A 4 cm long Er‐doped Al2O3 spiral waveguide amplifier was fabricated on a Si substrate, and integrated with wavelength division multiplexers within a total area of 15 mm2. When pumped with 9 mW 1.48 μm light from a laser diode, the amplifier shows 2.3 dB net optical gain at 1.53 μm. The gain threshold was 3 mW. The amplifier was doped with Er by ion implantation to a concentration of 2.7×1020 cm−3. The data agree well with calculations based on a model which includes the effects of cooperative upconversion and excited state absorption. For an optimized amplifier, net optical gain of 20 dB is predicted.