1 Micron Laser Overview

Nd:YAG Lasers

Lasing materials of solid-state lasers are synthetic crystal rods. The rods are pumped with energy, typically from xenon flash lamps or laser diode stacks. Most solid-state lasers use resonator cavities with external mirrors. For industrial applications, the most commonly used laser crystals are made of Nd:YAG. These lasers operate in the near-infrared at a wavelength of 1.06 μm. Power output can be as high as 5 kilowatts. The uses of Nd:YAG lasers include cutting, drilling, welding, scribing, and engraving. Materials processed by Nd:YAG lasers include carbon resin, ceramics, most metals, and most plastics. YAG lasers often are used in industrial welding applications and are power-scaled by placing rods in series. Unlike CO2 systems, YAG lasers can deliver power via a fiber delivery system. While the output fiber (and beam diameter) is larger than with a fiber laser, they still offer more flexible beam delivery than CO2 lasers. Indeed, fiber delivery at high power levels has permitted the Nd:YAG into areas where there is limited overlap with CO2. Integration is also easier than with CO2 lasers because the fiber can be used with robots. Consequently, primary advantages to the YAG are lower wavelength and the ability to use robotics. In addition, YAGs have a low plasma plume and, therefore, require no shield gas. Disadvantages to this type of laser include poor wall plug efficiency of approximately 3 percent and a large footprint (relative to diode and fiber laser systems). This larger footprint is due to its large cooling system. YAGs also have adequate beam quality for welding but not for cutting (since the achievable energy density is lower). One reason for the decrease in cut quality is because the beam quality for the Nd:YAG decreases when coupled into fiber.


Diode Lasers

Direct diode laser systems provide power via semiconductors connected to a fiber- or mirror-based delivery system. When these laser diodes are grouped or “stacked” in one- or two-dimensional arrays, the total power output can extend into the kilowatt range. The laser diode material is cleaved along natural facets, creating mirrored surfaces. Thus, the diode itself acts as the resonator cavity. Electric current provides the pump source, and wavelength is dependent on the semiconductor material from which the diode is made. With typically linear or rectangular beam shapes, industrial diode laser stacks are not suitable for drilling or cutting, where a narrowly focused beam is required. However, they are suitable for applications including continuous seam welding, brazing, cladding, heat treating, and soldering. Materials that can be processed by industrial diode lasers include a wide range of metals and plastics. Also, with shorter wavelengths (0.8-1.0 μm) than typical Nd:YAG (1.06 μm) or CO2 (10.6 μm) lasers, the absorption rate in aluminum and other metals is much higher and does not require the metal to be precoated, as is often the case with CO2 lasers. In addition to industrial uses, laser diodes are also used as the pump source in solid-state (crystal), disk, and fiber lasers. Diode lasers have a much higher efficiency than other types of lasers but also lower energy density (even with stacks and fiber coupling). Premature diode failure and, consequently, reliability concerns still plague diode lasers (and the systems that employ them to pump other materials). Higher costs result as diode stacks must be either replaced when they fail or duplicated in system designs for built-in redundancy. Maximum commercially-available power levels for the diode laser exceed 5 kilowatts.


Fiber Lasers

Fiber lasers have made huge advances in the last 2 years. While they have not gained widespread acceptance as an industrial tool, they show promise for some new applications. Their current primary use is in low-power applications. With a smaller footprint than other laser types, they are also very modular. Because the laser cavity is a conventional multimode fiber, small diameter fiber delivery is inherent to the system, and no air-to-fiber coupling losses result. And, at an emission wavelength of 1.07 μm, there is relatively low loss in the fiber. The fiber is typically made of fused silica, doped with Ytterbium (Yb), pumped by diode laser stacks, and capped by fiber Bragg gratings. With multiple layers of pig-tailed, single-emitter diode lasers, the laser is scalable beyond 10 kilowatts. The primary disadvantage to the fiber laser is the high cost of the many diode stacks (since single emitters currently can achieve only about 4 watts of power) and related reliability issues. The key to overcoming uncertain diode lifespan is redundancy, which will mask diode stack failure, although this further increases the cost and complexity of those laser systems.

Fiber lasers offer the preferred wavelength range for metal processing due to high material absorption. The 300-μm fiber also provides a very straight-sided beam profile, which is good for welds and extended depth of focus. A primary benefit to fiber lasers is their delivery via thin fiber, which can be manipulated using inexpensive robots.

Some anticipate applications where fiber lasers could eventually replace CO2 laser systems. For example, the shipbuilding industry currently uses 4- to 8-kilowatt CO2 laser systems. “But, in about 5 years, we could see fiber lasers used in these processes,” according to Stefan Heinemann (Fraunhofer USA [Center for Laser Technology]), due to the ability to manipulate the fiber output within confined



Disk Lasers

In the thin-disk laser system, the laser active medium is a very thin disk with less than 200-μm thickness. The Yb:YAG crystal is stimulated at the frontside via a diode laser stack in a quasi-end-pumped design. The backside is cooled over the whole area. Due to the small thickness, only a part of the pump beam is absorbed. In an optical system consisting of one parabolic mirror and one retro-reflective mirror (refer to diagram), the not-absorbed power on each disk will be imaged multiple times from each diode laser system to optimize efficiency. Typically, up to 32 passes of each pump beam are realized. A single disk can produce up to 3.5 kilowatts of power, and these lasers operate in the near-infrared at a wavelength of 1.03 μm. Through innovative designs, such as the one diagrammed above, multiple disks can be cascaded to achieve higher power levels.

Because the disk can be cooled over its entire backside, thermal lensing is kept to a minimum. Heat flow and temperature gradients predominantly occur on-axis, resulting in an almost homogeneous distribution of temperature and minimal distortions of the wavefront. Due to these advantages, it is possible to achieve excellent beam quality even at high laser powers. Depolarization caused by the active medium is also very low. Furthermore, simple power scalability can be achieved through variation of the pumped diameter and pump power.

Modular design allows the changing of different laser parameters, such as operational mode, power level, and beam quality. The laser module consists of a thin disk module and other components like a resonator or housing. Cooling the thin disk module is done with either a cooling unit or house water supply. The completely assembled module contains a coated and qualified thin disk, bonded on a heat sink and integrated with the optical system necessary for multipass imaging of pump radiation. The crystal can be assembled on a goniometer mount, enabling the use of the thin disk as a mirror in a resonator.

Disk lasers offer both higher efficiency and better beam quality than Nd:YAG lasers. The smaller beam, and resultant higher energy density, reduces kerf loss and allows faster cutting on thin steel. (Because thick steel cuts require wider kerfs for improved gas flow dynamics, however, disk lasers are less effective and slower on such samples.) In addition, the smaller-diameter fiber delivery system allows for manipulation of the laser output with inexpensive robots.

These lasers are considered by many to be the successor to Nd:YAG lasers in most welding applications. Disk lasers with long working distance and high beam quality have excellent long-term potential in high-power applications.