Directly modulated lasers with graded-κ grating for stable single-mode operation and bandwidth enhancement

We present a graded-κ grating technique that enables stable single-mode lasing and robust modulation-bandwidth enhancement in directly modulated lasers. Internal reflection at the interface between the active and passive regions typically degrades detuned-loading and photon-photon resonance effects, which limit high-speed operation. The proposed graded-κ structure suppresses these reflections by smoothing the photon-density and refractive-index transition, resulting in fluctuation-free bandwidth scaling and reliable activation of single-cavity etalon effects. The device achieves a 3-dB bandwidth of 47.5 GHz and supports 112-Gbps PAM4 transmission with TDECQ values lower than 2.9 dB up to 45 ℃, demonstrating its suitability for next-generation optical interconnect solutions in datacenters.