A little bit of history … First discovered back in 1963 by Elias Snitzer, the fiber laser has been replacing different types of laser, such as CO2 laser and solid-state laser, in a wide range of applications. Although it took nearly two decades for the first fiber laser to be commercialized, today it is one of the most used systems in the material processing industry as well as in telecommunications and sensing.
What is the reason for this delay? Well, to begin with, there was still a lot of untapped potential in fiber laser technology at the time. In contrast to most applications that require at least 20 watts, fiber lasers could only radiate a few tens of milliwatts. Furthermore, due to the lack of high-quality laser diodes, high-quality pump light could not be generated, which was a requirement for the targeted applications.
What is a fiber laser?
A fiber laser is a special type of laser that uses a rare-earth doped optical fiber for laser cavity, where the beam is generated within the fiber, unlike solid-state lasers which generate the beam through a gaseous cavity. For various applications, different types of rare earth may be used to generate the beam, such as Ytterbium for wavelength around 1 µm, Erbium for 1.5 µm, and Thulium in the 2 µm region. It also uses an undoped optical fiber for beam delivery. As a variation on the standard solid-state laser, fiber lasers offer several advantages over other laser technologies, such as ease of use, minimal maintenance required, high reliability, and high integration capability.
Laser diodes are the most common method of optically pumping fiber lasers, although other fiber lasers are also occasionally used. Most or all of the optics in these systems are fiber-coupled, with fibers connecting the different components. The diode pump source can be a single diode, or an array of pump diodes coupled by fiber. The doped fiber has a mirror cavity on each side, called Fiber Bragg Gratings, which are a distributed Bragg reflector used to reflect wavelengths of light. As the cavity is composed of doped optical fiber i.e., silica glass, it can then be coiled, meaning that the cavity can be meters long if desired.
Typically, the optical fibre structure used in fiber laser is a double-clad fiber where the inner cladding collects the pump light and guides it along the fiber. A fiber laser could be either end-pumped or side-pumped, where light is coupled into the side of the fiber depending on the design of the system.