A TERAHERTZ PHASE FREQUENCY CHANGER WITH THE CRYSTAL QUARTZ PHASE SECTIONS

A phase frequency changer (PFC) for quasi-optical transmission line with phase sections made of crystal quartz and polarizers executed on the basis of small-period wire gratings has been considered. The PFC is intended for operation in terahertz (THz) frequency region. It has been examined at the frequency f _o= 0.89THz. The influence of the differential phase shift deviation in the sections on the output signal spectrum and the influence of the sections mismatch on the reflection signal spectrum have been considered. In the frequency region ± 10% f _o the levels of the spurious spectral components of output signal are less than −40 dB with regard to the level of the useful signal of the shifted frequency. The levels of all spectral components of the reflection signal are less than −60 dB with regard to the level of the useful component of the PFC output signal.     

Microscope system with on axis programmable Fourier transform filtering

We propose an on-axis microscope optical system to implement programmable optical Fourier transform image processing operations, taking advantage of phase and polarization modulation of a liquid crystal on silicon (LCOS) display. We use a Hamamatsu spatial light modulator (SLM), free of flickering, which therefore can be tuned to fully eliminate the zero order component of the encoded diffractive filter. This allows the realization of filtering operation on axis (as opposed to other systems in the literature that require operating off axis), therefore making use of the full space bandwidth provided by the SLM. The system is first demonstrated by implementing different optical processing operations based on phase-only blazed gratings such as phase contrast, band-pass filtering, or additive and substractive imaging. Then, a simple Differential interference contrast (DIC) imaging is obtained changing to a polarization modulation scheme, achieved simply by selecting a different incident state of polarization on the incident beam.     

Modelling of diffractive structures in photorefractive Er/Yb–co-doped glass waveguides

The photorefractive properties of Ge-doped glasses are routinely exploited for the production of Bragg gratings in optical fibres. Today, the most common fabrication procedure relies on the phase-mask technique, which allows to produce (through UV irradiation) a periodical refractive index variation in Ge-doped fibre cores. The same methodology can be applied to integrated optical components in photorefractive glass planar substrates. If rare earth ions (such as erbium and ytterbium) are included in the glass matrix, both active and photorefractive properties can be combined in the same material. This fact allows for the possibility of realizing complex integrated optical components, such as Bragg-reflection lasers and amplifiers. In this work, a numerical analysis of a waveguide Bragg reflector is presented based on a homemade modelling technique.     

亚波长光栅偏振片的纳秒脉冲激光损伤特性

偏振片在诸多光学系统中有着重要的应用。亚波长介质光栅可用作正入射偏振片，在高能激光系统中有着广泛的应用前景。为了探究波长为1 064 nm的纳秒脉冲激光对于亚波长全介质光栅的诱导损伤特性，使用了粒子群优化算法结合严格耦合波分析设计了光栅的几何参数，计算表明亚波长光栅偏振片在入射光波长1 064 nm附近带宽0.5 nm内，平均消光比为1 500。使用了紫外曝光配合离子束刻蚀的工艺制备了HfO₂光栅，并对其纳秒脉冲激光损伤阈值进行了测试。测试结果表明S光损伤阈值约为P光损伤阈值的5倍，且都大于5 J/cm2。结果表明亚波长全介质光栅偏振片可广泛用于正入射激光系统中。     

Broadband circular polarizer based on high-contrast gratings

A circular polarizer, which is composed of periodic and two-dimensional dielectric high-contrast gratings, is designed theoretically such that a unity conversion efficiency is achieved at λ(0)=1.55 μm. The operation is obtained by the achievement of the simultaneous unity transmission of transverse magnetic and transverse electric waves with a phase difference of π/2, meaning that an optimized geometrical anisotropy is accomplished. By the utilization of the rigorous coupled-wave analysis and finite-difference time-domain methods, it is shown that a percent bandwidth of ～50% can be achieved when the operation bandwidth is defined as the wavelengths for which the conversion efficiency exceeds 0.9.     

利用遗传算法优化线性啁啾光栅性能

针对制作町用于40 Gb/s全光色散补偿的宽带线性啁啾光栅时出现带内群时延纹波波动较大等问题.提出了一种通过设计和改变切趾函数的参量来优化线性啁啾光栅的新方法.该方法实现简单.只需根据需要设计具有不同滚降特性的切趾函数,同时利用遗传箅法来优化切趾参量.结合传输矩阵法经过200代获得了低带内时延纹波的线性啁啾光栅.数值结果验证了采取非对称分段切趾法在保持反射谱宽和平坦性的同时可以抑制带内群时延纹波的优越性.利用该方法制作了反射谱工作带宽为1.06 nm、时延纹波不超过45.60 ps、可用于大容量密集波分复用系统(DWDM)色散补偿的线性啁啾光纤光栅.     

相位调制边带失衡问题研究

 高功率激光系统中一般利用相位调制器对单纵模激光进行频谱展宽以避免受激布里渊散射，当后继系统中的滤波器存在中心波长偏差时，则会导致相位调制产生的边带失衡，使光信号出现较为严重的高频起伏。基于前端系统模拟平台对此问题进行了数值模拟研究，结果表明滤波后的高频起伏随滤波器带宽变窄和中心波长偏差增大而加剧。分析表明0.2 nm带宽的滤波器出现0.05 nm左右的波长偏差时，将引起24.3%左右的顶部起伏。实验中根据分析将滤波器带宽从0.2 nm加大到0.4 nm，成功将脉冲顶部起伏从25.2%减小到了4.4%，对前端系统的研制起到了很好的指导作用。     

采用WDM技术的光纤Bragg光栅传感网络

采用绝对测量原理的波长调制技术,光纤Bragg光栅可组成并行、串行和阵列WDM拓扑结构.分析表明,光纤Bragg光栅网络的工作原理类似于一个多宽带平面镜.利用光谱仪可测量上述光纤Bragg光栅网络的反射谱,其中,光源是宽带为～40nm的掺饵光纤放大器.当网络中的光纤Bragg光栅受扰动后,受扰光栅的反射谱发生相应的变化,即Bragg波长发生相应的偏移.结果表明,当事先确定了光纤光栅的波长调制范围,反射的峰值波长能反应光纤光栅传感网络的信息.值得注意的是～3nm的波长调制范围可满足～100℃和～2000με的参量测量.     

Optical Pulse Compression Schemes That Use Nonlinear Bragg Gratings

Nonlinear optical pulse compression of picosecond pulses typically requires long lengths of optical fiber and multiple components. Periodic structures, such as fiber Bragg gratings, are highly dispersive at wavelengths outside of the photonic bandgap. This implies that such gratings can be used as very short all-fiber compressors. In this paper a number of such compression schemes are reviewed involving uniform and nonuniform fiber Bragg gratings, relying on both soliton and nonsoliton compression principles. Experimental results supporting the corresponding underlying theory are also presented. Finally, an extension of one of the compression schemes is shown, which allows the generation of adjustable high-repetition rate soliton trains.     

Optical Pulse Compression Schemes That Use Nonlinear Bragg Gratings

Nonlinear optical pulse compression of picosecond pulses typically requires long lengths of optical fiber and multiple components. Periodic structures, such as fiber Bragg gratings, are highly dispersive at wavelengths outside of the photonic bandgap. This implies that such gratings can be used as very short all-fiber compressors. In this paper a number of such compression schemes are reviewed involving uniform and nonuniform fiber Bragg gratings, relying on both soliton and nonsoliton compression principles. Experimental results supporting the corresponding underlying theory are also presented. Finally, an extension of one of the compression schemes is shown, which allows the generation of adjustable high-repetition rate soliton trains.     

Tunable optical switch for DWDM systems

Characteristics of optical gratings on Si/SiGe heterostructures for switchable optical band-pass filters will be discussed. The grooves of the gratings have an angle of 45° to the propagation of light. The bandwidth can be as low as 0.2 nm. By reducing the refractive index of the SiGe layer due to injected free carriers it is possible to move the transmission band to lower wavelengths. In dense wavelength division multiplexing (DWDM) systems transmission channels can be selected by tuning the current. Therefore, the structure can be used as an electronic adjustable switch.     

Transmissive quasi-optical Ronchi phase grating for terahertz frequencies

A transmissive, square-wave Ronchi phase grating has been fabricated from the dielectric polytetrafluoroethylene to diffract an ~0.7 THz beam quasi-optically. When illuminated by a coherent, cw terahertz (THz) source, the spot separation of the ±1 diffractive orders and the diffraction efficiency were measured as a function of THz frequency and rotation angle. The grating performance depends sensitively on the refractive index, whose value can be measured with an accuracy limited by the fabrication precision. The use of these gratings for polarization-insensitive quasi-optical imaging and phased arrays is discussed.     

Ultrabroad-bandwidth electro-optic modulator based on a cascaded Bragg grating

It is shown that the bandwidth of a traveling-wave electro-optical modulator can be greatly increased by matching of the group velocities of the optical and rf waves in the waveguides with cascaded Bragg gratings. A LiNbO>(3) Mach-Zehnder modulator with 1-V half-wave voltage and a bandwidth in excess of 100 GHz is proposed and its performance evaluated.     

Analysis of Interdigital Electro-optic Bragg Diffraction Grating

Some integrated optics devices can be made based on the interdigital electro-optic Bragg diffraction grating. The point-matching method is extended to the analysis of interdigital electro-optic Bragg diffraction gratings. This method provides a simple and fast analytic expression of the electric field in the structure. The field distributions are used to calculate the optical and electrical characteristic parameters of the gratings. The effects of finite conductor thickness have been taken into account in the analysis. It is shown that the simulation results agree well with the measured data.     

Dispersion compensation by microstructured optical devices in ultrafast optics

Dispersion compensation is a ubiquitous problem for the generation and application of ultrashort optical pulses. Early approaches to compensate for the spectral group-delay dependence in materials used prism and grating sequences for this purpose, but are limited in bandwidth. Microstructuring dielectric optical materials on the scale of the optical wavelength have developed as an alternative for inducing a desired spectral group-delay dependence. With this approach a nearly arbitrary dependence of group delay vs. wavelength can be compensated for. We will discuss different approaches to microstructured dispersion compensation, namely chirped Bragg gratings, different generations of chirped mirrors, quasi-phase-matching gratings, and arrayed-waveguide gratings. We will outline common limitations and discuss ideas to expand further the utility of these approaches. PACS 42.65 Re; 42.79 Wc; 42.81 Wg     

Simultaneous wavelength tuning and amplitude modulation in QPM optical parametric amplifiers

We repot simultaneous optical parametric amplifiers (OPA) and amplitude modulation of signal pulse in a monolithic periodically poled KTP (PPKTP) by electrically controlling the relative phase between the pump, signal and idler waves in a KTP dispersion section sandwiched between two PPKTP gratings, theoretically. The controlling electric field can be arbitrary direction. For an ultrashort pulse group velocity mismatch, group dispersion and diffraction must be regarded, simultaneously. The solutions of the truncated equations, including the above effects in the two PPKTP gratings of simultaneous frequency tuning and amplitude modulation OPA, were analyzed. It is shown that the intensity of output signal pulse of OPA depends on the thickness of the dispersion section, the magnitude and azimuth of controlling electric field and one evidences periodicity. For the certain angular dispersion there exist the largest frequency bandwidths of optical parametric amplifiers. Certain angular dispersion of a signal pulse can increase the frequency bandwidth, and broadband amplification of ultrashort pulse can be performed effectively.     

Optical phase front control in a metallic grating with equally spaced alternately tapered slits

A technique capable of focusing and bending electromagnetic （EM） waves through plasmonic gratings with equally spaced alternately tapered slits has been introduced. Phase resonances are observed in the optical response of transmission gratings, and the EM wave passes through the tuning slits in the form of surface plasmon polaritons （SPPs） and obtains the required phase retardation to focus at the focal plane. The bending effect is achieved by constructing an asymmetric phase front which results from the tapered slits and gradient refractive index （GRIN） distribution of the dielectric material. Rigorous electromagnetic analysis by using the two-dimensional （2D） finite difference time domain （FDTD） method is employed to verify our proposed designs. When the EM waves are incident at an angle on the optical axis, the beam splitting effect can also be achieved. These index-modulated slits are demonstrated to have unique advantages in beam manipulation compared with the width-modulated ones. In combination with previous studies, it is expected that our results could lead to the realization of ootimum designs for plasmonic nanolenses.     

叉指电光布拉格衍射光栅分析

叉指电光布拉格衍射光栅能够用于制作集成光学器件。将点匹配法扩展应用于又指电光布拉格衍射光栅分析,将光栅各区域的势函数表示为该区域中满足拉普拉斯方程的一系列基函数的级数,匹配边界上有限个点的边界条件以确定出级数项的系数,从而得到叉指电傲电场分布的解析表达式,在此基础上得到了叉指电光布拉格衍射光栅的一些重要的光学与电学特性参量。将分析结果与测量值进行了对比,发现二者十分吻合。所提出的分析方法简便快捷、精度高,所导出的电场分布的解析表达式和分析结果对叉指电光衍射光栅的优化设计具有重要的意义。     

双光栅快速扫描光学延迟线的色散补偿

光学相干层析成像(OCT)系统的纵向分辨力不仅与光源的带宽有关,而且与系统中两干涉臂间的色散匹配有关。如果色散没有得到精确匹配,将使光学相干层析成像系统的纵向分辨力达不到所预期的理论值。色散问题在超高分辨光学相干层析成像系统中尤为突出。提出了一种基于双光栅快速扫描光学延迟线(RSOD),用于光学相干层析成像系统的色散补偿。该方法中新增的光栅引入了光栅间距这一独立变量,其与常规单光栅快速扫描光学延迟线机构中的光栅离焦量一起,可使光学相干层析成像系统中的群速度色散(GVD)和三阶色散(TOD)同时得到补偿。分析了双光栅快速扫描光学延迟线的色散特性和色散调节原则,并提供了一个典型光学相干层析成像系统中的色散补偿实例。     

Analysis of Complex Dielectric Phase Gratings by Hybrid FEM-Gaussian Mode Expansion Technique

Hybrid technique of finite element method (FEM) and Gaussian mode expansion method is implemented for the analysis of transmission property of dielectric phase gratings used in holographic power combining systems. It takes advantages of the versatility of FEM and the simplicity of the mode expansion method. Apparently, this hybrid method is well suited for the analysis of phase gratings with complex profiles, which provide more degrees of structural freedom than those of classical rectangular groove gratings. The transmitted fields of an array of two and three fundamental Gaussian beams through the multi-stepped and curved gratings are calculated by the hybrid method. Numerical results show that much less phase distortion can be fulfilled for the reconstructed pseudo-plane wave by the modified phase gratings, especially the curved ones. This is advantageous for improving the combining efficiency in spatial millimeter wave and sub-millimeter wave power combining systems.