Abstract: An implantable medical device (1) is detailed that includes a housing (2) enclosing an electronic circuit and source of power, and an optical unit (3) sealingly coupled to the housing, the optical unit including monolithic block unit (4) made of a transparent ceramic material and comprising: a thin window (4w) defined by an inner surface and an outer surface, an outer mating structure (4om) for coupling a fibre optic to the monolithic block, and an inner mating structure (4im) for permanently coupling a light unit, and a light unit (5) rigidly mounted in the inner mating structure (4im) of the monolithic block unit. The light unit includes a light element including one or more of an inner light source (5L), and/or a photodetector (5d), the light element and a fibre optic engaged in the outer mating structure are in alignment with a corresponding reference point (4r) located on the thin window.