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-f iber implementation of a Mach-Zender interferometer with an imbalance of 30 ps. The impulse response of the optical frequency to injection c urrent 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 structu re with finite thermal impedance of the active region and should be ta ken into consideration in various wide bandwidth applications of direc t modulated semiconductor lasers.