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  • Optical Fiber ‘Nanospikes’ Effectively Trap, Focus Laser Light
Mar 2016
ERLANGEN, Germany, March 14, 2016 — Using laser light to manipulate a glass optical fiber tapered to a sharp point smaller than a speck of dust, in the middle of an optical fiber with a hollow core, has been demonstrated by a team from the Max Planck Institute for the Science of Light. Optical forces cause the sharp point, or “nanospike,” to self-align at the center of the hollow core, trapping it more and more strongly at the core center as the laser power increases.

The new work could increase applications for hollow-core fibers, a new class of fiber that features a hollow core rather than one made of glass like traditional optical fibers. Hollow-core fibers are especially good at handling high-power lasers, making them potentially useful for laser machining and cutting of metals, plastics, wood and other materials.

Simple, self-aligning method traps tiny tapered glass fiber inside hollow-core optical fiber, with potential applications in laser cutting and basic physics research.
Simple, self-aligning method traps tiny tapered glass fiber inside hollow-core optical fiber, with potential applications in laser cutting and basic physics research. Courtesy of Philip Russell/Max Planck Institute for the Science of Light.

To create the nanospike, the researchers started with an ordinary single-mode glass optical fiber about 100 μm in diameter. The fiber was heated so that it could be stretched to form a tapered portion and then the fiber’s tip was etched with hydrochloric acid to create a nanospike around 100 nm in diameter — smaller than the wavelength of visible light — and less than 1 mm long.

The nanospike was inserted into the hollow core fiber and a high-power 1064-nm laser beam was launched into the single-mode fiber, creating the optical trap. When the laser light entered the tapered portion of the fiber it began to spread out beyond the nanospike into the empty space inside the hollow core fiber. As the taper got smaller and smaller, the light began to sense the boundary of the larger fiber core, causing the light to reflect inward toward the tapered fiber. This reflected light exerted a mechanical force on the nanospike, forming an optical trap.

“Launching very high power laser light into an optical fiber, especially a hollow-core fiber, can be very difficult and usually requires extensive electronics and optics to maintain alignment,” explained Philip Russell, director at the Max Planck Institute for the Science of Light in Erlangen, Germany, and leader of the research team. “This can be accomplished with our new system by simply pushing the nanospike into the hollow core and then turning up the laser power slowly. Once the nanospike self-stabilizes, you can turn up the laser power and nothing will move or get damaged. “The nanospike is held in place by the light at exactly the right place to perfectly launch the light into the hollow core without any electronics or other systems to keep it in place,” Russell said. “If any of the components move a little, there’s no effect on the laser light because the nanospike self-aligns and self-stabilizes.”

Nearly 90 percent of the laser light was transferred from the nanospike to the hollow-core fiber, researchers said.

“The beauty of the nanospike is that it behaves like a very small particle, but because it is firmly attached to a strong piece of fiber at one end, it isn’t lost if it jumps out of the trap,” said Russell. “This system allows us to measure forces that are almost impossible to measure in other systems, making it feasible to explore of an area of fundamental physics that isn’t very well understood.”

In addition to efficiently coupling high-power laser light to hollow-core fibers, the new system offers an entirely new way to study the mechanical forces exerted by light, or optomechanics, especially at very low pressures. Scientists working to study optomechanical forces under high vacuum conditions have been hampered by the tendency of particles to jump out of optical traps as air pressure is lowered from atmospheric levels. The reasons for this tendency are not fully understood.

The research was published in Optica, a publication of The Optical Society (OSA) (doi: 10.1364/optica.3.000277).

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