Femtosecond IR laser fabrication of Bragg gratings in photonic crystal fibers and tapers

Fiber Bragg gratings were written in pure silica photonic crystal fibers (PCFs) and PCF tapers with 125-fs 800-nm infrared radiation. High reflectivities were achieved with short exposure times in the tapers. Both multimode and single-mode grating reflections were achieved in the fiber tapers.     

Plasmonic Metasurfaces for Simultaneous Thermal Infrared Invisibility and Holographic Illusion

In 1860s, Gustav Kirchhoff proposed his famous law of thermal radiation, setting a fundamental contradiction between the infrared reflection and thermal radiation. Here, for the first time an ultrathin plasmonic metasurface is proposed to simultaneously produce ultralow specular reflection and infrared emission across a broad spectrum and wide incident angle range by combining the low emission nature of metal and the photonic spin–orbit interaction in spatially inhomogeneous structures. As a proof‐of‐concept, a phase gradient metasurface composed of sub‐wavelength metal gratings is designed and experimentally characterized in the infrared atmosphere window of 8–14 µm, demonstrating an ultralow specular reflectivity and infrared emissivity below 0.1. Furthermore, it is demonstrated that infrared illusion could be generated by the metasurface, enabling not only invisibility for thermal and laser detection, but also multifunctionalities for potential applications. This technology is also scalable across a wide range of electromagnetic spectrum and provides a feasible alternative for surface coating.     

Bragg Gratings Made in Highly Nonlinear Bismuth Oxide Fibers With Ultrafast IR Radiation

High-quality Bragg gratings with refractive index modulations $>7times 10^{-4}$ were induced in highly nonlinear bismuth oxide glass fibers using ultrafast infrared radiation and a phase mask. Grating reflectivities approaching 95% for a 6-mm-long grating were achieved. Annealing tests show good thermal stability of the grating structure below 220 $^{circ}hbox{C}$ . The Bragg resonance shift with temperature is observed to be 3–4 times higher than similar gratings in silica fiber.      

Ultrafast IR Laser Writing of Strong Bragg Gratings Through the Coating of High Ge-Doped Optical Fibers

Strong fiber Bragg gratings were written through the standard polymer coatings of "off-the-shelf" high numerical aperture single-mode optical fibers after a few seconds exposure with femtosecond pulse durations of infrared radiation through a phase mask. While writing through the acrylate coating, we obtained index modulations of up to 1.4 x 10^-3 and 7 x 10^-4 with and without H₂ -loading, respectively.     

Effect of the Fiber Coating on the Radiation Sensitivity of Type I FBGs

We have studied the influence of the fiber coating on the radiation sensitivity of fiber Bragg gratings (FBGs). These FBGs are draw tower gratings written before applying the coating in a fiber with a photosensitive 18-mol.% GeO$_{2}$-doped core. Using a Co$^{60}$ source we irradiated polyimide, acrylate, and ormocer coated gratings as well as mechanically stripped ormocer coated FBGs up to a 40-kGy dose. This total dose level corresponds to a near-earth space radiation environment mission. We show that the coating type must be taken into account for correctly interpreting ionizing radiation effects on FBGs.      

铒镱共掺光纤二次谐波的产生

报道了一种在光纤中激发二次谐波振荡(SHG)的简单方法：利用紫外脉冲激光在载氢后的高浓度铒、镱共掺的光纤上刻制布拉格光栅。从而打破了石英光纤二阶极化率为零的固有限制，形成了极大的永久的二阶极化率，周期性的结构辅助实现了相位的匹配。在实验中，利用波长为980nm的半导体激光器(LD)抽运该光纤光栅，在只有几毫瓦的抽运功率下，便观察到了显著的蓝绿光辐射，而且强度随抽运功率的增加而增加。在显微镜下对光纤光栅发光现象进行了观测并拍摄了数码相片，在相片中观测到光纤光栅纤芯处的蓝绿光辐射，而非光栅区没有发光现象，同时观测到了紫外光辐射。     

辐射对准周期多层平板介质栅传输特性的影响

本文多模网络理论与模匹配相结合的方法对准周期多层平板介质栅的传输特性进行了系统的分析，深入讨论了辐射现象对准周期传输特性的影响，为各种介质栅结构的精确设计了依据。     

The Effect of Radiation on Transmission of Quasi-Periodic Multi-Layer Planar Dielectric Gratings

The transmission characteristics of quasi-periodic multi-layer planar dielectric gratings are systematically investigated by a method that combines the multimode network theory with rigorous mode matching method. The effect of radiation on the transmission characteristics of the multi-layer dielectric gratings is discussed in details, and some guidelines are given for accurate design of the gratings.     

Tunable Submillimeter Interferometers of the Fabry-Perot Type

Fabry-Perot Interferometers (FPI's) have been constructed for the far infrared and submillimeter wave region with metal grids as reflectors. Two-dimensional grids with square holes (metallic mesh) can be used for unpolarized radiation, too, and they are more convenient for the construction than one-dimensional (parallel wire) grids. The performance of several FPI's has been measured in the 100-600 /spl mu/ wavelength region. Q values in first-order range from 5 to 30, and peak transmissions up to 0.9 have been reached. The experimental results are in qualitative agreement with the theory of thin parallel wire gratings. The influence of the unevenness of the reflectors is studied theoretically. Applications of a submillimeter FPI include its use as the dispersion element in a spectrometer, as a narrow-band filter to check the radiation purity of a grating spectrometer, and as a separator of the harmonics from a crystal harmonic generator.     

局域光场增强的量子阱红外探测器（特邀）

量子阱红外探测器是继碲镉汞红外探测器之后又一重要的可以在中、长波段和甚长波段工作的红外探测器件。它在长波红外探测、多色探测及其焦平面技术方面表现出比碲镉汞红外探测器更具特色的优势,对量子阱红外探测器的研究将在很大程度上推动我国红外探测器技术的发展。这一探测器的突出优势是其材料均匀性好,制备技术成熟。但是由于量子效率偏低,且无法直接吸收垂直入射红外光,所以需要针对不同的红外探测波段,设计和制备各类光栅或微腔结构来进行光耦合及局域光场增强以有效提升探测器性能。如何更有效提升量子阱红外探测器的光耦合效率,降低暗电流,提高器件工作温度是仍然是目前研究的重点。文中着重介绍和总结了近5年来研究的局域光场增强的新型量子阱红外探测器,从提高探测器光耦合效率、降低器件暗电流和提高工作温度等方面重点讨论各种量子阱红外探测器的新结构和新机理,同时展望了这一探测器的未来发展方向。     

Two-dimensional bi-periodic grating coupled one- and two-colorquantum well infrared photodetectors

Multiple quantum well infrared photodetectors (QWIP's) with two-dimensional bi-periodic grating couplers have been demonstrated. For one-color QWIP's, the response linewidths of such QWIP's are found to be much wider than those of conventional grating coupled QWIP's. The bi-periodic gratings have also been applied to two-color QWIP's to separately optimize the absorption at two wavelength bands. The large difference in responsivities at the two wavelength bands for devices with single period gratings can be greatly reduced with the use of bi-periodic gratings     

Unified Approach for Coupling to Cladding and Radiation Modes in Fiber Bragg and Long-Period Gratings

Couplings of core mode to guided cladding and unguided radiation modes in fiber Bragg and long-period gratings are investigated by a unified approach based on complex mode matching method (CMMM). With the combination of the perfectly matched layer (PML) and the perfectly reflecting boundary (PRB), the continuous radiation modes can be well represented by a set of discrete complex modes so that simulation of coupling to radiation modes is greatly simplified and may be treated in the same fashion as guided modes. Numerical results of fiber Bragg and long-period gratings with refractive index of the outer cladding lower, equal, and higher than that of the inner cladding indicate that the unified approach is highly effective in the simulation of couplings to cladding and radiation modes.      

Analysis of second-order gratings

The results of an analysis of second-order gratings used as distributed Bragg reflectors in surface-emitting lasers are presented. The gratings provide reflection, output coupling, and power transmission to other gain segments for purposes of injection locking. The analysis determines these quantities for arbitrary-shaped grating teeth and includes the presence of a substrate reflector to reduce the radiated power in that direction. The reflector is shown to be effective, but only if it can be precisely positioned. Examples illustrating variations in dimensions, tooth shapes and heights, waveguide loss, and detuning are included. Second-order, square gratings, whose conventional coupling coefficient is identically zero, exhibit substantial reflectivity which is primarily produced by the radiation reaction. The analysis also forms the basis for calculating the far-field grating coupled radiation patterns     

Accelerated recording of negative index gratings in Ge-doped optical fibers using 248-nm 500-fs laser radiation

The fabrication of Bragg gratings in commercial B-Ge and high-Ge optical fibers using 248-nm 500-fs laser radiation is demonstrated. The underlying photosensitivity process is that of Type IIA, where negative index change gratings are recorded, following a nonmonotonic index change growth. Refractive index evolution curves for both average and modulated index changes are presented. Index changes of the order of 10/sup -3/ are obtained for the fibers exposed, under 2.4 kJ/cm/sup 2/ accumulated energy density. For comparative purposes grating exposures using 248-nm nanosecond excimer laser radiation are also appended. Thermal annealing studies performed revealed that Type IIA negative index gratings fabricated using femtosecond pulses exhibit enhanced thermal stability, enduring temperatures up to 650/spl deg/C.     

Volume current method for analysis of tilted fiber gratings

An analytic approach to calculate the radiation pattern (including the wavelength dependence, azimuthal distribution, and polarization dependence) of tilted fiber phase gratings using the volume current method (VCM) is introduced in this paper. Results for both uniform and apodized fiber gratings are derived and discussed, and a brief comparison to experimental measurements from practical devices is also made     

Analysis of grating-coupled radiation in GaAs:GaAlAs lasers and waveguides

Grating-coupled radiation in GaAs:GaAlAs lasers and waveguides is analyzed. A general formulation is developed for arbitrary-shaped gratings which need not be small in size. Two methods are used to solve the resulting equations in the case of rectangular-shaped gratings. The first is a perturbation technique and the second is iterative in nature. The iterative procedure converges to a numerical exact solution in many cases of practical interest and indicates that the perturbation results are quite accurate. Curves are presented for radiated power from traveling waves as a function of grating tooth height, tooth width, refractive index, waveguide thickness, and period for rectangular gratings in heterostructure waveguiding geometries. It is shown that radiation is not a monotonically increasing function of tooth height, but rather maxima occur when the teeth are half the optical wavelength in the material. Also, in particular geometries with an air:GaAs grating interface, radiated power of a mode can exceed 100 cm^-1.     

Narrow-depressed cladding fiber design for minimization of claddingmode losses in azimuthally asymmetric fiber Bragg gratings

The cladding and radiation mode loss characteristics of photo-induced gratings written in fibers with a narrow-depressed inner cladding are compared to the properties of gratings written in matched clad, photosensitive clad, and wide-depressed cladding fibers under varying grating azimuthal asymmetry conditions. Fibers with narrow-depressed claddings are found to have lower cladding mode losses for small degrees of asymmetry than previously proposed fiber designs for cladding-mode reduction     

Simple formulae for the transmittance of strip gratings

We present simple but accurate formulae for the transmittance through strip gratings of infinitesimal thickness. The formulae are valid irrespective of the angle of incidence of radiation on the gratings and can take into account the effects of a dielectric substrate. We demonstrate the range of applicability of the formulae by comparing their predictions with the results of rigorous calculations.     

高分辨率近红外成像光谱仪光学系统

研究了一种在近红外波段具备良好成像能力的双凹面光栅成像光谱仪系统。对这种串联光栅系统中存在的主要像差像散和彗差进行了分析,并计算获得了该系统的最优成像条件:两个光栅和柱面透镜的最优摆放位置。这种改进型的Wadsworth系统可以在全波段近似消除彗差和像散,具备良好的光学成像质量,并仅通过刻线密度较低的光栅即可实现高光谱分辨率。设计了一个工作于780~1 100 nm波段的成像光谱仪系统,其光谱采样达到0.92 nm/pixel,全视场调制传递函数在17 lp/mm的奈奎斯特频率下高于0.45,系统像差得到充分校正,且加工和装调公差比较宽松。研究结果分析证明了设计理论的正确性。