Encyclopedia > letter B > brightness converters

Brightness Converters

Author: the photonics expert Dr. Rüdiger Paschotta

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

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

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.

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 M² 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 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.

More to Learn

Encyclopedia articles:

Questions and Comments from Users

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.