A phase‐locked laser diode array with three index waveguides is described. The array has a novel structure which prevents channels from being etched back. As the result, the waveguide elements in the array are very uniform. A stable 180° phase mode oscillation is obtained up to 50‐mW output power in the single longitudinal mode and three supermodes are observed simultaneously above 50‐mW output power. The near‐field patterns and the far‐field patterns of these supermodes are in good agreement with the theoretical results.