Pair Of Fiber Bragg Gratings Research Articles

Multiplexing of fiber Bragg grating pairs for sensing based on optical low coherence technology

AbstractA new multiplexing scheme of fiber Bragg gratings (FBGs) sensing system is presented. Each sensing element is a pair of identical FBGs, with one working as sensing FBG and the other working as a reference. Multiplexing of the FBG pairs is achieved based on optical low coherence technology. A measurement of strain is demonstrated to verify good performance of the system. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 2437–2439, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23628

A Simple Passive Arc-Shape Temperature-Independent Load Sensor Using a Pair of Fiber Bragg Gratings

In this paper, a novel arc-shape FBG (Fiber Bragg Grating) load sensor is developed with a pair of FBGs embedded symmetrically in an elastic bending beam. The FBG pair is produced with peak wavelength separation of about 0.4 nm. The FBG with shorter wavelength is located above the neutral layer and another with longer wavelength below the neutral layer. As a result, the two peak wavelengths of the FBG pair move apart with lateral force applied but move together in the same direction when surrounding temperature varies, both in a linear manner. A very low temperature-dependency of only 0.04 pm/°C across the temperature range from 35°C to 82°C has been achieved, which makes it suitable for dynamic lateral force measurement even when the environmental temperature varies. Due to the fact that the sensor module consists of only the grating fiber with two FBGs and some elastic material, which are purely passive components, hence, the temperature effect of this sensor can be self-compensated without a need of any active temperature controller or other temperature-isolation mechanism, thus in principle, this sensor module is passively temperature-independent.

Coherence Multiplexing of Distributed Sensors Based on Pairs of Fiber Bragg Gratings of Low Reflectivity

A coherence multiplexing scheme for distributed sensors based on fiber Bragg grating (FBG) pairs is introduced. Each pair of identical FBGs forms a Fabry-Perot (FP) interferometer (FPI) and induces an additional optical path difference (OPD) that is proportional to the center-to-center interval between the two FBGs (one for sensing and the other for reference). The interference intensity reaches its maximum when the FPI-induced OPD is compensated by scanning one arm of a Michelson interferometer to a certain position. The variation of the measurand induces a mismatch between the central reflection wavelengths of two FBGs and consequently reduces interference intensity. To separate the interferometric signals for demultiplexing, the intervals between the two FBGs are preset to different values for different sensors. In order to improve the multiplexing ability of the system and reduce the crosstalk among the sensors, we use gratings of low reflectivity. Temperature sensing is demonstrated to show the high sensitivity (-1.92%/degC) and low crosstalk of our distributed sensing system.

A Micro-Vibration Measurement Method Based on Isostrain Cantilever and Cascade Self-Demodulated FBG Pair

A micro-vibration measurement method is proposed based on isostrain cantilever structure and cascade self-demodulated fiber Bragg grating (FBG) pair. The micro-vibration is measured with a cantilever structure which can induce a differential shift of FBG pair's Bragg wavelengths. The cross-sensitivity to temperature can be eliminated simultaneously. Both having the capability of self-demodulation, the topology of cascade FBG pair structure has an equivalent measure range, a simpler structure, and a better resolution capability than that based on a twice reflection FBG pair structure. With a preliminary experimental setup a high resolution better than 0.0003 mm can be achieved.

New sensing mechanisms using an optical time domain reflectometry with fiber Bragg gratings

A new optical time domain reflectometry (OTDR) sensing mechanism combined with fiber Bragg gratings (FBGs) is presented. The intensity-based optical fiber sensor is implemented between a pair of FBGs in an OTDR system. Based on the ratio from two reflecting intensities of the reference and the sensing FBGs, the appropriate measurable parameters associated with the intensity variations can be detected. By applying the Fresnel reflection signals caused by FBGs as the measuring indicator, the dynamic range and signal-to-noise ratio of an OTDR FBGs system are improved in comparison to the traditional method. Besides, by adapting the system using a double-light-source scheme, the spatial resolution limited by the ghost zone in a traditional OTDR system is also resolved. The sensors in an OTDR FBGs system could be configured for multiplexing and immunity to the variations from the light source and the lead-in/out fibers caused by the external loads. The new method using single-light-source and double-light-source schemes has been validated by fiber bending loss and refractive index measurements.

Wavelength Division Multiplexed Fiber Bragg Grating Vibration Sensor Array with Temperature Compensation

We have developed a novel method of wavelength division multiplexed operation of thermally stabilized fiber Bragg grating (FBG) vibration sensors in that simultaneous multipoint vibration detection with a temperature-insensitive operation can be achieved. One of the sensing elements of the arrayed sensors consists of a pair of FBGs, one is used for providing a narrow-spectrum light from a broad-spectrum incoherent optical source and the other is used for vibration sensing. The paired FBG configuration enables us to compensate the temperature effects that are commonly applied to the FBGs used. In the experiment, two sensing elements are arranged in tandem and the detection of simultaneous two-point multiple vibrations and the measurement of an elastic wave propagating in a metal bar are demonstrated.

Effect of cavity structure on the spectrum of the supercontinuum generated in a linear cavity pumped by a continuous-wave Raman fiber laser

With a 10-km single-mode fiber and a pair of fiber Bragg gratings (FBGs), we design four different linear cavity structures for supercontinuum (SC) generation using a cw Raman fiber laser (RFL) as pump. The output characteristics of the generated SC in these different constructs are compared when the RFL reaches its maximum output power. The broadest SC spectrum of 100 nm and the highest output power of 1.1 W are obtained when the nested cavity structure is employed. The influences of the feedback FBGs and cavity constructs on the SC generation are analyzed by considering the Raman effect and four-wave mixing.

Stable photonic millimeter-wave generation using an integrated reciprocating optical modulator

We investigated stability and phase noise of V-band millimeter-wave generated by using a hybrid integrated reciprocating optical modulator consisting of a pair of fiber Bragg gratings and a LiNbO/sub 3/ optical phase modulator. The amplitude fluctuation of generated millimeter-wave was less than 1% without using feedback stabilization technique. Phase noise of the millimeter-wave was -74.3 dBc/Hz at 10-kHz offset.

Experimental optimization of high power Raman fiber lasers at 1495 nm using phosphosilicate fibers

We report in this paper on the experimental optimization of high power Raman fiber lasers at the second Stokes order of a phosphosilicate (P-doped) fiber doped with 13 mol% P 2O 5. The lasers were pumped with a 20 W Yb double clad fiber laser at 1070.75 nm and the cavity used two pairs of fiber Bragg gratings (FBGs). One pair is highly reflective at 1248 nm to fully cascade the pump wavelength to the first Stokes order, and the other includes a partial reflective (PR) FBG at 1495 nm to couple the laser out of the cavity. A total of 20 laser configurations were constructed and tested, consisting of four PR FBGs couplers with reflection from 4% to 45% and five fiber lengths from 200 to 1000 m. It is found that the optimal configuration is that with a fiber length of 300 m and a 33% PR FBG. The maximum power obtained from this laser is 6.66 W at 1495 nm with a threshold of ∼1 W and a slope efficiency of 39.18%. The highest slope efficiency achieved with pump power up to 18 W is 55.6% in the shortest fiber laser of 200 m without any PR FBG but a cleaved end. It is observed that the input–output characteristics are almost linear for all the configurations with feedback of PR FBGs ⩾15%, indicating the stimulated scattering to higher Stokes orders is negligible. Laser line width is also observed to become broader with increase in pump power, fiber length and the reflection of the PR FBGs.

Integrated Reciprocating Optical Modulator for Optical High-Order Sideband Generation

We demonstrated optical high-order sideband generation using a reciprocating optical modulator consisting of a pair of fiber Bragg gratings and an optical phase modulator. The reflection band of the fiber Bragg grating had a steep edge of 0.02 nm. A twelfth optical high-order sideband component was generated from a 4.4 GHz electric signal. Millimeter waves generated as beat signals of the optical high-order sideband components were quite stable. Spectrum line widths were less than 300 Hz. The intensity modulation of the generated millimeter waves was also demonstrated by applying a baseband signal to the modulator without using another electric or optical modulator.

Performance of optical automatic gain control EDFA with dual-oscillating control lasers

The authors present a simple configuration for dual-oscillating lasers gain control erbium-doped fiber amplifier (EDFA) in which dual-oscillating lasers are formed by two pairs of fiber Bragg gratings (FBGs). Experimental results show that dual-oscillating lasers dramatically reduce steady and transient gain variations caused by spectral hole burning and relaxation oscillations, and the dual-oscillating lasers gain controlled EDFAs operate satisfactorily with channel’s add/drop frequency up to 43 kHz which is significantly larger than that with single-oscillating laser control. Furthermore, we present detail performance of the gain control EDFAs with different dual-oscillating control lasers. Experimental results show that the function of different dual-oscillating lasers is similar. Thus dual-oscillating lasers can be chosen with large spectral space in order not to affect the signals in optical communication window.

Fiber-Bragg-grating pair interferometer sensor with improved multiplexing capacity and high resolution

A new technique for the monitoring and multiplexing of fiber-Bragg-grating (FBG) sensors is described. The sensing elements - low-finesse Fabry-Perot interferometers formed with FBG pairs - are reflectively monitored with a linearly chirped Er:fiber laser. This reflected signal is convolved with a reference waveform which varies at a frequency proportional to the optical length of each sensor to 1) obtain a coarse measurement of FBG reflectance peak frequency and 2) refine this measurement using the phase of the convolved signal. Multiplexed sensors of different lengths are distinguished even though the reflectance spectra for the sensors may overlap or even coincide. A spectral resolution of 0.045 GHz (0.36 pm), corresponding to a temperature resolution of 0.035/spl deg/C, was achieved.

Monitoring fiber Bragg grating pair interferometer sensor with a modulated diode laser

A new technique of monitoring a fiber Bragg grating pair interferometer (FBGPI) sensor with a modulated diode laser is demonstrated. The sensing element includes a pair of fiber Bragg grating (FBG) sections separated with a short fiber cavity, so that the reflectance spectrum is a superposition of a gradually changing reflectance envelope over a sinusoidal modulation. Modulating the drive current of a diode laser produces laser chirping to scan over ∼2.6 fringes of the FBGPI reflectance spectrum. The spectral position of the sensor is precisely determined from the phase of the scanned interference pattern. It is an absolute measurement because the fringe order numbers of the interference pattern can be determined according to the average reflectance changes. Temperature measurements are performed over the range from 24 to 367 °C with a resolution of 0.004 °C.

High‐resolution fiber Bragg grating sensor system using linear‐cavity fiber laser scheme

AbstractA high‐resolution fiber Bragg grating (FBG) sensor system using a linear‐cavity fiber laser scheme is proposed and numerically studied. The proposed sensor system can improve the sensing accuracy by using a discriminating FBG pair in conjunction with a fiber laser scheme. This FBG sensor system can resolve the temperature up to 0.0005°C theoretically and can be applied for strain sensing and temperature sensing with high precision. © 2002 Wiley Periodicals, Inc. Microwave Opt Technol Lett 34: 323–325, 2002; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.10450

Self-referenced intensity-based fiber optic sensor system using fiber Bragg gratings

We present a method for measuring the intensity variation regarding the measured physical attributes of intensity-based fiber Bragg grating sensors. These sensors can be configured for multiplexing, thermal insensitivity, and immunity to noise from the light source and/or the lead-in/out fibers caused by the external load. To achieve these goals, the intensity-based optical fiber sensor is implemented between a pair of fiber Bragg gratings. Based on the ratio or dBm difference from the two reflecting intensities of the reference and the sensing fiber Bragg gratings, the appropriate measurable parameters associated with the intensity variations can be detected. Different from transmission type intensity-based sensors, a reflective type based on a pair of fiber Bragg gratings is introduced. To validate the proposed method, a transducer made by using an etched-cladding fiber is deposited onto the metal thin film for sensing metal corrosion. Based on the evanescent field of a fiber and a pair of fiber Bragg gratings, multiple embedded corrosion sensors could be designed for civil structural health monitoring. Also, the proposed method could be applied to a biosensing design on a transducer.

Adaptive dispersion slope equalizer using a nonlinearly chirped fiber Bragg grating pair with a novel dispersion detection technique

We realized the first adaptive-dispersion equalizer that equalizes dispersion over a wide wavelength range (6 nm) in the zero-dispersion wavelength region of a dispersion-shifted fiber (DSF). The equalizer is based on a pair of nonlinearly chirped fiber Bragg gratings, which are designed to equalize exactly the dispersion profile of a DSF. The dispersion changes were tracked using a technique that employs opposite dispersion fibers to identify the sign of the dispersion change. Unlike previous approaches, no additional sources or changes in the source wavelength are required. We demonstrate the adaptive equalization of the dispersion changes in an 83-km DSF, induced by temperature changes between -10/spl deg/C and 60/spl deg/C.

Multipoint temperature-independent fiber-Bragg-grating strain-sensing system employing an optical-power-detection scheme.

A temperature-independent fiber-Bragg-grating strains-sensing system, based on a novel optical-power-detection scheme, is developed and analyzed. In this system a pair of fiber Bragg gratings with reflection spectra either partially or substantially overlapping is placed side by side to form a temperature-independent strain-sensor unit. Conventional wavelength-interrogation techniques are not used here, and instead an optical-power-detection scheme is proposed to directly calibrate the measurand, i.e., the strain. Unlike the conventional approach in a multiplexed sensing system, the presented power-detection-based interrogation method does not need the fiber-Bragg-grating sensors to be spectrally separate. The only requirement is that the spectra of the two fiber Bragg gratings of each sensor unit in a multiplexed system be identical or slightly separate (slightly overlapping spectra would also work in the sensing scheme) and the source's optical power be sufficient for sensitive measurement. Based on a three-sensor-unit system, we demonstrate simple strain measurements of high linearity (+/- 0.4%), good sensitivity [2 microstrains (microS)], high thermal stability (+/- 0.8%), and zero cross talk. The effects of light source spectral flatness and fiber bending loss on measurement accuracy are also discussed.

A wavelength-tunable dispersion equalizer using a nonlinearly chirped fiber Bragg grating pair mounted on multilayer piezoelectric transducers

We realized a wavelength-tunable dispersion equalizer using a pair of nonlinearly chirped fiber Bragg gratings mounted on multilayer piezoelectric transducers. The operation region can be continuously tuned up to 7 nm by applying 50 V. This device enables us to equalize the second- and third-order dispersion of dispersion-shifted fibers with different zero dispersion wavelengths over 6 mn. This performance shows that our equalizer can be employed for adaptive equalization.

Pressure vessel wall thinning detection using multiple pairs of fiber Bragg gratings for unbalanced strain measurements

This paper proposes a method for using a pair of identical fiber Bragg gratings to measure the unbalanced strain factor in an isothermal-mechanical system. Cascading two identical fiber Bragg gratings, the unbalanced strain factor caused by the disturbance can be interrogated from the peak difference of the two reflecting Bragg wavelengths. If the mechanical system is in an isothermal environment, the thermal variations in the Bragg grating detection system can be automatically compensated. To verify the feasibility of the unbalanced strain detection using a pair of fiber Bragg gratings, the wall-thinning monitor in a pressure vessel system was investigated. The proposed technique is easy to install, immune to electro-magnetic and thermal interference, and highly sensitive in-time to remote monitoring for fiber optic smart structures. A finite element model (FEM) is used to simulate the wall thinning in a pressure vessel system. The minimum detectable unbalanced strain and cross-talk between two reflecting Bragg wavelengths were investigated.

Photonic signal processing of microwave signals using an active-fiber Bragg-grating-pair structure

A new active photonic signal processor which achieves a high-Q microwave bandpass response is presented. It comprises active fiber within a pair of fiber Bragg gratings, and produces multiple taps with precise delay-time characteristics. The impulse response has demonstrated well in excess of 270 taps. The filter response demonstrates high resolution, having a narrow-band response with a Q of 325. The processor is also tunable, in both passband width and frequency.