A fuel sensing system utilizes a fiber optic sensor comprising a membrane made of a direct band gap semiconductor material (such as gallium arsenide) that forms an optical cavity with an optical fiber inside a hermetically sealed sensor package located at the bottom of a fuel tank. The optical fiber inside the fuel tank is not exposed to the fuel. The optical cavity formed by the bottom surface of the membrane and the surface of the distal end of the internal optical fiber is capable of behaving as a Fabry-Pérot interferometer. Multiple light sources operating at different wavelengths and multiple spectrometers can be coupled to the confronting surface of the membrane via the optical fiber inside the fuel tank, a hermetically sealed fiber optic connector that passes through the wall of the fuel tank, and a fiber optic coupler located outside the fuel tank.