The number of connections that can be used per optical channel in an FBG sensing chain depends not only on the type of connections used, but also on the interrogator, the fiber type, and length, as well as on the optical signal losses, which can be caused by the installation process (cable path, micro-curvatures, and so on).
a) Connectors vs splices
Two possible connections can be used when connecting optical strain sensors in chains: connectors and splices.
Connectors are easier to use on site, as they literally mean“plug and play”. However, they pose higher losses to the optical signal and are more prone to degradation with time.
Splices are, on the other hand, definitive connections, a fusion of the two fibers that are stable throughout time and feature low optical losses. Nevertheless, splicing requires dedicated tools, trained professionals and longer installation time.
To minimize installation time and, at the same time, increase the number of sensors that can be connected in a chain of sensors, HBM FiberSensing offers pre-assembled arrays of sensors connected by splices protected to suit the application.
b) Fiber type and length
The fibers used on the two HBM FiberSensing's sensor lines are different: the fiber used on the FS Line sensors has 9µm, core and the fiber used on the OP Line sensors has 6µm core.
Sensors and cables from the FS line can run through kilometers without compromising the sensor signals, as the 9µm core fiber has very small attenuation losses. Fibers and cables from the OP line show higher attenuation losses and hence, should not run for long distances.
When the two types of fibers are connected together, even via splices, there are also high losses on the interface that limit the number of times that different types of fiber can be used within a chain.
c) Sensor reflectivity
FBG sensor measuring principle is based on a reflected spectrum of incident light. The signal reflected back is a percentage of the incident light. FS Line sensors have a reflectivity of around 65%, and OP Line sensors have a reflectivity < 15%. On calculating the losses, the reflectivity of the sensors should also be considered.
d) Interrogators’ dynamic range
The admissible losses on an optical sensor chain are dictated by the available dynamic range of the interrogator. The dynamic range can be perceived as a measurement of the signal-to-noise ratio of the optical spectrum for peak detection. Signals with high or very close loss values to the dynamic range will not be correctly acquired by the interrogator.
The technical characteristics of the sensors, interrogators and accessories should be checked carefully to optimize the performance of an optical sensing chain.
HBM can offer support on the right selection of components.