Laser diode modules

A laser diode module - introduction

A laser diode module is a laser whose light is generated by laser diodes, i.e. semiconductor materials. The diode lasers can produce light outputs up to more than 60 kW. Note that “diode laser” is a quite confusing name, “semiconductor laser module” or “semiconductor laser head" is a better name.

Single emitters

In the simplest form, a diode laser module consists of only a single laser diode, always with collimating and focusing optics. Single-emitter laser diodes are used, for example, in laser pointers, for optical data transmission or in CD and DVD scanners or burners. Single blue laser diodes have enough power to be used in CNC machines and 3D printers. Suitable for cutting and engraving materials such as wood, plywood, paper, cardboard, leather, rubber or plastics. Single-emitters are manufactured with powers up to a few watts, are mounted on a heat sink and often already contain beam collimation with a spherical or anamorphic lens or even a pigtailed fiber.

The single-emitter diode laser heads often have a special construction that is not used in the multi-emitter diode laser modules described below. The most important special forms of single-emitter lasers are:

Multi emitters

Since individual laser diodes can only be manufactured up to powers of a few watts and emit strongly diverging laser radiation, often several laser diodes are combined electrically and optically: Bars are used, which contain several individual emitters next to one another on a strip-shaped chip. These are electrically operated in parallel and mounted on a heat sink.

The 10-20 individual diodes (emitters) of such a bar each emit a laser beam which has approximately 40 ° (fast axis) in one direction and approximately 12 ° (slow axis) emission angle in the other direction. The fast axis has the highest possible and the slow axis relatively poor beam quality. To summarize these individual beams, they are after the collimation of the fast axis (by means of the micro-optical cylindrical lens) geometrically rotated by another micro-optics and arranged side by side and then the so-called slow-axis collimation is performed.

Diffraction limited or not depends on optics, not from the laser diode. A "laser beam" thus generated actually consists of several individual laser beams and has a much poorer beam quality than other lasers of the same power. Manufacturers offer both composite products and hermetically sealed lasers, some already with permanently mounted fiber optic cables or connections for fiber optic connectors (for example, an F-SMA socket). Applications of such laser diode modules: metal and plastic welding, selective hardening, soft and brazing, build-up welding, pumping of solid-state lasers, especially fiber lasers.

Several such bars can be combined into stacks, whereby the bars are electrically connected in series and the individual beams are combined optically. With such stacks, optical powers of 0.5-1 kW can be generated. To further increase of the performance, you can use the linear polarization and different wavelengths of several stacks: By means of dichroic and polarization-dependent mirrors, the radiation of several stacks (for example, four stacks with two different wavelengths, each mounted orthogonally to each other) can theoretically be mirrored into one another without quality and power loss. You can obtain laser beam sources with several kilowatts of optical power at comparatively high efficiency (20-30%). Such high powers are used for material processing (metal welding, hardening, remelting, powder application).

Advantages of laser diode modules:

  • Very compact design
  • Easy pumping by means of electric current
  • Comparatively high electrical/optical efficiency from 25 to over 50%
  • Long service intervals compared to lamp-pumped lasers
  • Coupling and transport of the radiation in optical cables possible
  • Very long service life, sometimes more than 30,000 hours
  • Low power degradation; much less than 1% / 1000 h when operating at rated current

Disadvantages of laser diode modules:

  • In comparison to other lasers, poorer beam quality (especially at high powers), therefore hardly suitable for cutting and only conditionally suitable for deep welding of metals
  • Strong beam divergence if not corrected by appropriate optics
  • Costs: Laser diodes, their mounting on a heat sink and the adjustment of the micro-optics are still a high-cost factor of a diode laser system

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