Researchers at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a new strategy for sending acoustic waves through water. By taking advantage of the dynamic rotation generated as acoustic waves travel, the orbital angular momenta, the researchers were able to pack more channels onto a single frequency, effectively increasing the amount of information capable of being transmitted. This new method could potentially open up the world of high-speed communications activities underwater, including scuba diving, remote ocean monitoring, and deep-sea exploration.
The researchers demonstrated this by encoding the letters that make up the word "Berkeley," in binary form and transmitting the information along an acoustic signal that would normally carry less data. It's comparable to going from a single-lane side road to a multi-lane highway.
While human activity below the surface of the sea increases, the ability to communicate underwater has not kept pace, limited in large part by physics. Microwaves are quickly absorbed in water, so transmissions cannot get far. Optical communication is no better since light gets scattered by underwater microparticles when traveling over long distances.
Low frequency acoustics is the option that remains for long-range underwater communication. Applications for sonar abound, including navigation, seafloor mapping, fishing, offshore oil surveying, and vessel detection. However, the tradeoff with acoustic communication, particularly with distances of 200 meters or more, is that the available bandwidth is limited to a frequency range within 20 KHz. This frequency limits the rate of data transmission to tens of kilobits per second, a speed that harkens back to the days of dialup internet connections and 56-kilobit-per-second modems.
The researchers adopted the idea of multiplexing, or combining different channels together over a shared signal, or multiplexing, is a technique widely used in telecommunications and computer networks. But multiplexing orbital angular momentum is an approach that had not been applied to acoustics until this study.
As sound propagates, the acoustic wave front forms a helical pattern, or vortex beam. The orbital angular momentum of this wave provides a spatial degree of freedom and independent channels upon which the researchers could encode data.
The rotation occurs at different speeds for channels with different orbital angular momentum, even while the frequency of the wave itself stays the same, making these channels independent of each other. That is why researchers could encode different bits of data in the same acoustic beam or pulse. They then used algorithms to decode the information from the different channels because they're independent of each other.
The experimental setup, located at Berkeley Lab, consisted of a digital control circuit with an array of 64 transducers, together generating helical wave fronts to form different channels. The signals were sent out simultaneously via independent channels of the orbital angular momentum. They used a frequency of 16 KHz, which is within the range currently used in sonar. A receiver array with 32 sensors measured the acoustic waves, and algorithms were used to decode the different patterns.
They modulated the amplitude and phase of each transducer to form different patterns and to generate different channels on the orbital angular momentum. For their experiment, the researchers used eight channels, so instead of sending just 1 bit of data, they could send 8 bits simultaneously. In theory, however, the number of channels provided by orbital angular momentum can be much larger.
The researchers noted that while the experiment was done in air, the physics of the acoustic waves is very similar for water and air at this frequency range. Expanding the capacity of underwater communications could open up new avenues for exploration. This added capacity could eventually make the difference between sending a text only message and transmitting a high-definition feature film from below the ocean's surface. Remote probes in the oceans could send data without the need to surface.
