Novel photodetectors may enable more efficient deployment of free-space optical communication, increasing its scope of use and making light-based wireless communication a practical choice for bringing internet services to remote areas of the world.

One of the primary challenges of implementing wireless communications has been how to precisely point a very small laser beam carrying the data at a tiny light detector that is some distance away.

Researchers at the Facebook Connectivity Lab demonstrated a method for using fluorescent materials instead of traditional optics to collect light and concentrate it onto a small photodetector with an active area of 126 cm². The photodetector demonstrated the ability to collect light from any direction and transmit data up to 2.1 Gb/s. 

To build the detector, the researchers doped optical waveguides with wavelength shifting dyes which were contained in a fluorescent optical fiber. Incident light (blue) was absorbed by the dye molecules and was emitted at a different wavelength (green). The fiber collected a portion of the emitted light and funneled the light to a small-area photodiode, creating a luminescent detector (LD) system that combined luminescent concentrators and the photodiode.

The intermediate scattering caused by the dye enabled efficient collection of light over a large area and enabled the light to be concentrated over an end facet with a reduced étendue.

“We demonstrated the use of fluorescent optical fibers that absorb one color of light and emit another color,” said researcher Tobias Tiecke. “The optical fibers absorb light coming from any direction over a large area, and the emitted light travels inside the optical fiber, which funnels the light to a small, very fast photodetector.”

The fast speeds are possible because there is a lapse of less than two nanoseconds between the blue light absorption and the green light emission.

“The fact that these fluorescent optical fibers emit a different color than they absorb makes it possible to increase the brightness of the light entering the system,” Tiecke said. “This approach has been used in luminescent concentrators for solar light harvesting, where the speed of the color conversion doesn’t matter. We showed that the same concept can be used for communication to circumvent pointing and tracking problems while accomplishing very high speeds.”

The use of luminescent concentrators increased the active area and response time of the LD, allowing high-data-rate communication rates without the need for accurate pointing and tracking. Since the detector has omnidirectional sensitivity and is insensitive to the spatial mode of the incident light, it has the potential to support mobile applications and to use diffuse light sources and multimode optical fields (e.g., arising from propagation through a turbulent atmosphere).

The researchers transmitted up to 2.1 Gb/s despite the system’s bandwidth of 100 MHz by incorporating orthogonal frequency division multiplexing (OFDM), which allowed them to modulate signals and encode digital data so that multiple data streams could be transmitted at once. Although OFDM is commonly used for wired and wireless communication, it is not typically used with laser communication.

With data rates of more than 2 Gb/s, the researchers’ approach to photodetection could simplify free-space optical communication and increase its scope of applications.

“A large fraction of people don’t connect to the internet because the wireless communications infrastructure is not available were they live, mostly in very rural areas of the world,” said Tiecke. “We are developing communication technologies that are optimized for areas where people live far apart from each other.”

If materials were developed that operate in the IR part of the spectrum, the new approach could theoretically allow free-space optical data rates of more than 10 Gb/s, Tiecke said.

“We achieved such high data rates using commercially available materials that are not designed for communications applications,” said Tiecke. “We want to get other groups interested in developing materials that are tailored for communications applications.”

In addition to working with partners to develop new materials, the research team is planning to move the technology out of the lab by developing a prototype that could be tested in a real-world situation. “We are investigating the feasibility of a commercial product,” said Tiecke. “This is a very new system, and there is a lot of room for future development.”

The research was published in Optica, a journal of The Optical Society (doi: 10.1364/optica.3.000787).