A holographic pressure sensing apparatus includes a first optical fiber with a diffractive element at its end face, and a light-coupling component for receiving from the first optical fiber end face first and second images respectively formed by interaction with the diffractive element of a first light of a first wavelength and a second light of a second wavelength. Displacement of the light-coupling component, toward or away from the first optical fiber end face, will adjust an overlap of the first and second images, such that a change in a measurement of said overlap will indicate a change of the pressure in the fluid surrounding the casing.