Integrated optical fiber sensing solutions for airframe monitoring applications

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 (Figure 1).

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 fibers with integrated sensing structures based on wavelength-selective mirrors (Bragg gratings). This approach offers several unique sensing features such as:
- insensitivity to electromagnetic interference
- light weight and low-cost
- multiplexing possibilities using a range of different Bragg wavelengths (wavelength division multiplexing) enabling quasi-distributed sensing measurements
making them ideal candidates for structural health monitoring applications.

Different challenges however still exist within the field of optical fiber sensors based on fiber Bragg gratings, especially when integrating in composite host materials. They are mainly related to the cost and the size of the equipment needed to read out the sensors (spectral analysis) as well as the cross-sensitivity (for example to temperature and pressure) of the fiber Bragg gratings. State-of-the-art solutions for these challenges are being developed within the research labs CMST (department Elis) and B-Phot (Vrije Universiteit Brussel) as well as within the research project Self Sensing Composites.

An interrogation technology based on miniaturized optoelectronic components is drastically limiting the dimensions of the read-out equipment enabling integrated measurements. Both driving and read-out components can be integrated in ultra-thin packages using CMST technology. An example of an integrated photodetector is shown in Figure 2 (total thickness is only 40 μm). The combination of different integrated optoelectronic components will lead to a fully embedded fiber Bragg grating interrogation system.

Advanced modeling and fabrication of the optical fiber based on microstructures is enabling cross-insensitive measurements of ambient parameters. Figure 3 shows a typical cross section of such a microstructured optical fiber (B-Phot). By modifying the microstructure, one can tailor the guiding properties of the fiber for specific applications and enhance its sensitivity to particular physical quantities.


Figure 1: Composite materials such as fiber reinforced plastics comprise more than 20% of the Airbus 380's airframe.


Figure 2: Ultra-thin integrated photodetector package, part of the miniaturized interrogation system.


Figure 3: SEM image of a cross section of a highly birefringent microstructured optical fiber (VUB design).

Objectives:

During the master thesis, the opportunities of these recent developments within the field of the interrogation technology combined with microstructured optical fiber sensors will be exploited in the field of composite (airframe) materials.

1) The first step is the design, fabrication and verification of a miniaturized optical fiber sensor read-out scheme based on low-cost optoelectronic components. Compatibility with the specialty (microstructured) optical sensing fibers and proper functioning within challenging environmental conditions (temperature variations, high pressure) are important requirements. Emphasis will be on microtechnology and cleanroom manufacturing as well as optical coupling to microstructured optical fibers.

2) Later in the development cycle, these newly developed sensing systems will be combined with composite host materials. This will be done in close collaboration with the research lab Mechanics of Materials and Structures (MMS, http://www.composites.ugent.be). Different surface-mount and embedding technologies will be investigated for this purpose.

Depending on the interest of the master student, the focus of this master thesis can be slightly shifted towards one of the specified topics.

Keywords:

fiber Bragg grating, microstructured optical fiber, miniaturized read-out, composite structures, modeling, fabrication, field tests

Location:

Ardoyen CMST (clean rooms) and at home. Experimental work on microstructured optical fibers in close collaboration with the Brussels Photonics Team (VUB).