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