This paper presents 3-D Fabry–Pérot (FP) cavities fabricated directly onto cleaved ends of low-loss optical fibers by a two-photon polymerization (2PP) process. This fabrication technique is quick, simple, and inexpensive compared to planar microfabrication processes, which enables rapid prototyping and the ability to adapt to new requirements. These devices also utilize true 3-D design freedom, facilitating the realization of microscale optical elements with challenging geometries. Three different device types were fabricated and evaluated: an unreleased single-cavity device, a released dual-cavity device, and a released hemispherical mirror dual-cavity device. Each iteration improved the quality of the FP cavity's reflection spectrum. The unreleased device demonstrated an extinction ratio around 1.90, the released device achieved 61, and the hemispherical device achieved 253, providing a strong signal to observe changes in the free spectral range (FSR) of the device's reflection response. The reflectance of the photopolymer was also estimated to be between 0.2 and 0.3 over the spectrum of interest. The dual-cavity devices include both an open cavity, which can interact with an interstitial medium, and a second solid cavity, which provides a static reference reflection. The hemispherical dual-cavity device further improves the quality of the reflection signal with a more consistent resonance, and reduced sensitivity to misalignment. These advanced features, which are very challenging to realize with traditional planar microfabrication techniques, are fabricated in a single patterning step. The usability of these FP cavities as thermal radiation sensors with excellent linear response and sensitivity over a broad range of temperature is reported. The 3-D structuring capability the 2PP process has enabled the creation of a suspended FP heat sensor that exhibited linear response over the temperature range of 20-120ºC; temperature sensitivity of ~50 pm/ºC at around 1550 nm wavelength; and sensitivity improvement of better than 9x of the solidly-mounted sensors.
Journal of Micromechanics and Microengineering
Jonathan W Smith et al 2020 J. Micromech. Microeng. 30 125007, doi: https://10.1088/1361-6439/abc0fd
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