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 generation, or the generated beam is {TAIL ~ 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 the beams have had problems such that { HEAD ~ a complicated optical system } is necessary for generation , or the generated beam is { TAIL ~ low quality }
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