永田君（B4）の成果が Results in Optics に受理されました。
フェムト秒レーザ照射によるコア改質ファイバの反射スペクトルの歪・温度特性を調査した永田君（B4）の論文が、Results in Optics に受理されました。
S. Nagata, Y. Matsushita, S. Shirai, K. Kikuchi, H. Lee, K. Goya, and Y. Mizuno, “Broadband reflection spectra from continuously femtosecond-laser-modified optical fibers for strain and temperature sensing,” Results Opt., accepted.
Fiber Bragg grating sensors are commonly interrogated at high speed by converting wavelength shifts into intensity changes using a narrow-linewidth laser placed on a spectral slope. However, the usable wavelength region is limited to the narrow slope around the Bragg reflection peak. In this study, we experimentally investigated a point-type optical fiber sensing structure based on continuous femtosecond-laser modification of the core of a silica single-mode fiber. The modified region generated a characteristic broadband reflection spectrum rather than a single narrow Bragg reflection peak. When strain or temperature perturbation was applied to the boundary between the modified and unmodified regions, the broadband spectral pattern shifted in wavelength while largely maintaining its overall shape. For a fiber with a 20-cm modified region, the strain and temperature coefficients were ~9.9 nm/% and 9.3 pm/°C, respectively. Fibers with modification lengths from 50 µm to 20 mm were also evaluated to clarify the length dependence of the reflection spectra and sensing responses. Measurable strain and temperature responses were obtained for modification lengths from 500 µm to 20 mm, whereas the 50-µm modified region did not provide a stable wavelength-shift response. Among the individual specimens examined, 5 mm was the shortest modification length that produced a clear broadband spectrum with weak input-direction dependence, although replicate specimens will be required to establish a statistically validated minimum length. These results indicate that continuously femtosecond-laser-modified fibers can provide broadband wavelength-dependent features for point-type strain and temperature sensing and may offer multiple candidate operating wavelengths for future intensity-based interrogation.
