Conventionally, there have been known several light beams whose central spot diameters when being focused are smaller than the diffraction limit, however, all of {HEAD ~ the beams} have had problems such that a complicated optical system is necessary for generation, or {TAIL ~ the generated beam} is low quality 
Conventionally , there have been known several light beams whose central spot diameters when being focused are smaller than the diffraction limit , however , all of { HEAD ~ the beams } have had problems such that a complicated optical system is necessary for generation , or { TAIL ~ the generated beam } is low quality
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