sorbed by the photodiode and generates an electrical signal, which becomes a measurement of the incoming laser power. This gives a fast, real-time way to monitor the power from the laser. Since these delicate photodiodes cannot survive laser powers of more than a few milliwatts, they cannot measure the full laser beam. During laser operation, a very tiny fraction of the laser’s light is diverted to the photodiode. The power measured by the photodiode is proportional to the total power in the laser, but that proportion is difficult to quantify because the ratio of diverted light to the total laser power (the “pickoff ratio”) is so small. For example, if a photodiode capable of measuring up to 10 mW of optical power were used to measure a 1-kW laser, only 1/100,000 of the laser’s power would be directed onto the photodiode. If we wanted merely a 10% uncertainty in the estimate of total laser power, we would need to know the power in the pick-off beam to a staggering 0.0001% of the total laser power. Thus, while the pickoff approach provides a fast and real-time measurement of laser power, its lack of calibration makes it poorly suited to high-accuracy power measurements. Welding power measurements by means of the present force-balance technology, however, are also not ideal due to the limited response time. Figure 5 shows an example of the laser power measured during a circumferential weld done on the stainless steel pipe with the setup described previously. The laser power was stepped up halfway through the weld. The figure shows both a power measurement from the RPM and the voltage from an uncalibrated photodiode. Clearly, the radiation pressure result is important because it measures accurate laser power, but the 5-s rise time (seen by the rounded corners on the rising edges of the RPM plot in Fig. 5) limits its ability to measure any changes in laser power that might occur on a timescale faster than a couple of seconds. This illustrates the continued usefulness of the pick-off power monitor as a fast measurement. We are considering potential improvements of the measurement speed of the scale. In the interim, perhaps the best result will be a hybrid where the radiation pressure power meter calibrates the photodiode in real time (eliminating concerns about drift in the pickoff ratio) to provide a fast and accurate measurement. Conclusion The results shown here demonstrate that a radiation-pressure-based approach of measuring the power output of a welding laser based on its push rather than the heat it generates is an exciting prospect. We showed that this technique can be used in a welding environment to achieve accurate real-time laser power measurements during a complete-joint-penetration weld. This radiation pressure technique provides accuracies and response times that meet or exceed the best specifications for thermal power meters but with the advantage of realtime measurement during a laser weld. Future development work will consider how to improve the response time through scale technology or in conjunction with other techniques (e.g., photodiode pickoff). Acknowledgment The authors thank A. Feldman for his work developing the Distributed Bragg Mirror. Note This work of the U.S. Government is not subject to U.S. copyright. 34 WELDING JOURNAL / MARCH 2016 Fig. 5 — Plot of optical power measured with the radiation-pressure power meter (red, upper curve) and the uncalibrated photodiode pickoff monitor voltage (blue, lower curve). WJ PAUL WILLIAMS (paul.williams@nist.gov) and BRIAN SIMONDS are physicists and JEFFREY SOWARDS is a metallurgist with the National Institute of Standards and Technology, Boulder, Colo. For info, go to www.aws.org/adindex
Welding Journal | March 2016
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