A fiber laser is generated within a flexible doped glass fiber that is typically 10 to 30 feet long and between 10 and 50 microns diameter. Ytterbium is usually used as the doping element. You do not have to align the medium to cavity mirrors, nor maintain optics and alignment. In fact, it’s such an efficient lasing process that this laser can be small, air-cooled, and provide high wall plug efficiencies. Fiber lasers offer great “focusability” and a range of beam qualities, which can be tuned for each welding application.
Continuous wave (CW)
With a CW laser, the laser output remains on until being turned off. For spot welding either a single weld or a seam, the laser output can be modulated – this means the laser is turned on and off rapidly. The CW laser’s peak power is the same as its maximum average power, so focused spot sizes are generally under 100 microns. CW fiber lasers are usually a good choice for general seam welding up to 1.5 mm depth for a 500W laser, high speed seam welding of same and dissimilar materials, and producing spot welds below 100 microns in diameter.
Quasi-continuous wave (QCW)
The QCW fiber laser’s peak power and pulse width characteristics are similar to those of the Nd:YAG laser. The QCW lasers offer single mode to multi-mode options with spot sizes from 0.02 to 1.0 mm. These lasers also shine in small spot size applications and penetration applications, although they really can handle many micro welding applications.
The nanosecond fiber laser is a relatively new addition to the family. Often used for laser marking applications, nanosecond fiber lasers actually make a very cost effective welding solution. The nanosecond laser provides multi-kilowatt peak power, but with pulse widths around 60-250 nanoseconds that can be delivered between 20-500 kHz. This high peak power enables welding of almost any metal, including steels, copper, and aluminum. The nanosecond fiber laser’s very short pulse widths means you can get very fine control for welding small parts. This one is also a good choice if you need to weld dissimilar materials.
In metal working applications, medium power Fiber Lasers in the range 50W to 2000W offer new degrees of operational freedom and process control. The ability to operate with pulse lengths continuously tunable from a few microseconds to full CW operation and with pulse repetition rates up to tens of kilohertz offers the applications engineer the ability to optimise the process conditions over a wide range of applications.
Due to their monolithic single mode fiber construction Fiber Lasers do not suffer from changes in focus position due to thermal lensing as the average power is changed, and don’t require periodic adjustment or tuning of the Laser cavity or component maintenance to ensure output stability.
Process repeatability: Laser welding/cutting/marking, engraving, cleaning are non-contact processes which eliminates potential problems caused by wearing parts, contact deformation or contamination.
Process control: The high beam quality and a resulting spot size control together with the continuously tunable average power of a Fiber Laser ensure that the energy is delivered only where it is needed and with exceptional control. This enables for example that Laser welds can be placed very close to polymer seals, glass-to-metal seals, soldered components, and thermally sensitive electronic circuits.
Hermeticity: Unlike soldering or brazing, Laser welding can provide high quality hermetic welds with high yield, both of which are fundamental requirements in the manufacturing of high value implantable medical devices.
Surface finish: In addition to the aesthetic quality, the smooth and pore-free surface finish achievable enables reliable autoclave sterilisation.
Traditional laser welding technologies, such as continuous-wave CO2 welding lasers are limited in terms of accuracy and undesired, high heat input into the weld. On the other hand, the limitations of traditional pulsed Nd:YAG or the newly developped pulsed QCW are the maximum welding speed, the minimal spot size that can be achieved and the electrical to optical energy conversion efficiency that can be obtained. Ever more applications are demanding a higher precision control, lower heat input and lower electrical energy consumption. Continuous Wave Fiber Laser Welding is a technology that offers those features.
In a fiber laser, the laser light is generated in an active fiber and guided to the work piece by means of a flexible delivery fiber, which acts as a “light guide”. The flexibility of the delivery of this laser beam is an important feature for many forms of material processing such as laser cutting, laser welding, laser marking and laser engraving.
Continuous Wave versus Pulsed Wave for metal welding
Fiber lasers are available with both type of energy delivery: Continuous and Pulsed.
As the name states, the Continuous Wave (CW) lasers deliver a continuous, uninterrupted output. This output can have an upslope (soft-start) when switched on, an energy modulation while active, and a downslope when switched off (crater filler). Of course, this type of laser can also be switched on and off to create pulses. However, the maximum power level can never exceed the average power.
In contrast, the Pulsed Fiber lasers deliver a pulse of energy which is typically ten to twenty times higher than their average power. For example, a laser can have 300 W average power and 4000 W peak power. These lasers are often referred to as Quasi Continuous Wave (QCW) Fiber Lasers.