Laser Interferometer Measures Performance, Efficiency of 5G Devices

An instrument that images acoustic waves over a range of frequencies and produces so-called movies of the waves will enable improved understanding of the acoustic vibrations. The ability to dynamically measure vibrations in mechanical resonators at microwave frequencies is critical for emerging technologies such as 5G wireless communications and quantum-state generation, storage, and transfer.

However, as 5G networks become predominant in wireless communications, the challenge of measuring these tiny acoustic waves is poised to increase. Further, the method commonly used to image vibration wavefields, continuous-wave laser interferometry, has been limited to vibration amplitudes that are insufficient for these rising technologies.

Developed by a team at the National Institute of Standards and Technology (NIST), the proposed instrument uses a stroboscopic interferometer with femtosecond (fs) laser pulses to optically sample the motion of a resonator while under coherent excitation. The instrument combines elements of continuous-wave interferometers and pump-probe systems to achieve significantly better resolution and bandwidth than either approach alone — enabling nanomechanical vibrations to be measured at superhigh frequencies.

The illumination source for the interferometer is a laser that pulses 50 million times per second, which is significantly slower than the vibrations being measured.

The interferometer compares two pulses traveling along different paths. The first pulse travels through a microscope that focuses the light on a vibrating micromechanical resonator; this light is then reflected back. The second pulse serves as a reference; its path is continually adjusted so that its length is within a micron of the distance traveled by the first pulse.

The reference laser pulses between 20 and 250 times more slowly than the frequency at which the micromechanical resonator vibrates. This allows the laser pulses illuminating the resonator to, in effect, slow down the acoustic vibrations, similar to the way a strobe light seems to slow down bodies in motion. This strobe light effect “freezes” the motion of the resonator at specific points of interest to the researchers.

Additionally, the slowdown allows acoustic vibrations that oscillate at GHz frequencies to be converted to MHz frequencies. “Moving to lower frequencies removes interference from communication signals typically found at microwave frequencies and allows us to use photodetectors with lower electrical noise,” researcher Jason Gorman said.

Movies made at NIST, using an optical interferometer, capture the atomic-scale vibrations of microresonators similar to those found in cellphones. Micromechanical resonators in the frequency range, measuring by an instrument developed by the NIST team, could be used to store fragile quantum information and to transfer the data from one part of a quantum computer to another. Courtesy of NIST.

When the two pulses meet, the lightwaves from each pulse overlap, creating an interference pattern. Subsequent pulses that enter the interferometer will change the interference pattern as the resonator vibrates. Researchers can measure the amplitude and phase of the vibrations based on the changes in the pattern. Each pulse lasts only 120 fs, providing continuous updates on the vibrations from the resonator.

The laser scans the resonator, sampling the amplitude and phase of vibrations across the entire surface of the device and producing high-resolution images over a range of microwave frequencies. By combining these measurements, averaged over multiple samples, the researchers can create 3D movies of the resonator’s vibrational modes.

The images and movies show whether a resonator is operating as expected. They highlight problem areas, such as places where acoustic energy is leaking out of the resonator, reducing its efficiency and causing information to be lost. By identifying problem areas, the NIST instrument provides scientists with the information they need to improve resonator design.

Using the new approach, the researchers measured acoustic vibrations up to 12 GHz with a nearly flat femtometer-scale noise floor and an arbitrarily fine frequency resolution, while maintaining a measurement laser power as low as 100 μW. The team believes that it may be able to extend the measurements to 25 GHz, which would provide the necessary frequency coverage for 5G communications and potential future applications using quantum information. The researchers reported that they could image acoustic vibrations that have an amplitude as small as 55 femtometers.

Micromechanical resonators in the frequency range measuring by the NIST instrument could be used to store fragile quantum information and to transfer the data from one part of a quantum computer to another. Establishing an imaging system that can routinely measure micromechanical resonators for such applications requires further research. However, the current study represents a milestone in assessing the ability of micromechanical resonators to accurately perform at the high frequencies that will be required for effective communication and quantum computing in the near future, Gorman said.

The research was published in Nature Communications (www.doi.org/10.1038/s41467-022-28223-w).