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Characteristics of a planar microcavity laser using rhodamine 6G with pulsed excitation is studied. Theoretical aspects of controlled spontaneous emission and oscillation in a planar microcavity laser are discussed. The measured spectrum and the angular divergence of spontaneous emission below threshold are in good agreement with theory. The angular divergence yields the radius of the cavity quasimode. The spontaneous emission coupling ratio obtained from the measured input‐output characteristics is in reasonable agreement with the theoretical value. The expression for the coupling ratio derived for a Fabry–Pérot‐type microcavity is shown to be essentially equal to that of a closed cavity or guided mode cavity derived by Yamamoto, Machida, and Björk [Phys. Rev. A 44, 657 (1991)]. The observed spectral behavior near the threshold approximately follows the Schawlow–Townes formula, but for a limited range of output power. The observed spectrum and divergence above threshold indicate incoherence much worse than the Schawlow–Townes limit.