2 μm Laser: From Research to Industrial Applications By Laurynas Šatas, 2020 While these lasers are considered eye safe, the same water absorption line located at these wavelengths makes such lasers some of the preferred tools for precision surgery. With direct or endoscopic medical applications, there are only two critical parameters that matter: coagulation and ablation depth. Small coagulation zones help doctors to contain bleeding and significantly improve procedural quality, whereas large ones can disturb the functionality of tissue. Precise control of ablation depth and affected zones is highly important in allowing professionals to ensure a high level of accuracy in precision surgery. It has been shown that the actual wavelength is also important for the ablation of biological tissue. A comparison between two of the most popular laser systems – Ho:YAG and Tm:YLF – suggests that the sweet spot is located somewhere around 2090 nm. At the same average powers, Ho:YAG lasers show almost twice the ablation depth while maintaining smaller coagulation zones than Tm:YLF lasers. In addition ability of Ho:YAG lasers to work in different regimes (CW and Q-switched with electrooptical or acousto-optical modulators [4]) and tune wavelength from 2090 to 2123 nm leads to extraordinary process control. Water Hemoglobin Melanin Penetration depth in tissue Figure 2. Absorption and penetration depth in water and other biological tissue constituents for different wavelengths [1]. High importance applications also include defense and security fields. IR guided missiles were responsible for 90% of damaged planes in conflict areas around the world over the past 20 years. Direct infrared counter measure (DIRCM) systems serve a crucial role in protecting planes from these heat-seeking projectiles. Therefore it is crucial to have a compact and reliable DIRCM system as it can save a plane and the lives of its crew. These systems come in two types: one operates in the first atmospheric window at wavelengths of around 2 micrometers, and a second is designed for the second atmospheric window of 3-5 µm. The latter is typically a high-complexity hybrid laser system containing a thulium-based diode-pumped fiber laser, Ho:YAG in a solid state and a zinc germanium phosphide (ZGP) optical parametric oscillator [3]. Challenges with DIRCM systems can be properly addressed only through multiple stages of optical pumping, which result a bulky and expensive system. Coagulation Hyperthermia Laser beam Over the past few decades, lasers have become an increasingly crucial tool for multiple applications. By selecting right tool and optimizing it’s parameters professionals can cover wide range of different technological tasks with optimum efficiency. Lasers operating at the wavelength range of 2 μm is highly demanding due to its unique properties. Water absorption increases steadily from 1.2 to 2.9 µm, resulting in wavelengths of around 2 µm still belonging to the so-called “eye safe” region – with water making up around 70% of corneal tissue. Through falling within this region and still being at suitable atmospheric window, lasers operating at the wavelength of 2 μm enable an entire range of applications and opportunities for free-space communication, remote detection (for atmospheric absorbers: H2O, CO2, N2O, etc.) and spectroscopic advances in environments that cannot be isolated from people. Ablation depth Coagulation depth Coagulation margin Ho:YAG and Tm:YLF laser light application at 10 W for 1, 5, 10 s [1]. Figure 1. Single-spot experiments (fiber-tissue distance d = 5 mm) on porcine liver tissue using Ho:YAG and Tm:YLF laser in CW mode (P = 10 W, t = 1, 5, and 10 s). Mean and standard deviations of ablation depth, axial coagulation depth and width of the radial coagulation e

Dielectric coatings Interference coatings are constituents that make optical glass into a low-loss window or laser mirror. Depending on the technology used, coating density can range from highly porous to near-as-bulk. In typical NIR nanosecond applications, it is known that more porous coatings are capable of surviving higher energy densities due to having lower stresses. However, in the spectral region where water absorption is critically important, technologies that are highly functional at 1 μm range do not work at 2 μm as well. Depending on the application, there can be several different...