A common multi-gas ring down detector incorporates a cavity that has a piezoelectric mirror and at least two displaced mirrors to define two different transit paths in the cavity. The two paths intersect at the piezoelectric mirror at different angles. Two different laser beams having first and second different wavelengths, can be coupled to the cavity, at different times, by driving the piezoelectric mirror axially. Beam outputs can be evaluated to establish the presence of selected gases in the cavity.