Diode-Pumped Lasers

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

• A lamp-pumped laser uses a certain gas discharge lamp (arc lamp or flash lamp) as the pump source.
• A diode-pumped laser, a laser pumped with some kind of laser diode.

This article discusses the latter category and also uses the term all-solid-state lasers.

Types of diode-pumped lasers

Most diode-pumped lasers are solid-state lasers (DPSSL=DPSSss laser=diode-pumped solid-state lasers). They are either bulk lasers that use some kind of laser crystal or large pieces of glass, or fiber lasers (although the term DPSSL is less commonly used with fiber lasers). The output power of these two categories ranges from a few milliwatts to several kilowatts (high-power lasers).

Less common are optically pumped semiconductor lasers (specifically VECSEL = vertical cavity surface-emitting lasers), and there are some relatively exotic diode-pumped gas lasers such as alkali vapor lasers.

Laser diode type

There are different types of laser diodes that can be used for diode pumping and vary greatly in terms of optical power:

• Low-power lasers (up to about 200 mW) can be pumped with small edge-emitting laser diodes. These lasers exhibit a beam quality that is essentially diffraction-limited, making it fairly easy for solid-state lasers to achieve the same effect.
• Wide-area laser diodes typically produce a few watts of power and are suitable for pumping solid-state lasers with output powers of several watts. Their beam quality is essentially asymmetrical, but it is still usually sufficient to achieve diffraction-limited laser output without the use of complex optics.
• High-power diode rods emit tens of watts (or even more than 100 W), allowing for higher output power, especially when several rows are combined. Their output beam is strongly asymmetrical and the beam quality is poor, so their radiation (brightness) is much lower than that of low-power diodes. Various types of beam shapers are used to make the beam symmetrical. This makes it easier to pump a body laser or couple light into an optical fiber.
• For the highest power, diode stacks are typically used. They are still of poor beam quality and low brightness, but can provide multiple kilowatts of power.

However, for Q-switched lasers with high pulse energies and low pulse repetition rates, quasi-CW operation with higher peak power over a limited time interval (e.g., 100 μs) is sometimes employed. Some pump diodes are explicitly optimized for this mode of operation.

Depending on the type of laser diode, different kinds of pump optics are used. It is also possible to use a fiber-coupled diode laser, which allows the actual laser head to be separated from another package that contains a pump diode, so that the laser head can become very compact.

Advantages of diode pumping:

There are miniature solid-state lasers with excellent efficiency, beam quality, spectral purity, and stability, some of which may even be battery-powered.

Diode-pumped, high-power solid-state lasers can provide kilowatt-level output power and fairly high beam quality. This applies especially to thin-disk lasers, but also to high-power fiber lasers and amplifiers.

Diode pumping is also essential for a large number of mode-locked lasers, for example, producing an average output power of well over 100W in sub-picosecond pulses, or picosecond pulses for telecommunications applications with pulse repetition rates up to 50 GHz.

limit

In the early days of diode pumping, the achievable output power was very limited – less than that of lamp-pumped lasers. However, in the interim, high-power diode rods and diode stacks have become very powerful, and the highest output power is now usually achieved by diode pumping.

The main disadvantage of diode pumping (compared to lamp pumping) is that the cost per watt of pumping power is much higher. This is serious for high power, especially for pulses that generate high energy, requiring a fairly high pump power for a limited time. For this reason, lamp pumps are still used in cases where high power and particularly high pulse energy are required. For example, lamp-pumped Q-switched Nd:YAG lasers are still widely used for laser marking and will not be replaced by diode-pumped lasers anytime soon. In some applications, even joule pulse energy and pump power in the tens of kilowatts are required, while low repetition rates (e.g., 10 Hz) allow for controllable thermal effects and acceptable low power conversion efficiency.

Laser diodes are not as electrically stable as gas discharge lamps. For example, they can be quickly destroyed by excessive drive currents or electrostatic discharges. However, when combined with properly designed electronics, this shouldn't happen. Problems can also come from optical feedback.

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Diode-pumped solid-state lasers have a very wide range of applications. All areas of application are mentioned in the laser application article.