Multigigahertz bandwidth FM response of two-electrode DFB lasers

Abstract

We report the experimental and theoretical characterization of the FM response of a two-section DFB laser emitting at 1.3 µm. Such tunable lasers have emerged as important components for various FDM and FSK optical communication systems. Experimentally determined FM responses for similar structures have been reported previously only for modulation frequencies below ~1 GHz. We extend these measurements of two-section DFB lasers into the multigigahertz frequency range. The FM response magnitude is large and flat for frequencies from 10 kHz up to several hundred megahertz. This is followed by a dip in the response at ~1 GHz and a relaxation resonance peak at a few gigahertz. However, this dip does not limit the speed of the device, as is evident from the flat AM response of this laser at 1 GHz. In proper bias conditions, an FM response magnitude >1 GHz/mA and a 3-dB FM bandwidth of ~5 GHz is demonstrated, including the shallow dip and increased response of the resonance; we believe this to be the widest FM bandwidth reported to date for such lasers. As the bias is increased in the modulated section, the resonance peak becomes damped and moves out to higher frequencies. Both the characteristic response dip found at ~1 GHz and the resonance at a few gigahertz depend strongly on the bias conditions. The laser can be biased in a regime where there is either a red or blue frequency shift with static current tuning. In comparison with the static red-shifted FM response, the blue-shifted response has a more severe dip at ~1 GHz and a strongly damped resonance. Consequently, a blue-shift-bias produces a much lower 3-dB bandwidth than a red-shift bias. The observed behavior is well represented by theoretical curves derived from a small-signal analysis of the coupled rate equations. This model characterizes the laser behavior by only a few parameters; namely, the relaxation frequency, the photon and carrier lifetimes, a damping factor, and the asymmetry between the two sections.