THINK YOU KNOW ABOUT FIBER LASER?

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 other type of 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 solid-state laser that uses a rare-earth doped optical fiber for laser cavity, where the beam is generated within the fiber, unlike gas lasers such as CO₂lasers, 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.

Schematic of a fiber laser:

Principle of a fiber laser operation

Light generated by the pumping diode travels through an optical fiber cavity where the fiber is doped with rare earth element to produce a specific wavelength. The electrons of particles in the doped fiber rise in energy as they interact with light, then fall back to their basic state when these electrons release their energy. These phenomena are referred as “electron excitation” and “electron relaxation” respectively.

As the cavity is also composed of Fiber Bragg Gratings, the light can then bounce back and forth in between, acting as a resonator. This creates “Light Amplification by the Stimulated Emission of Radiation,” or LASER.

Amplification occurs when photons hit other excited particles, and these particles also release photons. Since the Bragg gratings reflect photons back into the cavity and more pump light is sent in, an exponential number of photons are released. As a result of this stimulated emission of radiation, laser light is created.

A fiber laser can function in either continuous-wave (CW) or pulsed operation. A CW fiber laser emits a continuous beam with a constant light intensity. The energy released in the beam is thus constant over time.

In a pulsed operation, the laser produces a series of pulses at a certain width and frequency until stopped. The width of the pulses can vary from nanosecond to femtosecond (ultrashort pulses) matching the desired application. A pulsed fiber laser can produce a peak power greater than its average power as shown in the figure below.

The development of fiber laser has progressed in terms of output power, pulse energy and pulse width over the years. Due to the range of wavelengths that they can generate, fiber lasers are used nowadays in various applications including material processing, telecommunications, sensing, medical, and defense and security. These applications, and many more, greatly benefit from the use of fiber lasers. Follow our blog for more details or contact one of our experts to find out more about fiber lasers and fiber optics. Whatever your field is, fiber laser can most certainly facilitate the way in which you work!

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