Cavity Length of LASER and Interference

Crumbles

I have been working on an experiment that uses double exposure holography to determine sub-micrometer displacements. [see attached picture for details]

It turns out that the optical path lengths LRP and LOP do not have to be exactly but roughly equal for the light if you want the light from R at P and the light from O at P to be in phase. I have been told that the two optical path lengths LRP and LOP have to be equal to within a factor of the cavity length of the LASER in use.

I am confused as to why you can still have LRP slightly larger or smaller than LOP and still have light at P that is in phase. Any help/insights is much appreciated.

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