Scattering and Absorption Losses of Multimode Optical Fibers and Fiber Lasers

A cavity laser consists of an active medium that provides the required gain and a (usually open) external cavity furnishing the feedback for laser operation. A fiber laser also has gain and feedback, but instead of using the resonant modes of an open cavity it employs an optical fiber for guiding the radiation back and forth between the set of mirrors forming the cavity. 12 A fiber laser thus might be much narrower than a cavity laser since it need not allow space for the diffraction-limited beam to spread out in transverse direction. The width of the fiber laser is limited only by the loss of the fiber waveguide, which increases with decreasing fiber diameter. 1463 In this paper we calculate the mode losses of step-index fibers and use them to estimate the losses of fiber lasers. The losses are caused by scattering from the rough fiber wall and by the presence of a lossy cladding. Figure 1 shows a schematic of the fiber laser. We assume that plane mirrors are placed at the end of the fiber that also contains the gain mechanism for the laser. Figure la shows a fiber laser with plane mirrors positioned exactly perpendicular to the fiber axis, whereas Fig. lb shows a laser with slightly tilted mirrors so that the wave inside the fiber, indicated schematically by a light ray, interacts more strongly with the fiber wall. For simplicity, we assume that the mirrors and the medium inside the fiber do not cause scattering and that only the fiber walls are slightly rough. We also assume that the fiber is surrounded by a lossy cladding that causes power loss via the evanescent field tail of the guided wave penetrating into the cladding.