Showing papers on "Fiber Bragg grating published in 2001"

Microstructured optical fiber devices.

Abstract: We present several applications of microstructured optical fibers and study their modal characteristics by using Bragg gratings inscribed into photosensitive core regions designed into the air-silica microstructure. The unique characteristics revealed in these studies enable a number of functionalities including tunability and enhanced nonlinearity that provide a platform for fiber device applications. We discuss experimental and numerical tools that allow characterization of the modes of the fibers.

Optical delay lines based on optical filters

Abstract: Optical delay lines have some important applications, notably in optical communication systems and in phased arrays. These devices are based on the concept of optical group delay, which, in turn, can be understood as the property of an optical filter. Optical filters are well-understood devices and, in particular, their dispersive properties determine the group delay response. We review these dispersive properties and point out some of the inherent tradeoffs involved in generating large group delay. Fiber Bragg gratings and recent results on optical all-pass filters are used as examples.

On the synthesis of fiber Bragg gratings by layer peeling

Abstract: Two methods for grating synthesis which have appeared in the literature recently are compared directly. In particular, we point out the similarity between the two; both algorithms are based on propagation of the fields through the structure with simultaneous evaluation of the coupling coefficient according to simple causality arguments (layer-peeling algorithms). The first published method (the discrete layer-peeling algorithm) is reformulated in a simpler, more efficient way, and it is shown that its implementation can be made exact. For mathematical comparison, a derivation of the second method (the continuous layer-peeling algorithm) is presented. The methods are compared both mathematically and numerically. We find that the discrete layer-peeling algorithm is significantly faster and can be more stable than its continuous counterpart, whereas the continuous algorithm offers some advantages in flexibility.

Tilted short-period fibre-Bragg-grating-induced coupling to cladding modes for accurate refractometry

Abstract: We investigate the changes in the transmission spectrum of long period fibre gratings and tilted short-period fibre Bragg gratings versus the refractive index of the surrounding medium. The metrological characteristics of tilted short-period fibre Bragg gratings and an analytical method enabling their potential use in accurate refractometry are discussed.

Tilted short-period fibre-Bragg-grating-induced coupling to cladding modes for accurate refractometry

Abstract: We investigate the changes in the transmission spectrum of long period fibre gratings and tilted short-period fibre Bragg gratings versus the refractive index of the surrounding medium. The metrological characteristics of tilted short-period fibre Bragg gratings and an analytical method enabling their potential use in accurate refractometry are discussed.

Structural health monitoring of innovative bridges in Canada with fiber optic sensors

Abstract: This paper describes the development and application of fiber optic sensors for monitoring bridge structures. Fiber Bragg gratings (FBGs) have been used to measure static and dynamic loads on bridge decks and columns, including composite repairs for rehabilitation purposes. A new long gage concept that permits overall average strains to be measured has also been developed with gage lengths varying from 1-20 m. These gages can be bonded to the concrete structure or imbedded in the composite repair patch. Six projects undertaken by ISIS Canada to incorporate fiber optic sensing to monitor the structural health of bridges in Canada are described. Data will be presented for several bridges that indicate a measure of system reliability over several years in a hostile environment. The benefits of fiber optic sensors will be highlighted.

Corrugated long-period fiber gratings as strain, torsion, and bending sensors

Abstract: We present a novel corrugated long-period fiber grating whose transmission spectra are highly sensitive to the applied tensile strain, torsion, and bending due to the periodical index modulation created and changed by these mechanic forces. The induced index modulation can also be experimentally characterized by using a built-in fiber Bragg grating (FBG). The long period fiber gratings possess the following unique properties when used as sensors. As a tensile strain sensor, its resonance loss varies but resonance wavelength remains stable. As a torsion sensor, the wavelength varies with the applied twist rate. As a bending sensor, the cladding-mode resonance grows with the bending curvature.

Fiber-optic liquid-level sensor using a long-period grating

Abstract: A liquid-level sensor based on the refractive-index sensitivity of long-period fiber-optic gratings is proposed and demonstrated. The form of the transmission spectrum of the long-period grating is dependent on the fraction of the length of the long-period grating that is surrounded by the liquid. The sensor shows a large linear range, with sensitivity of 4.8% change in transmission per millimeter, for a long-period grating with a length of 40 mm and a periodicity of 400 μm.

Multiwavelength fiber laser sources with Bragg-grating sensor multiplexing capability

Abstract: Two erbium-doped fiber ring lasers (EDFRLs) wvith simultaneous emission at four different wavelengths are demonstrated. Both systems employ fiber Bragg gratings (FBGs) to select the operation wavelengths within the ring. The sensing capability of the FBGs has been taken advantage of, allowing for the sources to be used as sensor multiplexing schemes. The first system employs four FBGs in a tree filter topology, achieving four output channels with -5 dBm power each. The second system comprises an in-line filtering topology with active fiber segments within the filter. This second source yields 2-dBm output signals and allows for a higher number of lines to be easily added to the system. A comparison between both topologies is carried out, and their capability for sensor multiplexing is demonstrated.

Sampled fiber Bragg grating for simultaneous refractive-index and temperature measurement.

Abstract: We present a novel scheme for simultaneous measurement of temperature and refractive index by use of a single sampled fiber Bragg grating (SFBG). The intrinsic nature of a SFBG that possesses both fiber Bragg grating and long-period grating spectral responses permits the temperature effect to the measured solely from the former, whereas the refractive-index information is extracted from the latter. Using such a dual-parameter sensor, we successfully demonstrated measurement of the dependence on temperature of the refractive index of an aqueous solution of sucrose.

A comparative study of the performance of seven- and 63-chip optical code-division multiple-access encoders and decoders based on superstructured fiber Bragg gratings

Abstract: We report a range of elementary optical coding and decoding experiments employing superstructured fiber Bragg grating (SSFBG) components: first, we perform a comparative study of the relative merits of bipolar and unipolar coding: decoding schemes and show that the SSFBG approach allows high-quality unipolar and bipolar coding. A performance close to that-theoretically predicted for seven-chip, 160-Gchip/s M-sequence codes is obtained. Second, we report the fabrication and performance of 63-chip, 160-Gchip/s, bipolar Gold sequence grating pairs. These codes are at least eight times longer than those generated by any other scheme based on fiber grating technology so far reported. Last, we describe a range of transmission system experiments for both the seven- and 63-bit bipolar grating pairs. Error-free performance is obtained over transmission distances of /spl sim/25 km of standard fiber. In addition, we have demonstrated error-free performance under multiuser operation (two simultaneous users). Our results highlight the precision and flexibility of our particular grating writing process and show that SSFBG technology represents a promising technology not just for optical code division multiple access (OCDMA) but also for an extended range of other pulse-shaping optical processing applications.

A fibre Bragg grating refractometer

Abstract: An opto-chemical in-fibre Bragg grating (FBG) sensor for refractive index measurement in liquids has been developed using fibre side-polishing technology. At a polished site where the fibre cladding has partly been removed, a FBG is exposed to a liquid analyte via evanescent field interaction of the guided fibre mode. The Bragg wavelength of the FBG is obtained in terms of its dependence on the refractive index of the analyte. Modal and wavelength dependences have been investigated both theoretically and experimentally in order to optimize the structure of the sensor. Using working wavelengths far above the cut-off wavelength results in an enhancement of the sensitivity of the sensor. Measurements with different mode configurations lead to the separation of cross sensitivities. Besides this, a second FBG located in the unpolished part can be used to compensate for temperature effects. Application examples for monitoring fuels of varying quality as well as salt concentrations under deep borehole conditions are presented.

40-nm-wide tunable fiber ring laser with single-mode operation using a highly stretchable FBG

Abstract: We demonstrate a 750-Hz linewidth single-mode erbium-doped fiber (EDF) ring laser with wide tunability using a widely tunable fiber Bragg grating (FBG). The stable single-mode operation is realized by using the FBG as a narrow wavelength-selective element and 4 m of unpumped EDF as a saturable absorber in the cavity. The 40-nm continuous tuning range of 1522-1562 nm is achieved using a highly stretchable FBG that exhibits a filter tuning range of over 52 nm. The grating is prepared with chemically stripped deuterium-loaded fiber to eliminate degrading factors for the grating strength, thereby achieving the wide tunability. The tuning range represents a 3.5-fold increase in wavelength tuning over previous use of FBGs.

Structural Monitoring with Fibre Optic Technology

Abstract: This is an ambitious book aimed at introducing the relatively new concepts and possibilities of optical fibre sensors for structural monitoring to the uninitiated who have an engineering or general physics background. Measures draws the reader into the volume with a description of smart structures - the structural monitoring equivalent of artificial nervous systems - before a series of back-to-basics tutorials on optical theory and photonic technology. The emphasis on smart structures early in the book is a worthy attention-grabber since it elevates the subject of structural health monitoring above just another set of techniques for making engineering measurements. The promise is to `revolutionize engineering design philosophy' by creating `intelligence within otherwise inanimate structures'. In the latter two thirds of the book, the author steps through the main issues of structural monitoring using fibre optic sensors. Intensity-based, interferometric, polarimetric and spectral sensors (including the ubiquitous Bragg grating) are compared and contrasted. The hot topic of strain versus temperature discrimination in fibre sensors earns a whole chapter and several useful techniques for overcoming this cross-sensitivity are portrayed. Installation of sensors is also discussed with reference to retro-fit and co-manufacturing (embedding) approaches. Examples of concrete constructions such as bridges (a frequent theme in the book) and fibre-reinforced plastics such as glass-fibre and carbon composite materials are considered. A chapter on `short-gauge' sensors and applications deals in some depth with the Bragg grating as a strain sensor. The methods of multiplexing and interrogating these devices are explored with many examples from both Measures' own research and the work of other groups worldwide. The Beddington Trail bridge trial in Calgary, one of the first such installations of Bragg gratings, followed by the more ambitious Confederation Bridge, also in Canada, provide concrete examples of the technology's application. The material is marred somewhat by the inferior reproduction of some of the photographs, especially those showing field installations of the optical sensors. Other applications are not neglected. A description of trials aboard a Norwegian Naval vessel with composite hull monitored by Bragg gratings is also given. Interferometric sensors in similar applications trials are also covered in chapters on short and long gauge length devices. Distributed strain and temperature sensing techniques using Fourier transform, low coherence and stimulated backscattering are covered in the penultimate chapter, which draws together distributed measurement at a small physical scale in the form of intra-Bragg grating strain profile measurements (on the scale of millimetres) and measurements over kilometres using stimulated Brillouin scattering. In this reviewer's opinion the book dwells on strain monitoring in civil engineering structures at the expense of a broader scope, which could have included, for example, the detection of impacts or the acoustic emissions from crack propagation and other forms of structural damage. Nevertheless, this volume is an impressive collection of background and examples of real applications in heavyweight engineering. It adds significantly to the claim that fibre optic sensors have at last arrived. Peter Foote

Fabrication and characterization of narrow-band Bragg-reflection filters in silicon-on-insulator ridge waveguides

Abstract: We describe the design, fabrication and measurement of an integrated-optical Bragg grating filter, operating at a freespace wavelength of 1543 nm, based upon a silicon-on-insulator (SOI) ridge waveguide. The measured spectral response for a 4-mm long grating has a bandwidth of 15 GHz (0.12 nm), and shows good agreement with theoretical predictions.

Rectangular pulse generation based on pulse reshaping using a superstructured fiber Bragg grating

Abstract: We present a technique for the shaping of short pulses based on the use of superstructured fiber Bragg gratings (SSFBGs). We apply this technique to demonstrate the generation of 20-ps rectangular pulses by phase and amplitude profiling of 2.5-ps soliton pulses. Numerical calculations validate our experimental findings.

Use of 3000 Bragg Grating Strain Sensors Distributed on Four Eight-Meter Optical Fibers During Static Load Tests of a Composite Structure

Abstract: This paper describes the use of a fiber optic system to measure strain at thousands of locations along optical fibers where weakly reflecting Bragg gratings have been photoetched. The optical fibers were applied to an advanced composite transport wing along with conventional foil strain gages. A comparison of the fiber optic and foil gage systems used for this test will be presented including: a brief description of both strain data systems; a discussion of the process used for installation of the optical fiber; comparative data from the composite wing test; the processes used for the location and display of the high density fiber optic data. Calibration data demonstrating the potential accuracy of the fiber optic system will also be presented. The opportunities for industrial and commercial applications will be discussed. The fiber optic technique is shown to be a valuable augmentation to foil strain gages providing insight to structural behavior previously requiring reliance on modeling.

High-sensitivity pressure sensor using a shielded polymer-coated fiber Bragg grating

Abstract: We demonstrate a novel high-sensitivity pressure sensor, which is based on the use of a fiber Bragg grating (FBG) embedded in a polymer-filled metal cylinder with an opening on one side to enhance the pressure sensitivity. The measured pressure sensitivity of the fractional change in the Bragg wavelength of our experimental sensor is -3.41/spl times/10/sup -3/ MPa/sup -1/ which is approximately 1720 times higher than that can be achieved with a bare FBG. The linearity of our sensor is also good. This sensor should find applications in the area of low-pressure measurement.

Medical instrument for insertion into an examination subject, and medical examination/treatment device employing same

Abstract: In a medical instrument for insertion into an examination subject, as well as a medical examination device or treatment device employing such an instrument, the instrument has an elongated instrument body formed by a number of successively arranged rigid sections, with respective, successive sections being connected to one another via articulated joints which can be angled relative to one another. At least one optical fiber supplied with light is conducted along the instrument body, and at least one fiber Bragg grating is fashioned in the optical fiber in a region adjacent to a joint which is deformed to a degree corresponding to angling of the adjacent sections to the joint, thereby modulating the light in the optical fiber.

Acoustic emission detection using fiber Bragg gratings

Abstract: A systematic study of acoustic emission detection using fiber Bragg grating sensors has been carried out over the last year. In this, we attempt to use the fiber Bragg grating to sense the dynamic strain created by a passing ultrasonic wave signal. Our goal here is to see if such a sensor is possible, and if so, what the detection sensitivity and limitations will be. To answer these questions, we carried out several experiments involving the detection of simulated acoustic emission events. In the first experiment, we attach fiber Bragg grating to the surface of a piezoceramic resonator which is driven by a signal generator. We were able to detect the resulting surface vibration of the resonator up to 2.1 MHz. In the second experiment, we attach a fiber Bragg grating to the surface of an aluminum plate. We excite an acoustic wave using an ultrasonic transducer located at various positions of the aluminum plate. In this way, we demonstrated that the fiber Bragg Grating sensor is capable of picking up the signal coming from a distance (up to 30 cm) for up to 2.5 MHz. In a third experiment, we use the same fiber Bragg grating on aluminum plate set up, but set up an acoustic signal by either a gentle knock on the plate by a pin, or by breaking a pencil lead on the plate. We were able to detection acoustic emission set up by pencil lead breaking up to a frequency of 30 kHz. Higher frequency components were not detected mainly due to the limitation of available electronic equipment at this time (higher frequency band-pass filters and amplifiers. In all the above-mentioned experiments we use a match Bragg grating to demodulate the detected optical signal and use a dual channel scheme for electronic data acquisition and processing (a signal channel and a reference channel).

Embedded optical fiber Bragg grating sensor in a nonuniform strain field: Measurements and simulations

Abstract: This paper investigates the use of embedded optical fiber Bragg gratings to measure strain near a stress concentration within a solid structure. Due to the nature of a stress concentration (i.e., the strong nonuniformity of the strain field), the assumption that the grating spectrum in reflection remains a single peak with a constant bandwidth is not valid. Compact tension specimens including a controlled notch shape are fabricated, and optical fiber Bragg gratings with different gage lengths are embedded near the notch tip. The form of the spectra in transmission varies between gages that are at different distances from the notch tip under given loading conditions. This variation is shown to be due to the difference in the distribution of strain along the gage length. By using the strain field measured using electronic speckle pattern interferometry on the specimen surface and a discretized model of the grating, the spectra in transmission are then calculated analytically. For a known strain distribution, it is then shown that one can determine the magnitude of the applied force on the specimen. Thus, by considering the nonuniformity of the strain field, the optical fiber Bragg gage functions well as an embedded strain gage near the stress concentration.

Interrogation of fiber gratings by use of low-coherence spectral interferometry of noiselike pulses.

Abstract: We demonstrate an innovative method for a real-time interrogation of fiber Bragg gratings based on low-coherence spectral interferometry of noiselike pulses. By analyzing the spectral interference at the output of a Michelson interferometer we obtained the impulse response of the grating with a time resolution of ∼350 fs. Using the Gabor transformation, we could directly detect nonuniform regions inside the grating and could measure the spatial dependence of the resonance wavelength along the grating.

High-temperature sensitivity of long-period gratings in B-Ge codoped fiber

Abstract: We report an investigation of thermal properties of long-period fiber gratings (LPFGs) of various periods fabricated in the conventional B-Ge codoped fiber. It has been found that the temperature sensitivity of the LPFGs produced in the B-Ge fiber can be significantly enhanced as compared with the standard telecom fiber. A total of 27.5-nm spectral shift was achieved from only 10/spl deg/C change in temperature for an LPFG with 240-/spl mu/m period, demonstrating a first ever reported high sensitivity of 2.75 nm//spl deg/C. Such an LPFG may lead to high-efficiency and low-cost thermal/electrical tunable loss filters or sensors with extremely high-temperature resolution. The nonlinear thermal response of the supersensitive LPG was also reported and first explained.

A simple all-optical label detection and swapping technique incorporating a fiber Bragg grating filter

Abstract: This letter proposes and demonstrates a simple all-optical label detection and swapping technique incorporating a fiber Bragg grating (FBG) filter and a double sideband subcarrier multiplexed (DSB-SCM) signal. The optical datagram consisting of a baseband 2.488-Gb/s data payload and a subcarrier multiplexed 622-Mb/s label are optically processed to separate the two components, and then a new label content is encoded by subsequent optical modulation. The optically separated original label is detected by a square law detector without involving any high-frequency subcarrier circuitry. Bit-error-rate measurements confirm a successful and low-penalty label swapping technique, which maintains optical transparency for the data payload.

Thermal tuning of polymer optical fiber Bragg gratings

Abstract: Bragg gratings in polymethyl methacrylate (PMMA)-based polymer fibers were created and were tuned thermally. It is found that the tuning range is more than 18 nm over a temperature variation of 50/spl deg/C. More importantly, no hysteresis effect was observed as the gratings were heated up and cooled down.

Limitations of phase-shift method in measuring dense group delay ripple of fiber Bragg gratings

Abstract: The phase-shift method is an established technique for measuring the group delay of fiber-optic components. In devices, such as chirped fiber Bragg gratings, the group delay exhibits ripple as a function of wavelength. We have analyzed the dependence of the measured ripple amplitude on the modulation frequency and present a physical model, which gives an analytical formula for estimating the measurement error.

Superluminal optical pulse propagation at 1.5 microm in periodic fiber Bragg gratings.

Abstract: We report on the experimental observation of superluminal tunneling of picosecond optical pulses in a periodic fiber Bragg grating. Optical pulses of 380-ps duration, generated by an externally-modulated single-frequency erbium-ytterbium laser operating near $1.5\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ wavelength, were propagated at a group velocity greater than $\ensuremath{\sim}1.97$ times the speed of light in vacuum across a 2-cm long fiber grating. Owing to the very large ratio between the thickness of the barrier (2 cm) and the wavelength of probing optical pulses $(\ensuremath{\sim}1.5 \ensuremath{\mu}\mathrm{m}),$ our experiment allows for the observation of superluminal tunneling in the optical region by direct optoelectronic time-domain measurements.