We present a 445-nm InGaN blue diode laser in wavelength-tunable Littrow-type external cavity with a single longitudinal mode output, and a linewidth of 4.7 MHz. Its tuning range was estimated at 4 nm. The maximum output power was up to 20 mW, and the slope efficiency was approximately 0.36 W/A. The external cavity-diode laser linewidth was measured as 4.7 MHz with a lasing threshold to 8.1 MHz at higher applied current. The slope of the linewidth broadening with applied currents was calculated as 57 kHz/mA. And the wavelength tuning range was approximately 4.3 nm at low applied current and narrowed to approximately 2.6 nm at high applied current. A blue external cavity diode laser can deliver a single frequency beam within entire operation range.
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