Optical fiber sensors utilizing Brillouin scattering rely on the principle that the Brillouin frequency shift is a function of the local temperature or strain. Conventional optical fibers, such as standard telecommunications single-mode fibers, have been successfully used in these applications, and most typically in the time domain, such as with BOTDR. Such conventional fibers however are susceptible simultaneously to both temperature and strain, requiring either at least two fibers or specialized cabling to distinguish the effects of a local stress from those of a local change in temperature. Recently, methods utilizing fibers possessing at least two Brillouin frequency shifts, each with different temperature or strain coefficients have been proposed. However, realizing such fibers is challenging, requiring fibers with regions of very different compositions, all of which must have substantial overlap with the optical field, posing significant manufacturing challenges. We present several new specialty optical fibers based on novel and unconventional fabrication techniques with significant potential for use in distributed fiber sensor systems. First, we describe a class of fibers fabricated from materials whose Brillouin frequency shifts are immune to either temperature or strain, with a demonstration of the former using fiber derived from sapphire crystal, and modeling and measurements predicting the latter. The ‘Brillouin-athermal’ fiber enables the measurement of a local strain, independent of the local temperature. Second, we describe and demonstrate a novel group of longitudinally graded (chirped) fibers enabling easily-implemented frequency-domain systems; affording the potential to simplify and reduce the cost of Brillouin-based distributed sensors.
Numerous methods to increase the stimulated Brillouin scattering (SBS) threshold have been previously
implemented. Some are passive, based on acousto-optic fiber designs that incorporate longitudinally- or radially-tailored
optical and/or acoustic index profiles, leading to broadened Brillouin gain spectra (BGS) with reduced peak
gain. Some are active, relying on an applied temperature or strain distribution, also resulting in broadened BGS.
Broadening the laser spectrum still represents the most effective method to-date to obtain large-scale (> 20 dB)
decreases in the gain, but the suitability of this method depends largely on the application and system requirements
on the laser spectrum. Despite these technologies, some introduced only in the last decade, the vast majority of high-energy, narrow-linewidth fiber laser systems are still limited by SBS rather than the availability of pump power. We
present an alternative approach; rather than focusing on ‘suppressing’ SBS in waveguide or other designs, we
propose implementing materials with intrinsically low Brillouin gain. We focus on high-density, high-soundvelocity,
large acoustic-damping-coefficient, and low-photoelastic-constant materials wherein the correct balancing
of physical characteristics gives rise to extremely low Brillouin gain. In general, the approach requires the use of
compositions that would be considered to be highly unconventional and unachievable utilizing standard fiber
fabrication methods. For example, we describe recent results on sapphire-derived fibers (among other compositions)
wherein a Brillouin gain nearly 20 dB lower than those of more conventional fibers has been realized. Other
compositions will also be presented, including new results on a novel baria doped fiber, including others predicted to
have zero-valued photoelastic constants, and therefore zero Brillouin gain.
Increasing power levels and novel applications are demanding from fibers performance capabilities that have, to date,
not been realized. One such example arises from the nascent push towards the 10-kW power threshold for narrow linewidth fiber lasers designed for applications including coherently-phased laser arrays and spectroscopic lidars. It is well-known that Brillouin scattering still restricts continued power scaling in these systems, despite several recent advances in acoustic-wave Brillouin management. Accordingly, novel fibers possessing a Brillouin gain coefficient 10 dB or more less than previously demonstrated would be of great practical benefit if they comprise novel materials in simple geometries and are manufactured using industry-accepted methods. Introducing a new and effective approach to the management of Brillouin scattering, we present on all-glass optical fibers derived from silica-clad sapphire with alumina concentrations up to 55 mole percent; considerably greater than conventionally possible enabling the design of optical fiber possessing a series of essential properties. Markedly, a Brillouin gain coefficient of 3.1 × 10-13 m/W was measured for a fiber with an average alumina concentration of 54 mole percent. This value is nearly 100 times lower than standard commercial single-mode fiber and is likely the lowest ever specified value. This reduction in Brillouin gain is enabled by a number of key material properties of the alumina-silica system, amazingly even leading to a predicted, but not yet demonstrated, composition with zero Brillouin gain. Optical fiber materials with these and other crucial properties will be discussed in the context high energy fiber laser systems.