J.J. Ewing
Solid-state lasers are lasers whose active medium is typically a minority ion in a solid-state host. The host is most often a single crystal with about 1% of a different species, such as a neodymium ion (Nd3+), doped into the solid matrix of the host. For certain applications the host is a glass, the most important of these being glass doped with the erbium ion (Er3+), which is then pulled into a long thin fiber and used in telecommunications systems. Solid-state lasers use optical pumping by other lasers or lamps to produce a wide variety of laser devices. The population inversion (a condition in which the atomic population of the higher energy state is greater than the lower energy state) in solid-state lasers is created when the active medium absorbs photons from an intense light source.
One important class of solid-state lasers consists of the tunable and ultrafast lasers. These systems use solid-state lasers to excite them, and have other similarities in their thermal and optical properties. In this article, however, we address only the solid-state "drive lasers" for such tunable systems.
The typical optically pumped, solid-state laser system is much larger than a semiconductor device. The optical pump may either be a single, "high-power" semiconductor diode, a small array of semiconductor diodes on one chip, or a stack of arrays for a laser with outputs of up to 1000 W. Before the advent of laser diodes, solid-state lasers were pumped by lamps—either running continuously for modest power lasers, or pulsed flash lamps for laser output energy in the 1-Joule-per-pulse category. Since the number of arrays needed to power a 1-J-per-pulse laser is still quite pricey, lamps still have an important niche in the solid-state laser business.
The active medium of a solid-state laser consists of a passive host crystal and the active ion, and it is these components that give the laser its name. A "neodymium:YAG" (Nd:YAG) laser, for example, consists of a crystal of yttrium aluminum garnet (YAG) with a small amount of neodymium added as an impurity. It is the Nd ion (Nd3+ added in the form of Nd2O3 to the materials to make the single crystal) in which the population inversion is created, and which generates the photon of laser light. Typically the lasing ion is present at about 0.1% to 1% of the ion density of the metal ions of the host crystal or glass. There are times, however, when the host of the active ion in the solid-state laser is used as the name of the laser. The Nd:YAG is often called a YAG laser, harking back to a time when the only good solid-state laser was YAG with Nd3+ ion minority constituent.
Today, however, there are perfectly good solid-state lasers that use YAG as the host for Yb (ytterbium, lasing at 1.03 µm), Ho (holmium, 2.1 µm), Tm (thulium, 2 µm) or Er (erbium, 2.9 µm) ions. In all of these cases the host material is usually configured in a simple rod shape that is cut from a synthetically grown crystal or poured glass. For the important case of the erbium-glass medium, the glass with erbium minority constituent is drawn into a very long thin fiber and typically used as an amplifier in an erbium doped fiber amplifier (EDFA). In general, the vendor of a laser crystal is not the manufacturer of the laser itself.
The first solid-state laser, indeed the first laser ever, was the ruby laser. This laser used a flash lamp to create a population inversion in a cylindrical rod cut from large single crystals of Al2O3 containing small amounts of impurities in the form of Cr2O3 . This mixture has the common name of ruby, given to the natural gem material long before its chemical composition (or the concept of a laser) was known. After the ruby laser was demonstrated, other ion/host combinations were investigated.
Nd:YAG lasers
The Nd:YAG laser is the most prevalent of today's solid-state lasers. These systems can be found on the industrial work floor welding heavy metals, in the surgical suite performing delicate surgery, in the research laboratory making precise spectroscopic measurements, and on a satellite orbiting the planet Mars measuring the detailed topography. The Nd:YAG lasers found in each of these diverse applications have distinct characteristics that make them suitable for that use. The welding laser, for example, is likely to be a large, lamp-pumped laser, while the surgical system is likely to be a smaller, diode-pumped system that is frequency doubled to produce green light. The spectroscopic laser could also be a diode-pumped system, perhaps utilizing special resonators to reduce its bandwidth. And the Mars-orbiting laser would be Q-switched to produce short pulses for distance measuring.
The neodymium ion can be added as an impurity to a glass matrix, producing the ion/host combination of an Nd:glass laser. Although the same triply ionized neodymium ion does the lasing in both Nd:YAG and Nd:glass, the two lasers have little in common. Even the wavelengths of the two lasers are slightly different (because the electric fields in the two hosts shift the energy levels differently). Nd:glass is much better at storing energy than Nd:YAG, so Nd:glass lasers are used in high-energy, Q-switched applications. Large Nd:glass lasers can produce pulses of 100 J and greater, while Nd:YAG is limited to about 1 J from a single, Q-switched oscillator. On the other hand, the thermal conductivity of glass is much lower than that of YAG, so YAG lasers are preferred for high-average-power applications. A single Nd:YAG lamp-pumped oscillator, containing several Nd:YAG rods lined up in series, can produce an output of a kilowatt or more.
Thermal conductivity is the mechanism for removing waste heat from the interior of the laser host rod. A coolant then removes the heat from the rod surface. If the heat isn't removed fast enough from the interior of the crystal, the host overheats and distorts the optical quality of the crystal.
The triply charged erbium ion (Er3+) can be added as a minority to YAG, glass, and other hosts. In YAG, the lasing wavelength of 2.9 µm is "eyesafe," meaning it cannot damage the retina. The strong absorption of 3-µm light by tissue makes the Er3+:YAG ideal for medical applications. In glass, Er3+ can lase at 1.5 µm, a wavelength of minimal loss in optical fibers. Erbium-doped fiber amplifiers (EDFAs), glass fibers with Er3+ doping excited by semiconductor lasers, have been widely deployed in global telecommunication systems, where they directly boost the signal carried on fiberoptic cables. Before EDFAs were integrated into the fiberoptic-based phone system the optical signal in the fiber-based cables had to be converted to an electrical signal for amplification, and then back to optical for further transmission. The advent of EDFAs in the mid-1990s has revolutionized wideband telecommunications.
Another ion, one that absorbs the pump light more efficiently than the lasing ion, is sometimes added to these ion/host solid-state lasers. For example, a Ho:YAG laser might be "co-doped" with chromium (Cr3+) or thulium (Tm3+) ions that strongly absorb the pump light and transfer the energy to the lasing holmium ion. The EDFAs rely on energy transfer from Yb ions co-doped into the glass fiber.
Diode-pumped solid-state lasers
Historically, lamp-pumped lasers preceded their diode-pumped brethren by decades, though the simplicity of diode pumping makes the laser easier to explain. Lamp pumping uses a relatively inexpensive light source but is considerably less efficient than diode pumping. Diodes or diode arrays are more expensive per unit of energy but despite the cost of powerful semiconductor diode arrays, diode pumping often is preferable because it produces much less heat in the laser medium and has a significantly greater overall efficiency.
The energy level diagram for Nd3+ is a four-level system, meaning that the pump light goes into a state that is not the upper laser level, relying on the energy put in the laser to relax to the upper level in a billionth of a second or less. Similarly, at the bottom of the energy ladder, the lower level relaxes quickly, thus the name four-level laser. These levels aren't the only energy levels in the Nd ion, and some of the other levels can serve as the lower laser level of secondary lasing transitions, at 1338 and 946 nm. One can force the laser to lase at one of the secondary transitions by maximizing the feedback at that wavelength, and minimizing the feedback at competing laser wavelengths.
Many other ion/host combinations can be diode pumped. Neodymium in other hosts behaves similarly to the Nd:YAG case. Holmium lasers, with output in the 2- to 3-µm region, are often co-doped with thulium (Tm). The Tm ions efficiently absorb the pumping radiation from a laser diode at 785 nm, and transfer the energy to the lasing Ho ion. The laser diodes that supply this 785-nm pump lamp are similar to the diodes that pump Nd:YAG at 808 nm. Er:glass lasers can likewise be co-doped with ytterbium (Yb), which absorbs pump light at 960 nm and transfers the energy to the erbium.
But not all ion/host combinations are suitable for diode pumping. Ti:sapphire, for example, requires green pump light, at a wavelength too short to be generated efficiently by existing laser diodes. Moreover the storage time in Ti:sapphire is much shorter than that for Nd:YAG, and a huge number of diodes would be needed to pump energy into the Ti3+ ions. As such, Ti:sapphire lasers tend to be laser pumped, with either argon-ion lasers, CW diode-pumped Nd lasers or pulsed, lamp-pumped Nd lasers providing the optical pumping energy.
ACKNOWLEDGMENTS
Adapted with permission from Introduction to Laser Technology, B. Hitz, J. J. Ewing and J. Hecht; publisher IEEE, Piscataway, NJ.
J. J. EWING is president of Ewing Technology Associates (Bellevue, WA); e-mail: [email protected].