English: Diagram of a ruby maser microwave amplifier for a radio telescope in 1960. The ruby maser (Microwave Amplification by Stimulated Emission of Radiation) is a low noise amplifier used at microwave frequencies, invented by a team at Bell Labs led by H. E. D. Scovil in 1958, which works on the same principle as a laser. It consists of a rod of synthetic ruby in a microwave cavity, in a magnetic field between the poles of a magnet, cooled to cryogenic temperatures by liquid helium in the dewar shown, attached to a waveguide. A "pump" beam of microwaves at a higher frequency beamed into the crystal excites the chromium atoms in the ruby to a higher energy level, creating a population inversion. The weak incoming microwaves from the antenna travel through the circulator and down the waveguide to the ruby crystal, stimulating the excited atoms to emit their energy, amplifying the input waves, which travel back up the waveguide through the circulator to the receiver.