Spencer Chadderdon

Full-spectrum interrogation of fiber Bragg gratings at 100 kHz for detection of impact loading

This paper explains key innovations that allow monitoring of detailed spectral features of an FBG in response to impact loading. The new system demonstrates capture of FBG spectral data at rates of 100 kHz. Rapid capture of the entire reflection spectrum at such high reading rates shows important features that are missed when using systems that merely track changes in the peak location of the spectrum. The update rate of 100 kHz allows resolution of features that occur on transient time scales as short as 10 µs. This paper gives a detailed description of the unique features of the apparatus and processes used to capture the data at such a rapid rate. Furthermore, we demonstrate this interrogation scheme on a composite laminate system during impact.

Electric-field sensors utilizing coupling between a D-fiber and an electro-optic polymer slab.

This paper provides a detailed analysis of electric field sensing using a slab-coupled optical fiber sensor (SCOS). This analysis explains that the best material for the slab waveguide is an inorganic material because of the low RF permittivity combined with the high electro-optic coefficient. The paper also describes the fabrication and testing of a SCOS using an AJL chromophore in amorphous polycarbonate. The high uniform polymer slab waveguide is fabricated using a hot embossing process to create a slab with a thickness of 50 μm. The fabricated polymer SCOS was characterized to have a resonance slope of ΔP/Δλ=6.83E5 W/m and a resonance shift of Δλ/E=1.47E-16 m(2)/V.

Wavelength hopping due to spectral distortion in dynamic fiber Bragg grating sensor measurements

We demonstrate the measurement of wavelength hopping in dynamic fiber Bragg grating (FBG) sensor measurements and its effect on the interpretation of the dynamic behavior of a composite laminate. Strain measurements are performed with FBG sensors embedded in laminates, subjected to low-velocity impacts, with data acquired using a commercial peak wavelength following controller and a high-speed full-spectral interrogator recently developed by the authors. The peak follower response is theoretically predicted from the full-spectral interrogator measurements. We demonstrate that dynamic wavelength hopping does occur, that it changes the apparent dynamic behavior of the composite and that it can be directly predicted from the dynamic spectral distortion. We also demonstrate that full-spectral data acquisition at speeds lower than those required to fully resolve the dynamic event creates apparent measurement errors due to wavelength hopping as well.

Multiaxis electric field sensing using slab coupled optical sensors

This paper provides the details of a multiaxis electric field sensor. The sensing element consists of three slab coupled optical-fiber sensors that are combined to allow directional electric field sensing. The packaged three-axis sensor has a small cross-sectional area of 0.5 cm×0.5 cm by using an x-cut crystal. A method is described that uses a sensitivity-matrix approach to map the measurements to field components. The calibration and testing are described, resulting in an average error of 1.5°.

Improvements in electric-field sensor sensitivity by exploiting a tangential field condition

This paper presents improvements to slab-coupled optical fiber sensors for electric-field sensing. The sensors are comprised of a potassium titanyl phosphate (KTP) crystal mounted on a D-fiber. The improvements are based on changing the crystal orientation, which enhances sensitivity due to a combined increase in the effective electro-optic coefficient and electric-field penetration into the KTP crystal. The paper provides a detailed comparison of the improved sensor, which uses x-cut KTP to the previous sensor design using z-cut KTP. The measurements show an 8.6× improvement in the sensitivity.

Ion Trap Electric Field Characterization Using Slab Coupled Optical Fiber Sensors

AbstractThis paper presents a method for characterizing electric field profiles of radio frequency (rf) quadrupole ion trap structures using sensors based on slab coupled optical-fiber sensor (SCOS) technology. The all-dielectric and virtually optical fiber-sized SCOS fits within the compact environment required for ion traps and is able to distinguish electric field orientation and amplitude with minimal perturbation. Measurement of the fields offers insight into the functionality of traps, which may not be obtainable solely by performing simulations. The SCOS accurately mapped the well-known field profiles within a commercially available three-dimensional quadrupole ion trap (Paul trap). The results of this test allowed the SCOS to map the more complicated fields within the coaxial ion trap with a high degree of confidence as to the accuracy of the measurement. Figureᅟ

Single tunable laser interrogation of slab-coupled optical sensors through resonance tuning

This paper describes a method for tuning the resonant wavelengths of slab-coupled optical fiber sensors (SCOSs). This method allows multiple sensors to be interrogated simultaneously with a single tunable laser. The resonances are tuned by rotating a biaxial slab waveguide relative to an optical D-fiber. As the slab waveguide rotates, its effective index of refraction changes causing the coupling wavelengths of the slab waveguide and D-fiber to shift. A SCOS fabricated with potassium titanyl phosphate crystal as the slab waveguide is shown to have resonance tuning ranges of 6.67 and 22.24 nm, respectively, for TM and TE polarized modes.

Increasing dynamic range of a fibre Bragg grating edge-filtering interrogator with a proportional control loop

We present a fibre Bragg grating (FBG) interrogator that uses a microcontroller board and a tunable optical filter in a proportional control loop to increase dynamic range and achieve high strain sensitivity. It is an edge-filtering interrogator with added proportional control loop that locks the operating wavelength to the mid-reflection point on the FBG spectrum. The interrogator separates low-frequency (LF) components of strain and measures them with extended dynamic range, while at the same time measuring high-frequency (HF) strain without loss in strain sensitivity. In this paper, we describe the implementation of the interrogator and analyse the characteristics of individual components, such as the speed and voltage resolution of the microcontroller and the tunable optical filter. We measure the performance of the proportional control loop at frequencies up to 1?kHz and characterize the system using control theory. We illustrate the limitation of the conventional interrogator to measure strains greater than 40 ?? and demonstrate successful application of the proposed interrogator for simultaneous measurement of 450 ?? LF strain at 50?Hz superimposed with 32?kHz HF strain.

Slab coupled optical fiber sensor calibration

This paper presents a method for calibrating slab coupled optical fiber sensors (SCOS). An automated system is presented for selecting the optimal laser wavelength for use in SCOS interrogation. The wavelength calibration technique uses a computer sound card for both the creation of the applied electric field and the signal detection. The method used to determine the ratio between the measured SCOS signal and the applied electric field is also described along with a demonstration of the calibrated SCOS involving measuring the dielectric breakdown of air.

Multi-axis, all-dielectric electric field sensors

This paper presents innovations that reduce the dimensions and interrogation complexity of a previously developed multi-axis electric field sensor. These devices are based on slab coupled optical sensor (SCOS) technology. SCOS are sensitive to electric fields that are parallel to the optic axis of the electro-optic slab. Electric fields are measured in two axes by mounting SCOS devices, which have slabs with optic-axes perpendicular to the fiber (z-cut), orthogonal to each other. In order to reduce dimensions of the sensor, the third-axis is measured by having a slab with the optic-axis parallel to the fiber (x-cut). Since the resonant mode coupling of a SCOS device occurs at specific wavelengths whose spectral locations are determined in part by the effective refractive index of the modes in the slab, rotating a z-cut slab waveguide relative to the optical fiber will cause the spectral position of the resonance modes to shift. This method allows the resonance modes to be tuned to specific wavelengths, enabling a multi-axis SCOS to be interrogated with a single laser source.