Comparison of hyperspectral imaging spectrometer designs and the improvement of system performance with freeform surfaces

Chang Liu; Christoph Straif; Thomas Flügel-Paul; Uwe D. Zeitner; Herbert Gross

doi:10.1364/AO.56.006894

Applied Optics
Vol. 56,
Issue 24,
pp. 6894-6901
(2017)
•https://doi.org/10.1364/AO.56.006894

Comparison of hyperspectral imaging spectrometer designs and the improvement of system performance with freeform surfaces

Chang Liu, Christoph Straif, Thomas Flügel-Paul, Uwe D. Zeitner, and Herbert Gross

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Chang Liu, Christoph Straif, Thomas Flügel-Paul, Uwe D. Zeitner, and Herbert Gross, "Comparison of hyperspectral imaging spectrometer designs and the improvement of system performance with freeform surfaces," Appl. Opt. 56, 6894-6901 (2017)

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Abstract

Hyperspectral-grating-based imaging spectrometer systems with F/3 and covering the visual–near-infrared (420–1000 nm) spectral range are investigated for monitoring Earth’s environmental changes. The systems have an entrance slit of 24 μm and a 6.5 nm spectral resolution. Both smile and keystone distortions are smaller than 20% of the pixel pitch. We benefit from the development in freeform technology and design 15 different systems with the help of off-axis aspheric and freeform surfaces. The potential of each system is explored with the help of nonspherical surfaces. Cross comparisons between different system types are summarized to give their advantages and disadvantages. In the end, detailed tolerancing of one selected system is presented to show the feasibility for fabrication.

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	VNIR
Wavelength (nm)	420–1000
F number	3
X field in height (mm)	$\pm 12$
Spectral resolution (nm)	6.5
Smile/keystone distortion	$< 20 %$ of a pixel pitch
Pixel geometry (μm)	$24 \times 24$
MTF	$> 25 %$ at 20.8 cyc/mm across track
	$> 16 %$ at 20.8 cyc/mm along track
Grating efficiency	$\geq 70 %$

System	Freeform Location	Keystone	Smile	System Size ( ${mm}^{3}$ ) ( $x \cdot y \cdot z$ )	F	A	Grating Type	Angle (°)	Recomm.
Double TMA	M4, M5, M6	$< 25 %$	$< 5 %$	$130 \times 200 \times 300$	3	3	plane, reflection	14.11	0
Double TMA_TG	M4, M5, M6	$< 8 %$	$< 7 %$	$110 \times 220 \times 210$	3	3	plane, transmission	0	0
TMA+Schwarzschild	M4, M5	$< 17 %$	$< 10 %$	$100 \times 250 \times 400$	2	3	plane, transmission	0.43	0
TMA 1	—	$< 30 %$	$< 40 %$	$110 \times 190 \times 160$	0	3	plane, reflection	0	0
TMA 2	M2	$< 5 %$	$< 12 %$	$120 \times 190 \times 240$	1	2	plane, reflection	0	++
TMA 3	M1, M3	$< 5 %$	$< 20 %$	$80 \times 110 \times 150$	2	1	plane, reflection	0	+
TMA 4	M2, M3	$< 14 %$	$< 15 %$	$65 \times 85 \times 80$	2	1	plane, reflection	0	+
-+-TMA	—	$< 8 %$	$< 16 %$	$75 \times 200 \times 90$	0	3	plane, reflection	0	++
-+-TMA (integrated)	M3	$< 1.5 %$	$< 5 %$	$75 \times 160 \times 210$	1	2	plane, reflection	0	+
Folded TMA	M1, M2, M3	$< 8 %$	$< 7 %$	$40 \times 150 \times 210$	3	0	plane, reflection	0	0
Offner	—	$< 3 %$	$< 0.9 %$	$50 \times 110 \times 110$	0	2	convex, reflection	25.98	++
Schwarzschild	—	$< 0.15 %$	$< 0.25 %$	$50 \times 160 \times 140$	0	4	plane, reflection	17.73	++
Schwarzschild_TG	—	$< 4 %$	$< 8 %$	$50 \times 180 \times 120$	0	4	plane, transmission	0	+
Dyson 1	—	5%	8%	$35 \times 45 \times 110$	0	0	plane, transmission	20.18°	0
Dyson 2	lens rear surface	15%	5%	$25 \times 45 \times 110$	1	0	plane, transmission	20°	+

Mirror	Surface form error	RMS 18 nm/PV 140 nm
	Decenter	10 μm
	Tilt	0.008°
Grating	Period	1 nm
Grating	Substrate surface irregularity	PV 250 nm*

	M1		M2		M3
Tilt around x axis (°)	−0.008	0.008	−0.01	0.01	P2	P2
Tilt around y axis (°)	−0.008	0.008	−0.01	0.01	P2	P2
Tilt around z axis (°)	−10	10	−0.1	0.1	P2	P2
Decenter in x axis (mm)	−0.012	0.012	−0.015	0.015	P2	P2
Decenter in y axis (mm)	−0.012	0.012	−0.015	0.015	P2	P2
Decenter in z axis (mm)	−0.01	0.01	−0.012	0.012	P2	P2
Surface irregularity (mm)	−0.0001	0.0001	−0.0026	0.0026	−0.0001	0.0001
Radius of curvature (fringe)	−1	1	−1	1	−2	2

Grating Tolerances
Element tilt around x	−0.04	0.04
Element tilt around y	−0.04	0.04
Element decenter in x	−0.03	0.03
Element decenter in y	−0.03	0.03
Element decenter in z	−0.2	0.2
Surface tilt around x	−0.03	0.03
Surface tilt around y	−0.01	0.01
Surface decenter in x	−0.03	0.03
Surface decenter in y	−0.03	0.03
Surface decenter in z	−0.2	0.2
Substrate radius	−3	3
Substrate irregularity	−0.6	0.6
Grating radius	−3	3
Grating period	−0.016	0.016

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Applied Optics