In this work, we examine how the linewidth of high-power Yb-doped fiber lasers changes as a function of laser power. Four-wave mixing between the various longitudinal modes of the laser cavity tends to broaden the laser linewidth, while Bragg reflectors have a narrow bandwidth that limits the extent of this broadening. An analytical model taking into account these effects predicts that the laser linewidth scales as the square root of laser power, in agreement with numerical simulations . This model has been previously validated with a low-power Er-doped fiber laser  and with Raman fiber lasers . In this paper, we compare the measurements taken with Yb-doped fiber lasers at power levels ranging from a few watts to hundreds of watts with the model. The broadening of high-power fiber lasers deviate from the model. Experimental data show that the linewidth broadens as a power function (between 0.5 to 1) of the laser power. A simple modification of the model is proposed which fits all the experimental data.
The thermal degradation of double clad optical fiber coatings is known to be the prime limiting factor for the operation
of high power CW fiber lasers. In this paper, we conduct a study of thermal effects in high power CW fiber lasers. A
particular focus is put on heating at the splice points and in the doped fiber due to the quantum defect in 100-W class
CW fiber lasers. A theoretical model and experimental measurements taken with a high resolution IR camera on 125 to
400 μm diameter fibers are presented. Thermal contact resistance between the fiber and its heat sink are considered in
the conduction heat transfer model and measured for different geometries. Proper designs for cooling apparatus are
proposed and optimization of the active fiber is discussed. Some predictions for power scaling and temperature
management of fiber lasers to kW power level are also described.
A large number of high power CW fiber lasers described in the literature use large mode area (LMA) double cladding
fibers. These fibers have large core and low core numerical aperture (NA) to limit the number of supported modes and are typically operated under coiling to eliminate higher order modes. We describe here multimode (MM) high NA ytterbium doped fibers used in single mode output high power laser/amplifier configuration. Efficient single mode amplification is realized in the multimode doped fiber by matching the fundamental mode of the doped fiber to the LP<sub>01</sub> mode of the fiber Bragg grating (FBG) and by selecting the upper V-number value that limits the overlap of the LP<sub>01</sub> to the higher order modes. We show that negligible mode coupling is realized in the doped fiber, which ensures a stable power output over external perturbation without the use of tapers. Fundamental mode operation is maintained at all time without coiling through the use of FBG written in a single mode fiber. We show that such fiber is inherently more photosensitive and easier to splice than LMA fiber. We demonstrate an efficient 75W singlemode CW fiber laser using this configuration and predict that the power scaling to the kW level can be achieved, the design being more practical and resistant to photodarkening compared to conventional low NA LMA fiber.