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NIST-on-a-Chip: Photonic Sensors - Temperature and Light

photonic thermometer

Close-up of a photonic thermometer prototype, revealing the top of the chip-based sensor.

Photonic thermometry relies on the principle that certain thermally induced changes in the dimensions of an object (e.g., swelling), as well as its thermo-optic properties, affect the way that light moves through it.

The principal advantage of photonic sensor technology is that it is low-cost, lightweight, portable, resistant to mechanical shock and electromagnetic interference, and can be deployed in a wide variety of settings from controlled laboratory conditions to a noisy factory floor to the variable environment of a residential setting. NIST scientists are currently exploring several different photonic designs to measure temperature, all on the scale of a few hundreds of nanometers.

photonic thermometer
Prototype of a photonic thermometer held in a clamp.

One employs fiber Bragg gratings, devices which transmit or reflect different wavelengths depending on the temperature of the grating and the surrounding fiber-optic line. (For a technical description, read this paper.) Monitoring those changes is thus a sensitive measure of temperature. A second, related, technology involves tracking light as it moves through tiny Fabry-Perot cavities formed by Bragg gratings or vacancies in a photonic crystal (for a technical description, read this paper). In either case, thermally induced changes in cavity dimension correspondingly change the wavelengths that will resonate in the cavities. Measuring those wavelengths is a measure of temperature. Current crystal cavity designs have a resolution of around 50 mK now, and 1 mK should be possible in the near future. That makes the technology competitive with SPRTs, which can resolve differences in the range of 10mK. Fiber Bragg gratings are accurate to +/- 0.5 °C  over the range of -40 °C to 120 °C, equivalent to a Type J thermocouple.

Photonic thermometers
Scanning electron microscopy (SEM) images of two types of chip-based photonic thermometer sensor: a photonic crystal cavity (left) and a waveguide Bragg grating cavity (right). The inserts show a closer view of the central section – the signal-enhancing cavity – for each thermometer.

Yet another design by the photonic thermometry group employs a fiber-optic line in conjunction with a ring resonator: a closed loop of total-internal-reflection waveguide, about 10 micrometers in radius, placed adjacent to – but about 100 nm to 200 nm away from – the main optical fiber. The resonator will capture or “absorb” wavelengths propagating down the fiber from a laser source if the wavelengths are resonant with the optical properties and dimensions of the loop. Because temperature directly affects those properties and dimensions, the device can serve as a temperature sensor. At present, the ring resonators are accurate to about 40 mK, and 15 mK is expected soon.

In a somewhat related design, NIST scientists have devised a prototype of what could eventually become an absolute thermometer. They carve a small reflective cavity into a piece of silicon nitride less than one micrometer wide. When they shine a laser through the crystal, the light reflecting from the cavity experiences slight shifts in wavelength due to the beam’s temperature-induced vibrations, making the light’s color change noticeably. They are also able to separately detect the system’s zero-point (ground state) vibration. By comparing the relative size of the thermal vibration to the ground state motion, the absolute temperature can be determined.


Created April 10, 2017, Updated November 15, 2019