Artificially disordered birefringent optical fibers.

We develop and experimentally verify a theory of evolution of polarization in artificially-disordered multi-mode optical fibers. Starting with a microscopic model of photo-induced index change, we obtain the first and second order statistics of the dielectric tensor in a Ge-doped fiber, where a volume disorder is intentionally inscribed via UV radiation transmitted through a diffuser. A hybrid coupled-power & coupled-mode theory is developed to describe the transient process of de-polarization of light launched into such a fiber. After certain characteristic distance, the power is predicted to be equally distributed over all co-propagating modes of the fiber regardless of their polarization. Polarization-resolved experiments, confirm the predicted evolution of the state of polarization. Complete mode mixing in a segment of fiber as short as ∼ 10cm after 3.6dB insertion loss is experimentally observed. Equal excitation of all modes in such a multi-mode fiber creates the conditions to maximize the information capacity of the system under e.g. multiple-input-multiple-output (MIMO) transmission setup.

Single-mode Er-doped fiber random laser with distributed Bragg grating feedback.

We report the implementation of a one-dimensional random laser based on an Er/Ge co-doped single-mode fiber with randomly spaced Bragg gratings. The random grating array forms a complex cavity with high quality factor resonances in the range of gain wavelengths centered around 1535.5 nm. The reflection spectra of the grating array and the emission spectra of the laser are investigated for different numbers of gratings. The experimental results are compared qualitatively with numerical simulations of the light propagation in one-dimensional Bragg grating arrays based on a transfer matrix method. The system is pumped at 980 nm and the experimentally observed output radiation presents a typical laser threshold behavior as a function of the pump power. We find that the laser output contains several competing spectral modes.

Intermode light diffusion in multimode optical waveguides with rough surfaces.

A theoretical analysis of incoherent intermode light power diffusion in multimode dielectric waveguides with rough (corrugated) surfaces is presented. The correlation length a of the surface-profile variations is assumed to be sufficiently large (a less less than lambda/2pi) to permit light scattering into the outer space only from the modes close to the critical angles of propagation and yet sufficiently small (a less less than d, where d is the average width of the waveguide) to permit direct interaction between a given mode and a large number of neighboring ones. The cases of a one-dimensional (1D) slab waveguide and a two-dimensional cylindrical waveguide (optical fiber) are analyzed, and we find that in both cases the partial differential equations that govern the evolution of the angular light power profile propagating along the waveguide are 1D and of the diffusion type. However, whereas in the former case the effective conductivity coefficient proves to be linearly dependent on the transverse-mode wave number, in the latter one the linear dependence is for the effective diffusion coefficient. The theoretical predictions are in reasonable agreement with experimental results for the intermode power diffusion in multimode (700 x 700) optical fibers with etched surfaces. The characteristic length of dispersion of a narrow angular power profile evaluated from the correlation length and standard deviation of heights of the surface profile proved to be in good agreement with the experimentally observed changes in the output angular power profiles.

Nonreciprocal effects in the double passage of light through a random diffuser and a birefringent crystal.

We present a theoretical and experimental study of the scattering of light by double passage through a system that consists of a strong diffuser, a piece of birefringent crystal, and a plane mirror. We show that this arrangement can produce not only enhanced backscattering and satellite peaks but also satellite dips in the angular distribution of the mean intensity. The experiments are in agreement with theoretical results based on scalar diffraction theory in the paraxial approximation.

Observation of satellite peaks and dips in the scattering of light in a double-pass geometry.

We report the experimental observation of enhanced backscattering and satellite peaks and dips in light scattered by a system that involves double passage of waves through a random-phase screen and a birefringent crystal.

Coherent scattering by one-dimensional randomly rough metallic surfaces.

A critical evaluation of various theoretical techniques for calculating the reflectivity of one-dimensional metallic randomly rough surfaces is presented. We proceed by comparing experimental and rigorous numerical results with those obtained with three perturbation theories and the Kirchhoff approximation. The samples were fabricated in photoresist, and their metallized surface profiles constitute good approximations to Gaussian-correlated, Gaussian random processes. The correlation lengths of these surfaces range from approximately one third to approximately three times the infrared wavelengths employed. The results show that the phase-perturbation theory has wider applicability than the other perturbation theories and the results based on the Kirchhoff approximation.