The lasing characteristics of a tunable-single-frequency laser operated in conjunction with a tunable external cavity are investigated. The effect of phase error in the external cavity on the lasing frequency, linewidth, and output power is calculated by deriving the characteristic equation for the FP cavity as a traveling amplifier with an effective mirror concept. The results compare well with experimental data obtained with a 1.3-micron cleaved-coupled-cavity laser operated with a PZT-mounted external mirror. It is found that the diode laser and the external cavity may be successfully tuned open-loop if the feedback strength is appropriate and the tuning nonlinearities in the active and passive cavities are not too severe. This is demonstrated by maintaining a linewidth of 4 MHz while tuning continuously over 34 GHz, using an electrooptically tuned external cavity with a two-section DFB laser which had a 40-MHz linewidth under similar operating conditions with no external cavity.