Table of Contents
Infrared Spectral Regions and Their Applications
Infrared is usually divided into 3 spectral regions: near, mid, and far-infrared. The boundaries between the near, mid, and far-infrared regions are not agreed upon and can vary. The main factor that determines which wavelengths are included in each of these three infrared regions is the type of detector technology used for gathering infrared light.
Near-Infrared and Short-Wavelength Infrared
Near-infrared light is transmitted and focused to the sensitive retina in the same way as visible light, while not triggering the protective blink reflex.
The short-wavelength infrared is relatively eye-safe since such light is absorbed in the eye before it can reach the retina. Erbium-doped fiber amplifiers for optical fiber communications, for example, operate in that region.
Long- and Far-Infrared for Thermal Applications
The long-wavelength, infrared, followed by the far infrared (FIR), which ranges to 1 mm and is sometimes understood to start at 8 μm already. This spectral region is used for thermal imaging.
Mid-Infrared Lasers for Spectroscopy and Sensing
The mid-infrared spectral range is understood to include wavelengths from 3 μm to 8 μm. There are many absorption lines, e.g., of carbon dioxide (CO2) and water vapor (H2O). This spectral region is interesting for highly sensitive trace gas spectroscopy.
The mid-infrared lasers are of particular interest for in-situ and remote sensing of material composition, as many chemical species have absorption features in this wavelength range that are associated with molecular rotational-vibrational transitions. These include molecules such as H2O, CO2, N2O, CH4, CO, NH3, NOx, HCl, and many other compounds. Currently, in the mid-IR range, CO2 and solid-state lasers dominate in materials processing and medical treatment applications.
Recent developments in quantum-cascade laser technology, resulting in room temperature, high power, and single-mode laser sources, allow access to much stronger absorption bands of CO and CO2 in the mid-infrared lasers.
Important CO2 laser characteristics are high unsaturated gain, high-power output, and good efficiency.