Powerful lasers in the far-infrared wavelength range (47.6 and 57.2 μm) have been developed to measure the plasma density in the Large Helical Device at National Institute for Fusion Science and future plasma devices such as the International Thermonuclear Experimental Reactor. The intensification of these lasers has been done by cooling the laser tube wall, adding He as the buffer gas, and using a chemical-vapor-deposited diamond output window. The output powers for the 57.2 and 47.6 μm lasers have been found to be 1.6 and 0.8 W, respectively.