Once a laser beam is generated, beam delivery optics are needed to steer the beam from the resonator to the workpiece. It sounds like a pretty easy optical task. Just pick a mirror that can reflect most of the laser beam and then carefully orient the mirror so that the beam is sent in the proper direction. But the role of a beam-steering mirror is much more complex, especially in high-power CO2 laser systems. Reflectivity, absorption, polarization and water-cooling are four critical parameters that must be considered for each mirror in a beam delivery system. Picking the right mirror will ensure maximum uptime and the best process speeds and quality, whether the application is welding, cutting or drilling. Picking the wrong mirror will result in poor processing speeds, poor process quality and long downtimes spent in trying to troubleshoot these problems.
Reflectivity and absorption
When choosing mirrors a good place to start is to consider the reflectivity of the mirrors at 10.6 µm, the CO2 laser wavelength. Laser optic manufacturers can supply this data. The reflectivity of the mirrors in the system determines how the mirror will affect the process, directly or indirectly. For example, a common mirror often used in laser beam delivery systems is made of silicon or copper mirror with some type of enhanced coating. These standard enhanced dielectric coatings raise the natural reflectivity of the metal mirror to around 99.5 percent, a value that varies somewhat depending on beam polarization.
The first thing to note is that the reflectivity will reduce the total laser beam power by 0.5 percent, a value that may not be significant to most applications. For example, when cutting metal with a 3kW beam, losing 15 W of total power is not very significant to the cutting process. But if there are eight mirrors in the beam delivery system, the total loss in beam power will be a more significant 120 W. However, most of the time even this modest loss in power will not "make or break" a process.
Perhaps a more critical factor is the lost 15 W of laser power that is completely absorbed in the mirror coating and converted to heat energy. The heat energy raises the temperature of the optic. In most applications 15 W of heat energy is not significant. But as optics are used in the system, their environment contaminates them and they absorb more of the laser beam. More absorption leads to more heating in the optic and this in turn leads to thermal lensing. Thermal lensing can cause problems at the workpiece. It causes the focused spot to shift as the lens heats up and it can also cause the spot size to change. The operator will see poor or inconsistent cut quality when thermal lensing of a mirror (or lens) occurs.
This leads to a third consideration in selecting a mirror—water-cooling type. All mirrors can be water cooled. Silicon mirrors usually are cooled with a chill plate that contacts the entire back surface of the mirror. In more demanding applications direct cooling of the substrate is possible with copper mirrors that incorporate water-cooling channels in the substrate for the most efficient method of cooling. Of course, it is possible to purchase copper mirrors without water-cooling, in which case a chill plate can be used if water cooling is required.
Polarization effects
Of all the laser mirror parameters that an end user must consider before selecting the proper mirror, polarization effects are probably the most difficult to understand, because polarization cannot be "seen" (see sidebar, "What is polarization?"). Yet polarization problems with a laser beam can cause a variety of cutting problems, including uneven kerfs, slow cutting and overall poor cut quality. The key to understanding polarization effects is to understand that, in the beam delivery system, laser mirrors are designed to do one of two things: maintain the beam polarization or alter the beam polarization.
For laser cutting to have good quality, even kerfs and equal cutting speeds in all directions, a laser beam must have good circular polarization. This means that the electric field strength of the beam is the same in all directions, leading to even cutting in all directions. If the beam exiting the laser is circularly polarized, then the laser mirrors in the beam delivery system must maintain that polarization all the way to the workpiece.
Laser resonators generally produce a beam that is linearly polarized; that is, the electric field of the beam is oriented in one direction. If this type of beam is used for cutting, the resulting cuts will have kerf width and quality that vary, depending on the direction of the cut. In these cases at least one mirror in the beam delivery system must be designed to alter the polarization of the beam; that is, it must convert the polarization from linear to circular. Once this is accomplished, (usually by having the first mirror that the beam sees after leaving the resonator be the polarization altering mirror) then the remaining (downstream) mirrors in the system need to maintain this circular polarization.
Normally, laser system manufacturers specify the correct mirrors to use in the beam delivery system to achieve the optimum circular polarization at the workpiece. However, if the mirrors have been changed, or are of an unknown type, a careful review of all of the mirrors in the system will be required, in order to verify that the correct mirrors are being used in the correct positions in the beam delivery system.
Visible reflectivity considerations
Laser optic manufacturers produce mirrors with visible reflectivities ranging from about 30 percent to as high as 95 percent. This visible reflectivity, however, must be considered in combination with all of the other parameters of the mirror, primarily its reflectivity at 10.6µm, and its effect on the polarization of the laser beam. The user must choose the best balance between all of these parameters, to find the mirror or mirrors that give the best overall result in the intended application.
While the visible reflectivity of a laser mirror is not a major factor, it still should be carefully considered before the mirrors are selected. In many laser applications, the reflectivity of a beam delivery mirror at visible wavelengths is not critical. However, many systems are aligned by using a visible alignment beam, generated either by a HeNe laser operating at 0.6328 µm or a laser diode, which can operate at one of several "red" wavelengths, such as 0.635 µm, 0.650 µm or 0.670 µm. Some systems may also utilize a visible beam for safety or beam location purposes. For any of these applications, the reflectivity of each individual mirror becomes more critical, as the total number of mirrors in the system increases. For example, a beam delivery system with one mirror may be able to tolerate a visible reflectivity of only 40 percent from that one mirror, but if there are four such mirrors in the system, then the overall visible throughput of the system will be only 2.6 percent. In this case, the visible beam will probably be so faint as to render it useless for its intended application.
Conclusion
Choosing a mirror for a laser application is critical to ensure maximum performance and minimum downtime. As shown above, there are many factors to consider when selecting mirrors, including polarization issues, reflectivity, absorption, water cooling and visible reflectivity, to name a few. You can work with your optics supplier to determine which mirror has the best combination of features and a price that will fit your application.