Distributed-feedback (DFB) resonators [1, 2] are of interest for inherently producing narrow-linewidth laser emission, without the need of external cavities or filters. The single-frequency emission is obtained by designing an additional phase shift δϕdesign of π/2, thereby producing a resonance that is spectrally located at the centre of the reflection band. Conventionally, δϕdesign is introduced abruptly in the Bragg grating, but this method has the important disadvantage of causing spatial-hole burning in the laser resonator due to a high photon density where the phase shift is located. An attractive solution is to distribute the phase shift, e.g. by widening the waveguide region, thus increasing the effective refractive index .
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