Optical buffers are key components in optical packet switching network systems that prevent packet collision along the same virtual path and wait for header processing time. Conventional fiber loops for optical buffers present the drawback of unexpected oscillation at gain peak wavelength of the fiber loop. In this paper, an optical tunable delay line using a fiber ring with an acousto-optic frequency shifter and an erbium-doped fiber amplifier (EDFA) are demonstrated experimentally. The frequency shifter plays the role of preventing unexpected oscillation. Clear eye heights are visible in data packets through the delay line.