A Primer on Solid

Nov 17, 2023

The first laser ever constructed was a solid-state ruby laser built by Theodore Maiman in 1960. But solid-state lasers are not a historical curiosity. Rather, the technology has grown and diversified over the years, servicing a tremendous range of scientific, industrial, aerospace, defense, medical and life sciences applications.

Lasers are broadly classed by the state of matter of their lasing material (gain medium): gas, liquid, solid-state, and even plasma lasers. But it's common practice to use the term solid-state to refer only to lasers that use a crystal or glass gain medium. This host material is usually doped with ions to support population and thus laser action.

Pumping is the process of supplying raw energy to the laser crystal, which it then converts to laser light. The crystal is non-conductive, so pump energy is virtually always supplied to solid-state gain media in the form of light, rather than in the form of electricity. Early solid-state lasers were pumped by flashlamps. That situation changed dramatically with the introduction of diode laser pumping in the 1980s.

Diode lasers provide an intense source of light and the wavelength can be matched to the absorption of the gain medium. This results in a very efficient laser where a relatively large amount of the energy originally supplied to the laser (specifically the electricity used to power the diodes) ends up converted into laser light. Plus, diode pumping delivers tremendous reliability and lifetime advantages, a small footprint (size), and operational consistency.

However, lamp-pumping is still used with certain solid-state laser crystals. This is because lamp-pumped, solid-state lasers can produce very high pulse energies. Plus, the typical purchase price and cost per watt of lamp pump power is much lower than for diodes.

Solid-state laser resonators are mostly configured in the traditional manner. Namely, the gain material is placed between two mirrors to form an optical cavity. Sometimes the end(s) of the laser crystal is coated to become the mirror(s). The laser crystal itself can be in rod, slab, or thin disk form.

Because of the large number of different crystals available, there are many different types of solid-state lasers currently in use. It's not possible to describe them all here, and even categorizing them is difficult, since existing solid-state lasers cover an extremely broad space of output characteristics. But, for the purposes of this discussion, it's useful to break them down into three broad categories: continuous wave (CW) and nanosecond pulse width, ultra-short pulse, and ultra-fast lasers.

The most common solid-state lasers of this category are based on neodymium crystals, usually doped with either yttrium aluminum garnet (Nd:YAG), yttrium orthovanadate (Nd:YVO4), or yttrium lithium fluoride (Nd:YLF). The strongest laser fundamental output for all these crystals is in the infrared at around 1 μm.

These crystals are all in use because each produces somewhat different operating characteristics. For example, Nd:YVO4 is best suited for high peak power, high repetition rate pulsed lasers. In contrast, Nd:YAG typically delivers higher total pulse energy at lower repetition rates. Nd:YLF provides even higher pulse energies, usually at even lower repetition rates.

There are also several laser crystals that utilize holmium, thulium, ytterbium or erbium dopants instead of Nd. The crystals Er:YAG, Tm:YAG, Ho:YAG all lase at around 2 μm. This wavelength is strongly absorbed by living tissue containing water, making these laser types useful for a variety of medical applications.

Most of these crystals can be operated continuous wave (CW). But, the majority of materials processing and other industrial solid-state lasers are operated pulsed. Pulsing increases the peak power, which is critical for getting above the ablation threshold (minimum power needed to melt or vaporize) for many materials, particularly metals, or to produce a surface color change for marking.

The most widely used pulsing method is q-switching, typically implemented using an acousto-optic deflector that acts as a fast shutter inside the laser resonator. First, the q-switch is closed which prevents light from circulating within the laser cavity. During this time, the pump energy supplied to the laser crystal accumulates in it. Then the q-switch is rapidly opened. This allows the laser to operate. The stored energy is quickly converted into laser light and emitted in the form of a short pulse. This process is rapidly repeated.

Most of the commonly used q-switched, solid-state lasers produce pulse widths in the tens of nanoseconds range. They typically offer average powers of tens or hundreds of watts (in the infrared) and repetition rates in the 10s of Hz to about 300 kHz.

The high peak power achieved with pulsing also facilitates the use of nonlinear processes. A key one of these is frequency conversion that uses a crystal to generate harmonics of the initial light frequency. So, pulsed solid-state lasers that produce 1064 nm can be frequency multiplied to output 532 nm (second harmonic generation or SHG), 355 nm (third harmonic generation or THG), or even 266 nm (fourth harmonic generation or FHG). Frequency conversion isn't impossible with CW lasers, but it is not as simple to implement.

Another way of pulsing a solid-state laser is mode locking. This produces repetition rates in the many tens or hundreds of MHz. So, for many applications, the laser simply appears to be on continuously. As a result, these sources are often referred to as quasi-CW, or QCW. But, again, pulsing increases peak power, which in turn allows for frequency conversion, thus providing a relatively simple way to get shorter wavelength solid-state laser sources that are virtually CW. Semiconductor wafer inspection is an important application for these types of lasers.

While nanosecond pulse width solid-state lasers are widely used in materials processing, pulses in the picosecond and femtosecond regime can offer significant advantages for the most demanding precision processing tasks. These benefits include the ability to produce very small structures with virtually no heat affected zone, as well as compatibility with an extremely broad range of materials, even those that are transparent, such as glass.

Mode locking can be used to produce pulsewidths of about 10 ps or shorter. But, most mode locked lasers have pulse energies that are too low for material processing uses. However, this pulse energy can be increased through amplification.

This process usually starts with a "pulse picker" to select individual pulses from the high repetition rate mode locked laser output (e.g., every tenth pulse). These pulses are sent into a free space amplifier, most commonly in either a regenerative or multipass configuration. More than one amplifier stage can be used to reach even higher power.

While this approach may sound complex, commercial industrial ultra-short pulse (USP) lasers are extremely reliable, thanks to diode pumping, careful opto-mechanical design, and rigorous assembly protocols. Commercial picosecond USP lasers typically deliver pulse-widths under 15 ps, and power of up to 100W in the infrared. Both green and UV output is available as well. There are also USP lasers that have pulse widths of hundreds of femtoseconds, and output of tens of watts, also in the IR, visible and UV. Commercial USP lasers are used in many precision microelectronics cutting and drilling applications, for cutting OLED modules and displays, in medical device manufacturing, and even in watchmaking.

Mode locked solid-state lasers for scientific applications are typically referred to as ultrafast lasers. With pulse widths in the 10 fs to 200 fs range, these lasers have become workhorse tools for a wide range of investigations in physics, chemistry, biology and materials science. Ultrafast lasers are distinguished from industrial USP lasers in that they typically offer shorter pulse widths and much more control over output parameters, including wavelength, pulse width, and more.

The key to achieving these extremely short pulse widths is to use a gain crystal which outputs over a very broad range of wavelengths. The broader the output spectrum, the shorter the pulses can be. The most popular material currently in use is Ti:Sapphire. This material must be pumped with green light, so the pump source is typically a frequency doubled, CW, diode-pumped, solid-state laser. Recently ytterbium is proving a popular alternative to Ti:Sapphire.

While the most sophisticated and high-performance scientific ultrafast laser sources are quite complex, these products are mature and have leveraged all the advantages of microprocessor technology. As a result, they’re extremely reliable, and most of the output adjustments are performed through software control. This turnkey operation has allowed scientists in many disciplines to use them as they would any other instrument, without having to develop any special expertise.

Solid-state technology has literally been with the laser industry since day one. Ongoing innovations that deliver greater performance, as well as enhanced reliability and lower operating costs, continue to keep solid-state lasers relevant and gainfully employed.

This article was written by Jörg Heller, Product Line Manager, Coherent, Inc. (Santa Clara, CA). For more information, visit here .

This article first appeared in the March, 2022 issue of Photonics & Imaging Technology Magazine.