What is an unbalanced interferometer? I have read in some papers about a so-called unbalanced interferometer. This appears particularly in the context of experimentally verifying the Englert-Greenberger-Yasin duality relation. However, I can't figure out exactly what they mean by an unbalanced interferometer, and whenever I search for the definition of an unbalanced interferometer I only get more papers and I can't find a straightforward definition. So what exactly is an unbalanced interferometer?
 A: People might use the term in different contexts to mean slightly different things.
When you are interested in interference between two beams that have a different history (path), you normally want to ensure two things:


*

*equal intensity

*equal path length


Without the former, you would risk any interference pattern being "washed out" (because the pattern have an intensity associated with the weaker components, superposed on an intensity associated with the difference). Without the latter, you run into a problem of temporal coherence: if beams travelled different distances, then anything but perfect monochromatic light will exhibit some loss of coherence and reduced intensity of the interference pattern. In fact this effect can be used to accurately estimate the line width of a spectral line: plotting the intensity of the interference pattern as a function of the path difference tells you about the coherence length $L$; the relative line width is then related to the coherence lenght by $\frac{\delta \lambda}{\lambda}=\frac{\lambda}{L}$, or $\delta \lambda = \frac{\lambda^2}{L}$.
When either of these conditions is not met, the interferometer is "unbalanced". But which kind it is depends on the context.
A: It simply means the arms/ path length traversed by two split beams inside the interferometer is not equal resulting in destructive interference. 
