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The bend-induced optical loss in a monomode fiber has been studied as a continuous function of wavelength in the range 1.2-1.6 microm. The loss was observed to be an oscillatory function of wavelength. These observations were explained by using a model based on interference between the guided core mode in the fiber and a "whispering gallery" mode ejected from the core by the bend. The wavelengths of the minima in the oscillatory function were accurately predicted by the model when the propagation of optical power in the fiber buffer coating was taken into account.

The properties of charge carriers in solids can be modified greatly by their real and virtual interactions with optical phonons. These effects differ in two particular ways from the changes induced by interactions with acoustic lattice modes. Firstly, those optical modes which interact with carriers are closely monochromatic. Sharply defined structure can therefore appear in many experimental observations, including optical properties, photoconductivity, tunnelling, the magnetoresistance observed under quantum-limit conditions and a range of coupled mode phenomena embracing both single carriers and plasma modes. Secondly, in materials of a polar nature the coupling of carriers to the optical modes is particularly strong and the resulting scattering probabilities and changes in effective mass can be large. Nevertheless, significant effects produced by the optical modes may be observed in homopolar semiconductors such as silicon and germanium. This article reviews the consequences of the coupling between the carriers and the optical phonons. T h e theory of such interactions is developed at length and then applied to a wide range of experimental observations in semiconducting and in insulating solids.

A theory is presented of the observed dependence of exciton series spectra on crystal thickness [2,3]. When the latter is comparable to the Wannier exoiton radius the effective confinement of the exciton bound states, by the uncertainty principle, raises the kinetic energy. This confinement is introduced by the boundary condition which requires the Wannier wave function to vanish outside the crystal. Allowance is made for thick crystal anisotropy, and its effects on the binding energy calculated for the low excited states. Es wird eine