They are caused by the different paths taken by both brances which cause a phase difference. However, the fringes, disappear if linear polarizer filters of different orientation are placed in the two branches before unification in the second splitter. This is a extrapolation of quantum behavior on a macroscopic level, because the interference can occur even if only a single photon is moving across the system is in any given time. This may sound counterintuitive, but the photon actually interacts with itself because it takes both paths at the same time. Our project involves many more photons but the behavior is the same.
Interesting things happen when photons pass through our polarizers.
This tags the path taken by the photons. The tagging in a way makes the photon to take either path or the another, but not both at the same time. Which implies that it can not interact with itself. And finally it would not produce an interference pattern. The nature of this behavior is beautiful by itself.