Nonlinear propagation and self-switching of ultrashort opticalpulses in fiber nonlinear directional couplers: the normal dispersionregime

The propagation of short optical pulses in a nonlinear directional coupler operating in the normal dispersion regime is investigated. The specific example of a birefringent periodically rocked fiber filter where coupling occurs between the two orthogonally polarized modes that are aligned with the birefringence axes of the fiber is considered. In the simulations, a bell-shaped input pulse is injected into one polarization mode: for different input peak powers, the energy present in the same mode at the output of a coupler, which is one or two coupling lengths long, is calculated. The nonlinear temporal broadening, due to the combined effect of group velocity dispersion and self-phase modulation, sets a fundamental lower limit to the time width of the input pulses. When pulses shorter than this critical value are used, the power-dependent energy switching characteristics of a device one coupling length long gets substantially spoiled. Effects related to the finite spectral bandwidth of the fiber filter are discussed. Numerical simulations are compared to recent experimental results on the self-switching of picosecond pulses in the visible (615 nm) with a birefringent rocking fiber     

Optimization design and realization of Gaussian apodized sample gratings for narrowband wavelength selectivity filters

A sample fiber Bragg grating（SFBG） with Gaussian apodization profile is designed and optimized for the narrowband wavelength filter,and the relevant conclusions provide a guide for the further filter fabrication.With the grating length limited,a narrowband（5 GHz） grating filter used in the radio over fiber（ROF） system is fabricated,which has insertion loss of 3.463 dB,sideband suppression of 24.21dB,3 dB bandwidth of 40 pm,and 20 dB bandwidth of 106.3 pm.     

光纤光栅声光调制特性分析

高频超声波在紫外写入的光纤布拉格光栅(FBG)上传播，产生的长周期超声光栅调制光纤光栅，形成光纤超声超结构光栅。利用模式耦合理论，分析了光纤超声超结构光栅的光谱特性，得出光纤超声超结构光栅的反射谱存在多个反射峰，其反射峰的波长间隔由超声波的频率决定。因此，可以通过改变超声波的频率，调节一阶反射谱的反射波长，从而实现宽调谐范围的快速波长可调滤波器。该滤波器与通常的声光可调滤波器相比，带宽更窄，调谐速度更快。     

Tunable mode converter using electromagnet-induced long-period grating in two-mode fiber

We present an in-fiber mode converter that changes the propagation ratio of two core modes. A dynamic long-period grating is constructed in a two-mode fiber by the combination of an electromagnet with a nonmagnetic spring coiled around a stiff iron rod. The fundamental mode is converted to the LP₁₁ mode according to the voltage applied on the electromagnet. The mode-conversion wavelength can be tuned by stretching the coil spring. Compared to the coupling result with leaky cladding modes, the mode-conversion bandwidth expands from 5 nm to 90 nm and the drive voltage decreases almost by half.     

一种基于三芯光纤的模式复用器/解复用器的设计

提出了一种基于三芯光纤的模式复用器/解复用器,三个纤芯包括两个相同的单模纤芯和一个少模纤芯,在少模纤芯中写入长周期光纤光栅以实现单模纤芯中LP01模与少模纤芯中LP11模间的转换。同时从两个单模纤芯输入LP01模,能激发少模纤芯中两个简并的LP11,a模和LP11,b模。采用光束传播法分析了该模式复用器/解复用器的耦合特性及串扰。数值模拟结果显示,若以少模纤芯中LP11模输出能量大于-1dB的波长范围作为该模式复用器/解复用器的工作带宽,则能够达到23nm,并且少模纤芯中生成的其他模式的输出能量皆比LP11模的输出能量小-21dB以上。该模式复用器/解复用器能够实现对少模光纤的模式复用与解复用,并具有极低的串扰。     

基于双金属结构的光纤光栅温度不敏感滤波器

为提高光纤陀螺宽谱光源的平均波长稳定性,提出了一种用60μm超短光纤光栅制作带宽11.77 nm温度不敏感滤波器的方法。利用金属材料的热膨胀系数差,设计了双金属温度补偿结构,能够随温度的升高/降低对光纤光栅压缩/拉伸,有效地补偿光纤光栅由热光效应引起的波长变化。在30～60℃的温度范围内,光纤光栅的温度灵敏度系数为0.15 pm/℃,较未补偿前降低了60倍以上。该结构具有较好的温度不敏感性,可作为光源滤波器提高光源的平均波长稳定性并有望用于高精度光纤陀螺。     

Theory and simulation of passive modelocking dynamics using a long-period fiber grating

A novel modelocking technique is presented in which the intensity-dependent mode-coupling dynamics of a long-period fiber grating is used to achieve modelocking in a passive optical fiber laser. By an appropriate choice of the grating period, a resonant coupling occurs between co-propagating core and cladding modes, causing the low-intensity wings of the pulse to be transferred to the cladding mode and be attenuated. In contrast, the higher intensity peaks of a pulse are detuned from resonance by the nonlinearity and remain largely unaffected. Numerical studies of this pulse-shaping mechanism show that the laser produces stable mode-locked soliton-like pulses which are limited in bandwidth by the smaller of either the grating transmission bandwidth or amplifier bandwidth.     

窄线宽LD泵浦双包层光纤激光器

报道了LD泵浦的窄线宽双包层光纤（DCF）激光器，从理论和实验数值模拟了激光输出功率对输出镜反射率，光纤长度和吸收泵浦功率的依赖关系，进而进行了实验，实验中选用光纤布拉格光栅（FBG）作为输入腔镜，利用光纤端面菲涅耳反射作为输出腔镜，得到了窄线宽的单模激光输出。最大输出功率421mW，斜率效率78.2%，激光中心波长1086.92nm，谱线宽度0.16nm。     

光纤布拉格光栅耦合器型窄带上下话路滤波器的数值分析

从麦克斯韦方程组出发,通过将光纤波导扰动和光栅折射率调制扰动效应转换成等效的耦合系数,推导出光纤布拉格光栅耦合器的统一特征方程。采用打靶法并结合朗格-库塔(Runge-Kutta)数值积分对两种光纤布拉格光栅耦合器型上下话路滤波器的传输特性进行了详细的分析。结果表明,对于光栅破坏耦合器,利用光栅引入的强色散效应,滤波器的波长选择能力得到极大的提高,低于1 nm的带宽能够满足波分复用(WDM)系统的需求,并且当光栅调制的等效耦合远远超过两光纤消逝场间的耦合效率时,不仅有效地抑制了反射信号,使满足布拉格匹配条件的信号透射输出,还消除了旁瓣纹波;而对于光栅辅助耦合器,其非干涉本质使得器件性能更加稳定,并可通过调整光栅长度进一步改善滤波响应。     

Characteristics of cladding index modulated fiber gratings for ambient refractive index sensing

Characteristics of cladding index modulated fiber gratings for ambient refractive index sensing is studied in this paper. The modes propagation properties and coupling properties in cladding index modulated FBG are studied based on the classical coupling mode theory while considering interactions among multiple modes and developed on a three-layer step-index fiber geometry. Evolution of cladding mode resonance spectra of cladding index modulated FBG with the decreasing of fiber diameter is simulated and compared with the core grating. Sensing characteristics of cladding modes resonance of cladding index modulated FBG to the variation of ambient refractive index is detailed and emphasized for cladding modes resonances has a much narrower bandwidth. Results show that cladding index modulated fiber grating has better spectrum for ambient refractive index sensing applications under the same sensitivity with core index modulated fiber grating and may be applied to many field including biochemical sensing and environmental monitoring.     

基于色散光纤环级联结构的可调谐带通微波光子滤波器

提出并验证了一种宽调谐带宽的带通微波光子滤波器设计方案。该滤波器借助可调谐光纤光栅Sagnac环对宽带光源进行均匀切割,产生波长间隔可调的连续光载波作为滤波器的抽头,结合色散光纤环级联结构,实现滤波器的可重构性。研究结果表明,在光电调制器和光电探测器的频率带宽足够大的情况下,当光纤光栅Sagnac环的臂长差在0.50~8.28mm内变化、可调谐光纤延迟线的最小变化步长为0.01mm时,该方案能够实现滤波器中心频率在8.0506~1333.2000GHz内调谐,调谐步长为161.01MHz,边瓣抑制比达到27dB。     

Optimal design of grating-assisted directional couplers

In this paper, a rigorous leaky mode propagation method has been used to investigate the influence of the grating period and grating index profile on the design of grating-assisted directional couplers (GADC's). A detailed explanation of resonance condition and radiation loss in terms of electromagnetic field contribution in the grating region as a function of the grating period and profile is given. Optimal design parameters hare been found for well-defined structures in order to achieve either minimum coupling length or maximum coupling efficiency. A very fast method to extract the resonance condition in any grating-assisted structure by using a sinusoidal profile is proposed. Numerical results are presented for both moderately and strongly asymmetric structures in terms of normalized propagation constant, mode radiation loss, coupling length and coupling efficiency. Comparisons with grating period and coupling length predictions obtained by other methods are also shown. The rectangular profile with optimized duty cycle has been demonstrated to be the best choice in order to minimize the GADC coupling length     

超宽带低损耗SAW滤波器的优化设计

该文介绍了基于大机电耦合系数乐甫波模式的超宽带低损耗声表滤波器的开发,基底为15°YX-LiNbO3晶体和铜电极。设计采用修正的耦合模模型进行精确仿真以及模拟退火算法进行谐振器参数的优化。通过假指电极宽度/长度加权来抑制横向模式谐振造成的寄生响应,通过在滤波器表面涂覆黏性膜来有效抑制瑞利波模式造成的寄生凹坑,最终制作出一款中心频率约620 MHz,插损0.94 dB,-1 dB相对带宽14.9%,-3 dB相对带宽18.3%,带外抑制大于40 dB性能较优良的器件。     

基于高精细度光纤滤波器的双波长光纤激光器

提出了基于高精细度光纤环形滤波器的双波长窄线宽光纤激光器结构.在单波长光纤激光器的基础上,增加保偏光纤布拉格光栅(PM-FBG)和高精细度的光纤滤波器.其中保偏光纤布拉格光栅作为激光器的波长选择元件,可产生两个波长的激光输出.高精细度的光纤滤波器由两个光耦合器和一段弱抽运的掺铒光纤构成,掺铒光纤产生的增益和光纤时延使滤...     

Helical-grating two-mode fiber spatial-mode coupler

The authors investigate a helical-grating geometry for coupling the spatial modes in a two-mode fiber. The feasibility of making fiber grating filters based on this geometry is investigated. By adding mode strippers at the input and output of a helical-grating spatial-mode coupler, an optical notch filter is demonstrated with 0.3-dB waveguide loss, 24-dB extinction, and 24-nm spectral width. The effect of small ellipticity in the fiber core is also investigated, and it is shown that polarization-insensitive coupling can be achieved in a fiber with a sufficiently circular core     

Single-longitudinal-mode fiber ring laser employing an equivalent phase-shifted fiber Bragg grating

A novel single-longitudinal-mode (SLM) fiber ring laser that incorporates an equivalent phase-shifted fiber Bragg grating acting as an ultra-narrow bandpass filter in the laser cavity is proposed. The equivalent phase-shifted fiber Bragg grating has an ultra-narrow transmission bandwidth which ensures an SLM lasing. Stable SLM operation without mode hopping is demonstrated.     

Analysis of phase-matching conditions in flexural-wave modulatedfiber Bragg grating

Effective refractive indexes of the core mode and various cladding modes of a tapered single-mode step-index fiber were calculated for identifying the cladding modes of phase matching for coupling with the core mode, caused by a prewritten Bragg grating together with an applied flexural wave (microbending) in the tapered region. With these coupling mechanisms, the previously reported reflection wavelength switching could be well interpreted. Between the core mode and a cladding mode, the Bragg grating caused contradirectional coupling and the flexural wave resulted in codirectional coupling of either first- or second-order diffraction. Meanwhile, the period of the acoustic-induced flexural wave was calibrated to be a few hundreds of micrometers. Based on the phase-matching calculations, the relationship between cladding radius and flexural wave period with a chosen wavelength for reflection switching was provided. Such design considerations should be helpful in implementing wavelength switches, based on flexural wave modulation of fiber Bragg grating (FBG), for the applications of wavelength division multiplexing (WDM) fiber communications     

S-band gain flattened distributed fiber Raman amplifier with chirped fiber bragg grating filter

IPTV,video-phone,video-conference,distance learn-ing,distance medical ,e-governmental affairs ,etc .will be-come commonthings in people' s daily life with the fastdevelopment of Internet technology.During this processextra-capacity of fiber communication …     

传输矩阵法求解长短周期级联光纤光栅传输谱

基于光纤光栅的模式耦合理论,用传输矩阵法对长短周期级联光纤光栅的传输谱特性进行分析。先由耦合模方程分别得到长周期光纤光栅(LPFG)、连接光纤段和短周期光纤光栅(FBG)的传输矩阵,然后将它们相乘得到总的传输矩阵,最后再结合边界条件求得长短周期级联光纤光栅(CLBG)的传输谱。进一步对CLBG的传输谱进行了数值模拟,并通过实验进行了验证。实验结果表明,CLBG的传输谱中出现了由纤芯模和包层模耦合形成的两个谐振峰,与模拟计算的结果一致,证明了传输矩阵法求解CLBG传输谱的可行性,为CLBG在多参量传感和测量领域的广泛应用提供了理论支持。     

Electrically controllable long-period liquid crystal fiber gratings

An electrically controllable fiber-optic filter for broad-band rejection has been demonstrated by periodically poling a liquid-crystalline core in a hollow-core fiber by means of an external long-period-combed electrode. The periodically poled liquid-crystalline core in a period of 483 μm couples the fundamental core mode to the leaky cladding modes. A maximum loss dip of approximately 15-nm bandwidth and 6 dB band rejection has been obtained for a nonpolarized light with a combed 250-V external voltage modulation. The experimental result matches well with a numerical expectation, which has been analyzed by the discretized coupled-mode theory for an anisotropic perturbation