Interference of 1.48-µm light in multimode interference waveguides is made visible by imaging green and infrared upconversion luminescence from Er3+ ions dispersed in the waveguide. A two-dimensional mode density image can be derived from the data and agrees well with mode calculations for this structure. This new technique provides an interesting tool for the study of optical mode distributions in complicated waveguide structures and photonic band-gap materials.

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Journal Opt. Lett.
van den Hoven, G.N, Polman, A, van Dam, C, van Uffelen, J.W.M, & Smit, M.K. (1996). Direct imaging of optical interference in erbium-doped Al2O3 waveguides. Opt. Lett., 21, 576–578.