In this system, since the thicknesses of first birefringent crystal 231 and second birefringent crystal 232 are the same, {HEAD ~ any rotation} of {TAIL ~ birefringent displacer} 201 about the x axis would not introduce any optical path length change for beam component 233 and beam component 234 
In this system , since the thicknesses of first birefringent crystal 231 and second birefringent crystal 232 are the same , { HEAD ~ any rotation } of { TAIL ~ birefringent displacer } 201 about the x axis would not introduce any optical path length change for beam component 233 and beam component 234
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH