This chapter theoretically and experimentally discusses the performance of optical links with fixed optoelectronic devices, focusing on optical fibers with different characteristics, on the basis of the link power budget model. The eye diagram provides valuable information on the optical link performance, such as the signal distortion, rise time, noise margin, and timing jitter. The average error probability is the most useful criterion for evaluating the performance of digital communication systems. Silica‐based single‐mode fibers (SMFs) are widely used in long‐haul networks because of their extremely low attenuation and high bandwidth. The coupling loss of a fiber connection strongly depends on the core diameter of the optical fiber because the ratio of overlapping core regions is determined by the core diameter even for connections with the same misalignment. Gigabit Ethernet allows a power budget of 7.5 dB for multimode fiber (MMF) networks.