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Meet LANER: the "Network" Laser

It seemed like a simple idea: shine a laser through a complex network of optical fibers and see what pathway(s) the beam of light preferred. But once he got started, Giovanni Giacomelli realized that his project had opened the way to something much bigger—something that would eventually lead him to revisit the handful of laser designs currently in existence.

You see, a laser is much more than a glorified flashlight. Where an incandescent bulb or LED emits a range of light wavelengths in all directions, a laser puts out many photons of the exact same wavelength. Each of these identical photons points in the exact same direction, which is why lasers can stay bright over incredibly long distances. All of this is thanks to two main ingredients: a special atomic mechanism known as stimulated emission, and a feedback mechanism to amplify its effect.

Stimulated emission is essentially an elaborate photon-cloning mechanism where one photon of a very specific energy hits an atom and causes it to release another, identical photon. The trick with lasers is to produce enough of these cloned photons to create a powerful light beam. Traditionally, this is done by placing mirrors at both ends of a chamber that’s full of the active medium (the atoms which are used to clone photons) so that the photon beam bounces back and forth many times, gaining strength with each consecutive pass in a process known as amplification. One of the mirrors is typically semi-transparent, allowing for some of the light to escape and form a laser beam.

The same basic principle applies to another common setup: the ring cavity, in which the light circulates around and around in a circular pattern to gain strength rather than bouncing back and forth. In both cases, the beam increases in power by passing through the active medium many times in a single specially designed atrium known as the laser cavity. This basic design hasn’t changed much since the laser’s inception in the mid-twentieth century—which brings us back to Giacomelli’s project.

A simple ring laser. The laser beam is manipulated by three angled mirrors, which force it to circulate through the active medium (blue region) many times.
Image Credit: Gormé via Wikimedia Commons

Working in his lab at the Consiglio Nazionale delle Ricerche in Italy, Giacomelli and his colleagues, Stefano Lepri and Cosimo Trono, started off with an investigation of laser beams passing through a complicated network optical fibers. Initially they just wanted to study the way that laser light would propagate through the fibers and various other elements. Before long, however, they realized that with a little tweaking they could use the fiber network itself as a laser cavity! The team added several regions of active medium to the network and pumped it with energy from an external source, which started off the amplification process. Then they took highly detailed measurements at the end to see if the result matched the criteria for a laser.

This schematic shows the basic concept behind the researchers' work. Optical fibers (blue) are split and reconnected at nodes (green) and contain many small regions of active elements (red).
Image Credit: Giacomelli et al.
These measurements revealed that the photons had traveled throughout the fiber network, exploring it like hamsters in a tunnel complex. In keeping with their quantum nature, where two fibers connected the photons chose a path based on a very specific probability calculation. They looped through and around, roughly doubling in number each time they passed through a region of active medium—until a laser beam exited the system and confirmed the researchers’ hunch.

Inspired by the opportunities afforded by this new discovery, the research group dubbed their creation the LANER (LAsing NEtwoRk) to parallel the term laser. To describe the system theoretically, they dove into elements of quantum chaos, graph theory, and many other seemingly disparate branches of physics and math. Eventually they managed to create a framework that described not only the LANER but also—in its simplest form—the traditional laser cavity designs. In other words, the mathematics behind the LANER is actually a generalization of the laser itself!

While the project is still ongoing, the Italian team is starting new collaborations to delve further into this new field. Eventually they hope to produce new varieties of laser that can be modified for specific applications. By controlling such aspects as the length of the fiber segments and the location of network elements these lasers can be adapted to highlight specific modes (optical frequencies). You're not likely to see a complex nest of fibers if you take apart a cat toy laser pointer anytime soon, but the researchers are hopeful that the insight could yield new designs for research and industrial applications. After all, the original laser was once called "a solution in search of a problem"—so who knows what problems the LANER might help solve?

—Eleanor Hook

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