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NIST photonic thermometer speeds up temperature measurements

30 Aug 2023

Frequency comb approach allows faster operation of devices, potentially in parallel.

A team at NIST has built on its existing research into photonic thermometers and developed a way to speed up the operation of the devices.

The improvement could allow the design of multiple photonic probes operating in parallel, allowing them to be used in the study of complex processes and measure a number of physical properties simultaneously.

NIST's photonic thermometers rely on the way heat changes the dimensional and thermo-optic properties of miniaturized photonic resonator devices, such as ring resonators and photonic crystal cavities, creating an optical filter whose spectral characteristics can be a measure of temperature.

These sensors, perhaps alongside similarly miniaturized sensors for other properties such as strain and humidity, could be embedded into civil engineering constructions as they are built, according to NIST, helping to monitor how the structures behave.

But one challenge with the devices has been how to interrogate them effectively, inputting light and detecting the altered output as rapidly as possible. Traditional methods, with laser sources creating discrete input wavelengths, can be slow, expensive and bulky. Speed is particularly an issue for applications where the temperatures are changing rapidly, as when cells are given a dose of radiation therapy during cancer treatment.

The new study reported in Optics Letters tested the use of dual frequency combs, an approach in which two laser frequency combs with slightly different repetition rates are employed as an alternative to a broadband source. Dual-frequency combs have previously been employed to improve detection of greenhouse gases, and deployed for this purpose by NIST at its Boulder campus in 2021, but not previously used with photonic thermometers.

Close to commercialization

NIST's instrument requires near-IR light from 1520 to 1560 nanometers, which the project input as two combs with frequencies offset by amounts in the radio frequency (RF) range, creating effectively a single RF comb.

The temperature being experienced by the device leads to a "dip" in output in the optical regime, but the NIST approach means that the same effect is registered as a dip in the RF output too, making the drop in amplitude for a particular wavelength more noticeable, according to the project team.

"That's the advantage of the dual-comb approach," commented NIST's Adam Fleisher. "It compresses all of that optical information into the RF regime where it's easier to read out."

In trials, the team placed its photonic sensor in a thermos-like container called a fixed-point cell, chunks of pure material that are either melting or freezing but which maintain a very stable temperature while the phase change takes place.

"We used a fixed-point cell because we wanted temperature stability not to be the limiting factor in this experiment," said NIST's Tobias Herman. "If something was shifting or moving or noisy, we could rule out the temperature bath as the source. It was just there to be a stage on which the interrogator could shine."

With this setup, they were able to measure temperature to within ten thousandths of a kelvin, which the researchers say is sufficient for most industrial applications.

"It's very close to what we need it to be for commercialization," said NIST's Zeeshan Ahmed. "We already met a lot of the metrics we need to meet, like the accuracy and speed of the measurements. This study shows that you can take a small version of a dual comb system and get good enough answers for the application space you need."

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