[time-nuts] Sapphire whispering gallery mode masers?

[Gregory Maxwell]

The recent mention of WGM sapphire oscillators and recent threads
about time-nut constructable secondary frequency standards reminded me
of a paper I ran into a while back about a WGM maser.

The mechanical of the device were similar to an ordinary WGM
oscillator: cryocooled sapphire crystal in a vacuum. But
electronically it was pretty different, a dopant in the crystal could
be RF pumped at some frequency far away from one of the normal high-Q
modes of the oscillator and formed a three-stage laser whos emission
bandwidth included the normal (~10GHz) high-Q WGM, which it would then
happily lase at.  They reported that the signal power was
significantly higher than a AHM.

The advantage of the construction is that the maser level is
controlled by saturation, and so it was not very sensitive to the
intensity stability of the pump.

It seemed to me that it might be a possible candidate for a difficult
but achievable 'home' experiment for an oscillator with exceptional
short to medium term stability in a way that something like a hydrogen
maser isn't.  (Then again, I've never worked with anything cooler than
LN2).

Unfortunately I can't find the paper, but I doubt I dreamed it.