When areas sectioned by p circumferences centered on a predetermined position are set on a beam cross section of light input to the optical phase modulator 15, the more outside each of the areas is, the wider the radial width of the area, the amount of phase modulation is constant in {TAIL ~ each} of {HEAD ~ the areas}, and the amounts of phase modulation differ by π between two adjacent areas out of the areas 
When areas sectioned by p circumferences centered on a predetermined position are set on a beam cross section of light input to the optical phase modulator 15 , the more outside each of the areas is , the wider the radial width of the area , the amount of phase modulation is constant in { TAIL ~ each } of { HEAD ~ the areas } , and the amounts of phase modulation differ by π between two adjacent areas out of the areas
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