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