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