We have been studying a high finesse enhancement cavity for photon target of Laser Compton Scattering X-ray generation. It is very important to develop an extremely stable external optical cavity for laser Compton scattering. At the same time, a stable seed laser oscillator for an incident laser of an optical cavity is also very important tissue. Thus, we have been developing a stabil mode-locked Yb-doped fiber laser based on Non-Linear Polarization Rotation. We have generated laser pulses which have 102.9mW average power at repetition rate of 119MHz. Furthermore, we started accumulating lasers in the optical cavity, and we have already confirmed that our oscillator is able to accumulate in the cavity.