The compensation mechanism may be a glass plate that is disposed in an optical path of {HEAD ~ a polarization component} so that the optical path length of that polarization component can be made substantially equal to the optical path length of the other polarization component that traverses through {TAIL ~ a half wave plate} 
The compensation mechanism may be a glass plate that is disposed in an optical path of { HEAD ~ a polarization component } so that the optical path length of that polarization component can be made substantially equal to the optical path length of the other polarization component that traverses through { TAIL ~ a half wave plate }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL