基于少模长周期光纤叠栅的模式转换器

本文提出一种在同一段少模光纤上写入两个不同周期Λ1和Λ2的长周期光纤光栅构成叠栅的方法,实现了纤芯基模LP_(01)向高阶纤芯模LP_(11)模式转换的宽带宽的新型的全光纤模式转换器.利用有限元法和耦合模理论建立了模式转换器的理论分析模型.数值仿真分析了叠栅中两个子光栅周期间隔、光栅长度、耦合系数等光栅参数对模式转换器的影响.仿真分析和实验结果表明,通过改变两个子光栅的周期间隔来改变两个损耗峰的位置,形成一个损耗峰,从而可以实现宽带宽的模式转换器,其10dB带宽约是单栅的2倍.与传统的模式转换器相比,该转换器带宽宽、转换效率高,尺寸小、抗干扰能力强,可以在模分复用系统和光通信中得到广泛的应用.     

非对称双芯光子晶体光纤宽带模式转换器研究

提出了一种基于非对称双芯光子晶体光纤的新型宽带模式转换器。对光纤参数与模式有效折射率之间的分析,通过调整纤芯内层空气孔的直径及掺杂介质棒的折射率,使两个纤芯中需要转换的模式之间实现了宽带的相位匹配。数值模拟结果表明,一种长仅为5.9mm的双芯光纤,在保证传输损耗小于0.5dB的前提下,其工作带宽达160nm且偏振相关损耗小于0.28dB。     

基于高折射率液体填充的花瓣形微结构光纤可调滤模特性

提出一种新型的可调滤模光纤结构,利用纤芯模式与微结构包层形成的超模群之间的耦合实现选择性滤模,采用花瓣形包层结构使包层中传输的模式更容易产生高的泄漏损耗;提出以液体填充包层介质柱,使包层形成的超模群有效折射率区间可以通过环境温度来调节,从而达到可调选择性滤模目的.利用液体柱的LP11模所形成的超模群,有效增大了其工作带宽和温度调谐范围.数值模拟结果表明,采用长度仅为71.4 mm的滤模光纤,可以使特定的抑制模式损耗达到20 dB以上,而其他模式损耗均在1 dB以下.提出的光纤可以在少模光纤传输系统中作为滤模器使用,以降低模式转换器、复用器/解复用器以及光开关和光路由等的模式串扰.     

基于悬浮式双芯多孔光纤的太赫兹偏振分离器

提出了一种基于悬浮式双芯多孔光纤的低损耗、宽带太赫兹偏振分离器.通过纤芯的多孔结构实现器件的低损耗特性,利用两个纤芯微结构的正交关系实现宽带的单偏振模式匹配.结构参数设计采用折射率反转匹配耦合法;数值计算采用全矢量有限元法;光纤基底材料选择太赫兹波段低损耗环烯烃共聚物COC.首先对单芯高双折射悬浮式多孔光纤的色散、模式双折射、基模在空气中的能量分数、以及损耗等特性进行了分析,在此基础上,对悬浮式双芯多孔光纤偏振分离器的特性进行了详细研究.发现该偏振分离器的工作带宽超过1.5 THz(0.8 THz到2.3 THz).其偏振分离长度和吸收损耗随频率的增大而增大,在1 THz,分离长度仅为0.66 cm;x,y两偏振的消光比分别为-14.64 dB和-14.84 dB,两偏振模式的实际吸收损耗均小于0.12 dB.相对于其他双芯光纤偏振分离器设计,该结构具有宽带、低损耗、设计简单、拉制容易、以及抗环境干扰等优点.     

基于少模光纤的全光纤熔融模式选择耦合器的设计及实验研究

提出了三种基于少模光纤的全光纤熔融模式选择耦合器. 根据模式匹配原理采用单模光纤与少模光纤熔融连接方式, 运用耦合模理论及光束传播法模拟分析了模式选择耦合器的结构参数对模式选择及耦合特性的影响, 实现了单模光纤中基模到少模光纤中不同阶模式的转换, 以满足不同的应用需求. 实验上以2× 2熔融光纤耦合器为例, 采用对称和非对称熔融拉锥方式, 分别实现了从基模到LP₁₁, LP₂₁模式的转换. 实验结果表明所得到的LP₁₁, LP₂₁模式在1530–1560 nm的波长带宽范围内均有较高的模式纯净度, 且模式耦合效率高于80%, 与理论模拟结果基本一致.     

旋转光纤圆起偏器的特性分析

基于旋转光纤耦合模理论,对窄带和宽带旋转光纤圆起偏器的特性进行了计算分析.研究了注入光的偏振态、光纤固有线双折射和旋转速率对窄带圆起偏器最小工作长度的影响,并借助多包层光纤的分析方法,分析了旋转光纤各参量变化对宽带圆起偏器工作带宽的影响.结果表明:窄带圆起偏器的最小工作长度与光纤固有线双折射和光纤旋转速率有关,而与注入光的偏振态无关|改变光纤旋转速率可调节宽带圆起偏器的工作带宽,改变应力区到纤芯的距离可改变宽带圆起偏器的中心波长.     

一种具有低串扰低非线性的双沟槽环绕型十三芯五模光纤

信息化对高速大容量光纤网络的需求日益强烈,空分/模分复用是继波分复用之后可使通信容量翻倍的新一代光纤通信技术.本文提出了一种双沟槽环绕型十三芯五模光纤,以满足未来对高速大容量信息传输的需求.针对空分-模分复用中降低串扰的目标设计优化光纤,采用双沟槽环绕结构,将光能量更好地限制在纤芯内,从而减小芯间和模间串扰.利用全矢量有限元法与功率耦合理论相结合计算并分析多芯光纤的串扰和传输特性.经过优化结构参数,可使光纤在1.3—1.7μm波段内稳定传输LP01, LP11, LP21, LP02,和LP31 5个模式;信号在1.55μm波长处传输60 km时,对应于以上5个模式相邻纤芯的芯间串扰分别为–122.37,–114.76,–106.28,–100.68,–92.81 d B,相邻模式之间的有效折射率差大于1.026×10–3,芯间和模间串扰可以被有效抑制; 5个模式对应的非线性系数分别为0.74, 0.82, 0.88, 1.26, 0.93 W–1·km–1,均可保持低非线性传输.该光纤结构简单紧凑,可利用气相沉积法和堆叠法制备预制棒,进一步拉制成具有低串扰低非线性的十三芯五模光纤,可应用于长距离高速大容量光纤传输系统.     

基于多芯光纤的三模复用/解复用器的设计

根据模式耦合理论,设计了一种基于三芯光纤并支持LP_(01)、LP_(11a)、LP_(11b)三种空间模式传输的三模复用/解复用器。该三芯光纤由一个位于中央的三模纤芯和两个外部的单模纤芯构成。选择三模纤芯的结构参数,根据模式有效折射率匹配原理分别设计两个外部单模纤芯的参数。通过仿真分析了LP_(11a)模与LP_(01)模、LP_(11b)模与LP_(01)模的功率转换过程,确定最优的光纤长度为5.2mm。在C+L波段中所设计的三模复用/解复用器能提供50nm的工作带宽,其模式转换效率达到90%。该基于三芯光纤的三模复用/解复用器具有结构简单、模式转换效率高、插入损耗小、带宽宽等优点。     

具有双模特性的大模场面积微结构光纤的设计

本文提出了一种具有双模特性大模场面积的微结构光纤,通过有限元法计算其模场分布、基模有效模场面积及弯曲损耗特性,并分析了不同结构参量对限制损耗及有效模场面积的影响.计算结果表明:通过合理地改变光纤结构参量,可以使二阶模LP11、三阶模EH11截止,实现基模LP01、三阶模HE31双模传输,其中基模的有效模场面积可达700μm2.这种结构的光纤制作简单,在大容量光纤传输系统中具有重要应用.     

一种沟槽辅助气孔隔离的低串扰高密度异质多芯少模光纤

以单模光纤为基础的传统光通信系统的容量已趋近其理论极限,多芯少模光纤是突破现有传输容量瓶颈的一种有效方式.本文设计了一种低串扰5-LP模的弱耦合异质芯7芯光纤,采用沟槽辅助和气孔隔离相结合的方法,在标准125μm外径的情况下实现了芯间和模间的低串扰.利用有限元法计算了纤芯之间的串扰、有效模面积等.经过设计优化,光纤在光通信C+L波段可以稳定传输5个LP模式,其中LP₂₁与LP₀₂模之间的有效折射率差最小,且大于1.1×10^–3;光纤中LP₃₁模式的芯间串扰最大且低于–50 dB/km,因此该光纤可以同时实现模间和芯间的低串扰传输. 7个纤芯中5个LP模的有效模面积均大于86μm~2,在波长1550 nm处相对纤芯复用因子为57.63,该光纤可用于大容量高速光纤传输系统.     

Hybrid Wavelength and Code Division Multiple Access in Optical Networks

A review of schemes for multiple access in fiber optic networks shows that a hybrid of wavelength and code division multiple access (WCDMA) combines the best features of both. In particular, the hybrid scheme retains the large information carrying capacity of wavelength division multiple access (WDMA) and flexibility of code division multiple access (CDMA). In this paper WDMA, optical CDMA (OCDMA), and WCDMA networks are discussed. In OCDMA networks, concept of incoherent and coherent coding including inverse decoding and matched filter is introduced. The delay performance of both WDMA and WCDMA networks, under the simple suboptimum access protocols based on cyclic search, is computed. It has been shown quantitatively that tuning delay significantly affects the delay performance of both WDMA and WCDMA networks. Futhermore, delay performance of WCDMA networks is always better than the WDMA networks for the same tuning delay, load, and number of users.     

Guided-Wave Two-Dimensional Acousto-Optic Scanner Using Proton-Exchanged Lithium Niobate Waveguide

Two-dimensional scanning of a 0.6328 mum guided-light beam has been realized using noncolinear acousto-optic (AO) coplanar Bragg diffraction together with colinear AO guided-mode to substrate radiation-mode conversion in a Z-cut Xpropagation LiNbO3 proton-exchanged (PE) waveguide. The two surface acoustic (SAW) waves utilized are at the center frequencies of 500 and 200 MHz, propagating in the Y and X axes, respectively. Two-dimensional scanning of approximately 720 resolvable light beam spots, namely, 18 40 (horizontal vertical) scanning, has been demonstrated using a light beam of 1.0 mm aperture. The total number of resolvable beam spots can be greatly increased from 720 by simply utilizing SAW transducers of larger bandwidth and a light beam of greater aperture. It should also be possible to significantly increase the diffraction efficiency from 3 % by optimizing the parameters of the PE waveguide and the SAWs.     

Femtosecond Transform-Limited Pulse Generation by Compensating for the Linear Chirp of SPM Spectra in Dispersion Shifted Fibers

A simple technique of pulse compression, based on the linear chirp compensation of self-phase modulation (SPM) spectra in dispersion shifted fibers, is demonstrated. The optimization procedure is carried out, for a short span of a single-mode fiber, using a parabolic law, which describes the behavior of the squared output pulse width versus the pump peak power in the case of Gaussian pulses. The experimental results give a minimum pulse duration of 233 fs, which is in good agreement with the model. Shorter and coherent pulses, down to 90 fs, have been obtained by inserting an interference filter at the optical output.     

Effect ofγ-radiation on the spectral luminescence characteristics of impure cadmium tungstate crystals

Results of a study of the effect of γ-radiation on the spectral luminescence properties of cadmium tungstate crystals doped with silver, bismuth, and molybdenum cations are presented. Spectral characteristics of the nondnnnoped crystals are briefly described. Absorption and photo and X-ray luminescence spectra of the crystals taken before and after exposure to γ-radiation (5.5·10⁴ Gy) are compared. It is found that the spectral characteristics of the crystals doped with silver, bismuth, and molybdenum cations do not change markedly after the exposure. The relation between the type of impurity-induced defects, individual characteristics of the impurity cations, and the character of the effect of γ-radiation on the spectral luminescence properties of impure crystals is analyzed (preliminarily). Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 64, No. 1, pp. 55–60, January–February, 1997.     

Theoretical analysis and simulation of conjugate heights for dual-conjugate AO system in lidar

A multi-conjugate adaptive optics （MCAO） can offer a possibility of widening field of view （FOV） characterized by the isoplanatic angle, and the choose of conjugate height becomes a basic problem for MCAO, which influences the size of iosplanatic angle. Considering the application of lidar, the isoplanatic angle＇s expressions of two deformable mirrors （DMs） MCAO for uplink and downlink are deduced. The effects of conjugate heights for dual-conjugate AO are thoughtfully discussed, and the isoplanatic angles are further analyzed. The results show that the isopanatic angle varies with the conjugate height and reaches the maximum as the conjugate height is at the optimal altitude. Moreover, the optimal conjugate height changes with the propagation distance.     

Quick single-photon detector with many avalanche photo diodes working on the time division

Due to the limit of response speed of the present single-photon detector, the code rate is still too low to come into practical use for the present quantum key distribution (QKD) system.A new idea is put up to design a quick single-photon detector.This quick single-photon detector is composed of a multi-port optic-fiber splitter and many avalanche photo diodes (APDs).Au of the ports with APDs work on the time division and cooperate with a logic discriminating and deciding unit driven by the clock signal.The operation frequency lies on the number N of ports, and can reach N times of the conventional single-photon detector.The single-photon prompt detection can come true for high repetition-rate pulses.The applying of this detector will largely raise the code rate of the QKD, and boost the commercial use.     

Research on beamforming method for dual-line array with space mismatch between the two line arrays

The performance of port/starboard discrimination of dual-line array will be seriously degraded when the space between the two line arrays of the dual-line array is mismatched. The directivity function of the delay-sum and delay-subtract beamforming is deduced and the port/starboard discrimination of the delay-sum and delay-subtract beamforming is also studied both under the ideal status and the status under which the space between the two line arrays of the dual-line array is mismatched. Analysis results show the delay-subtract beamforming has better performance of port/starboard discrimination. At last, the sea trial results demonstrate the theoretical results.     

Sorting of bending magnets for the SSRF booster

The Shanghai Synchrotron Radiation Facility(SSRF)booster ring,a full energy injector for the storage ring,is deigned to accelerate the electron beam energy from 150MeV to 3.5GeV that demands high extraction efficiency at the extraction energy with low beam loss rate when electrons are ramping.Closed orbit distortion(COD)caused by bending magnet field uniformity errors which affects the machine performance harmfully could be effectively reduced by bending magnet location sorting.Considering the affections of random errors in measurement,both ideal sorting and realistic sorting are studied based on measured bending magnet field uniformity errors and one reasonable combination of bending magnets which can reduce the horizontal COD by a factor of 5is given as the final installation sequence of the booster bending magnets in this paper.     

Retrieval of ultraviolet skylight radiances and O3 slant column densities from balloon-borne limb spectrometer

With a novel light-weight and absolutely calibrated ultraviolet （UV） spectrometer, UV skylight radiances and O3 slant column densities are measured by balloon-borne limb measurements in Xinjiang area, China. UV skylight radiances measured at the height of 31 km are compared with the results from Modtran in the wavelength range from 290 to 420 nm. O3 slant column densities are derived from radiance spectra in the Huggins bands （320 - 335 nm） using differential optical absorption spectroscopy method. And the parameter exhibits a good correlation with the same value simulated by radiative transfer model （Tracy）. The O3 profile simultaneously measured by an O3 sonde is used as input in Tracy calculations. The O3 sonde is launched on the same balloon.