The Frequency Modulation Response of Vertical-Cavity Surface-Emitting Lasers: Experiment and Theory

The FM response of vertical-cavity surface-emitting lasers (VCSELs), i.e., the dynamic wavelength tuning behavior, is scrutinized. The FM amplitude and phase shift are measured up to 80 MHz for GaAs-, InP-, and GaSb-based VCSELs from 763 to 2300 nm. From measurements, it is found that the FM response consists of three components: intrinsic thermal tuning (dominating to several megahertz) with characteristic 1/√(if) behavior, the plasma effect (dominating from several megahertz), and a small effect (10-100 Hz) caused by the interaction of laser chip and submount. All effects are modeled and the measurement data are fitted to obtain effective thermal diffusivities, strength of the plasma effect, and time constant of the laser chip submount interaction. Comparing thermal models with different asymptotic behaviors, an approximation of the heat source in the laser with a nonzero thickness turned out to be necessary. Due to the plasma effect, with influence starting at 100 kHz, VCSELs cannot be considered a minimum phase system, which makes separate amplitude and phase measurements essential for device characterization. The “ N time constants model” is the proper choice for empirical description of the intrinsic thermal tuning component. The best fit coefficients to a rational frequency response are given for use in time-domain simulation programs.