Wavelength division multiplexers for space division multiplexing (SDM-WDM devices)

An optical coupler array for optical coupling of a plurality of optical fibers carrying light at least at two wavelengths W-1 and W-2 to an optical device, comprising:

• an elongated optical element having a first end operable to optically couple with said plurality of optical fibers and a second end operable to optically couple with said optical device,
• and comprising:
• a common single coupler housing structure;
• a coupling section;
• a plurality of longitudinal waveguides, including at least one first waveguide and at least one second waveguide, each of said plurality of longitudinal waveguides being positioned at a spacing from one another, each having a capacity for at least one optical mode of a mode field profile, and a corresponding propagation constant, and each being embedded in said common single housing structure, wherein at least one of said plurality of longitudinal waveguides is a vanishing core waveguide, each said vanishing core waveguide comprising:
• an inner vanishing core, having a first refractive index (N-1), and having a first inner core size (ICS-1) at said first end, and a second inner core size (ICS-2) at said second end;
• an outer core, longitudinally surrounding said inner core, having a second refractive index (N-2), and having a first outer core size (OCS-1) at said first end, and a second outer core size (OCS-2) at said second end, and
• an outer cladding, longitudinally surrounding said outer core, having a third refractive index (N-3), a first cladding size at said first end, and a second cladding size at said second end;
• and
• wherein said common single coupler housing structure comprises a medium having a fourth refractive index (N-4) surrounding said plural longitudinal waveguides, wherein a relative magnitude relationship between said first, second, third and fourth refractive indices (N-1, N-2, N-3, and N-4, respectively), comprises the following magnitude relationship:
• (N-1>N-2>N-3), wherein a total volume of said medium of said common single coupler housing structure is greater than a total volume of all said vanishing core waveguides inner cores and said outer cores confined within said common single coupler housing structure, and wherein said first inner vanishing core size (ICS-1), said first outer core size (OCS-1), and said spacing between said plurality of longitudinal waveguides, are simultaneously and gradually modified, in accordance with a profile, between said first end and said second end along said optical element, until said second inner vanishing core size (ICS-2) and said second outer core size (OCS-2) are reached, wherein said second inner vanishing core size (ICS-2) is selected to be insufficient to guide light therethrough, and said second outer core size (OCS-2) is selected to be sufficient to guide at least one optical mode, such that:
• light traveling from said first end to said second end escapes from said inner vanishing core into said corresponding outer core proximally to said second end, light traveling from said second end to said first end moves from said outer core into said corresponding inner vanishing core proximally to said first end,
• and wherein, in said coupling section located proximal to said second end, at least one said vanishing core waveguide is in coupling distance to another said longitudinal waveguide, said coupling distance and length of said coupling section are configured to couple light at least at wavelength W-1 of at least one core mode of said at least one said vanishing core waveguide with at least one core mode of another said longitudinal waveguide while continuing the propagation of the light at said wavelength W-2 in said another longitudinal waveguide.