A basic laser scanning galvanometer device appears to be very simple: two reflective mirrors driven by an electric motor, capable of reflecting the emitted laser beam to any point in a special area. If the composition of a focusing point lens and a movable mirror is used in the laser path in front of the scanning galvanometer instead of a flat field lens, the working range and working distance can be further improved.
In a variety of applications, the high-precision positioning speed of galvano scanner is required. For example, when the focal length of the lens is as short as 163mm, the scanning speed in a working area of 120×120mm² can reach 10m/s. Simple geometric calculations show that the small angle error of the scanning galvanometer glass is likely to cause a significant displacement of the laser beam on the working plane diagram. Therefore, high-level precision is necessary for the scanning galvanometer driver, mirror glass, or mirror mount. In addition, the motor and control electronics of the scanning galvanometer may generate heat, causing thermal drift and resulting in positioning errors. Therefore, if high precision and long-term reliability are required, one solution is to choose a scanning galvanometer with water-cooling function.
In laser marking applications, the types of reflective mirrors selected for the scanning galvanometer include quartz substrate raw materials, with a thickness between 2.0 and 7.0Mm, which is due to the mirror specifications and angular acceleration. The surface coating of the electrolytic solution shows sufficient reflectivity in the corresponding wavelength range. Such mirrors generally can withstand power up to 500W/cm², which is very suitable for traditional laser marking applications. The introduction of scan heads to other applications has led to new challenges, such as high polymer welding. This application requires precise control of the product workpiece temperature, usually based on non-contact accurate measurement using a high-temperature meter. For this technology, the radiation heat data signal of the product workpiece must be returned from the laser beam spot position to the sensor along the laser path of the laser. Because the dielectric layer in this wavelength range is transparent to the laser radiation source, the problem can be solved by adding an aluminum coating to the reverse side of the quartz substrate mirror. If it is necessary to expand the wavelength range, the scanning optical system must be adjusted.
For new applications with higher output power, such as remote control laser welding, remote control laser cutting, or scanning thermal treatment technology, which require output power from several hundred watts to several KW, new challenges for the scanning galvanometer scanning head are highlighted. Even if the reflectivity of the material mirror is very high, part of the light source is likely to scatter and be absorbed by the mirror substrate or surrounding components. For low-output-power lasers, this situation is easy to solve. However, high-power lasers may generate a lot of heat inside the equipment, resulting in significant thermal drift and unreliable long-term reliability fluctuations.
Therefore, the water-cooling function of the laser scanning mirror device is very necessary but generally cannot solve the problem. This is because it cannot prevent the thermal load of the quartz reflection mirror and the harm caused by it, such as causing adhesion deformation or loosening or common faults of the scanning galvanometer driver due to the heating of the motor rotor and rolling bearing. Therefore, new mirror glass technology is indispensable.