NASA's Orion Artemis II Optical Communications System (O2O) will provide operational laser communications between the ground and lunar orbit for the Artemis II crewed mission. In this work we describe a 40 W ground-based laser transmitter for the O2O system. The uplink transmitter operates in the optical C-band and uses an energy-efficient 32- PPM modulation format. Four spatial diversity channels are time-aligned and combined in the far field. Each channel produces up to 10 W of output power and contains both the communications signal and the 7 kHz modulated beacon signal required for acquisition. The transmitter delivers data at 10 Mbits/s and 20 Mbits/s channel rates, corresponding to the 250 MHz and 500 MHz slot rates respectively.