基于石英柱模型的光子晶体光纤异常布里渊散射特性的理论研究

通过石英圆柱模型,理论研究了小芯径光子晶体光纤中混合声波模式的色散、模式耦合以及声光相互作用,理论计算出了布里渊散射增益系数谱的双峰结构及其随抽运光波长和温度的演化规律. 理论分析表明光子晶体光纤中布里渊散射增益系数谱的双峰结构源于小芯径光子晶体光纤中混合声波模式之间的模式耦合. 通过温度改变导致的材料参数变化对声波模式色散特性的影响,特别是声波模式耦合点的移动,解释了双峰结构随外界温度的变化规律. 并且,通过理论计算与实验结果的对比讨论了石英圆柱模型的局限性和适用范围. 关键词： 布里渊散射 声光相互作用 模式耦合 光子晶体光纤     

Acoustic longitudinal mode coupling in w-shaped Al/Ge Co-doped fibre

This paper proposes a novel fibre structure aiming at distributed temperature and strain sensing. Utilizing Al₂O₃ and GeO₂ as dopants to form a w-shaped acoustic waveguide, it realizes modal coupling between longitudinal acoustic modes of its inner and outer core layers, leading to a dual-peak or multi-peak Brillouin gain spectrum. The relationship between the acoustic mode coupling properties and the fibre materials, doping concentrations and structural parameters are investigated, showing that the positions of mode coupling points in acoustic dispersion curves and the coupling intensities can be designed flexibly. A specific fibre design for the discriminative sensing of temperature and strain under a pump wavelength of 1.55 μm is given. The responses of its Brillouin gain properties on temperature and strain are analysed theoretically, demonstrating its potential for distributed fibre Brillouin sensing.     

Multi-wavelength Brillouin fiber laser using dual-cavity configuration

A simple technique for achieving multi-wavelength tunable multi-wavelength Brillouin fiber laser (BFL) based on dual-pass configuration is demonstrated. The BFL uses a piece of 10 km long non-zero dispersion shifted fiber (NZ-DSF) as a Brillouin gain medium to obtain odd-order Brillouin Stokes waves as an output with the line spacing of 0.16 nm (20 GHz) between each two consecutive waves by employing even-order Brillouin Stokes to improve output performance of the laser. With a BP of 15.3 dBm, at least 15 odd-order Brillouin Stokes and anti-Stokes lines are generated. The laser is room temperature stable, tunable over 50 nm wavelength range and has an optical signal to noise ratio of more than 30 dB at 1560 nm region. This Brillouin fiber laser can operate at any wavelength depending on the Brillouin pump (BP) wavelength used.     

纵模对光子晶体中类狄拉克点传输特性的影响

本文通过数值仿真分析了无序正方晶格光子晶体中类狄拉克点的光子传输特性. 结构中的无序是通过随机移动氧化铝介质柱的位置来实现. 研究发现, 由于纵模被激发出来, 在类狄拉克点及其附近无序对结构透射率的影响是不同的. 在类狄拉克点, 由于纵模的干扰, 透射率随着无序的增加而减小, 与通带的行为类似. 在不受纵模干扰的类狄拉克点附近, 透射率几乎不受无序的影响, 这主要是由于结构可以等效为近零折射率材料, 等效的波长非常大. 本文的研究结果有助于人们进一步理解光学纵模和零折射率材料.     

Improvement of four-wave mixing-based wavelength conversion efficiency in dispersion shifted fiber by 40-GHz clock pumping

40-GHz clock modulated signal as a pump to improve the efficiency of four-wave mixing (FWM)-based wavelength conversion in a 26.5-km dispersion shifted fiber (DSF) is investigated. The experimental results demonstrate that the conjugated FWM component has higher intensity with the clock pumping than that with the continuous-wave (CW) light pumping. The improvement of FWM-based wavelength conversion efficiency is negligible when the pump power is less than Brillouin threshold. But when the pump power is greater than Brillouin threshold, the improvement becomes significant and increases with the increment of pump power. The improvement can increase up to 9 dB if pump power reaches 17 dBm.     

Polarization-dependent two-color dispersion wave generation and evolution in polarization-maintaining photonic crystal fiber

We investigate the effect of the polarization state of the input pulses on the visible emissions in the anomalous dispersion region of polarization-maintaining photonic crystal fiber (PM-PCF), by using ～100 fs pump pulses whose central wavelength (1064 nm) is close to the second zero dispersion wavelength (1100 nm) of the fiber, where the soliton fission mechanisms play an important role. The experimental results show that the phase-matching two-color dispersive wave emission, one at 582 nm and the other at 600 nm, is polarization-dependent and frequency shift results from the different dispersion characteristics along the two orthogonal principal axes of PM-PCF. Furthermore, it is observed for the first time that the variation of the linear input polarization angles in 45° region almost has no influence on the output spectral profiles, and the break variation of the output spectrum exists when the angle between the polarization of the linear incident pulse and the fast-axis or the slow-axis of PM-PCF is 45°, which are attributed to the coupling between the two polarization modes in high birefringent PM-PCF.     

15波长输出的布里渊掺铒光纤激光器

多波长布里渊掺铒光纤激光器是一种新型的多波长光纤激光器,其原理是利用受激布里渊增益和掺铒光纤的线性增益,可以在常温下得到波长间隔约为0．08nm(～10GHz)的多波长输出。报道的布里渊掺铒光纤激光器,在布里渊抽运功率为1．7mW、980nm抽运功率为300mW的情况下得到稳定的15个波长(间隔～10GHz)的输出,这种激光器用作光传感器、光谱分析仪以及密集波分复用系统的光源。实验发现,输出波长的个数随着980nm抽运功率的增大而增加。另外,布里渊掺铒光纤激光器的信号功率主要来自于掺铒光纤的增益,而布里渊增益对它的影响不大。     

水中受激拉曼散射的能量增强及受激布里渊散射的光学抑制

为提高液体介质中受激拉曼散射的输出能量,提出了通过温度调控来抑制受激布里渊散射的方法,设计了532 nm多纵模宽带脉冲激光泵浦的受激拉曼散射发生系统,测量了不同温度下水中前向受激拉曼散射及后向受激布里渊散射的输出能量,分析了水温、泵浦激光线宽及热散焦效应对受激拉曼散射输出能量影响的物理机制.实验结果表明:通过降低水温可实现对受激布里渊散射过程的有效抑制,同时减小热散焦效应带来的光束畸变,从而有效提高受激拉曼散射的输出能量.研究结果对液体介质中的受激拉曼散射多波长转换具有重要意义.     

A new method to eliminate the influence of the backward defect infection and the position-dependent of the coefficient on distributed Brillouin fiber sensor

Two theoretical problems associated with the measurement of temperature or strain from the normalized power of Brillouin signal in a distributed fiber sensor are studied. One problem is the position-dependent of the coefficient relating the Brillouin power to the local temperature or strain. The other problem is the backward infection of the defect (local temperature or strain change). These two problems will degrade the measurement accuracy of a distributed Brillouin fiber sensor by changing the distribution of pump power. To eliminate these two problems, new reference functions are proposed to normalize the Brillouin signal. Simulation results by using fourth-order Runge-Kutta algorithm show that these new reference functions can eliminate the influence of these two problems on distributed Brillouin fiber sensor.     

Widely tunable linear-cavity multiwavelength fiber laser with distributed Brillouin scattering

We demonstrate a multiple wavelength Brillouin/erbium fiber laser in a linear cavity configuration.The laser cavity is made up of a fiber loop mirror on one end of the resonator and a virtual mirror generated from the distributed stimulated Brillouin scattering effect on the other end.Due to the weak reflectivity provided by the virtual mirror,self-lasing cavity modes are completely suppressed from the laser cavity.At Brillouin pump and 1480-nm pump powers of 2 and 130 mW,respectively,11 channels of the demonstrated laser with an average total power of 7.13 dBm can freely be tuned over a span of 37-nm wavelength from 1530 to 1567 nm.     

微纳光纤环形腔海水温度的谐振光谱分析

微纳光纤环形腔作为传感器件,具有灵敏度高、响应快的优点,因而越来越广泛地应用在传感领域。环形腔的谐振光谱直接反映外界环境的变化,因此谐振光谱分析对探测环境参数至关重要。从理论和实验研究了微纳光纤环形腔谐振光谱与海水温度的关系。首先,数值计算了基模(HE₁₁)两个垂直偏振态的传播常数随光纤直径和探测波长的变化关系,计算结果表明传播常数随光纤直径增大而增大,随波长增大而减小,同时计算了传播常数随海水温度的变化,海水温度越高,传播常数越大,表明海水温度的变化会影响到模式的传播常数,因而可以通过测量谐振光谱的变化得知海水温度的变化。其次,搭建了海水温度传感实验系统,获得了微纳光纤环形腔海水温度的谐振光谱,实验发现同时存在两套谐振峰,分别对应基模的TE模和TM模,两个偏振模式的传感灵敏度分别为5.54和5.24 pm·℃-1。最后,探讨了基模两个偏振态谐振光谱的产生原因,由于结型耦合区的扭曲耦合使得两个偏振态分离,并对两个偏振模式的谐振强度进行了分析。两个模式的谐振强度不同,而且随着波长的增加,一个模的谐振强度不断增强,另一个模的谐振强度逐渐减弱。这是由于不同耦合态的耦合系数和衰减决定的,而且他们随着波长而改变。实验结果与理论计算相吻合。     

Tunable all-optical delays via Brillouin slow light in an optical fiber

We demonstrate a technique for generating tunable all-optical delays in room temperature single-mode optical fibers at telecommunication wavelengths using the stimulated Brillouin scattering process. This technique makes use of the rapid variation of the refractive index that occurs in the vicinity of the Brillouin gain feature. The wavelength at which the induced delay occurs is broadly tunable by controlling the wavelength of the laser pumping the process, and the magnitude of the delay can be tuned continuously by as much as 25 ns by adjusting the intensity of the pump field. The technique can be applied to pulses as short as 15 ns. This scheme represents an important first step towards implementing slow-light techniques for various applications including buffering in telecommunication systems.     

基于受激布里渊散射的波长间隔可变多波长光纤激光器

提出了一个基于自激发受激布里渊散射的波长间隔可变多波长光纤激光器.利用单模光纤中自激发产生的非线性布里渊增益和掺铒光纤的线性增益组成混合增益光纤激光器,从而使光纤激光器在室温下产生稳定的多波长输出.改变双折射光纤环镜滤波器中保偏光纤的长度,可以实现波长间隔可变多波长激光产生,提高了多波长光纤激光器操作的灵活性和实用性.实验实现了波长间隔从0.8 nm至0.076 nm可变的多波长激光产生,波长数随波长间隔减小而增加,间隔为0.08 nm的激光波长数达86.     

Polarization-Independent Guided-Mode Resonance Filters under Oblique Incidence

We present the dispersion relation of guided-mode resonances in planar periodic waveguides, both for s-polarized （TF, mode） and p-polarized （TM mode） incident waves. For a fixed homogeneous planar waveguide, dispersion curves of the TE eigenmode cannot cross that of the TM eigenmode at all. That is to say, at a certain wavelength, TE and TM modes cannot be excited with the same propagation constant. Due to Bragg reflection in the planar periodic waveguide, dispersion curves of the TE leaky mode may intersect with that of the TM leaky mode in the first Brillouin zone. We employ these intersections to achieve polarization-independent guided-mode resonance filters.     

Stimulated Brillouin scattering of an electromagnetic wave by an acoustic-like mode in multi-ion species plasmas

Using an hydrodynamical model, analytical investigation of the stimulated Brillouin scattering of an electromagnetic wave by an acoustic-like mode has been done in a multi-ion species plasma. The acoustic-like mode is a new mode which propagates in a multi-ion plasma only. The non-linear dispersion relation and the growth rate of the excited modes are derived. The non-linearity arises through the motion of ions which is introduced through ponderomotive force. It is found that the growth rate can be controlled by several parameters like charge number, mass, density and temperature of the ions.     

Negative-index dispersion and accidental mode degeneracy in an asymmetric spoof-insulator-spoof waveguide

It has been recently demonstrated that negative-index dispersion and mode degeneracy can be achieved by manipulating a spoof-insulator-spoof(SIS) waveguide. In this paper, we propose a new SIS waveguide, which is composed of two spoof surface plasmon polaritons(SSPPs) waveguides drilled with periodic rhomboidal grooves. Both the symmetric and asymmetric cases are investigated. Our simulation results show that the asymmetric SIS waveguides are more significant.By tailoring the tilt of the rhomboidal grooves, the negative-index dispersion can be achieved and the microwave band gap(MBG) can be effectively modulated. At a critical tilt, there appears an accidental mode degeneracy at the edge of the first Brillouin zone. The excitation and propagation of the two coupled modes sustained by the asymmetric SIS waveguides are also demonstrated.     

涂覆石墨烯的椭圆形电介质纳米线光波导的模式特性分析

设计了一种涂覆石墨烯的椭圆形电介质纳米线光波导.采用分离变量法,在椭圆柱坐标系中,借助Mathieu函数,得到了色散方程.通过数值求解色散方程,可以得到模式的有效折射率和场分布,从而得到模式的传播长度.研究了工作波长、结构参数以及石墨烯的费米能对模式特性的影响,并给出了前五个模式的品质因数.计算表明,当波长从4.3μm增加到8.8μm,这5个模式的有效折射率的实部减小,基模和一阶模的传播长度增大,二阶模的传播长度先增大后减小.当改变纳米线结构参数半长轴和半短轴时,对基模和一阶模的模式特性影响较小,对二阶模的模式特性影响较大.当石墨烯的费米能从0.45 eV增加到0.72 eV时,有效折射率的实部减小,传播长度可以达到2μm左右.分离变量法得到的结果与有限元方法得到的结果完全一致.本文工作可以为基于涂覆石墨烯的电介质纳米线的光波导的设计、制作和应用提供理论基础.     

基于包层模谐振的三包层石英特种光纤温度传感特性研究

提出了一种基于包层模谐振的光纤温度传感器. 它是通过将三包层石英特种光纤(TCQSF)两端分别与普通单模光纤(SMF)电弧熔接构成的SMF-TCQSF-SMF结构. 根据耦合模理论, 首先将TCQSF等效为三个同轴波导, 按各波导模场的分布特点标量计算其传输模式的色散曲线, 并深入研究其耦合长度与传输谱线之间的关系; 其次根据光纤的热光效应及热膨胀效应, 分析计算该传感器的温度灵敏度; 最后选取耦合长度为一个拍长时的传感器进行温度传感实验. 实验结果表明, 在35-95 ℃的温度变化范围内, 其温度灵敏度为73.74 pm/℃, 与理论计算结果一致. 因此, 该传感器具有结构简单、制备容易、灵敏度高、包层模激发可控等优点, 可用于工业生产、生物医学等温度传感领域.     

Enhancement of stimulated Brillouin scattering of higher-order acoustic modes in single-mode optical fiber

Solving the elastic wave equation exactly for a GeO2-doped silica fiber with a steplike distribution of the longitudinal and shear velocities and density, we have obtained the dispersion, attenuation, and fields of the leaky acoustic modes supported by the fiber. We have developed a model for stimulated Brillouin scattering of these modes in a pump-probe configuration and provided their Brillouin gains and frequencies for an extended range of core sizes and GeO2 doping. Parameter ranges close to cutoff of the acoustic modes and pump depletion enhance the ratio of higher-order peaks to the main peak in the Brillouin spectrum and are suitable for simultaneous strain-temperature sensing.     

光子晶体光纤中布里渊散射声波模式特性的分析

推导了光子晶体光纤中声波微小位移波动方程; 研究了泵浦波长以及纤芯折射率对声波模式的影响; 应用石英圆柱模型研究了小芯径光子晶体光纤中纤芯直径对布里渊声波模式色散的影响. 结果表明在光子晶体光纤中, 纵向声波和横向声波共同作用产生质点声场, 两者相互耦合将产生混合声波模式; 可以通过改变泵浦波长或光子晶体光纤纤芯折射率来改变参与布里渊散射(BS) 过程的声波模式的传播常数; 随着光子晶体光纤(PCF) 纤芯直径的增大, 声波模式耦合程度得到加强, 相速度呈减小趋势, 且同一传播常数下, 声波模式数呈增多趋势; 随着泵浦波频率的增大, 声波相速度减小.