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Configuring Optical Sensors

One of the advantages of Fiber Bragg Grating (FBG) technology is its intrinsic multiplexing capability. The sensors can have both specific and different Bragg wavelengths and can be connected in series without compromising the correct reading of the measurements as long as the sensor signals do not overlap.

Sensors can be acquired individually, with or without connectors, or as pre-assembled arrays of sensors connected by fusion splices — a permanent connection between two fibers. Upon installation, sensors and/or arrays can be linked together to one of the interrogator's optical channels, but attention must be given to the selection of wavelengths and the power losses that cable lengths and connections impose on the signals.

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1. Sensor Selection

Several types of FBG based sensors are provided by HBK, and all of them can be combined on the same optical channel of the interrogator, as long as optical losses are within limits and wavelength signals do not overlap. The tips given below apply to strain measurements but can be easily transferred to other types of sensors:

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In order to choose the most suitable type of sensor, the following requirements must be taken into consideration:

a) Measurement range

Different sensor encapsulations may limit their measurement ranges. When selecting the right sensor, the required range of measurement can be a factor for excluding some sensors.

b) Mounting type

HBM offers solutions for the sensors to be glued, welded, embedded, or bolted to the specimen. The mounting type can interfere with the speed and cost of installation, it can also eliminate other options. Weldable gauges can be considered for installing sensors on metallic structures. Welding is a fast and efficient way of bonding sensors to a structure, and that is ready to use right after installation without the need to wait for adhesive curing. For composite materials, the two options are gluing or embedding. Bolt connections are normally not advised for fiber composites, as drilling damages the fibers, but it can be a good solution for concrete or metallic structures.

c) Robustness

There is a wide range of sensors available with varying degree of robustness. Sensors are designed with several cable options for different environments such as, for example, laboratory, outdoor, offshore or high voltage.

d) Cable bending radius and length

newLight HBK FiberSensing sensors simplify and combine the previously existing OP and FS lines. The two sensor lines used different optical fibers that granted them specific characteristics.
The OP Line used a fiber with high bending capability performing products, and fiber links that could be used in very narrow spaces with tight bending radius, both on the cable path with neglectable power losses, as well as on the sensor area suitable even for measuring bent surfaces. The FS Line showed a lower level of flexibility on the sensors and cables, but they could freely be used over several kilometres without significant optical losses.
newLight sensors use low bend loss, telecom compatible fiber that opens the possibilities for innovative sensor designs with a smaller footprint, curved surface applications, as well as the straightforward use of multiplexed sensors on the same fiber over long lengths.

e) Operating temperature

Strain measurements can be made in very different environments. For high or low temperatures, only some of the optical strain gauges are suitable.

f) Temperature compensation needs

FBG-based strain sensors are sensitive to temperature changes, and correction is advised.

HBK FiberSensing offers sensors with incorporated temperature compensation, for example, by using two FBGs operating in a push pull configuration. When selecting such a sensor, there is no need for an extra sensor for temperature compensation of the measurement. For all other sensors, an extra sensing element is necessary, for example:

  • A temperature sensor placed under the same temperature as the sensor to compensate: With a temperature measurement on the same location of the strain sensor, the thermal cross-sensitivity of the sensor (as stated on the data and calibration sheets) and the thermal expansion of the base material, means that the strain measurement can be corrected.
  • An optical compensation element: An FBG sensor with a known behavior with temperature, for example, a non-calibrated temperature sensor, can be used to compensate for the effect of temperature on the measurement.
  • An optical strain gauge that should be applied to the same material and subjected to equal temperature conditions, but not subjected to strain: The strain measured by this sensor is the temperature induced strain.
  • An optical strain gauge installed on the opposite surface of the specimen, where strain has the same value but a different signal: With sensors operating in this push-pull configuration, the effect of temperature can be nulled by combining both strain measurements.

The number of sensors needed in an array should consider the above options.

2. Wavelength Selection

The Bragg wavelength distance between two FBG peaks defines the maximum wavelength range that both FBGs can use during the measurement. If the signals overlap, measurements are compromised.

The sensitivity of the sensor will be the dominating influence on the Bragg wavelength shift. The used wavelength range is determined considering the measurement range of the sensor and its sensitivity. Still, parasitic effects such as thermal sensitivity and thermal expansion should also be considered when choosing sensor Bragg wavelengths to avoid interference between sensors.

When using sensors with two or more FBGs, the behaviour of both will affect the overall returned signal to the interrogator.

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Using as example a strain sensor, starting from the central wavelength of the sensor (λ0), the range of wavelength that needs to be reserved is from the minimum possible wavelength value of the sensor to the maximum:

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Pre-designed wavelengths are available. Nontheless, care should be taken when selecting the centre wavelength for higher measurement ranges

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3. Controlling Optical Losses

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.

knowledge, resource center, articles, strain measurement basics, the two strain gauge technologies from hbm

Optical Sensing Chain

The technical characteristics of the sensors, interrogators and accessories should be checked carefully to optimize the performance of an optical sensing chain.

HBK can offer support on the right selection of components.