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Water Aids Laser Materials Processing

Photonics Spectra
Apr 1998
Dr. Flavio Fontana

According to an ancient Roman saying, single drops of water can penetrate rocks. Inspired by this wisdom, a group of Swiss researchers at the Polytechnic Institute of Lausanne has discovered that a clever combination of hydrodynamics and laser engineering could lead to unprecedented results in the field of materials processing.

In the microjet system from the Polytechnic Institute of Lausanne, a water jet guides a laser beam to cut metal.

Protecting lenses

Materials processing scientists know that the limited depth of field of even the most elaborate laser-focusing lenses can be a real drawback in laser cutting and marking applications. Moreover, the delicate optical elements must be protected efficiently from vaporized metal and more massive debris. Until now these problems have caused objective difficulties in the spread of this branch of laser technology.

The solution proposed by the institute's B. Richerzhagen is quite simple: A thin jet of compressed water can act as a lightguide, transferring Nd:YAG beams over distances of several centimeters, yet retaining a cross section of about 70 to 100 µm.

This elementary solution offers many advantages over traditional approaches. A water jet is extremely resistant to laser damage, and water allows efficient cooling of the part processed by the laser. When a metal sheet is drilled by this method, it acquires remarkable mechanical properties because of lower tempering and has cuts with parallel edges. The "water fiber" can transfer laser radiation in sites such as deep holes and narrow grooves that cannot be reached by usual methods.

The system, developed by Synova S.A., a spinoff company of the institute, is based on a LASAG AG flashlamp pumped Nd:YAG laser with an average power up to 450 W. The heart of the system is its pump/nozzle unit, which produces the water jet and couples the laser beam into it with minimal losses.

Designers of this subunit have applied the principles of hydrodynamic engineering to avoid instabilities and oscillations of the very thin jet that could impair the overall efficiency of the coupling process and limit the maximum length over which laser light can be transmitted. In the current version, the system features a numerically controlled working table and computer supervision unit, making it easy for even nonspecialized operators to use.

This unconventional approach to laser cutting is enjoying increasing success among European manufacturers of high-tech mechanical parts. They appreciate the possibility of processing heat-sensitive materials and the reduced need for purging of waste metallic vapors, which the water absorbs nearly completely.

Continued research will investigate additional options such as in situ control of the jet displacement by electrostatic or magnetic means.

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