Abstract: The inventive quantum well opto-electronic devices of cavity length L.sub.C comprise, in addition to a "gain" section, a "loss" section that is optically coupled to the gain section but electrically substantially isolated therefrom. Through change of the electrical bias on the loss section the modal gain of the cavity can be changed. Devices according to the invention are useful in a variety of applications. Exemplarily, by making (in an inventive laser) L.sub.C /L.sub.A greater than about 20 (L.sub.A is the length of the loss section) and making the electrical isolation between the gain section and the loss section greater than about 3 k.OMEGA., fast amplitude modulation of the laser radiation is possibly. By applying appropriate AR coatings to the cavity of an inventive device, a fast variable-gain optical amplifier can be made. Finally, an inventive laser can be switched electrically between two widely spaced wavelengths, the wavelength difference being at least 10 nm.