Go Back

Solid State Lasers

Solid state lasers are a major class of lasers based on solid state gain media as opposed to gas phase media such as in CO2 lasers. The gain media can be made of crystals or glasses doped with rare-earth or transition-metal ions, as well as semiconductors. Solid state lasers may generate output powers between a few milliwatts and many kilowatts. They can be made in the form of bulk lasers, fiber lasers, or other types of waveguide lasers. The following few paragraphs give a brief explanation of the inner workings and characteristics of the generated outputs by these laser sources.
In terms of their potential for wavelength tuning, different types of solid state lasers differ very much. Most rare-earth doped laser crystals, such as Nd:YAG or Nd:YVO4, have a fairly small gain bandwidth in the order of 1nm or less, leaving a rather limited range of tuning possible. On the other hand, tuning ranges of tens of nanometers and more are possible with rare-earth doped glasses, and particularly with transition-metal doped crystals such as Ti:sapphire, Cr:LiSAF or Cr:ZnSe. Most importantly, most solid state lasers operate in the visible and near IR wavelengths (around 1μm) as opposed to CO2 lasers which operate around 10 μm.

Bulk lasers: The term bulk laser refers to a solid state laser with a bulk piece of doped crystal or glass as the gain medium. This distinguishes such bulk lasers from waveguide or fiber lasers (see below). In most cases, the gain medium is doped either with rare-earth ions or transition metal ions.

Figure 1. Typical setups of solid state bulk lasers, converting pump light (blue) into laser light (red): end pumped (upper figure) and side pumped (lower figure) versions.

As there is no waveguide structure, the beam radius in the gain medium is essentially determined not by the gain medium, but by the design of the laser resonator. This has important implications: 1) the resonator may be designed for a rather large effective mode area in the crystal so as to enable Q-switched operation with very high pulse energy; 2) alternatively, a small mode area allows for a low threshold pump power. However, it implies a strong beam divergence and can thus not be maintained over a large length of material. 3) The beam radius can be influenced by thermal lensing, and may thus change when the pump power is changed. The laser resonator is in most cases formed with laser mirrors placed around the crystal (see Fig. 1). It is also possible to use a laser crystal with a highly reflecting dielectric mirror coating on one side, which serves as a resonator end mirror. Also, there are monolithic solid state lasers where the beam path is entirely inside the crystal.

Fiber lasers: Fiber lasers are lasers with optical fibers as gain media. The gain medium is a fiber doped with rare-earth ions such as erbium (Er3+), neodymium (Nd3+), ytterbium (Yb3+), thulium (Tm3+), or praseodymium (Pr3+), and one or several laser diodes are used for pumping.

Fiber lasers can deliver output powers of hundreds of watts, sometimes even several kilowatts from a single fiber. This potential arises from a high surface-to-volume ratio (avoiding excessive heating) and the guiding effect, which avoids thermo-optical problems even under conditions of significant heating. With various methods of active or passive Q-switching, fiber lasers can be used for generating pulses with durations which are typically between tens and hundreds of nanoseconds and pulse energies of several millijoules. As fibers can be coiled and the light propagating in fibers is well shielded from the environment (e.g. concerning dust), fiber lasers can have a compact and rugged design which makes them attractive for a variety of laser marking applications.

Figure 2. Setup of a simple fiber laser. Pump light is launched from the left side through a dichroic mirror into the core of the doped fiber. The generated laser light is extracted on the right side.

Optical pumping methods: Many solid state lasers are pumped with flash lamps or arc lamps. Such pump sources are relatively cheap and can provide very high powers. However, they lead to quite low power efficiency, moderate lifetime, and strong thermal effects in the gain medium. For such reasons, laser diodes are very often used for pumping solid state lasers. Such diode-pumped solid state lasers (DPSS lasers, also called all-solid-state lasers) have many advantages, in particular, a compact setup, long lifetime, and often a very good beam quality. Therefore, their share of the market is rising rapidly.

Pulsed operation: The long upper-state lifetimes makes solid state lasers very suitable for Q-switching: the laser crystal can easily store an amount of energy which, when released in the form of a nanosecond pulse, leads to a peak power orders of magnitude above the achievable average power. Bulk lasers can thus easily achieve millijoule pulse energies and megawatt peak powers. High peak powers in turn open up direct routes for marking, cutting, and welding at relatively low average powers.

Solid state lasers in laser marking industry: The following are a few examples of solid state lasers for use in laser marking applications:

• Diode-pumped Nd:YAG (YAG lasers) or Nd:YVO4 lasers (vanadate lasers) often operate with output powers up to tens of watts. Q-switched versions generate pulses with durations of a few nanoseconds, millijoule pulse energies and peak powers of many kilowatts.
• Q-switched Nd:YAG lasers are still widely used in lamp-pumped versions. Pulsed pumping allows for high pulse energies, while the average output powers are often moderate (e.g. a few watts). The cost of such lamp-pumped lasers is lower than for diode-pumped versions with similar output powers. Yag and Vanadate lasers operate at 1064nm wavelength.
• Fiber lasers can generate high average output power, high power efficiency, good beam quality, and broad wavelength tunability. Their compact design is very favorable for making portable solutions for laser marking systems, e.g. integrated portable markers and readers.

Things to note: The good absorption properties of most materials at near IR wavelengths make solid state lasers very attractive for laser marking applications. The beam quality, compact design and maintenance free operation are of particular interest. The CW and Q-switched lasers cover a wide range of applications and fit most marking requirements. Simple fiber laser setups can be made from relatively cheap components, and they need fewer mechanical components. Ideally, a fiber laser setup should be made with fibers only, not involving any air space. Where this is possible, fiber lasers can be significantly cheaper and smaller than bulk lasers. The output may then conveniently be delivered to a fiber connector, which allows easy connection to other devices without any alignment procedures. However, the use of air spaces in fiber laser resonators can often not be avoided, e.g. when certain bulk optical elements need to be inserted into the laser resonator. In that case, the tight alignment tolerance of single-mode fibers and sometimes the high optical intensities at fiber ends may make the setup less robust than that of a bulk laser, in addition to more expensive.

In summary, solid state lasers offer compact, maintenance free, robust designs and can easily be integrated in various automated systems. These advantages, however, come at the expense of their cost relative to their gas phase (e.g. CO2) counterparts. Solid state lasers can be as much as twice as expensive as CO2 lasers.

Because of their compact design, solid state lasers offer great potential for integration in various inline automated systems. For large volume applications the inline application and marking can be incorporated into an automated line. Some laser manufacturers offer various solutions for automation and integration problems. In particular, they can integrate any standard marking products with a specific application.

Go Back