The second mirror is configured to receive {HEAD ~ a second incident laser beam} from {TAIL ~ the second end} and reflecting a second reflected beam to intersect the first reflected beam and to reflect as a fourth reflected beam from the fourth mirror 
The second mirror is configured to receive { HEAD ~ a second incident laser beam } from { TAIL ~ the second end } and reflecting a second reflected beam to intersect the first reflected beam and to reflect as a fourth reflected beam from the fourth mirror
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL 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 OTH OTH OTH