Pantazis Mouroulis

Recent advances in blazed grating fabrication by electron-beam lithography

Convex and concave diffraction gratings are required for concentric imaging spectrometer forms. Direct-write electron-beam lithography has proven to be an effective method for fabricating high-efficiency blazed gratings on non-flat substrates. Recently fabricated convex gratings have demonstrated relative efficiency greater than 90%, diffuse scatter and ghosts less than 5x10-4 of the main diffraction order, and zeroth-order wavefront error less than 1/4-wave at 633 nm. Such gratings can be fabricated on JPL’s JEOL JBX-9300FS electron-beam lithography system with a writing speed of approximately 1 to 2 cm2 per hour. The technique was recently used to fabricate flight-qualified gratings for the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument that is scheduled to fly on the NASA Mars Reconnaissance Orbiter in 2005.

Low-distortion imaging spectrometer designs utilizing convex gratings

Several designs are described that are based on the Offner concentric spectrometer form, with the grating formed on the convex secondary mirror. It is shown that these designs permit the reduction of spectral and spatial distortion to a small (approximately 1%) fraction of a pixel, while also providing a compact form with excellent optical correction. These designs can satisfy the needs and tight calibration requirements of imaging spectrometers for Earth remote sensing, over a broad spectral range from the ultraviolet to the thermal infrared. The practical realization of the designs owes much to the recent development of convex grating fabrication by electron-beam lithography.

Spectral and Spatial Uniformity in Pushbroom Imaging Spectrometers

Two simple and compact pushbroom spectrometer forms are described that can satisfy stringent spectral and spatial uniformity requirements in terms of minimizing distortion as well as the variation of the pixel spectral and spatial response functions.

Optical design for imaging spectroscopy

Design examples are given for imaging spectrometer systems operating in the solar reflected spectrum. Design considerations and procedures are outlined for incoherently and coherently coupled systems. The case of a partially coupled system is also examined and it is shown why it is generally to be avoided.

Optical design, performance, and tolerancing of next-generation airborne imaging spectrometers

We describe the optical design and performance of the Next-Generation airborne Imaging Spectrometer (NGIS) currently being constructed at Caltechs Jet Propulsion Laboratory. The new, high-resolution instrument incorporates a number of design advantages including a two-mirror anastigmatic telescope for simplified alignment and high throughput, as well as a concentric, multi-blazed grating for tailored broadband efficiency. A detailed tolerancing and sensitivity approach reveals tight requirements that must be satisfied for spectral calibration and boresight stability. This improved spectral and pointing stability, combined with high uniformity and high signal-to-noise ratio allows us to generate spectrometry measurements capable of answering challenging science questions.

Compact low-distortion imaging spectrometer for remote sensing

We describe a pushbroom imaging spectrometer having a number of attractive features for remote sensing applications, including compact and simple form, good image quality, high efficiency, and very low levels of distortion. These properties are made possible by the unique characteristics of convex gratings manufactured by electron-beam lithography. A laboratory prototype has been built and is under evaluation. If has an f-number of 2.8, covers a spectral band from 400 to 1000 nm with 3 nm spectral resolution and has 750 spatial elements across the entrance slit. Experimental results are shown that demonstrate very low distortion, on the level of 2 percent of a pixel.

A Compact, Fast, Wide-Field Imaging Spectrometer System

We present test results from a compact, fast (F/1.4) imaging spectrometer system with a 33° field of view, operating in the 450-1650 nm wavelength region with an extended response InGaAs detector array. The system incorporates a simple two-mirror telescope and a steeply concave bilinear groove diffraction grating made with gray scale x-ray lithography techniques. High degree of spectral and spatial uniformity (97%) is achieved.

Spectral response evaluation and computation for pushbroom imaging spectrometers

We discuss issues associated with computation and measurement of the spectral response function and spectral uniformity of pushbroom imaging spectrometers. Methods for accurate computation are presented and advantages or pitfalls of particular approaches are identified. We demonstrate the effect to which partial coherence effects can be neglected or approximated through simpler incoherent computations. The results are illustrated with spectral calibration data from a complete sensor.

Optical Design and Performance of the Ultra-Compact Imaging Spectrometer

We present the optical design and performance of the Ultra-Compact Imaging Spectrometer (UCIS) currently under development at Caltechs Jet Propulsion Laboratory. The new instrument demonstrates a low optical bench mass of less than 0.5 kg and compact size that enables Mars Rover or other in situ planetary applications. UCIS is an F/4, wide field (30°) design, covering the spectral range 600-2600 nm and is enabled by a simple all aluminum two-mirror telescope and Offner spectrometer. We discuss here the optical design and alignment method that enables this compact and low mass imaging spectrometer and demonstrate successful spectrometer alignment with smile and keystone levels at 2-3% of a pixel width.

Polarization compensating protective coatings for TPF-Coronagraph optics to control contrast degrading cross polarization leakage

The Terrestrial Planet Finder Coronagraph (TPF-C) for observing and characterizing exo-solar planets requiring star light suppression to 10-10 level demands optical aberrations and instrument stability to sub-nm levels. Additionally, wavefront polarization has to be tightly controlled over the 8m x 3.5m primary mirror aperture and 500nm - 800nm minimum bandwidth because the Deformable Mirror (DM) employed to control the wavefront can not correct simultaneously for the different wavefronts presented by two orthogonal uncorrected polarization fields. Further, leakage of cross polarization fields introduced by the various optical surfaces can degrade the image contrast. The study reported here shows mirror coating designs that reduce the phase difference between orthogonal polarizations reflected by a mirror surface to less than 0.6 deg over the bandwidth and aperture which may encounter a maximum angle of incidence of about 12 deg at a curved mirror. Such designs mitigate the contrast degradation due to cross polarization leakage. Simulations show that required contrast levels can be achieved with such coatings.