The overall objective is to demonstrate the concept of Yb:YAG crystalline fiber MOPA laser and investigation the technical feasibility toward 50 mJ single frequency MOPA system in the Phase-II research. Onyx Optics crystalline fiber waveguides are made from true rare-earth doped YAG crystals with Adhesive-Free Bond (AFB®) technology. Compared with silica or phosphate glasses, the YAG crystalline fiber waveguides have the following advantages: (i) YAG crystal has at least one order of magnitude lower Stimulated Brillouin scattering (SBS) gain coefficient than silica or phosphate glasses (10-15 to 10-12 vs 10-11 m/W), which directly leads to the SBS free power being at least one order of magnitude higher than the glass fibers; (ii) YAG crystal has much higher thermal conductivity than glass fibers (10.7 vs 1.38 W/m degree C). Therefore, much shorter fiber length that is only about one tenth of the glass fiber can be used for the same pumping conditions and the SBS threshold can be further increased; (iii) Due to the shortening in the fiber length, straight fiber can be practically used for high power amplification. Therefore, large single mode area (LSMA) can be more easily achieved. Considering the high pulse energy requirement in a future LIST mission, only large mode area (LMA) Yb:YAG fibers with core size >40 mm will be fabricated and investigated in the Phase-I research. The fibers will be double clad to increase the pumping power and efficiency, while maintaining near diffraction limit laser beam quality.