Further, {TAIL ~ even the conventional techniques} for generating approximate Bessel beams are associated with difficulties in {HEAD ~ optical techniques} 
Further , { TAIL ~ even the conventional techniques } for generating approximate Bessel beams are associated with difficulties in { HEAD ~ optical techniques }
OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD