This paper describes the construction of a single-mode diode laser having an external resonator, stabilized by the resonance of saturated absorption in the D₁-line of the K atom, and presents the results of investigation of its fluctuation characteristics and efficiency of pumping ³⁹K vapors. Results show that a hitherto unattainable sensitivity of the magnetometric sensor of 1.8×10^-15 T Hz ^1/2 , and that in the range of intensities need for optical pumping of ³⁹K, amplitude fluctuations are reduced to shot fluctuations.