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