Nonlinearities in high-index-coupled λ/4-shifted distributed-feedback semiconductor lasers

Distributed-feedback λ/4-shifted laser diodes with index coupling factor κ as high as 300 cm^-1 have been tested. The linewidths have been compared with the results of two computer models of the device including nonlinear effects. The fundamental role of local intensity-dependent effects, which take place inside the cavity, is pointed out. It explains the absence of any saturation of the curve of the linewidth against inverse power that is usually expected in this kind of device at high injection levels