Lasers are typically identified by their gain medium and are often classified by the radiating species that give rise to stimulated emission. These radiating species can include atoms and molecules in a dilute gas, organic molecules dissolved at relatively low concentration in liquid solutions, semiconductor materials, and dielectrics such as crystalline solids or glasses that are doped with a high concentration of ions. These laser categories are generally referred to as gas, liquid, semiconductor, and solid-state.
For gas lasers, population inversion is typically achieved by applying a voltage across a glass or ceramic tube that contains the gain medium which is either a low-pressure gas or gas mixture. The voltage produces an electric field within the tube which induces an electrical current. These electrons collide with the gas atoms, thereby exciting them to higher energy levels that will serve as the upper laser level. The lower laser level typically decays to the ground state much faster than the upper level, thereby creating a population inversion between the two . Since the radiating species are very dilute, the resulting laser transitions have very narrow spectral bandwidths and operate at well-defined wavelengths. Due to the wide variety of gaseous media, the range of operating wavelengths can vary from the UV for excimer lasers, through the visible (VIS) range for argon ion and HeNe lasers to the MIR range for CO2 lasers.
Gas lasers have historically been deployed in a wide variety of applications but have been largely supplanted by DPSS lasers and laser diodes except for specialized applications. The exceptions to this trend are for CO2 and excimer lasers which still play significant roles in the laser processing and medical eye surgery markets.
Certain organic dye molecules can act as radiating species for lasing since they have sufficiently long lifetimes in their upper energy levels and can therefore radiate energy from that level instead of losing energy due to collisions. To ensure the proper concentration of radiating species are present, the dye molecules (typically in powder form) are dissolved in a solvent at a concentration of about one part in ten thousand. Due to this solution form, the system is known as a liquid dye laser. Dye lasers are optically pumped by either flashlamps or other lasers. Each dye molecule, due to its overlapping electronic/rotational/vibrational transitions, has a wide homogeneously broadened gain spectrum on the order of 30-50 nm. By utilizing many different dye molecules, the laser can be tuned over a wide spectrum in the UV, VIS, and near-infrared (NIR). Combining this broad gain bandwidth with a frequency-selective element allows wide tunability coupled with a narrow spectral bandwidth. As a result, the dye laser has traditionally been used for various spectroscopic applications.
Dye lasers require significant maintenance due to the decomposition of the dye when dissolved in its solvent. Therefore, DPSS lasers coupled with nonlinear frequency conversion have largely replaced dye lasers in many applications.
A semiconductor laser is often referred to as a laser diode since it operates like a diode with current flowing in the forward direction of the junction. By injecting charge carriers into the region of space defined by the junction, recombination radiation can occur. Provided this current injection is strong enough, a population inversion can be achieved and stimulated emission will occur. Due to the large refractive index difference between the semiconductor material and air, the semiconductor crystal surface can possess sufficient reflectivity to act as its own resonator cavity. These two characteristics, electrical pumping and compact laser design, coupled with the maturity of the semiconductor manufacturing process, has enabled laser diodes to gain a number of advantages over other types of lasers, including high power and efficiency, small size, as well as compatibility with electronic components.
Unsurprisingly, they are one of the most important classes of lasers in use today, not only because of their use in applications such as optical data storage and optical fiber communication, but also because they serve as pumping sources for solid-state lasers.
The term solid-state laser refers to a laser whose gain medium consists of active ion species introduced as impurities in an optically transparent host material (typically crystals or glasses). As detailed in Light-Matter Interactions in Lasers and Required Components for Lasing, materials for laser operation should possess strong and spectrally-narrow transition cross-sections, strong absorption bands for pumping, and a long-lived metastable state. Ions that have optical transitions between states of inner, incomplete electron shells generally exhibit these characteristics. However, these ions must be protected or shielded from other ions to prevent loss of these desired characteristics. This is accomplished by incorporating the ions in a solid host material whose lattice allows for ion doping levels sufficient for a gain medium while simultaneously shielding the ions from one another.
According to Required Components for Lasing, solid-state lasers achieve their population inversion through optical pumping, which can be accomplished by using a flashlamp or direct pumping from another laser source such as a laser diode or a DPSS system.
There are many varieties of solid-state lasers due to the range of host materials available and the numerous dopant ions. Beyond this spectral agility, the variations in gain media enable systems to exhibit many different characteristics including very high output powers, lower powers with very good spatial beam quality, CW output with remarkable power stability, or ultrashort pulses with ps or fs durations. This flexibility enables solid-state lasers to be used in applications ranging from multiphoton microscopy to light detection and ranging (LIDAR) to material processing/marking/cutting and even laser fusion.
When a solid-state gain medium is fabricated into an optical fiber and a resonator is integrated, a fiber laser is formed. This class of lasers is usually treated separately from the “bulk” solid-state lasers discussed above due to the unique light-guiding properties of fibers, e.g., strong spatial confinement over long distances. A fiber laser is defined as a laser where the optical fiber is itself the gain medium which can be distinguished from having another type of laser or gain medium simply being coupled to an optical fiber
Q-switch pulsed laser are relatively simple in structure and cost-effective, which can meet most marking applications.
MOPA pulsed laser are relatively complex in structure, but with more flexible control and more functions, which are suitable for higher-end precision marking.
- Q-Switch: The acousto-optic Q switch compresses the output continuous laser energy into a very narrow pulse and emits it. (the pulse width is relatively fixed)
- MOPA: (Master Oscillator Power-Amplifier) The seed signal light and pump light with high beam quality are coupled into the double-clad fiber through a certain method to amplify, so as to achieve high power amplification of the seed light source (pulse width can be adjusted) )