More specifically, {TAIL ~ an axicon} is necessary to generate approximate Bessel beams and the axicon has difficulty in obtaining {HEAD ~ 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 a sufficient quality for the purpose mentioned above 
More specifically , { TAIL ~ an axicon } is necessary to generate approximate Bessel beams and the axicon has difficulty in obtaining { HEAD ~ 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 a sufficient quality for the purpose mentioned above
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