We report on discrete mode laser diodes designed for narrow linewidth emission and demonstrate a linewidth as low as 96 kHz. A discrete mode laser diode with a minimum linewidth of 189 kHz was also characterised in a coherent transmission setup using quadrature phase shift keying modulation. Similar performance to an external cavity laser is demonstrated at baud rates as low as 2.5 Gbaud. The effect of increased linewidth on transmission performance is also investigated using lasers with linewidths up to 1.5 MHz. © 2012 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).