Heterodyne interferometer with unequal path lengths
  Laser interferometry is an extensively used diagnostic for plasma experiments. Existing plasma interferometers are designed on the presumption that the scene and reference beam path lengths have to be equal, a requirement that is costly in both the number of optical components and the alignment complexity. It is shown here that having equal path lengths is not necessary - instead what is required is that the path length difference be an even multiple of the laser cavity length. This assertion has been verified in a heterodyne laser interferometer that measures typical line-average densities of $\sim 10^{21}/\textrm{m}^2$ with an error of $\sim 10^{19}/\textrm{m}^2$. 
Deepak Kumar (1) and Paul M. Bellan (1) ((1) Applied Physics,   California Institute of Technology.)