An implantable medical device comprising: (a) a housing defining an inner volume separated from an outer environment, and enclosing an electronic circuit and source of power, and (b) an optical unit sealingly coupled to the housing, said optical unit consisting of: • a monolithic block unit made of a transparent ceramic material which is transparent to wavelengths comprised between 300 and 2200 nm, and comprising: ∘ a thin window defined by an inner surface facing the inner volume of the housing and an outer surface facing the outer environment, ∘ an outer mating structure located on the side of the outer surface of the thin window, for coupling an outer fibre optic in alignment with a corresponding reference point of the inner or outer surface of the thin window, and ∘ an inner mating structure located on the side of the inner surface of the thin window, for permanently coupling a light unit, and • a light unit comprising a light element including one or more of an inner light source, and/or photodetector, and/or inner fibre optic, said light unit being rigidly mounted in the inner mating structure of the monolithic block unit, such that one of the inner light source, and/or photodetector, and/or inner fibre optic is in alignment with said corresponding reference point of the inner or outer surface of the thin window. (a) a housing defining an inner volume separated from an outer environment, and enclosing an electronic circuit and source of power, and (b) an optical unit sealingly coupled to the housing, said optical unit consisting of: • a monolithic block unit made of a transparent ceramic material which is transparent to wavelengths comprised between 300 and 2200 nm, and comprising: ∘ a thin window defined by an inner surface facing the inner volume of the housing and an outer surface facing the outer environment, ∘ an outer mating structure located on the side of the outer surface of the thin window, for coupling an outer fibre optic in alignment with a corresponding reference point of the inner or outer surface of the thin window, and ∘ an inner mating structure located on the side of the inner surface of the thin window, for permanently coupling a light unit, and • a light unit comprising a light element including one or more of an inner light source, and/or photodetector, and/or inner fibre optic, said light unit being rigidly mounted in the inner mating structure of the monolithic block unit, such that one of the inner light source, and/or photodetector, and/or inner fibre optic is in alignment with said corresponding reference point of the inner or outer surface of the thin window. • a monolithic block unit made of a transparent ceramic material which is transparent to wavelengths comprised between 300 and 2200 nm, and comprising: ∘ a thin window defined by an inner surface facing the inner volume of the housing and an outer surface facing the outer environment, ∘ an outer mating structure located on the side of the outer surface of the thin window, for coupling an outer fibre optic in alignment with a corresponding reference point of the inner or outer surface of the thin window, and ∘ an inner mating structure located on the side of the inner surface of the thin window, for permanently coupling a light unit, and ∘ a thin window defined by an inner surface facing the inner volume of the housing and an outer surface facing the outer environment, ∘ an outer mating structure located on the side of the outer surface of the thin window, for coupling an outer fibre optic in alignment with a corresponding reference point of the inner or outer surface of the thin window, and ∘ an inner mating structure located on the side of the inner surface of the thin window, for permanently coupling a light unit, and • a light unit comprising a light element including one or more of an inner light source, and/or photodetector, and/or inner fibre optic, said light unit being rigidly mounted in the inner mating structure of the monolithic block unit, such that one of the inner light source, and/or photodetector, and/or inner fibre optic is in alignment with said corresponding reference point of the inner or outer surface of the thin window.