Mode selective/filtering microbend sensor structure for use in distributed and quasi distributed sensor systems

This paper describes a novel optical fibre microbend sensor architecture which my be utilised in distributed and quasi distributed measurement. The actual sensor element is graded index multimode fibre coupled to the measurand field through the usual microbend inducing structures. However, the feed to the sensing section is through a single mode fibre spliced to the multimode fibre to ensure that only the lowest order spatial mode is launched. Similarly the receiver is also coupled to the sensing element through a single mode fibre. The single mode within multimode fibre propagates with minimal mode coupling with source to receiver losses of typically 0.5 dB for short sensors ranging to approximately 0.3dB per each additional kilometre of sensing fibre. The sensitivity of this structure to microbend induced losses has been thoroughly characterised. Typically the optical power loss for a given microbend structure and force is about three to six times higher in this architecture than for conventional fully mode filled microbend sensor. Further since the microbend operates only on the lowest order mode of the sensor fibre operation of the sensors does not depend on initial modal distribution in sensing fibre. The structure is also almost totally insensitive to the macrobend induced losses and allows a variety of novel designs in microbend inducing structures. Additionally, the use of spatial mode filters allows effective control over concatenation effects that are common in microbend sensors.