Increased intensity noise in light transmitted through optical fiber is attributed to the conversion of phase modulation to intensity modulation by SBS. The conversion is caused by a phase shift of the optical carrier relative to the sidebands.Among optical nonlinearities in optical fibers, stimulated Brillouin scattering (SBS) is dominant due to its low power threshold. SBS is a nonlinear process, which generates a backscattered Stokes wave by the parametric coupling between the pump wave and electrostriction-induced traveling acoustic wave [1]. Because of power transferred to the backward Stokes wave, SBS limits the maximum power that can be transmitted through an optical fiber [2]. SBS also increases noise into the transmitted light. The increased noise and the significant power loss of the transmitted wave make SBS the main impairment in long-distance optical fiber communication systems. There have been several reported theories to model the noise of the transmitted light: Stokes and anti-Stokes Brillouin scattering [3], Guided acoustic-wave Brillouin scattering [4], and a phase shift of the optical carrier relative to the sidebands due to an asymmetric Brillouin gain spectrum [5]. Here, we report experimental results to determine the source of the increased noise of the transmitted light. IM PM Attn. Circulator Attn. Photo-900/o 1550 nm detector CW Laser QCoupler\(( C upler 25 km single-25 km single-EOM EDFA rnode Fiber mode Fiber (a) (b) Fig. 1. (a) Setup of the relative intensity noise (RIN) measurement of transmitted light through 25 km of single-mode fiber.Measurements are made for continuous wave (CW), phase-modulated, and intensity-modulated light, modulated by an electro-optic modulator (EOM). (b) The phase noise of the source is measured by delayed self-homodyne using the asymmetrical Mach-Zehnder interferometer, which is substituted for the 25 km fiber length. The relative delay time is much larger than the coherence time of the source. 1:19 dBm; ,00t (a) 2:12 dBrn(b) ; 3: 8 dBm.~~~~~~~~~~~~~~~~~~3 8dm -110 -~~~~~~~~~~~~~~~~~~~~~~~110 -120 -120 S~~~~~~~~~~~~~~Z Z 1o&30 130 -140 -~~~~~~~~~~~~~~~~~~~~~~140 -1SO .150 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Frequency (GHz) Frequency (GHz) Fig. 2. Measured RIN spectra of light after transmission through 25 km single-mode fiber for two laser sources. The average optical launch powers are 8, 12, and 19 dBm. a) Laser A, a distributed feedback semiconductor (DFB) laser, and (b) Laser B, a solid-state laser. The Brillouin threshold is about 7.3 dBm for Laser A, and 6 dBm for Laser B.We measure the relative intensity noise (RIN) of the light after transmission through a 25 km single-mode fiber for two different continuous wave (CW) I550-nm lasers. The setup is shown in Fig. 1 and the RIN is shown in Fig. 2. At launch powers above the SBS threshold, significant broadband noise is seen for Laser A, while Laser B shows 140

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We propose a distributed optical-fiber sensing scheme using four-photon mixing pulse walk-off. We use a copropagating system in which only a pump wave is launched into the fiber. We analyze the performance by including the effects of fiber birefringence, group-velocity dispersion, and self-phase modulation. A short system with high sensitivity and good resolution can be built. The useful operating length is limited by self-phase-modulation-induced pulse broadening.

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