On the Various Time Constants of Wavelength Changes of a DFB Laser Under Direct Modulation

The temporal behavior of the optical frequency emitted by several DFB lasers under direct square wave modulation was measured using an all-fiber implementation of a Mach–Zender interferometer with an imbalance of 30 ps. The impulse response of the optical frequency to injection current modulation was found to contain a time constant as short as 10–20 ns, together with a few longer ones. The existence of such a short time constant is consistent with a thermal analysis of a laser structure with finite thermal impedance of the active region and should be taken into consideration in various wide bandwidth applications of direct modulated semiconductor lasers.