Overview of diffraction gratings technologies for spaceflight satellites and ground-based telescopes

Optical components such as diffraction gratings are widely used in Space-flight satellites to analyse light properties from soft-X rays to Infra-Red. The diffraction gratings are one of the key optical components of space-flight instruments and have to exhibit very high optical performances.

Spherical and paraboloidal diffraction gratings are considered for use on an experiment involving a collection of satelliteborne spectrographs that will investigate astrophysical plasmas in the spectral region from approximately 100 to 300 A. We shall use eight spectrographs: Wadsworth mounts in near-normal incidence with effective focal lengths of 3 m, each centered on a chosen wavelength in this spectral range. Ray tracings show that paraboloidal gratings used in the Wadsworth mount have a significant increase in resolution over spherical gratings of the same effective focal length, used in the same mounting, as long as the spectral ranges are small (approximately 10% of the central wavelength). This increase in resolution is due to the spatial extent of coma for a paraboloidal grating that is less than the circle of least confusion of spherical gratings.

Whilst the atmosphere places a limit on the successful applications of many branches of astronomy, it becomes an invaluable tool for the detection of very high energy γ-rays. This thesis is concerned with reducing the systematic uncertainties inherent to using the atmosphere as a detector of very high energy radiation. The interaction processes important to high energy particles are met in the first chapter. The second chapter explores how these interaction processes are responsible for generating observable Cerenkov radiation that can be detected by ground based telescopes. A description of one of these atmospheric Cerenkov telescopes, the University of Durham Mark 6 telescope, is given in chapter 3. A timing analysis was performed on data obtained with this telescope of the high mass X-ray binary Centaurus X-3 and the findings are given in chapter 5. The result of the test for orbital modulation of the VHE γ-ray signal has implications for the possible site of VHE γ-ray emission in this system and for the analysis strategy required to test for modulation of the VHE signal at the pulsar period. One of the findings of the timing analysis was a need for more accurate flux estimates and spectral energy measurements of the VHE γ-ray signal, which requires a greater understanding of the systematic errors inherent to the atmospheric Cerenkov technique. The effective collecting area of a Cerenkov telescope is related to the generation and attenuation of Cerenkov photons in the atmosphere. Uncertainties in the magnitude of the effective area result in errors in the deduced flux, whilst uncertainties in the function of effective area with energy result in errors of the spectral slope determined for any source. By using an inappropriate model for the atmosphere in simulations of atmospheric Cerenkov telescopes a systematic error can be introduced into calculations of the effective area. Chapter 6 compares the effective areas obtained from several model atmosphere types, including a contemporary model of the conditions at the Mark 6 site from data taken with atmospheric monitoring equipment employed in the Mark 6 operations. The findings from this work are then all drawn together in the final chapter, along with a discussion of the future atmospheric monitoring work that will go in to the next generation of atmospheric Cerenkov telescope installations.

We show here that a highly effective diffraction grating can be embedded into a nanoparticulate TiO2 film via imprinting combined with TiCl4 treatment. A thin TiO2 film spin-coated on a glass substrate was patterned by imprinting and this patterned layer was TiCl4-treated with a higher concentration than for a thicker over-coated TiO2 film. Due to the refractive index difference between the two layers, the incident light could be strongly diffracted. Different types of gratings have been incorporated into the films. A line grating with a 2 μm period and 1.2 μm thickness generated diffracted beams up to 3rd-order, with total diffraction efficiencies over 90% at 633 nm. This method has potential applications in TiO2-based photovoltaic and photocatalytic devices.

Diffraction gratings are frequently used in dye-laser cavities as wavelength-tuning elements. These gratings often limit the maximum laser output energy because of their low damage thresholds. We have measured the damage characteristics of both ruled and holographically produced gratings, under a variety of conditions. Using the single-shot-persite mode, the samples were irradiated by 30-ps, 1.064-μm pulses having a spot size of 0.5-mm radius. It was found that holographic gratings have damage thresholds from 1.5 to 5.0 times higher than similar ruled gratings. Thresholds for S-polarized light (E parallel to grooves) were higher by factors of 1.5 to 6. For the same type grating, gold coatings yielded higher thresholds than aluminum, although this is wavelength dependent. For holographic gratings, replicas have slightly higher thresholds than masters. Dependence upon groove spacing was weak. Data are presented to show a variety of comparisons between different types of gratings, including two different manufacturers and usage at higher orders of incidence.

An optical etching technique for producing small diffraction gratings, in which a thin metallic film set at an angle to the axis of a ruby laser cavity acted as a Q-switch in the operation of that laser, was previously reported. Here we report a comprehensive investigation of the formation of such etched gratings by the effect of laser light on a thin film external to the laser cavity. A time resolved investigation has been made of the development of such gratings in a number of metallic films, and the effect of film thickness, incident laser intensity, and angular orientation of the film has been studied.

A high-efficiency extreme ultraviolet (EUV) imaging spectrometer has been constructed and tested. The spectrometer employs a concave toroidal grating illuminated at normal incidence in a Rowland circle mounting and has only one reflecting surface. The toroidal grating has been fabricated by a new technique employing an elastically deformable submaster grating which is replicated in a spherical form and then mechanically distorted to produce the desired aspect ratio of the toroidal surface for stigmatic imaging over the selected wavelength range. The fixed toroidal grating used in the spectrometer is then replicated from this surface. Photographic tests and initial photoelectric tests with a 2-D pulse-counting detector system have verified the image quality of the toroidal grating at wavelengths near 600 A. The results of these initial tests are described in detail, and the basic designs of two instruments which could employ the imaging spectrometer for astrophysical investigations in space are briefly described, namely, a high-resolution EUV spectroheliometer for studies of the solar chromosphere, transition region, and corona and an EUV spectroscopic telescope for studies of nonsolar objects.

Principles for the design of pendulum-type ruling engines and their potential application for the fabrication of non-classical ruled diffraction gratings with large apertures are considered. For spherical diffraction gratings, the derivation of the analytical dependences for the determination of the geometrical shape of the curvilinear projections of their grooves, their curvature radii, and variable pitch is presented. Expressions for the coefficients of first–third-order aberrations are obtained.

Holographic gratings are widely used in spectrographs and monochromators, since they make it possible to control the most important aberrations. However, there are at most three degrees of freedom in determining the positions of the two mutually coherent point sources relative to the grating substrate for a planar recording system. Recording in an aspheric wavefront by means of ellipsoidal mirrors has been used to provide a larger number of free parameters. The aspherical optics approach, with its production and testing, is a complex task, and this solution is not suitable for optical production. Recording by means of spherical diffraction gratings was proposed for apparatus operating on a Rowland circle. Since only recording in copropagating beams was considered, the application of the method was limited to certain particular optical systems. The recording of concave gratings in counterpropagating beams by means of a complex system consisting of several lenses was proposed in Ref. 3. The use of such refractive optics with any holographic recording increases the scattered light level and degrades the quality of the final grating. Recording by means of a spherical diffraction grating and counterpropagating recording beams is free from all the disadvantages mentioned above. The holographic recording apparatus using counterpropagating beams proposed in Ref. 7 is more workable. However, from the standpoint of correcting aberrations, the use of this system is extremely limited.

There are several high-performance adaptive optics systems that deliver diffraction-limited imaging on ground-based telescopes, which has renewed interest in single-mode fiber (SMF) spectroscopy for exoplanet characterization. However, the fundamental mode of a telescope is not well matched to those of conventional SMFs. With the recent progress in asphere manufacturing techniques, it may be possible to reshape the fundamental mode of a SMF into any arbitrary distribution. An optimization problem is set up to investigate what the optimal mode field distribution is and what the fundamental throughput limit is for SMF spectroscopy. Both single-object spectrographs and integral-field spectrographs are investigated. The optimal mode for single-object spectrographs is found to be the aperture function of the exit pupil; for integral-field spectrographs, the optimal mode depends on the spatial sampling of the focal plane. For dense sampling, a uniform mode is optimal; for sparse sampling, the mode of a conventional SMF is near optimal. With the optimal fiber mode, a high throughput ( > 80 % ) can be achieved when the focal plane is (super) Nyquist sampled. For Nyquist sampled cases, the optimal mode has almost 20% more throughput than a conventional SMF.

In this Letter, we propose a type of continuous phase vortex gratings (CPVGs), which are able to generate a series of vortex beams with equal or proportional diffracted energies and different topological charges (TCs). A set of CPVGs with dimensions (60 × 60 × 1.1 μm3) are directly designed by mathematical equations, which avoids the use of iterative algorithms. The CPVGs are fabricated by femtosecond laser direct writing (FsLDW) with photoresist, and the experimentally generated vortex beams are in good agreement with the theoretical designs, exhibiting high optical efficiencies. In addition, we realized a CPVG onto the tip of a composite fiber for integrated optical systems. Our work paves the way for applications in optical communications, optical manipulations, and high-performance integrated optics.

<strong>12.1 Introduction</strong> Astronomers require larger and larger radiation-gathering apertures for their telescopes to penetrate farther into the mysteries of the universe. Large telescopes collect more photons and thus see fainter objects. These large apertures also increase the angular resolution across the object, as we saw in Chapter 10. For practical reasons such as telescope mass and thermal inertia, astronomers who build ground-based telescopes now partition primary mirrors greater than about 8 m into smaller segments, with each segment aligned so that its surfaces fall on the desired concave surface of the larger mirror. Large space- and ground-based telescopes are often divided into two stages. Stage 1 is the large collector, which forms an imperfect image in complex amplitude and phase. Before an image is detected, stage 2 of the optical system takes this imperfect image and removes wavefront aberrations using WFSC. This principle was discussed in Chapter 11. In Chapter 5 we learned that the number of reflections in the optical system needs to be small in order to minimize the power that is absorbed by the mirrors and that does not reach the focal plane. Consequently, rather than correcting the wavefront at an image of the telescope pupil, many telescopes correct the wavefront at the secondary mirror of the astronomical telescope before the primary focus. Correcting the wavefront at the secondary limits the FOV of the telescope that can be corrected. However, most astronomical applications today use a narrow FOV (limited by the expensive digital array detector size or by atmospheric turbulence), so wavefront control (WFC) at the secondary is sufficient. Frequently, the optical system for large telescopes is divided into three parts: the large precision telescope radiation collector, a metrology and WFSC system (see Chapter 11), and the instrument that analyzes the radiation. The motivation behind this configuration is to reduce the cost of the telescope system. For ground-based telescopes, the two expensive subsystems are the dome and the very stiff massive mechanical structure required to hold the mirror, the secondary, and its supports.

Highly confined waves of surface plasmon polaritons (SPPs) in monolayer graphene are efficiently excited using an etched diffractive grating on silicon. In this paper, an inhomogeneous graphene-covered grating is proposed and the excitation of which is analyzed as an analogy with the two-level system in quantum mechanics. Based on the equivalent circuit theory, the excitation of plasmonic waves in an etched diffractive grating is equivalent to the resonance of an oscillator. Thus, according to the governing equation, the electric currents for the inhomogeneous graphene-covered grating is correspond to the ket of energy states in the two-level system. In addition, the excitation of SPPs in an inhomogeneous graphene-covered grating is numerically analyzed to validate the theoretical model. Our work offers a new inroad of the excitation of SPPs in an inhomogeneous graphene-covered grating in an analogy with the two-level system.

Ly{alpha} photons that escape the interstellar medium of star-forming galaxies may be resonantly scattered by neutral hydrogen atoms in the circumgalactic and intergalactic media, thereby increasing the angular extent of the galaxy's Ly{alpha} emission. We present predictions of this extended, low surface brightness Ly{alpha} emission based on radiative transfer modeling in a cosmological reionization simulation. The extended emission can be detected from stacked narrowband images of Ly{alpha} emitters (LAEs) or of Lyman break galaxies (LBGs). Its average surface brightness profile has a central cusp, then flattens to an approximate plateau beginning at an inner characteristic scale below {approx}0.2 Mpc (comoving), then steepens again beyond an outer characteristic scale of {approx}1 Mpc. The inner scale marks the transition from scattered light of the central source to emission from clustered sources, while the outer scale marks the spatial extent of scattered emission from these clustered sources. Both scales tend to increase with halo mass, UV luminosity, and observed Ly{alpha} luminosity. The extended emission predicted by our simulation is already within reach of deep narrowband photometry using large ground-based telescopes. Such observations would test radiative transfer models of emission from LAEs and LBGs, and they would open a new window on the circumgalacticmore » environment of high-redshift star-forming galaxies.« less

The features of the Talbot effect using the phase diffraction gratings have been considered. A phase grating, unlike an amplitude grating, gives a constant light intensity in the observation plane at a distance multiple to half of the Talbot length ZT. In this case, the subject of interest consists in so-called fractional Talbot effect with the periodic intensity distribution observed in planes shifted from the position nZT/2 (the so-called Fresnel images). Binary phase diffraction gratings with varying phase steps have been investigated. Gratings were made photographically on holographic plates PFG-01. The phase shift was obtained by modulating the emulsion refraction index of the plates. Two types of gratings were used: a square grating with a fill factor of 0.5 and a checkerwise grating (square areas with a bigger and lower refractive index alternate in a checkerboard pattern). By the example of these gratings, the possibility of obtaining in the observation plane an image of a set of equidistant spots with a size smaller than the size of the phase-shifting elements of the grating (the so-called Talbot focusing) has been shown. Clear images of spots with a sufficient signal-to-noise ratio have been obtained for a square grating. Their period was equal to the period of the grating. For a grating with a checkerwise distribution of the refractive index, the spots have been located in positions corresponding to the centres of cells. In addition, the quality of the resulting pattern strongly depended on the magnitude of a grating phase step. As a result of the work, the possibility to obtain Talbot focusing has been shown and the use of this effect to wavefront investigation with a gradient sensor has been demonstrated.