![]() This is done using a series of phase-locked loops and a high performance quartz oscillator. The coherent receiver is used to amplify the signal and change the frequency. The microwave signal coming out of the maser is very weak (a few pW) and the frequency is extremely stable but can not be changed.A small fraction of the signal in the microwave cavity is coupled into a coaxial cable and then sent to a coherent receiver.The resonant frequency of the microwave cavity is exactly tuned to the hyperfine structure of hydrogen: 1420 405 751.768 Hz. This combination of amplification and feedback is what defines all oscillators. The stimulated emission amplifies the microwaves on each pass through the beam. ![]() A high quality factor microwave cavity confines the microwaves and reinjects them repeatedly into the atom beam.From this state, the atoms can decay to the lower state and emit some microwave radiation. 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. This is done in a way that is very similar to the famous Stern-Gerlach experiment. 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 by submitting the gas at low pressure to an RF discharge (see the picture on this page). ![]()
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