Near-Field Measurements and Mode Power Distribution of Multimode Optical Fibers

This paper analyzes the impact of some of the most common uncertainties arising in the computation of modal transfer function (MTF) and mode power distribution (MPD) in multimode fibers (MMFs). The evaluation of these parameters is carried out following a procedure that is currently being tested in the framework of an international round robin before becoming a recognized standard. MTF and MPD are key parameters for the determination of device excitation conditions and for the determination of whether a so called ¿equilibrium modal distribution¿ (EMD) has been achieved. The procedure evaluates these parameters by relying on near-field (NF) measurements, and this paper investigates the effects related to the nonlinearity of the camera used to grab the images, the image scale factor, and the defocus. In particular, the experimental assessment of these effects evidences the fault-tolerant properties of NF measurement provided some precautions are taken in the images' acquisition and elaboration. Then, given the importance of operating the devices under an EMD, some typical techniques to achieve such a condition are reviewed, and an experimental realization of an effective mode conditioner is presented.