As fiber optic technology continues to advance, it faces challenges from both its physical properties and its use of infrastructure. One emerging high-speed solution being developed at Stevens Institute of Technology uses lasers to transmit data through readily available open space, with the potential of expanding past the limitation of fibers into a system known as optical free space communications.

Dr. Rainer Martini has overcome a number of free space challenges to develop a high-speed communications technology that is not limited by a physical conductor. With an optical system that is stable enough, satellites may one day convert to laser technology, resulting in a more mobile military and super-sensitive scanners, as well as faster Internet for the masses.

As Director of Stevens Ultrafast Laser Spectroscopy and Communication Laboratory and Associate Professor of Physics and Engineering Physics, Dr. Martini confronts the challenge of creating ultrahigh-speed free space communications in the MIR spectrum, a range in which researchers work at the fringes of the laws of physics and material properties in developing faster systems of data transmission.

Eventually the team hopes to extend the reach into the terahertz spectrum as well. But first Dr. Martini and his team faced a fundamental problem: optically-induced modulation of lasers.

Optical Modulation of Lasers
A laser's beam must be optically modulated in order 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 - for the same reason that Rush Limbaugh gets fuzzy during a rainstorm.

Now, Stevens researchers led by Dr. Martini 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 the barriers of dust and fog 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 gigabytes of data per second.

A paper explaining the work, "Optically induced fast wavelength modulation in a quantum cascade laser," was recently published in Applied Physics Letters. The paper was later featured in the research highlights of Nature Photonics. In addition, Laser Focus World Magazine created a feature news story on the results for its November issue.

Before the development of the optical technique, frequency or amplitude modulation of middle infrared quantum cascade lasers was limited by electronics, which are barely capable of accepting frequencies of up to 10 GHz by switching a signal on and off. Beyond that frequency, the electronics simply could not keep up.

"When you try to switch the current on and off that fast, it doesn't work. It just drops off at around 10 GHz," explains Dr. Martini. "So we started to think of another way to switch it on an off, and that is how we came up with a complete different version: that is direct optical modulation."

Breakthrough Leads to Greater Control

Last year, Martini and his team at Stevens, The Innovation University, 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 amplitude modulation system employed a second laser to modulate the amplitude of the middle infrared laser - in essence using light to control light.

Specifically the front facet of the middle infrared quantum cascade laser is illuminated with a femtosecond near infrared laser. This system allows for fast optical amplitude modulation without electronics to hinder the process. But the team still faced the problem of reliability, so they turned to optical frequency modulation. "FM transmitted data is not affected by the environmental elements that affect AM data," Martini says.

The recent success allows modulating specifically the emission frequency of the laser - allowing a much more reliable transmission. "But," Martini qualifies, "This was much more difficult to achieve and to prove."

The new research into optically-induced frequency modulation gives researchers even greater control over lasers. The technique allows them to optically induce frequency modulation - a process that allows them to control the laser's "color" along the electromagnetic spectrum. This signal will not suffer due to environmental factors that an AM signal is subject to.

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

Martini's work in free space optics was also featured in a second article in the November issue of Laser Focus World Magazine. Kumar C. Patel, CEO of Pranalytics in Santa Monica, California, refers to Dr. Martini's study that showed that an FSO system with an 8.1 m source showed 2 to 3 times greater transmission than 1.3 and 1.5 m sources during fog formation and after a short rain event. This research helps establish free-space optics as a viable medium for communications.

A Creative Approach
"Dr. Martini's creativity and persistence have yielded great advances in laser optics," says Dr. Michael Bruno, Dean of the Charles V. Schaefer, Jr. School of Engineering and Science. "As the first person to explore amplitude and frequency modulation, he opened the doors to faster, clearer, free space communications. Today, he continues to advance a field he created."

Doctoral candidate Anderson Chen is pursuing his thesis with Dr. Martini in the Ultrafast Laser Spectroscopy and Communication Laboratory. Anderson says he is constantly inspired by his mentor's drive to find unique solutions to existing problems: "He comes up with these ideas that no one else has thought of. Sometimes the idea doesn't work, but he plays with it and tries to make it work. He is very, very, bright. Being around that, you can't help but absorb a little and want to do better."

As Dr. Martini looks ahead, one problem to be solved is the development of a new signal detector, which can detect AM as well as FM up to highest frequencies. The current detector is only capable of detecting frequencies up to 10 GHz, but Dr. Martini is confident that a new detector will make the system capable of frequencies well beyond 10 GHz.

"The one question that everyone asked was 'how fast can you get data transmitted?' That depends on how much bandwidth you have. Those lasers theoretically should go up to 100 GHz and beyond," Martini says.

In terms of communications, that could mean 100 gigabytes of data per second. Tweaking these systems so they could apply to fiberoptics or satellite data would be a boon to free space communications.

As pioneers into the evolving world of free-space optical communications, Dr. Martini and his team continue to search for new solutions in making research an every-day reality. One area of focus could take the lasers below ground by integrating the system into existing fiber optics networks, enabling high speed laser communications both above and below ground.

The team is also busy developing a phase control detector to complement their recently-created phase control emitter, which will create an entirely phase-controlled system, and enabling researchers to manage every aspect of the system.

Such a control is well know from radar and radio systems - yet unprecedented in optical systems. This could open a whole new world of possibilities including enhanced chemical and biological detection by up to 1,000,000 times, and facilitate integration into products.

For Dr. Martini, it is all a matter of perseverance as he explores this new frontier. "There is proof of concept that we can do it," Martini says. "The question now is what limitations are there?"