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)
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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F location represents the locations of the freeform surfaces in each system, in which M is used to represent mirror. Keystone represents the keystone distortion, Smile represents the smile distortion, F represents the number of freeform surfaces, and A represents the number of aspheres. The last column gives recommendations of the systems based on the targeted application and the system performances. “0” means not recommended, “+” means somewhat recommended, and “++” means recommended.
Table 3.
Typical Tolerances According to the Usual Manufacturing Conditiona
Mirror
Surface form error
RMS 18 nm/PV 140 nm
Decenter
10 μm
Tilt
0.008°
Grating
Period
1 nm
Substrate surface irregularity
PV 250 nm*
The substrate surface irregularity is measured on a 6 in. (15.24 cm) mask blank.
Table 4.
Curved Mirror Tolerances of the Double-Pass TMA System After Coupling M1 and M3a
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
P2 represents picking up from M2, which means all position tolerances of mirror 3 are the same as mirror 2.
F location represents the locations of the freeform surfaces in each system, in which M is used to represent mirror. Keystone represents the keystone distortion, Smile represents the smile distortion, F represents the number of freeform surfaces, and A represents the number of aspheres. The last column gives recommendations of the systems based on the targeted application and the system performances. “0” means not recommended, “+” means somewhat recommended, and “++” means recommended.
Table 3.
Typical Tolerances According to the Usual Manufacturing Conditiona
Mirror
Surface form error
RMS 18 nm/PV 140 nm
Decenter
10 μm
Tilt
0.008°
Grating
Period
1 nm
Substrate surface irregularity
PV 250 nm*
The substrate surface irregularity is measured on a 6 in. (15.24 cm) mask blank.
Table 4.
Curved Mirror Tolerances of the Double-Pass TMA System After Coupling M1 and M3a
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
P2 represents picking up from M2, which means all position tolerances of mirror 3 are the same as mirror 2.