A power meter for non-destructively measuring the power of a high energy er beam. The power meter includes a cell disposed in the flow path of a gas containing a laser radiation-absorbing species. The absorption coefficient of the absorbing species is small enough to allow the beam to pass through the cell essentially unaltered. The concentration of the absorbing species is modulated to modulate its absorption of the laser beam power and produce acoustic waves in the gas which can be detected and measured to give an absolute measurement of the power in the high energy laser beam.
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