Optical coupler for optical fibres

Claim 1

A releasable optical coupler for coupling light into or from an intermediate part of an optical fibre without damaging the fibre, said fibre being constituted by a core surrounded by a cladding with a lower refractory index, said coupler comprising: first and second transparent coupler parts and forming first and second gratings facing each other, respectively, said gratings being constituted by two regular successions of ridges and furrows formed respectively on oppositely facing surfaces of said two transparent parts, the ridges of one grating facing the furrows of the other; pressure means for pressing the ridges against two opposite sides of the optical fibre in order to form alternate bends in a portion of the fibre and thus to impart to this portion of the fibre an undulating form about an average axis, so as to couple modes with high propagation constants which propagate in the core to modes with low propagation constants which propagate in the cladding, said first and second coupling parts together forming an index adapter constituted by a transparent medium whose index is not substantially lower than that of the cladding so as to couple said modes which propagate in the cladding to light which propagates in said transparent medium and which constitutes an inner set of rays, the rays of this set passing in the vicinity of the axis of the fibre along the whole length of a segment of this axis and all forming a same predetermined angle with this axis; and said index adapter having an optical surface in the form of a tip of revolution about said axis to provide coupling between this set of rays and a light beam which is parallel to this axis or converges at a point aligned on this axis.