Optical fiber-based corrosion sensor systems
An investigation into optical fiber sensing methods for monitoring metallic corrosion 1s presented. A series of sensing techniques are presented and explored, and two techniques are identified as possible candidates for a corrosion sensing system. The first is a technique based on the phenomenon of surface plasmon resonance, a technology useful in the sensing of changes in the index of refraction of a material in a localized area. The second involves the use of a metal-clad fiber sensing region where the standard core/cladding fiber geometry is modified to have a silica core and a metal cladding.
A series of improvements to the current surface plasmon devices lead to the choice of a multimode cylindrical geometry for the sensor fiber. Under the correct conditions, energy in the TM modes in the fiber will transfer to a surface plasmon mode coupled to the surface of a metal film on the fiber. Monitoring the spectral output of the sensor provides information on the change in index of refraction of the surrounding material or the remaining thickness of the metal film applied to the fiber.
The metal-clad sensor involves replacing the standard dielectric cladding with a metal in the sensing region. The loss of power through this region is directly related to the thickness of the metal cladding, which decreases with corrosion. A theoretical analysis is performed and compared with experimental results which demonstrate the feasibility of the sensing technique.
Both the surface plasmon approach and the metal-clad approach are demonstrated as corrosion presence indicators, that is, they cannot currently resolve the level of corrosion, but just indicate when a certain amount of corrosion has occurred. The metal-clad technique is therefore identified as the most practical technique due to its relatively simple operation and easily understandable output response to corrosion. Future improvements are presented for the metal-clad sensor to help resolve the total amount of corrosion and a method is demonstrated for the repeatable fabrication of the metal-clad sensor.