nis ball and how fast it is moving. In the same way, when light reflects from a mirror, the mirror is pushed. Since all light travels at the same speed (about 299,792 km per second), the force felt by the mirror will depend only on the number and energy of the photons (light particles) hitting it in a given amount of time. (Think of billions of trillions of infinitesimally small tennis balls hitting the mirror every second). It turns out the force on the mirror is conveniently proportional to the optical power of the laser light that is being reflected. While this effect may seem insignificant, there are several familiar examples of radiation pressure. For instance, the tail of a comet points away from the sun in part due to the radiation pressure from the sun pushing on the gas and ice that make up the tail; spacecraft on long missions must correct for the force of sunlight in order to arrive at their target; and microscopic objects are manipulated by the force of light in a technique known as “optical tweezing.” With increasing numbers of highpower lasers (1 kW and above) being used in laser welding operations, and the availability of scale technologies that can accurately measure changes in mass as small as 1–10 μg, radiation pressure can be easily measured. Practically, we have found that for multikilowatt lasers, their force on a mirror can be measured simply with a commercial scale. Putting Theory Into Practice We use a prototype device that we call a “radiation pressure power meter” (RPM) based on a commercially available mass-measuring scale and a highquality (Distributed Bragg Reflector) mirror that reflects more than 99.9% of the incident laser light. The scale has a unique design that allows it to operate sideways (that is, unlike conventional scales that measure a force in the downward direction, this scale can measure a push in the horizontal direction). The scale can sense a change in mass as small as 10 μg. By reflecting the welding laser light from a mirror that is attached to the scale, we can measure the force imparted by the light without absorbing the light (less than 0.1% is absorbed). By recording the light force (radiation pressure), we measure the optical power of a laser beam. For perspective, 10 W of laser light causes a force of 66.7 nN (nanoNewtons), which is roughly the weight of an eyelash, 1 kW of light pushes with 6.67 μN (about the weight of a grain of sand), and 100 kW generates a force equal to the weight of about two staples (667 μN). The prototype radiation pressure power meter was added to our welding workstation as shown schematically (an overhead view) in Fig. 2 and with a photograph of its implementation in Fig. 3. Because our current prototype was designed for a horizontally travelling laser beam, we modified the welding setup by removing the light delivery (“process”) fiber from the vertically positioned weld head and used an optical collimator to establish a collimated laser beam (all of the light travelling in an essentially parallel direction). This beam was reflected from the sensing mirror in our RPM and then was focused by a lens onto the workpiece. Of course, accurate scales are notorious for their difficulty in operating in a vibrating environment, in the presence of air currents, or if their temperature is changing significantly. Inside a laser welding workstation, all three of these can be a problem. The scale was mounted to the inner floor 32 WELDING JOURNAL / MARCH 2016 Fig. 3 — Radiation pressure power meter (RPM) in welding workstation. The focused laser light exits the cover glass (false-colored red beam) and is focused onto the workpiece (pipe).
Welding Journal | March 2016
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