Advanced modelling of a moderate-resolution holographic spectrograph

In the present article we consider an accurate modeling of spectrograph with cascade of volume-phase holographic gratings. The proposed optical scheme allows to detect spectra in an extended wavelength range without gaps providing relatively high spectral resolution and high throughput. However, modeling and minimization of possible cross-talks between gratings and stray light in such a scheme represents a separate task. We use analytical equations of coupled wave theory together with rigorous coupled wave analysis to optimize the gratings parameters and further apply the latter together with non-sequential raytracing algorithm to model propagation of beams through the spectrograph. The results show relatively high throughput up to 53% and absence of any significant cross-talks or ghost images even for ordinary holograms recorded on dichromated gelatin.Comment: 9 pages, 8 figures. Accepted for publication in Applied Optics, v. 56, N21, 201

Design and optimization of a dispersive unit based on cascaded volume phase holographic gratings

We describe a dispersive unit consisting of cascaded volume-phase holographic gratings for spectroscopic applications. Each of the gratings provides high diffractive efficiency in a relatively narrow wavelength range and transmits the rest of the radiation to the 0th order of diffraction. The spectral lines formed by different gratings are centered in the longitudal direction and separated in the transverse direction due to tilt of the gratings around two axes. We consider a technique of design and optimization of such a scheme. It allows to define modulation of index of refraction and thickness of the holographic layer for each of the gratings as well as their fringes frequencies and inclination angles. At the first stage the gratings parameters are found approximately using analytical expressions of Kogelnik's coupled wave theory. Then each of the grating starting from the longwave sub-range is optimized separately by using of numerical optimization procedure and rigorous coupled wave analysis to achieve a high diffraction efficiency profile with a steep shortwave edge. In parallel such targets as ray aiming and linear dispersion maintenance are controlled by means of ray tracing. We demonstrate this technique on example of a small-sized spectrograph for astronomical applications. It works in the range of 500-650 nm and uses three gratings covering 50 nm each. It has spectral resolution of 6130 - 12548. Obtaining of the asymmetrical efficiency curve is shown with use of dichromated gelatin and a photopolymer. Change of the curve shape allows to increase filling coefficient for the target sub-range up to 2.3 times.Comment: 10 pages, 7 figures. Proc. SPIE 10233. Conference "Holography: Advances and Modern Trends V" (Prague, Czech Republic

Optical scheme of small-size curved detector spectrofluorometer

We present two versions of a compact spectrograph optical scheme based on aberration-corrected concave holographic grating. It is shown that performance of a widely-known flat-field spectrograph scheme can be significantly improved by means of use of a curved detector and introduction of an additional condition of coma correction.The spectrograph provides spectral resolution up to 0.23 nm over an extended working range of 250-900 nm and can be used for fluorescence analysis of waste water

Advanced modeling of a moderate‐resolution holographic spectrograph

International audienceIn the present article we consider an accurate modeling of a spectrograph with a cascade of volume‐phase holographic gratings. The proposed optical scheme allows us to detect spectra in an extended wavelength range without gaps, providing relatively high spectral resolution and high throughput. However, modeling and minimization of possible cross‐talk between gratings and stray light in such a scheme represents a separate task. We use analytical equations of the coupled‐wave theory together with rigorous coupled‐wave analysis to optimize the gratings parameters and further apply the latter together with a non‐sequential ray‐tracing algorithm to model propagation of beams through the spectrograph. The results show relatively high throughput up to 53% and the absence of any significant cross‐talk or ghost images, even for ordinary holograms recorded on dichromated gelatin

Optical Designs with Curved Detectors for Fiber Bragg Grating Interrogation Monitors

In this paper, we evaluate the application of curved detectors and freeform optics technologies for fiber Bragg gratings (FBGs) interrogation monitors design. It is shown that, in a high-dispersion spectrograph scheme, the camera part operates in special conditions, which result in a field curvature change. This field curvature can be compensated by the use of a curved detector. When used together with freeform optics, the curved detectors allow for reduction of the number of optical components to two or even one element by merging their functions. Three design examples for the range of 810–860 nm reaching the spectral resolution limit of 89–139 pm at NA=0.14 are presented to demonstrate the achieved performance and the technological trade-offs

Optical Design of a Slitless Astronomical Spectrograph with a Composite Holographic Grism

International audienceIn the present work, we consider an optical design of a slitless spectrograph for an existing 0.5 m-class telescope. This design concept has a number of advantages such as compact size, simplicity, and simultaneous coverage of a large field of view. A challenge with this design is correcting aberrations caused by placing a dispersing element in a converging beam. To overcome this issue, we propose to use a composite grism, which represents a combination of a prism and a volume-phase holographic grating, the latter which is split into zones with independently optimized parameters. We demonstrate two designs of such a grism. In both designs, the spectrograph operates in the range of 450–950 nm in an F/6.8 beam and covers a field of view of 35.6' × 7.2'. Through advanced modeling, it is shown that a composite grism having four rectangular zones with different thickness and index modulation depth of the hologram and recorded with an auxiliary deformable mirror decreases the astigmatic elongation by a factor of 85, increases the spectral resolving power by 4.4 times, and reaches R1389 while increasing the average diffraction efficiency by a factor of 1.31. If we reduce the number of zones to only two, replace the deformable mirror with two static corrector plates, and fix the hologram thickness, the corresponding performance gains still remain high: the astigmatism is reduced by a factor of 61, the spectral resolving power is up to 1.7 times higher, reaching R1067, and the efficiency is increased by a factor of 1.27. This shows that the proposed design allows the construction of a simple and compact instrument, providing high performance over the entire field of view and spectral range

Digital Holographic Positioning Sensor for a Small Deployable Space Telescope

We propose a system and a technique to measure misalignments of individual optical elements in a deployable telescope for a small satellite, and we demonstrate its application for a 2230 mm F10.3 Ritchey–Chretien type telescope. The system uses laser point sources at the edge of each of the deploying segments to form an interference pattern at the telescope image sensor. By processing the interferograms in the Fourier domain and comparing them to the computed ones, it becomes possible to measure the position of the primary mirror segments with a precision of 0.8μm and the axial displacement of the secondary mirror with the precision of 0.1 nm. Such a system would allow to build a deployable telescope fitting in a 1U CubeSat and align it in space with a sufficient accuracy, thus providing a gain in collecting power, reaching a factor of 2.49 with respect to a single mirror telescope fitting on a 1U CubeSat

A Tribute to Sergey Odinokov

“My main goal was to point out new phenomena and spread the ideas, which will become the starting points for new research” [...

A Tribute to Sergey Odinokov

“My main goal was to point out new phenomena and spread the ideas, which will become the starting points for new research” [...