Comment by bkloppenborg

We've been doing optical interferometry for quite some time. Homodyne techniques (see facilities, CHARA, VLTI, COAST, NPOI, SUSI) in which the light is interfered with itself are quite common. Heterodyne methods (one facility, ISI) in which the incoming light is mixed with a stable laser and downconverted to longer wavelengths are uncommon though.