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The fraction of spontaneous emission going into an oscillating laser mode has been calculated. It is shown that this fraction strongly depends on the strength of astigmatism in the laser output beam. Therefore the spontaneous emission factor in planar stripe lasers with narrow stripe is in the order of 10-4and by one order of magnitude larger than in injection lasers with a comparable active layer volume and with a built-in index waveguide. It is shown that the spontaneous emission factor is approximately proportional to the solid angle of laser radiation and nearly independent of the transverse active layer dimensions. Owing to the large spontaneous emission factor, the spectral width of narrow planar stripe lasers is significantly broader compared to narrow stripe lasers with a built-in index waveguide. In addition the large spontaneous emission coefficient also yields a much stronger damping of relaxation oscillations.