Non-minimum phase reconstruction from amplitude data in fiber Bragg gratings using an adaptive simulated annealing algorithm

Adaptive simulated annealing algorithm used for phase reconstruction in fiber Bragg grating (FBG) is presented. Theory and numerical simulations show that the non-minimum phase of the FBG can be reconstructed by using this method.     

Numerical approach for designing a Bragg grating acousto-optic modulator using the finite element and the transfer matrix methods

The finite element and transfer matrix methods are applied in the design of a Bragg grating acousto-optic modulator. For simulation purposes, the device is taken as a single structure, composed of the silica horn and the fiber Bragg grating. The approach allows the strain field to be completely characterized along the whole structure and leads to a better understanding of the influence of the horn dimensions on the design and performance of the modulator. Results obtained using the two methods show an excellent agreement with experimental data in similar structures of the same dimensions.     

Hetero-core fiber Bragg grating for simultaneous measurement of strain and bending based on cladding mode ratiometric detection

A simple but practical method using multimode-single mode fiber Bragg grating (FBG) for cladding mode based simultaneous measurement of strain and bending is proposed and investigated experimentally. Experimental results show that the intensity ratio between cladding mode and FBG resonance of hetero-core FBG show monotonous changes following with the increase of deflection, by which strain and bending discrimination can be achieved by measuring the wavelength shift of FBG resonance and ratiometrically detecting the intensity changes between cladding modes and FBG resonance.     

一种新型的利用重构等效啁啾超结构光纤光栅消啁啾技术研究

提出了一种利用重构等效啁啾超结构光纤光栅对啁啾光脉冲进行频域消啁啾和时域脉宽压缩的方法.由于重构等效啁啾技术可实现任意物理可实现滤波特性的光纤光栅,因此所提出的新型消啁啾方法可以针对任意啁啾模型的脉冲.仿真结果表明,对于脉宽为20 ps,啁啾系数为-5,啁啾模型为线性、高斯型、洛仑兹型的啁啾高斯脉冲,其被消啁啾后时间带宽积分别由初始的225,265,250下降到0458,0708,0731,脉宽压缩效果明显.针对商业软件给出的增益开关分布反馈半导体激光器输出光脉冲的模型,实际制作相应的重构等效 关键词： 重构等效啁啾 光纤光栅 啁啾 增益开关分布反馈半导体激光器     

Temperature-insensitive transverse load sensing with improved accuracy using stress induced birefringence effects of fiber Bragg grating

Different from the basic sensing theory of the fiber Bragg grating (FBG) sensors based on direct spectral analysis, a new method exploiting the polarization properties of FBG for temperature-insensitive transverse load sensing is proposed. In this paper, the birefringence effects on FBG under transverse load are analyzed. The evolution of the polarization dependent loss (PDL) of FBG with wavelength for the transmitted signals with respect to the transverse load is also studied theoretically and experimentally. We proposed utilizing the PDL evolution of FBG to precisely recover the wavelength spacing between the two peaks of x-mode and y-mode for accurate transverse load measurement. Good agreements between experimental results and numerical simulations had been obtained.     

Low-threshold performance of fiber Bragg grating external-cavity semiconductor lasers using genetic algorithms

The lowest threshold current of the external-cavity semiconductor laser with fiber Bragg grating using genetic algorithms is investigated. Effects of the external cavity length, coupling efficiency and anti-reflection coating reflectivity on static characteristics, such as L-I curves and side mode suppression ratios (SMSR) are investigated by using multi-mode rate equation. It is found that the reflectivity of the anti-reflection coating for low-threshold performance needs to be increased at the cost of the stability of the fiber grating external-cavity semiconductor laser. The optimal external cavity length obtained by genetic algorithms can be used to obtain the lowest threshold current for sacrificing a little SMSR. However, with the decrease of AR-coating reflectivity, output powers and SMSRs are slightly dependent on the external cavity length.     

Synthesis of chirped apodized fiber Bragg grating parameters using Direct Tabu Search algorithm: Application to the determination of thermo-optic and thermal expansion coefficients

In this paper, Direct Tabu Search (DTS) is proposed to synthesize the physical parameters of a fiber Bragg grating (FBG) numerically from its reflection response. A reflected spectrum is being calculated by using the Transfer Matrix Method (TMM). Direct search based strategies are used to direct a tabu search. These strategies are based on a new pattern search procedure called Adaptive Pattern Search (APS). In addition, the well-known Nelder-Mead (NME) algorithm is used as a local search method at the final stage of the optimization process. Direct Tabu Search (DTS) is applied for reconstruction of a raised cosine chirped fiber Bragg grating (CFBG) and a Gaussian multi channel fiber grating. The method is then used to synthesize a CFBG from its reflectivity taken at different temperatures. It gives a good estimate of the thermal expansion coefficient and the thermo-optic coefficient of the fiber.     

Characteristics of strain transfer and the reflected spectrum of a metal-coated fiber Bragg grating sensor

Previous researchers have simulated strain transfer and spectrum of normal fiber Bragg grating (FBG) sensors with a polymer coating bonded on the structure. They only considered the shear stress in a polymer coating for the simulation. However, for metal-coated FBG sensors, not only shear stress but also axial stress in the metal coating should be reflected into the calculation because its axial stiffness is no longer negligible. Thus, the author investigated the strain transfer and reflected spectra of metal-coated FBG sensors by considering both shear stress and axial stress. The strain transfer analysis involved evaluating the strain profiles along the sensor by plotting an analytical solution, and validating the evaluated profiles with the results obtained by a finite element analysis (FEA). The solution was also verified by the experiments that used aluminum-coated FBG sensors bonded on a carbon fiber reinforced polymer (CFRP) composite specimen. A transfer-matrix (T-matrix) formulation and coupled mode theory were used to simulate the reflected spectra of metal-coated FBG sensors for the evaluated strain profile. In addition, the effect of mechanical and geometric parameters of the sensor was examined. The findings revealed that the strain transfer characteristics and reflected spectra deteriorated with increases in the thickness and Young's modulus of the metal coating due to the consideration of axial stress. It is the opposite results for the normal FBG sensor with a polymer coating. Furthermore, the results also indicated that the decrease in bonding thickness resulted in improved strain transfer and signal characteristics. Moreover, a bonding length of 14 mm was suitable in suppressing an asymmetric shape of the reflected spectrum and in achieving an accurate measurement. The results of the parametric study are expected to contribute to improve the measurement accuracy of metal-coated FBG sensors in actual applications. The analytical methodology can be usefully employed in the design of a metal-coated FBG sensor system.     

The L-band EDFA of high clamped gain and low noise figure implemented using fiber Bragg grating and double-pass method

The L-band erbium-doped fiber amplifier (EDFA) of low noise figure and high clamped-gain using gain-clamped and double-pass configuration is presented in this paper. A total of five different configurations of EDFAs by reflection scheme with single forward pumping schemes are examined and compared here. Among these configurations, we first find the configuration of 1480-nm pumped L-band EDFA with optimum gain and noise figure value. To further minimize the gain variation, a fiber Bragg grating (FBG) with 1615-nm center wavelength and 1-nm bandwidth is determined and added in double-pass L-band EDFA. The gain variation and maximum noise figure of EDFA while channel dropping is investigated. As the number of channel dropping from 32 to 4, the L-band type-A EDFA keep the variation of gain within 2.9 dB and the maximum noise figure below 5 dB with each channel’s input power of −23 dBm.