Corrugation-Pitch-Modulated Distributed Feedback Lasers with Ultranarrow Spectral Linewidth

We demonstrate that the spectral linewidth of distributed feedback (DFB) semiconductor lasers can be reduced by suppressing the longitudinal spatial hole-burning (SHB) effect. We confirm that the minimum spectral linewidth of lattice-matched multiple-quantum-well λ/4-shifted DFB (lattice-matched MQW-λ/4-DFB) lasers is limited by the SHB effect and we show that a corrugation-pitch-modulated (CPM) grating structure reduces the SHB effect while maintaining stable single-mode oscillation: at an output power of 25 mW, a lattice-matched MQW-CPM-DFB laser gives a spectral linewidth of 56 kHz. We further introduced the strained MQW structure into the CPM-DFB laser to obtain a narrower spectral linewidth. Introducing a 1.0% compressively strained MQW active layer into a CPM-DFB laser gives a spectral linewidth of 3.6 kHz at 55-mW output power and a linewidth floor (residual linewidth for extrapolated infinite output power) of 2 kHz and results in a linewidth-power product of 140 kHz·mW.