Square microcavity laser with an output waveguide is proposed and analyzed by the finite-difference time-domain （FDTD） technique. For a square resonator with refractive index of 3.2, side length of 4μm, and output waveguide of 0.4μm width, we have got the quality factors （Q factors） of 6.7 ×10^2 and 7.3 × 10^3 for the fundamental and first-order transverse magnetic （TM） mode near the wavelength of 1.5μm, respectively. The simulated intensity distribution for the first-order TM mode shows that the coupling efficiency in the waveguide reaches 53%. The numerical simulation shows that the first-order transverse modes have fairly high Q factor and high coupling efficiency to the output waveguide. Therefore the square resonator with an output waveguide is a promising candidate to realize single-mode directional emission microcavity lasers.
