568 Microwave Engineering
13.8.2 Types of Masers
These can be classied into (i) atomic beam masers which include ammonia
maser, free electron maser and hydrogen maser, (ii) gas masers which include
rubidium maser and (iii) solid-state masers which include Ruby maser. Some
of these masers are described below:
13.8.2.1 Ammonia Maser
These masers use ammonia (NH
3
) gas as working material. Since ammo-
nia molecules are already distributed in two different (E
1
and E
3
) levels, no
pumping is required. With the help of a dc magnetic eld the higher energy
molecules are focused into a microwave cavity resonator. The frequency
of this resonator corresponds to natural frequency of ammonia molecules.
Inside this cavity there will be downward t