The present invention consists in an optical device comprising an optical component and an optical medium with different refractive indices, said optical component being at least partly immersed in said optical medium, which device is characterized in that said optical medium contains a salt that is in the liquid state in the range of temperatures at which said optical component is used and stored, i.e. in a range of temperatures generally from -40.degree. C. to +200.degree. C. The optical component may in particular be a Bragg long-period grating (LPG) or a photonic crystal fiber.