Optical-fiber-based Raman imaging probes are traditionally made with silica optical fibers. Unfortunately, silica has strong broadband background noise at low wavenumbers of less than 500 to 1700 cm-1 (which are characteristic of many biological proteins, fatty acids, and hydrogen-bonded chemicals, as well as oxidizing metals) and obscures the weak Raman signal received from a material under test. This is why current Raman imaging fiber probes have two silica fibers—one to bring the light to the sample and another to collect the light and filter it through additional optical elements in the probe. This introduces a limitation on the miniaturization of Raman imaging probes.
To address these challenges, researchers at EPFL (Lausanne, Switzerland) and Baidu Shenzhen R&D Center (Shenzhen, China) have constructed an improved small-diameter (60 µm) Raman imaging probe using multimode sapphire optical fiber, taking advantage of its inherent low fluorescence, narrow Raman peaks, high temperature and corrosion resistance, and large numerical aperture (NA). The probe successfully images polystyrene beads, carbon nanotubes, and calcium-sulfate (CaSO4) agglomerations with a spatial resolution of 1 µm in a 30 µm field of view. The sapphire fiber is used for both excitation and collection of the Raman signal without any focusing optics. In one measurement, the closely spaced Raman spectral peaks of a polystyrene bead (1005.4 cm-1) and a CaSO4 agglomeration (1011.1 cm-1) were easily distinguished. Reference: S. Deng et al., Opt. Express,27, 2, 1090–1098 (2019).
