No abstract

Abstract:The output of high power fiber amplifiers is typically limited by stimulated Brillouin scattering (SBS). An analysis of SBS with a chirped pump laser indicates that a chirp of 2.5 × 10 15 Hz/s could raise, by an order of magnitude, the SBS threshold of a 20-m fiber. A diode laser with a constant output power and a linear chirp of 5 × 10 15 Hz/s has been previously demonstrated. In a low-power proof-of-concept experiment, the threshold for SBS in a 6-km fiber is increased by a factor of 100 with a chirp of 5 × 10 14 Hz/s. A linear chirp will enable straightforward coherent combination of multiple fiber amplifiers, with electronic compensation of path length differences on the order of 0.2 m. ©2012 Optical Society of America

We measure the Rayleigh scattering spectrum in an optical fiber. Our data suggest that the narrower gain bandwidth and frequency shift that we observe are due to guided entropy modes in the fiber.

Backward Rayleigh scattering in optical fibers due to the fluctuations that are "frozen-in" to the fiber during the manufacturing process may limit the performance of optical sensors and bidirectional coherent optical communication systems. In this manuscript we describe a comprehensive model for studying intensity noise induced by spontaneous Rayleigh backscattering in optical systems that are based on self-homodyne detection. Our model includes amplitude and frequency noise of the laser source, random distribution of the scatterers along the fiber, and phase noise induced in fibers due to thermal and mechanical fluctuations. The model shows that at frequencies above about 10 kHz the noise spectrum is determined by the laser white frequency noise. The laser flicker frequency noise becomes the dominant effect at lower frequencies. The noise amplitude depends on the laser polarization. A very good agreement between theory and experiment is obtained for fibers with a length between 500 m to 100 km and for a laser with a linewidth below 5 kHz.

Rayleigh scattering (RS) adds noise to signals that are transmitted over optical fibers and other optical waveguides. This noise can be the dominant noise source in a range between 10 Hz and 100 kHz from the carrier and can seriously degrade the performance of optical systems that require low close-in noise. Using heterodyne techniques, we demonstrate that the backscattered close-in noise spectrum in optical fibers is symmetric about the carrier and grows linearly with both input power and fiber length. These results indicate that the RS is spontaneous and is due to finite-lifetime thermal fluctuations in the glass. © 2013 Optical Society of America OCIS codes: 190.4370, 290.5870. A new class of radio-frequency (RF)-photonic applications uses lasers and optical media, such as optical fibers to generate, transmit, and process ultralow-noise RF signals.The performance of systems, such as optoelectronic oscillators, optical-fiber time and frequency-transfer systems, microresonator-based oscillators, and fiber sensors is limited in part by scattering in the optical media [1][2][3][4]. It has been shown that Rayleigh scattering (RS) induces noise in a 10-100 kHz bandwidth around the optical carrier in optical fibers [5,6]. Previously, we measured the Rayleigh backscattered noise spectrum in single-mode optical fibers, and we showed that the gain relative to the input relative intensity noise (RIN) matched the theoretically expected spectrum for stimulated RS in bulk materials, but with a much narrower spectrum [7]. A narrower spectrum is expected from processes that are driven by the transverse intensity gradient of the light in the optical fiber, as is the case for guided acoustic wave Brillouin scattering (GAWBS). For this reason, we refer to the Rayleigh scattering process in this case as guided entropy mode Rayleigh scattering (GEMRS). Our work was carried out using standard SMF-28e communication fiber.The experiments to date have not resolved the important question of whether we are observing a stimulated process or a spontaneous process. While the gain spectrum that we observed is reminiscent of a stimulated process, the analogous process of GAWBS is a spontaneous process. The answer to this question has profound implications for the physical origin of the scattering and the best methods for suppressing it. If the process is stimulated, then the source of the noise is amplification of the laser noise. In this case, the output noise spectrum should be asymmetric about the carrier. If it is upshifted, then we would infer that the scattering is primarily electroabsorptive; if it is downshifted, then we would infer that the scattering is primarily electrostrictive [8]. In either case, we anticipate an exponential growth of the output spectrum as the input light power or the length of the fiber increases. By contrast, if the process is spontaneous, then the carrier is scattered by finite-lifetime thermal fluctuations in the optical fiber-as opposed to the frozen-in thermal fluctuations that have an infinite life...