Here is a drawing of our microwave spectrometer:
The microwaves are injected into the cavity at the (left)
mirror,
the pulse of radiation builds up a brief standing wave
in the cavity provided the second mirror is n/2 wavelengths distance form
the first mirror. Molecules of interest are injected into the cavity
from above, and absorb part of the standing wave, a consequent emission
signal is then recorded after 'listening' at the same antenna where the
signal was sent in.