Any light that is outside the short coherence length will not interfere.
The significance is that interference will be strong within a coherence length of the source, but not beyond it.
However in this case the coherence lengths are associated with "mixed" combinations of density fields.
The coherence length is simply the distance that the laser light can travel in the coherence time.
The coherence length of the laser determines the depth of field which can be recorded in the scene.
White light has only a very limited coherence length.
The extent of the fringes depends on the coherence length of the source.
It does not pay to use a nonlinear crystal much longer than the coherence length.
The most important parameters of the light source are its wavelength and coherence length.
In order for interference to occur, the path length difference must be smaller than the coherence length.