Beam quality measured in design
23 April 2009
Solid-state lasers are the same as any other product – the customer needs to trust the quality of the beam and other parameters. Mike Mason, Vice President of Technology at Powerlase, discussed with EPD the methods by which lasers are tested to ensure that performance can be guaranteed. Powerlase manufactures high power diode-pumped, solidstate lasers for industrial applications; these lasers can range from 100 W – 1600 W average power and up to several Megawatts of peak power with repetition rates of tens of kHz.
Powerlase defines laser development and design through the needs of their customers and therefore, their testing stage is largely directed by the end-user application. The most important parameters, as for most lasers, are power, pulse duration, beam quality and divergence, and in some cases, numerical aperture.
Power measurement is quite straightforward, with the use of thermal power meters which measure the temperature of the beam and equate it to the power output of the laser. The pulse duration is measured with fast rise time (1-2 ns) photodiodes with the pulses displayed on a Tektronix digital scope for visual clarity. However, assessing the beam quality is a much more tricky matter. The M^2 factor is a common measure of the beam quality of a laser, and indirectly determines how small the beam spot size can be. A beam with an M^2 factor of 1 will have a Gaussian profile, with higher M^2 values limiting the degree to which the beam can be focused for a given beam divergence angle. Powerlase use an opto-mechanical system manufactured by Spiricon which measures the beam profile and divergence and meets the ISO standard for the M^2 measurements. Finally, initial system design is based on results from extensive computational modelling thus ensuring the reliability of the laser during the testing phase.
Powerlase lasers are developed for industrial applications primarily for use in the materials processing and microelectronics markets for flat panel displays, microelectronics, automotive and aerospace sectors. Dr. Mason finds an important aspect of laser testing is consistency of measurements between the customers, suppliers and Powerlase. For example, engineers at Powerlase must ensure that signal generators provide trigger signals in the same way the customers intend to use them in their own applications. Similarly, the exact operating conditions must be understood between the customer and Powerlase in order to determine the output parameters and avoid any performance disputes. Therefore, agreeing specifications with customers before the delivery of the laser is paramount. Additionally, turning physics into finished products requires robust engineering systems; early lasers and demonstration units often face problems in the field, but testing these lasers by replicating problems and solving them in-house is what finally makes the system reliable.
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