Effects of grating order and tooth rounding in laterally coupled distributed feedback lasers

We demonstrate the results of an analysis of laterally coupled distributed feedback (LC-DFB) lasers with higher order gratings, including the effects of radiating partial waves. For a given fabrication resolution, first-order gratings, if they can be reliably manufactured, always provide the strongest coupling. However, at resolutions requiring higher order gratings, if duty cycles of >0.5 are used, the lowest grating order is not always the one with the strongest coupling. An analysis of the rounding of the grating teeth showed that the required threshold gain was increased by nearly 20% when the rectangular grating was rounded in fabrication.     

Geometric micron-moiré

A high spatial resolution and high-sensitivity geometric micron-moiré method is presented. The method is based upon classical geometric moiré method and microscopic principle, the frequency of the specimen and reference gratings used in this method can be higher than 1000 lines/mm. By a microscopic moiré system, the high-frequency gratings could be magnified to a low-frequency level, so geometric moiré patterns can be formed. Experiments show that both the spatial resolution and the sensitivity can reach or exceed micron-level.     

Use of spherical gratings in a toroidal grating mount for vuv-soft x-ray synchrotron radiation

A toroidal grating monochromator (TGM) based beamline was designed earlier and it is being setup on the 450 MeV Indus-1 synchrotron radiation source. This beamline operates in 40–1000 Å wavelength range and gives a moderate resolution (～500) at high throughput. Now-a-days, spherical gratings are giving very high resolution in some complex monochromators. We have evaluated the use of spherical gratings in the simple rotation mount of a TGM. The motivation has been to evaluate if the existing TGM beamline, without any hardware changes, can be used with the much cheaper spherical gratings. The two monochromators, employing toroidal and spherical gratings respectively, have been compared by performing detailed ray tracing calculations. Our study shows that, with the same mechanical hardware, a real spherical grating monochromator gives better performance compared to the conventional TGM with respect to the spectral resolution and the flux. Furthermore, for moderate resolution this improvement is realized even when the exit slit is not moved to keep the spherical grating in-focus.     

Spectral-mismatch-induced resolution limit of interferometric fiber Fabry-Perot sensor system

The mismatch of fiber Bragg gratings (FBGs) in spectral profiles can lead to a severe degraded resolution of the constructed fiber Fabry-Perot (FFP) sensor system through its effect on the fringe visibility. The variation of visibility induced by spectral mismatch and the corresponding phase resolution limit are analyzed theoretically and experimentally. Theoretical analyses are based on the approximation of Gaussian profiles to the reflection spectra of weak FBGs, especially with consideration of side lobes. The investigation provides an insight into the evolution of the fringe visibility caused by spectral mismatch, and shows good agreement with experimental results. An optimum phase resolution of about 55 μrad/Hz^1/2 above 100 Hz is achieved for a nearly 4 m-long FFP sensor by matching spectral profiles of the gratings and balancing path length differences of the tandem interferometers.     

Sub-wavelength imaging of photo-induced refractive index pattern in chalcogenide glass films

Mapping of refractive index patterns with sub-wavelength resolution is achieved using Near-field Scanning Optical Microscopy (NSOM) in reflection mode. Imaging of index pattern is performed on surface gratings photo-imprinted in As₂S₃ films. The NSOM is adapted with a near infrared laser which wavelength (785 nm) is chosen to be within the transparency window of the glass film therefore allowing consistent measure of reflected light. Quantitative measurements of photo-induced index changes can then be obtained from knowledge of the initial film index. Images of gratings with a period of 0.5 micron are easily collected therefore demonstrating sub-wavelength spatial resolution. The technique permits to concurrently obtain a topographic image and index image of the gratings thereby permitting to quantify the extent of photodarkening and photoexpansion simultaneously. It is shown that relief gratings tend to vanish in films aged in air for several months however the index gratings remain.     

Excimer laser-induced etching of sub-micron surface relief gratings in fused silica using phase grating projection

Phase grating projection using a Schwarzschild objective is presented in conjunction with a process that allows laser-induced etching of transparent materials at the interface of liquids, in order to generate periodical surface relief structures with sub-micron resolution in fused silica. The achieved sinusoidal gratings exhibit a period of 780 nm and a depth of up to 180 nm, having a roughness lower than 5 nm r.m.s. The depth and roughness of the gratings are related to the applied laser fluence and pulse number. In addition to the grating formation, an overall removal of material at higher laser fluences was found. Received: 3 December 2001 / Accepted: 5 December 2001 / Published online: 11 February 2002     

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。     

Design of aberration-corrected flat-field holographic concave grating by parameter optimization

Based on the aberration theory of concave grating [1], [2], [3], a method to design aberration-corrected flat-field holographic concave gratings by parameter optimization is introduced. And the influence of different parameters such as the incident angle, effective grating grooves and grating radii on the aberrations at diffracted order ?1 is discussed. The results indicate that for a concave grating working in a given spectral range, we can correct aberrations by optimizing the incident angle. In addition, flat-field holographic concave gratings with higher resolution and lower aberration can also be designed by choosing the effective grating grooves and larger grating radius properly.     

一种制作超高频全息光栅的新方法

为了满足测量的高分辨率和灵敏度，基于光栅为关键元件的光学测量方法要求光栅有很高的频率。在已有的高折射率介质制栅方法的基础上提出了一种新的制作超高频全息光栅的方法，该方法制栅准确、简单、方便，其特点是所制光栅的频率与激光波长及介质的折射率无直接关系，而是等于制栅光路所得频率与两倍母栅频率之和。实验表明，利用该方法制得１００００线／ｍｍ的光栅是完全可能的     

High spectroscopic qualities in blazed ion-etched holographic gratings

Blazed ion-etched holographic gratings with a high absolute diffraction efficiency, good diffracted wavefront, high resolution, and low stray light have been produced. The performances were proved to be the same as, or superior to, commercially available blazed gratings produced by a ruling technique.     

A high-resolution spatial heterodyning interference spectrometer

A novel spectrometer which has a resolution of 0.013 nm with high throughput is described. The system uses a Michelson interferometer in which the mirrors have been replaced with Littrow-mounted diffraction gratings. First-order diffracted beams returning from the gratings interfere at the interferometer output to produce a spatial interferogram which is heterodyned about an optical frequency determined by the setting angles of the gratings. The heterodyning process leads to increased resolution, but limits the free-spectral range of the instrument to, in our case, about 3 nm. This is sufficient for characterizing narrow-band sources such as laser diodes. We present the theory of operation of the instrument and some sample spectra obtained from a sodium vapor lamp, a HeNe discharge, and a 670 nm diode laser.     

Tapered Mach–Zehnder interferometer based on two mechanically induced long-period fiber gratings as refractive index sensor

A Mach–Zehnder interferometer formed in single mode fiber is implemented. The interferometer is built by two mechanically-induced long-period gratings. In addition, a fiber taper in the middle section is inserted. The spectral properties of the whole system are analyzed. Visibility of the interference fringes up to 0.80 (the higher ever reported using mechanically-induced long-period gratings) with fringe spacing in the 4.1 to 0.86 nm range are experimentally demonstrated. The proposed device allows reducing the fiber diameter of the section between gratings with a minimal effect in the interference fringe spacing. The sensitivity of the interferometer to external refractive index changes was also studied. It is experimentally shown that, due to the nature of the cladding mode excited, it is necessary to taper the fiber to improve the system sensitivity to external refractive index. Fiber tapers with different diameter, inserted between the long-period gratings pair were fabricated and tested for measuring external refractive index changes. A maximum resolution of 2.3×10^?4 RIU in a refractive index range from 1.36 to 1.402 is achieved.     

Spectral characterization of fiber gratings with high resolution

We demonstrate a high-resolution technique to measure the optical magnitude and phase responses of fiber gratings. The technique employs single-sideband modulation of an optical source and has spectral resolution in the hertz regime. The technique is demonstrated by measurement of the phase ripples of unapodized and apodized chirped gratings as well as the transmission spectrum of a pi-phase-shifted grating. Although it is demonstrated on fiber gratings, the technique is applicable to any optical device.     

Optic sensors of high refractive-index responsivity and low thermal cross sensitivity that use fiber Bragg gratings of >80 degrees tilted structures

For the first time to the authors' knowledge, fiber Bragg gratings (FBGs) with >80 degrees tilted structures have been fabricated and characterized. Their performance in sensing temperature, strain, and the surrounding medium's refractive index was investigated. In comparison with normal FBGs and long-period gratings (LPGs), >80 degrees tilted FBGs exhibit significantly higher refractive-index responsivity and lower thermal cross sensitivity. When the grating sensor was used to detect changes in refractive index, a responsivity as high as 340 nm/refractive-index unit near an index of 1.33 was demonstrated, which is three times higher than that of conventional LPGs.     

摆动刻蚀法制作高衍射效率凸面闪耀光栅

通过摆动离子束刻蚀方法,制作了用于短波红外高光谱成像光谱仪的凸面闪耀光栅。该方法通过在光栅子午方向上进行摆动刻蚀,解决了凸面光栅子午方向的闪耀角一致性问题。建立了摆动刻蚀模型来分析摆动速度、束缝宽度等工艺参数对槽型演化的影响,并计算了优化的刻蚀工艺参数。制备了基底尺寸为67 mm,曲率半径为156.88 mm,刻线密度为45.5 gr/mm,闪耀角为2.2°的凸面闪耀光栅,并对其表面形貌及衍射效率进行了测量。实验结果表明,摆动刻蚀法能够制作出闪耀角一致性好、衍射效率高的小闪耀角凸面光栅,满足成像光谱仪对光谱分辨率和便携性的使用要求。     

Regularization of jump points in applying the adaptive spatial resolution technique

The performance of the adaptive spatial resolution technique is improved by making the resolution function of the coordinate transformation as smooth as possible. To this end, the smoothness of the resolution function is probed and a quantitative criterion is proposed to make the jump points; which were conventionally equidistant from each other, regularized. The here-proposed regularization is applied to two different recent formulations and its effects on the overall convergence rate and on the presence of numerical artifacts in analysis of highly conducting gratings are studied. Dielectric and metallic gratings at optical and microwave frequencies are considered and the helpfulness of the proposed technique is discussed.     

基于X射线塔尔博特效应的纳米光栅制作模拟研究

沿纳米多层膜生长方向切割可制成周期只有几纳米而厚度几十微米的切片多层膜光栅. 基于该切片多层膜光栅塔尔博特自成像效应的X射线光刻是一种新型的纳米图样制作方法. 已有学者用该方法完成了百纳米结构光栅的制作. 采用严格耦合波方法, 本文模拟计算了切片多层膜光栅在满足塔尔博特自成像条件下的后表面光场分布, 详细讨论三个影响光栅后表面成像质量的重要参数:光栅厚度、材料厚度所占比例和多层膜周期. 模拟结果表明, 光栅厚度不仅影响X射线透射率, 还会改变像面条纹衬比度. 材料厚度比的大小直接决定像面是否存在清晰条纹, 选取合适的材料厚度比, 得到了前人实验中近场反常成像现象. 计算还表明, 在一定条件下, 采用周期更小的多层膜光栅有望获得更高分辨率的纳米图形, 这说明使用塔尔博特效应制作更加精细的纳米结构图形具有可行性.     

Highly resolved EUV beam-foil spectra of silicon

Spectra of foil-excited ions at tandem energies in the EUV region show many unclassified lines. In order to provide a basis for term analysis, high resolution spectra of Si are taken using a ruled 600 l/mm grating in 3^rd diffraction order and two holographic gratings of 1,200 l/mm resp. 3,600 l/mm in first diffraction order. With the latter grating the resolution power of a 2.2 m grazing-incidence monochromator is sufficient to measure fine-structure splittings of inter-shell transitions of O VI–O VIII and Si X–Si XII in the wavelength rangeλ < 20 nm. The performance of the gratings is demonstrated.     

Picosecond linear optical pulse shapers based on integrated waveguide Bragg gratings

We demonstrate a general linear pulse-shaping technique based on integrated III-V Bragg gratings (BGs). Such a technique allows for the synthesizing of complex waveforms with picosecond resolution using a compact single-waveguide design. This approach is experimentally demonstrated by fabricating and testing a series of integrated ultrafast optical pulse shapers based on BG geometries acting as time-domain code generators operating at 500 Gbits/s.     

Torsion sensing with a fiber ring laser incorporating a pair of rotary long-period fiber gratings

We experimentally demonstrate a fiber ring laser for high-resolution torsion measurement, where the laser cavity consists of a Mach-Zehnder interferometer formed with a pair of long-period fiber gratings written in a twisted single-mode fiber by a CO₂ laser. The emitting wavelength of the laser provides a measure of the rate of the torsion applied to the grating pair, while the direction of the wavelength shift indicates the sense of the applied torsion. The narrow linewidth and the large side-mode suppression ratio of the laser can provide a much more precise measurement of torsion, compared with passive fiber-optic torsion sensors. The torsion sensitivity achieved is 0.084 nm/(rad/m) in the torsion range ± 100 rad/m, which corresponds to a torsion resolution of 0.12 rad/m, assuming a wavelength resolution of 10 pm for a typical optical spectrum analyzer. The ultimate resolution of the sensor is limited by the linewidth of the laser and could be an order of magnitude higher.