Traditionally, laser scanner galvanometers have been mainly used in fields such as marking and rapid prototyping with laser beam of limited wattage. Such systems provide high-quality beams, but the laser power is limited to hundreds of watts. Now, with the appearance of high-brightness lasers such as fiber lasers and disc lasers and even the use of some low-brightness high-power diode lasers, the application of scanning galvanometers has been extended to several kilowatts. The scanning device must be capable of handling the high-power range of applications without affecting precision and speed, which is a challenging task for scanning galvanometer manufacturers.
With a thickness ranging from 2.0 to 7.0 mm, this depends on the mirror size and angular acceleration. Electrolytic coatings provide sufficient reflectivity within the corresponding wavelength range. These mirrors typically withstand power densities of up to 500 W/cm2, which is more than enough for traditional marking applications. The introduction of laser scanner galvanometers into other applications such as polymer welding has brought other challenges, requiring precise control of the workpiece temperature, usually through non-contact high-temperature measurement with sensors returning heat radiation signals through the laser beam path, for example, through a mirror surface of the scanning galvanometer. The typical wavelength range for high-temperature measurement is from 1.7 to 2.2 m. An aluminum coating layer on the backside of the quartz substrate solves the problem since the dielectric layer in this wavelength range is penetrable by laser radiation. To extend the wavelength range, the scanning optical system needs to be adjusted.
In higher power new applications such as laser remote welding, remote cutting, or scanning heat treatment, requiring hundreds of watts to even several kilowatts of power, this poses new challenges for laser scanner galvanometers scan heads. Even if the reflective mirror's reflectivity is high (especially after being coated with aluminum), some light rays may still be transmitted and absorbed by the mirror substrate or surrounding components. These conditions are well-handled for low-power lasers. However, high-power lasers may generate a lot of heat inside the device, resulting in significant thermal drift and unstable long-term stability variations. Therefore, the cooling function of the scanning device is necessary but not sufficient. This is because it is unable to avoid the thermal load of the quartz reflection mirror and its resulting effects, such as causing deformation or softening of the glue layer or driving failures of the scanning galvanometer due to heat generated by the rotor and bearings. Therefore, new mirror surface technologies are essential.
One of the major drawbacks of quartz is its low thermal conductivity, which results in poor cooling performance. Silicon-based materials, such as silicon or silicon carbide, provide higher thermal conductivity. Since the strength of carbon-silicon-based materials is higher, they allow for the reduction of the thickness of the dielectric layer even though their density is higher, which still reduces the overall mass. Wide-band reflection aluminum coatings may be coated directly on the dielectric layer between the coated electric medium film and the silicon-based material if an opaque substrate such as Si or SiC is used. Careful model calculations on the mechanical design of the reflective mirror substrate can yield an optimized design in terms of stability, weight, thermal conductivity, and moment of inertia.