Introduction: Laser glass welding – how USP laser welding improves the laser beam weld quality
Laser glass welding is a well-established process in many industries, where laser power is used to heat glass and melt it selectively, causing two glass pieces to bond together in a high strength bond. Originally, laser glass welding utilized CO2 lasers, as these were the first high power sources and CO2 radiation has strong absorption in glass. This strong absorption limits the CO2 laser beam weld to a surface weld, forcing the laser to melt the entire glass top-to-bottom, making the process slow.
The advent of high power ultra short pulse (USP) lasers enables novel laser glass welding concepts based on non-linear absorption of the laser in narrow, well defined focal regions. This has the potential to create strong welds by melting only the weld area, without wasting energy on heating the entire glass bulk. However, the high-power densities required for the USP laser glass process tend to demand high NA focusing, limiting the depth of focus and thus the laser beam weld depth to a narrow layer.
In this article we review some diffractive optics beam shaping solutions that can enhance laser glass welding with UPS lasers, enabling extended depth of focus and a thicker welding layer while suppressing the emergence of processes such as cracking and trapped gas bubbles.
Laser glass welding with USP laser- typical process flow, challenges and constraints
Typical high output power lasers used in welding are USP fiber lasers, mostly in the IR ~1um wavelength. The two glass surfaces to be welded are then brough in as close contact as possible, and the laser is typically applied from the top by focusing it on the bottom of the upper glass piece, using a high NA objective.
The USP laser generates a melt pool in the glass only where the laser spot is at focus, as this is where the nonlinear absorption occurs. While local, relatively “cool” and fast, this type of USP laser welding does pose several challenges:
- Due to practical constraints, there is always some air gap between the glasses. Overcoming this gap is generally referred to as “bridging”. In USP laser welding, the melt pool rapid cooling creates strong tensile stress on the weld when the melted glass that expanded into the gap cools and resolidifies. This can create cracks in the weld when the gap is too large.
- Another cause of welding cracks in UPS laser welding application is the dynamics of glass heating. When the glass is still cold, the UPS laser often creates cracks that are later absorbed by the melt pool and eliminated. However, if the melt pool is elongated (large depth and narrow waist, cracks can propagate far beyond the melted area and are not eliminated. This means that high NA focusing must be used in laser glass welding to make sure the depth of focus is of an order similar to the beam waist.
- These bridging mechanics and crack formation limit the types of material and glass surface quality ranges that can be welded, as high-rate laser glass welding machines cannot afford clamping glass pieces together, or welding only high flatness and optically polished glass.
Laser beam shaping solutions to enhance USP glass welding
Holo/Or diffractive optical elements can shape and split a laser beam to generate any desired intensity distribution. This flexibility opens various options for overcoming laser glass welding constraints:
- Diffractive Beam splitters can turn a single welding spot into and array (or ring) of spots. This can improve the laser beam weld seam quality by spreading the energy over several melt pools while still maintaining non-elongated melt pool shape. This allows for a larger melt pool that can bridge larger gaps without cracking due to elongation.
- Multifocal DOEs, specifically Bifocal Does (BF), can split the focal spot into two foci with pre-set separation. This can allow melting both glass surfaces simultaneously, thus reducing cracking and decreasing the effective bridging gap by half.
- Spiral phase plates can be used to generate a more uniform melt pool, due to the ring-shaped heating they enable , allowing for reduction of cracking and increased bridging distance.
Conclusion
Laser glass welding using ultra short -pulse lasers is a promising process with many applications that is gradually being adopted in more industrial applications. USP glass laser welding applications include laser welding of end caps for fiber, glass hermetic sealing of microchips that are implanted in the body and many other high-end applications.
The USP laser beam weld process is challenging and requires overcoming various process related issues such as cracking, bridging gap and the need for tight focusing. Diffractive optical elements can modify the laser beam shape in a manner that enhances the USP laser glass welding process, enabling welding over larger bridging gaps and at higher speed.SP
