The remaining step in utilizing optical amplification to create an optical oscillator is to place highly reflecting mirrors at each end of the amplifying medium so that a wave in a particular spatial mode will reflect back upon itself, gaining more power in each pass than is lost due to transmission through the mirrors and diffraction. When these conditions are met for one or more longitudinal modes, then radiation in those modes will rapidly build up until gain saturation occurs, resulting in a stable continuous laser-beam output through the front (typically 99% reflecting) mirror.