General considerations in unbalanced fourth-order interference

Interferometry has been used widely in sensing application. However, the technique is limited by the finite coherence time of the light sources when the interference paths are not balanced. Higher-order interference effects involve intensity correlations between multiple detectors and may have the advantage over the traditional second order interference effect exhibited in only one detector. We discuss various scenarios in fourth-order interference with unbalanced delays in different paths. We find in some cases, interference effect persists even when the delays are much larger than the coherence time of the sources. We also extend the discussion to non-stationary pulsed fields, which needs to consider the pulse shape and requires a different treatment. These results will be useful in remote sensing applications.