紧凑型宽带TE01-TE02模式转换器

在传播常数随半径变化的情况下近似分析了两模转换条件,得到了模式转换器的初始结构,然后调用迭代法进行优化,最终能得到结构紧凑、带宽较宽、转换效率高的模式转换器。设计了两周期的Ka波段和一周期的W波段TE01-TE02模式转换器,带宽（转换效率95%以上频带宽度）分别达到2.47,8 GHz。CST仿真结果与耦合波仿真程序计算结果吻合较好,验证了该方法的有效性。     

紧凑型宽带TE01-TE02模式转换器

在传播常数随半径变化的情况下近似分析了两模转换条件,得到了模式转换器的初始结构,然后调用迭代法进行优化,最终能得到结构紧凑、带宽较宽、转换效率高的模式转换器。设计了两周期的Ka波段和一周期的W波段TE01-TE02模式转换器,带宽（转换效率95%以上频带宽度）分别达到2.47,8 GHz。CST仿真结果与耦合波仿真程序计算结果吻合较好,验证了该方法的有效性。     

Resultant mode pattern and polarization in a LP01, LP02 two-mode linearly birefringent optical fibre

Modal interference between the LP₀₁ and LP₀₂ polarization modes propagating along a linearly birefringent bimodal optical fibre is considered. On the basis of Jones matrix formalism, radial Stokes parameters containing information about the resultant polarization and mode pattern are derived in the monochromatic and the quasimonochromatic cases. Particular cases with different polarization inputs are considered. The application of the obtained results to fibre-optic sensors and wavelength filter structures is discussed.     

Efficient broadband silicon-on-insulator grating coupler with low backreflection

We design and fabricate an efficient broadband grating coupler on a 400 nm thick silicon-on-insulator wafer. The measured coupling loss is 3 dB when coupling to a single-mode fiber at 1310 nm wavelength with TE polarization. The spectral FWHM and backreflection are determined to be 58 nm and -27 dB, respectively.     

宽带非均匀半径渐变TE0n-TE0(n+1)模式转换器的设计研究

基于耦合波理论,对两类半径渐变圆波导TE0n-TE0(n 1)模式转换器进行理论分析、数值计算和仿真模拟.均匀半径渐变波导高功率模式转换器,采用中心频率为17.14 GHz、六周期TE02-TE03模式和中心频率为34.30 GHz、六周期TE01-TE02模式两种设计参数.非均匀半径渐变波导高功率模式转换器,采用中心频率为34.30 GHz、六周期TE01-TE02模式为设计参数.研究的重点在于保证足够转换带宽的情况下,力求最大化模式转换效率和模式纯度.通过理论分析和模拟,这种新型的非均匀半径渐变波导模式转换器在转换效率和带宽方面都明显优于传统的模式转换器,峰值转换效率达99.5%,转换带宽超过1.5 GHz.     

Design for broadband high-efficiency grating couplers

In this Letter, we propose general optimization methods to design broadband high-efficiency grating couplers for planar waveguides. We attribute the coupling bandwidth to the mismatch of effective indices between the diffracted beam and the actual grating structure around the operation wavelength for fiber to waveguide excitation. The coupling bandwidth formula is deduced. A simple parameter-separate optimization procedure is proposed for general layered grating couplers for high coupling efficiency. Using our principle, we optimized a grating coupler for a horizontal slot waveguide operating at wavelength 1.55 μm for TM polarization. The grating coupler has 1 dB bandwidth of 60 nm and coupling efficiency of 65% with incident light from single-mode optical fiber (SMF) at 8°.     

TE01-TE11模式变换器的迭代设计方法

对模式变换器的迭代设计方法做了改进：给出了合理的迭代步长初始值取值方式;在迭代时只考虑了贡献较大的几条耦合线,而忽略了贡献小的耦合线,从而改善带宽。利用改进的迭代设计方法设计了波导直径与自由空间波长比值为2.765 5的TE01-TE11模式变换器,总长度为600 mm,相对带宽为6%,中心频率转换效率为99.14%,总计算时间为40 s。利用CST软件进行了仿真验证,结果吻合较好。     

Design and characterization of single mode circular photonic crystal fiber for broadband dispersion compensation

In this paper, we present a single mode circular photonic crystal fiber (C-PCF) for broadband dispersion compensation covering 1400 to 1610 nm wavelength band over the telecommunication windows. Investigations of guiding properties are carried out using finite element method (FEM) with circular perfectly matched layer boundary condition. Numerical study reveals that a negative dispersion coefficient of about −386.57 to −971.44 ps/(nm km) is possible to obtain over the wavelength ranging from 1400 to 1610 nm with a relative dispersion slope (RDS) of about 0.0036 nm⁻¹ at 1550 nm wavelength. In addition, the single mode behaviour of C-PCF is demonstrated by employing V parameter. According to simulation, it is found that the proposed C-PCF acts as a single mode fiber within 1340 to 1640 nm wavelength. Moreover, effective dispersion, relative dispersion slope, birefringence and confinement loss are also presented and discussed.     

Optimized overmoded TE01-to-TM11 mode converters for high-power millimeter wave applications at 70 and 140 GHz

Mode coupling in bent, oversized, smooth-wall circular waveguides was studied by means of numerical integration of coupled-mode differential equations in order to optimize high-power TE01-to-TM11 mode transducers at 70 GHz and 140 GHz. Such mode transformers are used in the mode conversion sequence TEOn to TE01 to TM11 to HE11 for generating the almost perfectly linearly polarized Gaussian-like HE11 mode from circular electric TEOn gyrotron modes. This quasi-optical HE11 hybrid mode is in many respects ideal for electron cyclotron resonance heating (ECRH) of magnetically confined plasmas in thermonuclear fusion research and for other technical applications. Curvature and ellipticity coupling as well as ohmic attenuation of 6 coupled modes (TE01, TM11, TE11, TE12, TE21, TM21) are included in the coupling matrices. Integral expressions were used for deriving the coupling coefficients for arbitrary modes in bent, smooth-wall waveguide. Lowest level of unwanted spurious modes together with highest transmission efficiency (shortest arc length) is achieved with sinusoidal curvature distribution instaed of constant curvature. The calculated conversion efficiencies of 98.0% at 70 GHz and 95.2% at 140 GHz (interior waveguide diameter D=27.8 mm for 200 kW transmission lines) are in excellent agreement with the measured values of (97.6±0.4)% and (95±1)%, respectively.     

TE03 to TE01 mode converters for use with a 150 GHz gyrotron

Two-step high-power TE03-to-TE01 mode transducers (TE02 is the intermediate mode) were designed to transform the TE03 output mode of the 150 GHz KfK gyrotron into the fundamental circular symmetric TE01 mode for low-loss propagation and further conversion into the linearly polarized quasioptical HE11 mode. The advanced perturbation structures of the ruppled-wall mode converters (with input- and output-diameter D=27.8 mm) were optimized by numerically solving the proper coupled-wave differential equations. The overall TE03-to-TE01 conversion efficiency at 150 GHz was calculated to be ₀=99.1% for a long converter (total length L=2.67 m) and ₀=97.8% for a short converter (L=1.07 m); ohmic attenuation is included. The corresponding theoretical bandwidth (for 95%) is ±1.4 GHz and ±2 GHz, respectively. Low-power experimental data are in excellent agreement with these computed values.     

宽带非均匀半径渐变TE0n-TE0(n+1)模式转换器的设计研究

基于耦合波理论,对两类半径渐变圆波导TE_0n-TE_0(n+1)模式转换器进行理论分析、数值计算和仿真模拟.均匀半径渐变波导高功率模式转换器,采用中心频率为17.14GHz、六周期TE₀₂-TE₀₃模式和中心频率为34.30GHz、六周期TE₀₁-TE₀₂模式两种设计参数.非均匀半径渐变波导高功率模式转换器,采用中心频率为34.30GHz、六周期TE 关键词： 模式转换器 耦合波理论 非均匀半径渐变 转换带宽     

Highly efficient excitation of LP_(01) mode in ring-core fibers by tapering for optimizing OAM generation

We have demonstrated the highly efficient excitation of the linearly polarized mode(LP01)in ring-core fibers(RCFs)by tapering the spliced point between the RCF and the standard single-mode fiber(SMF)to optimize all-fiber orbital angular momentum(OAM)generation.The tapering technique has been investigated theoretically and experimentally.Before tapering,only 50%of light can be coupled from SMFs to RCFs.The modal interference spectrum with an extinction ratio(ER)of～9 dB is observed,showing that higher-order modes are excited in RCF.By tapering the spliced point,90%of light is coupled,and the ER is minimized to be～2 dB,indicating that the higher-order modes are effectively suppressed by tapering.Such tapered spliced points of RCF–SMF are further applied for all-fiber OAM generation.The efficiencies of OAM+1 and OAM?1 generation are found to be enhanced by approximately 11.66%and 12.41%,respectively,showing that the tapered spliced point of the RCF–SMF is a feasible way to optimize OAM generation.     

Generation of optical frequency comb squeezed light field with TEM_(01) transverse mode

Nonclassical optical frequency combs find tremendous utility in quantum information and high-precision quantum measurement. The characteristics of a type-I synchronously pumped optical parametric oscillator with the TEM_(01) transverse mode below threshold are investigated and a squeezing of 0.7 dB for an optical frequency comb squeezed light field with the TEM_(01) transverse mode is obtained under the pump power of 130 m W. This work has a promising application in three-dimensional space-time measurement.     

Ultrashort-pulse investigation of the propagation properties of the LP(11) mode in 1.55-mum communication fibers

The coupling of an ultrashort laser pulse into a single-mode optical communication fiber gives rise to two propagating pulses as a result of the excitation of two guided modes, the fundamental, LP(01) , and the leaky, LP(11) . Such a phenomenon provides a new approach to the study of the propagation properties of the LP(11) mode. An experiment with tunable 110-fs pulses at a wavelength near 1550 nm is described. Information about the group velocity, the polarization-rotation length, the attenuation coefficient, and the cutoff wavelength of the LP(11) mode is obtained in a simple and direct way for various fibers.     

高功率弯波导TE_(01)—TM_(11)模式变换临界角分析

在模式耦合理论的基础上 ,采用传统的波导轴线圆弧弯曲的方法 ,对TE0 1—TM11模式变换器的临界角进行了全面的优化分析。得出在临界角情况下 ,若考虑多模因素 ,则不能使TE0 1—TM11的能量发生全转换 ,而真正的最优化能量全转换角在临界角的附近 ,且转换的效率与弯波导曲率相关。     

Efficient temperature compensation strategies for guided wave structural health monitoring

The application of temperature compensation strategies is important when using a guided wave structural health monitoring system. It has been shown by different authors that the influence of changing environmental and operational conditions, especially temperature, limits performance. This paper quantitatively describes two different methods to compensate for the temperature effect, namely optimal baseline selection (OBS) and baseline signal stretch (BSS). The effect of temperature separation between baseline time-traces in OBS and the parameters used in the BSS method are investigated. A combined strategy that uses both OBS and BSS is considered. Theoretical results are compared, using data from two independent long-term experiments, which use predominantly A₀ mode and S₀ mode data respectively. These confirm that the performance of OBS and BSS quantitatively agrees with predictions and also demonstrate that the combination of OBS and BSS is a robust practical solution to temperature compensation.     

Numerical analysis and optimization of a dual-concentric-core photonic crystal fiber for broadband dispersion compensation

In this article, a novel dual-concentric-core photonic crystal fiber (DC-PCF) for dispersion compensation is presented. The proposed DC-PCF has relatively high negative dispersion over a wide wavelength range, which covers E, S, C, L and U telecommunication wavelength bands. The validity of the proposed design is carried out by employing a 2-D finite difference frequency domain method (FDFD) with perfectly matched layers (PML). By using the numerical method, the dispersion profile of the DC-PCF is optimized in terms of three air-hole diameters to achieve desirable negative dispersion. The influence of the location of ring-core, the number of air-hole rings on dispersion and loss characteristics are also studied.     

Schemes for complete compensation for polarization mode dispersion up to second order

The required structure and elements of polarization mode dispersion (PMD) compensators for complete second-order compensation are investigated by use of a general PMD vector formalism of concatenated PMD elements based on Mueller matrices and Stokes vectors. The investigation shows that two- and three-stage compensators with five independent parameters can compensate for polarization-dependent chromatic dispersion as well as the depolarization component of second-order PMD.     

相位修正的耦合模理论用于计算光纤Bragg光栅法布里-珀罗腔透射谱

基于光纤Bragg光栅(FBG)反射复振幅相移对FBG法布里-珀罗腔透射谱的影响,分析了传统耦合模理论计算均匀FBG反射复振幅相移产生误差的原因.引入折射率分布初始相位参数描述FBG折射率分布纵向的微小偏移,用真实的反透射系数代替简明形式的反透射系数,对传统耦合模理论进行了修正,增加了与折射率分布初始相位参数有关而与波长无关的相位因子.在此基础上进一步对计算非均匀FBG的传输矩阵法的相位进行了修正.修正后的快速计算结果用于FBG法布里-珀罗腔透射谱的计算,可反映折射率分布初始相位参数对透射峰波长位置的影响,与Rouard 算法及实验值均有较好的一致性. 关键词： 光纤Bragg 光栅 法布里-珀罗腔 耦合模理论     

Timing compensation for an excimer laser solid-state pulsed powermodule (SSPPM)

For certain applications, it is critical to minimize variations in the (throughput) timing between trigger and the output pulse of a magnetic modulator. A circuit is described that maintains a relatively constant delay over a large operating voltage range (600-1150 V) and temperature range (25 °C-65 °C) range, The circuit operates by sampling the charging voltage and magnetic switch temperature just prior to the start switch trigger. Those parameters are then used to calculate the appropriate amount of delay to add into the low-level trigger chain to ensure that the delay stays constant over the voltage and temperature operating range. Although other approaches can be conceived and implemented, this particular design is relatively simple and inexpensive and meets the desired performance goals. Data presented show that the ideal correction function is nonlinear in nature and, as a result, simple linear approximations are limited in their ability to minimize the timing variations. Improvements to the original circuit use a multiple, piece-wise, linear approach in order to obtain better performance. The results are that an initial timing variation of almost 3 μs has been reduced to a total variation of less than 100 ns