A superluminescent diode (SLD) which simultaneously generates incoherent optical radiation and amplifies this radiation has been made from double-heterostructure (DH) AlxGa1-xAs-GaAs material previously used to make lasers and light-emitting diodes having superior characteristics. The device configuration was similar to that of the stripe-geometry DH injection laser, except that optical feedback was suppressed by providing absorption for the backward waves in the cavity. A single-pass gain device resulted. The stripe contacts were 12.5 μm or 25 μm in width, and from 0.125 to 1.5 mm in length. The single-pass gain was found to be proportional to ( , where β is a gain coefficient depending on the pump current and is the stripe length. The structure was found to favor low-order modes, and the half-power radiation beam angle in the junction plane was measured to be . The radiation field was slightly TE polarized, and it exhibited a spectral width of 50-85 Å. When operated in the pulsed mode, the longest diodes (1.5 mm) radiated 60-mW peak power at a peak-current density of 6.5 kA/cm2(2.5-A current) and about 10 mW at 3 kA/cm2. The external differential quantum efficiency was 3.5 percent. Due to the relatively narrow radiation pattern of the SLD, more than 80 percent of the radiation from this source has been coupled into a multimode optical fiber with a numerical aperture of 0.63. These measured characteristics were found to conform to those calculated on the basis of a simple model.