Twisted light can provide 100-times faster fiber capacity and secure wireless data transmission to moving platforms like UAVs

Rajesh Uppal September 10, 2021 Comm. & NW, Photonics Comments Off on Twisted light can provide 100-times faster fiber capacity and secure wireless data transmission to moving platforms like UAVs 573 Views

Broadband fiber-optics carry information on pulses of light, at the speed of light, through optical fibers. But the way the light is encoded at one end and processed at the other affects data speeds. Basically, light or any other electromagnetic radiation has energy defined by its frequency and momentum defined by its wavelength. Current state-of-the-art fiber-optic communications, use only a fraction of light’s actual capacity by carrying data on the colour spectrum. Rising demands of data traffic of last two decades have given rise to new communication technologies that can carry more information.

The traditional solutions have been based on different advanced multi-level modulation formats such as m-ary phase-shift keying (m-PSK) and m-ary quadrature amplitude modulation (m-QAM) as well as various multiplexing techniques such as wavelength-division multiplexing (WDM), orthogonal frequency-division multiplexing (OFDM), time-division multiplexing (TDM) and polarization-division multiplexing (PDM).

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.

Researchers have demonstrated that four light beams with different values of orbital angular momentum and encoded with 42.8 × 4 Gbit s−1 quadrature amplitude modulation (16-QAM) signals can be multiplexed and demultiplexed, allowing a 1.37 Tbit s−1 aggregated rate and 25.6 bit s−1 Hz−1 spectral efficiency when combined with polarization multiplexing.

In another paper researchers have demonstrated bidirectional transmission systems using twisted lights multiplexing in fiber by exploiting twisted lights multiplexing. Researchers have experimentally demonstrated a full-duplex data transmission link using twisted lights multiplexing over 1.1-km OAM fiber. The downlink and uplink transmit OAM+1 and OAM−1 modes carrying 20-Gbit/s quadrature phase-shift keying (QPSK) signals, respectively.

Mode-division multiplexing (MDM), a subset of space-division multiplexing

This orthogonality is of crucial benefit for a communications engineer. It implies that multiple independent data-carrying optical beams can be multiplexed and simultaneously transmitted in either free-space or fiber, thereby multiplying the system data capacity by the total number of beams. Moreover, since all the beams are in the same frequency band, the system spectral efficiency (i.e., bits/s/Hz) is also increased. These multiplexed orthogonal OAM beams are a form of mode-division multiplexing (MDM), a subset of space-division multiplexing

(A) Multiple OAM beams are coaxially transmitted through free space. (B) Each orthogonal OAM beam carries an independent data stream.

MDM has similarities to wavelength-division multiplexing (WDM), in which multiple independent data-carrying optical beams of different wavelengths can be multiplexed and simultaneously transmitted. WDM revolutionized optical communication systems and is ubiquitously deployed worldwide. Importantly, MDM is generally compatible with and can complement WDM, such that each of many wavelengths can contain many orthogonal structured beams and thus dramatically increase data capacity.

Optical fiber transmission

MDM can be achieved in both free-space and fiber, with much of the transmitter and receiver technology being similar. However, the channel medium is different, which gives rise to the following distinctions:

(a)
There is no beam divergence in light-guiding fiber.

(b)
Fiber has various kinds of inhomogeneities, and coupling can occur among modes within a specific mode group or between mode groups, thereby creating deleterious interchannel crosstalk; typically, intramodal group crosstalk is higher than intermodal group crosstalk.

The excitement around using MDM for capacity increase originally occurred primarily in the fiber transmission world, especially in research laboratories. There was much important work using LP modes as the modal set in fiber. However, since there was significant modal crosstalk when propagating through conventional-central-core few-mode fiber, MIMO-like DSP was used with impressive results to mitigate crosstalk.

OAM has also been used as the modal basis set for fiber transmission, both for central-core and ring-core few-mode fibers. Importantly, the modal coupling itself can be reduced in the optical domain by utilizing specialty fiber that makes the propagation constants of different modes quite different, thus reducing intermodal coupling. Such fibers include ring-core and elliptical-core fibers, and 10’s of modes with low crosstalk have been demonstrated. These specialty fibers have produced exciting results, but they are structurally different than conventional fiber and thus require a little more resolve in order for them to be widely adopted.

Free-space links

As compared to RF links, optics in general can provide: (a) more bandwidth and higher data capacity due to the higher carrier wave frequency, and (b) better beam directionality and lower divergence, thus making eavesdropping more difficult. When incorporating MDM using OAM multiplexing, such optical links can potentially achieve capacity enhancement and increased difficulty to eavesdropping. This lower probability of intercept stems from the issue that any misalignment causes intermodal coupling, such that it is extremely difficult for an off-axis eavesdropper to recover the signals, and even an on-axis eavesdropper would need to know the modal properties in order to recover the data, again fairly difficult. In addition, these free-space applications share some common desirable characteristics, including: (1) low size, weight and power (SWaP), which can be alleviated by advances in integrated OAM devices; and (2) accurate pointing, acquisition and tracking (PAT) systems, which helps limit modal coupling and crosstalk

These advantages have generated interest in free-space MDM communications in the following scenarios:

(i)
Atmosphere: OAM multiplexing can potentially benefit communication to: (a) unmanned aerial vehicles, for which distances may be relatively short range and a key challenge is to miniaturize the optical hardware, and (b) airplanes and other flying platforms, for which distances may require turbulence compensation and highly accurate pointing/tracking.

(ii)
Underwater: Blue–green light has relatively low absorption in water, thereby potentially enabling high-capacity links over ∼100 m. Note that radio waves simply do not propagate well underwater, and common underwater acoustic links have a very low bit rate. For underwater OAM links, challenges include loss, turbidity, scattering, currents, and turbulence. An interesting challenge is transmitting from above water to below the water, such that the structured optical beam would pass through inhomogeneous media surrounding the interface, including nonuniform aerosols above water, the dynamically changing geometry of the air–water interface, and bubbles/surf below the surface.

(iii)
Satellites: OAM multiplexing may have interesting advantages for up–down links to satellites. However, cross-links that are ultralong might necessitate extremely large apertures due to the increased beam divergence of higher-order modes

80-Gbit/s 100-m Free-Space Optical Data Transmission Link via a Flying UAV

Researchers have also explored the use of orbital-angular-momentum (OAM)-multiplexing to increase the capacity of free-space data transmission to moving platforms, with an added potential benefit of decreasing the probability of data intercept.

The UAVs have been employed recently for many civilian and military purposes. They require high capacity data links to their control station to transmit the video, IR and radar data gathered by their sensors. In addition to the need for high-speed communications, there is also the desire to minimize the probability of possible interception of the data exchange in order to achieve enhanced privacy and security. Due to the higher carrier frequency of the lightwave, FSO communications generally holds the promise of having both higher capacity and lower probability of intercept (LPI) than radio frequency (RF) and millimeter-wave techniques.

They have experimentally demonstrate and characterize the performance of an OAM-multiplexed, free-space optical (FSO) communications link between a ground station and a moving unmanned-aerial-vehicle (UAV). Researchers were able to achieve a total capacity of 80 Gbit/s up to 100-m-roundtrip link by multiplexing 2 OAM beams, each carrying a 40-Gbit/s quadrature-phase-shift-keying (QPSK) signal.

Challenges

While optical angular momentum techniques have already been used to transmit data across cables, transmitting twisted light across open spaces has been significantly more challenging for scientists to date. Even simple changes in atmospheric pressures across open spaces can scatter light beams and cause the spin information to be lost.

The researchers examined the effects on both the phase and intensity of OAM carrying light over a real link in an urban environment to assess the viability of these modes of quantum information transfer. Their free space link, in Erlangen, Germany, was 1.6km in length and passed over fields and streets and close to high-rise buildings to accurately simulate an urban environment and atmospheric turbulence that can disrupt information transfer in space – a thorough approach that will be instrumental in moving OAM research forward.

Conducting this field tests in a real urban environment, has revealed exciting new challenges that must be overcome before systems can be made commercially available. Previous studies had indicated the potential feasibility of OAM communication systems, but had not fully characterized the effects of turbulent air on the phase of the structured light propagating over links of this length.

The turbulent atmosphere used in this experiment highlighted the fragility of shaped phase fronts, particularly for those that would be integral to high-bandwidth data transfers. This study indicated the challenges future adaptive optical systems will be required to resolve if free space optics are to eventually replace fiber optics as a functional mode of communication in urban environments and remote sensing systems.

For the case of OAM multiplexing in FSO communications between a ground station and a moving platform faces a few major challenges arising from the special structured nature of the OAM beams themselves. Researchers found that challenge include the following:

(a) Alignment: Low inherent crosstalk and power-coupling loss generally relies on accurate on-axis detection of the multiple OAM beams, thereby necessitating more tracking sophistication for an OAM-multiplexed link over a single conventional Gaussian-based link;

(b) Turbulence: Turbulence resulting from the atmosphere or from a UAV’s propellers could significantly distort the OAM beam’s phase front, thus resulting in increased received power fluctuations and channel crosstalk, as compared to recovering a single conventional Gaussian beam

Quantum communications

Another important advantage of OAM orthogonality is that one can use OAM mode order as a data encoding scheme. For example in the case of a quantum communication system, an individual photon can carry one of the many different OAM values; this is similar to digital data taking on one of many different amplitude values. A binary data symbol (i.e., one data bit) has two values of “0” and “1”, whereas an M-ary symbol may have many more possible values ranging from “0” to “M−1”. The number of data bits per unit time would be log2M. If each photon can be encoded with a specific OAM value from M possibilities, the photon efficiency in bits/photon can be increased. This has the potential to be quite useful for quantum communication systems which are typically photon “starved” and of which qubits commonly can be encoded on one of only two orthogonal polarization states.

Concept of orbital-angular-momentum (OAM)–based quantum data encoding. Within each symbol period, a Gaussian photon is converted to one of the M OAM states, resulting in information encoding of up to log2M bit/photon. The accumulated intensity structure image is recorded using a single-photon sensitivity, low-noise–intensified charge-coupled device camera.

A larger alphabet for each qubit is, in general, highly desirable for enhancing system performance. However, there is much research needed to overcome the challenges in fielding an OAM-encoded quantum communication system, such as: (i) mitigating coupling among orthogonal states, and (ii) developing transmitters that can be tuned rapidly to encode each photon on one of many modes.

References and Resources also include:

https://www.rmit.edu.au/news/all-news/2018/oct/faster-internet

https://physicsworld.com/a/twisted-light-gains-angular-momentum-through-self-torque/

https://www.degruyter.com/document/doi/10.1515/nanoph-2020-0435/html