Laser-based Mid-infrared Sources and Applications

3Fiber Mid‐IR Lasers

3.1 Introduction

Fiber lasers can produce, in a compact device, outputs with remarkable beam quality and high output power and can operate with long life, high power efficiency, and low cost of ownership. In many cases, fiber lasers are replacing solid‐state lasers in research, defense, and industry. Extending the emission wavelength into the mid‐infrared is one of the avenues of fiber laser development, which will clearly benefit numerous existing and future applications, e.g. in the field of medicine, remote sensing, welding of polymers, and for pumping longer‐wavelength mid‐IR or terahertz optical parametric oscillators.

Fiber lasers are typically pumped by laser diodes with free‐space or fiber‐coupled output (Figure 3.1) and consist of a special optical fiber whose core has been infused with active ions that create the optical gain. For mid‐IR fiber lasers, the dopants in the gain medium are rare‐earth ions such as thulium (Tm³⁺), erbium (Er³⁺), holmium (Ho³⁺), or dysprosium (Dy³⁺) [1].

Although the gain media for fiber lasers are similar to those of solid‐state bulk lasers, the waveguiding effect and the small effective mode area usually lead to substantially different properties. For example, fiber lasers often operate with much higher laser gain and higher resonator losses. Also, the spatial beam quality in fiber lasers is typically close to diffraction‐limited. Similar to solid‐state crystalline lasers, the lasing wavelength is determined by the ...