Several interferometric fiber-optic sensing applications have been proposed that require extremely high sensitivity. For example, electric field and magnetic field sensing. It has commonly been supposed that for such applications one could simply use extremely long lengths of fiber to obtain the desired sensitivity. It is shown that this is not the case and that one encounters a maximum length beyond which no further increase in sensitivity is obtained for a given frequency, due to the finite propagation speed of light in the fiber. This length is given by ℓmax = (v/2fm ), where v is velocity of propagation in the fiber and fm is the frequency to be sensed. FEr 100kHz this length is approximately one kilometer for fused silica fiber. It is found that linear length scaling projections can be used up to R provided the sensitivity so obtained is reduced by a factor of (2/π) = 0.637. IncrWREing the amount of fiber beyond this maximum length has the surprising result of actually decreasing the sensitivity and eventually one can obtain no sensitivity at all at twice L Implications of these results in terms of minimum required transducer sensitivities are discussed and related to measured short length sensitivities. In addition, methods to circumvent this limitation and some of their consequences are presented.

Date Published: 28 April 1987
PDF: 3 pages
Proc. SPIE 0718, Fiber Optic and Laser Sensors IV, (28 April 1987); doi: 10.1117/12.937524

Published in SPIE Proceedings Vol. 0718:
Fiber Optic and Laser Sensors IV
Ramon P. DePaula; Eric Udd, Editor(s)