Diode-pumped Lasers

Author: the photonics expert Dr. Rüdiger Paschotta

Virtually all optically pumped lasers fall into one of two categories:

• lamp-pumped lasers, having some kind of gas discharge lamp (arc lamp or flash lamp) as the pump source
• diode-pumped lasers, which are pumped with some kind of laser diodes

This article treats the latter category, for which the term all-solid-state lasers is also used.

Types of Diode-pumped Lasers

Most diode-pumped lasers are solid-state lasers (DPSSL = DPSS lasers = diode-pumped solid-state lasers). These are either bulk lasers, using some kind of laser crystal or bulk piece of glass, or fiber lasers (although the term DPSSL is less common for fiber lasers). Both categories span a range of output powers from a few milliwatts to multiple kilowatts (→ high-power lasers).

Less common are optically pumped semiconductor lasers (particularly VECSELs = vertical external cavity surface-emitting lasers), and there are also some relatively exotic types of diode-pumped gas lasers, e.g. alkali vapor lasers.

Types of Laser Diodes for Diode Pumping

There are different types of laser diodes which can be used for diode pumping, and very much differ in terms of optical power:

• Low-power lasers (up to roughly 200 mW) can be pumped with small edge-emitting laser diodes. These exhibit an essentially diffraction-limited beam quality and make it fairly easy to achieve the same for the solid-state laser.
• Broad area laser diodes typically generate several watts and are suitable for end pumping solid-state lasers with output powers up to a few watts (see Figure 1). Their beam quality is substantially asymmetric, but normally still sufficient for achieving a diffraction-limited laser output without using complicated optics.
• High-power diode bars emit tens of watts (or even more than 100 W), allowing for higher output powers, particularly when several bars are combined. Their output beam is strongly asymmetric, and the beam quality is poor, so that their radiance (brightness) is much lower than that of lower-power diodes. Various types of beam shapers are used to symmetrize the beams. This makes it easier either to pump a bulk laser or to couple the light into a fiber.
• For the highest powers, diode stacks are often used. These have a still worse beam quality and lower brightness, but can provide multiple kilowatts. They are usually suitable only for side pumping.

In most cases, the pump diodes are operated continuously. This applies to all continuous-wave and mode-locked lasers, and also to many Q-switched lasers. However, quasi-continuous-wave operation with higher peak power for limited time intervals (e.g. 100 μs) is sometimes used for Q-switched lasers with a high pulse energy and low pulse repetition rate. Some pump diodes are explicitly optimized for such an operation mode.

Depending on the type of laser diode, different kinds of pump optics are used. It is also possible to use fiber-coupled diode lasers, which make it possible to separate the actual laser head from another package containing the pump diodes, so that the laser head can become very compact.

Advantages of Diode Pumping

The main advantages of diode pumping (in contrast to lamp pumping) are:

• A high electrical-to-optical efficiency of the pump source (of the order of 50%) leads to a high overall power efficiency (→ wall-plug efficiency) of the laser. As a consequence, small laser power supplies are needed, and both the electricity consumption and the cooling demands are drastically reduced, compared with those for lamp-pumped lasers.
• The narrow optical bandwidth of diode lasers makes it possible to pump directly certain transitions of laser-active ions without losing power in other spectral regions. It thus also contributes to a high efficiency and there's also to reduce problems with thermal lensing and other detrimental thermal effects.
• Although the beam quality of high-power diode lasers is not perfect, it often allows for end pumping of lasers with very good overlap of laser mode and pumped region, leading to high beam quality and power efficiency. In the domain of slab lasers, it allows edge pumping instead of face pumping, which brings important advantages.
• Diode-pumped low-power lasers can be pumped with diffraction-limited laser diodes. This allows the construction of very low-power lasers with reasonable power efficiency, i.e., with a very small electrical pump power, as is important for battery-powered devices.
• The lifetime of laser diodes is long compared with that of discharge lamps: typically many thousands of hours, often even well above 10 000 hours. However, various factors (e.g. voltage spikes, optical feedback and insufficient cooling) can lead to drastically reduced lifetimes. The exchange of laser diodes is also substantially more costly than that of discharge lamps.
• The compactness of the pump source, the power supply and the cooling arrangement makes the whole laser system much smaller and easier to use.
• Diode pumping makes it possible to use a very wide range of solid-state gain media for different wavelength regions, including e.g. upconversion lasers. For many solid-state gain media, the lower brightness of discharge lamps would not be sufficient.
• The low intensity noise of laser diodes leads to low noise of the diode-pumped laser.

Achievements

The benefits of diode pumping have lead to amazing achievements. Some examples are:

• There are miniature solid-state lasers with excellent efficiency, beam quality, spectral purity, and stability. Some of those may even be battery-powered.
• Diode-pumped high-power solid-state lasers can deliver kilowatt output powers with fairly high beam quality. This applies particularly to thin-disk lasers, but also to high-power fiber lasers and amplifiers.
• Diode pumping is also essential for a large variety of mode-locked lasers, generating e.g. well over 100 W of average output power in sub-picosecond pulses, or picosecond pulses for telecom applications with a pulse repetition rate up to 50 GHz.

Limitations

In the early years of diode pumping, the output powers achievable were very limited – smaller than those of lamp-pumped lasers. In the meantime, however, high-power diode bars and diode stacks have become very powerful, and the highest output powers are now usually achieved with diode pumping.

The main disadvantage of diode pumping (as compared with lamp pumping) is the significantly higher cost per watt of pump power. This is severe for high powers, and particularly for generating high-energy pulses where a rather high pump power is required for limited time. For this reason, lamp pumping is still used in cases where high powers and particularly high pulse energies are needed. For example, lamp-pumped Q-switched Nd:YAG lasers are still widely used for laser marking, and will not soon be replaced with diode-pumped lasers. For some applications, even joule-level pulse energies and many tens of kilowatts of pump power are required, while low repetition rates (e.g. 10 Hz) make the thermal effects manageable and the low power conversion efficiency acceptable.

Laser diodes are electrically less robust than discharge lamps. They may e.g. be quickly destroyed by excessive drive currents, or by electrostatic discharges. In conjunction with properly designed electronics, however, this should not happen. Problems can also arise from optical feedback.

Applications

Diode-pumped solid-state lasers have a very wide range of applications. Indeed, they are used in all of the areas mentioned in the article on laser applications.

More to Learn

Encyclopedia articles:

• lasers
• laser crystals
• solid-state lasers
• all-solid-state lasers
• lamp-pumped lasers