Improving the Modulation Bandwidth in Semiconductor Lasers by Passive Feedback

We explore the concept of passive-feedback lasers (PFLs) for direct signal modulation at 40 Gb/s. Based on numerical simulation and bifurcation analysis, we explain the main mechanisms in these devices that are crucial for modulation at high speed. The predicted effects are demonstrated experimentally by means of correspondingly designed devices. In particular, a significant improvement of the modulation bandwidth at low-injection currents can be demonstrated

I. Introduction

Directly modulated semiconductor lasers are of great interest for low-cost transmitter applications in short-reach and very short-reach optical data transmission systems. Mainly single-section distributed feedback (DFB) laser modules are considered for this purpose. However, until now, the system application of these lasers for data rates beyond 10 Gb/s per channel is limited by the optical modulation bandwidth (MBW), which is related to the relatively slow carrier–photon (CP) interaction as it appears in the relaxation oscillation of such lasers.