Both beams are injected into {HEAD ~ an unstable laser resonator aperture} in {TAIL ~ the center} of a primary mirror in the resonator 
in
Both beams are injected into { HEAD ~ an unstable laser resonator aperture } in { TAIL ~ the center } of a primary mirror in the resonator
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH