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.
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.
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.