Coplanar electrode directly modulated lasers with different cavity length

Abstract

A ridge-waveguide (RW) directly modulated laser (DML) with a coplanar electrode structure is proposed and measured in this paper. Since utilizing the coplanar electrode can significantly reduce parasitic capacitance, the frequency responses of DMLs exhibit a quite low roll-off. Shortening the cavity length L is attractive to enhance the high-speed performance of DMLs, but it will increase the mirror loss and the chip resistance at the same time. We analyzed the static and dynamic characteristics of 200-μm, 250-μm, and 300-μm lasers experimentally and theoretically to investigate the impact of L on the coplanar-electrode DML. Shortening the cavity length can improve the relaxation oscillation frequency f r effectively. The 200-μm DML achieves a bandwidth of 15.3 GHz. The f r is the most important limiting factor for the laser to obtain a wide bandwidth. Decreasing L and increasing the differential gain are promising for the further improvement of f r . However, shortening L causes a more severe thermal effect and the deterioration of the linearity. This trade-off must be considered in the future study.