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Spontaneous emission in the direction perpendicular to the lasing direction is experimentally examined in AlGaAs TJS lasers which lase in the fundamental transverse and single longitudinal mode. It is found that the spontaneous emission intensity uniformly saturates above threshold, spatially as well as spectrally. Spontaneous emission peak energy is compared to the lasing energy in a series of lasers with a different Al content in the active region. The energy difference between two energies is shown to be 50 meV and to be independent of the Al content from 0 to 0.26. The absorption coefficient in the active region is calculated from the spontaneous emission spectral shape, which is reasonably explained with the doping condition of the TJS lasers.