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 are '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, HBK FiberSensing offers pre-assembled arrays of sensors connected by splices protected to suit the application.
b) Fiber length
newLight sensors are compatible with telecomunication fiber that is optimized to link very long distances without important signal degradation. We can achieve tens of kimometres of distance between sensors and interrogators without compromising the measurement.
c) Sensor reflectivity
The FBG sensor measuring principle is based on a reflected spectrum of incident light. The signal reflected back is a percentage of the incident light. The FBG sensors available in the market have different reflectivities – some above 65%, others below 5%, depending on the supplier or employed fabrication technique. newLight sensors have 20% reflectivity. 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.
e) Interrogators’ peak detection
Adding up all the above-mentioned effects of distance, joint losses and reflectivities one can get complex signal spectrums that are demanding for the interrogators.
Some interrogators feature different gain steps that will allow the 'magnifying' of signals to read smaller peaks, but this compromises the acquisition of sensors that are immune to some of the losses. In this case, the more controlled losses there are per optical connector, the easier it is to prevent measurement issues, but this means that there will be compromises: the number of sensors, or the number of joint connections or the cable lengths must be reduced.
Alternatively, interrogators with Smart Peak Detection can be selected. This exclusive peak detection algorithm ensures the use of the full dynamic range within each band that is defined for the operation of each FBG peak, making the coexistence of high peaks and low peaks on the same optical connector a peaceful one.