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Information Sheets
Laser Sources
The aim of these few paragraphs is to introduce some of the more common industrial laser types and to attempt to put them into some sort of order so that inexperienced readers can broadly familiarise themselves with the main technologies.
Of the many varieties of industrial lasers available today most are either based upon gas or solid state technology; that is to say that the medium within which the laser action takes place is either a gas or a solid. The laser ‘light’ to emerge from a source has the characteristics of being highly directional and essentially monochromatic (single wavelength). These ‘coherent’ characteristics distinguish laser light from other sources with which we are familiar, such as sunlight and man-made sources such as incandescent light bulbs or fluorescent tubes. The reason why laser light is so different is indicated by the words behind the acronym LASER – light amplification by stimulated emission of radiation; the laser is essentially an optical amplifier: just as feedback from a microphone into an audio amplifier produces a high pitched squeal from the speakers, so carefully aligned mirrors in the laser provide feedback to the optical amplifier to produce a laser beam.
Lasers can be pulsed or continuous wave (CW)
The laser beam can be focused to a tiny spot using a lens or curved mirror – resulting in an extremely intense focused spot. This focus can then be manipulated by a variety of optical and mechanical techniques and used to process materials, be it to cut, weld, drill, ablate, harden, mark, expose or engrave.
See units.
Gas Lasers
The two most important industrial gas lasers are CO2 and Excimer lasers, and these happen to occupy the extremes of the wavelength range under consideration.
Excimer lasers provide output in the UV part of the spectrum (at discrete wavelengths between 157nm and 351nm). Such radiation is strongly absorbed in most materials. The lasers are pulsed, delivering up to about a Joule of energy in a pulse of duration of around 10ns. The laser pulses are often imaged through a mask and used in the micro-machining of thin layers of organic or ceramic materials by ablation.
CO2 lasers were the first lasers to be applied to a commercialised industrial application – die board manufacture, wherein fine cuts are made in wooden forms to allow blades to be accurately inserted to produce a tool to press out cardboard boxes. The CO2 laser output is more commonly continuous, but can be pulsed. The output is in the far infrared part of the spectrum, around 10µm wavelength and average powers in excess of 10kW are commercially available. There are several distinct types of CO2 lasers available – sealed, semi-sealed and flowing gas.
Unlike most of the solid state lasers that follow, the output wavelength of the UV and IR gas lasers described above cannot be delivered by fibre optic. One of the attractions of the near IR and visible-beam lasers described below is that these wavelengths have negligible losses when transmitted down flexible fibres, which provide the most convenient delivery, particularly if using robot manipulation to bring the laser power to the workpiece.
Solid State Lasers
The first laser, demonstrated by Theodore Maiman in 1960 was a solid state ruby laser, the ruby crystal was pumped by a flashlamp. Today, some of the most important industrial solid state lasers are based around the rare earth element, Neodymium (Nd). The Nd atoms displace a small percentage of the atoms in a suitable host crystal lattice to make a single crystal of rare earth doped laser medium – common hosts are YAG and YVO4 (‘Vanadate’). These Nd:YAG and Nd:YVO4 lasers are optically pumped, either by arc lamps or by laser diodes.
Traditional
Until recently, the commercial world of these lasers was dominated by one crystal geometry: the rod. Rod-shaped crystals of perhaps 15:1 aspect ratio, the size of a pen, have served industrial laser users well for over 30 years.
The rod design has a drawback: the quality of the laser beam generated can be compromised by the inherent difficulty of extracting the heat produced during operation. Heat from within the body of the rod has to be removed by water cooling the surface of the rod; this is not an ideal process. An improved surface to volume ratio makes it is easier to cool material in the form of an extremely long rod (i.e. a fibre) or an extremely short rod (i.e. a disc) and in recent years industrial lasers based on these two geometries have begun to reach the market.
Fibre Laser
Glass fibres are doped with a rare earth element, often Ytterbium (Yb) and pumped with laser diodes to produce commercial fibre laser modules of up to a few hundred Watts of continuous wave (cw) power at 1060nm – 1070nm, with a high beam quality (M2 close to 1). These fibres are sometimes bundled together to produce multimode outputs of 10kW and beyond. Such fibre lasers can also have pulsed outputs.
Disc laser
Thin discs of Yb:YAG crystal are also diode laser pumped to produce cw powers of up to several kW, which are multimode, but of better beam quality than equivalent rod-format lasers. Single mode versions of these disc lasers can produce around 100W of power with excellent beam quality, M2close to 1, which can be ‘Q-switched’ – more on Q-switching later.
The majority of solid state lasers supplied today are, however still rod-based. For industrial purposes these can be classified as cw, pulsed and Q-switched. Of these the highest average power is available from cw (Nd:YAG) solid state rod lasers. These can be pumped by diode lasers or by arc lamps and overlap in performance with the Disc and Fibre lasers mentioned above, albeit with somewhat lower beam quality (higher M²).
Pulsed sources
In a flash lamp pumped Nd:YAG laser each high energy pulse of white light from the tube produces a laser pulse from the Nd:YAG rod. Typical outputs from these lasers are up to 100J or so per pulse and pulse durations are in the range of of a millisecond (ms) – most typically between a tenth of a ms and ten ms (0.1ms – 10ms). These industrial pulsed YAG lasers can operate at repetition rates from single pulses up to several thousand pulses per second (‘several kHz’), subject to maximum average powers of several hundred Watts.
Q-switching
Q-switching involves optically switching the laser while pumping is continued: in this way energy is built up within the laser medium and then released in a short, powerful pulse. The favoured means of interrupting the laser action in cw pumped solid state lasers is acousto-optic deflection. The appeal of using an acousto-optic switch is that it can be precisely controlled by a microprocessor at very high frequency (on-off rates).
Q-switched Nd:YAG and Nd:YVO4 rod-lasers are often operated at thousands or tens of thousands of pulses per second (Hz) and can achieve peak powers of more than a thousand times the average power that a given laser is capable of. Such lasers are usually capable of delivering average or cw powers in the range of tens to hundreds of Watts. These high peak powers, albeit lasting for only very brief pulses [typically in the range of a few nanoseconds (ns) up to a microsecond (µs)], can be very valuable for many material processing applications.
A summary of pulse durations from several solid state laser technologies is shown in figure 2.
Laser pulse duration and wavelength both have a profound effect on materials processing, be it cutting, marking or surface modification. For example, in the case of laser wavelength it may be better to tailor the laser wavelength for maximum absorption in the material being processed. The rod-type, Q-switched, diode pumped solid state (DPSS) lasers described above have the characteristics of high peak power and comparatively good mode quality. The near infrared wavelength of this type of laser can be converted with reasonable efficiency using a non-linear crystal. Such a crystal, when inserted into the laser, can cause wavelength of Nd:YAG (1064nm) to be converted into half this wavelength (533nm, green light) – this process is known as ‘frequency doubling’. Frequency tripling and frequency quadrupling is also available from commercial laser suppliers, providing wavelengths in the ultraviolet region of the spectrum, at 355 and 266nm respectively. Conversion efficiencies in the region of 10% to 60% are commonly offered; the higher order conversions being generally less efficient. These frequency-doubled and frequency-tripled wavelengths are included in figure 1.
Diode lasers
Diode lasers, widely used in telecommunications and in every CD/DVD player, are the largest unit volume laser type manufactured. Diode lasers consist of a p-n junction of semiconductor material, based for example on Gallium Arsenide (Ga As). Such tiny ‘chips’ resemble Light Emitting Diodes (‘LEDs’), but are able to emit coherent laser light at one of a number of wavelengths from red (around 660nm) to near IR (980nm). However, the output from each tiny element is limited to a fraction of a Watt and for industrial applications arrays of such independent elements are fabricated: a row on a semiconductor strip or ‘bar’. A bar can output up to a few hundred Watts. Further, these bars can be assembled into stacks, having a continuous wave output of several kilowatts. Such sources are compact, highly efficient (as much as 50% ‘wallplug to optical’ efficiency) and require little or no maintenance. There is however a limitation to their application in that is that such arrays provide relatively poor quality beams – they cannot be focused as tightly as the other laser types discussed here. Diode lasers are ideal efficient pump sources for other laser types, but are usually only used directly in applications that do not require high power density, such as heat treatment, plastic welding or conduction-limited metal welding.