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, some part of {TAIL ~ the incident laser beam} being reflected by the optical member to return to the beam splitter and {HEAD ~ 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 , some part of { TAIL ~ the incident laser beam } being reflected by the optical member to return to the beam splitter and { HEAD ~ the other part } being transmitted through the optical member
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