We have investigated some of the properties of distributed-feedback (DFB) lasers made in active thin-film optical waveguides. By optically pumping these guides with spatially modulated light we have obtained narrow-line tunable oscillation. Operation in single-mode waveguides is well behaved and understood. Measurement of the oscillating wavelengths can be used to determine accurately the parameters that characterize the optical waveguide. This DFB measurement technique offers several advantages over the prism-coupler measurement technique for use with active films.