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A vector solution is obtained for the fields of the lowest-order modes in a three-layer elliptic fiber waveguide which is valid in the case of a small ellipticity of the boundaries between the layers, but is not restricted in respect of the differences between the refractive indices of the core and claddings. This solution is used to study mode birefringence as a function of the fiber waveguide parameters. It is shown that for a given core and an outer cladding, a waveguide with the W profile ensuring a high concentration of the energy of the lowest-order modes exhibits a stronger mode birefringence than a waveguide with a two-step profile. Near the limit of single-mode behavior an increase in the thickness of the inner cladding enhances the mode birefringence of a W-type waveguide, but reduces the corresponding birefringence of a two-step waveguide. The sensitivity of a three-layer waveguide to elliptic distortions of the boundaries increases on increase in the difference between the refractive indices of the three media forming the waveguide.

The kinematic approximation is used in an investigation of the theoretical solution of the problem of diffraction of arbitrarily polarized light waves by static volume phase holograms in uniaxial crystals. The diffusion, drift in a field, and photogalvanic mechanisms of photorefraction are allowed for and no restrictions are imposed on the orientation of the optic axis of a crystal. Several special cases are considered for crystals of the symmetry classes 3m and 4mm when measurements of the intensity of the diffracted radiation can be used to estimate the contribution of the various mechanisms to the process of formation of a hologram and the conditions are established for the reconstruction of the polarization of an arbitrarily polarized object wave.

Mode birefringence of an elliptic fiber is described by a formula valid in the case of a small ellipticity of the core and an arbitrary difference between the refractive indices of the core and cladding. This formula is derived using approximate vector solutions of the Maxwell equations for two dominant orthogonally polarized modes. It is shown that there are considerable changes in the mode birefringence due to changes in the parameters of the single-mode regime. A study is made of the difference between the group velocities of the dominant modes. It is shown that the ranges of the parameters of a single-mode elliptic fiber in which the mode birefringence is maximal and the intermode dispersion minimal overlap near the limit of the singlemode regime. A comparison is made with the published results.

We investigate the frequency shift for phase-matched four-photon mixing processes in low mode-number GeO 2 -doped silica glass fibers as a function of dopant concentration, core radius, index profile and Nd:YAG laser pump wavelength for different mode combinations. The ways for the Stokes-anti-Stokes frequency tuning over wide visible and near-IR spectral region are presented.

An investigation was made of the frequency shift in stimulated four-photon processes involving various combinations of LP modes in fiber waveguides made of quartz glass. The dependence of the shift on the parameters of the waveguides, refractive index profile, and pump wavelength were determined. Propagation of the pump photons in various modes gave rise to a nonmonotonic dependence of the frequency shift on the core radius, but when the pump photons were carried by one mode, the frequency shift decreased on increase in the radius throughout the region where it varied and where phase locking was possible. A dip in the refractive index profile reduced considerably the frequency shift. The shift increased on increase in the pump wavelength and also on increase in the difference between the refractive indices of the core and cladding.