adiabatic coupler of the present invention can be accomplished with higher extinction coupling less power into unwanted modes of the microfiber than prior art couplers further prior arrangements did not take advantage of the benefit of controlling the spatial location of the shifted mode to reduce overlap with the central rod or glass surface of the microfiber reducing loss as an example consider a curved adiabatic coupler such as coupler 10 with constant radius r and bend radius r z which is changing from a very large value r 0 at z 0 corresponding to the optical fiber input coupling section to the coil radius r according to the law here l is the characteristic length of the transition region bending is modeled with a vector beam propagation model bpm by introduction of a local effective index variation corresponding to the local optical fiber curvature as shown in fig8 a which illustrates the conformal transformation of a curved optical fiber into a straight optical fiber in this example the fiber radius is set to r 10 u03bcm and the final coil radius is set to r 1 mm fig8 b shows the results of numerical simulation surface plots for the transition lengths l 1 2 and 6 mm in surface plot 1 l 1 mm and the launched fundamental mode of a straight microfiber experiences non adiabatic transformation which causes excitation of at least two modes e 00 and e.