South African researchers to discuss spatial multiplexing at OSA Laser Congress

The Optical Society (OSA) says that researchers from the University of Witwatersrand in South Africa will present their research into the development of spatial multiplexing technology at the upcoming OSA Laser Congress. The team will discuss an approach that enables optical signal transmission in more than 100 spatial modes.

The Optical Society (OSA) says that researchers from the University of Witwatersrand in South Africa will present their research into the development of spatial multiplexing technology at the upcoming OSA Laser Congress. The team will discuss an approach that enables optical signal transmission in more than 100 spatial modes.

The University of Witwatersrand team, led by Professor Andrew Forbes, has investigated the use light with an orbital angular momentum, which the OSA says gives it a twisted, or helical, shape. The researchers manipulated a pair of variables to create the high number of modes. In the first, the team varied the number of twists (of "azimuthal degrees of freedom," as they call them) to create different spatial modes. However, prior research has shown the use of azimuthal degrees of freedom alone doesn't create enough modes to significantly increase optical transmission bandwidth.

Professor Forbes' team addressed this limitation by pairing azimuthal degrees of freedom with the second variable, "radial degrees of freedom." Because all the resultant modes are orthogonal to each other, the signals don't interfere with each other and can be separated at the receive end.

The researchers say that this is the first time two spatial degrees of freedom have been used to optically encode information. "We created 35 spatial modes encoded in three different wavelengths, producing 105 total modes," says Carmelo Rosales-Guzmán, research fellow and first author of the paper.

The team developed a spatial light modulator to enable such transmission. One spatial light modulator shapes the laser output into the various modes at the transmit end and another reverses the process at the receiver.

"One of the advantages of our approach is that we only need a single detector to demultiplex all the spatial modes to recover all the information," explains Rosales-Guzmán. "This is faster than other approaches for increasing bandwidth that need multiple detectors."

The researchers will report that they have demonstrated that this approach can transmit data with 98% efficiency in a laboratory free-space optical network. The team believes the approach should work in optical fiber as well.

As part of a demonstration of the technique, the team used it to encode a grayscale and color image. Each image was transmitted pixel by pixel; each pixel was recovered to reconstruct the image. For the grayscale image, each gray level was linked to a separate spatial mode, allowing transmission of 105 gray levels.

"In this demonstration, sending a 10,000-pixel image took 5 to 7 minutes," reports Rosales-Guzmán. "However, we could increase that speed by sending two or four pixels at the same time or by using many more wavelengths."

Rosales-Guzmán says the university crew is already working to commercialize the technology. "We are working with a company in South Africa that already makes a device that has the ability to use different spatial modes for free space communication," he says. "We are interested in trying to increase the bandwidth of their device to four times what it is capable of now."

Angela Dudley, an honorary academic at the university and a visiting scholar in Andrew Forbes' lab, will present the research in a paper titled, "Free-space communication with over 100 spatial modes" as part of the Free Space Optical Communications I session of the Application of Lasers for Sensing & Free Space Communication Conference. Dudley will speak on Thursday, November 3 from 9:15 to 9:30 AM in Room 1 of The Westin Boston Waterfront. The conference is one of three colocated meetings that will take place between October 30 and November 3 at the OSA Laser Congress. The other two are the Advanced Solid State Lasers Conference and the Laser Applications Conference. The OSA also will host an Executive Forum.

Information about the event is available at the OSA Laser Congress website.

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