Progress in Electromagnetic Research Symposium 2004, Pisa, Italy, March 28 - 31 727 Light Diffusion in a Multimode Optical Fiber with a Rough Surface E. I. Chaikina, A. G. Navarrete, S. Stepanov, E. R. Méndez Division de Fisica Aplicada Centro de Investigación Científica y de Educación Superior de Ensenada México e-mail: chaikina@cicese.mx T. A. Leskova Department of Physics and Astronomy University of California, Irvine, California, U.S.A. e-mail: leskova@duey.ps.uci.edu Abstract We report results on inter-mode light power transfer in essentially multimode optical fibers with rough surfaces. In particular, dispersion of an initially narrow mode spectrum profile excited by oblique incidence of a plane wave is investigated. Experimental results are interpreted using equations for the incoherent inter-mode power diffusion in k-space. Introduction Propagation of a coherent light in multimode perfectly conducting randomly corrugated waveguides has been a topic of intensive theoretical study in recent years [1-4]. The main motivation for that work was the possibility of observing, simultaneously, different regimes of light propagation: ballistic, diffusive, and light localization. This random waveguide configuration with surface roughness differs essentially from that utilized in experiments on propagation of electromagnetic waves of the GHz spectral region through hollow metal tubes with volume heterogeneities [5,6]. Due to the relatively large attenuation that occurs in the reflection of light from real metal surfaces, comparison of similar optical experiments (with surface or volume scattering in metal waveguides) with the theory could be very difficult, if possible at all. More natural optical waveguides (both single- and multi-mode) are obviously dielectric waveguides, i.e. optical fibers. In previous works, we have reported some experimental results on the propagation of coherent light through multimode optical fibers with etched (i.e. optically rough) surfaces [7,8]. In particular, leakage of the light out of the fiber, followed by the formation of a narrow central angular maximum in the transmitted light, in response to a more or less uniform initial mode excitation was observed. A theoretical model of incoherent inter-mode light power transfer was proposed, which resulted in a diffusion type partial differential equation [8] (earlier [9] similar equations were introduced for metal waveguides). In this paper we present new experimental results concerning the dispersion of initially narrow distributions of light in k-space, accompanied by a numerical analysis. Experimental configurations and results The experimental samples were prepared by etching chemically the fused silica nuclei of a 3M TECS TM FT-200-EMT multi-mode optical fiber of 200 μm diameter with the plastic jacket and cladding removed. Statistical characteristics of the fiber surface processed in this way, and in particular, the correlation length and the standard deviation of the roughness were estimated from the surface profiles obtained by an electron scanning microscope and a DEKTAK 3 ST profilometer. The surface profile cannot be modeled as a single-scale random process (i.e. it is fractal-like) and from our measurements it is difficult to characterize its lateral scale. Thus, at present, we can only report on the standard deviation of heights δ of the surface profile. Its value, depending on the duration of the etching (up to tens of minutes), was in the range m μ 5 . 0 02 . 0 - . In the experiments presented below we used two sources of red coherent radiation: a single-mode linearly polarized cw semiconductor laser diode with the wavelength m μ λ 655 = and a He-Ne laser with the wavelength m μ λ 8 . 632 = . To excite the broad angular spectrum of the propagating fiber modes the input end of the fiber (still covered by the plastic cladding and jacket) was illuminated by a collinearly propagating laser beam focused with a 40x microscopic objective - Figure 1. By filling the entire angular aperture of the utilized fiber (N.A = 0.39) we excited about 250x250 of the fiber modes.