Orbital angular momentum multiplexing

Orbital angular momentum multiplexing is a way to send many signals at once using light or radio waves. It uses a special property of these waves called orbital angular momentum, which is different from the spinning motion of light. This special property lets scientists create many more channels for information than ever before, limited only by the tools we use.

Unlike other methods that can only handle two states, orbital angular momentum can work with many more states because it uses a wider beam of light. This means we could send much more data in the future. However, this method is still being tested and has only worked in labs so far.

Some people think orbital angular momentum might just be a clever way to use old methods in new ways. But recently, scientists have seen signs that this property might exist naturally in space, which is exciting and still being studied.

History

Scientists showed how to use a special property of light called orbital angular momentum (OAM) for sending signals in 2004. After that, people studied this idea in two ways: using radio waves and using light for sending information.

Radio frequency

Terrestrial experiments

In 2011, an experiment showed that OAM multiplexing could carry two different radio signals over a distance of 442 meters. Some people believe that OAM does not offer much advantage over traditional radio systems that use MIMO, because these systems can mimic some of OAM's properties.

In 2012, there was debate among researchers about whether OAM multiplexing was truly different from traditional methods. Some thought it was just a version of MIMO, while others believed it was a unique phenomenon that had been proven in experiments.

In 2014, researchers used OAM and polarization together to send data over 8 millimetre-wave channels, achieving a speed of 32 Gbit/s across just 2.5 meters. Interest in using OAM for long-distance microwave communication has decreased since 2015, as studies showed it did not provide significant benefits over traditional methods in terms of capacity or antenna use.

Radio astronomy

In 2019, scientists found evidence that OAM radio signals had traveled from near the M87* black hole, more than 50 million light-years away. This suggests that OAM information can travel vast distances.

ISAC

Recently, OAM has been studied for use in integrated sensing and communications (ISAC). This means using one electromagnetic wave for both sending data and sensing the environment. OAM beams can carry multiple data streams and create different patterns that help locate objects. Experiments have shown that by changing OAM modes, we can create beams that help in both communication and finding targets. Research also looks at how these beams work in radar and antenna systems, noting similarities to traditional MIMO methods.

Optical

Scientists have tested a special way to send information using light, called OAM multiplexing. In 2012, they showed they could send information very fast—up to 2.5 trillion bits per second—using eight different signals in one beam of light, but only over a short distance of about one meter. Researchers are working to make this work over longer distances for things like free-space optical communication.

Right now, this kind of multiplexing doesn’t work well with the usual long-distance fiber wires because they only carry one type of light shape. Special wires or many types of light shapes are needed instead. Even then, changes in the wires can mix up the signals. Because of these challenges, this method hasn’t been used for long-distance communication yet. However, scientists have made progress, showing that special wires can carry these signals stably for up to 50 meters, and they continue to improve this distance.

One way to make this work over long distances involves complex computer processing. Researchers have also created small chips that can do this, which could make it easier to use in real communication systems in the future.

Practical demonstration in optical-fiber system

In 2013, scientists showed they could send multiple signals at once over a 1.1 kilometer path using a special type of fiber wire. They also combined this method with another way to send many signals at once, called Wavelength-division multiplexing (WDM), though they used only two signals with this method at the time.

Practical demonstration in conventional optical-fiber systems

In 2014, researchers showed they could send these special signals over five kilometers using regular fiber wires that people already use. They used computer processing to fix mixing problems inside the wires. This is promising because it could work with wires already in use, even though it needs a lot of computing power.

In 2018, a team made this technology much smaller—about the size of a small chip—which could make it easier to use in real networks. They believe this could make fiber wires carry at least 100 times more information than they do now, and maybe even more in the future.