A maser is a device that amplifies electromagnetic waves by stimulated emission. The principle of operation of a maser is similar to that of a laser, but the wavelength of the amplified radiation is usually much longer than that of a laser. Masers are used in radio astronomy and as microwave amplifiers in communication systems.
The first maser was built by Townes and Schawlow in 1954. It consisted of a rod of ruby surrounded by an electromagnet. When microwaves were passed through the rod, they caused the atoms in the ruby to emit light at a wavelength of about 694 nanometers. This light was then amplified by stimulating other atoms to emit more light at the same wavelength. The output from the maser could be made very intense, and it was soon realized that masers could be used as powerful microwave amplifiers.
Since then, masers have been built using many different materials, including water vapor, ammonia, carbon dioxide, and semiconductor crystals such as gallium arsenide. The wavelengths of the amplified radiation can range from about 1 millimeter (30 gigahertz) to 1 meter (300 megahertz).
Masers are used in radio astronomy to study distant objects such as quasars and pulsars. They are also used as local oscillators in some types of radio receivers, and as microwave amplifiers in long-distance telephone and data links.