Historically, high-energy (Joule-class) nanosecond YAG lasers have been confined primarily to laboratory research facilities and government labs. System designs were unique for each facility and the low number of systems worldwide made the learning curve for system improvement slow. It is only in recent years that the maturity of the technology has reached a state that makes it possible to consider these lasers for industrial applications.
In order to be successful in an industrial environment, these systems need to be user friendly and reliable. Subassemblies must be modular and easy to service. Finally, they must be capable of monitoring their performance parameters and maintaining them consistently with a high duty cycle. One of the emerging applications for this class of laser system is laser shock peening. While first demonstrated in the mid-1970s, it was only in the late 1990s that the technology was advanced to the point of commercial acceptance.
Shock Peening Lasers
Laser shock peening is a technique that generates compressive stress on the surface of a metal part, greatly enhancing its resistance to metal fatigue and micro-cracking. A sacrificial layer is applied to the surface of the target and is covered by a transparent layer - usually water. The laser energy is absorbed by the sacrificial layer and the plasma is contained by the water layer. The shock-wave is transmitted into the target material, creating compressive stress in the surface of the part.
Continuum (Santa Clara, CA) has been supplying laser systems for laser shock peening applications since the late 1990s. With an installed base of over 30 systems, the company has worked closely with the early adopters of the technology in developing the right tools for the job. While early systems used many of the same components as traditional laser sources, more recent systems have advanced to include features that make their introduction into true industrial environments possible, including distributed intelligence and monitoring systems.
High-energy YAG lasers consist of an oscillator and a series of amplifiers working together to create the necessary energy for the application. Repetition rates can range from 10 Hz to 50 Hz and more. Figure 1 shows a laser that delivers in excess of 25 J at1064 nm, and in the configuration shown, outputs 4 J at 532 nm in a 4-ns pulse.
The system architecture is modular, with distributed microprocessing throughout. Each oscillator or amplifier set has a dedicated microprocessor that monitors temperature and is available to provide controls for the system’s Q-switch, as well as accessories such as an intra-cavity shutter, automated attenuator, external interlocks, pyro detectors, and management of harmonic generators.
The microprocessor in each power supply controls the recirculating water supply, monitoring deionization (DI) resistance, flow, and temperature. It also manages the high voltage delivered to the lamps. Improvements in design of the core supply technology have led to longer lamp lifetimes, with up to a five-fold increase in some systems.