brightness converters (original) (raw)

Definition: optically pumped devices where the generated optical output has a higher brightness than the optical pump source

Categories: article belongs to category photonic devices photonic devices, article belongs to category laser devices and laser physics laser devices and laser physics

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DOI: 10.61835/rka Cite the article: BibTex BibLaTex plain text HTML Link to this page! LinkedIn

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What are Brightness Converters?

The output power of a laser device is always lower than the pump power, since some energy losses are always occurring in laser operation. Nevertheless, the brightness (more precisely: the radiance) of an optically pumped laser can be far higher than the brightness of its pump source, if the pump source has a poor beam quality (thus a low brightness) while the beam quality of the output beam is much better. In such cases, a laser can be said to have the function of a brightness converter.

Examples of Brightness Converters

Some examples of lasers serving as brightness converters are:

A solid-state bulk laser (e.g. a rod laser or a thin-disk laser) may be end-pumped or side-pumped with high-power laser diodes and emit an essentially diffraction-limited beam, having a significantly increased brightness. This effect is typically more pronounced for a side-pumped laser utilizing the very diffuse light from an arc lamp or flash lamp, even though the output beam quality is then also often not diffraction-limited.
A large brightness enhancement can also be achieved with a high-power fiber laser based on a double-clad fiber. The pump cladding may guide thousands of modes, so that e.g. a diode bar with very high _M_2 factor can be used. The single-mode fiber core allows for diffraction-limited beam quality, and the power conversion efficiency can be higher than 80%.
An optically pumped vertical external cavity surface-emitting laser (VECSEL), which is a kind of semiconductor laser, can also generate a diffraction-limited output. Its requirements concerning pump beam quality are very low since the pump radiation is absorbed in a broad but very thin layer, so that its beam divergence does not matter very much (even though a too high beam divergence can result in problems with geometric constraints in the laser construction).
High-power optical parametric oscillators can in principle also be operated as brightness converters. Although most OPOs are pumped with close to diffraction-limited beams, it is possible to pump an OPO with a highly multimode beam while still generating a diffraction-limited signal beam. The idler beam quality will then be poor, but this may not matter.

In many cases, the function as a brightness converter is essential for laser applications. For example, many solid-state lasers are used in laser material processing because their pump light, having too low brightness, could not be used directly (although direct diode lasers are gaining traction).

Another frequently required function of a solid-state laser is pulse generation e.g. via Q-switching. This might in principle also be considered as a kind of brightness converter function since relatively long pump pulses can be converted into far shorter laser pulses, effectively concentrating energy in time. However, the term is usually reserved to improved spatial concentration of light.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a brightness converter in the context of lasers?

A brightness converter is an optically pumped laser that produces an output beam with a far higher brightness (radiance) than its pump source. It achieves this by converting light from a source with poor beam quality into a laser beam with much better beam quality.

What are common examples of brightness converters?

Examples include solid-state bulk lasers (e.g., rod or thin-disk lasers), high-power fiber lasers with double-clad fibers, and vertical external-cavity surface-emitting lasers (VECSELs), all of which can be pumped by low-brightness laser diodes to produce a high-quality beam.

How can a fiber laser significantly enhance brightness?

A double-clad fiber laser uses a large pump cladding that can accept low-quality light from thousands of modes, for example from a diode bar. This light pumps a single-mode fiber core, which then produces a diffraction-limited output beam, resulting in a large brightness enhancement.

Why is brightness conversion essential for applications like material processing?

Many applications, such as laser material processing, require a beam which can be focused to a very small spot. The pump light from sources like diode bars often has too low brightness for this. A laser acting as a brightness converter generates a high-quality beam which is suitable for such applications.

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