A novel method has been developed to stabilize the amplitude of a multiwavelength laser beam. The method is based on the use of an acousto‐optic tunable filter to diffracted light which has more than one wavelength. The number of the wavelengths in the diffracted light is controlled by the number of the rf signals which are applied into the filter. Because the intensity of each wavelength in the diffracted beam is dependent on the power of the corresponding applied rf signal, the total amplitude of the multiwavelength diffracted beam can be stabilized, i.e., adjusted at a constant amplitude and maintained at that level, by individually controlling the intensity of each corresponding wavelength by use of an rf driver with a feedback loop for each wavelength. In addition, the method also provides a unique means to amplitude‐modulate one (or more) wavelength(s) of a multiwavelength beam at a specific frequency while keeping the other wavelengths at a constant amplitude level.