Temperature sensors with the capability to resolve fast temperature fluctuations are needed for turbulence study and control. The speed of a temperature sensor is fundamentally determined by the heat transfer process between the sensing element and the ambient environment. Our fiber-optic temperature sensor has a fast response with a plunge-into-water time constant of ~0.5 ms and a temperature resolution of 0.2-0.5 m℃. The sensing element is a tiny silicon pillar functioning as a Fabry-Perot (FP) interferometer attached to the tip of an optical fiber. Temperature fluctuations change the refractive index of silicon and consequently the cavity length of the FP interferometer, which can be measured by monitoring the reflection spectrum of FP interferometer.
The fast response of the sensor is made possible by the small size of the sensing element and the large thermal diffusivity of the silicon material. The sensor also benefits from the very large refractive index and thermo-optic coefficient of silicon that results in a high temperature resolution.
The fiber-optic temperature sensor can be adapted for use as a bolometer to measure plasma radiation in fusion devices. Plasma radiation has a broad spectrum ranging from soft x-ray to near infrared. The fiber-optic bolometer is made by attaching a gold disk to the silicon pillar of the temperature sensor described above. The gold disk collects the plasma radiation and heats up the silicon pillar and the temperature rise from radiation is measured by the silicon temperature sensor. Fiber-optic bolometers are inherently immune to the strong electromagnetic interference (EMI) often present in magnetic confinement thermal fusion devices, which is a major limiting factor to the performance of resistive bolometers. They have additional advantages of small footprint and can be made into large 2-D arrays for applications in plasma imaging.