Complex-coupled λ/4-shifted DFB lasers with a flat FM response

Theoretical analysis of the wavelength tuning characteristics shows that the carrier effect in λ/4-shifted DFB lasers with in-phase complex grating can produce a large red shift by a self-suppressing mechanism of the spatial hole-burning effect while maintaining stable single-mode operation. The FM response of these lasers can thus avoid having a dip at lower modulation frequencies, where the thermal red shift opposes the carrier blue shift of conventional lasers. An in-phase complex-coupled λ/4-shifted DFB laser with a ridge structure is described that gives a flat FM response from 10 kHz to over 20 GHz (3-dB bandwidth) achieved by using the self-suppressing mechanism of the spatial hole-burning effect. This bandwidth is the widest ever achieved for frequency modulation