The two-photon absorption spectrum of ruby and its role in distributed optical fibre sensing

The two-photon absorption spectrum and cross-section (δ) for ruby (Cr3+:sapphire) have been measured over the 0.8–1.2 μm wavelength range at 25°C. Absolute values of δ were obtained by comparing the fluorescence yield for two-photon excitation with that for single-photon excitation of the same ionic transition under identical illumination conditions. The maximum values of δ are 4.6×10−3 GM for o-polarisation and 5.9×10−3 GM for e-polarisation at 840 nm. The relevance of these measurements to distributed optical fibre sensing is briefly discussed