RELATION BETWEEN THE COHERENCE LENGTH AND MODAL NOISE IN A GRADED-INDEX MULTIMODE FIBER FOR WHITE-LIGHT INTERFEROMETRIC SYSTEMS

The results of a comparative experimental study on the effect of modal noise induced by the modal coupling effect in a graded-index multimod e fiber that is illuminated by a light source with a tunable coherence length are reported. It has been shown that, in the coherence-length region of 30-80 mum, the value of the signal-to-noise ratio in an inte rferometric system could be reduced by the perturbation-induced modal noise, and, as the coherence length increases, the more the signal-to- noise ratio decreases. However, when the value of the coherence length is in the region of 80 mum and upward, the value of the corresponding signal-to-noise ratio reduction is seen to vary in only a very small range (<2 dBV). Under this condition the modal noise induced in the fi ber cannot be suppressed by the incoherent nature of the light source, thus showing the practical limitation of the use of multimode fibers in an interferometric system with a low-coherence light source.