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.