More specifically, an axicon is necessary to generate approximate Bessel beams and the axicon has {HEAD ~ difficulty} in obtaining a highly accurate element and adjusting {TAIL ~ 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 
in adjusting
More specifically , an axicon is necessary to generate approximate Bessel beams and the axicon has { HEAD ~ difficulty } in obtaining a highly accurate element and adjusting { TAIL ~ 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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