an optical member for partially reflecting and partially transmitting an incident beam, the laser beam reflected by the beam splitting means being incident onto the optical member, {HEAD ~ some part} of {TAIL ~ the incident laser beam} being reflected by the optical member to return to the beam splitter and the other part being transmitted through the optical member 
an optical member for partially reflecting and partially transmitting an incident beam , the laser beam reflected by the beam splitting means being incident onto the optical member , { HEAD ~ some part } of { TAIL ~ the incident laser beam } being reflected by the optical member to return to the beam splitter and the other part being transmitted through the optical member
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