Showing papers on "Fiber Bragg grating published in 2002"

Optical fiber long-period gratings with Langmuir–Blodgett thin-film overlays

Abstract: An overlay material was deposited by the Langmuir–Blodgett technique onto a single-mode optical fiber containing a long-period grating. The long-period grating exhibits characteristic attenuation bands in its transmission spectrum whose central wavelengths were observed to depend on the optical thickness of the overlay material, even for materials that have a refractive index higher than that of silica.

Relative humidity sensor with optical fiber Bragg gratings

Abstract: A novel concept for an intrinsic relative humidity (RH) sensor that uses polyimide-recoated fiber Bragg gratings is presented. Tests in a controlled environment indicate that the sensor has a linear, reversible, and accurate response behavior at 10-90% RH and at 13-60 degrees C. The RH and temperature sensitivities were measured as a function of coating thickness, and the thermal and hygroscopic expansion coefficients of the polyimide coating were determined.

Switchable multiwavelength erbium-doped fiber laser with cascaded fiber grating cavities

Abstract: We propose a new concept to produce switchable multiwavelength oscillations with a cascaded fiber Bragg grating overlapping cavity erbium-doped fiber laser. The laser can be designed to operate in multiwavelength or in wavelength switching modes. The different lines have different thresholds. For three-wavelength oscillation, the output powers are linear or piecewise linear functions of the input pump power. An output with a variation of only 1.5 dB has been achieved with an input pump power of 100 mW. For wavelength switching, three single-wavelength and two dual-wavelength operation regions have been obtained toy adjusting the input pump levels.

Delamination detection in CFRP laminates with embedded small-diameter fiber Bragg grating sensors

Abstract: Newly developed small-diameter fiber Bragg grating (FBG) sensors were applied for the detection of the delamination in carbon fiber reinforced plastic (CFRP) cross-ply laminates Since the diameter of the FBG sensor was very small, the sensor was easily embedded into the 0° ply along the reinforcing carbon fibers The reflection spectra from the FBG sensor were measured at various delamination lengths through four-point bending test The form of the spectrum changed sensitively, as the delamination length increased Moreover, the spectrum was calculated theoretically for confirmation of the measured spectrum The calculated result reproduced the change in the measured spectrum very well Then, we proposed the intensity ratio in the spectrum as an effective indicator for the prediction of the delamination length This indicator has a potential to be applied to other laminate configurations

Bandwidth control of long-period grating-based mode converters in few-mode fibers.

Abstract: Control of the group-velocity differences between two distinct modes in a few-mode fiber can be used to define the spectral characteristics of long-period gratings written in them. Using this effect, we report the demonstration of strong mode conversion (>99%) with long-period fiber gratings over what is believed to be a record bandwidth of 63 nm. These novel spectra are obtained from gratings written in specially designed few-mode fibers in which the grating phase-matching condition is satisfied over a large spectral range. We show that the bandwidths of such mode converters can be tailored by suitably altering the design of the few-mode fibers. The polarization-dependent coupling for the mode converters varies by less than 0.004% over the entire spectrum.

High-power cladding-pumped Tm-doped silica fiber laser with wavelength tuning from 1860 to 2090 nm

Abstract: A high-power double-clad Tm-doped silica fiber laser, pumped by two beam-shaped and polarization-coupled diode bars at 787 nm, was wavelength tuned by use of an external cavity containing a diffraction grating. The Tm fiber laser produced a maximum output power of 7 W at 1940 nm for 40 W of incident diode power and was tuned over a wavelength range of 230 nm from 1860 to 2090 nm, with >5-W output power over the range 1870-2040 nm. The prospects for further improvement in performance and extension of the tuning range are discussed.

Fiber-loop ring-down spectroscopy

Abstract: Pulsed, visible and near-infrared laser light is coupled into an optical fiber, which is wound into a loop using a fiber splice connector. The light pulses traveling through the fiber-loop are detected using a photomultiplier detector. It is found that once the light is coupled into the fiber it experiences very little loss and the light pulses do a large number of round trips before their intensity is below the detection threshold. Measurements of the loss-per-pass and of the ring-down time allow for characterization of the different loss mechanisms of the light pulses in the fiber and splice connector. This method resembles “cavity ring-down absorption spectroscopy” and is well suited to characterize low-loss processes in fiber optic transmission independent from power fluctuations of the light source. It is demonstrated that by measuring the ring-down times one can accurately determine the absolute transmission of an optical fiber and of the fiber connector. In addition it is demonstrated that the tech...

Real-time cure monitoring of smart composite materials using extrinsic Fabry-Perot interferometer and fiber Bragg grating sensors

Abstract: Real-time cure monitoring of composite materials is very important to improve the performance of advanced composite materials. It is very difficult to monitor the cure process online using conventional methods. Fiber optic sensors in smart composite materials provide a unique opportunity to monitor the cure process of composite materials in real time by using embedded sensors. In this paper, extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors are embedded in carbon/epoxy composite laminates and used to monitor the cure process simultaneously. Furthermore, measurements of residual strains of composite laminates during the cure have been performed. The results show that both EFPI and FBG sensors can be used to monitor the strain development of composite laminates with and without damage during cure. An excellent correlation between the EFPI and FBG sensors is presented.

Fabrication of directly UV-written channel waveguides with simultaneously defined integral Bragg gratings

Abstract: A new technique for UV direct writing of Bragg gratings in planar silica is presented. In this method the Bragg gratings and the channels are defined simultaneously, conferring advantages in flexibility of design and grating performance. Photosensitive Germanium doped silica-on-silicon produced by Flame Hydrolysis Deposition was used.

Fiber optic Bragg grating sensor based on hydrogels for measuring salinity

Abstract: We report a new type of optical salinity sensor with a fiber Bragg grating (FBG) coated with hydrogels. The sensing mechanism in this device is based on mechanical stress that is induced in the chemically sensitive water swellable polymers (hydrogels) coating when the water escapes from it. The stress in the hydrogels coating stretches and shifts the Bragg wavelength of the FBG. Varying the composition of the gel, the sensor can be used to response to different trigger stimulus. Here the sensors for measuring salinity are demonstrated.

Dependence of temperature and strain coefficients on fiber grating type and its application to simultaneous temperature and strain measurement

Abstract: We report an investigation of the dependence of the temperature and strain coefficients on the grating type for fiber Bragg gratings that are UV inscribed in B/Ge-codoped fiber with and without hydrogenation. The results reveal that all types of grating exhibit similar strain sensitivities but markedly different temperature sensitivities, greater for gratings inscribed in hydrogen-free rather than hydrogenated fiber and substantially less in type IA gratings than all others. The sensitivity characteristics of these gratings have been used to implement a new type of dual-grating sensor for simultaneous measurement of temperature and strain that has properties superior to those of previously reported structures.

Compact diode-pumped passively Q-switched tunable Er-Yb double-clad fiber laser.

Abstract: Highly-efficient repetitive passive Q-switching of a cladding-pumped Er-Yb fiber laser has been demonstrated by employing an external cavity configuration containing a Co2+:ZnS crystal as saturable absorber. Energies up to 60µJ in pulses of duration as short as 3.5ns (FWHM), corresponding to a peak power >10kW, have been generated, and the maximum slope efficiency with respect to absorbed pump power was 13%. Using a bulk diffraction grating in Littrow configuration to provide wavelength selective feedback, the passively Q-switched fiber laser was tuned over 31nm from 1532nm to 1563nm. The prospects for further improvement in performance will be discussed.

Cavity-ring-down principle for fiber-optic resonators: experimental realization of bending loss and evanescent-field sensing.

Abstract: A novel measurement principle for fiber-optic sensing is presented. Use of a cavity-ring-down scheme enables measurements of minute optical losses in high-finesse fiber-optic cavities. The loss may be induced by evanescent-field absorption, fiber bending, fiber degradation, Bragg gratings, or any other effect that might change the fiber transmission or cavity reflector properties. The principle is proved to be rather insensitive to ambient perturbations such as temperature changes. A high-sensitivity measurement of loss due to bending is presented as a proof-of-principle. With a cavity finesse of 627 a sensitivity for induced loss of 108 ppm (4.68 × 10-4 dB) is achieved. Preliminary measurements of evanescent-field absorption are also discussed.

Tunable microfluidic optical fiber

Abstract: We describe a class of active, tunable optical fiber that incorporates multiple microfluidic plugs into interior fiber microchannels. The propagation characteristics of certain optical modes of these fiber waveguides can be usefully manipulated by controlling the positions and optical properties of these plugs, using actuators and pumps formed on the fiber surface. These hybrid microfluidic/silica structures offer versatile tuning capabilities in a format that preserves the advantages of conventional, passive optical fiber. As an example, we report an all-fiber, narrowband filter, whose transmission wavelength and attenuation are independently adjustable via microfluidic tuning.

Effect of thermal residual stress on the reflection spectrum from fiber Bragg grating sensors embedded in CFRP laminates

Abstract: When FBG sensors are embedded in CFRP laminates, the reflection spectrum from the FBG sensors splits into two peaks because of the non-axisymmetric thermal residual stress. This deformation of the spectrum will lead to misreading in strain measurements or crack detection in the laminates. In the present research, three types of FBG sensors: uncoated normal, polyimide-coated normal, and polyimide-coated small-diameter FBG sensors, were embedded in CFRP cross-ply laminates, and reflection spectra from the sensors were measured during the fabrication process of the laminates. Through the comparison of results obtained for the three FBG sensors, it was found that the effect of thermal residual stress on the reflection spectrum could be decreased when the optical fiber was coated with polyimide and its diameter was small in the present laminate configuration and embedment position. Furthermore, these changes of the spectra during the curing process could be simulated by theoretical calculation considering the birefringence effect.

Characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors

Abstract: The characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors are discussed. This sensor is able to detect macro bending with the transmitted power variation of the first side mode in the blazed fiber Bragg grating. Since an incident ray experiences different variations of tilt angles with respect to bending direction, the blazed fiber Bragg grating has different coupling efficiencies of the first side mode, which can be reduced considerably in the case of twisted blazed fiber Bragg gratings.

Discrete gap solitons in modulated waveguide arrays.

Abstract: We suggest a novel concept of diffraction management in waveguide arrays and predict the existence of discrete gap solitons that possess the properties of both conventional discrete and Bragg grating solitons. We demonstrate that one can control both the soliton velocity and the propagation direction by varying the input light intensity.

High-energy femtosecond stretched-pulse fiber laser with a nonlinear optical loop mirror.

Abstract: A stretched-pulse fiber laser with a nonlinear optical loop mirror (NOLM) that produces 100-fs pulses with 1-nJ energy is demonstrated. These results constitute a 30-fold increase in pulse energy over previously reported femtosecond fiber lasers with a NOLM. Compared with previous stretched-pulse lasers, this laser offers a cleaner spectrum and improved stability, with comparable pulse duration and energy. Implications for the construction of truly environmentally stable lasers are discussed.

Demonstration of a four-channel WDM/OCDMA system using 255-chip 320-Gchip/s quarternary phase coding gratings

Abstract: In this letter, we report the fabrication and application of 255-chip 320 Gchip/s quaternary phase superstructure fiber Bragg gratings (SSFBGs) for optical code generation and recognition in a four-channel wavelength-division-multiplexing (WDM)/optical code-division-multiplexing (OCDM) experiment. Individual users of the system operate with different coding schemes, repetition rates, and wavelengths. Our experiments show that a single SSFBG can be used to perform simultaneous optical decoding and wavelength channel selection.

Cavity-enhanced spectroscopy in optical fibers

Abstract: Cavity-enhanced methods have been extended to fiber optics by use of fiber Bragg gratings (FBGs) as reflectors. High-finesse fiber cavities were fabricated from FBGs made in both germanium/boron-co-doped photosensitive fiber and hydrogen-loaded Corning SMF-28 fiber. Optical losses in these cavities were determined from the measured Fabry-Perot transmission spectra and cavity ring-down spectroscopy. For a 10-m-long single-mode fiber cavity, ring-down times in excess of 2 ms were observed at 1563.6 nm, and individual laser pulses were resolved. An evanescent-wave access block was produced within a fiber cavity, and an enhanced sensitivity to optical loss was observed as the external medium's refractive index was altered.

Continuous-wave fiber optical parametric oscillator

Abstract: We report continuous-wave operation of singly resonant fiber optical parametric oscillators. In a cavity formed by 100 m of highly nonlinear fiber and two fiber Bragg gratings, the pump power threshold was 240 mW; the output wavelength could be tuned over 80 nm by tuning of the pump. We also obtained an internal conversion efficiency of 30%, compared with the maximum theoretical value of 50%, by use of a 1-km-long cavity.

New approach to robust optics for HEL systems

Abstract: A new material for optics is being developed that promises to be far more robust than alternative materials. It is a photo-thermo-refractive (PTR) glass in which Bragg gratings (holograms) can be written in the interior (not the surface) of the glass. The gratings are permanent as they are not removed by illuminating them with light at other wavelengths or by heating unless the temperature exceeds 400 degree(s)C. This technology can be used to make diffractive elements such as spatial filters, attenuators, switches, modulators, beam splitters, beam samplers, beam deflectors, selectors of particular wavelengths (notch filters, add/drop elements), spectral shape formers (gain equalizers), spectral sensors, angular sensors, Bragg spectrometers, and transverse and longitudinal mode selectors in a laser resonator. The PTR Bragg grating has been exposed to a 100 W, 1096 nm beam focused to 100 kW/cm2 spot for 10 minutes without exhibiting any temperature rise. The pulsed laser damage threshold has been measured to be within 30% of that of the best silica glass used in high power 1064 nm systems. The useful spectral range of this glass is from 350 nm to 2.8 microns.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Simultaneous measurement of strain and temperature using a fiber Bragg grating and a thermochromic material

Abstract: A new scheme for simultaneous measurement of temperature and strain using fiber Bragg grating (FBG) and a thermochromic material is presented. It consists of a dual system with two different sensing elements included in the same fiber: a fiber Bragg grating to measure the strain and a thermochromic effect based optical fiber sensor to measure the temperature variations and to compensate the cross sensitivity of the fiber Bragg grating with temperature. Experimental results of the optical dual system are shown and discussed.

Surface-emission laser diode operable in the wavelength band of 1.1-1.7 micrometers and optical telecommunication system using such a laser diode

Abstract: A surface-emission laser diode includes a distributed Bragg reflector tuned to wavelength of 1.1 μm or longer, wherein the distributed Bragg reflector includes an alternate repetition of a low-refractive index layer and a high-refractive index layer, with a heterospike buffer layer having an intermediate refractive index interposed therebetween with a thickness in the range of 5-50 nm.

Temperature monitoring of nuclear reactor cores with multiplexed fiber Bragg grating sensors

Abstract: In-core temperature measurement is a critical issue for the safe operation of nuclear reactors. Classical thermocouples require shielded connections and are known to drift under high neutron fluence. As an alternative, we propose to take advantage of the multiplexing ca- pabilities of fiber Bragg grating (FBG) temperature sensors. Our experi- ments show that sensitivity to radiation depends on both the radiation field and the grating characteristics. For some FBGs installed in an air- cooled graphite-moderated nuclear reactor the difference between the measurements and the readings of calibrated backup thermocouples was within the measurement uncertainty. In the worst case, the differ- ence saturated after 30 h of reactor operation at about 5°C. To reach megagray per hour level gamma-dose rates and 10 19 neutron/cm 2 flu- ences, we irradiated multiplexed FBG sensors in a material testing nuclear reactor. At room temperature, FBG temperature sensors can sur- vive in such radiation conditions, but at 90°C a severe degradation is observed. We evidence the possibility to use FBG sensing technology for in-core monitoring of nuclear reactors with specific care under well- specified conditions. © 2002 Society of Photo-Optical Instrumentation Engineers.

L-band multiwavelength fiber laser using an elliptical fiber

Abstract: In this letter, we propose and demonstrate a simple and novel technique to get multiwavelength operation in the L-band by using an elliptical core erbium-doped fiber. The principle of operation is based on the anisotropic gain effect in a polarization-maintaining fiber. Stable multiwavelength operation is achieved at room temperatures. The lasing lines may be controlled by a polarization controller.

A holey fiber-based nonlinear thresholding device for optical CDMA receiver performance enhancement

Abstract: We demonstrate the use of an optical thresholder based on a short length of holey fiber to achieve enhanced code recognition quality in a 255-chip 320-Gchip/s superstructured fiber Bragg grating-based optical code-division multiple access code:decode system. The nonlinear thresholder is based on bandpass filtering of spectrally broadened components generated by self-phase modulation and assisted by the Raman effect in an 8.7 m length of highly nonlinear holey fiber. Error free penalty free system performance is obtained with complete recovery of the original input pulse shape.