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, {HEAD ~ the amount} of phase modulation is constant in {TAIL ~ each} of the areas, and the amounts of phase modulation differ by π between two adjacent areas out of the areas 
is constant in
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 , { HEAD ~ the amount } of phase modulation is constant in { TAIL ~ each } of the areas , and the amounts of phase modulation differ by π between two adjacent areas out of the areas
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