A wide range of applications have emerged for tunable ytterbium fiber lasers as single-frequency sources for spectroscopic applications, pumping source of Pr:ZBLAN amplifier, and Tm:ZBLAN upconversion laser, material processing, and military applications. In this paper, a 975-nm high power fiber coupled high power diode laser module of up to 5 W end pumping a ytterbium-doped multimode D-shaped fiber laser has been investigated. This used a Fabry-Pérot cavity with output coupler reflectivities of 80%, 60%, and Fresnel reflection of 4%. The output laser wavelength was tuned over a wide range of more than 50 nm, from 1041 to 1094 nm for cavity lengths from 1 to 10 m, respectively. An optical-to-optical slope efficiency of 45% was found for a 1-m cavity length, which increased to 60% for a 4-m cavity length. The maximum slope efficiency of 82.1% for a cavity length of 2 m was measured with the Fresnel reflection output coupler, and the measured lowest threshold pump power for this high gain cavity configuration was 130 mW. The threshold lasing pumping powers of 4.3, 4.5, and 4.7 W were dependent on the output coupler reflectivities of 80%, 60%, and Fresnel reflection of 4%, respectively. Also, self-pulsation phenomena were observed only at higher level pumping powers of more than 4 W and at longer cavity length.