Fiber lasers and fiber amplifiers

Nowadays, lasers have become ubiquitous in our daily lives. But why are lasers such indispensable light sources across a multitude of applications? It boils down to their remarkable ability to concentrate high-intensity light within an exceptionally narrow spectral band, ensuring coherence over distances of hundreds of kilometers.

Laser technology has advanced to encompass nearly the entire electromagnetic spectrum, spanning from ultraviolet to mid-infrared wavelengths. This broad spectrum of wavelengths proves beneficial across various sectors of our lives, including healthcare, telecommunications, industry, entertainment, and more.

Fundamentals of fiber laser set-up

To build a laser, primary it is required a gain medium, which could be a solid, liquid, gas, or semiconductor, serving to amplify light through stimulated emission. This medium necessitates an energy source, whether electrical current, flash lamps, or another laser for excitation to higher energy states. An optical cavity or resonator, comprising highly reflective mirrors, encloses the gain medium, enabling photons to stimulate further emissions as they bounce back and forth. One mirror within the cavity functions as a partially reflective output coupler, permitting a portion of the light to exit as the laser beam.

Supporting infrastructure includes a power supply, control electronics, and potentially a cooling system to regulate temperature and prevent overheating. Additional optical components such as lenses and beam splitters facilitate beam manipulation as per specific requirements. The design and constituent elements vary depending on the laser type and intended application, encompassing solid-state, gas, semiconductor, and dye lasers, each distinguished by unique characteristics and operational necessities. [1]

                                             
                                                        Figure 1. Simplified schematic of DPSSL. 

Various configurations of laser resonators are available, including plane planar resonators, concentric resonators, confocal resonators, and ring resonators. The latter, illustrated in Figure 1, stands as the primary setup employed in the majority of our laser systems. These setups are constructed based on a unidirectional ring-cavity design, composed of three mirrors forming a triangle with two equal-length sides. Pumping is directed into the cavity through the curved mirror (M3) from pump diodes. Within the cavity, a Nd:YAG laser crystal with a 1% doping level and a length of 3 mm is utilized. Additionally, an optical isolator is integrated into the cavity to ensure unidirectional operation.

In fiber lasers, the active medium comprises a fiber doped with rare-earth (RE) ions. Among the RE- ions utilized for doping optical fibers are neodymium (Nd3+) [2], erbium (Er3+) [3], ytterbium (Yb3+) [4], thulium (Tm3+) [5], bismuth (Bi3+) [6], holmium (Ho3+) [7], dysprosium (Dy3+) [8], and praseodymium (Pr3+) [9]. The spectrum achieved using these RE-ions is broad, encompassing radiation from ultraviolet (UV) to near-infrared (IR) wavelengths.

Single-frequency fiber lasers

A fiber laser is considered single-frequency when it maintains only one oscillating mode without other frequency components in the cavity or output. Its linewidth ranges from hundreds of Hz to several MHz. Two typical configurations, illustrated in Fig.2, distributed Bragg reflector (DBR) lasers and distributed feedback (DFB) lasers.

Figure 2. Typical schematic of a (a) DBR laser (b) DFB laser (Courtesy of Dr. E. Balliu, HÜBNER Photoncs). 

Fig. 2 shows the structure of a distributed Bragg reflector (DBR), where two spliced fiber Bragg gratings (FBG), one narrowband (NB- FBG) and the other broadband (BB-FBG), are used for optical feedback and optical filtering. It is essential that the reflective spectrum of the NB-FBG falls entirely within that of the BB-FBG, which in principle can be replaced by a dielectric mirror. The single-frequency operation in this type of laser requires a short cavity, thus limiting output power levels to hundreds of mW. When the FBG is built into the active fiber with a phase shift (Fig. 2(b)), the active fiber can be longer than in a DBR laser, resulting in higher output power at a single frequency in the order of some hundreds of mW [10].

What are fiber amplifiers?

The output power of lasers is often limited, making power scaling essential for numerous applications that demand high output powers. To attain higher output power for single-frequency lasers, amplification of light from a laser source (seed laser) can be accomplished through a fiber amplifier in a master oscillator fiber power amplifier (MOPA).

A fiber amplifier serves the purpose of boosting the output power of a laser cavity, utilizing various types of laser sources as previously outlined. The medium employed to amplify the power is an optical fiber doped with rare-earth (RE) ions. The most commonly used configuration in fiber amplifiers is the master oscillator power amplifier (MOPA) architecture, designed to enhance the power of a signal transmitted from a laser (oscillator) by optically pumping an active medium. Amplification occurs solely when the population of atoms in the low energy level is excited (inverted) by a power source (pump) to the higher level; stimulated emission surpasses absorption. To enable this, the pump laser wavelength must fall within one of the absorption bands of the active medium. Consequently, the amplitude of the input signal increases proportionally by a linear coefficient known as the amplifier gain, as illustrated in Fig. 3.

Figure 3. Simplified schematic of light amplification (Courtesy of Dr. E. Balliu, HÜBNER Photonics). 

Our fiber amplifer, Ampheia Series, is a fiber amplfier, in which the laser source emits single-frequency light and the active medium is an optical fiber doped with Yb3+-ions.

In case you are interested in femtosecond fiber lasers, then we recommend you read more about our VALO Femtosecond Series lasers, with the markets shortest and cleanest femtosecond pulses.

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