2.1

Double-pass prism-based spectrometer

This paper [1] describes the design and the performance of the double-pass prism-based spectrometer designed to provide spectrum over the spectral domain 400-2500 nm. The F-number is equal to 4.3, the GSD is equal to 5.7 m and the ground swath is equal to 2.86 km. The design comprises one fused silica prism as a dispersive element, one retro-reflecting mirror and two fused silica focal reducer lenses as shown in Figure 1. The total length of this instrument is equal to 400 mm. The magnification is equal to 0.48. The dispersion is equal to 8.2 nm/pixel. The spectrum is spread on 256 pixels.

Figure 1.

double-pass prism-based spectrometer optical design layout.

The detector is made of one thinned HgCdTe detector that comprises 1024 x 256 pixels. This detector can operate over the specified spectral interval, from 400 nm to 2.5 μm. The HgCdTe grows on CdZnTe substrate, but the CdZnTe blocks the shorter wavelength below 850 nm. The thinned HgCdTe is produced by removing of the CdZnTe substrate. We consider that the capability for one detector to cover the full specified spectral interval is very interesting.

2.2

Compact Hyperspectral Prism Spectrometer (CHPS)

The Compact Hyperspectral Prism Spectrometer (CHPS) [3], [4] has been developed under the NASA’s Sustainable Land Imaging-Technology (SLI-T) program. This instrument operates over the spectral domain 400-2500 nm and the design includes prisms to avoid the straylight induce by the gratings and to reduce the sensitivity of the polarization for inland and coastal water observations. This instrument provides continuous high-fidelity spectroscopic information across the solar reflected portion of the electromagnetic spectrum at the spectral sampling range from 1.3 nm/pixel to 11 nm/pixel.

The CHPS volume is equal to 500x500x500 mm³ that is 30 times smaller than Landsat-8. The CHPS airborne version (CHPS-AB) provides 2.5 m GSD with 1.41 km swath at an altitude of the aircraft equal to 4 km. The CHPS space borne version will provide a GSD equal to 30 m at a spacecraft altitude equal to 704 km. The CHPS-AB dispersive elements include a curved aplanatic prism in a double-pass configuration following by one spherical mirror. Two additional fused silica elements are used as field lenses. The first field lens is located next to the slit. The second field lens is placed in front of the focal plane as shown in Figure 2. The detector format is 2k x 2k pixels with 12 μm pixel size. The dispersion of the CHSP-AB varies between 2.3 nm/2binning pixels to 21.3 nm/2binning pixels.

Figure 2.

Optical design layout of CHPS. The front telescope is on the left side of the line. The spectrometer is on the right side of the line.

3.1

Wide field freeform Offner spectrometer

The compact spectrometer presented in [7] has been designed to provide a large field of view (27 degree) and a spectral range equal to 240-2500 nm thus covering the UV, VNIR and SWIR bands. The GSD is specified to be equal to 4 m in the VNIR and 7.5 m in the SWIR.

The spectrometer is an Offner that comprises one grating, one filter, three mirrors and two folding mirrors. The spectrometer layout is presented in Figure 3. The primary mirror M1 is a freeform surface, the reflective grating (M2) is manufactured on a convex spherical surface and the tertiary mirror M3 is spherical and concave.

Figure 3.

Wide field freeform Offner spectrometer optical design layout with three channel focal planes.

M3 reflects the incident beam toward the filter set that is considered as the most critical part of the instrument. This filter comprises one filter for the UV band, one filter for the VNIR band and one filter for the SWIR band. These multilayer filters are used to select spectral band incident on each detector.

This instrument includes folding mirrors located in front of the image plane to spatially separate the beam in the UV, VNIR and SWIR spectral bands. The beam corresponding each spectral band is focused on a detector. The pixel sizes are equal to 13.5 μm in UV-VNIR and is equal to 25 μm in the SWIR. The dispersion in the UV-VNIR (respectively in the SWIR) is close to 4 nm/pixel (respectively equal to 25 nm/pixel). The volume of the payload (telescope + spectrometer) is equal to 316 x 160 x 170 mm³. The flight altitude is equal to 30 km.

3.2

Compact freeform Offner spectrometer

The paper [8] presents the advantages provided by freeform optics to reduce the volume and increase the image and spectral performance of an Offner spectrometer. The spectral domain is equal to 200-1500 nm, the beam aperture is equal to f/3.8 and the spectral dispersion is equal to 100 nm/mm. The grating density is equal to 150 line/mm and the FOV is defined by the slit length that is equal to 10 mm.

The starting point is the all-spherical Offner - Chrisp spectrometer that presents a problem of astigmatism over the spectral domain [200, 500 nm]. This instrument can provide an acceptable image quality over [500 nm, 1100 nm] only. The volume of this spectrometer is equal to 530 cm³. The author shows that if we reduce the volume to 100 cm³ by keeping spherical surfaces only, the image quality is very bad and the spectrometer cannot be used.

This spectrometer is optimized in a first time by changing the spherical surfaces to fourth- and sixth-order aspherical surfaces. After optimization, the spectrometer presents a better image quality over [500 nm, 1100 nm] and the volume of the spectrometer is reduced to 100 cm³. Finally, the spectrometer is optimized for freeform surfaces. The all-freeform Offner spectrometer comprises only freeform surfaces. The performance is diffraction limited over the full specified FOV and the full specified spectral domain for a volume still equal to 100 cm³ Figure 4. By keeping the same size, the design can be optimized to work at [200 nm, 1900 nm] with 10 mm slit or at [200 nm, 1500 nm] with 20 mm slit.

Figure 4.

Freeform Offner spectrometer optical design layout with the wavefront error map over 0.5-1.1 μm spectral domain.

3.3

Conical diffraction Offner hyperspectral imaging spectrometer (CDO-HIS)

The paper [9] describes an Offner hyperspectral imaging spectrometer designed to observe the global ocean. This instrument includes one convex blazed grating to produce a “nearly non distortion and high spectral fidelity image”. In particular, the Offner configuration has been selected to get a compact solution with high performance and a low distortion in wide spectral band.

The CDO-HIS comprises a push-broom imaging covering [400 nm, 900 nm]. The F-number is equal to 4 and the FOV is equal to 0.9 degree that is very close to the FOV of the instrument we plan to design in the framework of the CNES study. The slit width (respectively length) is equal to 0.1 mm (respectively 1 mm). The dispersion width is specified to be equal to 10.4 mm and the spectral resolution is specified to be better than 5 nm. The Keystone and Smile are specified to be lower than 0.1 pixel.

The design is based on an all-spherical Offner configuration as shown in Figure 5. The mirrors are spherical and the diffraction grating is conical. The authors mention that this grating was fabricated by holographic lithography/ion-beam-etching techniques. The grating blaze angle is equal to 6.4°. The grating radius of curvature is around 50 mm with diameter of 16 mm. The grating groove shape was optimized to improve the 1st order diffraction efficiency and the broadband response. The calculation had been done through a finite element analysis of the groove profile. The grating efficiency has been measured and is higher than 0.5 over the full specified spectral band. One filter is installed in front of the detector to minimize the straylight due to the spectral order overlapping.

Figure 5.

CDO-HIS spectrometer optical design layout.

The authors state that the theoretical system is almost diffraction limited with a very low level of distortion (less than 0.025 pixel).

The spectral resolution, smile distortion and key stone distortion have been measured at three different FOVs and at different spectral lines over the specified spectral domain. The average linear dispersion is 0.0206 mm/nm. The lowest spectral resolution is equal to 4.76 nm. The smile distortion is equal to 0.28 nm or 5.6% of the pixel size. The keystone distortion is equal to 0.18 nm or 3.6% of the pixel size.

4.1

Double-pass freeform TMA

The paper [17], which is the reference document mentioned by CNES, presents a comparison of 15 different systems: one Offner design, two Schwarzschild designs, two Dyson designs and ten TMA designs. The specified spectral domain is equal to 420 – 1000 nm. The spectral dispersion is equal to 6.5 nm/pixel. The aperture is equal to f/3. The distortion is accessed by using two criteria: the smile and the keystone. The distortion is specified to be less than 20% of the pixel size, equal to 24 μm. The authors conclude that the following systems present better performance: the double-pass TMA, the Schwarzschild design and the Offner design.

We propose to focus on the double pass TMA that presents the advantage of including a plane diffraction grating as represented in Figure 6. The mirrors M1 and M3 are aspherical surfaces that could be manufactured on the same substrate to simplify the assembly process and relax the tolerances. The mirror M2 is the eighth-order freeform surface described by generalized Forbes polynomials. The authors propose to place one filter in front of the detector to eliminate the unwanted light from the diffraction orders. The system volume is equal to 120 x 190 x 240 mm³. The theoretical image quality is diffraction limited and the keystone is less than 5% and the smile is less than 12% of the pixel size.

Figure 6.

Double-pass TMA optical design layout.

5.1

Compact Wide Swath Imaging Spectrometer (CWIS)

This paper [22] presents the compact wide swath imaging spectrometer (CWIS). The instrument is a push-broom imaging spectrometer operating over the spectral domain [380 nm, 2510 nm]. The swath is equal to 52 degree, the slit length is equal to 48 mm that covers 1600 pixels in the FOV direction. The aperture number is equal to F# = 1.8 and the specified spatial resolution is comprised between 0.3 and 20 m depending on the spectral band. The specified signal to noise ratio is higher than 2000 at wavelength equal to 450 nm.

This spectrometer is a Dyson design with a volume equal to 325x150x150 mm³ and a mass less than 7 kg. The CWIS spectrometer includes one achromatic doublet made of CaF2 and Fused-silica glasses to provide acceptable performance over the full specified FOV and spectral domain. The optical design layout is presented in Figure 7.

Figure 7.

CWIS spectrometer optical design layout.

The most critical component of the spectrometer is the convex diffraction grating. The profile of the grating groove has been designed to provide a maximum grating efficiency over the full specified spectral domain. The groove profile differs significantly from the classical groove shape and requested specific manufacturing methods.

This concave grating was fabricated by the Jet Propulsion Laboratory, using electron beam lithography technique and diamond turning, then post polishing to minimize stray light. The grating diameter is equal to 125 mm and the sag is greater than 6 mm. The theoretical efficiency is close to 0.62 at λ=1750 nm and drops to about 0.49 at the wavelength λ=2500 nm. The efficiency is lower than 0.3 at the wavelength range below 1200 nm.

The image quality performance is compatible with a pixel size of 30 μm. The smile and keystone distortions are about 1% of the pixel size.

5.2

LWIR Dyson

In the paper [19], the authors describe one Dyson spectro imager operating in the LWIR spectral domain, from 7.8 μm to 13.4 μm. It is important to mention that this spectral domain is different from the VNIR and SWIR spectral domains specified for our study. However, we consider that the LWIR Dyson spectrometer proposed in this paper identifies interesting options to design a compact Dyson spectrograph that could be used to design a similar instrument operating in the VNIR and SWIR spectral bands.

The F-number is equal to 2.5. The Dyson lens and aspherical lens are made of Zinc Selenide. The layout of the modified Dyson spectrometer is shown in Figure 8. The slit and the focal plane are located at a given distance close to the plane surface of the Dyson lens. This, in order to make sure that the slit and the Focal Plane Array can be accommodated. The air distances between the Dyson lens and the slit or the focal plane introduce some spherical aberration that degrades the performance.

Figure 8.

LWIR Dyson spectrometer optical design layout.

The authors propose two solutions to correct the spherical aberration: introducing an aspherical grating into the design or an aspherical lens in front of the spherical concave grating. The design presented in the paper includes a ZnSe aspherical lens located in front of the grating as represented in Figure 8.

The spot image over the specified spectral band and over the full slit is well contained in one pixel of 40 μm. The smile and keystone are less than 2 μm. The volume of the spectrometer is equal to 130x50x50 mm³, making this solution particularly interesting for our study.

6.1

Concave dual-period and variable-period gratings

In the paper [23], the authors present the performance of gratings manufactured at Durham University’s Precision Optics Laboratory by using a five-axis diamond-turning machine. This machine can manufacture i) multi-blaze structure gratings to maximize the diffraction efficiency over a larger spectral range and ii) variable spaced grooves to consider and compensate the projection effect of the grating’s lines on a curved surface. The other advantages are the possibility to manufacture large metallic objects in substrates (such as nickel) thermally stable and compatible with cryogenic operations.

There are also two main limitations. The first limitation is the wearing of the diamond tip that could affect the homogeneity of the groove profile on the large surface thus impacting the grating efficiency. The second limitation is the thermal variation that could occurred during long time machining and that could induce surface errors.

The diameter of the grating is equal to 40 mm. The grating groove density is equal to 100 line/mm. The grating is optimized to work at 1st diffraction order on the visible band [400 nm, 700 nm]. The resolving power is equal to 4500. The spectrometer F-number is equal to 6.6.

Four freeform grating are considered in this study. The gratings are presented in Figure 9. The Grating 1 is made of Aluminum RSA 6061 with constant line frequency optimized for λ=588 nm. The Grating 2 is made of Aluminum RSA 443 and plated with NiP with a constant line frequency optimized for λ=588nm. The Grating 3 is made of RSA 443 and NiP with dual blazed structure optimized for λ=470 nm and λ=1047 nm. The Grating 4 is made of RSA43 and NiP with a variable period optimized for λ=588 nm. The maximum differentiation of the variable period is equal to 20 nm.

Figure 9.

Freeform diffraction gratings manufactured using a 5-axis CNC machine.

The NiP was identified as the substrate offering the lowest roughness and the best profile quality. The NiP offers an average roughness of 2.5 nm RMS per facet. The typical groove profiles measured shows that the profile machined on NiP is sharper than RSA6061.

6.2

Convex blaze grating

This paper [25] present a specific kind of spectrometer based on an Offner configuration and that includes a multi-slit system located in an intermediate plane. The BATMAN spectrometer was developed for 3.6m Telescopio Nazionale Galileo ground base telescope. The “Smart slit” is programmable to select the section on the FOV that will be sent to the spectrometer. The spectral domain covers 400-800 nm. The central wavelength is at 580 nm. The spectrometer aperture number is equal to 4. The volume of the spectrometer is equal to 1.4 x 1.2 x 0.75 m³ and the mass is equal to 400 kg. The particularity of this instrument is that the multi-slit system is an active MOEMS component. This instrument has been designed for astronomical observations over the visible spectral band only. The proposed instrument is thus outside the scope of our study.

This paper presents the development status and the performance of a convex blazed grating which is very interesting in the framework of the development of freeform compact spectrometers. The rectangular grating manufacturing realization method was firstly done on the flat substrate as shown in Figure 10. The Sol-Gel is used to replicate and transfer the shape of the original grating onto the substrate. To apply this method to the curve substrate is to use a flexible replica with an additional process of reactive ion etching as shown in Figure 11. The grating coating is protected silver from Optics BALZERS. The BATMAN has a grating size of 63.5 mm, radius of curvature of 225 mm, 5.7 degree of blaze angle and 300 line/mm.

Figure 10.

Grating replication process on a flat substrate.

Figure 11.

Grating replication process on a curve substrate.

6.3

Surrey Satellite Technology (SSTL) spectrometer

The SSTL spectrometer [27] is dedicated to monitoring atmosphere chemistry by using the absorption spectra of atmospheric gas species. The spectral resolution is equal to 0.5 nm in UV and VIS bands. The spectral resolution is equal to 0.1 nm in NIR and SWIR bands. The spatial resolution is equal to 300 m with 150 km swath width. The instrument includes silicon detectors for the UV, VIS and NIR bands and uses on HgCdTe detector for the SWIR band.

Two design of the spectrometer were proposed. The first spectrometer is specified for the Oxygen-A band 750 – 775 nm spectral interval (NIR band). The second spectrometer is specified for the SWIR band covering the spectral band of 2305-2385 nm. The SWIR spectrograph comprises one blazed grating used in a quasi-Littrow configuration that provides an efficiency over 50 %.

The NIR spectrographs includes one immersed grating manufactured on the face of a prism as a dispersive element. The optical design layout is presented in Figure 12. The incident angle is comprised between 50 and 60 degree incidence angle in total internal reflection close to the Littrow condition that induces a high efficiency. It is worth to mention that collimator and the focusing lenses are identical in the proposed design.

Figure 12.

SSTL-NIR spectrometer layout

6.4

MicroCarb

The objective of the CNES MicroCarb Satellite [28] flight mission is to monitor the carbon dioxide in Earth’s atmosphere. The optical system has been designed to provide spectrum over four spectral bands: 764-768 nm, 1602-1619 nm and 2037-2065 nm for carbon dioxide and 1660-1672 nm for oxygen. One additional visible camera is installed before the spectrograph to image the observed scene.

The payload mass is 60 kg. The volume is 1200 x 700 x 450 mm³. The telescope and the spectrometer are all made of freeform optics. The design is presented in Figure 13. There are three freeform mirrors for the telescope and three freeform mirrors for the spectrometer. These optics have been fabricated by Safran Reosc, with a surface size that varies between 50 and 200 mm. The amplitude of the freeform surfaces is comprised between 100 and 1000 μm PTV. After manufacturing, the surface roughness is equal to 2 nm and the WFE is better than 20 nm RMS.

Figure 13.

MicroCarb spectrometer layout

The entrance sub-pupils are implemented to limit the field of each spectral band. In this optical system, the pupil separation prism (PSP) separates the entrance pupil into sub-pupil. Each sub-pupil will provide one beam for each spectral band. The field mask is implemented to eliminate the light from other sub-pupils.

The beam from the telescope is incident on the entrance slit and the Pupil Alignment Prisms (PAP). The PAP aims to re-centering the entrance sub-pupil for each spectral band. The beam incident on the folding mirror FM1 is reflected toward the M3sp, M2sp and M1sp respectively. The M1sp reflects the beam toward the Echelle grating in near-Littrow configuration. The Echelle grating disperses the beam toward the M1sp, M2sp and M3sp respectively. The M3sp reflects the light toward the folding mirror FM2 and the detectors.