High Brightness Laser Diodes

Author: the photonics expert Dr. Rüdiger Paschotta

While low-power laser diodes can exhibit a very high beam quality, the beam quality of high-power laser diodes is normally substantially lower. Therefore, the obtained radiance (brightness) is often much less improved than the output power, or even lower than that of a low-power laser. However, there are high brightness laser diodes which are optimized for a particularly favorable combination of high output power and good beam quality, resulting in a particularly high brightness (more precisely: high radiance).

Types of High Brightness Laser Diodes

The term does not indicate with what technology the high radiance is achieved. In fact, the term is used for quite different types of laser diodes, and particularly for the following categories:

• Most frequently, the term is used for optimized broad area laser diodes, particularly in the form of tapered laser diodes.
• The same principles can be applied to diode bars. However, their radiance is in most cases substantially lower, even if the contained emitters have a very high radiance, since their emission is usually mutually incoherent, and there is some dead space between the emitters.
• Further, there are fiber-coupled diode lasers which are optimized in terms of radiance. That optimization does not necessarily apply to the laser diodes themselves, but can also be done with particularly efficient fiber launch systems.

The radiance of diode laser systems can also be substantially increased with methods of beam combining – in the form of coherent beam combining or spectral beam combining. In such cases, one should actually use the term high brightness diode lasers instead of high brightness laser diodes.

Applications of High Brightness Laser Diodes

A typical application of high brightness laser diodes is the optical pumping of solid-state bulk lasers. A particularly high radiance is required in cases where the pump radiation needs to be strongly focused – for example, in diode-pumped titanium–sapphire lasers. Also, there are cases where a relatively long laser crystal is needed, and the beam quality of the pump radiation then needs to be high enough to avoid substantial divergence within the length of the crystal.

Fiber lasers can also benefit from high brightness pump sources. For example, these allow one to launch a higher pump power into a double-clad fiber. In other cases, one can use a double-clad fiber with smaller pump cladding, exhibiting stronger pump absorption, so that one can use a shorter length of fiber – as can be beneficial e.g. for minimizing nonlinear effects.

The development of pump sources with higher brightness is one of the most important reasons why the performance levels of solid-state bulk and fiber lasers have been substantially raised in recent years. Note that such kind of performance increases should not be considered as resulting from power scaling; they result from component optimization rather than from the application of a scaling procedure.

A high level of radiance is also often required for laser material processing. The substantial technical advances of recent years have more and more enable the use of high-power diode lasers in this area. They are called direct diode lasers, in contrast to lower-brightness lasers which can only be used in conjunction with a diode-pumped laser, functioning as a brightness converter.

More to Learn

Encyclopedia articles:

• broad area laser diodes
• tapered laser diodes
• diode bars
• direct diode lasers