Optical systems comprising multi-core optical fibers for realizing direct core to core coupling

A multicore optical fiber comprising:

• a plurality of cores disposed in a cladding, wherein:
• the plurality of cores comprise a first core, a second core, and a third core;
• the first core comprises a first propagation constant β1, the second core comprises a second propagation constant β2, the third core comprises a third propagation constant β3, the cladding comprises a cladding propagation constant β0, and
• 
• β
• 1
• -
• β
• 2
• 
• 
• β
• 2
• -
• β
• 0
• 
• <
• 1
• 2
• ;
• the first core and the second core comprise a modal mismatch M12;
• M
• 12
• =
• 
• β
• 1
• -
• β
• 2
• 
• 2
• ⁢
• C
• 12
• where C12 is a coupling coefficient for coupling from the first core to the second core;
• and
• M12 is greater than 10.

A method of coupling light from a first core of a multicore optical fiber to a second core of the multicore optical fiber, the method comprising:

• directing a plurality of photons generated by a photon generator optically coupled to an input end of the multicore optical fiber comprising a plurality of cores disposed in a cladding into the first core of the plurality of cores, wherein:
• the plurality of cores comprise the first core and the second core;
• and
• the first core comprise a first propagation constant β1, the second core comprises a second propagation constant β2, the cladding comprises a cladding propagation constant β0, and
• 
• β
• 1
• -
• β
• 2
• 
• 
• β
• 2
• -
• β
• 0
• 
• <
• 1
• 2
• ;
• and
• engaging the multicore optical fiber with a modulator, thereby generating a long period fiber grating comprising a grating period Ω in the multicore optical fiber, wherein (2π)/(|β1−β2|)=Ω such that the first core is coupled with the second core.