Design for an aberration-corrected concave grating for a mid-infrared long-slit spectrometer

A new design for an aberration-corrected concave grating for the spectral region near 10 mum is presented. It was designed for use in the ground-based astronomical medium-resolution (lambda/Dlambda ~ 100) Mid-Infrared Camera and Spectrometer (MICS). It provides a flat focal plane for a wide spectral range (7.5-13.5 mum) with small aberrations, permitting efficient long-slit observations in the mid-infrared region. It permits a simple design of the spectrometer without collimator and camera mirrors, which is quite advantageous for cryogenic instruments. The grating has variable spacing grooves to reduce aberrations. In addition, the grating surface figure is designed to be toroidal and in the direction perpendicular to the grooves, aspherical, to suppress the aberrations further over a wide spectral range. The angle of the grooves is also varied to yield better efficiency near the blaze angle. The grating was fabricated by high-quality ultraprecision machining, which made these features possible. Test observations confirmed that the designed spectral resolution was achieved.     

Characteristics of an absolutely calibrated flat-field extreme ultraviolet spectrometer in the 10-130 A range for fusion plasma diagnostics

A flat-field extreme ultraviolet (EUV) spectrometer with a nominal 2400 grooves/mm aberration-corrected ruled grating has been developed to analyze the emission spectrum in the wavelength range of 10 to 130 A from large helical device (LHD) plasmas. Spectral properties such as resolution, sensitivity, contribution of higher-order light, and background stray light have been studied using emission spectra mainly from intrinsic impurities, e.g., C and Fe. It is found that the spectrometer well resolves closely existing spectral lines of highly ionized medium- and high-Z impurities even in a very short wavelength range such as 10 to 20 A. As a result, it allows one to study the charge state distribution of elements in high-temperature fusion plasma. The ruled grating was then replaced by a laminar type holographic grating for the comparative study. The spectral resolution for the ruled grating (Dlambda approximately 0.08 A at 33.73 A) is clearly better than the holographic grating (Dlambda approximately 0.13 A at 33.73 A). Both gratings well suppress the higher-order light, e.g., the second-order light is only less than 11% of the first-order light for C vi(33.73 A). Relative sensitivity calibration with the wavelength has been done using bremsstrahlung continuum from the LHD plasmas. Absolute intensity calibration has been done by comparing the spectral intensities directly with the absolutely calibrated 1200 grooves/mm EUV spectrometer in the overlapping range of 90-120 A due to the absence of a good branching pair in 10-130 A. As a typical result on the present spectrometer well-resolved n=2-3 full transition arrays from Ne- to Li-like ions are measured for Fe and Ti and wavelengths of the spectral array are tabulated for each charge state. Spectroscopic comparison is also made between the 1200 grooves/mm and 2400 grooves/mm gratings in a range of 50-130 A.     

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.     

Applications of holographic gratings to two-dimensional spectroscopy

Calculations are presented for a set of aberration-corrected holographic concave gratings for possible use in space mission instruments. It is concluded that, for the visible range 0.26-1.02 μm when detectors with a 15-μm pixel size are used, a total field of view of ～1° is possible at an aperture of ?/7.0. For the infrared regions 0.9-2.5 μm and 2.4-4.2 μm, in which detector arrays have larger pixel sizes of 30-40 μm, a spatial field of view of ～2° at ?/7.0 and ～1.0° at ?/3.5 can be achieved. An exploration of spectrum lengths showed that performance starts to fall off sharply for lengths over 15 mm at a focal length of 150 mm (i.e., a spectral angular extent of～6°).     

Optimal implementation of a microspectrometer based on a single flat diffraction grating

An analytical model has been developed and applied to explore the limits in the design of a highly miniaturized planar optical microspectrometer based on an imaging diffraction grating. This design tool has been validated as providing the smallest possible dimensions while maintaining acceptable spectral resolution. The resulting planar spectrometer is composed of two parallel glass plates, which contain all components of the device, including a reflective slit and an imaging diffraction grating. Fabrication is based on microelectromechanical system technology and starts with a single glass wafer; IC-compatible deposition and lithography are applied to realize the parts in aluminum, which makes the microspectrometer highly tolerant for component mismatch. The fabricated spectrometer was mounted directly on top of an image sensor and takes up a volume of only 50 mm(3). The measured spectral resolution of 6 nm (FWHM) in the 100 nm operating wavelength range (600-700 nm) is in agreement with a model calculation.     

High-resolution two-grating spectrometer for dual wavelength spectral imaging

A two-grating high-resolution spectrometer for dual wavelength imaging is demonstrated based on the standard Czerny-Turner mounting with an auxiliary grating and a mirror. A two-dimensional charge-coupled device (CCD) detector in the spectrometer focal plane allows simultaneous detection of two spectral intervals. Each spectrometer grating is driven by a high-precision stepper motor interfaced to a computer via home-made software. The software allows fast tuning of the gratings to a desirable spectral interval anywhere between 200 nm and 800 nm. The spectral interval widths are 2-3 nm for a 'high-resolution' (2400 grooves/mm) grating and 4-5 nm for a 'low-resolution' (1200 grooves/mm) grating. The resolution varies between 0.01 nm and 0.02 nm depending on the grating used. The performance of the spectrometer is demonstrated by detecting spectrally resolved images from a back-illuminated template and from a laser-induced plasma. The spectrometer can be useful for two-line spectroscopic diagnostics or can be expanded for multi-element spectral analysis.     

Replicated arrays of hybrid elements for application in a low-cost micro-spectrometer array

Abstract

We report on the fabrication and characterization of replicated hybrid elements for low-cost micro-spectrometer array applications. An array of hybrid elements, where one surface combines the fairly large dimensions of refractive microlenses with the submicron features of a diffraction grating, was successfully replicated by hot embossing. The parameters are: lens diameter=990 μm, height=60 μm, grating period=1 μm, linewidth=300 nm and grating depth=2 μm. These replicated spectrometer elements showed a maximum resolution of 2.25 nm and a stray-light suppression better than 30 dB.     

Polarization and efficiency of a concave multilayer grating in the 135-250-Å region and in normal-incidence and SeyaNamioka mounts

A molybdenum/silicon multilayer coating was applied to a holographic ion-etched blazed grating substrate that had 2400 grooves/mm and a radius of curvature of 2.2 m. Scanning probe microscopy yielded the same surface microroughness (5 ? rms) before and after deposition of the multilayer. The efficiency and polarization performance of the grating was measured by synchrotron radiation in the 135-250-? wavelength region. In the second grating order and the second Bragg order of the multilayer coating, the peak normal-incidence efficiency was 7.5% at a wavelength of 147 ?, representing a groove efficiency of 27%. At an angle of incidence of 35°, the polarization performance of the grating was 95%-100% in the 210-250-? wavelength region. In a Seya-Namioka spectrometer mount at an angle of incidence of 30°-40°, the grating is a nearly perfect polarizing optical element in the wavelength bands between 125 and 300 ?, which are covered by the multilayer coating.     

Imaging spectrograph for interstellar shocks: a narrowband imaging payload for the far ultraviolet

We present an imaging spectrometer developed for narrowband imaging at 1035 A with high (approximately 1-arc sec) spatial resolution over a modest field of view (approximately 5 arc min). The instrument is based on a conventional Gregorian telescope with aberration-corrected holographic rulings on the secondary optic. These aberration-correcting rulings enable stigmatic imaging in diffracted light with a minimum number of optical elements, thereby maintaining a high system efficiency. The capabilities of this instrument allow us to map the distribution of UV-emitting material in the hot (approximately 300,000 K) plasma from shocks in supernova remnants. Although this design is optimized for imaging near 1035 A, the basic concept can be applied to provide narrowband imaging or long-slit imaging spectroscopy at any wavelength. In addition, a larger field of view is possible with a corresponding loss in spatial resolution.     

High-power widely tunable thulium fiber lasers

Applications requiring long-range atmospheric propagation are driving the development of high-power thulium fiber lasers. We report on the performance of two different laser configurations for high-power tunable thulium fiber lasers: one is a single oscillator utilizing a volume Bragg grating for wavelength stabilization; the other is a master oscillator power amplifier system with the oscillator stabilized and made tunable by a diffraction grating. Each configuration provides >150 W of average power, >50% slope efficiency, narrow output linewidth, and >100 nm tunability in the wavelength range around 2 μm.     

Design of a high-resolution extreme-ultraviolet imaging spectrometer with aberration-corrected concave gratings

A high-resolution extreme-ultraviolet imaging spectrometer is designed for the Japanese solar mission Solar-B. A spherical varied-line-space (SVLS) grating and a toroidal uniform-line-space (TULS) grating are chosen as candidates for use in the spectrometer to yield high spectral and spatial resolution within the spectral range 25-29 nm. The spectral image-focusing properties and the mechanical tolerances for fabrication and alignment are compared for the two types of grating. The SVLS design is found to be superior to the TULS design for off-plane spectral images and in ease of fabrication and optical alignment.     

Image Evaluation of the High Resolution VUV Spectrometer at SURF II by Ray Tracing

A high resolution VUV spectroscopic facility has been in use for several years at SURF II, the Synchrotron Ultraviolet Radiation Facility at the National Institute of Standards and Technology in Gaithersburg, Maryland. At this facility, a combination of three cylindrical mirrors is utilized to focus the light originating in the storage ring onto the horizontal entrance slit of the spectrometer. The spectrometer uses a 6.65 m concave grating having a groove density of 4800 lines/mm in the off-plane Eagle mounting. In preparation for the installation of an array detector in the exit image plane, a ray tracing program has been formulated and spot diagrams have been constructed by plotting the coordinates of the points of intersection of the diffracted rays with the image plane, which is tangent to the Rowland circle. In creating the spot diagrams, we have considered both parallel and tilted configurations of the entrance slit with respect to the grating grooves. It is shown that the line widths of the spectral images can be reduced when the entrance slit is properly tilted. Finally, we have estimated the spectral widths of the images when they are recorded on an array detector placed tangent to the Rowland circle. We conclude that an image spectral width of 0.41 pm to 0.88 pm in first order can be achieved over the wavelength region of 40 nm to 120 nm.     

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.     

Visible and infrared mapping spectrometer for exploration of comets,asteroids, and the saturnian system of rings and moons

The science objectives and system design for a Visible and Infrared Mapping Spectrometer (VIMS), an imaging spectrometer for planetary exploration, are described. The instrument development is the result of a collaboration between scientists and engineers from the United States, Italy, and France. NASA has selected VIMS as a facility instrument for the Comet Rendezvous Asteroid Flyby (CRAF) Mission, and as a candidate facility instrument for the Cassini Mission to explore Saturn and its rings and moons. (A facility instrument is provided by the project for the use of a science team which was selected on the basis of individual competitive science proposals.) VIMS covers the spectral range from 0.35 to 5.2 μm with a nominal spatial resolution of 0.5 milliradians (pixel size) and spectral resolutions (spectral width) of 7 nm at 0.35–1.0- μm wavelength and 16nm at 1.0–5.2-μm wavelength. The nominal pixel size is 0.5 × 0.5 mrad². The two separate missions are scheduled to be launched by NASA from the Eastern Test Range at Cape Canaveral, Florida, using the Titan IV Launch Vehicle. The planned launch date for the Cassini Mission is November 1995. The planned launch date for the CRAF Mission is February 1996.     

一种微型光纤光谱仪的研制及其性能测试

报道了一种自行研制的微型光纤光谱仪及其性能测试结果.基于C-T(czerny-turner)成像系统,将通过光纤导入的待测光进行分光后成像于线阵CCD(charge-coupled device)探测器上.采用FPGA(field-pro-grammable gate array)作为主处理器实现CCD的驱动、数据采集与处理,通过USB 2.0接口将数据传送至上位机.根据USB的接口协议及数据格式,编写了一套光谱仪测试软件.利用汞灯作为测试光源,对自制的光纤光谱仪性能进行了仔细分析.结果表明,当采用缝宽为30μm的狭缝时,仪器的波长准确度小于0.3nm,光谱分辨率可达1.1nm.     

Imaging spectroscopy for two-dimensional characterization of auroral emissions

A large throughput transmission spectrometer, with a grating on a prism as the diffraction element, has been developed to study altitude distributions of auroral emissions. The imaging spectrometer disperses spectrally in one dimension while spatial information is preserved in the orthogonal direction. The image is projected onto a CCD array detector. Image processing methods have been developed to calibrate for wavelength, uniform field, spectral sensitivity, curvature of field, and spatial mapping. Single images are processed to represent a measured signal brightness in a unit of Rayleighs/pixel, from which area integrations can be made for desired spatial-spectral resolution. System performance is ~1.5-nm resolution over a 450-nm bandwidth (420-870 nm). Two spectrometer systems of this design were operated simultaneously, one with additional optical instruments and an incoherent scatter radar at Sondrestrom, Greenland, and the other at Godhavn, Greenland, which lies 290 km to the northwest and nearly in the magnetic meridian of Sondrestrom. The developed system, calibration method, and examples of performance results are presented.     

Raman spectroscopy using a spatial heterodyne spectrometer: proof of concept

The use of a spatial heterodyne interferometer-based spectrometer (SHS) for Raman spectroscopy is described. The motivation for this work is to develop a small, rugged, high-resolution ultraviolet (UV) Raman spectrometer that is compatible with pulsed laser sources and that is suitable for planetary space missions. UV Raman is a particular technical challenge for space applications because dispersive (grating) approaches require large spectrographs and very narrow slits to achieve the spectral resolution required to maximize the potential of Raman spectroscopy. The heterodyne approach of the SHS has only a weak coupling of resolution and throughput, so a high-resolution UV SHS can both be small and employ a wide slit to maximize throughput. The SHS measures all optical path differences in its interferogram simultaneously with a detector array, so the technique is compatible with gated detection using pulsed lasers, important to reject ambient background and mitigate fluorescence (already low in the UV) that might be encountered on a planetary surface where samples are uncontrolled. The SHS has no moving parts, and as the spectrum is heterodyned around the laser wavelength, it is particularly suitable for Raman measurements. In this preliminary report we demonstrate the ability to measure visible wavelength Raman spectra of liquid and solid materials using an SHS Raman spectrometer and a visible laser. Spectral resolution and bandpass are also discussed. Separation of anti-Stokes and Stokes Raman bands is demonstrated using two different approaches. Finally spectral bandpass doubling is demonstrated by forming an interference pattern in both directions on the ICCD detector followed by analysis using a two-dimensional Fourier transform.     

Radiation filters and emitters for the NIR based on periodically structured metal surfaces

Abstract

The spectrally selective optical properties of wavelength selective radiation emitters and filters based on periodically microstructured metal surfaces were investigated. Metal surfaces were structured by the use of a holographic mask and subsequent etching processes. Due to the microstructure, thermally excited surface plasmons couple to electromagnetic radiation. Therefore a structured tungsten surface can act as a selective radiation emitter. The calculation of the absorptance by a rigorous diffraction theory allows the prediction of the emissivity of such structures. The angle dependent emissivity of tungsten gratings with periods of 1.4 μm and 2.0 μm was measured. A peak emissivity of 70% at a wavelength of 1.6 μm was achieved. Band pass filters for the near infrared spectral range based on perforated metal films were investigated theoretically and experimentally. Filters with a grating period of 2.0 μm were produced. A peak transmittance of 80% at a wavelength 2.9 μm was achieved. The optical properties of the diffractive elements described partly show a strong angle dependence