While clinicians primarily utilize OCT-A to qualitatively assess retinal microvasculature, researchers in recent years have aimed to develop more quantitative approaches.[29] These include the development of several vascular metrics that aim to quantify vascular features such as density and morphology. Though not exhaustive, below are several commonly used metrics.

Laser communication in space is the use of free-space optical communication in outer space. Communication may be fully in space (an inter-satellite laser link) or in a ground-to-satellite or satellite-to-ground application. The main advantage of using laser communications over radio waves is increased bandwidth, enabling the transfer of more data in less time.

A substantial market for laser communication equipment may establish when these projects will be fully realized.[73] New advancements by equipment suppliers is enabling laser communications while reducing the cost. Beam modulation is being refined, as its software, and gimbals. Cooling problems have been addressed and photon detection technology is improving.[citation needed] Currently active notable companies in the market include:

Extravascular signal from exudative edema - macular edema may appear as hypo- or hyper-reflective on structural OCTs. Hyper-reflective fluid, a form of exudate, is thought to be caused by suspended particles. OCT-A devices may capture the motion of these particles, leading to extravascular artifact. Transudative macular edema, in contrast, will not appear on OCT angiograms.

In June 2018, Facebook's Connectivity Lab (related to Facebook Aquila) was reported to have achieved a bidirectional 10 Gbit/s air-to-ground connection in collaboration with Mynaric. The tests were carried out from a conventional Cessna aircraft in 9 km (5.6 mi) distance to the optical ground station. While the test scenario had worse platform vibrations, atmospheric turbulence and angular velocity profiles than a stratospheric target platform the uplink worked flawlessly and achieved 100% throughput at all times. The downlink throughput occasionally dropped to about 96% due to a non-ideal software parameter which was said to be easily fixed.[32]

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On 29 November 2020, Japan launched the inter-satellite optical data relay geostationary orbit satellite with high speed laser communication technology, named LUCAS (Laser Utilizing Communication System).[34][35]

In January 2013, NASA used lasers to beam an image of the Mona Lisa to the Lunar Reconnaissance Orbiter (LRO) roughly 390,000 km (240,000 mi) away at night from the Next Generation Satellite Laser Ranging (NGSLR) Station at NASA's Earth-based Goddard Space Flight Center. To compensate for atmospheric interference, an error correction code algorithm similar to that used in CDs was implemented.[14]

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Therefore, only a brief description of current uses is available in this article and further information should be looked in the available ophthalmology journals.

Free spaceOptics technology

In May 2005, a two-way distance record for communication was set by the Mercury laser altimeter instrument aboard the MESSENGER spacecraft. This diode-pumped infrared neodymium laser, designed as a laser altimeter for a Mercury orbit mission, was able to communicate across a distance of 24,000,000 km (15,000,000 mi), as the craft neared Earth on a fly-by.[9]

It has been reported as a useful tool for evaluating optic disc perfusion in glaucomatous eyes, since attenuated peripapillary and macular vessel density was detectable in pre-perimetric glaucoma patients. Therefore, there is enthusiasm about the role of OCT-A in early detection of glaucomatous damage. Moreover, the quantitative data from these retinal vessels may prove useful in analysing metabolic activity from the inner layers of the retina and thus provide further advances in monitoring function and progression in this disease.[14][24]

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In November 2013, laser communication from a jet platform Tornado was successfully demonstrated for the first time. A laser terminal of the German company Mynaric (formerly ViaLight Communications) was used to transmit data at a rate of 1 Gbit/s over a distance of 60 km and at a flight speed of 800 km/h in daylight. Additional challenges in this scenario were the fast flight maneuvers, strong vibrations, and the effects of atmospheric turbulence. The demonstration was financed by EADS Cassidian Germany and performed in cooperation with the German Aerospace Center DLR.[17][18][19]

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Vessel Area Density – a unitless measure that reflects the proportion of the OCT angiogram that is occupied by vessels of all caliber. This is typically accomplished by first binarizing the image such that the area occupied by vessels is comprised of white pixels and the avascular area is comprised of black pixels. A proportion of the white pixels divided by the total pixels in the image provides a measure of vessel density.

Free SpaceOptics companies

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Projection Artifact - Projection artifact is inevitable and is related to light that traverses blood vessels and is reflected back by deeper layers (e.g. pigment retinal epithelium), which will appear in the final deep image with a similar vascular pattern as the overlying superficial vessels.[30]

One asset of this OCT-based approach is that it provides a quantitative analysis of the retinal vessels (in addition to the qualitative analysis done on standard angiography). Moreover, and contrary to the "2-D" conventional angiograms, OCT-A technology provides "3-D" imaging information of the macula and visualizes peripapillary capillaries that supply the retinal nerve fiber layer.[6]

Optical coherence tomography angiography (OCT-A) has emerged as a non-invasive technique for imaging the microvasculature of the retina and the choroid. The first clinical studies using this innovative technology were published in 2014 .[1]

Assuming available laser technology, and considering the divergence of the interferometric signals, the range for satellite-to-satellite communications has been estimated to be approximately 2,000 km (1,200 mi).[83] These estimates are applicable to an array of satellites orbiting the Earth. For space vehicles or space stations, the range of communications is estimated to increase up to 10,000 km (6,200 mi).[83] This approach to secure space-to-space communications was selected by Laser Focus World as one of the top photonics developments of 2015.[84]

In December 2014, NASA's Optical Payload for Lasercomm Science (OPALS) announced a breakthrough in space-to-ground laser communication, downloading at a speed of 400 megabits per second. The system is also able to re-acquire tracking after the signal is lost due to cloud cover.[27] The OPALS experiment was launched on 18 April 2014 to the International Space Station (ISS) to further test the potential for using a laser to transmit data to Earth from space.[28]

In November 2001, the world's first laser intersatellite link was achieved in space by the European Space Agency (ESA) satellite Artemis, providing an optical data transmission link with the CNES Earth observation satellite SPOT 4.[6] Achieving 50 Mbps across 40,000 km (25,000 mi), the distance of a LEO-GEO link.[7] Since 2005, ARTEMIS has been relaying two-way optical signals from Kirari, the Japanese Optical Inter-orbit Communications Engineering Test Satellite.[8]

Multiple companies and government organizations want to use laser communication in space for satellite constellations in low Earth orbit to provide global high-speed Internet access. Similar concepts are pursued for networks of aircraft and stratospheric platforms.

The main advantages are the shorter acquisition time and that it is a non-invasive process. Fluorescein and indocyanine-green angiography require an injectable dye (which takes time to reach retinal vessels, and may be associated with systemic adverse effects and even anaphylatic reactions[2]).

In May 2022, TeraByte InfraRed Delivery (TBIRD) was launched (on PTD-3) and tested 100 Gbit/s comms from 300 mile orbit to California.[38]

Segmentation Error - Automated segmentation of a structural abnormal retina is an unavoidable limitation of OCT-A imaging. In cases of pigment epithelial detachments (PED), one should look carefully for segmentation errors and manually edit layers if deemed necessary for a correct interpretation.[31]

In November 2014, the first ever use of gigabit laser-based communication as part of the European Data Relay System (EDRS) was carried out.[20] Further system and operational service demonstrations were carried out in 2014. Data from the EU Sentinel-1A satellite in LEO was transmitted via an optical link to the ESA-Inmarsat Alphasat in GEO and then relayed to a ground station using a conventional Ka-band downlink. The new system can offer speeds up to 7.2 Gbit/s.[21] The Laser terminal on Alphasat is called TDP-1 and is still regularly used for tests. The first EDRS terminal (EDRS-A) for productive use has been launched as a payload on the Eutelsat EB9B spacecraft and became active in December 2016.[22] It routinely downloads high-volume data from the Sentinel 1A/B and Sentinel 2A/B spacecraft to ground. So far (April 2019) more than 20000 links (11 PBit) have been performed.[23] As of May 2023, EDRS has over one million minutes of communications[24] with more than 50,000 successful inter-satellite links.[25][26]

The detection and evaluation of choroidal vascular membranes may be similarly achieved with OCT-A in uveitis, as in the other causes referred above.[28]Specifically in inflammatory conditions, OCT-A has the advantage of acquiring three-dimensional data, and potentially improving our understanding of the pathophysiology of these diseases as well as their follow-up and management. However, multimodal imaging is still the option of choice in the diagnosis and management of uveitis.[8]

In 2006, Japan carried out the first LEO-to-ground laser-communication downlink from JAXA's OICETS LEO satellite and NICT's optical ground station.[10]

Media Opacities - media opacities such as corneal scarring, cataracts, posterior capsular opacification, and vitreous floaters may lead to signal attenuation and shadowing artifact.[3] These may obscure portions of the OCT angiogram or lead to diffuse reduction in image quality.

In April 2020, the Small Optical Link for International Space Station (SOLISS) created by JAXA and Sony Computer Science Laboratories, established bidirectional communication between the ISS and a telescope of the National Institute of Information and Communications Technology of Japan.[33]

The first LEO-to-ground lasercom demonstration using a Japanese microsatellite (SOCRATES) was carried out by NICT in 2014,[29] and the first quantum-limited experiments from space were done by using the same satellite in 2016.[30]

Free SpaceOptics equipment

On 20 January 1968, the television camera of the Surveyor 7 lunar lander successfully detected two argon lasers from Kitt Peak National Observatory in Arizona and Table Mountain Observatory in Wrightwood, California.[3]

In June 2021, the U.S. Space Development Agency launched a two 12U CubeSats aboard a SpaceX Falcon 9 Transporter-2 rideshare mission to Sun-synchronous orbit. The mission is expected to demonstrate laser communication links between the satellites and a remotely controlled MQ-9 Reaper.[37]

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OCT-A technology uses laser light reflectance of the surface of moving red blood cells to accurately depict vessels through different segmented areas of the eye, thus eliminating the need for intravascular dyes.[2] The OCT scan of a patient's retina consists of multiple individual A-scans, which when compiled into a B-scan provides cross-sectional structural information. With OCT-A technology, the same tissue area is repeatedly imaged and differences are analyzed between scans (over time), thus allowing one to detect zones containing high flow rates (i.e. with marked changes between scans) and zones with slower, or no flow at all, which will be similar among scans.[3]

Motion Artifact -  OCT-A examination is motion-sensitive and requires patients to be able to reasonably fixate, which may be difficult to obtain for the visually impaired.[11] Excessive motion of the eye can lead to motion artifacts as seen in the image. This can be overcome to some extent with a tracking feature present on most devices.

Vessel Skeletal Density – One limitation of vessel area density is the variability of vessel caliber among OCT angiograms in different eyes. For example, one eye may by chance have a greater number of larger vessels than another within a set window, thereby giving a false impression that it has increased density. Vessel skeletal density adjust for this variability by iteratively deleting outer pixels of each vessel such that each individual vessel, regardless of size, is represented by only a single line of pixels.

Francesco Pichi and collaborators recently reviewed the uses and importance of OCT-A in uveitis.[8] For the sake of this article, only a brief summary will be reported.

Advantages offree spaceopticalcommunication

In outer space, the communication range of free-space optical communication is currently of the order of hundreds of thousands of kilometers.[1] Laser-based optical communication has been demonstrated between the Earth and Moon and it has the potential to bridge interplanetary distances of millions of kilometers, using optical telescopes as beam expanders.[2]

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As a fast, safe and noninvasive procedure to assess the chorioretinal microvasculature, OCT-A has been increasingly used in retinal diseases. The number of studies reporting new findings and utilities is exponentially growing.

Light is emitted through either a spectral domain OCT (SD-OCT), with a wavelength of near 800nm; or a swept-source OCT (SS-OCT), which utilizes a longer wavelength, close to 1050nm. Longer wavelengths have a deeper tissue penetrance, but a slightly lower axial resolution. OCT-A employs two methods for motion detection: amplitude decorrelation or phase variance. The former detects differences in amplitude between two different OCT B-scans. Phase variance is related to the emitted light wave properties, and the variation of phase when it intercepts moving objects. To improve visualization and reduce background noise from normal small eye movements, two averaging methods - split spectrum amplitude decorrelation technique and volume averaging - were developed.[4][5] These OCT-A algorithms produce an image (3mm2 to 12mm2) that is segmented, by standard, into four zones: the superficial retinal plexus, the deep retinal plexus, the outer retina and the choriocapillaris. Applied to the optic disc it includes its full depth.[6][7]

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In 2008, the ESA used laser communication technology designed to transmit 1.8 Gbit/s across 40,000 km (25,000 mi), the distance of a LEO-GEO link. Such a terminal was successfully tested during an in-orbit verification using the German radar satellite TerraSAR-X and the American Near Field Infrared Experiment (NFire) satellite. The two Laser Communication Terminals (LCT)[11] used during these tests were built by the German company Tesat-Spacecom,[12] in cooperation with the German Aerospace Center (DLR).[13]

However, some limitations and artifacts are important to consider in the interpretation of OCT-A images (see section below).

In September 2013, a laser communication system was one of four science instruments launched with the NASA LADEE (Lunar Atmosphere and Dust Environment Explorer) mission. After a month-long transit to the Moon and a 40-day spacecraft checkout, daytime laser communications experiments were performed over three months during late 2013 and early 2014.[15] Initial data returned from the Lunar Laser Communication Demonstration (LLCD) equipment on LADEE set a space communication bandwidth record in October 2013 when early tests using a pulsed laser beam to transmit data over the 385,000 km (239,000 mi) between the Moon and Earth passed data at a "record-breaking download rate of 622 megabits per second (Mbps)",[16] and also demonstrated an error-free data upload rate of 20 Mbit/s from an Earth ground station to LADEE in lunar orbit. The LLCD is NASA's first attempt at two-way space communication using an optical laser instead of radio waves, and is expected to lead to operational laser systems on NASA satellites in future years.[16]

The first successful laser-communication link from space was carried out by Japan in 1995 between the NASDA's ETS-VI GEO satellite and the 1.5 m (4 ft 11 in) National Institute of Information and Communications Technology (NICT)'s optical ground station in Tokyo achieving 1 Mbit/s.[5]

Laser communications in deep space will be tested on the Psyche mission to the main-belt asteroid 16 Psyche, launched in 2023.[39] The system is called Deep Space Optical Communications (DSOC),[40] and is expected to increase spacecraft communications performance and efficiency by 10 to 100 times over conventional means.[40][39]In April 2024, the test was successfully completed with the Psyche spacecraft at a distance of 140 million miles.[41]

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LunaNet is a NASA and ESA project and proposed data network aiming to provide a “Lunar Internet“ for cis-lunar spacecraft and installations. The specification for the system includes optical communications for links between the Earth and the Moon as well as for links between lunar satellites and the lunar surface.

Corporations like SpaceX, Facebook and Google and a series of startups are currently pursuing various concepts based on laser communication technology. The most promising commercial applications can be found in the interconnection of satellites or high-altitude platforms to build up high-performance optical backbone networks. Other applications include transmitting large amounts of data directly from a satellite, aircraft or unmanned aerial vehicle (UAV) to the ground.[45]

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In February 2016, Google X announced to have achieved a stable laser communication connection between two stratospheric balloons over a distance of 100 km (62 mi) as part of Project Loon. The connection was stable over many hours and during day and nighttime and reached a data rate of 155 Mbit/s.[31]

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Japan's National Institute of Information and Communications Technology (NICT) will demonstrate in 2022 the fastest bidirectional lasercom link between the geosynchronous orbit and the ground at 10 Gbit/s by using the HICALI (High-speed Communication with Advanced Laser Instrument) lasercom terminal on board the ETS-9 (Engineering Test Satellite IX) satellite,[42] as well as the first intersatellite link at the same high speed between a CubeSat in LEO and HICALI in GEO one year later.[43] As of May 2024[update], a Full Trasceiver type terminal compatible for CubeSat has been designed and in development. CubeSOTA is expected to launch during the Japanese fiscal year 2025 with the terminal for "demonstrating various scenarios, including LEO–ground, LEO–HAPS, and LEO–LEO." CubeSOTA "will be the first in-orbit validation of the terminals."[44]

It may also prove useful as a tool for ocular blood flow research and thus to help uncover non-IOP related mechanisms in this disease.

In 1992, the Galileo probe proved successful one-way detection of laser light from Earth as two ground-based lasers were seen from 6,000,000 km (3,700,000 mi) by the out-bound probe.[4]

On December 7, 2021, NASA's Laser Communications Relay Demonstration (LCRD) launched as part of USAF STP-3, to communicate between geosynchronous orbit and the Earth's surface.

Vessel Diameter Index – this value reflects the average vessel diameter within an OCT angiogram and is calculated by dividing vessel area density by vessel skeletal density.

Secure communications have been proposed using a laser N-slit interferometer where the laser signal takes the form of an interferometric pattern, and any attempt to intercept the signal causes the collapse of the interferometric pattern.[80][81] This technique uses populations of indistinguishable photons[80] and has been demonstrated to work over propagation distances of practical interest[82] and, in principle, it could be applied over large distances in space.[80]