Optical frequency-domain reflectometry for microbend sensor demodulation

The operation of an incoherent optical frequency-domain reflectometer for m onitoring the continuous Rayleigh backscatter ina multimode optical fiber i s presented. A simple but effective model to predict the value of beat freq uencies arising in the system when excited by a linearly frequency-swept am plitude modulation has been developed. We have verified the model's predict ions by experimental measurement of beat frequencies and modulation depth i ndices of different lengths of standard graded-index multimode optical fibe r. Demonstration of the system sensitivity to the detection of microbending loss is then discussed. In particular the detection of loss in a hydrogel- based water-sensing cable allows an alternative interrogation to convention al optical time-domain reflectometry techniques to be implemented. We demon strate that the incoherent optical frequency-domain reflectometer is capabl e of detecting and locating sections of increased loss in a multimode optic al fiber, and we discuss the fundamental limits on spatial resolution and d ynamic range. (C) 2000 Optical Society of America OCIS codes: 060.2370, 290 .5870.