Miniaturized fiber-optic biometer improves radiation measurement for fusion applications

Accurate measurements of rapidly changing radiated power help researchers understand and control fusion plasmas. A bolometer is a device that can be used for this purpose when incident radiation causes its temperature to rise. Fiber-optic bolometers (FOBs) offer optical readout that resists electromagnetic interference, but heat diffusion along a protruding silica fiber can slow their thermal response.

Wang et al. demonstrated a miniaturized FOB using lithographically fabricated silicon pillars and femtosecond laser micromachining. A 20-micrometer-diameter silicon pillar and a 150-micrometer-diameter gold absorber disk reduce thermal mass. Radiation heats the disk and pillar, changing the reflected optical signal. Positioning the fiber nearly flush with its copper holder suppresses slow, distributed heat diffusion.

“The conventional bolometer equation is not new,” said author Ming Han. “The contribution is an FOB architecture engineered so that this simple equation becomes accurate enough to directly reconstruct millisecond-scale radiation waveforms.”

Under the tested conditions, the redesigned sensor behaves approximately as a first-order system, meaning its temperature relaxes toward equilibrium with one dominant time constant. Radiation waveforms can therefore be calculated sample by sample from the temperature and its rate of change, without deconvolution. This provides a path toward real-time measurements and simplifies calibration to two parameters: steady-state responsivity and time constant.

Compared with the prior design, the time constant fell from about 200 milliseconds to 27.5 milliseconds in vacuum. The sensor reconstructed isolated 1-millisecond radiation pulses, with pulses 2 milliseconds or wider showing higher fidelity.

“Real-time radiation measurements could help regulate impurity injection to protect divertor surfaces while limiting radiation losses from the plasma core,” said author Morgan Shafer.

The group next looks to develop new fabrication techniques that retain the small geometry and first-order behavior under conditions of high temperature, vacuum, and radiation exposure.

Source: “Miniaturized fiber-optic bolometer with reduced thermal time constant exhibiting first-order thermal dynamics,” by Xiaoli Wang, Babak Moeinimaleki, Mohammed Alshammari, Qiwen Sheng, Musaddeque Syed, Seungsup Lee, Andrew Dvorak, Morgan W. Shafer, and Ming Han, Review of Scientific Instruments (2026). The article can be accessed at https://doi.org/10.1063/5.0350044 .