Heavily doped active fibers based on the soft phosphate glass offer an attractive gain medium for compact and high-power laser oscillators. We report a passively modelocked fiber oscillator at 1.5μm based on such active fiber. The standing-wave laser cavity consists of a 20 cm-long piece of the side-pumped active phosphate fiber which is heavily co-doped with Er and Yb ions, and a low-ratio fused coupler. The length of the all-fiber laser cavity is 65 cm. The modelocked operation of the oscillator is started and sustained by a Semiconductor Saturable Absorber Mirror (SESAM), and no additional pulse narrowing mechanism is used. In order to avoid a premature over-saturation of the SESAM, the fiber end which is butt-coupled to the SESAM is adiabatically tapered which expands the propagating fiber mode and decreases the power density incident on the absorber substantially. The stable modelocked operation of the laser oscillator occurs in the range between 0.65 W and 2.3 W of the average output power, which is limited by the maximum available pump power at 975 nm. The peak pulse power is limited by the saturated SESAM at ∼450 W, and the pulse width grows from 11psec to 35 psec as the pump power is increased. At the pulse repetition rate of 160 MHz, the pulse energy reaches 14.4 nJ. Our laser oscillator combines the convenience of the all-fiber construction with the power performance previously achievable only with the modelocked bulk-optic laser oscillators or more complex systems involving fiber amplifiers.