Milchberg and his lab report using an “air waveguide” to enhance light signals collected from distant sources. These air waveguides could have many applications, including long-range laser communications, detecting pollution in the atmosphere, making high-resolution topographic maps and laser weapons.
Because light loses intensity with distance, the range over which such tasks can be done is limited. Even lasers, which produce highly directed beams, lose focus due to their natural spreading, or worse, due to interactions with gases in the air. Fiber-optic cables can trap light beams and guide them like a pipe, preventing loss of intensity or focus.
Milchberg’s air waveguides consist of a “wall” of low-density air surrounding a core of higher density air. The wall has a lower refractive index than the core—just like an optical fiber. In the Optica paper, Milchberg, physics graduate students Eric Rosenthal and Nihal Jhajj, and associate research scientist Jared Wahlstrand, broke down the air with a laser to create a spark. An air waveguide conducted light from the spark to a detector about a meter away. The researchers collected a strong enough signal to analyze the chemical composition of the air that produced the spark.
The signal was 1.5 times stronger than a signal obtained without the waveguide. That may not seem like much, but over distances that are 100 times longer, where an unguided signal would be severely weakened, the signal enhancement could be much greater.
Illustration of an air waveguide. The filaments leave ‘holes’ in the air (red rods) that reflect light. Light (arrows) passing between these holes stays focused and intense. Credit: Howard Milchberg
The team shone four lasers in a square arrangement, heating air molecules and creating a low-density ring around a denser core of air. Light bounces around the dense core just like in a fiber.
The air fiber lasts for a few milliseconds – more than enough to send a signal. “This is an extremely long time from the vantage point of a laser,” says Milchberg. The results are published in the journal Optica.
So far the team has tested air fibers over a range of 1 meter. These delivered a signal 50 per cent stronger than through air alone over the same distance. Sending signals further gives light more chance to spread, so in theory, a 100-metre air-fiber could deliver a signal 1000 times stronger than sending it through air alone.
The team also transmitted a laser with 100 times more energy than those used to make the fiber. And they were able to receive signals: small flashes of light from the other end were detected. This suggests the fiber could be used for remote sensing, which could include detecting explosives at a distance.
Collection of weak signals from remote locations is the primary goal and the primary hurdle of optical standoff detection schemes such as light detection and ranging and laser-induced breakdown spectroscopy. Typically, the measured signal is enhanced using large-NA collection optics and high-gain detectors. We show that the signal in remote detection techniques can be enhanced by using a long-lived air waveguide generated by an array of femtosecond filaments. We present a proof-of-principle experiment using an air plasma spark source and a ∼1 m air waveguide showing an increase in the collected signal of ∼50%. For standoff distances of 100 m, this implies that the signal-to-noise ratio can be increased by a factor ∼104.
SOURCE – New Scientist, Optica, University of Maryland