The wave-guiding properties of organic semiconductor lasers operating at 63
0nm have been analysed. Calculations for gain-guided lasers show that singl
e lateral mode operation can be supported in structures having active regio
n widths of under 6 mu m, albeit with confinement highly dependent on activ
e layer thickness, Ridge-guided structures are investigated as a means of p
roviding single-mode operation with stable lateral confinement at high-gain
values. It is shown that ridge heights of about 0.03 mu m are sufficient t
o provide lateral confinement of about 0.5.