A sufficiently large electrical potential is applied between {TAIL ~ the anode} and cathode to cause a discharge current therebetween which results in the production of a pair of counter propagating laser beams within {HEAD ~ the block} 
A sufficiently large electrical potential is applied between { TAIL ~ the anode } and cathode to cause a discharge current therebetween which results in the production of a pair of counter propagating laser beams within { HEAD ~ the block }
OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD