An analytic method for fast design of aperiodic fiber Bragg grating (AFBG) devices by use of Fourier mode coupling (FMC) theory is presented. AFBG devices, which are often used in practice, include the superstructure, phase-shifted FBG, and the apodized FBGs with Gauss, cosine, raised-cosine, Hamming, or Blackman windows, and include the in-fiber Fabry–Pérot cavities formed by two AFBGs. The analytic solutions to the reflectivities of these AFBG devices are obtained on the basis of FMC theory, from which closed-form expressions for the relation between the AFBG structure parameters and their reflection properties are derived. According to these expressions, an analytic algorithm is established to quickly design AFBG devices with the desired reflection properties, which has linear complexity. The design of each AFBG device was achieved in ∼50 ms on a general PC, and the reflection properties of the designed AFBG device were confirmed by using the transfer matrix method. This indicates that the analytic design methods proposed are highly efficient and sufficiently accurate.
A novel bend-insensitive LPFG sensor written by high-frequency CO2 laser pulses is demonstrated, for the first time, to our knowledge. Experiment results shows that the bend-sensitivity of the novel LPFG depends strongly on curved orientation. The resonant wavelength shifted merely —0. 01 8 nm even for a curvature of 1 . 1m-1at the most bend-insensitive position. Such a bend-insensitive sensor could be used to solve the problem of cross-sensitivity between bend and other measurands, such as temperature, strain or refractive index, which is an unsolved problem for LPFG sensors in practice. In addition, the bend sensitivity of the LPFG can be adjusted by selecting its circular positions.
In this paper, a single long-period fiber grating (LPFG) element written by focused high-frequency CO2 laser pulses is demonstrated for simultaneous measurement of transverse load and temperature, for the first time, to our knowledge. Temperature and transverse load can be directly measured by detecting the resonant wavelength shift and the resonant peak amplitude change of the LPFG respectively, as there are two special circular orientations along the fiber axis where the resonant peak amplitude change of the LPFG has a linear relationship with the load applied and is insensitive to the resonant wavelength shift. Such a sensor could solve the cross-sensitivity problem between transverse load and temperature due to the unique feature of this type of LPFG, i.e. the wavelength-load-sensitivity of the LPFG strongly depends on loading orientations due to the non-uniform distribution of the refractive index on the cross-section of the LPFG caused by the thermal shock effect and other effects of the high-frequency CO2 laser pulses exposure method used.
A novel fibre-optic sensing system used for temperature- strain discrimination is presented. This system consists of an extrinsic Fabry-Perot interferometric sensor (EFPI) and a chirped in-fibre Bragg grating (CFBG) in series. The EFPI and the CFBG are wavelength-division multiplexed to provide strain and temperature information, respectively. The wavelength-shift of the CFBG induced by temperature change at the 1.55μm region is interrogated with an intensity- based scheme, allowing fast measurement of temperature. The cavity length change of the EFPI is measured at the 1.3μm region, allowing strain to be measured without cross-talk from the temperature sensor, i.e, the CFBG. Experimental results show that the achieved accuracies for strain and temperature measurement are ±20με and ±2°C, respectively.
Simultaneous strain and temperature measurement for advanced 3D braided composite materials using fiber-optic sensor technology is demonstrated, for the first time. These advanced 3D braided composites can virtually eliminate the most serious problem of delamination for conventional composites. A tandem in-fiber Bragg grating (FBG)/extrinsic Fabry-Perot interferometric sensor (EFPI) system with improved accuracy has been used to facilitate simultaneous temperature and strain measurement in this work. The non- symmetric distortion of the optical spectrum of the FBG, due to combination of the FBG and the EFPI, is observed for the first time. Experimental and theoretical studies indicate that this type of distortion can affect the measurement accuracy seriously and it is mainly caused by the modulation of the periodic output of the EFPI. A simple method has been demonstrated to improve the accuracy for detection of the wavelength-shift of the FBG induced by temperature change. A strain accuracy of ~+/- 20(mu) (epsilon) and a temperature accuracy of ~+/- 1 degree(s)C have been achieved, which can meet the requirements for practical applications of 3D braided composites.
In this paper, we describe low-noise Er-doped optical fiber amplifiers (EDFAs) achieved by exploring novel long period fiber gratings (LPFG) fabrication technique is based on the thermal shock effect of high-frequency CO2 laser pulses at several kHz. By inserting such a LPFG in the Er-doped fiber, the ASE noise of the EDFA has been reduced considerably. Two low-noise EDFAs have been constructed and demonstrated. The ASE noise of the two EDFAs used as a pre- amplifier and an in-line amplifier has been successfully reduced from 4.0dB to approximately 3.5dB and from approximately 4.8dB to approximately 4.3dB with a small- signal gain increase of approximately 7dB for the in-line amplifier, respectively.
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