MIR imaging bundles of ordered silver halide polycrystalline fibres for thermal transmission and imaging

In this study, we propose a way for miniaturization of mid-infrared (MIR) fibre bundles while maintaining low crosstalk and transmission losses. Such miniaturization of fibres is important when applying MIR in thin polycrystalline AgCl_0.25Br_0.75 fibres with a diameter of 110 µm, made using custom-designed extrusion components. These fibres were then mechanically assembled in an optical bundle of seven hexagonally arranged fibres. Transmission of MIR from a heat source through this bundle was compared with similar bundle of seven fibres with a diameter of 300 µm. It was found that both bundles are transparent to MIR in the spectral range of 2–20 µm corresponding to temperature range from –130 to + 1150 °C. They also have low crosstalks (' 5%) and negligible bending losses. Miniaturization, expectedly, leads to higher spatial resolution in the thinner 110-µm fibre bundle (4.5 lines mm⁻¹) compared to the thicker 300-µm fibre bundle (1.7 lines mm⁻¹). However, optical losses along the fibre are higher in thinner bundle (0.4 dB m⁻¹) than in the thicker bundle (0.2 dB m⁻¹). Yet, both 110 µm and 300 µm fibre bundles are suitable for acquiring thermal images. The overall diameter of the smaller fibre bundle (400 µm) can be advantageous for endoluminal and extracorporeal thermography towards personalized diagnosis and localized thermal treatments for personalized therapy. Such miniaturization can also facilitate in situ, real-time imaging in harsh environments and additive manufacturing.