Maser

I

Introduction

Maser, acronym for microwave amplification by stimulated emission of radiation, a device that amplifies or generates microwaves or radio waves. A maser producing radiation in the optical region is called a laser.

II

Principle of Operation

As in lasers, amplification of radiation in masers is obtained by stimulated emission. This occurs when a photon induces an excited atom or molecule to fall to a lower energy state while emitting a photon of the same frequency as the incoming photon. The emitted photon travels in the same direction and in phase with the incoming photon, which is not absorbed during the interaction. The amplitudes of the two waves add up, and amplification of the incoming wave has taken place. Masers make use of those transitions in molecules or crystals that correspond to the energies of microwave or radio frequencies.

III

Types of Masers

The first maser oscillator was developed by the American physicists Charles Hard Townes (1915- ), James P. Gordon, and Herbert J. Zeiger in 1954, and made use of the frequency of the ammonia molecule. This frequency corresponds to the energy of the photon emitted when the nitrogen atom moves from one side to the other of the triangle formed by the three hydrogen atoms in an ammonia molecule. The hydrogen maser makes use of the frequency corresponding to that of the photon released when the spin of the proton in a hydrogen atom flips over with respect to the spin of the atom's electron. Paramagnetic masers use energy transitions corresponding to the orientations of the magnetic moments of paramagnetic ions in crystalline substances placed in an external magnetic field. Different frequencies can be obtained by varying the magnetic field, thus allowing the tuning of a paramagnetic maser from less than a megacycle to several hundreds of megacycles.

IV

Applications

Because of the high stability of the generated frequencies, masers serve as time standards in atomic clocks. Masers are also used as low-noise radio frequency amplifiers in satellite communication and radio astronomy.

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