Lamp-pumped lasers

Solid-state lasers doped with insulators are basically optically pumped, and the pump source is usually a laser diode or some kind of gas discharge lamp; In rare cases, tungsten halogen lamps are used, which are not gas-discharge lamps but are similar to ordinary incandescent lamps.

The type of lamp

Gas discharge lamps used for laser pumping can be divided into two categories: arc lamps and flash lamps. Arc lamps are optimized for continuous-wave operation, while flash lamps generate pump pulses for free-running or Q-switched lasers.Note that the term flash is sometimes mistakenly used in place of an arc lamp, which is not necessarily based on arc discharge, but may use glow discharge.

Both types of lamps basically consist of a glass tube filled with some gas (such as krypton or xenon, at a few atmospheres) with a metal electrode at each end. Flashing lamps typically have a capacitor that provides electrical energy, and the energy is transmitted through a number of electronic components (pulse-forming networks) that affect the duration of the pump pulses.

For the pumping of solid-state lasers, linear lamps with electrode spacing between 5 and 15 cm are typically used. Short-arc lamps typically have electrode spacing of less than 1 mm and sometimes have carbon electrodes for other purposes. The shape of the lamp's electrodes depends on the mode of operation: flash lamps have a round cathode, while pointed cathodes are more suitable for arc lamps, which operate at a lower current. The applied current density can also have a significant impact on the spectrum produced; Arc lamps that run continuously usually exhibit a distinct linear spectrum, while flashes that enter the arc state have a smoother spectral shape.

Pump chamber

The laser crystal of a lamp-pumped laser is usually a relatively long side-pumped rod that adapts to the length of the lamp. In many cases, the laser bar and lamp are placed in an elliptical pump cavity with reflective walls, resulting in a large percentage of the pump light that can be absorbed by the laser bar. The excess heat is carried away by the cooling water, and additional filter glass can be used to protect the laser bar from the ultraviolet rays emitted by the gas discharge lamp. Another type of solid-state laser suitable for lamp-pumped is the end-pumped slatted laser.Here, an array of lamps pumps a slab through its large face, possibly from both sides. The pump light may be injected through a layer of cooling water.

In any case, the low brightness (more precisely: low emission) of the lamp limits the choice of geometry. For example, terminal pumping requires higher radiation and is therefore only possible with a laser diode (or sometimes some other type of laser).

Gain medium

Regarding the gain medium, the most common lamp-pumped laser is the Nd:YAG laser. Krypton lamps are mostly used in this case because the emission of krypton is strong in the region between 750 nm and 900 nm, and Nd:YAG has strong absorption lines in this region. Other neodymium-doped laser gain media, such as Nd:YLF and Nd:YVO4, are also suitable. They have a relatively wide absorption band and are a quaternary laser medium, so they can be used at moderate pump intensities and utilize a large portion of the lamp's spectrum. Less common lamp-pumped lasers are based on alexandrite (using xenon lamps), Ti:sapphire, Cr:LiSAF, or laser dyes.

Typically, lamp-pumped lasers require larger laser crystals. This makes it difficult to apply certain crystal types because it is difficult to obtain large chunks of high-quality crystals. A very large ceramic laser gain medium is suitable.

Advantages and disadvantages of lamp pumping

Although diode-pumped lasers have become very common due to many advantages, lamp-pumped lasers will continue to be used for a long time.

The main advantages are:

• Very high pump power (especially peak power) can be generated, and very high pulse energies (e.g. hundreds of joules) can be generated.
• Compared to laser diodes, the price per watt of pump power of the lamp is much lower.
• The luminaires are quite robust, e.g. unaffected by voltage spikes or currents.

The disadvantages are:

• Laser projectors typically have a very limited lifespan – often hundreds or thousands of hours. However, the lamp life achieved is highly dependent on the operating parameters.
• Lasers are inefficient – often only a few percent at most. The consequence is not only higher power consumption, but also higher heat loads, so a more powerful cooling system and a strong thermal lensing effect are required, making it more difficult to obtain good beam quality.
• The power supply of lamp-pumped lasers involves high voltages, which raises additional safety concerns.
• The low radiation (compared to diode lasers) and wide emission wavelength range exclude many solid-state laser gain media. For example, quasi-three-level lasers are difficult to pump with lamps.
• The lamp pump source is relatively noisy, resulting in a high level of laser noise. For example, Q-switched lamp-pumped lasers typically exhibit relatively strong pulse-to-pulse fluctuations.
• Due to the complexity of the process of lamp-pumped lasers, it is often unrealistic to develop a comprehensive physical model to quantitatively predict the performance of the laser. In contrast, the performance of many diode-pumped lasers is more predictable; In simple cases, even a couple of fairly simple equations are sufficient.

However, device life, power efficiency, cooling, and thermal lensing are not really important issues, for example, when the flash is operating at a low pulse repetition rate and low average power, as required in engraving and marking systems. In this case, the advantages specific to the lamp clearly prevail. On the other hand, high-power CW-lamp-pumped lasers, as well as pulsed lasers operating at high pulse repetition rates, have been largely replaced by diode-pumped devices, such as more compact rod lasers, thin-disk lasers, and fiber lasers.