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