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Direct Diode Lasers

Acronym: DDL = direct diode laser

Definition: diode lasers which are directly used e.g. for material processing

Category: laser devices and laser physics


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Direct diode lasers are laser devices where the output of laser diodes is directly used for an application – frequently in laser material processing, e.g. in the form of laser cutting or laser welding. This is in contrast to using diode-pumped lasers, where the diode laser radiation is used for pumping another laser, typically a solid-state laser, the output of which is then sent to the application. That additional laser usually acts as a brightness converter: although some of the optical power is lost, the beam quality of its output is so much higher than that of the diode laser(s) that the brightness (more precisely: the radiance) is still higher. This allows for increased optical intensities on workpieces, for example.

The direct use of diode lasers is attractive for different reasons:

  • Such laser sources are overall simpler and more compact, also potentially much cheaper than alternative solutions.
  • Their wall-plug efficiency is substantially higher, as conversion losses are avoided. That can greatly reduce the electric energy consumption.
laser cutting with diode laser
Figure 1: Laser cutting in stainless steel with a direct diode laser. Source: Fraunhofer ILT, Aachen, Germany.

Used Diode Lasers

Direct diode lasers are usually based on gallium arsenide technology, which allows for emission wavelengths of typically between 0.8 μm and 1 μm.

Many direct diode laser applications require substantial output powers. One uses not only single diode bars, which are suitable for powers of the order of 100 W, but also diode stacks, containing multiple diode bars and delivering powers of 1 kW or even several kilowatts. It is also possible to do spectral beam combining with several diode stacks operating at somewhat different wavelengths.

The improved radiance of high-power laser diodes has different origins. One factor is the continuous increase of radiance of diode bars and diode stacks through improvements of their design. For example, there are tapered laser diodes and tapered amplifiers with much increased beam quality. Another factor are improved methods and components for beam combination of the radiation of many laser diodes.

Advantages and Limitations

However, the beam quality of high-power laser diodes has for many years been clearly too low for direct application in laser material processing, e.g. laser welding. The rapid technical progress in this area has in recent years increased their beam quality and radiance to such high levels that many direct diode applications have become possible. Direct diode lasers are therefore now even used for metal cutting and welding, particularly for conduction welding of relatively thin metal sheets. Laser soldering and brazing, laser cladding and some laser surface treatments also belong to their applications.

Free Space or Fiber Coupling

In the same way as solid-state lasers, direct diode laser products can either emit a free-space laser beam or can be fiber-coupled. Beam delivery through an optical fiber is beneficial for many applications, and becomes feasible if the beam quality of the source is sufficiently high.

See also: diode lasers, diode-pumped lasers, solid-state lasers, brightness converters, beam combining, laser applications, laser cutting

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