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In this paper we report on experiments aimed at the development of a highly responsive sensor based on long period gratings (LPGs) operating at high order cladding modes which exhibit a turn-around-point in their phase matching diagrams. These high order cladding modes are accessed by decreasing the period of the grating within standard optical fiber according to theoretical simulations. The sensors tested detect hydrogen through a Pd coating on the fiber cladding. As expected from simulations these sensors exhibit a near eight fold increase in response to 1% hydrogen when operated near the phase turning point. This increase in response is, however, not selective to hydrogen and a combination of increased thermal noise and broadened resonance condition serves to decrease the signal to noise ratio such that any gain in theoretical sensitivity is counter-acted by a loss in fidelity of the recovered (or recorded) signal. The highest sensitivity (shift of a LPG lossband) recorded was in the order of 470 pm for an exposure to 1% H-2 at 70 degrees C, which represented a shift of similar to 1/150 of the FWHM of the LPG loss band.

In this paper, we describe a technique to characterize long period fiber gratings (LPG) by use of a correlation between the measured transmission spectrum and a theoretical model. The model is robust enough to require only a-priori knowledge of the fiber design and the inscription period. We demonstrate the application of this technique on an example LPG resulting in the calculation of the dispersive core index, length, and strength of the grating to better than experimental error. The technique is further expanded to allow for the modeling of metal jacketed long period gratings by use of an extended version of Erdogan's model.

Smart materials with integrated sensing capabilities are now ubiquitous in many structures and devices manufactured from composite materials and they offer enhanced safety, reliability and efficiency in such smart devices. This paper explores the application of embedded sensors to components manufactured using additive layer manufacturing (ALM) technology. ALM offers the ability to create physical parts with little or no restriction in shape and complexity. In this paper, optical fiber sensors incorporating fiber Bragg gratings are embedded inside a component made by, and during a powder-bed-based, layer-by-layer, additive manufacturing process. A commercial EOS P730 system is used, where a laser is employed to sinter the polymeric powder into a 3D component. The fiber embedding approach is based upon insertion of a “fiber-carrier” component, which replaces a removable “place-holder” component during an interruption of the ALM build process. Tensile test specimens fabricated this way are subjected to extended cyclic tensile loading trials at low strain levels of up to 580 μe . The test specimens demonstrate stable and reproducible responses over a period in excess of 720 days and 311000 load cycles. Polyimide and acrylic jacketed fibers are trialled, and the resulting deformations of the component through internal stresses depending on the fiber jacket type are discussed.

A self-referenced optical fiber refractometer using a ball lens as a sensor head has been developed and characterized. A 350-μm ball lens created at the tip of a single mode fiber has been coated with a 40-nm optically thin layer of palladium that reacts with hydrogen to form a hydride, which has a lower reflectivity than pure palladium. Optical reflectance measurements from the tip of the ball lens were performed to determine the hydrogen response. The change in reflectivity is proportional to the hydrogen concentration in the range 0% to 1% hydrogen in air with a detection limit down to 10 ppm (1σ) in air. This technique offers a simple sensor head arrangement, with a larger sampling area (∼40 times) than a typical single-mode fiber core. A statistical image analysis of a palladium film, with cracks created by accelerated failure, confirms that the anticipated sensor area for a ball lens sensor head has a more predictable reflectivity than that of a bare fiber core.

We investigate the phase matching conditions and sensitivities of higher order metal jacketed long period gratings (LPGs). These higher order modes have been previously demonstrated to have flatter, and therefore more sensitive, phase matching conditions leading up to the phase matching turning point. We demonstrate this increased sensitivity as applied to a Pd jacketed LPG hydrogen sensor illustrating an improvement in both the refractive index and temperature sensitivity (of the 17th order mode) of an order of magnitude over the lower order (1st-9th) modes. © (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Additive layered rapid manufacturing offers the ability to create physical parts with little or no restriction from shape complexity. Modelling and validation of this new technology requires embedded sensors which challenges conventional sensor technology. In this work, fibre Bragg gratings [FBG] have been embedded inside components made from a polymer during the manufacturing process of layer-by-layer additive manufacturing technology by laser sintering of powder layers using a commercial EOS P730 system. The technique is based on the insertion of a fibre carrier component, replacing a removable ‘place holder’ during an interruption of the build process. Tensile test specimens have been subjected to extended cyclic tensile loading trials at low strain levels of up to 400 µ-strain. The test specimen show stable and reproducible response over a period in excess of 330 days and 142000 load cycles. Polyimide and acrylic jacketed fibres have been trialled and the effect of minor deformations of the component through internal stresses depending on the fibre jacket type is discussed.

A multicore fibre (MCF) sensor to measure the radial deformation of a compliant cylinder under compression is presented. The sensor is connectorised and need not be permanently bonded to the test object. A differential measurement technique using FBGs written into the MCF makes the sensor temperature insensitive. FBG measurement of axial strain of a cylinder under compression is also reported.

Two-axis curvature measurement is presented using Bragg gratings in a fourcore optical fiber. Dynamic differential strains between core pairs were measured using arrayed waveguide gratings to track arbitrary free vibration of a cantilever.

Distributed transverse load sensing, along an extended length of polarization-maintaining (PM) fibre, has been demonstrated using high spatial resolution (2 cm) optical frequency domain reflectometry (OFDR) which is based on recording a characteristic Rayleigh scatter 'fingerprint' of a single mode fibre. Application of a transverse load to a PM fibre induces localized differential changes of the refractive index of slow and fast axes, which can be monitored simultaneously and separately. The magnitude of the refractive index variation affects the distributed Rayleigh scattering, permitting the determination of scale and/or direction of load, relative to the PM fibre axes, by cross correlation with a previously recorded 'fingerprint'. Load sensitivity as a function of load direction, relative to the PM fibre axes, and the linearity of induced refractive index changes with a load of up to 16 N mm−1 for the optimum load orientation are presented. The application of this technique towards a load-sensing mat is discussed.

We investigate the optical properties of Pd thin films of the thickness 20-100nm deposited on Si wafer via RF sputter coating. The Pd samples are characterised using white light interfermometry for thickness and ellipsometry for refractive index. We demonstrate the independence of refractive index on film thickness above 20nm. Considerable discrepancy is found between our measurement and previously published complex refractive indices for both bulk and RF sputter coated Pd, indicating a high degree of dependence on deposition technique.