Groove profile modification of blazed gratings by dip coating with hardenable liquids

We modify groove profile of various blazed gratings with groove densities as great as 3600 lines/mm by dip coating with hardenable liquids with the aim of reducing the blaze angle. The groove profiles resulting from coatings with different layer thickness are measured by atomic force microscopy. A highly reproducible blaze angle reduction to as high as a factor of 6 is achieved with mechanically ruled as well as ion-beam-etched holographic blazed gratings. Blaze angles, to as small as 0.7 deg, which are required for vacuum-UV and soft-x-ray applications but can hardly be formed with sufficient groove profile accuracy by direct ruling, are realized with this coating technique.     

Efficiency calibration of the first multilayer-coated holographic ion-etched flight grating for a sounding rocket high-resolution spectrometer

We have fabricated the four flight gratings for a sounding rocket high-resolution spectrometer using a holographic ion-etching technique. The gratings are spherical (4000-mm radius of curvature), large (160 mm x 90 mm), and have a laminar groove profile of high density (3600 grooves/mm). They have been coated with a high-reflectance multilayer of Mo/Si. Using an atomic force microscope, we examined the surface characteristics of the first grating before and after multilayer coating. The average roughness is approximately 3 A rms after coating. Using synchrotron radiation, we completed an efficiency calibration map over the wavelength range 225-245 A. At an angle of incidence of 5 degrees and a wavelength of 234 A, the average efficiency in the first inside order is 10.4 +/- 0.5%, and the derived groove efficiency is 34.8 +/- 1.6%. These values exceed all previously published results for a high-density grating.     

Fabrication and characterization of a diffraction-grating transducer in thin polybutadiene rubber film for sensing dynamical strain

Surface relief gratings are holographically fabricated in thin polybutadiene rubber films produced by both spin coating and dip coating on glass and metal substrates. These thin-film gratings are characterized for their application as efficient transducers for detecting dynamic strain in solids. The performance of these rubber-grating transducers is compared to surface-mounted fiber Bragg gratings for a range of frequencies between 50 Hz and 30 kHz. Dynamic-strain sensitivity around 1 nepsilon/radicalHz is recorded for thin rubber-film grating transducers.     

分布反馈量子级联激光器的光栅制备

利用全息曝光方法制备了分布反馈量子级联激光器的光栅掩模,选择和发展了恰当的用于InGaAs/InP材料的光栅腐蚀优化工艺,得到腐蚀规律,讨论了腐蚀机制。在量子级联激光器的InGaAs/InP层上制备光栅得到分布反馈量子级联激光器,其单模特性较好,信噪比大于30dB。     

Optimum parallel-face slanted surface-relief gratings

Using a combination of rigorous coupled-wave analysis and simulated annealing, parallel-face slanted surface-relief gratings (PFSSRGs) are optimized. For substrate-mode optical interconnects, profiles are presented for both polymer and silicon PFSSRGs for both TE and TM polarizations at normal incidence with grating periods designed to give a 45 degrees output angle in the negative-first forward-diffracted order. The resulting diffraction efficiencies range from 70% to 99%, with a majority of the optimized profiles yielding over 90%. Optimized polymer profiles for TE and TM polarizations exhibit similar high diffraction efficiencies, but the TM profiles generally require greater groove depths. Silicon profiles optimized for TM polarization have greater diffraction efficiencies than those for TE polarization. Profiles that can feasibly be fabricated are identified, and sensitivities to groove depth, filling factor, slant angle, and incident angle are shown to be modest.     

Metal Gratings with Dielectric Coating of Variable Thickness within a Period

Abstract

The rigorous theory of diffraction is applied to metallic gratings with dielectric coating. The modulation depth of the profile between coating and air is assumed to be lower than the modulation depth of the profile between metal and coating. We call this levelling. For monochromator mounting together with a special coating thickness levelled single-coated gratings with blaze show a marked increase of the efficiencies for wavelengths greater than the blaze maximum wavelength. Optimal improvements run over two octaves of the wavelength.     

Optimized condition for etching fused-silica phase gratings with inductively coupled plasma technology

Polymer deposition is a serious problem associated with the etching of fused silica by use of inductively coupled plasma (ICP) technology, and it usually prevents further etching. We report an optimized etching condition under which no polymer deposition will occur for etching fused silica with ICP technology. Under the optimized etching condition, surfaces of the fabricated fused silica gratings are smooth and clean. Etch rate of fused silica is relatively high, and it demonstrates a linear relation between etched depth and working time. Results of the diffraction of gratings fabricated under the optimized etching condition match theoretical results well.     

High-efficiency reflective diffraction gratings in fused silica as (de)multiplexers at 1.55microm for dense wavelength division multiplexing application

We describe high-efficiency, high-dispersion reflection gratings fabricated in bulk fused silica illuminated by incident lights in the C + L bands as (de)multiplexers for dense wavelength division multiplexing (DWDM) application. Based on the phenomenon of total internal reflection, gratings with optimized profile parameters exhibit diffraction efficiencies of more than 90% under TM- and TE-polarized incident lights for 101-nm spectral bandwidths (1520-1620 nm) and can reach an efficiency of greater than 97% for both polarizations at a wavelength of 1550 nm. Without loss of metal absorption, without coating of dielectric film layers, and independent of tooth shape, this new kind of grating should be of great interest for DWDM application.     

Comparison of mechanically ruled versus holographically varied line-spacing gratings for a soft-x-ray flat-field spectrograph

Recent progress in the design of aspheric wave-front recording systems has permitted the manufacture of holographic gratings with highly variable groove densities that are suitable for flat-field spectrographs. A holographic grating thus recorded was processed to produce a laminar profile by use of reactive-ion etching. Measurements are reported of the absolute diffraction efficiency of this grating and of a comparable mechanically ruled grating. It is found that the holographic grating is much more effective in suppressing the higher orders. The spectral resolution was determined by use of a carbon Kalpha x-ray generator and a spectrograph with an imaging detector. The spectral resolution of the holographic grating was approximately 3 times worse than that of the ruled grating.     

Fabrication of holographic blazed grating with Fourier synthesis exposure

A simple, direct, and universal fabrication method for holographic blazed gratings is proposed and a 79 lines/mm holographic echelle grating with 4 μm depth quasi-triangular grooves experimentally demonstrated as an example of a blazed grating fabricated directly by Fourier synthesis exposure. With the method, we converted a periodic grating groove profile into the exposure energy spatial distribution on the surface of a photoresist with the response characteristics of the particular photoresist. Then the exposure energy could be decomposed into a series of sinusoidal functions using Fourier series, and realized by superposing a series of two-beam interference patterns. In contrast with mechanically ruled gratings, the fabrication process of holographic gratings is quite quick and low cost, and especially no Rowland ghosts appear in their dispersion spectrum. The reported work will offer a new approach for blazed grating fabrication with a holographic process.     

Convex grating types for concentric imaging spectrometers

The properties of convex gratings fabricated by electron-beam lithography are investigated. Three grating types are shown. The first is a single-panel, true blazed grating in which the blaze angle stays constant relative to the local surface normal. This grating provides high peak efficiencies of approximately 88% in the first order and 85% in the second order. The second grating has two concentric panels, with each panel blazed at a different angle. This type permits flexibility in matching the grating response to a desired form. The third type has a groove shape that departs from the sawtooth blazed profile to increase the second-order bandwidth. All these types are difficult or impossible to produce with conventional techniques. The gratings compare favorably with conventional (holographic and ruled) types in terms of efficiency and scatter. Simple scalar models are shown to predict the wavelength response accurately. These gratings allow the optical designer to realize fully the considerable advantages of concentric spectrometer forms.     

Measurement of photoresist grating profiles based on multiwavelength scatterometry and artificial neural network

We employed a grating profile measurement method, which is based on the combination of multiwavelength scatterometry and artificial neural network, to determine the critical dimensions of submicrometer-period photoresist gratings with wavy sidewall profiles. Six laser beams in three wavelengths and two orthogonal polarizations were adopted for the scatterometry measurement, and the incident angle of each beam was chosen following principles that we propose for achieving high sensitivity. We measured diffraction efficiencies of a large number of photoresist gratings made on glass substrates and high-reflectivity multilayer substrates coated with a chromium thin-film layer, and determined the grating groove parameters using a neural network model. The experimental results are statistically compared with results extracted from scanning electron micrographs. Good agreements between the indirect, neural network predicted results and the direct, scanning electron microscopy results are obtained.     

Multilevel diffractive elements in SiO2 by electron beam lithography and proportional etching with analogue negative resist

Abstract

A negative low-contrast electron beam resist X AR-N 7700/18 is introduced, which provides a nearly linear dose-to-depth curve at electron acceleration voltages below 20 kV, and is therefore an excellent material for fabrication of multilevel diffractive optics. Direct electron beam recording at 12.5kV is used to pattern analogue surface profiles in this resist, and proportional reactive ion etching (RIE) is employed to transfer the profile into the SiO₂ substrate. It is shown that errors in resist-profile scale can be corrected by proper adjustment of the radio-frequency power or chamber pressure in RIE. The reproducibility of the profile scale is better than 2%. Transmission-type blazed gratings, array illuminators, and pattern projection elements are demonstrated.     

Electron-beam-fabricated asymmetric transmission gratings for microspectroscopy

Asymmetric transmission gratings operating in the resonance domain are designed by modeling of the dose-controlled electron-beam lithography process with Gaussian convolution. We aim to exceed some efficiency limit eta(s) over a specified spectral range and to maximize eta(s). The resultant continuous-profile gratings are fabricated by electron-beam lithography and proportional reactive-ion etching into SiO(2). We demonstrate gratings with good signal-to-noise ratio and a diffraction efficiency greater than 40% for wavelengths from 400 to 750 nm.     

Fabrication of optical gratings by shrinkage of a rubber material

Ordered wavy surface structures generated by deposition of a metal thin film on a pre-stretched PDMS plate were fabricated and its potential application for optical gratings was proposed. The orientation of the generated structures was always perpendicular to the pre-stretched direction and the pitch of the structure could be adjusted ranging from 4.5 μm to 6.8 μm by controlling the strength of the pre-stretched strain and the thickness of the surface metal film. Based on these periodic structures, various optical gratings were demonstrated. With a slight modification of the fabrication scheme, gratings with different orientations can be fabricated on both sides of the PDMS plate, the double-sided gratings, could be fabricated. It is believed the current method has the potential for the fabrication of a large-scale diffractive grating at lower costs.     

Small angle X-ray scattering measurements of spatial dependent linewidth in dense nanoline gratings

Small angle X-ray scattering (SAXS) was used to characterize the cross section of nanoline gratings fabricated with electron beam lithography (EBL) patterning followed by anisotropic wet etching into a single crystal silicon substrate. SAXS results at normal incidence clearly bear the signature of positional dependent linewidth within the gratings; such non-uniformity is subsequently confirmed with scanning electron microscopy. The proximity effect of EBL is believed to be the cause of the spatial variations of linewidth. To quantitatively fit the SAXS results the linewidth near the periphery of the patterned field needs to be 80% greater than that in the central region, whereas the cross section of nanolines can be modeled as a simple rectangular shape, as expected from the anisotropic wet etching process.     

Mo/Si multilayer-coated ruled blazed gratings for the soft-x-ray region

Two Mo/Si multilayer-coated blazed gratings have been fabricated for operation at soft-x-ray wavelengths above the Si L edge, λ ≥ 12.4 nm, at (near) normal incidence. The sawtooth profile of the grating structure was mechanically ruled into a 200-nm Au film that was deposited onto a plane glass substrate. To smooth the rough Au surface and to prevent interdiffusion of the Au film with the upper Mo/Si multilayer, a carbon film was evaporated onto the Au grating surface of one of the gratings before the deposition of the multilayer coating. We matched the multilayer grating, working on blaze in the third diffraction order, in which an absolute diffraction efficiency of 3.4% at a wavelength of 14 nm was measured, whereas only 1.1% was achieved for a similar grating (without a carbon interlayer). These efficiencies are higher than those obtained for other ruled blazed gratings reported in the literature. As a result of the multilayer and grating periodicity, the wavelength of diffraction can be tuned bya rotation of the grating, which is important for application in a soft-x-ray monochromator.     

Diffraction gratings of photosensitive ZrO2 gel films fabricated with the two-ultraviolet-beam interference method

Photosensitive ZrO(2) gel films were patterned with a two-beam interference method by use of a 325-nm-wavelength He-Cd laser for the first time to our knowledge. The ZrO(2) gel films were prepared from Zr(O-n-C(4)H(9))(4) chemically modified with benzoylacetone. We fabricated uniform gratings with a 0.5-mum period on Si or SiO(2) substrates by etching the gel films in ethyl alcohol after UV irradiation. A maximum diffraction efficiency of 28% was attained with the grating fabricated on Si substrate under a Littrow mounting condition by use of a 633-nm-wavelength He-Ne laser. Blazed gratings could also be fabricated.     

Surface ablation of RbTiOPO4 by femtosecond laser

We report here the results obtained in surface ablation of RbTiOPO₄ single crystals by femtosecond laser. We fabricated and characterized one-dimensional (1D) diffraction gratings with different lattice spacings of 15 and 20 μm, and with a sub-modulation of the period introduced in the later. The optical and electronic microscopy characterization and filling factor analysis of these diffraction gratings are reported. We also show that the roughness generated on the grooves by the ablation process can be improved by chemical etching.