Laser Tech to Improve Free Space Communications

A high-speed communications technology not limited by a physical conductor such as fiber optics has been developed using laser light to transmit data through readily available open space. This optical free space communication could lead to faster Internet for the masses, to supersensitive scanners and could result in a more mobile military.

Dr. Rainer Martini's quantum cascade laser at the Ultrafast Laser Spectroscopy and Communication Laboratory. (Images: Stevens Institute of Technology)

Dr. Rainer Martini, director of the Ultrafast Laser Spectroscopy and Communication Laboratory and associate professor of physics and engineering physics at Stevens Institute of Technology, hopes to extend the reach into the terahertz spectrum, but said that he and his team first need to face a fundamental problem: optically induced modulation of lasers.

A laser's beam must be optically modulated to transmit large amounts of data. Optically induced amplitude modulation (AM) of mid-infrared lasers was realized by researchers at Stevens a few years ago, but AM signals are at the mercy of dust and fog. Now, Stevens researchers have developed a technique to optically modulate the frequency of the beam as well (frequency modulation: FM), resulting in a signal that is disrupted significantly less by environmental factors. The new research stands to revolutionize communications, rendering environmental barriers meaningless and allowing mobile units not tied to fiber optic cable to communicate in the range of 100 GHz and beyond, the equivalent of 100 GB of data per second.

Martini, director of the Ultrafast Laser Spectroscopy and Communication Laboratory at Stevens Institute of Technology.

Electronic modulation of a mid-infrared quantum cascade laser is limited to 10 GHz, and optical modulation of frequency and amplitude offers a viable alternative. Last year, Martini and his team developed a method to optically induce fast amplitude modulation in a quantum cascade laser, a process that allows them to control the laser's intensity. Their system used a second laser to modulate the amplitude of the mid-infrared laser – in essence using light to control light. But the team still faced the problem of reliability, so they turned to optical frequency modulation.

Doctoral candidate Anderson Chen (left) is pursuing his thesis with Martini.

"FM-transmitted data is not affected by the environmental elements that affect AM data," Martini said. The recent success allows modulating specifically the emission frequency of the laser – allowing a much more reliable transmission. "But this was much more difficult to achieve and to prove."

Their optical approach has a number of applications, including frequency modulation in a mid-infrared free space communications system, wavelength conversion that will transform a near-infrared signal directly into a mid-infrared signal, and frequency modulation spectroscopy.

For more information, visit: www.stevens.edu