Technology The maser is based on the principle of stimulated emission proposed by Albert Einstein in 1917. When atoms have been induced into an excited energy state, they can amplify radiation at the proper frequency. By putting such an amplifying medium in a resonant cavity, feedback is created that can produce coherent radiation. [blockquote][/blockquote]Some common types of masers
First, a beam of atomic hydrogen is produced. This is done by submitting the gas at low pressure to an RF discharge (see the picture on this page).
The next step is "state selection"—in order to get some stimulated emission, it is necessary to create a population inversion of the atoms. This is done in a way that is very similar to the famous Stern-Gerlach experiment. After passing through an aperture and a magnetic field, many of the atoms in the beam are left in the upper energy level of the lasing transition. From this state, the atoms can decay to the lower state and emit some microwave radiation.
A small fraction of the signal in the microwave cavity is coupled into a coaxial cable and then sent to a coherent receiver.
The microwave signal coming out of the maser is very weak (a few pW). The frequency of the signal is fixed and extremely stable. The coherent receiver is used to amplify the signal and change the frequency. This is done using a series of phase-locked loops and a high performance quartz oscillator.
Terminology The meaning of the term maser has changed slightly since its introduction. Initially the acronym was universally given as "microwave amplification by stimulated emission of radiation," which described devices which emitted in the microwave region of the electromagnetic spectrum. The principle and concept of stimulated emission has since been extended to more devices and frequencies. Thus the original acronym is sometimes modified, as suggested by Charles H. Townes, to "molecular amplification by stimulated emission of radiation." Some have asserted that Townes's efforts to extend the acronym in this way were primarily motivated by the desire to increase the importance of his invention, and his reputation in the scientific community. When the laser was developed, Townes and Schawlow and their colleagues at Bell Labs pushed the use of the termoptical maser, but this was largely abandoned in favor of laser, coined by their rival Gordon Gould. In modern usage, devices that emit in the X-ray through infrared portions of the spectrum are typically calledlasers, and devices that emit in the microwave region and below are commonly called masers, regardless of whether they emit microwaves or other frequencies. Gould originally proposed distinct names for devices that emit in each portion of the spectrum, includinggrasers (gamma ray lasers), xasers (x-ray lasers), uvasers (ultraviolet lasers), lasers (visible lasers),irasers (infrared lasers), masers (microwave masers), and rasers (RF masers). Most of these terms never caught on, however, and all have now become (apart from in science fiction) obsolete except for maser and laser. In the early 1960s JPL developed a Maser to provide ultra low noise amplification of S band signals received from deep space probes. This Maser used refrigerated Hydrogen to chill the amplifier to four degrees Kelvin. Amplification was achieved by exciting a ruby comb with a 12 GHz Klystron. In the early days it took days to chill [and remove impurities from] the Hydrogen lines. Refrigeration was two stage with a large Linde unit on the ground and a crosshead compressor within the antenna; final injection was at 3000psi through a six thou inch [micrometer adjustable] entry to the chamber. The whole system noise temperature looking at cold sky [2.7K] was 17 degrees K. This gave such a low noise figure that the Mariner 64 Spacecraft could send pictures of Mars back to Earth even though the spacecraft transmitter was only 15 Watts and the signal received was -169 dbm