  Recently, on-chip single-mode laser emission has attracted considerable
research attention due to its wide applications. While most of single-mode
lasers in coupled microdisks or microrings have been qualitatively explained by
either Vernier effect or inversed Vernier effect, none of them have been
experimentally confirmed. Here, we studied the mechanism for single-mode
operation in coupled microdisks. We found that the mode numbers had been
significantly reduced to nearly single-mode within a large pumping power range
from threshold to gain saturation. The detail laser spectra showed that the
largest gain and the first lasing peak were mainly generated by one disk and
the laser intensity was proportional to the frequency detuning. The
corresponding theoretical analysis showed that the experimental observations
were dominated by internal coupling within one cavity, which was similar to the
recently explored inversed Vernier effect in two coupled microrings. We believe
our finding will be important for understanding the previous experimental
findings and the development of on-chip single-mode laser.
