<p>Erbium-doped waveguide amplifier (EDWA) is important for long-haul optical communication, whose gains by population inversion require high Er³⁺ densities and limited concentration quenching. Herein, we demonstrate the first example of erbium clusters named Er₅(DBM)₁₀ and Er₉(acac)₁₆ for high-gain EDWAs, in virtue of their multi-erbium cores for “spatial energy confinement”. The single-crystal packing diagrams and photophysical investigations indicate that highly concentrated Er³⁺ ions in the clusters and enlarged intermolecular distances simultaneously confine and share excited-state energies in the cores via Er-Er energy migration, and suppress collisional quenching, leading to increased emission intensities and elongated lifetimes. As consequence, under 377 nm LED pumping, compared to its mononuclear congener Er(acac)₃, Er₉(acac)₁₆ markedly increases output optical intensity of its evanescent-field waveguides by 464%, corresponding to a state-of-the-art maximum relative gain reaching 7.5 dB/cm at 1535 nm. This work demonstrates the superiority of clusters with a unique spatial confinement effect for high-performance integrated optics.</p>