Intensity modulated proton-implanted top surface-emitting vertical-cavity lasers (VCSEL's) with a small-signal modulation bandwidth of 12 GHz butt-coupled to multimode fibers are investigated as light source for optical interconnection, At 10-Gb/s pseudorandom data rates the bit-error rate (BER) remains under 10(-11) after transmission over 500 m of graded index multimode fiber, Optimum transmission behavior is achieved for linearly polarized nearly single-mode laser operation with a side-mode suppression of better than 25 dB under modulation, Spectral characterization indicates that linearly polarized single-mode light output is essential for good BER performance.