电子技术专业外文翻译4.docx

外文翻译 外文原文： Considerations for modalBirefringenceThere are two major aims of this chapter. Firstly, to provide a more explicit understanding of the meaning of polarization and modal birefringence in fibres such as microstructured fibres which have large refractive index contrasts. Secondly, in the instance of bound modes, to provide clear restrictions on the expected behaviour of polarization in the modes of birefringent fibres. Most conclusions that may be only strictly valid for bound modes, are in fact approximately valid for leaky modes with small confinement losses. Note that even when confinement loss is high for practical purposes one usually finds that Im(neff ) Re(neff ).Thus the proceeding analysis is approximately applicable to microstructured fibres supporting leaky modes.3.1 Local polarization The concept of polarization in optical waveguides is subtle in its difference to the polarization of a freely propagating plane wave. Consider the reduced fields e, h which are obtained from the total fields (3.1)by eliminating the dependence on time, t, and the propagating direction, z. They satisfy the vector wave equations [Snyder and Love, 1983, pp. 591] (3.2)The refractive index distribution used to create modes in optical waveguides results in two key differences. Firstly, the magnitude and direction of the reduced electric and magnetic field vectors may vary at different points in the waveguide cross-section.Secondly, the orthogonality between the transverse components of electric and magnetic fields are not preserved at all points in the cross-section. However, in the limit of small variation in refractive index the mode solutions are expected to locally resemble a plane travelling wave. Hence we expect to only deal with these two differences when refractive index contrasts are large. In this latter regime it is the local polarization of the electromagnetic field in an arbitrary waveguide mode that has an unambiguous meaning. Birefringence defines the insta