Theoretical study on coupling effects of modulation depth between two photorefractive phase gratings with an external applied field

We used the perturbation expanding method to the hopping model and studied coupling effects of the modulation depth between two photorefractive phase gratings stored in one point with an external applied DC electric field . It has been found that the modulation depth of one of the two gratings seriously affects the spatial-charge field of the other grating.     

不同折射率调制的光纤光栅的特性及其应用

介绍了九种不同折射率调制的光纤光栅的折射率分布及其光谱特性,建立了各种光纤光栅的数学模型,并分析了它们的优缺点及其应用特点。给出了各种光纤光栅之间的联系,并指出两种或多种单一折射率分布的光纤光栅的组合形式将是以后的发展趋势。以折射率分布为出发点,深入地讨论了各种光纤光栅的周期、折射率调制深度、中心反射光波长或透射光波长、带宽、反射率等参数与其功能的关系,为实际系统的设计提供了参考。     

Submicrometer grating fabrication in fused silica by interferometric laser-induced backside wet etching technique

Submicrometer period fused silica gratings were produced by two-beam interferometric laser-induced backside wet etching technique (TWIN LIBWE). The fourth harmonic of a Nd:YAG laser beam was spatially filtered in two steps, and the smoothened beam was split into two parts and interfered at incident angles of 60°, 30°, 14°, and 7.7°, respectively, on the backsides of fused silica plates that were in contact with a liquid absorber. The periods of the produced fused silica gratings were, respectively, 154 nm, 266 nm, 550 nm, and 990 nm. In the next step, TWIN-LIBWE setup was completed by using a coupling rectangular prism in order to reach immersion setup, which made possible to fabricate 104 nm period fused silica grating. This is the smallest laser-generated grating constant in fused silica at present. The morphology of the etched gratings was characterized by atomic force microscope. Important parameters (modulation depth, low-pass filtered waviness, quality factor) of the produced gratings were determined. Evolution of the grating parameters was also studied in the 990 nm case: the dependence of modulation depth, waviness, and quality factor on the number of laser pulses was investigated.     

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.     

Mystery of the double limit in homogenization of finitely or perfectly conducting periodic structures

We show that the difference in the effective medium properties of perfectly or finitely conducting short-period gratings is because for finitely conducting gratings it is possible to completely homogenize the electromagnetic field vector components, contrary to perfectly conducting gratings. As a consequence, for aluminum in the microwave domain, two possible effective media can be found, depending on whether the feature dimensions are much larger or shorter than the depth of field penetration inside the metal.     

双带通啁啾光纤光栅光谱特性仿真及分析

针对啁啾光纤光栅的带通滤波特性,设计了纤芯/包层复合结构的啁啾光纤光栅滤波器.基于光纤耦合模和传输矩阵理论及坐标变换方法,对其光谱特性进行仿真分析,验证了该光栅的双带通滤波特性;对周期为536、537、538nm,长度为8、10、12mm,折射率调试深度为0.0002、0.0003、0.0004,啁啾系数为0.05、0...     

Polymer gratings based on photopolymerization for low-order distributed feedback polymer lasers

Novel polymer distributed feedback(DFB)gratings are fabricated based on photopolymerization to reduce lasing threshold of polymer lasers.A photopolymer formulation sensitive to 355-nm ultraviolet(UV)light is proposed for the fabrication of polymer gratings and it can be used to form polymer films by spin-coating process.A very low surface-relief depth ranging from 12.5 to about 1.0 nm has been demonstrated with a refractive-index modulation of about 0.012.The experimental results indicate that such polymer gratings have promising potentials for the fabrication of low-order DFB organic semiconductor lasers.     

Phase Modulation Characteristics Analysis of Optically-Addressed Parallel-Aligned Nematic Liquid Crystal Phase-Only Spatial Light Modulator Combined with a Liquid Crystal Display

An optically addressed parallel-aligned nematic-liquid-crystal spatial light modulator (PAL-SLM) has been studied as a dynamic phase-only light modulation device. The phase modulation characteristics of the PAL-SLM using a liquid crystal display (LCD) as an addressing mask were investigated by analyzing diffraction efficiencies resulting from binary gratings projected from the LCD. A more than 2π phase-only modulation depth was achieved. The highest first-order diffraction efficiency of approximately 38% was also obtained; this is close to the theoretical limit. The experimental results of diffraction efficiencies depending on the phase modulation depth are in good agreement with the simulation for the system operation.     

Controllable grating fabrication by three interfering replicas of single femtosecond laser pulse

The controllable periodic M-shape gratings are fabricated on the surface of silica glass by three coplanar interfering beams from a single femtosecond pulse. The configuration of the M-shape periodic structure is characterized by optical microscopy and atomic force microscopy. The experimental results and the theoretical simulation show that the period and the modulation depth ratio between the neighboring grooves of the fabricated gratings can be controlled by adjusting the collision angles and pulse energy of the three beams, respectively.     

Fabrication of 550 nm gratings in fused silica by laser induced backside wet etching technique

A series of 550 nm spacing gratings were fabricated in fused silica by laser induced backside wet etching (LIBWE) method using the fourth harmonic of a Q-switched Nd:YAG laser (wavelength: λ = 266 nm; pulse duration: FWHM = 10 ns). During these experiments we used a traditional two-beam interference method: the spatially filtered laser beam was split into two parts, which were interfered at a certain incident angle (2θ = 28°) on the backside surface of the fused silica plate contacting with the liquid absorber (saturated solution of naphthalene-methyl-methacrylate c = 1.85 mol/dm³). We studied the dependence of the quality and the modulation depth of the prepared gratings on the applied laser fluence and the number of laser pulses. The surface of the etched gratings was characterized by atomic force microscope (AFM). The maximum modulation depth was found to be 180-200 nm. Our results proved that the LIBWE procedure is suitable for production of submicrometer sized structures in transparent materials.     

Angular Multiplexing Gratings Characterization in Dichromated Gelatin and Thoughts on Diffractive Optical Memory

We were mainly concerned here with multiple exposure gratings that allow interactions and coupling between gratings. We started the study of interactions and coupling with five gratings recorded in dichromated gelatin (DCG). Having determined experimentally the characteristic parameters of these five gratings, we used a method developed by Oxford University to synthesize the modulation profile. Entering these parameters of each grating into our computing programs (based on the alternative theory of diffraction in modulated media) and by varying the amplitude modulation, the latter was deduced by adjusting the numerical curve to the experimental one. We then extended the study to nine multiplexed gratings. This study allowed us to know the range of the amplitude modulation such that a certain number of angular multiplexing can be done in DCG. Several undesirable effects which we had experimentally observed appeared then during the reconstruction of any one of the multiplexed images, such as cross-talk effect. We noted the good concordance between experimental and theoretical results. We finally illustrated cross-talk effect in a diffractive memory where, if the amplitude modulation was first calculated, the number of multiplexing for an angular range could then be determined and the angular separation achieved to avoid cross-talk.     

用相位掩模法制作光纤光栅的技术

准分子激光照射相位掩模后,在近场形成周期的条纹分布,利用近场光强对光敏光纤进行曝光,可以在纤芯中写人光栅。对光纤光栅折射率分布进行合理假设,运用耦合模理论计算了光纤光栅的反射率,对其反射率与相关参数的关系进行了讨论,并且分析了用相位掩模技术制造光纤光栅的影响因素,得出要获得窄带宽高反射率光栅需有适当的折射率调制和大的光纤光栅长度的结论。另外还导出了掩模板后表面的近场光强分布和光纤光栅的周期公式。     

Surface plasmon resonance spectroscopy on rotated sub-micrometer polymer gratings generated by UV-laser based two-beam interference

Two-beam interference method was applied to generate gratings having periods of 416 nm and 833 nm by the forth harmonic of a Nd:Yag laser on thin poly-carbonate films spin-coated onto silver layer-covered substrates. The dependence of the modulation depth on the fluence and number of laser pulses was investigated by atomic force microscopy. A secondary pattern appeared on very thin polymer layers thanks to the “p” polarized laser beam illumination induced self-organized processes. The conditions of the emergence of grating-coupling caused additional plasmon resonance peak were determined for the sub-micrometer periodic polymer gratings. Surface plasmon resonance measurements were performed in attenuated total reflection arrangement to determine the effect of the angle between the plasmon propagation direction and the polymer groves on the grating-coupling. The effect of the modulation depth on the grating-coupling caused additional resonance minimum was also analyzed. We found coupling phenomena according to our calculations, the differences between the measured and theoretically predicted resonance curves were explained by the scattering on the complex surface structure.     

Observation of self-diffraction by gratings in nematic liquid crystals doped with carbon nanotubes

Diffraction gratings were studied in cells of the homogeneously aligned liquid-crystal E7 doped with multiwall carbon nanotubes. These phase gratings were induced by interference modulation of two coherent optical beams, in conjunction with an applied dc field that was perpendicular to the unperturbed director axis. Self-diffraction was observed at all angles of incidence of the writing beams, including normal incidence. A superior nonlinear-index coefficient of 5x10(-2)cm(2)/W was obtained after passage of a 44-mW/cm(2) beam through a film with a grating constant of 18 mum under an external voltage of 15 V. The observed phenomenon depends strongly on the applied dc field, and the memory effect in a nematic film depends strongly on the grating constant.     

Switchable two-dimensional gratings based on field-induced layer undulations in cholesteric liquid crystals

We propose switchable two-dimensional (2D) diffractive gratings with periodic refractive-index modulation arising from layer undulations in cholesteric liquid crystals. The cholesteric cell can be switched between two states: (1) flat layers of a planar cholesteric texture and (2) a square lattice of periodic director modulation associated with layer undulations that produces 2D diffraction patterns. The intensities of the diffraction maxima can be tuned by changing the applied field. The diffractive properties can be optimized for different wavelengths by appropriately choosing cholesteric pitch, cell thickness, and surface treatment.     

A flexible fast 3D profilometry based on modulation measurement

This paper proposes a flexible fast profilometry based on modulation measurement. Two orthogonal gratings through a beam splitter are vertically projected on an object surface, and the measured object is placed between the imaging planes of the two gratings. Then the image of the object surface modulated by the orthogonal gratings can be obtained by a CCD camera in the same direction as the grating projection. This image is processed by the operations consisting of performing the Fourier transform, spatial frequency filtering and inverse Fourier transform. Using the modulation distributions of two grating patterns, we can reconstruct the 3D shape of the object. In the measurement process, we only need to capture one fringe pattern, so it is faster than the MMP and remains the advantages of it. In the article, the principle of this method, the setup of the measurement system, some simulations and primary experiment results are given. The simulative and experimental result proves it can restore the 3D shape of the complex object fast and comparatively accurate. Because only one fringe pattern is needed in the testing, our method has a promising extensive application prospect in real-time acquiring and dynamic measurement of 3D data of complex objects.     

Plasmon resonances in deep nanogrooves of reflective metal gratings

Metallic gratings with very deep and narrow grooves are fabricated and their reflection spectra are characterized, which explicitly show high-order standing-wave-like resonances of surface plasmons in the cavity of nanogrooves. The effect of in-plane surface plasmon resonance is also observed, which is shown to have only a minor role on the reflection of such gratings, unlike that for shallow gratings. Using numerical simulations, the cavity resonances and their effects on the reflection of the gratings are identified and further analyzed. As field is more enhanced in the nanogrooves under cavity resonance conditions, the enhancement is also found to be dependent on the grating period, i.e. the strongest enhancement takes place for higher-order resonance modes for smaller grating period. For gratings with shallower grooves, comparable enhancement of field is also achievable by proper design of the grating period. The study suggests that field enhancement can be realized at selective wavelengths in a wide spectrum range using cavity resonances in the deep nanogrooves of metal gratings, and the position for field-enhancement can be tuned by the depth and period of the gratings.     

Observation of type I and type II gratings behavior in polymer optical fiber

We report the first observation of type I and type II fiber grating behaviors in making polymer optical fiber Bragg gratings. Our observation reveals that there are two distinctive stages in the fabrication of polymer optical fiber Bragg gratings. Notably the two stages of grating formation correspond to low and high-index modulation gratings, which match well with those in silica fiber grating fabrication. Thus we refer them as type I and type II polymer fiber gratings, following the same nomenclatures for different types of silica fiber gratings. In addition, the characteristics and formation mechanism for type I and type II polymer fiber gratings are also examined.     

大视场消色差温度补偿型风成像干涉仪调制度的分析与计算

简要论述了大视场、消色差、温度补偿型风成像干涉仪用于高层探测的基本原理.对自行设计的风成像干涉仪的调制度进行了理论分析与计算,推导了调制度的理论表达式.采用计算机模拟,分析了仪器调制度随入射角的变化关系.当调制度大于075时,风成像干涉仪视场角可达129°,充分显示了大视场、高调制度的显著特点.分析了玻璃折射率对调制度的影响,得到了调制度取得最大值时玻璃的优化组合;分析了温度对调制度的影响,得到了调制度随着温度变化的曲线.此研究为高层大气探测理论的发展、大气探测以及大视场、消色差、温度补偿型风成像干涉仪的研制提供了重要的理论依据及实践指导. 关键词： 干涉成像技术 迈克尔逊干涉仪 调制度 双光束干涉     

Sub-500-nm patterning of glass by nanosecond KrF excimer laser ablation

The surface of flint glass of type F2 is patterned by nanosecond KrF excimer laser ablation. Strong UV absorption provides a comparatively low ablation threshold and precise ablation contours. By using a two-grating interferometer, periodic surface patterns with 330 nm period and 100 nm modulation depth are obtained. This method enables the fabrication of 7 mm×13 mm wide grating areas with perfectly aligned grooves without the need of high-precision sample positioning. By double exposure, crossed gratings with adjustable depths in the two orthogonal directions can be generated.