Waveguides Widen Light Beams

This mode expansion device is made of a linear waveguide, a slab waveguide, and a grating. Light enters the device through the linear waveguide. When the waveguide makes contact with the slab, the light expands laterally. The grating then converts the expanded light into waves through free-space. (Source: S. Kelley, NIST)

By using light waves instead of electric current to transmit data, photonic chips show advanced funda­mental possi­bilities in many areas from time­keeping to telecommu­nications. But for many appli­cations, the narrow beams of light that traverse these circuits must be substan­tially widened in order to connect with larger, off-chip systems. Wider light beams could boost the speed and sensi­tivity of medical imaging and diagnostic proce­dures, security systems that detect trace amounts of toxic or volatile chemicals and devices that depend on the analysis of large groupings of atoms.

Vladimir Aksyuk and his colleagues from National Institute of Standards and Tech­nology NIST, including researchers from the Univer­sity of Maryland NanoCenter in College Park, Maryland, and Texas Tech Univer­sity in Lubbock have now developed a highly effi­cient converter that enlarges the diameter of a light beam by 400 times.

The slab maintains the narrow width of the light in the vertical dimension, but it provides no such constraints for the lateral, or sideways, dimension. As the gap between the waveguide and the slab is gradually changed, the light in the slab forms a precisely directed beam 400 times wider than the approxi­mately 300 nm diameter of the original beam. In the second stage of the expansion, which enlarges the vertical dimension of the light, the beam traveling through the slab encounters a dif­fraction grating. This optical device has periodic rulings or lines, each of which scatters light. The team designed the depth and spacing of the rulings to vary so that the light waves combine, forming a single wide beam directed at nearly a right angle to the chip’s surface.

Importantly, the light remains collimated, or precisely parallel, throughout the two-stage expansion process, so that it stays on target and does not spread out. The area of the colli­mated beam is now large enough to travel the long distance needed to probe the optical pro­perties of large diffuse groupings of atoms. Working with a team led by John Kitching of NIST the researchers have already used the two-stage converter to success­fully analyze the properties of some 100 million gaseous rubidium atoms as they jumped from one energy level to another. That’s an important proof-of-concept because devices based on inter­actions between light and atomic gasses can measure quantities such as time, length and magnetic fields and have appli­cations in navi­gation, communi­cations and medicine.

“Atoms move very quickly, and if the beam moni­toring them is too small, they move in and out of the beam so fast that it becomes difficult to measure them,” said Kitching. “With large laser beams, the atoms stay in the beam for longer and allow for more precise measure­ment of the atomic properties,” he added. Such measure­ments could lead to improved wave­length and time standards. (Source: NIST)

Reference: S. Kim et al.: Photonic waveguide to free-space Gaussian beam extreme mode converter, Light: Sci. & App. 7, 72 (2018); DOI: 10.1038/s41377-018-0073-2

Link: Center for Nanoscale Science and Technology, National Institute of Standards and Technology NIST, Gaithersburg, USA

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