1.55-μm DFB lasers with narrow ridge stripe and second-order metal surface gratings by holographic lithography

We report a high-performance operation of laterally coupled distributed feedback (LC DFB) lasers with a narrow ridge width of 2μm, based on second-order metal surface gratings by holographic lithography. Through theoretical analysis, the laser structure was optimized in terms of ridge width of laser, order of grating, and isolation layer around the ridge for more stable and improved device operation. By carefully controlling the photoresist patterning and etching, the second-order metal surface gratings of δ=489 nm with a duty factor of 0.25 could be achieved. The 300-μmlong laser exhibits an output power of 9mWand a threshold current of 24.8mA at 25 °C and 100mA in a continuous-wave mode, with still 2mW at 60 °C. Stable single-mode operation with a sidemode suppression ratio up to 41 dB, a temperature-tuning coefficient of 0.08 nm/K, an electrical power-tuning coefficient of 14.97 nm/W was observed for the LC DFB laser with a self-aligned SiO ₂ isolation layer, estimating a gain coupling coefficient of ∼27 cm ^-1. The beam full-width at half-maximum of far-field patterns are approximately 23.1 and 42.2° for the horizontal and the vertical directions, respectively.