Blaze Wavelength Research Articles

The numerical analysis of low diffraction efficiency phenomenon at the blaze wavelength for echelle gratings

In this paper, we explain and unify the low diffraction efficiency behavior of echelle gratings at blaze wavelengths under Littrow mount in terms of the metal refractive index (RI). The similarity in the diffraction efficiency (DE) behavior (e.g., efficiency decline and valley-like diffraction curves for low-efficiency orders) of echelle gratings with different metals at TM polarization points the research line of sight to the RIs. Further, the numerical calculations based on the rigorous vector diffraction theory show that the efficiency behavior of the echelle grating at TM polarization is extremely sensitive to the imaginary part of the RI. This is because there is a series of low light-collecting ability (LCA) regions as the imaginary part of the RI changes in the echelle grating, resulting in several disturbed DE zones. In addition, the increase in groove depth improves the LCA at both polarizations, which moves away from the low LCA region at the TM polarization. This leads to low polarization sensitivity and higher DE for echelle gratings, further revealing the essence of the efficiency improvement by increasing the groove depth. From the RI to the LCA, these results will give new concepts to designing and fabricating broadband high-efficiency echelle gratings.

Effect on the diffraction efficiency from the surface absorption of echelle grating in Littrow mounting.

[Opt. Express31, 26156202310.1364/OE.495525] shows an anomalous absorption of the echelle grating in TM polarization light near the pseudo-Brewster angle. On top of that, more generally, we relate the absorptions of echelle grating to the Al materials with an absorption spectral band. The blaze diffraction efficiencies (DEs), absorption strengths and electric field distributions, and the energy of non-blaze diffraction orders of the echelle are analyzed in detail. The computing reveals that the interaction between the incident light for TM polarization and the echelle structure leads to amplifying the absorption strength of Al materials with an absorption spectral band from visible to near IR. The deepening groove depth not only suppresses the absorption strength of the Al-echelle grating under TM polarization closer to the absorption spectra of Al materials but improves the light-collecting ability (LCA) at both polarizations. Therefore, the DE differences of different blaze wavelengths for the wideband blaze are explained. The Ag materials echelle with lower absorption is to further validate the results. From the point of view of the effects of absorption and LCA, the novel echelles with high DE can be designed and fabricated.

Design and Fabrication of Silicon-Blazed Gratings for Near-Infrared Scanning Grating Micromirror

Blazed gratings are the critical dispersion elements in spectral analysis instruments, whose performance depends on structural parameters and topography of the grating groove. In this paper, high diffraction efficiency silicon-blazed grating working at 800–2500 nm has been designed and fabricated. By diffraction theory analysis and simulation optimization based on the accurate boundary integral equation method, the blaze angle and grating constant are determined to be 8.8° and 4 μm, respectively. The diffraction efficiency is greater than 33.23% in the spectral range of 800–2500 nm and reach the maximum value of 85.62% at the blaze wavelength of 1180 nm. The effect of platform and fillet on diffraction efficiency is analyzed, and the formation rule and elimination method of the platform are studied. The blazed gratings are fabricated by anisotropic wet etching process using tilted (111) silicon substrate. The platform is minished by controlling etching time and oxidation sharpening process. The fillet radius of the fabricated grating is 50 nm, the blaze angle is 7.4°, and the surface roughness is 0.477 nm. Finally, the blazed grating is integrated in scanning micromirror to form scanning grating micromirror by MEMS fabrication technology, which can realize both optical splitting and scanning. The testing results show that the scanning grating micromirror has high diffraction efficiency in the spectral range of 810–2500 nm for the potential near-infrared spectrometer application.

High-efficiency X-ray multilayer-coated blazed gratings with shifted boundaries.

A new design for a high-efficiency multilayer-coated blazed X-ray grating with horizontal-shifted (non-conformal) boundary profiles is proposed. The investigation of the grating design is carried out using an integrated approach based on rigorous numerical calculations of light diffraction by gratings with realistic boundary profiles obtained from simulations of multilayer grating growth. By varying the incidence angle of the deposition flux, one can set the direction and magnitude of the boundary profile shifts over a wide range of values. It is shown that the diffraction efficiency of the blazed gratings with shifted boundary profiles may be substantially higher than the efficiency of gratings with conformal boundaries, which are, moreover, much more difficult to produce. High-efficiency gratings with shifted boundaries can be obtained when the deposition is mainly on the blaze facet with a high inclination of the deposition flux, as opposed to widely used near-normal deposition methods. The maximum absolute efficiency of a W/B4C 2500 mm-1 grating with a blaze angle of 1.76° and an anti-blaze angle of 20°, working at a blaze wavelength of 1.3 nm and having shifted realistic boundary profiles, obtained using our integrated approach is 23.3%, while that of a grating with the ideal (triangular) boundary profile and the same shifts is 25.3%, and that of an ideal conformal profile is only 22.2%. The maximum absolute efficiency of 40.2% of a 2500 mm Cr/C grating with a blaze angle of 1.05° and a realistic anti-blaze angle of 10°, working at a blaze wavelength of 0.83 nm and having ideal shifted boundaries, is higher than the maximum efficiency of the similar grating having ideal conformal boundaries with a non-realistic anti-blaze angle of 80°.

Optimization of a multiblaze grating in reflection using a free-form profile.

Diffraction gratings are very important in hyperspectral imaging. Their desired diffraction efficiency is dictated by the sensitivity of the detector in the spectral band of interest and the luminance of the scene to be observed. If the desired diffraction efficiency curve is established in this spectral band of interest, the remaining work is to design a diffraction grating that meets this demand. This paper is concerned with blazed gratings in reflection, and the geometry of the grating will therefore depend on this reference curve, the spectral band, and the optimization order. The simplest form is a grating with a uniform profile that is optimized at a single-blaze wavelength. It is a monoblaze grating. When such grating cannot meet the requirements in terms of diffraction efficiency, a multiblaze grating optimized at several blaze wavelengths is required. The objective of this paper is to propose a method of optimization of this multiblaze grating, i.e., how to find the number of blaze wavelengths necessary as well as their value to answer the requirements in term of diffraction efficiency.

Development of High-Resolution Fiber Spectrometer for LIBS Detection

This paper introduces a homemade high-resolution miniature fiber optic spectrometer for LIBS detection. The circuit part of the system is mainly controlled by a cortex-M3 chip, transmits data to the personal computer (PC) through the Universal Serial BUS (USB2.0), and a set of spectrometer test software is finished. The optical system is based on Czerny-Turner imaging system, using 1800grv/mm plane diffraction grating, 16μm slit and 2048 pixels linear array CCD as detector, high-resolution spectral output in the range of 335-455nm is required. By precisely calibration and testing, the precision of the system is 0.05nm, and the resolution of the wavelength at the blaze wavelength can up to 0.1nm which can fulfill the general LIBS detection.

Broadband and Efficient Diffraction

Surface topography dictates the deterministic functionality of diffraction by a surface. In order to maximize the efficiency with which a diffractive optical component, such as a grating or a diffractive lens, directs light into a chosen order of diffraction, it is necessary that it be "blazed". The efficiency of most diffractive optical components reported so far varies with the wavelength, and blazing is achieved only at a specific nominal energy, the blaze wavelength. The existence of spurious light in undesirable orders represents a severe limitation that prevents using diffractive components in broadband systems. Here we experimentally demonstrate that broadband blazing over almost one octave can be achieved by combining advanced optical design strategies and artificial dielectric materials that offer dispersion chromatism much stronger than those of conventional bulk materials. The possibility of maintaining an efficient funneling of the energy into a specific order over a broad spectral range may empower advanced research to achieve greater control over the propagation of light, leading to more compact, efficient and versatile optical components.

Restoration of absolute diffraction efficiency and blaze angle of carbon contaminated gratings by ultraviolet cleaning

An UV cleaning technique has been used for the removal of carbon contamination from grating surface, using oxygen free-radicals generated by the absorption of UV radiation (172 nm) in atmospheric oxygen. The extent of restoration of absolute diffraction efficiency (ADE) and grating blaze angle of a carbon-contaminated variable line-spaced concave grating (average line spacing of 1200 lines/mm, blaze angle 3.2 deg, and blaze wavelength 10 nm) has been studied over a 5-70 nm wavelength range. The contamination (due to prolonged use in extreme-ultraviolet spectrograph) resulted in a drastic reduction in the diffraction efficiency of grating, along with a change in the blaze angle. The UV cleaning led to the restoration of blaze angle as well as to an increase in the ADE by more than an order of magnitude for the first two diffraction orders (i.e., from ~0.2 to ~7.2%) for the first diffraction order and 0.1 to ~5% for the second diffraction order at the blaze wavelength. The study is useful for the restoration of carbon-contaminated costly optics.

Deep UV imaging by spectrometric full-colour cathodoluminescence microscopy

A new deep-UV cathodoluminescence (CL) microscopy is tried on studying design and fabrication of recent solid-state light-emitting (LE) devices that emit near 200 nm light. CL microscopy has enough energy for electronic excitation of deep-UV emitting devices and visualizes distribution map of their electronic structures. The deep-UV imaging system is constructed on our own spectrometric full-colour CL microscope that collects CL spectra at all pixel points of the specimen during regular SEM observation, and at the end of one frame scan, a set of CL spectra is accumulated in the control computer. One frame scan of 512x512 pixels needs 8 to 80 sec, which is unbeatable by any other commercial CL microscopes. Full-colour CL micrograph is constructed using the spectra. The detector, a 32-channel photoelectron multiplier array, has higher sensitivity in UV region than any solidstate linear sensors, and a grating of 300-nm blaze wavelength instead of one at 500-nm blaze wavelength for visible lights, provide CL micrographs of 180-700 nm lights for recent deep-UV LE devices. Some unforeseen light emissions between 200-350 nm regions are observed on various materials and they are discussed correlating with the structure observed in SEM micrographs.

Analysis of the influence of the passive facet of blazed transmission gratings in the intermediate diffraction regime

Blazed diffraction gratings are of enormous practical importance for imaging and analyzing hybrid optical systems. The intermediate diffraction regime is characterized by the transition from the scalar to the rigorous electromagnetic theory. An effect known as shadowing occurs and reduces the diffraction efficiency. Based on rigorous calculations for optimized sawtooth-shaped and binary-multilevel blaze profiles, we deduce a semianalytical model describing the shadowing phenomenon for the general case of oblique incidence. We discuss illumination both from air and from the substrate. Though a multilevel blaze possesses a discrete substructure, our shadowing model remains valid, if substructural effects are neglected. We find that electromagnetic effects due to the passive blaze facet lead to the efficiency reduction, and the blazing efficiency shows a linear dependence on the ratio of blaze wavelength to grating period. Our shadowing model is applied to predict the performance of a Littrow-like blazing condition in transmission geometry as, e.g., for a diffractive solid immersion lens.

Impact on Diffraction Efficiency of Binary Optical Elements for Spot Exposure Superposing Ratio

In fabrication of free form optical surfaces, spot exposure is a typical mode in laser photolithography. Proper selection of spot exposure superposing ratio is the key to optimize etching quality and writing speed. In this paper, a new composite diffractive grating structure is modeled for the influence on diffraction efficiency of binary optical elements caused by spot exposure superposing ratio. Composite grating can be transformed into one simple rectangular grating and two triangular gratings. Angular spectrum theory is used to describe the mathematical expression of the composite diffractive grating. Through simulation analysis, the relationships between diffraction efficiency and the other three items as grating depth, spot radius, and blaze wavelength, has been shown. Simulation results indicate that shape error caused by spot exposure ratio impact the diffraction efficiency greatly in deep etching, and the transmittance will reach maximal point approximately while spot exposure ratio is over 60%. This conclusion can be taken as the theoretical foundation for exposure frequency selection in laser photolithography figure generation.

Broadband blazing with artificial dielectrics.

The efficiency of conventional diffractive optical elements with échelette-type profiles drops rapidly as the illumination wavelength departs from the blaze wavelength. We use high dispersion of artificial materials to synthesize diffractive optical elements that are blazed over a broad spectral range (approximately 1 octave) or for two different wavelengths.

Aberration-corrected concave grating for the mid-infrared spectrometer aboard the Infrared Telescope in Space

A mechanically ruled aberration-corrected concave grating was developed for use in the low-resolution mid-infrared spectrometer aboard the cryogenically cooled Infrared Telescope in Space. The design and the performance testing of the grating are reported. The spectrometer requires a wide spectral range (4.5-11.7 µm) and a wide field of view (8 × 8 arcmin) with a low wavelength resolution (Δλ ≤ 0.3 µm). The aberration-corrected concave grating provides a flat focal plane with a small aberration in the spatial direction compared with those caused by the finite size of the entrance slit. It also permits a simple design for the spectrometer, which is advantageous for applications in space cryogenic instruments. The measurements of the wavelength resolution and the spatial resolution are shown to be in good agreement with the predicted performance. The diffraction efficiency of the grating is more than 80% at the blaze wavelength (6 µm) and fairly high (>30%) over the entire wavelength range in question. The grating produces polarization of less than 10% for λ < 6.4 µm and of 10-20% for 6.7 µm <λ 9.7 µm. These results indicate the potential applicability of this type of grating to the wide-field IR spectroscopic observations.

Results from several persistent train spectra intercompared

Some results following from two contemporary photographical programs (Slovakia and Japan) for persistent meteor train spectra are compared. It shows that even though both programs are not too different according to their lenses and films used, the spectral interval detected is very different. In this respect prism as a dispersion element is more favourable than the grating with blaze wavelength near 610 nm. However, some results previously assumed as typicai for presented persistent trains-e.g. the forbidden Herzberg O2 emission, the NO2 continuum and OH red emissions-seem to be substanciated especially due to higher and linear dispersion of those grating spectra.

High-Resolution Hadamard Transform Spectrometer Using a High-Precision Encoding Mask

We have carried out an experiment to obtain limited resolving power using a high-precision Hadamard mask and a commercially available half-meter diffraction grating spectrometer, which has a diffraction grating with the blaze wavelength of 2 µm and total groove number of 15600. A 2×255-slot encoding Hadamard mask (total number of slots: 510) was prepared. The size of the 255-slot encoding mask was 5.18×5.00 mm2 and the width of one slot was 20.32 µm. With this system, mercury emission lines were observed in the wavelength range of 1.685 to 1.715 µm. The resolving power was determined by the experiment to be larger than 6560, while the theoretical value was about 13000.

Electromagnetic study of transmission gratings

The performances of triangular groove photoresist gratings used in transmission are studied in the visible and near-infrared regions with the electromagnetic theory. The mounting considered here associates triangular groove gratings on the hypotenuse face of a right-angle prism in a configuration, usually called a grism, in such a way that for a chosen wavelength the deviations of the prism and the grating compensate. To assist designers of spectrometric systems, we have covered a complete range of blaze wavelengths and, consequently, of blaze angles. We studied the influence on grating efficiencies when the line density is increased, and the optimal choice angle of incidence is discussed.

A Blazed Holographic Concave Grating for Monochromators

Abstract The theory of blazed holographic gratings given in a previous paper is applied to the design of a blazed holographic aberration-corrected spherical concave grating. Two solution types are found which cover different ranges of blaze wavelength and aberration. The use of the refractive indices of the substrate, and the medium in front of the photoresist, as optimising parameters is demonstrated. Four ray-traced solution examples are given which include the distributions of blaze angle and wavelength across the grating surface.

Theory of Blazed Holographic Gratings

Abstract Geometric theory of the blazed aberration-corrected holographic grating is derived. This involves generalising the holographic component of the light-path function to permit aberration correction for recording geometries that arrange the two recording wavefronts to be incident from opposite sides of the grating substrate, thereby realising triangular or blazed groove profiles. The equation for the effective grating constant and an equation for the blaze angle are combined to yield a general expression for the recording angles in terms of the blaze angle, and the refractive indices of the grating substrate and the medium in front of the photoresist. This equation may then be solved simultaneously with the holographic aberration equations, to introduce the effective blaze angle and refractive indices as solution parameters. Also included are appendices that give essential equations for ray tracing, and for computing the blaze angle and blaze wavelength at any point on this type of grating.

Spectral Efficiency of the SpectraSpan III Echelle Grating Spectrometer

The efficiency of an echelle grating spectrometer incorporating prism cross-dispersion is empirically evaluated over a broad wavelength range incorporating a large number of orders. A derived parameter, the spectrometer efficiency, proportional to the spectrometer luminosity, conveniently gauges dynamic features of the echelle efficiency. Characteristics of efficiency over broad wavelengths suggested by earlier work are clearly observed. The shift of blaze wavelength with decreasing order of diffraction, predicted in a recent model of echelle grating efficiency, is experimentally documented.

Optical demultiplexer using a silicon concave diffraction grating

A demultiplexer composed of a concave diffraction grating and a multimode slab waveguide is attractive since it has many advantageous features. However, this type of demultiplexer has had high demultiplexing losses until now, because the concave diffraction gratings used had poor diffraction efficiency. A silicon concave diffraction grating has been developed to overcome this problem, manufactured by cylindrically bending a thin silicon plane diffraction grating. The diffraction efficiency of this grating was 82% at a blaze wavelength. The five-channel demultiplexer was assembled with this grating as well as with a multimode slab waveguide and a fiber array. Its branching loss was in the 1.4-1.8-dB range.