M.Sc Thesis

M.Sc StudentDavid Asaf
SubjectLow-Frequency Transmitted Intensity Noise Induced by
Stimulated Brillouin Scattering in Optical Fibers
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Moshe Horowitz
Full Thesis textFull thesis text - English Version


Stimulated Brillouin scattering (SBS) is a non linear physical process in which a light wave (pump wave) that propagates in a medium backscatters a Stokes wave whose frequency is downshifted from that of the incident light by an amount set by the nonlinear medium. The generation of the backscattered wave is manifested through interaction with optical density variations of the medium. Hence, even if the light wave that enters the medium is of high spectral purity, the backscattered Stokes wave will become noisy.

Due to its low threshold in optical fibers, SBS is considered a dominant nonlinear effect which limits the performance of light-wave systems using single-mode fibers. Therefore, SBS in optical fibers have been intensively studied in the last decades. The majority of previous studies have investigated the spectral characteristics of the backscattered Stokes wave. The transmitted noise, however, did not get as much attention as the reflected one. Works that have investigated the transmitted noise were mainly interested in its spectral behavior in high frequencies. Nevertheless, the recent developments in ultra-low noise generators make the investigation of transmitted noise in optical fibers important. The study of low-frequency noise induced by Brillouin scattering is important for transferring ultra-low noise signals in fibers and for generating ultra-low noise signals in optoelectronic oscillators (OEOs). To decrease phase noise in OEOs, the length of the OEO cavity and the optical power should be both increased. However, when the signal power and the fiber length are increased, Brillouin scattering can induce noise in the low-frequency region that is important for applications that are based on OEOs (0-100 kHz). Hence, SBS can limit the performance of OEOs.

This work studies theoretically and experimentally the spectral properties of low-frequency transmitted intensity noise induced by SBS in optical fibers. In fibers with a length of 25 km the Brillouin scattering induces transmitted intensity noise with a bandwidth on the order of tens of kHz. The power spectral density of the noise may be stronger than the shot noise in the photo-detector even when the optical power is significantly lower than the Brillouin threshold. The low-frequency transmitted intensity noise is caused due to depletion of the pump wave by the stochastic Brillouin wave. Since pump depletion occurs over a long distance, noise with a narrow bandwidth is generated in the transmitted wave. When the pump power is high enough, the spectrum of the induced noise contains features such as hole burning at low frequencies and ripples.