We describe a model for polarisation dispersion in single-mode fibres of arbitrary length and configuration that is based on the existence of principal states of polarisation. These are two orthogonal input states of polarisation whose corresponding output states exhibit zero dispersion in their state of polarisation to first order.

Phenomenological approach to polarisation dispersion in long single-mode fibres

Origins and control of polarization effects in single-mode fibers (A)

Recent research on fibers with very small or very large birefringence for polarization-dependent applications is reviewed. The nature of random coupling between normal modes of polarization is analyzed and discussed in connection with various applications.

Polarization in optical fibers

The polarization state of light in single-mode fibers is very sensitive to any perturbation which is not symmetric about the fiber axis. While this is a source of noise, drift, or signal fading in some applications, it can also be exploited in novel guided-wave devices. The basic birefringences that couple the two modes and change the polarization state along the fiber are reviewed. The three cases of uniform, phase-matched, and random coupling are considered. Polarization preservation in both low- and high-birefringence fibers is achieved by reducing this coupling. In addition to polarization-state changes, bireftingent fibers can quickly reduce the polarization degree of nonmonochromatic light if both modes are excited, a characteristic that greatly simplifies evaluation of the degree of polarization preservation in these fibers. Current evaluations of the birefringence and the polarization-holding ability of state-of-the-art fibers are discussed, and it is concluded that fibers with good polarization-holding properties are becoming available.

Origins and control of polarization effects in single-mode fibers

Loss, coupling, and delay differences among the modes of multimode fibers influence their transmission characteristic in a complicated way An approximation of the modes by a continuum leads to a comprehensive description of these interrelations We relate the mode power distribution to the far-field output and calculate these distributions as functions of the fiber length and the input We report measurements of the far-field distributions at various lengths of a cladded low-loss multimode fiber A comparison of theory and experiment yields a quantitative estimate of the mode coupling involved We associate this coupling with random irregularities of the fiber configuration and straightness, and construct a quantitative model of such irregularities

Optical power flow in multimode fibers

In dielectric waveguides operating at optical frequencies, the primary cause of time dispersion of narrow pulses can be mode conversion. In this paper we argue that under certain assumptions a dielectric waveguide acts as a linear system in intensity. That is, given the intensity input, the intensity output is equal to the input convolved with an intensity impulse response. We show that contrary to intuition, the width of the impulse response gets narrower when coupling between guided modes increases. Using the perturbation results of D. Marcuse, we obtain an interesting model of energy propagation down imperfect guides. We conclude that the intensity response width increases as the square root of the guide length for sufficiently long guides and approaches a gaussian shape for sufficiently long guides. We conclude from the theory that the dispersion in dielectric wave-guides may be orders of magnitude below that which was previously expected in guides of sufficiently long length having properly controlled large amounts of mode conversion. These theoretical results have not yet been verified experimentally.

S. D. Personick

Papers

Time dispersion in dielectric waveguides