It is quite difficult to excite higher order modes precisely in hollow core PCF, because of the difficulty of matching the fields in both amplitude and phase. This becomes increasingly difficult as the mode order rises. Spatial light modulators work rather well for low order modes, but for high order modes we have found prism-assisted side-coupling to be extremely useful both for measuring the modal refractive indices of the modes (very difficult to do any other way) and for mapping out the transverse field intensity patterns.
Control over the transverse intensity distribution of the light inside hollow core fibres is important, since is allows shaping of the trapping potential in which particles or atoms can be trapped and guided. We have developed a versatile method for selective mode excitation of desired guided modes in photonic crystal fibers, using holograms electronically generated by a spatial light modulator. The method enables non-mechanical and completely repeatable changes in coupling conditions. We have excited higher order modes up to LP31 in hollow-core photonic crystal fibers. The reproducibility of the coupling allows direct comparison of the losses of different guided modes in both hollow-core bandgap and kagome-lattice photonic crystal fibers [Euser (2008)].
Prism-coupling through the microstructured cladding is used to selectively excite individual higher order modes in hollow-core photonic crystal fibers (PCFs) [Trabold (2014)]. Mode selection is achieved by varying the angle between the incoming beam and the fiber axis, in order to match the axial wavevector component to that of the desired mode. The technique allows accurate measurement of the effective indices and transmission losses of modes of arbitrary order, even those with highly complex transverse field distributions that would be extremely difficult to excite by conventional endfire coupling. It also allows the field patterns of single higher order modes to be efficiently excited (see figure below). Such modes can then be used to seed a Raman amplifier in a gas-filled hollow core PCF, for example, resulting in a source of high power higher order modes [Menard (2015)].