COHERENCE OF LIGHT AT THE OUTPUT OF A FIBER WAVE-GUIDE ANALYZED IN THE SPACE-FREQUENCY DOMAIN

Coherence of light in the space-frequency domain is investigated at the exit face of a weakly guiding fibre waveguide a's well as in its far zone by using the coherent mode representation. Two limiting cases of fibre waveguide excitation by cross-spectrally pure, quasimonochromatic, spatially incoherent and coherent sources are considered in the evaluation of spectral changes occurring in the free space propagation of light in the paraxial regime from the exit face of the fibre waveguide to its far zone. In particular, the blue and the red shifts as well as the spectra modifications in the far zone of two-mode, weakly guiding, step-index fibre waveguide are presented for both spatially incoherent and coherent exciting sources. A frequency dependence of the modes is also considered which leads to a substantial reduction in spectral shifts. Moreover, it is revealed that a variable spectral modulation is present in the case of the fibre waveguide excitation by a spatially coherent source.