MEASUREMENT OF UP-LEAD AND DOWN-LEAD FIBER SENSITIVITY CAUSED BY THE LEAD IN A MULTIMODE FIBER IN AN INTERFEROMETRIC SYSTEM

The results of a comparative experimental study on the effect of the m odal noise induced by lead-in fibers in an extrinsic interferometric s ystem, illuminated by high- and low-coherence light sources, respectiv ely, are reported. When the up-lead fiber was subject to a perturbatio n, the sensitivity of the system was reduced by 20.9 dB through the us e of a high-coherence source, and by 1.8 dB through the use of a low-c oherence source. When the down-lead fiber was perturbed, the sensitivi ty dropped by 30.3 dB and 4.9 dB for high- and low-coherence sources, respectively. The results from the experimental analysis supported qua litatively by simple theory show that the use of a low-coherence light source can greatly suppress the modal noise induced in both the up- a nd down-lead fibers, if the coherence length of the light source used is less than the optical path difference between two adjacent fiber mo des. This shows the practicality of the use of multimode fibers in an interferometric system with a suitable light source.