Optical fiber coupler with low loss and high coupling coefficient and method of fabrication thereof

An optical fiber coupler, for use between a first optical waveguide device of a first numerical aperture, and a second optical waveguide device having a second numerical aperture, higher than said first numerical aperture, each of the first and second optical waveguide devices having a corresponding end face surface oriented toward one another, said optical fiber coupler having a capacity for at least one optical mode, and comprising:

• an elongated optical fiber element having a longitudinal axis, a first end comprising a first face surface, selectively connectable to the end face surface of the first optical device positioned along said longitudinal axis, an opposite second end, comprising a second face surface, seletively connectable to the end face surface of the second optical device also positioned along said longitudinal axis, sequentially said optical fiber element, such that said elongated optical fiber element is positioned between the first and second optical waveguide devices along said longitudinal axis, a coupler core having a first refractive index, a first core size at said first end, and a second core size at said second end;
• and a coupler cladding having a second refractive index, a first cladding size at said first end face surface, and a second cladding size at said second end face surface, said second core and cladding sizes being substantially smaller than said first core and cladding sizes, said coupler cladding being surrounded by a medium having a third refractive index, wherein said first refractive index is greater than said second refractive index, wherein said second refractive index is greater than said third refractive index, wherein a difference between said second and said third refractive indices is greater that a difference between said first and said second refractive indices, wherein said first core size and said first cladding size are gradually reduced, in accordance with a predetermined reduction profile, between said first end face surface and said second end face surface along said optical fiber element, until said respective second core size and said second cladding size are reached, wherein said second core size is selected to be insufficient to guide light, said second cladding size is selected to guide the at least one mode of the optical waveguide device mode capacity, such that, light traveling from said first end to said second end escapes from said core into said cladding proximal to said second end, and exits from said second end face surface into the sequentially positioned second optical device end face surface, and light traveling from said second end to said first end moves from said cladding into said core proximal to said first end, and exits said first end face surface into the first optical device end face surface, so that said first end forms a low numerical aperture waveguide and said second end forms a high numerical aperture waveguide.