Diode pumped disk laser pushed YAG into a new era

What causes a disk shape so ideal? How does the disc laser technology break through the beam quality of the rod laser and limit the length of the fiber? In response to these questions, one will understand how the disk laser has pushed YAG laser technology into a new era.

When it comes to diode pumped disc lasers, many products and processing methods are implicit in the word "disc". Because of their unique shape and the ability to solve the beam quality and the transmission problem, the disc laser is the most innovative choice. In addition to improving the efficiency of electric power, beam quality, focal length, dispersion and spot size and other properties, there are many obvious advantages when using diode pumped disk laser to process materials.

Disk feature

High beam quality from diode pumped disk laser (compared with lamp pump and diode pumped laser rod) main laser medium and the heat dissipation capability of. YAG rods (ca. 6 mm in diameter, length of 150 mm) are usually cooled by water through the periphery of the rod. The result is that the outer side of the rod is relatively cold, and the central part of the rod is still hot, with the highest temperature in the center line. The temperature difference causes thermal gradient and thermal distortion in the rod, which limits the beam quality of the rod YAG laser. By comparison, the disc (about 14 mm in diameter, thickness of 0.2 mm) on the water-cooled copper block (heat sink) above, it also acts as a mirror. Because the surface of the disc is mounted on the heat sink, the disk is very thin (the area is highly dependent on the laser stimulated volume), so the cooling efficiency is very high. In addition, the laser can be coupled in series to increase the output power of a disk laser.

In addition to good beam quality a lot, compared with rod laser, disk laser has other important advantages: allowing the use of long fiber disk laser beam transmission. Although it is more complicated to explain this phenomenon completely from physics, we can make some simple explanations from the following aspects. First, the back reflection (from the workpiece or from the end of the optical fiber) can be returned to the laser cavity. The back reflected light can be used to pump the laser material (rod or disc) further, and the pump volume is related to the volume of the YAG material. In a ytterbium doped YAG (Yb:YAG) disc, the volume of the laser material is much smaller than that of the rod like Nd:YAG laser material (about 4240mm3). Rod back pumped may cause extremely high peak power. Such peak power may damage the internal optical components and optical fiber cables. The back reflection is a function of the length of the fiber, the core diameter of the fiber and the power of the laser. The longer the fiber, the smaller the core, the higher the power, the greater the back reflection. Because of the small size of the YAG and the lasing of the back reflected energy, the length of the optical fiber can not be ignored.

Disk laser (the same is true of diode pumped laser rod) one of the advantages of the last is very worthy of note: light from laser diode pumped disk. This lamp (such as huguangdeng) pump laser compared advantages especially. The difference between them is very narrow wavelength range of diode laser (almost all of the laser light, and useful) light wavelength range is very wide, which most of the laser is useless, but have no use of energy. This is the basic reason that the total electro-optic efficiency of the disc laser is greater than 15%, and the total electro-optic efficiency of the typical lamp pump laser is only 3% to 4% (the total electro-optic efficiency of the diode pumped rod laser is about 10%).

Beam quality advantage

In power density on laser beam focusing ability and on the workpiece, the beam quality becomes very important. However, the beam must first be focused into the fiber, and the beam quality is the most important factor in determining the optical fiber of the transmitted beam. Beam quality, sometimes referred to as the "beam parameter product", or BPP, is a product of the beam radius and the beam divergence angle. The beam is focused into the fiber, the beam radius refers to the radius at the entrance of the focused beam in the optical fiber, and the beam divergence angle is equal to half of the coupling beam semiangle. In order to produce the laser device is stable, the focal spot at the entrance of the fiber must be smaller than the fiber core diameter, small enough to be used to plug replacement (replacement mode fiber cable, at the scene without any adjustment ability).

Core diameter (C diameter) and focal spot on the workpiece (d) has a direct relationship, and therefore also the power density, this relationship can be expressed as:

D = C (f/fc).

Among them, f is the focal length of the focusing optical system, and FC is the focal length of the collimating optical system.

So core diameter is smaller, the smaller the diameter of the focal spot. However, it is not enough to have the ability to focus on a small spot. Also consider the power density problem. Power density (Pd) is defined as the laser power (P) of the unit focal spot area (area = Pi d2/4)

Pd =4 P / PI D2

So when the diode pumped disk laser (Trumpf type HLD 4002 power on the workpiece up to 4 kilowatts, the diameter of fiber is 200 m) and lamp pumped laser rod (Trumpf model HL 4006, power on the workpiece up to 4 kilowatts, the diameter of fiber is 600 m) were compared spot size and power density, superior beam quality, can be generated by the power density or the focal length is clear.

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