Highly efficient single longitudinal-mode oscillation capability of gain-coupled distributed feedback semiconductor lasers-advantage of asymmetric facet coating

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Abstract
The gain-coupled (GC) distributed feedback (DFB) laser is found to possess an uncommon and useful feature; unlike the quarter-wave phase-shifted (index-coupled) DFB laser, it is possible to enhance single-facet power extraction efficiency by applying an asymmetric antireflection-high-reflection (AR-HR) coating without demolishing its intrinsic single-mode property. A 1-90% coating gives more than 92% of output power from one of the facets while preserving Delta gL, the difference between the lowest and second lowest normalized threshold gains, larger than 0.2 in 90% of the devices.