Optical fiber having an elevated threshold for stimulated brillouin scattering

#The present invention relates generally to optical fibers and, more particularly, to optical fibers having an increased threshold for stimulated Brillouin scattering.

#In many applications, such as high power fiber optic industrial lasers and scaleable high power fiber optic phased array laser systems, it is desirable to transmit optical signals having substantial amounts of power via optical fibers. Unfortunately, stimulated Brillouin scattering oftentimes limits the amount of power that can be transmitted via an optical fiber such that, even as additional input power is provided, the output power remains relatively fixed at the threshold at which stimulated Brillouin scattering commences.

#In general, stimulated Brillouin scattering is a phase-matched parametric amplification process involving the coupling of a optical wave, an acoustic wave and a backward propagating Stokes wave. In this regard, variations in the index of refraction of an optical fiber induced by pressure differences created by an acoustic wave traveling along the optical fiber can cause a portion of the optical wave to be backscattered, thereby creating the backward propagating Stokes wave. The backward propagating Stokes wave essentially robs power from the optical wave so as to limit the power of the optical signals that can be transmitted via the optical fiber. With reference to quantum physics, stimulated Brillouin scattering can therefore be described by the transfer of a photon from the optical wave into a new Stokes photon of lower frequency and the creation of a new phonon that adds to the acoustic wave.

#Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:

#FIG. 1 is a graph illustrating the transfer of input power from the optical signals propagating along an optical fiber in a forward direction to backscattered optical signals propagating in the opposite direction;

#FIG. 2 is a graph depicting the overlap between the fundamental optical and longitudinal acoustic modes of a conventional optical fiber having a core doped with GeO2;

#The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.

#Referring now to FIG. 3, an optical fiber 20 according to one embodiment of the present invention is depicted that exhibits an elevated threshold for stimulated Brillouin scattering relative to conventional optical fibers. The optical fiber includes a longitudinally extending core 22 having a first index of refraction and a first acoustic wave propagation velocity. The optical fiber also includes a cladding 24 surrounding the core and extending lengthwise therealong. Like the core, the cladding includes a second index of refraction and a second acoustic wave propagation velocity.

#While both the core 22 and the cladding 24 are typically formed of silica, at least one of the core and the cladding is preferably doped such that the first index of refraction of the core is greater than the second index of refraction and the cladding such that the optical waves and, in particular, the fundamental mode of the optical waves are confined within and guided through the core. According to the present invention, at least one of the core and the cladding are also doped such that the first acoustic wave propagation velocity of the core is greater than the second acoustic wave propagation velocity of the cladding. In this regard, the acoustic wave propagation velocity of a material is determined by the square root of the ratio of material density to the elastic stiffness constant. By doping at least one of the core and the cladding such that the first acoustic wave propagation velocity of the core is greater than the second acoustic wave propagation velocity of the cladding such that the acoustic waves and, in particular, the fundamental mode of the acoustic waves is not confined within and guided through the core. Instead, the optical fiber 20 of the present invention anti-guides the acoustic waves such that the acoustic waves are permitted to radiate laterally or radially out of the core and through the cladding toward the exterior surface of the cladding.