Selective mode injection and observation for few-mode fiber optics

By etching the cladding along several millimeters of circular- or elliptical-core few-mode fiber optics, we gain access to the core and can inject extremely pure modes. The same etching technique allows one to measure the modal purity at the far end of the fiber. Modal purities of -26 dB have been obtained. We also demonstrate a holographic technique that allows all the light of each mode to be focused to an independent spot.