Researchers Use Light to Measure and Image RF Fields

Optical wavelengths used to probe GHz RF energy June 19, 2014
RF simulation software can provide a graphic display of RF fields; however, measuring them usually involves large anechoic chambers and structures on the order of the wavelength of the RF fields to be mapped, limiting the smallest features that can be measured.
New theoretical and experimental work by researchers at the National Institute of Standards and Technology (NIST) and the University of Michigan have developed an innovative method to overcome this limitation by using laser light at optical wavelengths to measure and image RF fields at frequencies of 104.77 GHz and 17.04 GHz. The new technique uses a pair of highly stable lasers and rubidium atoms as tunable resonators to map and potentially image electric fields at resolutions far below their RF wavelengths.
These measurements could be useful in measuring, evaluating and developing metamaterials and metasurfaces-structures. NTIA said it could also help measure and optimize properties of densely packaged electronics and lead to new microscopy systems and imaging sensors.
The measurements are made using rubidium atoms in a hollow glass cylinder, which has two overlapping laser beams traversing its length that act as stimulants and filters. A red laser excites the atoms, which initially absorb all the light. A tunable blue laser then excites the atoms to one of many possible higher energy ("Rydberg") states which have properties such as extreme sensitivity and reactivity to electromagnetic fields.
The NIST news release NIST Technique Could Make Sub-wavelength Images at Radio Frequencies describes how the RF measurements are accomplished once the lasers are turned on: "Next an RF field—at the frequency to be mapped or imaged—is applied. This field alters the frequency at which the atoms vibrate, or resonate, changing the frequencies at which the atoms absorb the red light. This change in the absorption is easily measured and is directly related to the electric field strength at that part of the cylinder. By moving the cylinder sideways on a track across the narrow laser beams, researchers can map the changing field strength across its diameter. The blue laser can be tuned to excite the atoms to different states to measure the strength of different RF frequencies."
The imaging technique for measuring RF is a spin-off of an ongoing NIST effort to develop a method to directly link electric field measurements to the International System of Units.

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