Industrial applications for femtosecond laser sources

By Louise May, Senior Applications Engineer, Luxinar

Femtosecond laser sources represent a technology that has revolutionised materials processing due to their ability to provide both power and control.  In order to fully utilise the available power and translate it to high throughput processing, femtosecond lasers must be used in conjunction with high-speed motion systems such as galvo and polygon scanners.  The spatial overlap of consecutive laser pulses on the workpiece is a critical parameter for optimal machining quality, whether cutting, engraving, surfacing, marking or scribing.  While conventional methods typically result in inconsistency and overprocessing of complex profiles due to the acceleration and deceleration of the motion system, pulse-on-demand technology allows individual pulses or controlled pulse bursts to be placed with precision and evenly spaced, regardless of variations in motion speed.  Although alternative approaches such as skywriting? may be used to produce consistent results, these introduce a significant amount of dead time to the process, resulting in lower overall process speeds and longer cycle times.

Meanwhile the extremely short pulse width of the femtosecond laser source can virtually eliminate heat diffusion to the surroundings of the processed region, provided the pulse width is shorter than the time taken to transfer heat to the bulk material.  The process is commonly referred to as “cold ablation”.  Additionally, the pulse-on-demand feature effectively allows the user to control the repetition rate to avoid heat accumulation in the material, whilst maintaining a constant pulse energy.  This minimises the formation of a heat-affected zone (HAZ), enabling ultrahigh precision micro- and nanofabrication of various components with high throughput and repeatability.

Luxinar has patented a femtosecond laser source, the LXR series, that will allow entry into new markets and applications in a diverse range of industries that were previously inaccessible with our range of CO2 laser sources.  Sample processing using this ultrashort pulse laser is currently being carried out at the University of Hull (UK) in its department of Physics and Mathematics.  This article outlines just a few of the potential industrial applications for our femtosecond lasers.

Sample processing at the University of Hull

The research group at the University of Hull has extensive expertise in laser material interactions, and their laboratories are equipped with specialist equipment including an advanced motion system for sample handling.  In addition they have access to characterisation techniques such as scanning electron microscopy (SEM).  Processes they have investigated to date with the LXR series include flat cutting of metals, plastics and composites, tube cutting of metals and biodegradable plastic materials for medical devices such as coronary stents, and high-quality bitmap marking.  Several potential applications are outlined below.

Sub-surface glass marking for the pharmaceutical industry

Glass marking is an application where the difference between femtosecond and conventional CO2 lasers is clear; both systems are useful in different ways.  The CO2 laser wavelength is absorbed so strongly in glass that the entire interaction happens at the surface.  While this is an advantage in some applications where surface marking is desirable (such as head keepers in beer glasses), it means that it is simply impossible to mark below the surface.  However, this is not the case with femtosecond lasers, which are able to mark in the bulk of optically transparent materials.  Although glass is transparent at the laser wavelength, nonlinear absorption effects allow the formation of a dense plasma at the focal point, which then becomes strongly absorbing and produces marking in the volume of the material.  In this way, the LXR 120-1030 has successfully marked 0.5mm below the surface of a piece of glass.  The mark is made using closely spaced dots to form characters less than half a millimetre high; in fact, the marking is barely visible to the naked eye.  Potential applications include syringe and vial marking for the medical and pharmaceutical industries; sub-surface marking is particularly attractive in this setting, as the codes are indelible and not subject to wear or damage. 

Glass cutting for the automotive industry

Thick glass cutting is an interesting application where femtosecond and CO2 lasers are used in combination.  Traditional mechanical scribe and break techniques are still commonplace in this industry, or scribing and cutting can be performed using a CO2 laser alone; however the quality of results is not optimal in either case.  While both of these processes are quick and efficient, the edges of the glass are typically left with numerous microcracks.  Furthermore, the traditional methods cannot easily produce curved shapes or rounded contours, which are often required for automotive rear view and exterior mirrors, for example.  Superior results can be produced by using a femtosecond laser to scribe the glass.  This process requires high energy, which may be delivered as a single pulse or as a fast, high-energy burst.  A CO2 laser is then used to supply thermal energy which initiates cleaving along the scribed line.  The glass is separated cleanly with little or no damage due to microcracking at the edge, and curves, straight lines and closed contours can be produced.

Scribing metal foils

Another application making use of the ultrashort pulse laser’s precision is the scribing of metal foils for RFID and mobile device antenna applications, where aluminium and copper are commonly used.   The laser is used to selectively remove the conductive metal layer from a carrier or substrate material, typically paper or polyimide, to fabricate antennas and other flexible circuit components.  The constant drive towards miniaturisation calls for narrow lines to be scribed as close together as possible.  However, because the adjacent lines must be electrically isolated from one another, the metal must be removed very cleanly from the substrate, with no damage to the surrounding area and no burrs or rough edges which may form short circuits between device features.  The LXR 120-1030 has succeeded in scribing lines in aluminium foil which are 50µm apart (measured from centre to centre), without significant damage to the carrier material.  Again, this result would simply not be possible with a CO2 laser.

Cutting and drilling carbon fibre composites

Carbon fibre composites are now commonplace in the automotive and aerospace industries.  While these materials exhibit desirable properties including a high strength-to-weight ratio, drilling to form rivet holes for joining parts and other machining is problematic.  Mechanical processing can result in delamination and matrix damage, as well as considerable tool wear and breakage.  On the other hand, their anisotropic thermal and optical properties present significant problems for conventional laser machining.  The thermal conductivity of the fibres is much higher than that of the epoxy matrix, while the matrix material is vaporised more readily.  Laser processing often leaves the fibres exposed as a result, compromising the strength of the material around the cut. 

This is where the femtosecond laser comes in; heat diffusion is greatly reduced, so cutting or drilling can be accomplished leaving the epoxy resin intact in the surrounding material.  This means that holes can be drilled and features cut out without compromising the strength of the carbon fibre sheet, which is one of the key reasons for choosing this material in the first place.  The avoidance of heat accumulation is critical in this case.  Samples of carbon fibre composite were cut successfully using the LXR 120-1030, operating the laser at a demand frequency of 1MHz to maximise the available pulse energy.  Pulse-on-demand capability was used to control spatial overlap and to limit the actual repetition rate seen by the material. Overheating was minimised, reducing HAZ and limiting vaporisation of the epoxy resin alongside the cut; this allowed structural integrity of the material to be retained.

Summary

Femtosecond laser technology has opened a variety of new processing opportunities in industries including automotive, electronics and pharmaceuticals, which are simply not possible with CO2 laser sources.  With features such as high beam quality, flexible control, pulse on demand and burst mode selection for process optimisation and high throughput, as well as a short pulse width that minimises thermal damage, this laser is attractive for many high-precision applications.  We are only just beginning to discover the endless possibilities for this uniquely flexible femtosecond laser source.