10-21] SOLID STATE AMPLIFIERS 575 



Since then, remarkable progress has been made, and these types of devices 

 will certainly be used in many radar and communication systems of the 

 future. 



MASERS (Molecular Amplification by Stimulated Emission of 

 Radiation). Amplification is accomplished in a MASER by establishing 

 an excess population density in an excited molecular energy state so- that 

 the assembly of molecules emits electromagnetic energy in making transi- 

 tions to a lower energy state when stimulated by a low-power signal. 

 Coherent amplification of stimulating frequencies corresponding to the 

 energy-level change is obtained in this manner. Solid-state MASERS are 

 constructed using paramagnetic salts as the active material, with the 

 external electromagnetic field interacting with the magnetic moments of 

 bound electrons or their spin levels. For this reason such MASERS are 

 often called paramagnetic amplifiers. Normally, a very strong and uniform 

 d-c magnetic field is required to separate the spin levels sufficiently. Also, 

 very low operating temperatures are normally required to provide usable 

 efficiencies and the low noise operation of MASERS. For some applications, 

 it may be possible to obtain the required low temperatures by magnetic 

 cooling with only a moderately cold environment. To date, the auxiliary 

 equipment needed to provide the d-c magnetic field and the cooling has led 

 to rather bulky designs. 



Various possibilities exist for the choice of energy levels and methods for 

 producing inverted populations of molecules in which there are more than 

 a normal number in a high-energy level state, and the assembly of molecules 

 is in a condition to emit radiation when stimulated. The simplest type is a 

 two-level device. In the most common design for a two-level MASER a 

 strong auxiliary microwave pulse at the transition frequency inverts the 

 populations at the two levels by causing more upward than downward 

 transitions, since there are normally more molecules in the lower level. 

 Two-level MASERS normally provide only intermittent operation, since 

 amplification is not possible while the inverting pulse is being applied, 

 although some two-level systems have been constructed which operate 

 continuously. 



In three-level MASERS, a continuous pumping signal is used to equalize 

 the populations of the lowest and highest levels. An inverted population 

 condition is thus produced between one of these levels and a third inter- 

 mediate level. This method provides continuous operation since the 

 pumping signal is at a different frequency than that which is emitted. 



Another possibility which has been proposed as a means of obtaining an 

 emissive state is to pump with polarized light of a wavelength corre- 

 sponding to an optical transition of the active material. The light from the 

 pump is absorbed and shortly reradiated as unpolarized light. This results 



