Influence of gap width on mode discrimination in cleaved coupled cavity lasers

Recent publications on the C3 laser appear to have presented contradictory results for the optimum gap width to give maximum mode discrimination and thus enhance single-mode behaviour. We give here a simple analysis which shows that the optimum width is equal to an odd multiple of a quarter wavelength in cases where loss in the gap can be neglected, but to an even multiple for cases with significant gap loss.