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