Multi-axial optical waveguide grating sensors for (composite) airframe monitoring

Problem definition

Composite materials are used in a wide array of application markets such as aviation, automotive, biomedical implants, construction materials, etc. by virtue of their strength, stiffness and lightness. A major part of these structures do not allow for traditional - electrical - sensing systems. Because of this lack of structural health monitoring systems, many composite structures are replaced today purely for precautionary reasons at one hand or after failure of the construction on the other hand. Adding optical sensing intelligence to these structures not only prolongs their lifetime but also significantly decreases the environmental impact through a reduced use of raw materials and energy savings.

We are therefore investigating the use of optical waveguides with integrated sensing structures based on wavelength-selective mirrors (gratings). Different optical fabrication technologies are available within the research lab CMST (department Elis), http://www.cmst.be. One of these technologies is the fabrication of polymer optical waveguides which can be made mechanically flexible and combined with integrated optoelectronics (lasers and detectors) to read out the sensors.

Goal of this thesis

Currently, several technologies are available at Cmst for fabricating waveguides and grating structures (see figure) in various polymer materials and the basic concept has been proven. These technologies can be used as a starting point for this thesis. However, for obtaining so-called multi-axial sensors, i.e. sensors that can measure stress or strain in different directions, more than 1 sensing element is needed, as illustrated in the figure below.


Graphical summary of the thesis goals

Typical waveguide grating sensor + reflection spectrum showing different reflection peaks.
Possible multi-axial sensing configurations


In general, different possibilities for the implementation of sensing structures in optical waveguides will be studied. The first step within the development of these optical sensors is mechanical and/or optical simulation of the sensing principle, the surrounding light guiding medium and the composite host material. Secondly, the sensing system itself will be designed and fabricated using the CMST cleanroom capabilities. Later in the development cycle, these newly developed sensors can be integrated in an optical sensing/interrogation system and combined with composite host materials so that multi-axial stress can be measured in a real-life environment. This will be done in close collaboration with the research lab Mechanics of Materials and Structures (MMS, http://www.composites.ugent.be).

The work is divided in the following parts:

- Design/modeling/simulation of the sensing structures and the interaction with the sensing medium.
- Fabricating and validating the sensing concepts and work towards a multi-axial sensing system. An ideal sensor allows measuring strain in all directions. Therefore, we will integrate for example different grating sensors in a single waveguide, or combine various waveguides with a different orientation. Tuning the orientation of the grating and the size of the waveguide can for example by exploited to obtain reflection peaks at different wavelengths.
- Testing the full sensing system in a real composite environment.

Together with the master student, the focus of the master thesis can be slightly shifted towards one of the specified parts.

Practical details (see also Plato)

  • Location: clean rooms (Zwijnaarde, campus Ardoyen) + at home
  • Number of students: 1 or 2
  • Number of master theses: 1
  • Contact: Jeroen Missinne

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