Optical fibre with a protective covering

An optical fiber comprising:

• a silica core covered with a silica cladding of a refractive index lower enough than the refractive index of said core to inhibit light from entering said cladding from said core;
• said optical fibre having a mechanical protective covering of a plastic material passing rapidly from liquid state to solid state, being obtained by extrusion through a draw plate around a punch through which the optical fibre is passed;
• said draw plate and punch having calibrating parts, with the calibrating part of the punch protruding beyond the calibrating part of the draw plate about 0.5 mm to 1 mm;
• and the extruded plastic material having been submitted at the outlet of the calibrating part of the draw plate to a very rapid cooling and thereby to centrifugal shrinkage, whereby a substantial radial clearance is formed between the silica cladding and the covering of between 1 and 10 microns, but for pin-point contacts at spaced locations between said cladding and said covering.