Microlaser Flies Along Hollow Optical Fiber

Researchers created a flying microlaser by launching laser light into a hollow-core photonic crystal fiber to optically trap a whispering gallery mode microparticle. The microparticle contains a gain medium that causes it to lase when excited by a second laser. As the microlaser is propelled down the fiber, the spectrum of the lasing light shifts with changing temperature, allowing the microlaser to act as a location-sensitive temperature sensor. (Source: R. Zeltner, MPISL)

For the first time, researchers have optically trapped and propelled a particle-based laser for centi­meters inside an optical fiber. The new flying microlaser enables highly sensitive tempera­ture measure­ments along the length of the fiber and could offer a novel way to precisely deliver light to remote and inacces­sible locations. “The flying microlaser could poten­tially be used to deliver light inside the body,” said Richard Zeltner of the Max Planck Institute for the Science of Light, Germany.

“By inserting a fiber into the skin, a microlaser emitting at a suitable wavelength could deliver precisely posi­tioned light for use with light-activated drugs. The concept could also be applied in optofluidic lab-on-a-chip devices to provide a light source for various bioanalysis techniques or for on-chip temperature measurements with high spatial reso­lution”, Zeltner said. Researchers led by Philip St.J. Russell reported that the flying microlaser can perform position-resolved tempera­ture sensing with a spatial reso­lution on the order of millimeters. This demon­stration showed the flying microlaser’s usefulness for distributed sensing, an approach that performs real-time conti­nuous sensing along an optical fiber.

The flying microlaser is based on a whispering gallery mode resonator, a small particle that confines and enhances certain wave­lengths of light. “This is the first demon­stration of distributed sensing using a whis­pering gallery mode resonator,” said Zeltner. “This unique approach to sensing poten­tially opens many new possibilities for distributed measure­ments and assessing physical properties remotely with high spatial resolution. For example, it could be useful for tempera­ture sensing in harsh environ­ments.”

A critical component for creating the flying microlaser was a special type of fiber known as hollow-core photonic crystal fiber. As the name implies, this fiber features a central core that is hollow rather than solid glass like traditional optical fibers. The hollow core is surrounded by a glass micro­structure that confines light inside the fiber. “For quite some time, our research group has been deve­loping the tech­nology necessary to optically trap particles inside hollow-core photonic crystal fibers,” said Shangran Xie, a member of the research team. “In this new work, we were able to apply this technology not just to trap a particle but also to induce it to act as a laser that can be used for sensing over long distances in a fiber.”

To create the flying micro­laser, the researchers launched laser light into a water-filled hollow core fiber to optically trap the micro­particle. Like the materials used to make traditional lasers, the micro­particle incor­porates a gain medium. The researchers excited this gain medium using a second laser beam, causing the micro­particle to emit light, or lase. The particle position along the fiber is controlled using optical forces generated by the trapping laser or by intro­ducing a flow of water inside the core.

To test the new system’s ability to sense temperature changes, the researchers propelled the lasing micro­particle along two regions of the fiber heated to 22 degrees Celsius above room tempera­ture. By measuring shifts in the lasing wave­lengths emitted from the micro­particle, they could precisely detect changes in tempera­ture as the micro­laser moved through the fiber. The sensor detected tempera­ture changes of just under 3 degrees Celsius and provided a spatial resolution of a few millimeters.

“The spatial reso­lution of this distri­buted sensor is ultimately limited by the size of the particle,” said Zeltner. “This means that, potentially, we could achieve spatial resolution as small as several micro­meters over very long measurement ranges, which is a huge advantage of our system compared with other types of distri­buted temperature sensors.” Using laser Doppler veloci­metry, the researchers determined that the particle moved at a speed of 250 microns per second during the experiment. They say that using a fiber filled with air rather than water could increase the propul­sion speed to centimeters or even meters per second.

Although the micro­particles used in the experiment tend to lose their ability to lase after about a minute due to photo­bleaching, the researchers say that micro­particles with different gain materials could solve this problem. They are also exploring whether multiple micro­lasers could be mani­pulated inside the fiber simul­taneously and are working on improve­ments of the particle position detection scheme. “With the increasing commerciali­zation of hollow-core photonic crystal fibers, all the tech­nology we need to turn this system into a practical sensor is already available,” said Zeltner. (Source: OSA)

Reference: R. Zeltner et al.: Flying particle microlaser and temperature sensor in hollow-core photonic crystal fiber, Opt. Lett. 43, 1479 (2018); DOI: 10.1364/OL.43.001479

Link: Max Planck Institute for the Science of Light, Erlangen, Germany

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