A method, apparatus and computer-readable storage medium are described for a tunable laser source that produces a desired frequency modulated optical waveform with a precision within 0.01 percent over a bandwidth greater than about 50 gigaHertz. An apparatus includes a tunable laser having one or more drive inputs for affecting an optical frequency of light output by the laser; and an optical detector. Multiple optical paths are configured to direct light output by the laser onto the optical detector. A laser controller is configured to provide to a drive input a loopback signal based on a measured or predetermined difference in optical dispersion among the plurality of optical paths and a detector signal output from the optical detector. In some embodiments, a ranging device includes the tunable laser source.