The energy density distribution of the lasing mode inside 1.6-mm-long (a) uniform grating structure and (b) sampled grating structure with sampling duty cycle of 60%. Only half of the structure is simulated for saving computational memory. (c) The envelopes of normalized energy density intensity inside 1.6-mm-long uniform grating structure and sampled grating structure. The inset shows the detail of the energy density distribution near the lasing facet.