“Thin Air” Optical Fiber: Groundbreaking Experiment Proves it can Operate Continuously

Researchers at the University of Maryland (UMD) have demonstrated a continuously running optical fiber made of thin air.

The most common optical fibers are strands of glass that tightly trap light over long distances. However, these fibers are not well suited for directing extremely high-energy lasers due to damage to the glass and scattering of laser energy from the fibers. In addition, the need for a physical support structure meant that the fiberglass had to be laid long before the optical signals could be transmitted or collected.

Howard Milchberg and his group in the UMD Departments of Physics, Electrical and Computer Engineering and the Research Institute of Electronics and Applied Physics have demonstrated an optical routing method that overcomes both limits, using ultrashort auxiliary laser pulses to sculpt optical fiber guides in the air itself.

These short pulses form a ring of high-intensity light structures called “filaments,” which heat air molecules to form an extended ring of hot, low-density air surrounding an undisturbed central region; This is exactly the structure of the refractive index of optical fibers. By using the air itself as the fibre, very high average forces can be directed. To collect distant optical signals to detect pollutants and radioactive sources, for example, air waveguides can be arbitrarily “unstructured” and directed at the speed of light in any direction.

In an experiment published in January X physical reviewGraduate student Andrew Goffin and colleagues from Melchberg’s group showed that this technique can form 50-meter-long aerodynamic waveguides that last for tens of milliseconds until they dissipate from cooling by the surrounding air.

Created with only one watt of average laser power, these waveguides can theoretically direct mW laser beams, making them exceptional candidates for directed energy. The waveguide method is directly scalable to 1 km and longer. However, the waveguide-generating laser in this work fired a pulse every 100 ms (10 Hz repetition rate), with cooling dissipation of more than 30 ms, leaving 70 ms between shots with no pneumatic waveguide. This is an obstacle to directing a continuous wave laser or collecting a continuous optical signal.

In a new note in opticsAndrew Goffin, Andrew Tartaro, and Melchberg show that by increasing the repetition rate of the waveguide-generating pulses up to 1,000 Hz (a pulse every millisecond), the air waveguide is kept constant by heating and deepening the waveguide faster than the surrounding air can be cooled. The result is a continuously operating pneumatic waveguide that can direct an injected continuous wave laser beam. As the waveguide is deepened by repetitive generation, the confining efficiency of the guided light improves by threefold at the highest repetition rate.

Continuous wave optical guiding greatly improves the usefulness of airwaveguides: it increases the maximum average laser power that one can transmit and preserves the guiding structure for use in the continuous collection of distant optical signals. Because kilometer-scale and longer waveguides are wider, cooling is slower and a much lower repetition rate of 1 kHz would be necessary to maintain the guide. This more lenient requirement makes it easy to achieve air wave guidance at kilometer and longer ranges with current laser technology and modest power levels.

“With a laser system suitable for waveguide generation, continuous long-distance guidance should be easy to implement,” Goffin said. “Once we have that, it’s only a matter of time before we send out continuous, high-energy laser beams and detect pollutants from miles away.”