Novel applications of sub-surface laser machining

ABSTRACT Lasers can uniquely be used to create physical changes inside a bulk material. Traditional manufacturing processes are limited to surface modifications, but a laser can be focused at any location inside a material transparent to that wavelength. Using sub surface machining methods with ultrashort pulse lasers two practical applications are demonstrated. First, a laser is used to sever short-circuite d wires embedded deep inside a th ick piece of glass, effectively repairing a defective wire network. Second, subsurface bar-coding was shown to produce readable markings. Surface laser markings were shown to weaken the glass, but subs urface marking had virtually no effect on strength. Keywords: sub-surface, laser, barcode, marking, cutting, transparent materials, transparent armor, femtosecond, ultrashort pulse 1. INTRODUCTION In gift shops all over the world, blocks of glass can be found with 3-D images laser etched inside them. Marking inside a transparent material is a novelty application of lasers and a manufacturin g capability that no other tool offers. This method can be utilized to make practical marks such as barcodes inside bulk materials. Also, using the same subsurface laser processing principle, manufacturing mistakes made in transparent materials can be corrected. This paper examines how this technique can be utilized for two very different applications for which there are few alternatives. Most automobiles have heating elements in the form of thin wires bonded to the rear window as a defrosting agent to remove condensation, ice and snow. They connect to an electrified bus and the current running though the network of wires generates heat to defrost the window. Typically these are surface mounted systems, but in some cases the heating wires are embedded within the glass. Transparent armor (more commonly referred to as bulletproof glass) can be several inches thick and the tungsten wire defrosters are embedded as a network throughout the glass. Transparent armor is naturally more expensive and harder to manufacture than conventional windows. Recently a set of windows produced for military use were found to have a wiring flaw. The windows contained a network of tungsten wires less than a thousandth of an inch thick (25 microns) connected to a bus that ran just inside the perimeter of the window. The ends of the wires near the peri meter were not installed properly creating a short circuit. These unwanted connections occurred from extraneous wire in the glass. The windows ranged in thickness from with the wires embedded at a depth between 10mm and 55mm. When voltage was applied to the defrosting system, short circuits in the wiring would cause localized hot spots which would glow brightly and crack the glass; causing increasing damage with continued use. Not only would this reduce the efficiency of the defroster, but would weaken the armor, putting US soldiers at risk. The glow of the short circuits made them easy to identify, but a difficult problem to fix. The Pennsylvania State University El ectro-Optics Center (EOC) was approach ed to investigate if lasers had a potential solution to fix these windows, rather than scrap them. The EOCs laser micromachining lab holds a variety of common and exotic lasers to provide process development research for manufacturers. Using ultrashort pulse laser technology to maximize instantaneous power at the focal point within the glass and minimize thermal conduction to prevent cracking, experiments were conducted attempting to sever the short circuits. The second application for subsurface laser processing relates to bar-codi ng items. In manufacturing processes for high value critical parts it is often required that an item is individually labeled for tracking purposes. In the event of