Mode-selective optical fiber couplers

A mode-selective optical fiber coupler comprising:

• an input face and an output face separated by a coupling length L;
• a first propagation waveguide extending from the input face to the output face, the first propagation waveguide comprising a first core, a first inner cladding concentric with the first core, and a first outer cladding surrounding the first inner cladding, the first outer cladding having a first outer surface comprising a first arcuate segment and a first engagement segment;
• and
• a second propagation waveguide extending from the input face to the output face, the second propagation waveguide comprising a second core, a second inner cladding concentric with the second core, and a second outer cladding surrounding the second inner cladding, the second outer cladding having a second outer surface comprising a second arcuate segment and a second engagement segment, wherein:
• the first engagement segment and the second engagement segment are joined along the coupling length L;
• the first propagation waveguide and the second propagation waveguide are tapered from the input face and the output face to a midpoint of the coupler such that a tapered width dimension Wt of the coupler at the midpoint is less than a full width dimension Wf of the coupler at the input face and at the output face;
• when a first optical signal consisting of an LP01 mode coupled into the first core at the input face propagates through the first propagation waveguide and is emitted from the first core at the output face, an LP01 loss of the coupler in the first propagation waveguide at the output face is less than 1.0 dB for wavelengths in a spectral band having a bandwidth of less than or equal to 100 nm with a wavelength range from greater than or equal to 800 nm to less than or equal to 950 nm;
• and
• when a second optical signal comprising an LP11 mode coupled into the second core at the input face is cross-coupled to the first core and is emitted from the first core at the output face, an LP11 loss of the coupler at the output face is less than 1.5 dB for wavelengths in the spectral band having the bandwidth of less than or equal to 100 nm with the wavelength range from greater than or equal to 800 nm to less than or equal to 950 nm.