Spectral stability analysis of weakly coupled external-cavity semiconductor lasers

The spectral stability of weakly coupled external-cavity semiconductor lasers is studied theoretically. The carrier density modulation induced at the frequency of the external-cavity mode spacing f
ex
plays an important role in the spectral instability. Therefore, the laser under optical feedback reveals different spectral behaviors, depending on the relation between the frequency of the external-cavity mode spacing and the relaxation resonance frequency f
R
of the carrier density. For a better spectral stability, f
ex
must be larger than f
R
.