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 how light normally spins.

This special property can let scientists create many more channels for information than before. It could help us send much more data in the future. But this method is still new and has only been tested in labs.

Some people think this might just be a new way to use old methods. But scientists are excited because they think this property might exist naturally in space, and they are still studying it.

History

In 2004, scientists found a way to use a special property of light, called orbital angular momentum (OAM), to send signals. After this, people studied how to use this idea with radio waves and with light to send information.

Radio frequency

Terrestrial experiments

In 2011, an experiment showed that OAM multiplexing could carry two different radio signals over a short distance. Some people think that OAM does not have many advantages over traditional radio systems that use MIMO.

In 2012, researchers debated whether OAM multiplexing was different from traditional methods. Some thought it was just a type of MIMO, while others believed it was a special phenomenon.

In 2014, researchers used OAM and polarization together to send data over a very short distance. Interest in using OAM for long-distance communication has decreased, as studies showed it did not have big benefits over traditional methods.

Radio astronomy

In 2019, scientists found evidence that OAM radio signals had traveled from near the M87* black hole, a very faraway place. This suggests that OAM information can travel very long distances.

ISAC

Recently, OAM has been studied for use in integrated sensing and communications (ISAC). This means using one wave for both sending data and sensing the environment. OAM beams can carry many data streams and 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.