Solid-state optically pumped microwave masers

Optically pumped, CW microwave masers were successfully demonstrated using CaF
2
and SrF
2
doped with divalent thulium as the impurity ion. The optical pumping method made use of the polarization properties of the broad absorption bands of Tm
²⁺
to selectively pump the ground state maser levels. Cavity masers were operated as oscillators at 1.4°K at frequencies from 9.2 to 18 GHz and at 9.2 GHz for temperatures of 4.2°K. The figure-of-merit of this material based on the gain bandwidth product is equivalent to that of the conventional maser materials, ruby and Cr
³⁺
(Fe
³⁺
) in rutile, at 10 GHz and 4.2°K but has a higher figure-of-merit above 10 GHz. This material can be used to make CW masers up to about 30 GHz. The limitation on tunability of this type of maser is restricted to the microwave circuit since no tuning of the pump frequency is required. The upper operating temperature based on reasonable pump powers is, for intrinsic reasons, 6°K. For higher operating temperatures or higher signal frequencies at helium temperatures, a new material must be found having properties similar to the
F
center in the alkali halides but being more stable and in a more desirable host.