As technology is moving towards space division multiplexing in optical fiber to keep up the trend in rate increase over time and to avoid an imminent capacity crunch, it is of paramount interest to estimate the potential gains of this approach. As more spatial channels are being packed into a single fiber, their increased crosstalk necessitates the use of MIMO to guarantee reliable operation. In this paper, we consider the capacity of the optical channel. We exploit the analogy between an optical fiber and a model from mesoscopic physics – a chaotic cavity – to obtain a novel channel model for the optical fiber. The model captures both random distributed crosstalk and mode dependent loss, which are described within the framework of scattering theory. Using this model and tools from replica theory, random matrix theory, we derive the capacity of the fiber optical MIMO channel.