Using Photon Momentum To Measure High Cw Laser Power And Pulse Energy

The inherent momentum of photons generates a force when a laser beam reflects from a mirror. Since this force is linearly related to the laser beam's optical power, we can use this radiation pressure effect as an optical power meter for lasers. We have successfully demonstrated this approach as a high-accuracy CW laser power meter for powers ranging from 1 kW to 50 kW. Here we investigate the possibility of using the same experimental setup to measure the pulse energy in high energy laser pulses. Without access to a high energy pulsed laser, we perform preliminary tests with high energy pseudo pulses using a short (10 ms to 1 s) duration square wave output from a CW Yb-doped fiber laser for average powers from 2.5 kW up to 10 kW. We tested "pulse" energies from 50 J to 10 kJ. We demonstrate theoretically that integrating the measured force on a mirror when these pulses are reflected equates to the energy per pulse even for pulse durations shorter than the force sensor's response time. Experimentally, we show a linear relationship between the launched pulse energy and its value measured with radiation pressure. Our measurement noise floor is on the order of 100 J per pulse and we find a preliminary discrepancy of 8 % between the radiation pressure measurement and the known pulse energy. Using a modified scale with an analog output, tested with magnetically emulated pulses brings this discrepancy down to approximately 1 %.