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Direct-diode lasers: they've got the power

04 Jun 2009

High-power direct-diode lasers have emerged as a competitive option for applications like cladding, welding and heat treating. Stuart Woods of Coherent explains why the basic construction and operation of direct-diode systems translate into low cost-of-ownership.

Over the past decade, improvements in diode-laser fabrication and packaging technology have enabled the construction of compact, integrated systems with several kilowatts of output. These high-power direct-diode laser (HPDDL) systems offer a combination of excellent electrical efficiency, low maintenance and ease of integration, all of which contribute to a low cost of ownership unmatched by any other laser technology. As a result, HPDDLs are making inroads in a variety of materials processing applications. Before reviewing those applications in more detail, it's helpful to revisit the fundamentals of HPDDL construction and operation.

Powering up

The edge-emitting diode laser is a semiconductor device that converts electrical energy directly into laser light. Typically, the highest output power is achieved in the near-infrared, most commonly at either 808 or 980 nm. While an individual diode laser produces only up to a few watts of output power, numerous emitters can be fabricated on a single, monolithic semiconductor substrate (called a "bar") for a total output as high as 100 W.

Because of the small size of diode-laser bars, they can be effectively cooled with a low volume of circulating water and a chiller. Figure 1 shows a mounting configuration for diode-laser bars called a microchannel-cooled package (MCCP). Here, the diode-laser bar is mounted on a plate having internal water-circulation channels. The MCCP contains two large water ports, for input and output, each with an O-ring at their respective edges. The O-rings provide a water-tight seal when two MCCPs are placed against each other, a feature which enables multiple MCCPs to be stacked together and water circulated through the entire assembly. The figure also shows just such an assembly of several MCCPs. The resulting stack is an extremely compact assembly that can deliver several kilowatts of power.

Unlike many other lasers, which produce a round, focusable Gaussian beam, the output of a diode-laser stack comes from numerous individual emitters spread over an area several square millimetres in size. Furthermore, diffraction at the micron-sized rectangular emitters causes each to produce an elliptical beam with asymmetrical divergence. Because of this, specialized optics must be employed in order to convert the raw output into a far-field format useful for most applications.

These collection optics utilize both cylindrical and spherical lenses to gather and collimate the output of the entire stack. This light can then be focused onto the workpiece directly (termed free-space delivery) or channelled into a single optical fibre, enabling remote (up to 30 m) delivery of the laser source from the processing area. A typical output beam from a free-space HPDDL system might be 12 × 1 mm at its point of focus, while a fibre-delivered system might produce a round spot that is only a few millimetres in size. Also, fibre delivery generally yields a higher power density (laser energy per unit area) at the workpiece compared with free-space delivery.

The diode difference

Besides their output geometry, there are other ways in which HPDDLs differ from more traditional industrial lasers. For starters, the wallplug (electrical conversion) efficiency is many times higher in diode lasers than for any other laser type, including CO2, lamp-pumped solid state (LPSS), diode-pumped solid state (DPSS) and even fibre lasers (because fibre lasers consist of a fibre cavity pumped by diode lasers, and it is impossible for the fibre-laser cavity to convert the diode pump light with 100% efficiency). The primary benefit of this efficiency is lower operating cost for the system, because less electricity is required to produce a given amount of output power. Of course, this reduced power consumption also decreases the carbon footprint of the laser's operation.

HPDDLs are physically compact and lightweight when compared with most other industrial lasers (see photo). This in turn lowers their integration costs. In addition, a closed-loop cooling system can be connected to the diode stack affording a typical operating lifetime of tens of thousands of hours. Closed-loop cooling also makes it possible to use a water and coolant mixture specifically optimized to match with the metallurgy of the MCCPs, so as to further maximize their lifetime. This approach is utilized in some commercial HPDDLs, such as the Coherent HighLight 1000F laser system.

As a result, HPDDLs offer substantially lower cost-of-ownership when measured against other laser technologies. Equally, the initial capital cost is usually lower for an HPDDL than for another laser type of equivalent output power.

Technology trends, applications

HPDDL technology trends are centred on improving fibre-coupled brightness without any sacrifice in reliability or increase in the already low cost of ownership. In particular, the diodes that make up these stacks and the fibres that are married to them are both evolving together so as to maximize light-coupling efficiency. For example, complete bars are being tailored for fibre coupling from their initial design. This eliminates the need to make the individual diode emitter as bright as possible, and to then couple that emitter to a singlemode fibre, which is then combined into a fibre bundle.

One thing is certain: HPDDLs offering kilowatts of power from fibres that are hundreds of microns in diameter are just around the corner. Over time, it's inevitable that fibre-coupled bars will take on single emitters at the application and pumping level, while delivering lower cost per watt with equivalent reliability.

Despite the positive outlook, some applications are out of reach. The beam characteristics of HPDDLs preclude focusing to the submicron or even submillimetre spot sizes achieved with many other laser types. Consequently, they are not suitable for high-precision micromachining or metal cutting. However, the size, cost and optional fibre delivery of HPDDLs makes them an attractive option for tasks requiring intense heating over a well defined area of a few millimetres in size. Typical examples are cladding, heat treating and welding.

Cladding involves the creation of a new surface layer on a part, with that new layer having a different composition to the base metal. The process is used to improve the surface and near-surface properties (e.g. wear, corrosion or heat resistance) of parts, or to resurface a component that has become worn. Typically, the clad material is supplied in powder or wire form, which is thermally melted and fused onto the substrate (figure 2). For maximum adhesion, the clad layer should form a true metallurgical bond with the substrate material (although dilution of the base material with the clad material is not desirable, since this would change the bulk properties).

The output of the free-space delivered HPDDL is well matched to the needs of many cladding applications. Specifically, the focused line beam is used to melt the clad material, while either the beam or part is translated. With powder-based cladding, the long axis of the line beam is oriented perpendicular to the scan direction, enabling large areas to be processed rapidly. Alternatively, for wire-feed cladding, it is usually advantageous to orient the beam with the short axis in the direction of travel. Both powder-based or wire-feed cladding of smaller areas can be done with a fibre-coupled HPDDL as well.

All told, HPDDLs offer advantages over traditional cladding methods such as arc welding and thermal spraying. They deliver a true metallurgical bond with lower dilution than arc-welding-based techniques, and their low heat input minimizes the potential for part distortion. They also produce a better-quality bond than thermal-spraying methods. Furthermore, unique cladding compositions can be optimized for good wear resistance and low friction coefficients, all at a low application cost.

Laser welding is an established technology currently performed with DPSSLs, LPSSLs and fibre lasers, as well as HPDDLs. Automotive body welding, in particular, is dominated by LPSS technologies. Fibre lasers have been considered for this application but, in general, they have been found to be too bright and too expensive.

Compared with all of these other lasers, the primary advantage of HPDDLs is the substantial cost-of-ownership savings derived from their electrical efficiency. Put simply, HPDDL technology can be optimized to deliver the right brightness and power for a specific welding task at the lowest cost of ownership for the end-user.

This is further aided by the fact that HPDDLs have an instant "on" capability, eliminating standby power consumption. Even larger savings result from reduced maintenance costs and downtime, which are orders of magnitude lower for the HPDDL than other welding lasers.

Indeed, their small size and low weight mean that these laser systems can be mounted directly onto robot arms and moved relatively quickly. Alternatively, optical-fibre delivery allows a high level of flexibility in terms of where the laser system is located; it also enables beam delivery into tight or hard-to-access spaces. Typical uses are therefore under-hood welding of components in automobile manufacturing or cylindrical containers like batteries.

Heat treating (or case hardening) involves rapid heating and cooling of a thin surface layer of a metal (usually steel or cast iron). This process generates a change in crystalline properties resulting in greater hardness compared with the bulk substrate. Case hardening is widely used with machine tools and on bearing surfaces of moving parts.

CO2 laser systems have found limited use in case hardening for many years. The laser's primary advantage is that it produces rapid heating and cooling over a spatially well defined area. This yields rapid processing, precise control over case depth and minimal part distortion (eliminating the need for post-processing). However, CO2 lasers suffer from three drawbacks that have limited their widespread use. First, the 10.6 µm wavelength is not well absorbed by most metals, so parts must typically be painted with an absorptive material before processing. Also, the beam size of CO2 lasers is not well matched to the needs of most case-hardening applications, which typically require processing of an area several square millimetres in size. Finally, the cost of CO2 lasers does not compare favourably with alternative non-laser techniques for hardening.

HPDDLs directly address all three of these drawbacks and are, in many ways, an ideal source for heat treating. In addition, the option of fibre delivery provides greater flexibility in implementation versus what can be achieved with CO2 lasers. The photograph on the left, for example, shows the spatial selectivity with which the process can be performed.

To sum up: HPDDLs now offer an attractive alternative to other lasers and traditional techniques for a number of materials processing applications. Furthermore, their low cost-of-ownership and increasingly high fibre-coupling power levels promise a very bright future indeed.

• This article originally appeared in the June 2009 issue of Optics & Laser Europe magazine.

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