Miniaturized fiber-optic bolometer with reduced thermal time constant exhibiting first-order thermal dynamics

Accurate measurement of transient radiation power is critical for plasma diagnostics in magnetic confinement fusion, where rapid radiation events require high temporal resolution and reliable signal reconstruction. Fiber-optic bolometers (FOBs) based on silicon Fabry-Perot interferometry are attractive alternatives to resistive bolometers because of their compact size, electromagnetic-interference immunity, remote sensing capability, and compatibility with high-temperature and high-vacuum environments. However, conventional FOB designs can exhibit slow and non-first-order thermal responses because of sensing-head thermal capacitance and distributed heat transport in the protruding silica fiber stub, often requiring deconvolution to recover transient radiation profiles. Here, we demonstrate a miniaturized FOB fabricated by femtosecond laser micromachining, featuring a ∼20 μm-diameter silicon pillar and a ∼150 μm-diameter gold absorber disk. The reduced absorber and pillar dimensions decrease sensing-head thermal capacitance, while a flush fiber-copper-holder configuration eliminates the protruding fiber stub and suppresses distributed fiber thermal modes. Compared with previously reported FOBs, the device reduces the cooling time constant from ∼200 to ∼27.5 ms in vacuum while preserving a responsivity of ∼7.8 mK/(W/m2) and a noise-equivalent power density of ∼0.28 W/m2. The thermal response is experimentally shown to be well approximated by a first-order lumped model, enabling direct reconstruction of incident power density using the bolometer equation. Square-wave and millisecond-scale Gaussian radiation transients reconstructed without deconvolution show close agreement with independently measured reference signals, demonstrating ∼1 ms temporal resolution for real-time radiation diagnostics in fusion plasmas.