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