Laser Processing Heads
In laser material processing, one generally needs to direct a focused laser beam to a workpiece. The unit used for that is called a laser processing head. Depending on the particular application, it can more specifically called a cutting head, drilling head, welding head, soldering head, etc.
A processing head can be considered as the end part of a beam delivery system. As the most important functionality is often concentrated in the head, the term beam delivery system sometimes applies to that part only, ignoring the path from the laser source to the head – particularly if it is simple and short.
The radiation from a laser – most frequently, an industrial laser – may be sent to a single processing head, or to multiple heads: either in an alternating fashion (time sharing) or with a distribution of optical power on the different heads (by beam splitters).
Functions and Features of Laser Processing Heads
While the basic function of applying a laser beam may seem simple, laser processing heads often need to offer a number of additional features, reached with various kinds of accessories.
In many cases, the laser light must be focused to a small spot on the workpiece, e.g. because only a small spot needs to be processed, or because only that way sufficiently high optical intensities can be achieved. Usually, the beam focus must be on or near the surface of the workpiece.
Usually, the laser light is sent to the processing head in the form of an approximately collimated beam (i.e., with relatively large beam diameter and low beam divergence), and is then focused with a lens or a curved mirror within the head. Focusing with lenses is preferred at low power levels, but curved mirrors allow for better power handling. A potential disadvantage of using mirrors is the introduced astigmatism for non-normal incidence.
The distance from the focusing element to the beam focus is approximately equal to its focal length. In some cases, the focusing element can be exchanged for using different values of the focal length, which generally affect not only the beam radius in the focus but also its distance from the exit of the processing head, i.e., the working distance (see below). It is not common to use adjustable optics (like a zoom objective in a photo camera) for obtaining adjustable beam parameters.
Various parameters of the used laser source enter the beam delivery system must be compatible with those of the processing head:
- A head is usually made for a specific wavelength region, where the used mirrors, lenses, anti-reflection coatings etc. work well. Common versions work at 1.06 μm (for YAG lasers and fiber lasers) or at 10.6 μm (CO2 lasers), but there are also processing heads for green light (532 nm, 515 nm) or for the 2-μm region.
- For a free-space beam system (the usual solution for CO2 lasers and excimer lasers, for example), the input beam should usually be approximately collimated with an appropriate beam radius and beam quality factor M2. (Note that a too small beam diameter at the input may result in a larger spot in the beam focus.)
- In the case of delivery with a high-power fiber cable (which is common for solid-state lasers including fiber lasers and direct diode lasers), normally only the fiber core diameter and the numerical aperture are relevant. However, details of the launch conditions determine the power distribution over the guided fiber modes, and the obtained focus spot size depends on that.
- The optical power must not exceed the rating of the head. Besides the laser average power, for pulsed lasers the peak power may also be limited.
Laser sources with high beam quality allow the use of small optical components and thus also a more compact and lightweight design of the processing head.
Many lasers can or should not be frequently switched on or off; one then uses a beam shutter for temporarily blocking the beam. In some cases, a beam shutter is integrated into a processing head. Alternatively, it may be inserted directly after the laser source, or in the laser housing.
Multiple Output Beams; Pilot Beam
In most cases, a processing head has a single output beam only. However, there are situations where it is beneficial to have several beams:
- Multiple beams with the same power and other parameters can be used to process multiple points on the workpiece simultaneously.
- For some applications, one can switch between two different beams, for example with different optical wavelengths and/or different beam parameters.
- There are processes where the combination of a few beams is used in a special configuration. For example, dual-spot techniques use two closely spaced laser beams with equal power and diameter e.g. for welding. Trifocal welding or brassing is done with a main beam with large diameter, preceded by two more tightly focused beams.
A significantly improved quality can be reached with such refined processes.
Some laser heads inject an additional low-power pilot beam from a visible laser in order to clearly indicate the hit spot. Care must be taken to avoid inaccuracies due to chromatic aberrations or misalignment, for example.
The working distance (or stand-off distance) is the distance between the output end of the processing head and the surface of the workpiece. In some cases, the working distance needs to be rather small, e.g. because otherwise sufficiently strong focusing would not be possible, or because one needs to apply a process gas (see below) from a close distance.
In other cases, a substantially larger working distance is required, for example for remote welding (usually without process gas). For such remote operations, specialized processing heads have been developed, and one tentatively needs laser sources with higher beam quality.
In cases with a small effective Rayleigh length of the beam (i.e., for strong focusing and/or poor beam quality), it may be necessary to accurately stabilize the working distance with an automatic feedback system, involving some kind of distance sensor (typically a capacitive sensor). Otherwise, the longitudinal focus position may be too far inside or outside the material.
Beam Positioning and Scanning
Accurate positioning of the laser beam during processing is of course of essential importance – particularly in cases of laser micromachining, where the tolerances can be far below 1 mm. Position parameters can involve the distance to certain features on the workpiece (e.g. soldering pads or already produced seams), or e.g. the distance to the edge of a workpiece.
Some laser processing heads have a fixed beam path; the beams can then be moved only by moving the whole laser head. Alternatively, one may move the workpiece against the fixed beam.
Other processing heads have an integrated laser scanner, containing some movable optics for scanning the direction of the output beam in one or two dimensions. Sometimes, the <$z$> position (longitudinal position) of the focus can also be adjusted in some range. Essentially two different kinds of technical solutions are used for scanning laser heads:
- The movable scanner mirror(s) may be mounted after the focusing lens or mirror, working with the already converging beam. The fixed beam position on the focusing element has the advantage that that element does not have to be that large, and its optical aberrations are less critical. This configuration is often used in cases with a large working distance, and when variable beam directions (plus variable angles of incidence on a plane workpiece) are acceptable.
- Alternatively, one may first send the beam to the movable mirror and thereafter through the focusing element. One then has the option to use an f–theta lens, with which one can achieve a constant beam direction despite the variable location on the workpiece. However, the diameter of such lenses is limited, therefore also the accessible working area (as long as the workpiece is at a fixed position).
Rapid beam scanning is also often used as a means for flexibly generating various kinds of beam profiles. When the beam is scanned with a high enough frequency, the resulting average intensity profile may be the only relevant thing for the process, and this can easily be modified through the scanning parameters – not requiring any changes in the optics.
There are also laser trepanning heads containing a specialized kind of scanner, which rapidly moves the beam focus around a circle. This is useful for cutting holes with larger (although still limited) diameter.
Particularly high flexibility is needed and some cases, where not only the laser spot position in a plane must be varied, but also the direction from which the beam approaches the workpiece. In some cases, the laser head is mounted on a robot arm, allowing five or six degrees of freedom. For example, it is sometimes possible to rotate the laser head around one axis, introduce tilts with another axis, and translate the device in any direction.
Due to a frequently encountered challenging combination of demands, such as high precision and high speed, refined electromechanical technology is employed for such purposes.
Some laser-based manufacturing processes require the application of a process gas:
- In many cases, this is an inert gas such as nitrogen or argon, protecting the surfaces from oxidation by the oxygen of ambient air.
- For laser cutting, one sometimes uses air or even concentrated oxygen, exploiting the additional heat input from burning of the blown-out material. Another function of the process gas (whether it is an inert gas or oxygen) is to blow away the material, substantially supporting the cutting process. Because a well directed high-pressure gas jet is required, it needs to be applied from a close distance. However, there are also remote cutting processes which usually work without a process gas.
There are different ways of sending the process gas to the workpiece:
- In some cases, the laser head has a kind of nozzle through which both the laser beam and the process gas are sent. That is suitable, for example, for cutting and drilling processes.
- In other cases, the process gas is applied from the side, then flowing along the workpiece surface. That approach is appropriate e.g. for welding or hardening processes, where the surface remains intact.
In some cases, the workpiece does not need to be supplied with a process gas, but one applies a cross-jet within the processing head for protecting the optics (see below).
Apart from process gas, some processes require a feed of solid material. Some typical examples:
- Laser soldering requires some solid solder, provided in the form of a wire, small balls or powder. Often, a wire feeder or a powder delivery unit is attached to the side of the processing head. When using the solder bumping technique, one applies small balls of solder through a capillary.
- Some processes of laser surface modification also require the application of some powder, for example, before or during the laser process.
While in some cases such materials are provided through a kind of nozzle together with the laser beam, in other cases materials are supplied from the side. The processing results can be sensitive to the direction from which the material is applied (e.g. relative to the direction of movement of the laser head).
Protecting the Optics
Some laser beam machining processes (particularly laser cutting and drilling) produce considerable amounts of hot debris, fumes and the like, and those constitute a hazard for any nearby optics, particularly in situations with a small working distance. When materials are deposited on a focusing lens, for example, they cannot only reduce the laser power in the focus and spoil the beam profile, but also lead to immediate destruction of the lens due to thermal cracking.
A first layer of protection is often achieved with a process gas, which blows the problematic materials in a direction such that it cannot easily get into the opening of the processing head. That, however, is not always sufficient, and sometimes no process gas is applied. In some cases, the process head also contains an internal cross-jet below any optical elements or other sensitive components.
Another frequently used measure is that an anti-reflection coated optical window protects the scanner and/or the focusing optics. If the protection window is spoiled by deposited material, it can be exchanged more easily and at a much lower cost than if the other optical components would be affected. Also, no re-alignment of the beam is necessary. Nevertheless, one tries to avoid too frequent spoiling of the protection window.
Many laser-based processes work with rather high optical powers; the used processing head then obviously must be able to handle that power level. Besides the risk of damage, at high power levels there can be problems with thermal lensing effects, shifting the focus position as the laser power increases, or after a high-power beam is switched on. Many kilowatts of CO2 laser radiation can be handled with water-cooled metal mirrors, while for shorter wavelengths one may e.g. use high-reflectivity dielectric mirrors on a substrate like sapphire with high thermal conductivity.
For pulsed lasers, where the peak power is usually orders of magnitude higher than the average power, there can be a peak power limitation, e.g. set by the optical damage threshold of the optics in combination with the beam diameter.
The risk of damage to the processing head may be seriously increased when a high-power beam is injected with faulty alignment. Therefore, do you care has to be applied when mounting a processing head or when doing operations on the laser source.
Particularly sensitive parts are scanner mirrors because these should be lightweight for enabling quick movements, but this together with the challenge to provide good thermal contact of movable parts makes it difficult to handle high optical powers. Achieving a very high reflectivity of the used mirrors is an important measure because that reduces the amount of deposited heat.
Process Heads for Ultrafast Lasers
Some of the applications of laser material processing work with ultrafast lasers, i.e., with pulse durations in the picosecond or even femtosecond regime. These mostly work in the 1-μm wavelength regime, just as other solid-state lasers, although there are some green frequency-doubled versions as well as ultraviolet versions.
While a beam delivery system based on mirrors should usually work with such a laser, an ordinary high-power multimode fiber cable cannot be used, basically because of the strongly mode-dependent group delay: one would obtain a sequence of many pulses, each one with a different time delay and spatial profile, and each carrying only a fraction of the total optical energy. On the other hand, a conventional single-mode fiber could also not be used because of the high peak power, which would cause excessive nonlinear effects or instant destruction. Therefore, special hollow-core fibers have been developed which can be used to transmit ultrashort pulses with substantial peak powers and pulse energies of hundreds of microjoules. Chromatic dispersion may need to be compensated with extra means. The chromatic aberrations of a focusing lens may be another point of concern; one may need to use achromatic optics, depending on the optical bandwidth of the pulses and focusing details.
Specialized and Multi-purpose Processing Heads
Many laser processing heads are highly optimized for a specific manufacturing process and part type, particularly in industrial mass production.
However, there are also more versatile multipurpose processing heads, where various details can be modified, e.g. with adjustable optics, by exchanging optical and other components, or by attaching accessories like wire feeds. Some can be used with moderate reconfiguration efforts, even for different processes, such as welding and soldering.
There may also be the option of exchanging whole laser heads on a machine. However, the disadvantage of that approach is that there is a serious risk of contamination of optics, making necessary expensive and time-consuming repairs of the beam delivery system, particularly when the laser head must be exchanged in a not very clean industrial environment. Therefore, a flexibly reconfigurable processing head can be desirable, even if it does not perfectly fit all needs.
Because many laser-based processes are sensitive to the intensity profile of the laser beam, and the profile may change e.g. due to technical problems in the laser source or the delivery system, some laser processing heads have integrated beam diagnostics. Typically, a small proportion of the optical power is directed to the diagnostics camera using a beam splitter. The recorded beam profiles may be displayed on a screen or automatically processed.
For high quality results, it is often required to utilize one or several means of process diagnostics. Some examples:
- A camera, possibly equipped with an optical filter for removing light in certain spectral regions, can provide an image of the processed region, which can be used for monitoring various details. It may also be used for accurately positioning the workpiece before the processing begins, and for inspecting the processing results, e.g. welding seams. Data from a digital image sensor of the camera may be used in automated software for process monitoring and quality control.
- A simple photodetector may be sufficient e.g. for estimating the process temperature, for example in order to automatically adjust the laser power or the movement speed of the head.
- In some cases, a spectrometer for spectral analysis, e.g. of the radiation of the laser-induced plume can give valuable additional insight. For example, one can detect the presence of certain chemical elements and automatically stop a process under certain conditions.
- Some means of illumination (e.g. with LEDs) can aid the inspection at times where the laser beam is switched off.
Such tools are often attached to the side of a process had as optional accessories.
An industrial processing head must have a robust housing, preventing the escape of dangerous radiation as well as the intrusion by all kinds of dirt. Unfortunately, it is often not possible to completely close the housing, e.g. because of the need to transmit a process gas or a solder. Even if that is not the case, it would often not be wise to close the bottom part with an optical window because it would be too much at risk of being quickly spoiled.
A housing may need to allow one to open a section of the head, e.g. for replacing optical parts.
In modern industrial settings, laser-based manufacturing systems often need to provide a suitable software interface for (a) remotely controlling the whole machine including the laser head, (b) for acquiring various data with information on the workpieces (before and after processing) and (c) for details of the laser process. Monitoring the health condition of the machine system can include aspects like laser power and beam quality, beam distortions caused by the optics (e.g. by a spoiled optical window), misalignment, temperature conditions and gas flow.
With the automatic processing of such a multitude of signals, various kinds of faults which can affect the processing quality can be identified and fixed more rapidly, and this is of course essential for reliable and efficient production.
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