  Coupling light into microdisk plays a key role in a number of applications
such as resonant filters and optical sensors. While several approaches have
successfully coupled light into microdisk efficiently, most of them suffer from
the ultrahigh sensitivity to the environmental vibration. Here we demonstrate a
robust mechanism, which is termed as end-fire injection. By connecting an input
waveguide to a circular microdisk directly, the mechanism shows that light can
be efficiently coupled into optical microcavity. The coupling efficiency can be
as high as 0.75 when the input signals are on resonances. Our numerical results
reveal that the high coupling efficiency is attributed to the constructive
interference between the whispering gallery modes and the input signals. We
have also shown that the end-fire injection can be further extended to the
long-lived resonances with low refractive index such as n = 1.45. We believe
our results will shed light on the applications of optical microcavities.
