A system is provided for conversion of a first electromagnetic field into a desired second electromagnetic field, for example for coupling modes between waveguides or into microstructured waveguides. The system includes a complex spatial electromagnetic field converter that is positioned for reception of at least a part of the first electromagnetic field and that is adapted for conversion of the received field into the desired electromagnetic field, and wherein at least one of the first and second fields matches a mode of a microstructured waveguide. An advantage of the system is that the full effect of an incident light beam may be utilized for exciting a desired complicated mode of a specific waveguide. Another advantage is that the power of the incident beam may be coupled into one specific mode and not others, whereby a high mode suppression ratio may be achieved.