We propose an acousto-optically tuned external-cavity laser diode (ECLD) based on the Littrow configuration to expand the wavelength-scanning range and eliminate mechanical motions. The scanning range and tuning rate of our prototype were 68.85 nm and more than 100 kHz, respectively, without any mode hop. Moreover, we implemented this improved ECLD in an optical coherence tomography imaging system and conducted a measurement of the two-dimensional thickness distribution of a thin glass plate via continuous wavelet transform analysis.