Coupled-cavity tunable-single-frequency diode lasers are used in conjunction with an electronically tunable external cavity to increase the range of a coherent lidar system. Three-terminal DFB lasers and cleaved-coupled-cavity lasers are used which allow continuous wavelength tuning over a range greater than 2A by direct modulation of the injection current. This tuning range results in a distance resolution of less than a centimeter, without signal averaging or other enhancement techniques. By using an electrooptic waveguide phase modulator in the external cavity we have maintained a narrowed laser linewidth while simultaneously tuning the solitary laser. We have achieved a coherence length of 50 meters, a tuning range of 40 GHz, and a simultaneous tuning range-coherence length product of 750 GHz-m.