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 a complicated optical system is necessary for generation, or {HEAD ~ the generated beam} is {TAIL ~ low quality} 
is
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 a complicated optical system is necessary for generation , or { HEAD ~ the generated beam } is { TAIL ~ low quality }
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