making it difficult to fully capture the higher-order OAM beam at a limited-sized receiver aperture. Power loss obviously occurs if the beam power is not fully captured, but even modal coupling can occur due to the truncation of the beam’s radial profile. This truncation can result in power being coupled to higher-order p modes.
There are several approaches to potentially mitigate coupling and crosstalk in free-space OAM-multiplexed systems, including
Electrical digital signal processing (DSP): Crosstalk due to modal coupling has many similarities to crosstalk that occurs in multiple-transmitter-multiple-receiver (i.e., multiple-input multiple-output, MIMO) radio systems. Multiple optical modes are similar to parallel radio frequency (RF) beams that experience crosstalk. Similar to electronic DSP that can undo much of the crosstalk in MIMO RF systems, these DSP approaches could also be used for mitigating OAM modal crosstalk .
Adaptive optics: Adaptive optics, such as by using digital micromirrors, spatial light modulators (SLMs) or multi-plane-light-converters (MPLCs), can mitigate modal crosstalk. For example, if atmospheric turbulence causes a certain phase distortion on an optical beam, an SLM at the receiver can induce an inverse phase function to partially undo the effects of turbulence. Typically, there could be a feedback loop, such that a data or probe beam is being monitored for dynamic changes and the new phase function is fed to an SLM.
Modifying transmitted beams: The modal structure of the transmitted beams themselves can be modified. In this approach: (a) the medium is probed by taking power measurements and determining the system modal coupling and channel crosstalk matrix, and (b) transmitting each beam with a combination of modes that represent the “inverse matrix”, such that the received data channels would have little crosstalk
Researchers have introduced a smart quantum technology for the spatial mode correction of single photons, reported in March 2021
Researchers from Louisiana State University have introduced a smart quantum technology for the spatial mode correction of single photons. In a paper featured on the cover of the March 2021 issue of Advanced Quantum Technologies, the authors exploit the self-learning and self-evolving features of artificial neural networks to correct the distorted spatial profile of single photons.
The authors, PhD candidate Narayan Bhusal, postdoctoral researcher Chenglong You, graduate student Mingyuan Hong, undergraduate student Joshua Fabre, and Assistant Professor Omar S. Magaña?Loaiza of LSU — together with collaborators Sanjaya Lohani, Erin M. Knutson, and Ryan T. Glasser of Tulane University and Pengcheng Zhao of Qingdao University of Science and Technology — report on the potential of artificial intelligence to correct spatial modes at the single-photon level.
“The random phase distortion is one of the biggest challenges in using spatial modes of light in a wide variety of quantum technologies, such as quantum communication, quantum cryptography, and quantum sensing,” said Bhusal. “In this paper, we use artificial neurons to correct distorted spatial modes of light at the single-photon level. Our method is remarkably effective and time-efficient compared to conventional techniques. This is an exciting development for the future of free-space quantum technologies.”
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