Modeling and testing of static pressure within an optical fiber cable spool using distributed fiber Bragg gratings

Based on the force analysis, we establish a theoretical model to study the static pressure distribution of the fiber cable spool for the fiber optic guided missile (FOG-M). Simulations indicate that for each fiber layer in the fiber cable spool, the applied static pressure on it asymptotically converges as the number of fiber layers increases. Using the distributed fiber Bragg grating (FBG) sensing technique, the static pressure of fiber cable layers in the spool on the cable winding device was measured. Experiments show that the Bragg wavelength of FBG in every layer varies very quickly at the beginning and then becomes gently as the subsequent fiber cable was twisted onto the spool layer by layer. Theoretical simulations agree qualitatively with experimental results. This technology provides us a real-time method to monitor the pressure within the fiber cable layer during the cable winding process.     

Sampled phase-shift fiber Bragg gratings

A phase-shift fiber Bragg grating (FBG) with sampling is proposed to generate a multi-channel bandpass filter in the background of multi-channel stopbands. The sampled noire fiber gratings are analyzed by Fourier transform theory first, and then simulation and experiment are performed, the results show that transmission peaks are opened in every reflective channel, the spectrum shape of every channel is identical.It can be used to fabricate multi-wavelength distributed feedback (DFB) fiber laser.     

Characteristics of hydrogenated fiber Bragg gratings

By utilizing both hydrogen loading for photosensitivity enhancement and a phase mask for holographic exposure, we have fabricated fiber Bragg grating with controllable reflectance and bandwidth. The evolution of hydrogen diffusion into a single-mode optical fiber before exposure, and out of the fiber after exposure was characterized, and the results were consistent with the theoretical modeling. The shifting of Bragg wavelength and thermal reliability were found related to the hydrogen behavior inside the fiber. One solution to prevent Bragg wavelength from drifting after the grating was formed and hence to increase the grating's reliability was to anneal it so that the residual hydrogen was forced out of the optical fiber in a short time. Received: 12 June 1996/Accepted: 17 June 1996     

高能激光辐照下光纤布拉格光栅响应特性

为探索光纤布拉格光栅在高能激光参数测量方面的应用,通过实验和数值模拟研究分析了其在高能激光辐照下的响应特性。通过解传热方程,获得了光纤布拉格光栅在高能激光辐照下的热响应特性。利用一个光纤光栅解调仪,记录了光纤布拉格光栅在高能激光辐照下的波长漂移现象。实验结果表明：在一定的功率密度范围内,光纤布拉格光栅的最大波长漂移与其所受的辐照功率密度成线性关系,该结果与理论模拟相符。     

Fibonacci quasi-periodic superstructure fiber Bragg gratings

Superstructure fiber Bragg gratings constructed following the Fibonacci sequence are proposed and simulated by using the transfer matrix method. The reflection spectrum has a multi-fractal structure as a function of the wavelength and is symmetric to the center. The reflectivity has self-similarity and scaling with respect to the Fibonacci sequence order at the central wavelength. For the transmission spectrum, the interaction of the core-cladding mode is investigated and the particular wavelengths for which the resonance condition is satisfied are located by varying the grating parameters. The potential application of the quasi-periodic structure as fiber sensors is also proposed.     

Tunable Bragg gratings for fiber lasers

A tuning method of a fiber Bragg grating by the bending compression-tension is considered. With this method, a relative tuning range of the resonance wavelength of the grating of about 5% has been obtained. The changes in the shape of the spectrum and in the grating reflectivity during the tuning process are studied. The reflectivity of the fiber Bragg grating has been found to considerably increase with increasing degree of compression. It is shown that such an increase is caused by compression-induced changes in the refractive index modulation amplitude in the grating. The fiber Bragg grating developed has been applied for tuning the radiation wavelength of an ytterbium fiber laser. The tuning range of the laser has been found to be 45 nm around the central wavelength 1080 nm.     

Quantum photodetachment of hydrogen negative ion in a harmonic potential subjected to static electric field

Photodetachment of negative ions has attracted immense interest owing to its fundamental nature and practical implications with regard to technology. In this study, we explore the quantum dynamics of the photodetachment cross section of negative ion of hydrogen H-in the perturbed one dimensional linear harmonic potential via static electric field. To this end,the quantum formula for total photodetachment cross section of the H-ion is derived by calculating the dipole matrix element in spherical coordinates. In order to obtain the detached electron wave function, we have solved the time-independent Schr¨odinger wave equation for the perturbed Hamiltonian of the harmonic oscillator in momentum representation. To acquire the corresponding normalized final state detached electron wave function in momentum space, we have employed an approach analogous to the WKB(Wenzel–Kramers–Brillouin) approximation. The resulting analytical formula of total photodetachment cross section depicts interesting oscillator structure that varies considerably with incident-photon energy,oscillator potential frequency, and electric field strength as elucidated by the numerical results. The current problem having close analogy with the Stark effect in charged harmonic oscillator may have potential implications in atomic and molecular physics and quantum optics.     

Analysis of sampled fiber Bragg gratings in polarization-maintaining fiber

A novel type of sampled fiber Bragg gratings （FBGs） written in polarization-maintaining fiber （PMF） is proposed. The reflection spectrum, time delay, and group velocity dispersion （GVD） of the gratings are analyzed. In addition, the reflection spectrum is optimized by apodization. The scheme of multi-wavelength output based on the gratings is proposed, which could be used as a multi-wavelength polarization filter in the density wavelength division multiplexed （DWDM） system.     

Numerical reconstruction stability of fiber Bragg gratings

Traditional algorithms for solving the inverse scattering problem for a fiber Bragg grating are described, and a new numerical method for this problem is developed. The method is based on the fast inversion of a matrix using its Toeplitz symmetry and on a special “inner-bordering” procedure. It is shown that the method is equally efficient as the well-known discrete layer peeling method but exceeds the latter in accuracy, especially for high-reflectance gratings. The stability of the proposed method with respect to initial data errors is demonstrated.     

取样布拉格光纤光栅的传输矩阵

根据在一般坐标系下均匀布拉格光纤光栅的传输矩阵,得到了取样布拉格光纤光栅的传输矩阵。利用傅里叶变换得到了取样布拉格光纤光栅的谐振方程。结果表明,在不考虑平均折射率变化的情况下,谐振峰的位置是由光栅的周期和取样周期共同确定的,与取样时的占空比、光栅长度和耦合系数没有关系。类似于物理光栅,取样布拉格光纤光栅也存在缺级现象,给出了出现缺级的条件。     

光纤Bragg光栅的频谱特性

利用数值模拟方法分析了光纤Bragg光栅的结构参量对其频谱特性的影响,研究了旁瓣效应与κL值的关系,通过实验给出了第一、第二旁瓣的反射率与κL值的函数关系.实验测得频谱同理论模拟结果基本吻合,所得结论可为设计与制作高边模抑制比的光纤光栅提供参考依据.     

Switchable multi-wavelength erbium-doped fiber lasers by using cascaded fiber Bragg gratings written in high birefringence fiber

A novel and simple Erbium-doped fiber laser by using cascaded fiber Bragg gratings (FBGs) written in high-birefringence fibers for switchable multi-wavelength operation is proposed. Due to two-peak reflection with orthogonal polarizations of the FBG, the polarization hole-burning effect in the cavity is greatly enhanced. Experimental results show that the stable dual- and three-wavelength lasing operation with very narrow wavelength separation (0.39 nm) can be generated at room temperature. The configuration is simple and flexible. Only by adjusting polarization controllers (PCs), the laser can be switched among the stable single-, dual- and three-wavelength lasing operations.     

Local coupling-coefficient characterization in fiber Bragg gratings

We propose a new method for characterizing the local parameters of fiber Bragg gratings. This method combines measurement of the complex impulse response by optical low-coherence reflectometry and reconstruction of the complex coupling coefficient by layer peeling. Application of the method to a nonhomogeneous grating shows that the local coupling coefficient can be precisely determined with an axial resolution below 20 microm and a maximum error of less than 5% for amplitude and phase, respectively.     

Motion characteristics of Bragg grating solitons in rectangle-apodized fiber Bragg gratings

We introduce both concave and convex rectangular apodizations in the middle of fiber Bragg gratings to achieve slow light. Based on the nonlinear coupled mode equations (NLCMEs), the transmission characteristics of grating solitons in rectangle-apodized gratings are numerically simulated and analyzed. The rectangular apodization can change the grating coupling coefficient to give rise to slow and capture the solitons in gratings. The effects of the soliton energy parameters, the width of rectangular apodization and the variation of the coupling coefficient on the soliton transmission are presented. The results show that, the velocity of solitons can be slowed down, and the capability to capture a soliton depends on the energy of input solitons, coupling coefficient, and the rectangular width. Two kinds of soliton capture methods are proposed and compared with each other.     

Investigation of a multiwavelength raman fiber laser based on few-mode fiber Bragg gratings

We propose and experimentally demonstrate a simple multiwavelength Raman fiber laser based on few-mode fiber Bragg gratings without additional multichannel filters. The multiwavelength Raman laser output has a high extinction ratio of more than 45 dB. The multiwavelength output is so stable that the peak power fluctuation is less than 0.5 dB. The number of lasing wavelengths can be adjusted, corresponding to the properties of few-mode Bragg gratings with multiple resonant wavelengths. By the thermal tuning method, the lasing wavelength of each channel can be effectively controlled, and the laser's tunability is measured to be 10.5 pm/ degrees C.     

Self-similar amplification in fiber Bragg gratings written in fiber amplifiers

We show, by using numerical simulations, that self-similar pulses with a duration on the order of few nanoseconds and an energy on the order of 10?μJ can be obtained at the output of a fiber Bragg grating (FBG) written in a fiber amplifier. The evolution of the amplified pulses is determined by the combined effect of Kerr nonlinearity, normal-dispersion, gain, and gain saturation, which limit the pulse energy. The output pulse mainly depends on the initial pulse energy rather than on the initial pulse profile. The reduced group velocity in FBGs can significantly increase the total gain for a given amplifier length. Hence we find that the proposed amplification scheme can be highly advantageous for amplification of nanosecond-scale pulses in fiber amplifiers.     

Multiwavelength erbium-doped fiber ring lasers with overlap-written fiber Bragg gratings

Eight-wavelength Er-doped fiber lasers with lasing wavelength separations of ~1.6 and ~0.8 nm , respectively, have been achieved by use of overlap-written fiber Bragg gratings (OWFBG's) in the fiber lasers and by cooling of the Er-doped fiber with liquid N(2) . Our experiment shows that by utilizing the OWFBG's to select the lasing wavelengths one can achieve fiber lasers with lasing wavelengths and lasing wavelength separations that match the International Telecommunication Union channel-allocation grid well.     

Strong phase-controlled fiber Bragg gratings for dispersion compensation

Dispersion-compensating fiber Bragg gratings with approximately 99.9% reflectivity that are made by continuous apodization and phase control are demonstrated. These strong dispersion-compensating gratings provide precision second-order, third-order, or even more complex dispersion compensation, as well as sufficient transmission isolation to be used at add-drop stages without additional filtering. A 99.84% grating with a constant approximately 700-ps/nm dispersion and a 99.94% grating with dispersion varying linearly from 1000 to -1000 ps/nm are demonstrated.