Optimal cavity lengths and duty cycles for second-order laterally-coupled distributed feedback lasers

Using laterally-coupled gratings in a DFB laser produces a more manufacturable design that can be easily monolithically integrated with a variety of other photonic devices. Higher order gratings further improve the manufacturing tolerances, at the cost of more optical loss compared to first-order gratings. We analyze the effect of duty cycle and cavity length on the threshold gain, Bragg detuning, normalized gain difference and optical field flatness of laterally-coupled DFB lasers with second-order gratings. A duty cycle of 0.7 and a cavity length of 300 mum was found to result in a high-performance laser design. The duty cycle can be used to trade off low threshold gain (for duty cycles ~0.8) and high mode discrimination (for duty cycles ~0.5) when designing these lasers.