Search

In this paper we measure for the first time the full-spectral response of a fiber Bragg grating (FBG) sensor subjected to vibration. We consider two cases: with and without an initial spectral distortion due to non-uniform strain along the length of the FBG. Previous work has measured only the dynamic response at a single wavelength which is valid when no spectral distortion is present. FBG sensors are mounted near the notch tip on a double edge notch specimen that is also subjected to harmonic vibration. We measure the full-spectral response of the FBG at 100 kHz applying an interrogator recently developed by the authors. The measurements of the FBG response with an initial spectral distortion clearly show the transient response and are verified through simulation. Finally, we demonstrate that the use of the high-speed, full-spectral interrogator permits the separation of the spectral distortion and the harmonic vibration from the FBG response signal through classical filtering and can therefore be applied to measure non-uniform strain fields in noisy environments.

The push-pull slab-coupled optical sensor measures a localized single electric field component with the advantage of reducing adverse strain effects for sensing in harsh vibrating environments. Its miniature size makes it ideal for compact environments.

This paper presents an overview of non-intrusive electric field sensing. The non-intrusive nature is attained by creating a sensor that is entirely dielectric, has a small cross-sectional area, and has the interrogation electronics a long distance away from the system under test. One non-intrusive electric field sensing technology is the slab coupled optical fiber sensor (SCOS). The SCOS consists of an electro-optic crystal attached to the surface of a D-shaped optical fiber. It is entirely dielectric and has a cross-sectional area down to 0.3mm by 0.3mm. The SCOS device functions as an electric field sensor through use of resonant mode coupling between the crystal waveguide and the core of a D-shaped optical fiber. 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. An electric field changes the refractive index of the slab causing a shift in the spectral position of the resonant modes. This paper describes an overview of the SCOS technology including the theory, fabrication, and operation. The effect of crystal orientation and crystal type are explained with respect to directional sensitivity and frequency response.

Ion traps are widely used in the field of mass spectrometry. These devices use high electric fields to mass-selectively trap, eject, and count the particles of a material, producing a mass spectrum of the given material. Because of their usefulness, technology pushes for smaller, more portable ion traps for field use. Making internal ion trap field measurements not yet feasible because current electric field sensors are often too bulky or their metallic composition perturbs field measurements. Using slab coupled optical sensor (SCOS) technology, we are able to build sensors that are compatible with the spacing constraints of the ion trap. These sensors are created by attaching a nonlinear crystal slab waveguide to an optical fiber. When a laser propagates through the fiber, certain wavelengths of light couple out of the fiber via the crystal and create “resonances” in the output light spectrum. These resonances shift in proportion to a given applied electric field, and by measuring that shift, we can approximate the electric field. Developing a sensor that can effectively characterize the electric fields within an ion trap will greatly assist in ion trap design, fabrication, and troubleshooting techniques.

An efficient full-wave analysis technique for one-dimensional optical domains, known as the recursive Green's function method (RGFM), is presented for evaluation of distributed feedback (DFB) laser cavities with arbitrary material profiles. The method first constructs the Green's function of an inhomogeneous domain and subsequently uses Green's theorem to determine the laser optical field, lasing wavelength, and threshold gain. The technique is applied to investigate the performance of three DFB laser structures: a chirped-grating configuration, a modulated stripe width design, and a reduced duty cycle complex-coupled device. These structures are evaluated in terms of their single-mode lasing behavior and the uniformity of the optical field within the cavity.

We present a new type of fiber Bragg grating (FBG) that can be used in high-temperature sensing applications. We use the flat side of a D-shaped optical fiber as a platform to etch the grating into the surface of the fiber. Because the grating becomes a physical feature of the fiber, it is not erased at high temperatures as are standard FBGs. These surface relief fiber Bragg gratings will operate up to high temperatures. We provide a brief explanation of the fabrication process and present our results for operation up to 1100/spl deg/C.

We present a fiber Bragg grating (FBG) interrogation method using a micro-controller board and optical filter that achieves high strain sensitivity and high dynamic range. This interrogation method allows high sensitivity detection of ultrasonic waves superimposed on low-frequency (on the order of 100Hz) vibrations of arbitrary magnitude. One possible application is in-situ structural health monitoring of windmill blades exposed to strong winds by using FBG sensors for detection of ultrasonic waves. Interrogator operation is based on the edge filtering method using a broadband source, fiber Fabry-Perot filter and a micro-controller board which acts as a control feedback loop that locks the filter wavelength to the mid-reflection point on the FBG spectrum. Wavelength locking method allows high sensitivity for edge filtering of high-frequency waves, while the feedback signal is the measurement of low-frequency vibration with high dynamic range. The concept of the interrogator operation and different implementations are described and discussed with experimental results.

This paper presents improvements to slab-coupled optical fiber sensors (SCOS) for electric-field sensing. The improvements are based on changing the crystal cut and orientation of the slab waveguide in combination with altering the input light polarization. Traditional SCOS are fabricated using z-cut potassium titanyl phosphate crystals and are operated with TM polarized light. They have been shown to detect fields as low as 100 V/m. By using an x-cut crystal and TE polarized light, the sensitivity to electric fields is increased 8x due to, primarily, an increase in electric field penetration into the slab by exploiting a tangential boundary condition, and secondly, an increase to the effective electro-optic coefficient of the slab.

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.

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.© (2012) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

In this study we evaluate the measurements of a fiber Bragg grating (FBG) sensor subjected to a non-uniform static strain state and simultaneously exposed to vibration loading. The full spectral response of the sensor is interrogated in reflection at 100 kHz during two loading cases: with and without an added vibration load spectrum. The static tensile loading is increased between each test, in order to increase the magnitude of the non-uniform strain field applied to the FBG sensor. During steady-state vibration, the behavior of the spectral shape of the FBG reflection varies depending on the extent of non-uniform strain. With high-speed full spectral interrogation, it is potentially possible to separate this vibration-induced spectral change from spectral distortions due to non-uniform strain. Such spectral distortion contains valuable information on the static damage state of the surrounding host material.

This paper presents a high repetition rate fiber Bragg grating (FBG) interrogation system that is able to capture the entire reflection spectrum at a rate of up to 300 kHz. The system uses a high speed MEMS based tunable optical filter that is driven with a sinusoidal voltage. The time varying FBG reflection spectrum in transmitted through the tunable filter. The time varying signal is then mapped into time varying reflection spectra. This interrogation system is used during two dynamic strain tests, in which the reflection spectra are measured at a repetition rate of 100 kHz. The first test is the impact of a woven carbon composite and the second test is on an electromagnetic railgun.

Ultra small magnetic field sensors are created using magneto-optic slab waveguides coupled to optical fiber. The magneto-optic material used is bismuth-doped rare earth iron garnet (Bi:RIG). By etching close to the core of D-type optical fiber and attaching a magneto-optic material, light transfers from the fiber to the slab waveguide at specific wavelengths. The wavelengths that couple depend on the refractive index of the slab that changes in the presence of a magnetic field. When a field is applied, the wavelength coupling shifts and a resulting change in power can be detected. The sensors reported in this paper detect magnetic fields as low as 11 A/m.

Modern electronics are often shielded with metallic packaging to protect them from harmful electromagnetic radiation. In order to determine the effectiveness of the electronic shielding, there is a need to perform non-intrusive measurements of the electric field within the shielding. The requirement to be non-intrusive requires the sensor to be all dielectric and the sensing area needs to be very small. The non-intrusive sensor is attained by coupling a slab of non-linear optical material to the surface of a D shaped optical fiber and is called a slab coupled optical fiber sensor (SCOS). The sensitivity of the SCOS is increased by using an organic electro-optic (EO) polymer.

This paper demonstrates the value of D-type optical fibers (D-fibers) in a variety of sensing applications. The principal advantage of the D-fiber is that it allows for interaction with light traveling in the core of an optical fiber with materials or structures placed in contact with the fiber. This permits stimulus sensitive materials to be placed on the D-fiber to interact with the light in the core of the fiber. The presentation shows that this feature of D-fibers can be used to create alternatives to sensors formed in standard optical fibers for measuring temperature, strain, and shape change. In addition, D-fiber sensors have been fabricated to measure chemical concentrations, and electric fields.

A mathematical model is presented that predicts birefringence changes in an optical fiber as the cladding is removed. This model approximates a highly elliptical fiber core with a rectangular dielectric waveguide. The birefringence calculations obtained with the model compare well with experimental evidenceobtained with real-time birefringence monitoring during cladding removal by chemical etching. The information is used to control the amount of cladding removed from a D fiber to within approximately 0.05 microm for use in the production of passive optical fiber components.

This paper presents a means for the high repetition rate interrogation of fiber Bragg gratings (FBG's). The new system highlights a method that allows a tradeoff between the full spectrum capture rate and the wavelength range and/or the spectral resolution of the technique. Rapid capture of the entire reflection spectrum at high interrogation rates shows important features that are missed when using methods that merely track changes in the peak location of the spectrum. The essential feature of the new system is that it incorporates a MEMs tunable filter driven by a variable frequency openloop sinusoidal source. The paper demonstrates the new system on a laminated composite system under impact loading.

An electric-field sensor is specially fabricated with an optical D-fiber that utilizes weak evanescent coupling with a lithium niobate slab waveguide. Resonant modes with a Q-factor of ~13,000 yield high sensitivity for detecting electric fields.

A fiber-optic electric field sensor was developed to measure electric field up to 18 MV/m. The sensor uses resonant coupling between an optical fiber and a nonlinear electro-optical crystal. The sensing system uses high dielectric strength materials to eliminate dielectric breakdown. A postprocessing nonlinear calibration method is developed that maps voltage change to wavelength shift and then converts the wavelength shift to electric field using the transmission spectrum. The nonlinear calibration method is compared against the linear method with electric field pulses having magnitudes from 1.5 to 18 MV/m.

Surface-relief fiber Bragg gratings exhibit substantially more polarization dependence than standard fiber Bragg gratings. Using D-fiber with different core orientations, surface-relief gratings are analyzed and fabricated to determine the polarization dependence. We show that the largest Bragg reflection occurs for the polarization state with a dominant TE field component parallel to the flat surface of the fiber. The polarization dependence is adjusted by changing the index of refraction of the surrounding media and by fabricating the surface relief grating using rotated core D-fiber.

We use a surface relief fiber Bragg grating with a polydimethylsiloxane layer as a volatile organic compound chemical sensor. Sensitivity of ~4000 ppm is demonstrated of dichloromethane in a gas state.

Aimed at test and evaluation needs on high power microwave (HPM) weapons, we describe new developments on miniature all-dielectric optical field sensors with flat RF sensing response from ~ MHz to 12 GHz, with negligible field perturbation, good sensitivity (~70 mV/(mH√z), and >100dB dynamic range. Present devices use a 20 mm long sensing region in an integrated optical (IO) waveguide Mach-Zehnder interferometer (MZI) using electrooptic (EO) polymer for the waveguide. The fiber-coupled optical transmitter/receiver utilizes common optical communication technology. The incident HPM RF field induces an instantaneous change in the index of refractive of the polymer that is converted into an optical intensity modulation in the MZI device. The poled EO polymer requires no electrodes nor metallic antennas that can distort the field under test. We characterized the frequency response and polarization sensitivity of the field sensor, and both agree well with modeling predictions. Common fabrication limitations result in devices with sensitivity to thermal drift. New sensor designs are being developed with remote bias control that also can provide self-calibration. To further reduce the sensor size and insertion loss, beneficial for array applications, an "in-fiber" field sensor is being developed. The core of a D-shaped fiber is partially removed and replaced with EO polymer. Such a device may use polarization modulation sensing, or be configured in similar MZI structures as the IO waveguide sensors.

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface-relief FBGs are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. We describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors.

A new fiber sensor integrated monitor to be used in an embedded instrumentation system is proposed and its operating features are examined. The system integrates a fiber sensor together with a tunable MEMS filter, superluminescent light emitting diode and microcontroller creating a high-speed, low cost, low power smart sensor. The device has applications to a variety of fiber sensing technologies and, as an example, is integrated with a fiber Bragg grating for temperature sensing.

We use a single surface relief fiber Bragg grating as a bend sensor with two degrees of freedom. By monitoring the Bragg efficiency and Bragg wavelength, an effective multi-axis bend sensor can be realized.

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface relief fiber Bragg gratings (SR-FBGs) are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. In this work we describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors. We show that SR-FBGs resist much higher temperatures than standard FBGs by demonstrating their operation up to 1100 degrees Celsius.

We present a method of replacing a section of the core of a D-shaped optical fiber with sensing materials as a platform for various extrinsic fiber sensors. In this configuration light guides within the sensing material allow for strong interaction between the sensing material and the optical field. Initial experimental results indicate that replacing the fiber core with polymer enhances its temperature sensitivity by at least a factor of 5. The new technique is promising as a means for incorporating various sensing materials into the path of a beam traveling in an optical fiber.

We use the flat side of a D-shaped optical fiber as a platform to etch a fiber Bragg grating (FBG) into the glass surface. We present our results for operation up to 1100 degrees Celsius.

This presentation introduces a technique for replacing the core of an optical with a nonlinear polymeric material. This creates a platform that allows for interaction between light guided by an optical fiber and external influences, including electric fields and other controlled or sensed environmental changes.

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.

The need to miniaturize traditional optical devices and to incorporate them into a fiber environment has led to the study of in-fiber lasers with doped fiber cores and distributed- feedback grating cavities. This paper implements a recursive Green's function approach to model the threshold gain for distributed feedback in-fiber lasers. It is shown that the Dyson equation based model, which was originally applied to diode lasers, can also be used to model distributed feedback fiber lasers. A one-dimensional recursive Green's function process using Dyson's equation is used to obtain threshold gains and resonant frequencies for in-fiber, distributed feedback lasers. The use of Dyson's equation allows exact solutions to arbitrary accuracy for any grating shape. The recursive Green's function process allows rapid calculation of periodic or uniform structures of any size, yet does not easily yield the electric field values inside the grating structure. Also, the recursive method is easily applied to chirped gratings and phase sections between grating pairs, and can be extended to higher orders.

We have designed an input coupler to couple light into the core of a single mode optical fibers without severing the fiber. This design uses two coupling steps. The light is first diffracted into a planar waveguide constructed on the surface of the fiber using diffraction gratings. The light in the planar waveguide is then coupled to the core of the optical fiber through directional mode coupling.

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.

A method for producing a distributed feedback D fiber dye laser is presented. Existing fiber dye lasers are produced by placing a tapered single mode fiber into a dye solution. The evanescent field of a doubled YAG laser beam traveling in the fiber is used to pump the dye into fluorescence. Optical feedback and lasing action are achieved using external mirrors or a resonant fiber ring. In our scheme a segment of D fiber is etched to within a few microns of the fiber core. The etched fiber is placed in a solution of R6G dye. The etching is controlled using an in situ monitoring technique we have developed. An argon ion laser beam is focused into the fiber. Because of the small core size of the D fiber we use, there is sufficient energy in the evanescent field to cause the dye to be fluorescent. Diffraction gratings are produced generated by a holographic technique. We create diffraction gratings with periods of less than .3 (mu) using this technique. A miniature fiber dye laser could provide a compact source for short laser pulses for use in spectroscopy and communications.

The authors use a surface-relief fiber Bragg grating with a polydimethylsiloxane (PDMS) layer as a volatile organic compound chemical sensor. A PDMS layer is used because it is compatible with the optical properties of the grating and exhibits good chemical selectivity. As the analyte is absorbed the refractive index of the PDMS changes, causing the Bragg wavelength to shift, and this shift is correlated to chemical type and concentration. The direction and amount of the Bragg wavelength shift is dependent on the absorbed chemical. The authors demonstrate chemical differentiation between dichloromethane and acetone in gaseous states.

We present a new optical fiber sensor interrogator specifically designed for an embedded instrumentation system. The proposed system consists of a super luminescent diode as a broadband source, a high speed tunable micro-electro-mechanical system (MEMS) filter, photodetector, and an integrated microprocessor for data aggregation, processing, and communication. The entire system is integrated together in a compact package to create a fiber "smart" sensor. The system is capable of interrogating a variety of multiplexed fiber sensors, processing the data, and communicating the results digitally. As an example, the system has been calibrated with an array of fiber Bragg grating sensors.

This paper presents the analysis of a 2 cm long in-fiber polymer waveguide formed on the platform of a D-shaped optical fiber. Numerical simulations provide an understanding of the major loss mechanisms for feasible in-fiber polymer waveguide geometries. The primary loss mechanism is determined to be excitation of slab modes on the flat surface of the fiber with transition geometry being the next major contribution to loss.

We present a method for fabricating an in-fiber electro-optic polymer waveguide within a D-shaped optical fiber. A combined process of selective chemical etching and spin coating creates a 2-cm in-fiber poly(methyl methacrylate)-DR1 dye polymer waveguide section with an overall insertion loss of micro 1.6 dB at 1550 nm. Numerical simulations show that, for in-fiber polymer waveguides to have low loss, the polymer layer's thickness must be kept below a certain value so that it will not support slab waveguide modes. Long transition regions between the unetched fiber and the polymer waveguide section also reduce loss. We analyze the efficiency of an in-fiber polymer waveguide by simulating its theoretical performance as an electro-optic modulator.

A mathematical analysis that predicts the leakage parameters of diffraction gratings based on D-shaped fiber as a function of fiber and grating parameters is presented. The development approximates the elliptical fiber core with a rectangular dielectric waveguide. The effective-index method is used to transform the waveguide into an equivalent slab waveguide, and the leakage parameter of the grating is determined by using an approximate approach. A discussion of the relation between grating characteristics and waveguide parameters is presented.

This paper describes a novel technique for fabricating diffraction grating devices in optical fibers. The diffraction gratings are formed on fibers having a D-shaped cross sectional profile (D-type fibers) produced by Andrew Corporation. It is advantageous to fabricate in-fiber devices since they are smaller than currently available channel waveguide devices, and more compatible with fiber optics technology. The fiber optics communications industry has a pressing need for both passive and active devices which will allow the processing of optical signals without the need of converting them to electrical signals. Among these devices are fiber to fiber couplers, directional couplers, filters, modulators, and wave division multiplexers. Development of a method for fabricating diffraction gratings in optical fibers to perform these functions is an important first step in the creation of such devices.