Simon Thibault

Large-deviation achromatic Risley prisms pointing systems

As part of the Infrared Eye project, this article describes the design of large-deviation, achromatic Risley prisms scanning systems operating in the 0.5 - 0.92 and 8 - 9.5 μm spectral regions. Designing these systems is challenging due to the large deviation required (zero - 25 degrees), the large spectral bandwidth and the mechanical constraints imposed by the need to rotate the prisms to any position in 1/30 second. A design approach making extensive use of the versatility of optical design softwares is described. Designs consisting of different pairs of optical materials are shown in order to illustrate the trade-off between chromatic aberration, mass and vignetting. Control of chromatic aberration and reasonable prism shape is obtained over 8 - 9.5 μm with zinc sulfide and germanium. The design is more difficult for the 0.5 - 0.92 μm band. Trade-offs consist in using sapphire with Cleartran® over a reduced bandwidth (0.75 - 0.9 μm ) or acrylic singlets with the Infrared Eye in active mode (0.85 - 0.86 μm). Non-sequential ray-tracing is used to study the effects of fresnelizing one element of the achromat to reduce its mass, and to evaluate detector narcissus in the 8 - 9.5 μm region.

Enhanced optical design by distortion control

The control of optical distortion is useful for the design of a variety of optical system. The most popular is the F-theta lens used in laser scanning system to produce a constant scan velocity across the image plane. Many authors have designed during the last 20 years distortion control corrector. Today, many challenging digital imaging system can use distortion the enhanced their imaging capability. A well know example is a reversed telephoto type, if the barrel distortion is increased instead of being corrected; the result is a so-called Fish-eye lens. However, if we control the barrel distortion instead of only increasing it, the resulting system can have enhanced imaging capability. This paper will present some lens design and real system examples that clearly demonstrate how the distortion control can improve the system performances such as resolution. We present innovative optical system which increases the resolution in the field of view of interest to meet the needs of specific applications. One critical issue when we designed using distortion is the optimization management. Like most challenging lens design, the automatic optimization is less reliable. Proper management keeps the lens design within the correct range, which is critical for optimal performance (size, cost, manufacturability). Many lens design presented tailor a custom merit function and approach.

Advanced fiber coupling technologies for space and astronomical applications

For almost three decades, the problem of coupling light into a single-mode fiber or waveguide has been studied extensively both from the theoretical and experimental points of view. Today, commercial optical design software permit an accurate evaluation of single-mode fiber-coupling efficiency to be performed for various systems. More specifically, optical communication (satelllite-to-satellite or satellite-to-ground) and stellar interferometry (ground- or space-based) are the two main categories of applications that are discussed here. Coupling theory has recently been the subject of renewed attention due to the use of single-mode fibers in stellar interferometers, where wavefront aberrations caused by atmospheric turbulences have an impact on fiber-coupling efficiency, stability and phase. This impact, and the extent to which it can be minimized with adaptive optics, is a crucial piece of information for this application. Advanced fiber-coupling devices (AFCD), incorporating an adaptive optics system, are necessary to achieve the performance goals. Applications not involving atmospheric propagation, although in principle simpler, nevertheless present optical design challenges due to the low level of insertion loss sometimes required. Such applications would greatly benefit from a coupling system that reshapes the irradiance distribution in the coupled beam in order to better match the Gaussian mode of the single-mode fiber, and thus achieve higher coupling efficiency.

GPI: cryogenic spectrograph optics performances

The science instrument for GPI (Gemini Planet Imager) is a cryogenic integral field spectrograph based on a lenslet array. The integral field nature of the instrument allows for a full mapping of the focal plane at coarse spectral resolution. With such a data cube, artifacts within the PSF such as residual speckles can be suppressed. Additionally, the initial detection of any candidate planet will include spectral information that can be used to distinguish it from a background object: candidates can be followed up with detailed spectroscopic observations. The optics between the lenslet array and the detector are essentially a standard spectrograph with a collimating set of lenses, a dispersive prism and a camera set of lenses in a folded assembly. We generally refer to this optical set as the spectrograph optics. This paper describes the laboratory optical performances over the field of view. The test procedure includes the imaging performances in both non dispersive and dispersive mode. The test support equipments include a test cryostat, an illumination module with monochromatic fiber laser, a wideband light source and a test detector module.

Effects and prediction of stray light produced by diffractive lenses

Correlation between computational models and corresponding experimental data of stray light produced by unwanted diffraction orders of diffractive lenses is described. It is shown that the attractive scalar model under Fresnel approximation does not agree with experimental data. The validity of Fresnel approximation for multi-order focusing optics is derived to ex[plain the limitation of the scalar approach. Geometrical models with both coherent and incoherent summation of each diffraction orders coupled with efficiency estimation is used with success. The model takes into account the local diffraction efficiency and also of the number of diffraction orders supported locally by the structure. This geometrical optics model can be used in standard optical design software. Therefore it may be recommended as an additional tool for stray light analysis at the design stage of hybrid refractive/diffractive optical systems. In a second part, the impact of unwanted diffraction orders when diffractive optics is used in visible imaging system is discussed. Several experimental observations about the use of plastic/diffractive lens in relation to different fabrication process used to build the lens are also discussed. From those observations, we recommend an approach to reduce the stray light produced by the diffractive lens. In conclusion, a serious stray light analysis is mandatory in the design of visible imaging systems using diffractive lenses.

F-sin(theta) lens system and applications

Introducing the desired amount of distortion in an optical system is possible and relatively straight forward for optical designer. Known f-theta lenses used in laser scanner system is one of the most popular lenses of this type. With the development of DWDM system, compact spectrometer and various grating based devices, the interest in solving optical problems with innovative optical design has been a revival. On the other hand, it is possible to simplify considerably the task of the software by designing more sophisticated optical element. The F-sin(theta) lens is such a device which can be used in various grating based devices to improve the performance. This optical device is intended to provide a new lens or lens group which can be used with a diffraction grating to provide a linear output plane with the wavelength (lambda), but also with the order of diffraction (m), or with the grating spatial frequency, or with the inverse of the index of refraction (1/n). Based on theoretical consideration, we calculated which amount of distortion is desired to get linear output plane for a grating based device. We present singlet, achromatic doublet and mirror having this characteristic. These F-sin(theta) lenses present satisfactory distortion characteristic and have an improved correction for the curvature of image plane. F-sin(theta) lens will be also name F-lambda lens.

MIRCAMOS: a mosaic IR camera and multi-object spectrograph for CFHT

We present the preliminary conceptual design of a Mosaic IR Camera and Multi-Object Spectrography (MIRCAMOS) for the Canada-France-Hawaii Telescope. The instrument houses 4 Hawaii-2 2048 by 2048 HgCdTe detectors sensitive between 0.8 and 2.5 micrometers . The optics is all reflective, featuring a warm corrector with fast tip/tilt capability and 4 cryogenic optical trains. The pixel scale is 0.20 inch/pixel yielding a field of view of 13.7 feet by 13.7 feet. Z, J, H or K band spectroscopy at R approximately 1500 is obtained with a single grating setting. A cryogenic slit wheel unit featuring several positions for multi-object custom masks is mounted within a separate cryostat designed to be thermally cycled within 8 hours for rapid exchange of MOS masks. Each mask can hold up to approximately 300 slitlets distributed over a FOV of 7 feet by 13.7 feet. MIRCAMOS is very competitive compared with similar instruments planned for 8- 10 m telescopes.

Optical design and tolerances of wide-field imagers for astronomical survey

We present three designs and tolerances of wide-field imagers (3030 arc-minutes or larger) for astronomical surveying. Two infrared cameras (CPAPIR and PANORAMIX II) were designed for the 0.8-2.4 μm band and a third one (WIRCAM) for the visible and near-infrared band extending from 410 nm to 950 nm. The cameras are installed on the telescopes of the Canada-France-Hawaii (Hawaii, USA) and Mont Megantic Observatories (Quebec, Canada). The three cameras are compact, use only spherical refractive components and have an internal pupil accessible for insertion of filtering components. A Lyot stop must be used in the infrared camera for background rejection. For PANORAMIX II, a set of filters is used at the internal pupil. Correction of the large off-axis aberrations generated by the telescopes, wide spectral coverage, material choices, cryogenic temperature and alignment were the main design challenges. Also, tolerancing was particularly critical for the infrared cameras because they are cryogenically cooled, thus forbidding adjustment of internal components. The cameras’ theoretical performances are presented in terms of point-spread function, encircled energy and distortion.

Optical design of CPAPIR, a cryogenic IR camera for OMM

The optical design of the wide-field infrared camera CPAPIR (Camera PAnoramique Proche InfraRouge) for the Mont Megantic Observatory (OMM) has been completed. CPAPIR will be a unique wide-field camera at the OMM. It has a full field of view of 0.71 degrees, an instantaneous field of view of 0.88 arc-seconds, and a spectral coverage of 0.85 - 2.5 μm. The camera is operated under vacuum and at cryogenic temperature. The performance (image quality, vignetting, cold stop efficiency, ghost analysis and tolerancing) of CPAPIR has been optimized at cold temperature using cryogenic indices of refraction and coefficients of thermal.

Design of an infrared integral field spectrograph specialized for direct imaging of exoplanets

Quasi-static speckles are the main source of noise preventing the direct detection of exoplanets around bright stars. We are investigating the use of an infrared (1.5-2.4 μm) integral field spectrograph (IFS) specialised for speckle suppression and the detection of self-luminous giant planets. This paper presents the optical design and laboratory results obtained with a TIGER-type IFS prototype based on a microlens array. A similar IFS will be used for the Gemini Planet Imager (GPI). Preliminary speckle-suppression performances of the IFS are presented along with a prism design that allows keeping a constant spectral resolution over multiple wavebands as it will be required for GPI.