Optical imaging of multimode interference patterns with a resolution below the diffraction limit

For the first time the optical interference pattern in multimode interference couplers operating at 1485 nm is made visible using an optical microscope. This is done by imaging the green light (519 and 545 nm) which is generated by upconversion at high pumping power levels in waveguides with a high erbium concentration. As the intensity of the green light is roughly proportional to the fourth power of the pump signal intensity it thus becomes possible to image the 1485 nm intensity distribution with a resolution limited by the diffraction limit for 519 and 545 nm light.