The laser radiation is reflected back and forth within the optical resonator with partial waves from individual passes overlapping each other. When the wavelength of the radiation field is a multiple of twice the distance between the mirrors, the partial waves overlap constructively, otherwise they overlap destructively. This leads to a wavelength selection - the resonator thus restricts both the direction of propagation and the frequency of the laser light. The non-linear interaction of the laser beam field with the active medium reduces the bandwidth in addition to limiting the frequency. In highly stable lasers, a bandwidth of under 1 Hz can be achieved (at an average frequency of approximately 5x1014 Hz).
One of the mirrors is usually highly reflective while the other mirror is finitely reflective and lets some of the radiation through. The transmission from this output mirror can be anything from less than 1% to 70 - 80% depending on the type of laser. The optimal value for maximum power output depends on the intensification in the active laser medium and on the losses due to absorption, scattering and diffraction. Too large a transmission reduces the intensity of the radiation field within the resonator and thus in the active volume: spontaneous absorption and other processes which relax the higher laser energy level, prevail over stimulated emissions. If the transmission is too low, the radiation field will be reflected back and forth even more. Absorption at each pass due to contamination, scattering and diffraction, reduces the radiation - the losses increase with each pass (some of the laser radiation does not strike the mirror and is also lost). The amplification and the losses determine the optimal value for the transmission.