Diode-pumped disk laser YAG to a new era

What makes the disc shape so ideal? Disk laser technology is how to break the rod laser beam quality and inherent length limitations of the fiber? Answering these questions, one would understand how disk lasers push YAG laser technology to a new era.
When it comes to diode-pumped disc lasers, many products and treatments are implicit in the word "disc." Their unique shape and the ability to solve the beam quality and transmission problems of rod lasers make disk lasers the most innovative choice. In addition to improving the electrical efficiency, beam quality, focal length, dispersion, and spot size performance, there are many significant advantages when using diode-pumped disc lasers for material processing.

Disc features

The higher beam quality obtained from diode-pumped disk lasers (as compared to lamp and diode-pumped rod lasers) is primarily related to the heat dissipation capability of the laser medium. The YAG rod (about 6 mm in diameter and 150 mm in length) is usually circulated through the periphery of the rod with water. The result of this is that the stick's outer periphery is relatively cool, while the stick's center is still hot and the centerline is at its highest temperature. This temperature difference causes thermal gradients and thermal distortion in the rod, limiting the beam quality of the rod-shaped YAG laser. In contrast, the disk (about 14 mm in diameter and 0.2 mm in thickness) is mounted on a water-cooled copper block (heat sink) and serves as a mirror. Because the disk surface is mounted on the heat sink, the disk is very thin (high ratio of area to laser stimulated volume), the cooling efficiency is high and the resulting thermal gradient is negligible. In addition, disk lasers, like rod lasers, can also be coupled together in series to increase their output power.

In addition to the much better beam quality, disc lasers have another important advantage over bar lasers: disc lasers allow the use of longer fibers to transmit the beam. Although the physical explanation of this phenomenon is more complex? However, we can make some simple explanations from the following aspects. First, back reflections (generated from the workpiece or from the end face of the fiber) can be returned to the laser cavity. This back-reflected light can be used to further pump the laser material (rod or disk) and the amount of pumping is related to the volume of the YAG material. In a ytterbium-doped YAG (R) disk, the lasing volume of the laser material is negligible compared to the rod-shaped, back-reflected energy, so the fiber length is no longer limited by this phenomenon.

The last advantage of disk lasers (diode-pumped rod lasers) is also noteworthy: the pump disk's light energy comes from a diode laser. This is especially advantageous when compared to lamp (eg arc lamp) pump lasers. The difference between them is that diode lasers produce a narrow wavelength range (almost all light energy is useful for the laser), while the lamp's wavelength range is very wide, most of which is useless to the laser and produces unwanted thermal energy. This is the basic reason why the total electro-optical efficiency of a disk laser is more than 15%. The typical electro-optical efficiency of a typical lamp pump laser is only 3% to 4% (compared to about 10% for a diode-pumped rod laser).

Beam quality advantages

When it comes to beam focusing power and laser power density on a workpiece, the beam quality issue becomes very important. However, the beam must first be focused into the fiber, and the quality of the beam is the most important factor in determining the fiber size of the transmitted beam. The beam quality, sometimes called the "beam parameter product," or BPP, is the product of the beam radius and the half-angle of the beam, expressed in millimeters - milliradians. For focusing a beam into an optical fiber, the beam radius refers to the radius of the focused beam at the entrance of the optical fiber while the half-angle of the beam equals the half-angle of the coupled beam. In order to create a more stable laser device, the focal spot at the entrance of the fiber must be smaller than the fiber core, so small that the fiber can be replaced in a plug-and-play fashion (cable replacement in the field without the need for any adjustment).

The core diameter (φc) is directly related to the focal spot (d) on the workpiece and is therefore also related to the power density, which can be expressed as follows:

d = φc (f / fc)

Where f is the focal length of the focusing optical system and fc is the focal length of the collimating optical system.

Therefore, the smaller the core diameter, the smaller the focal spot diameter. However, the ability to focus light into small spots is not enough. Also consider the power density problem. The power density (Pd) refers to the laser power (P) per area of the focused spot (area = πd2 / 4)

Pd = 4 P / πd2

So, with a diode-pumped disc laser (Trumpf model HLD 4002, with a power of up to 4 kW on a workpiece and a diameter of 200 μm) and a lamp-pump rod laser (Trumpf model HL 4006, with up to 4 kW on the workpiece, Diameter fiber is 600 μm), the superiority of the beam quality can be clearly seen by the power density or focal length that is produced when spot size and power density are compared.

Higher power density can be used to produce high-speed, narrower welds, or by using long focal length lenses, resulting in lower speeds and wider welds. The advantage of using a long focal length lens compared to a short focal length lens is that it allows the processing optics to be kept away from welding fumes and spatter and produces a much greater depth of focus (or larger processing window).

For welding applications, the most significant depth of focus (depth of field) is the so-called 5% definition. The 5% depth of focus (L5%) is defined as a range within which the spot size does not vary by more than 5%. In other words, if the "in focus" spot is 0.1 mm and the depth of focus is 5%, then the spot size will not be larger than 0.105 mm. L5% depends on the focal spot size (d), the output beam quality (BQexit) and the laser wavelength (λ, the pump YAG at 1064 nm and the diode pump disk laser at 1030 nm). The relationship between them is given by:

L5% = d2 / (6λBQexit)

Therefore, the beam quality of any laser can directly indicate its focusing ability, and the relevant parameters such as spot size, focal length and depth of focus.

Long-distance welding

Over the years, the unique concept of welded parts has been evolving in the field of CO2 lasers. This soldering process, well-known for long-distance soldering, is only possible with increased power and good beam quality such as the Trumpf TLF 6000 HQ, 6 kW laser, near-Gaussian distribution. Due to the very good beam quality of the disk laser, long-distance soldering can also be achieved with YAG lasers.

Unlike the CO 2 laser remote welding system, a barrel-mounted structure with flying optics is used to deliver the beam. YAG long distance welding utilizes fiber optic beam delivery to couple the light to a scanning head mounted on a robot . There are many differences between long distance CO2 and YAG welding and these differences should be considered when evaluating any remote welding system. They also provide clues for further research and analysis.

The excellent beam quality of a disk laser allows the beam to be transmitted through a smaller diameter optical cable so that a smaller focal spot diameter (higher power density for fast processing) can be produced for a given focal length or, Set the spot size, can produce greater processing depth of field (larger processing window, more stable and longer welding capabilities).