A maser is a type of technology similar to a laser. Where a laser is a directed energy tool or weapon based on light amplification by the stimulated emission of radiation, a maser instead uses microwave amplification, or electromagnetic fields instead of coherent light beams, to produce similar results. The concept behind the maser was first conceived in 1953 when the US physicist Charles Townes built one, but it is based on an earlier understanding of stimulated emission of radiation principles established by Albert Einstein in 1917. The technology has been used in developing extremely precise clocks, in astronomical research, and has use in potential weapons applications.
Atomic beam masers usually use a form of gas that is charged to an excited state in a chamber, which causes that gas to emit radiation that is stored by a resonator, where it is later channeled out of the chamber through a small opening. The maser created by Townes was an ammonia maser, and its power generation was very low at 0.00001 watt. The beam of energy it produced was so precise, however, that it was seen to be useful as an extremely accurate clock that could show a correct time to the second for hundreds of years.
Where the maser is based on a gas, it only interacts along a range of narrow frequencies that is characteristic of the gas. This can make it a very precise amplifier in areas of research that study such a gas, however. The hydrogen maser is very effective at amplifying and measuring weak signals in space in radio astronomy. The hydrogen maser has also been used as a form of atomic clock more accurate than the one based on ammonia, and models as of 2011 are capable of not losing or gaining one second of erroneous time measurement in more than 100,000 years of recording.
Where amplification of radiation or radio wave signals is required over a band of different frequencies, a solid state maser is used. The ruby maser is an example of this, based on principles that a ruby laser also uses. The ruby crystal is excited in the presence of a magnetic field to tune a signal being studied along a specific frequency, and interacts well with very weak signals. This gives ruby masers ideal applications for amplifying signals sent back to Earth by distant space probes sent to other planets, or satellites orbiting at a fraction of a distance to the Moon, such as geosynchronous satellites. Such a maser has also been used to measure radio waves emitted by planets such as Venus to study the chemical composition of its atmosphere and its temperature.
Other types of commonly used masers in research include the rubidium and microwave varieties. The rubidium maser is gas-based, using a vaporized form of the silver-white metallic element. It is a useful laboratory tool in optical pumping, where electrons in an atom are excited to a known quantum state so that they can be studied. Microwave masers are used in cosmology research to study the cosmic microwave background radiation in space that formed as a result of the early expansion of the universe. This is known as the research field of microwave radiometry.