The pulse lasing in the broadband spectral range 420–590 nm was obtained from two squeezed together heavily Si doped Al0.74Ga0.26N/AlN/Al2O3 structures with external cavity under pump radiation with λp = 266 nm wavelength. These structures were grown by molecular beam epitaxy on (0001) oriented thick sapphire substrates. Stimulated emission at room temperature was observed with optical pumping thresholds 50 kW/cm2 in the semiconfocal cavity. The linear transverse-electric polarization dominates in the output emission. The optical gain coefficients values of (0.12–6) × 103 cm−1 for Al0.74Ga0.26N:Si films with inhomogeneous luminescence broadening were measured at 10–600 kW/cm2 pump power density radiation with 266 nm wavelength, 8 ns pulse duration and 10 Hz repetition rate.