More specifically, an axicon is necessary to generate approximate Bessel beams and the axicon has difficulty in obtaining a highly accurate element and adjusting an optical axis in use, and therefore, a great technological burden is required for generating approximate Bessel beams of {HEAD ~ a sufficient quality} for {TAIL ~ the purpose} mentioned above 
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More specifically , an axicon is necessary to generate approximate Bessel beams and the axicon has difficulty in obtaining a highly accurate element and adjusting an optical axis in use , and therefore , a great technological burden is required for generating approximate Bessel beams of { HEAD ~ a sufficient quality } for { TAIL ~ the purpose } mentioned above
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