Flat FM response without thermal dip for an optimized two section phase tunable DFB laser

The authors point out both theoretically and experimentally that photon distribution in two-active section lasers can greatly influence the FM response of such devices. Optical communication systems based on frequency shift keying (FSK) require laser sources featuring high FM efficiency for low spurious amplitude modulation, high bandwidth for high bit rate operation and flat FM response to avoid any degradation induced by long transmitted data sequences. These requirements are not fully met by standard distributed feedback (DFB) lasers, mainly because thermal effects induce a dip with phase inversion in the FM response at a relatively low frequency. To overcome this problem, a new two-segment DFB structure is proposed emitting at 1.5 μm with high FM sensitivity and flat FM response up to 1 GHz. The design of this structure relies on the optimum use of the effect of the inhomogeneous photon distribution along the cavity