MIT Lincoln Laboratory demonstrates novel laser
at technology expo

Lincoln Laboratory team at Wait, What? forum  
The Lincoln Laboratory team that demonstrated the 101-element laser system at the Wait, What? forum was, left to right, Franklin Jose, Juan Montoya, Andrew Benedick, Niyom Lue, and Christopher Aleshire. The monitors in the back show, clockwise from top left, a thermal profile of the cylinder through which the beam passes; an image of a metronome that illustrates how the beam can track a moving object; an image of the concentrated bright spot which results from the combining of 101 optical elements; and an image of the two-dimensional array of emitters arranged on a rectangular grid.

The visitors at the America's Center Convention Complex in St. Louis, Missouri, between 9 and 11 September included researchers from universities and national labs working in diverse fields—physics, computer science, paleontology, engineering, astronomy, genetics; entrepreneurs from the high tech industry; representatives from government agencies; and even U.S. Secretary of Defense Ashton Carter. What brought together such an eclectic mix of attendees was the Wait, What? technology forum hosted by the Defense Advanced Research Projects Agency (DARPA).

Wait, What?, an exposition featuring demonstrations and topical sessions, was so named to lightheartedly describe the anticipated reaction of the audience to the pioneering technologies on display and under discussion. The forum was also designed to inspire cross-disciplinary approaches to creating future innovations.

Researchers from MIT Lincoln Laboratory's Laser Technology and Applications Group were invited by DARPA to Wait, What? to demonstrate the advanced, 101-element optical phased array that they had developed under the agency's sponsorship. The team—Juan Montoya, Andrew Benedick, Niyom Lue, Franklin Jose, and Christopher Aleshire—appreciated the chance to showcase their work and to hear what new ideas other scientists and engineers are investigating. "I enjoyed seeing how different companies are innovating," said Montoya. "One interesting idea was how the concept of open-source software is being transitioned to hardware."

Lue was interested in DARPA's strong support of and commitment to new possibilities. "There were focuses on major and unexpected advances in many fields—new materials, surveillance, navigation, sensors, communications, computing, artificial intelligence, robotics, energy, and biomedical engineering." His colleague Benedick welcomed conversations with current and potential DARPA program managers: "Interacting with them one on one and showing the quality and creative nature of the work at Lincoln Laboratory is very important to ensuring that we continue to have an opportunity to contribute to or lead DARPA-funded programs."

At the Laboratory's booth, the engineers conducted demonstrations to highlight the capability of this unique fiber laser that coherently combines an array of 101 optical emitters to produce a powerfully bright single beam. "I had a number of people ask me questions about our technology, as well as other potential laser technologies and their applications," said Benedick. The attendees who stopped at the display were shown the contrast between the scattered speckle pattern produced by a random distribution of the light from 101 emitters and the focused bright spot created by merging the beams. A high-brightness, concentrated beam can enhance various applications; for example, metal can be cut or welded with a laser if the beam intensity is high, and a high-energy beam that can propagate over longer distances than can a diffuse array of beams could improve the range of laser communication systems. 

Arati Prabhakar (far left), director of the Defense Advanced Research Projects Agency, visited the display booth where Lincoln Laboratory staff members Juan Montoya (left) and Niyom Lue explained the technical advancements that have led to the laser system that combines 101 optical elements to create a single, high-brightness beam. Arati Prabhakar (far left), director of the Defense Advanced Research Projects Agency, visited the display booth where Lincoln Laboratory staff members Juan Montoya (left) and Niyom Lue explained the technical advancements that have led to the laser system that combines 101 optical elements to create a single, high-brightness beam.

The researchers explained to visitors that the challenge in developing this laser is to have the individual beams from all 101 emitters arrive at precisely the same time to a designated point in the far-field plane. To achieve this simultaneous arrival, all the path lengths of the 101 emitters need to be matched to much less than a 1 μm wavelength (less than 1/50th the diameter of a hair). The research team solved the synchronization of the beams by maneuvering a set of phase modulators, which sped up or slowed down the beams such that they all arrived together to create a bright central spot at a target.

Montoya said that an attraction at the laser display was the hands-on activity: visitors could "steer" the beam by using a game controller. "Behind the scenes, an algorithm converted an attendee's input signals to the appropriate phase-control adjustment such that the combined beam could be rapidly scanned over a moving object," he explained. Then, the set of horizontal scans was processed to reconstruct an image of the object. "Our phase-control algorithm then enabled the phased-array beam to track the moving target."

After they showed visitors how to steer a beam and how to compensate for the random fiber path-length variations in an environment that does not have turbulence and atmospheric disturbances, the Lincoln Laboratory team demonstrated beam propagation in a more challenging environment. They injected hot air into a segment of a beam path so viewers could watch on a monitor how the beam degraded because of the disturbance to it caused by the air's motion. "Our control algorithm compensated for the disturbance by iteratively applying a correction to all 101 emitters in order to optimize the central intensity of the beam," said Montoya, who further explained that the algorithm predistorts the 101-element beam before it propagates through an atmospheric disturbance.

At Wait, What? the five Lincoln Laboratory researchers took some time to explore what other scientists and engineers were investigating. The robotics demonstrations captured a lot of crowd attention because, as Jose described, "Robotics tend to make a fine spectacle. Understanding how much work goes into programming and building robots makes the final prototypes that much more impressive." Among the prototype systems he checked out were microrobots programmed to perform specific functions in a microfactory and macroscale robots designed as quadrupeds to provide mobile support for land-based military operations.

Benedick was impressed with a concept being investigated by PARC to mount microchips on prestressed glass substrates that would under thermal loading fracture themselves and the microchips, thus protecting the data on the chips. "One could imagine using this technology in your cellphone to ensure that once the phone was turned in for recycling, the information on the phone was destroyed."

The laser development team also talked with other research groups working on systems for directed energy. "Relevant to my work was a phased array of fiber lasers that was used over a 7 km path," said Jose. "Videos and slides showed how the thermal effects in the fibers, as well as atmospheric effects, were compensated for over this long range, yielding constructive interference between all of the array elements and producing a bright spot of light at an intended target." Montoya was interested in a cooling technology being developed for diode lasers that could be applied to fiber lasers, which are the type used in the Laboratory's research.

Wait, What?, which its website called "a crucible for generating ideas that can stretch current conceptual horizons" and which attracted hundreds of visitors from dozens of organizations, was, according to Lue, "a great opportunity to share ideas on potential technologies." Jose agreed, saying that the "visibility at the expo was a bonus," allowing the Laboratory to demonstrate to a broad community, including high-level defense officials and rising young scientists, the unique capabilities it can contribute to future technology.

Posted October 2015

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