Comparison of different polymers and printing technologies for realizing flexible optical waveguide Bragg grating strain sensor foils

Waveguides with Bragg gratings realized on a flat polymer foil are promising candidates for advanced strain sensors since such a planar approach allows precise positioning of multiple sensors in various well-defined directions, in the same foil. As such, an optical version of an electrical strain gage can be realized. Herein, several parameters are discussed which define the behaviour of such sensor foils, in particular the grating design, including the wavelength of operation and mechanical and optical properties of the used polymers. Epoxy and Ormocer®-based Bragg grating sensors operating at 850 nm and 1550 nm wavelength were realized using nano-imprint lithography and laser direct-write lithography and their strain and temperature sensitivities were compared. Finally, it is demonstrated that optical strain gage rosettes can be realized by multiplexing 3 angularly displaced sensors in the same waveguide on a single foil

Thin and flexible polymer photonic sensor foils for monitoring composite structures

Thin and flexible photonic sensor foils are proposed, fabricated and tested as a promising alternative for monitoring composite structures. Sensor foils are implemented using two different optical polymers and as such optimized for multi-axial sensing and embedding within composite materials respectively. It is first shown that those sensor foils allow multi-axial strain sensing by multiplexing a multitude of Bragg grating sensors in a rosette configuration. Secondly, those sensors can be realized in very thin foils (down to 50 μm) making them suitable for embedding in composite materials during their production. This was proven by visually inspecting and by testing the functionality of the embedded sensors. Finally, owing to their low Young’s modulus and flexibility, polymer sensor foils can be bent to small curvature radii and withstand large elongations. Herein, the sensors are bent down to a radius of 11 mm, and elongated by 1.4% without losing functionality

Bragg-grating-based photonic strain and temperature sensor foils realized using imprinting and operating at very near infrared wavelengths

Thin and flexible sensor foils are very suitable for unobtrusive integration with mechanical structures and allow monitoring for example strain and temperature while minimally interfering with the operation of those structures. Electrical strain gages have long been used for this purpose, but optical strain sensors based on Bragg gratings are gaining importance because of their improved accuracy, insusceptibility to electromagnetic interference, and multiplexing capability, thereby drastically reducing the amount of interconnection cables required. This paper reports on thin polymer sensor foils that can be used as photonic strain gage or temperature sensors, using several Bragg grating sensors multiplexed in a single polymer waveguide. Compared to commercially available optical fibers with Bragg grating sensors, our planar approach allows fabricating multiple, closely spaced sensors in well-defined directions in the same plane realizing photonic strain gage rosettes. While most of the reported Bragg grating sensors operate around a wavelength of 1550 nm, the sensors in the current paper operate around a wavelength of 850 nm, where the material losses are the lowest. This was accomplished by imprinting gratings with pitches 280 nm, 285 nm, and 290 nm at the core-cladding interface of an imprinted single mode waveguide with cross-sectional dimensions 3 × 3 µm2. We show that it is possible to realize high-quality imprinted single mode waveguides, with gratings, having only a very thin residual layer which is important to limit bend losses or cross-talk with neighboring waveguides. The strain and temperature sensitivity of the Bragg grating sensors was found to be 0.85 pm/µε and −150 pm/°C, respectively. These values correspond well with those of previously reported sensors based on the same materials but operating around 1550 nm, taking into account that sensitivity scales with the wavelength

Flexible thin polymer waveguide Bragg grating sensor foils for strain sensing

This paper demonstrates that epoxy-based single mode polymer waveguides with Bragg gratings can be realized in very thin (down to 50 micron) polymer foils which are suitable for strain sensing when integrated inside glass fiber reinforced polymer composite materials. The single mode waveguides were fabricated using laser direct-write lithography and the gratings were realized using nanoimprint lithography. These steps were performed on a temporary rigid carrier substrate and afterwards the functional layers were released yielding the thin, flexible sensor foils which can be laser-cut to the required dimensions. The Bragg grating-based polymer waveguide sensor foils were characterized before and after embedding into the composite. As expected, there was a blue shift in the reflection spectrum because of residual strain due to the embedding process. However, the quality of the signal did not degrade after embedding, both for 50 and 100 micron thick sensor foils. Finally, the sensitivity to strain of the embedded sensors was determined using a tensile test and found to be about 1 pm / microstrain

Characterization of the modal parameters of composite laminates using innovative ultrathin polymer waveguide sensor foils

The use of composite materials, like glass- and carbon- fiber reinforced polymers, is expected to increase exponentially in the coming years. Consequently, in order to monitor the structural health of these materials, the development of new sensing devices is rapidly accelerating. For this purpose, our research groups have recently developed new ultra-thin polymer waveguide sensors which can be exploited to measure both uniaxial and multiaxial strains occurring in composite components. These sensing foils are manufactured by creating Bragg gratings in waveguides realized in flat polymeric substrates, which makes their placement and alignment easier compared to traditional fiber optic sensors. Moreover, using a non-straight waveguide it is possible to spatially multiplex the sensing gratings in such a way that an optical strain rosette can be created. This paper investigates the suitability of the proposed polymer waveguide sensors for the estimation of the modal parameters of composite components

Multipurpose Polymer Bragg Grating-Based Optomechanical Sensor Pad

Flexible epoxy waveguide Bragg gratings are fabricated on a low-modulus TPX™ polymethylpentene polyolefin substrate for an easy to manufacture and low-cost optomechanical sensor pad providing exceedingly multipurpose application potentials. Rectangular EpoCore negative resist strip waveguides are formed employing standard UV mask lithography. Highly persistent Bragg gratings are inscribed directly into the channel waveguides by permanently modifying the local refractive indices through a well-defined KrF excimer laser irradiated +1/-1 order phase mask. The reproducible and vastly versatile sensing capabilities of this easy-to-apply optomechanical sensor pad are demonstrated in the form of an optical pickup for acoustic instruments, a broadband optical accelerometer, and a biomedical vital sign sensor monitoring both respiration and pulse at the same time

In-situ deformation monitoring of aerospace qualified composites with embedded improved draw tower fibre Bragg gratings

Aerospace certified fibre reinforced plastics (FRPs) are extreme performing construction materials, which today are increasingly applied in primary structures of the new generation aircrafts (e.g. Boeing 787, Airbus 350, Bombardier C-Series), such as the fuselage, the wings and the fin. An interesting aspect on the technological point of view of sensing is that airplane manufacturers such as Airbus and Boeing are looking at incorporating health-monitoring systems (such as optical fibre sensors, especially fibre Bragg gratings) that will allow the airplane to self-monitor and report maintenance requirements to ground-based computer systems. However, one has to realize that the mechanical behaviour of anisotropic FRPs is significantly different compared to conventional isotropic construction materials. In this dissertation, the author focuses on monitoring the strain and (permanent) deformation in carbon reinforced plastic laminates with embedded fibre Bragg gratings. The research is divided in two main parts. In the first part of this research, the existing fibre draw tower technology is utilized, to manufacture an improved version of the existing in-line high quality, draw tower fibre Bragg gratings (DTG®s). With respect to accurate measurements and structural integrity, the research focuses on reducing the total diameter of the optical fibre, so the incorporation in the reinforcement fibres is enhanced and the distortion in the composite is reduced. The author elaborates in detail the methods of strain and temperature calibrations and the different setups which are applied. Additionally, with respect to the high temperatures during the composite manufacturing process, the thermal stability of the DTG®s is studied at elevated temperatures (>300°C). In the second part, the author embeds the DTG®s in specific types of thermoset and thermoplastic carbon reinforced plastic laminates. The author applies the embedded DTG®s in several stages of the composite lifetime. Starting with the monitoring of the composite manufacturing process and ending with fatigue testing until failure of the composite laminates. During the different experiments, the sensors are subjected to high temperatures, high pressures, extreme longitudinal strains and transverse strains and in the mean time, they are employed to very accurately measure (multi-axial) strains inside composites at microstrain level (~10 6)