There is a huge difference between a maser (ultra low noise microwave preamp..used until about 20 years ago in radioastronomy, now only to be found in museums (eg with MPI for radioastronomy in Bonn,Germany) .
The other aspect..a microwave oven has a miserable spectrum I would not really call this coherent. I dont understand you question"heats only in one direction.."are you talking about a nonreciprocal device(isolator/circulator)