The amplitude noise of optically pumped cw FIR lasers is drastically reduced by using a FIR ring resonator instead of a linear resonator. Laser emission switches from the clockwise mode to the counterclockwise mode at the FIR line center, a feature permitting precise frequency stabilization of FIR lasers. With an auxiliary retroreflector for one of the modes, unidirectional output occurs across the whole laser tuning width.