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Fiber lasers are widely used as a high-power source as they offer significant advantages over conventional lasers, such as excellent output beam quality, superior thermal management, high efficiency, all-fiber monolithic architecture, and compact and reliable design.
Figure 1: Architecture of a typical fiber laser
A typical laser consists of a gain medium with a pumping mechanism to produce population inversion, a pair of mirrors to provide feedback, and a wavelength-selective element.
Similarly, in a typical fiber laser, as shown in Fig. 1, the gain medium is a rare-earth doped fiber optically pumped by pump diodes. The role of the mirror is provided by a pair of fiber Bragg gratings (FBGs), with highly reflecting (HR) (>99%) on one side and low reflecting (LR) (~ 8%) on the output side. The FBGs reflect at a fixed wavelength and in a narrow wavelength range, thus also acting as a wavelength-selective element. The FBGs are written directly in a fiber matched with the gain fiber to reduce any loss due to splicing.
Fiber lasers are optically pumped using pump diodes, and multiple pump diodes can be combined into the gain fiber using a pump combiner. The fiber output is generally angle-cleaved to prevent any feedback due to Fresnel reflections in the glass-air interface back coupled into the fiber.
Figure 2: Figure showing the emission wavelengths and power scalability of common rare-earth dopants in silica fibers.
Various rare-earth elements are doped into glass fibers, such as Ytterbium (Yb) providing gain in 1050 - 1120 nm, Erbium (Er) or Erbium-Ytterbium (Er-Yb) in 1530 - 1590 nm, and Holmium/Thulium (Ho/Tm) in 1900 - 2100 nm. Out of all the different rare-earth dopants that have been used, only Yb and Tm doped in single-mode silica fibers have been scaled to >1 kW of continuous wave (CW) power levels from a single laser module.
There are substantial white spaces in output wavelengths of rare-earth doped fiber lasers in the 1000 - 2000 nm range. One way is to use Cascaded Raman fiber lasers (CRFLs) to fill these gaps.
References:
M. N. Zervas and C. A. Codemard, "High Power Fiber Lasers: A Review," IEEE Journal of Selected Topics in Quantum Electronics 20(5), 219–241 (2014).
D. J. Richardson, J. Nilsson, and W. A. Clarkson, "High power fiber lasers: current status and future perspectives [Invited]," J. Opt. Soc. Am. B 27(11), B63 (2010).
L. Dong and B. Samson, Fiber Lasers : Basics, Technology, and Applications, First edition (CRC Press, 2016).
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