Corrugated long-period fiber gratings as strain, torsion, andbending sensors

We present a novel corrugated long-period fiber grating whose transmission spectra are highly sensitive to the applied tensile strain, torsion, and bending due to the periodical index modulation created and changed by these mechanic forces. The induced index modulation can also be experimentally characterized by using a built-in fiber Bragg grating (FBG). The long period fiber gratings possess the following unique properties when used as sensors. As a tensile strain sensor, its resonance loss varies but resonance wavelength remains stable. As a torsion sensor, the wavelength varies with the applied twist rate. As a bending sensor, the cladding-mode resonance grows with the bending curvature     

导模共振光栅参量对共振波长和线宽的影响研究

为了能够设计出具有反射功能的导模共振光栅，采用光栅的等效介质理论、平面波导理论以及严格耦合波法，进行了理论分析和实验验证，设计了在TE偏振下波长850nm处具有反射共振的导模共振光栅。利用严格耦合波法，计算并分析了光栅参量、入射角以及波导层厚度对共振波长和线宽的影响。结果表明，随着占空比的增大，共振波长会红移，而共振线宽会随着占空比的增大先增后减，占空比为0.5时线宽能达到最宽；共振波长会随着光栅周期和波导层厚度的增大而增大，但线宽几乎不变，当周期从490nm增加到520nm时，共振波长红移了将近50nm，而当波导层厚度从217nm增加到251nm时，共振波长红移了将近25nm；光栅厚度变化对共振波长和共振线宽影响很微弱，当入射角是垂直入射时仅有一个共振峰，但是当入射角不为0°时会出现两个共振峰，并且两个共振波长随着入射角度的变大一个会蓝移而另一个则红移。该研究为实际制备反射导模共振光栅提供了理论指导。     

光子晶体光纤布拉格光栅谐振特性的研究

利用模式理论和传输矩阵法,对基于包层空气孔排布为正八边形对称结构的光子晶体光纤光栅(PCFG)的模式截止特性以及谐振特性进行了研究,得出了此种光子晶体光纤光栅的模式截止条件和可传导的模式,并利用得到的模式截止条件,提出利用相位匹配条件设计相应的光纤光栅周期来得到满足需要的谐振波长,同时给出了谐振峰的反射谱图。研究表明,通过合理选择此种光子晶体光纤光栅的结构参数,可以实现需要的单模传输以及定制波长处的主谐振峰的谐振。所研究的基于包层空气孔排布为正八边形对称结构的锗掺杂光子晶体光纤光栅有望更好地应用于光子晶体光纤光栅传感领域。     

多相移和级联光纤光栅透射光谱特性研究

基于耦合模理论,利用传输矩阵法,系统地分析了多相移和级联相移光纤光栅的透射谱特点.结果表明,相移光纤光栅可在透射谱阻带中打开一个或多个透射窗口,多个相移时,透射窗口数等于相移的个数.改变相移量的大小可以改变输出光波长,而且随着相移量的增加,多相移和级联情况下,透射窗口向长波方向移动.光栅的级联使得其在布喇格衍射波长处,打开一个极窄的透射窗口,透射率与级联个数有关,当级联数为4时,透射率最高.     

Theoretical investigation of 2D periodic nanoplasmon structures

The problem of diffraction of electromagnetic waves by 2D periodic metal gratings is solved with allowance for the finite permittivity of a metal in the optical band. The developed mathematical model is based on the solution of the vector integro-differential equation of diffraction by 3D dielectric bodies by means of the Galerkin method. It is noted that the dependence of the scattered field amplitude on the wavelength has a resonance character and that the resonance wavelengths can substantially exceed the dimensions of a grating cell. The application of the method of approximate boundary conditions for the calculation of gratings containing nanodimensional metal layers is justified. It is demonstrated that a grating with small reflection and transmission factors under the plasmon-resonance conditions can be created.     

光栅参数对折射率剖面非均匀取样光纤光栅的影响

利用传输矩阵法,数值分析了光栅参数对折射率剖面非均匀的均匀光纤光栅及取样光纤光栅的透射谱、偏振关系损耗和偏振模色散的影响。计算结果表明,均匀光纤光栅的光栅条纹可见度及光栅周期对其偏振关系损耗和偏振模色散影响很大;改变取样光纤光栅的取样周期及光栅段长度,透射谐振峰的幅度、偏振关系损耗和偏振模色散的最大值均随取样光纤光栅长度的增加而递增。与靠近中心波长的谐振峰相比,折射率剖面非均匀导致的双折射对取样光纤光栅远离中心波长的谐振峰的影响较小。     

Modes in s.a.w. grating resonators

It is shown that s.a.w. gratings on the surface of an isotropic medium may support modes that are confined underneath the grating. The 3-dimensional resonant-mode patterns are derived for the Rayleigh wave in a high-Q-factor resonator consisting of two gratings separated by ?/2, where ? is the grating periodicity, in the limit when the grating width is large compared with a wavelength. The resonance frequencies of the different modes are presented.     

Compensation of dispersion in optical fibers for the 1.3-1.6 μmregion with a grating and telescope

The transmission of ultrashort optical pulses over long distances in optical fibers is limited by pulse broadening due to group velocity dispersion. A grating and telescope dispersion compensator with group velocity dispersion of equal magnitude and opposite sign can compensate for the fiber dispersion. The possible benefits of such dispersion compensation in the 1.3-1.6-μm wavelength region are investigated. The results show that compensation of first-order dispersion at 1.55 μm in a fiber with zero dispersion near 1.3 μm is primarily limited by the second-order dispersion of the grating and the telescope compensator. For a wavelength slightly greater than the zero dispersion wavelength, both the first- and second-order group velocity dispersion can be canceled by the grating and telescope dispersion compensator, allowing transmission exceeding 100 Gb/s over 100 km     

Transmission characteristics of arrayed waveguide N×Nwavelength multiplexer

To realize practical wavelength division multiplexing (WDM) systems, a high-performance N×N wavelength multiplexer is introduced that is based on an arrayed-waveguide grating. Its transmission characteristics are theoretically derived and experimentally confirmed. A prototype is constructed using the previously proposed techniques that attain low insertion loss and polarization independent operation. It has 16 channels (N=16) with a spacing of 0.8 mn, or 100 GHz, in the 1.55-μm band. Frequency relation between input and output ports, free spectral range, and passband width are determined. A demonstration of IM-DD pulse transmission shows that there is no degradation of bit error rate resulting from the finite passband width and crosstalk of the multiplexer. It is confirmed that the multiplexer can realize highly reliable N-channel WDM and WDM-based N×N interconnect optical networks     

Effects on extraordinary transmission of TE wave through varying structure of sub-wavelength metallic gratings without host media

A model of sub-wavelength metallic grating without host media is proposed. Under the excitation of TE polarized light, the extraordinary transmission is also found, and their transmission energy distributions corresponding to different structural parameters of this model are calculated systematically by using finite difference time domain (FDTD) method. The influence of slit width, grating thickness and grating period on the location of transmission peak is obtained. By studying these relations, it is found that Fabry-Perot-like (FPL) effect of the slit is the main physical reason of this extraordinary transmission. Varying the slit width can cause the change of reflection phase transition at both ends, and then the characteristics of FPL resonance of slit cavity are affected. The surface mode of metallic gratings has less effect on the location of transmission peak.     

基于周期压力单模光纤长周期光栅的折射率传感特性研究

利用交替放置的两个相同周期性刻槽板,对放置于套管中的单模裸纤和待测媒质施力,形成了具有复合光波导结构的长周期光纤光栅(LPFG)。实验研究了不同折射率待测媒质对LPFG传输谱的影响。结果表明:复合波导光栅的光谱呈现双谐振峰现象,且中心谐振波长随待测媒质折射率增大向短波长方向移动;包层模阶次越高,其中心谐振波长移动量越大。当折射率从1.33变化到1.43时,LP14包层模的中心谐振波长变化了7.2nm,所对应的传感器折射率灵敏度约为2.78×10-4 RIU。     

Exact solutions to the problem of wave diffraction from a Bragg grating with an apodized asymmetric, symmetric, or antisymmetric coupling factor

The coupling-wave model is applied to obtain an exact analytic solution to the problem of diffraction of a plane electromagnetic wave from a nonharmonic Bragg grating with a spatially modulated refractive index n(z) = n ₀ + Δn(z)cos(2πz/d + ?). Apodization for such gratings is described by continuous functions of the form n(z) = ±Δn/[1 ? n(z ? L/2)] (d, L, ?, and Δn = const are the period, length, phase, and amplitude of a grating, respectively) or piecewise-continuous symmetric and antisymmetric analogues constructed from these functions. Conditions ensuring suppression of oscillations in the reflection and transmission spectra of these gratings are found. It is shown that, when the coupling factor of a grating is antisymmetric, a narrow transparency band is observed within the forbidden transmission band located near a Bragg resonance. An analytic expression is obtained for the dependence of the width of the transparency band on the grating’s parameters. Gratings with an antisymmetric coupling factor can be used in narrowband frequency-selective Bragg transmission filters.     

强度解调的光纤光栅智能结构动态监测技术

将长周期光纤光栅(LPFG)粘贴于试件上构成光纤光栅型智能结构,研究了当试件受到垂直周期力载荷而发生弯曲并随之振动时LPFG的动态响应特性.利用光纤布拉格光栅(FBG)的反射特性获得确定波长处的窄带光作为LPFG的入射光源,通过对固定波长处的光功率探测,实现对试件从2～2 000 Hz范围内的振动监测.由实验采集到的振动信号时域波形及其频谱表明,基于弯曲的LPFG振动传感器具有很好的动态响应特性,其频谱与激振信号频率完全吻合,能够应用于智能材料与结构的健康监测.     

Tuning Bragg wavelength by writing gratings on prestrained fibers

Bragg gratings at a specific wavelength are made using an excimer KrF laser and a phase mask. The wavelength can be varied for fine tuning and multiplexing applications by straining the optical fiber during UV illumination. When the strain is removed, the grating formed is at a smaller wavelength than that dictated by the phase mask for the unstrained fiber. This technique was demonstrated by writing two gratings located at the same point in the optical fiber. The transmission from the first grating was used as a means for in situ absolute wavelength tuning. The second grating made with approximately 0.2% axial strain was at 1534.54 nm. Once the grating was made, the strain was removed from the fiber leaving the second grating tuned to 1532.04 nm, a wavelength shift of 2.5 nm     

Temperature control-free full-mesh wavelength routing network (AWG-STAR) with CWDM AWG-router

AWG-STAR is a star-shaped network that utilizes a uniform-loss cyclic frequency arrayed-waveguide grating (AWG) as a wavelength router to obtain full-mesh interconnection between surrounding wavelength-division-multiplexing nodes. In this paper, we describe a temperature control-free AWG-STAR network that employs a coarse wavelength-division-multiplexing AWG-router. The center wavelength and transmission band of the AWG-router are designed based on the light source wavelength shift that results from changes in operating temperature. The system employs bidirectional transmission to avoid coherent crosstalk from adjacent channels. A cost-effective and small-scale full-mesh network is realized for metropolitan areas.     

薄膜太阳电池二元光栅结构

提出了采用二元光栅结构提高太阳能薄膜衍射效率的技术方法。利用对透射率函数的傅里叶变换计算出垂直入射时二元光栅的衍射光场分布。研究了不同阶次二元光栅在可见和近红外波段的衍射零级光强和总衍射效率随波长变化的规律。以硅基太阳能薄膜为例,分别计算了普通相位光栅和二阶、三阶二元光栅随波长零级光强的变化规律和衍射效率。结果表明,与普通相位光栅结构相比,二元光栅结构能够在较宽波段提高光衍射效率。     

具有温度补偿的LPFG功率解调系统的设计与实现

传统的利用光纤Bragg光栅(FBG)反射光作为长周期 光纤光栅(LPFG)入射光的解调方式,无法避免温度的影响。本文提出了一种具有温度补偿功 能的LPFG功率解调方案,将FBG的Bragg波长由压电陶瓷(PZT)的驱动电压控制,使得FBG谐振 波长始终跟随LPFG谐振波长, 以此抵消温度对传统功率解调中的影响。这种解调方案适用于LPFG透射光谱随物理量变化谱 形也发生变化的传感,如微 弯、横向负载等特性。利用本系统对LPFG的横向负载特性进行试验,结果表明,采用此解调 方案获得的实验数据具有与 光谱仪相同的功率变化趋势,能够始终监测LPFG的谐振峰幅值,实现温度补偿的功率解调, 更适合于动态解调。对LPFG 的温度特性进行功率解调实验,结果也验证了解调系统具有温度补偿功能。     

A Frequency-Dependent LOD-FDTD Method and Its Application to the Analyses of Plasmonic Waveguide Devices

Detailed frequency-dependent formulations are presented for several efficient locally one-dimensional finite-difference time-domain methods (LOD-FDTDs) based on the recursive convolution (RC), piecewise linear RC (PLRC), trapezoidal RC (TRC), auxiliary differential equation, and ${mmb Z}$ transform techniques. The performance of each technique is investigated through the analyses of surface plasmon waveguides, the dispersions of which are expressed by the Drude and Drude-Lorentz models. The simple TRC technique requiring a single convolution integral is found to offer the comparable accuracy to the PLRC technique with two convolution integrals. As an application, a plasmonic grating filter is studied using the TRC-LOD-FDTD. The use of an apodized and a chirped grating is found quite effective in reducing sidelobes in the transmission spectrum, maintaining a large bandgap. Furthermore, a plasmonic microcavity is analyzed, in which a defect section is introduced into a grating filter. Varying the air core width is shown to exhibit tunable properties of the resonance wavelength.      

基于M-Z滤波和LPFG的液体折射率传感特性研究

利用飞秒激光逐线刻写方式在HI1060光纤中制作谐振波长为1548.5 nm,谐振强度为7 dB,栅区长度为3.2 mm的长周期光纤光栅。通过纤芯错位熔接方法在HI1060光纤中制作马赫曾德-干涉仪(MZI),对LPFG的透射光谱进行滤波优化。设计使用不同折射率的酒精溶液、氯化钠溶液和蔗糖溶液分别对基于MZI滤波的LPFG的折射率特性进行了测试和研究。实验中随着三种溶液折射率增加,LPFG的谐振波长发生红移,该LPFG在酒精溶液、氯化钠溶液和蔗糖溶液中的折射率灵敏度分别为301.78 nm/RIU、138.80 nm/RIU和132.67 nm/RIU。     

Chirping of the waveguide grating router for free-spectral-range mode selection in the multifrequency laser

Because the transmission through each input-output combination of a conventional waveguide grating router consists of a periodic sequence of equal-height passbands spaced by the router free spectral range, the absolute laser oscillation wavelength and single-passband oscillation can be difficult to control for all the channels in the multifrequency waveguide grating router laser. However, the router can be designed to have one dominant passband, with the neighboring passbands experiencing lower transmission coefficients for all the input output combinations by means of chirping of the waveguide grating. Such chirping is discussed theoretically and is demonstrated experimentally.