- In recent years, there has been an explosive growth of data traffic in data-center interconnections. A significant amount of the installations within data centers are graded-index multimode fibers (GI-MMFs). These fibers enable the use of mode-division multiplexing to increase the fiber's capacity. The heart of such a multiplexing scheme is a spatial multiplexer. In this paper, we propose a simple tilted Gaussian beam multiplexer for GI-MMF using low-cost commercially available lenses. We derive an analytical expression for the power coupling coefficients using a tilted Gaussian beam launch into a GI-MMF. We use the analytical expression to perform an optimization algorithm for tilted Gaussian beams as a spatial multiplexer with GI-MMF. The optimization was performed for 3 × 3, 4 × 4, and 5 × 5 multiple-input-multiple-output (MIMO) systems. Typical power coupling matrices exhibit maximum crosstalk of -12.5 dB with strong coupling efficiency that is greater than -0.6 dB.