Download Advances in diamond-turned surfaces enable unique cost

Advances in Diamond Turned Surfaces Enable Unique Cost Effective
Optical System Solutions
Joshua M. Cobba, Lovell E. Comstockb, Paul G. Dewaa, Mike M. Dunna, Scott D. Flinta
a
Corning Tropel, 60 O’Connor Rd, Fairport NY 14450-1376
b
Corning NetOptix, 69 Island St Keene NH 03431-3529
ABSTRACT
Corning has developed a number of manufacturing and test techniques to meet the challenging requirements of
imaging hyperspectral optical systems. These processes have been developed for applications in the short-wave visible
through long-wave IR wavelengths. Optical designs for these imaging systems are typically Offner or Dyson
configurations, where the critical optical components are powered gratings and slits. Precision alignment, system
athermalization, and harsh environmental requirements, for these systems drive system level performance and production
viability.
This paper will present the results of these techniques including all aluminum gratings and slits, innovative
grating profiles, snap together self-aligning mechanical designs, and visible test techniques for IR systems.
Keywords:
Hyperspectral imaging, Offner spectrometer, Diamond turning, Diffraction grating
INTRODUCTION
Imaging Spectroscopy or Hyperspectral Imaging has become a powerful technique for many fields such as
remote sensing. Early spectrometers such as the Multispectral Scanner flown on the Landsat series of satellites had
limited spectral and spatial resolution; 6 wavelength and 4 spatial bands1. More recent designs take advantage of high
resolution two dimensional detector arrays and diffractive element in the entrance pupil of the imaging system2345.
Future possibilities include compact, lightweight imaging spectrometers that perform over a range of environmental
conditions.
Corning has developed many useful manufacturing and test processes that enable future possibilities today.
These processes include novel grating fabrication methods and diamond machining alignment datum into the substrate of
the optical surface. These fabrication techniques allow “snap-together” systems that maintain precision alignment of the
optical system without requiring tedious adjustments. The materials compatible with the diamond machining process
allow the system to maintain alignment even over temperature extremes. Finally, Corning’s experience with precision
optical system metrology allows testing at the system level to validate the alignment and the performance of the optics.
This paper will present analysis of Offner imaging spectrometer design6, grating efficiency data, alignment
data, and visible wavelength testing of IR systems.
OFFNER SPECTROMETER DESIGN
The monocentric Offner design form offers some unique advantages in spectrometer design. Initially designed
as a microlithography relay optic, this elegant form eliminates Seidal aberrations by using three mirrored surfaces that
are concentric. This configuration will relay an arc object to an arc image at 1X magnification where both arcs are
Ground-based and Airborne Instrumentation for Astronomy, edited by Ian S. McLean, Masanori Iye,
Proc. of SPIE Vol. 6269, 62691L, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.672207
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concentric with the axis of the optical system. Figure 1 illustrates the conceptual layout with the object being a slit at the
upper left and the image being shown in the lower left of the figure 1. In this case, the primary mirror and the tertiary
mirror are different surfaces with different radii. In many cases, the primary and tertiary can be part of the same curved
surface.
Primary
Slit
Secondary
CCD
Tertiary
Figure 1Offner imaging relay
When this form is used in the design of a spectrometer, the diffraction grating is placed on the curved secondary mirror.
This then spreads the spectrum of the slit onto a CCD camera. This is shown in Figure 2.
The monocentric symmetry is broken when
imaging a straight slit instead of a curved slit, and
the design of the mirror curvatures must be
compromised for different image heights due to the
dispersion of the diffraction grating. Consideration
must be made for the amount of spatial distortion
and the amount of spectral distortion in the image.
The spatial distortion manifests itself in two forms:
smile distortion and keystone distortion.
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Figure 2 Offner spectrometer dispersion
Smile distortion is a measure of the bow in the
image of the line slit while keystone distortion is a
measure of the change in the length of the image of
the line slit, both as a function of wavelength.
Figure 3 shows an example of how smile distortion
varies across a wavelength band, and Figure 4
shows an example of the variation of keystone
distortion.
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Smile Distortion
Keystone Distortion
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W avelength (microns)
W avelength (microns)
Figure 3 “Smile” distortion of Offner spectrometer Figure 4 Keystone distortion of Offner Spectrometer
Spectral distortion shows the variation in dispersion on the CCD plane as a function of wavelength. This is an important
parameter because the calibrated height of the image at the CCD is related to the wavelength, and any spectral distortion
will make the conversion from CCD position to wavelength non-linear.
The optical system is also telecentric at the slit and at the CCD plane. Thus, the objective lens that forms the image on
the slit must also be telecentric to match the pupil locations. It is also advantageous to have excellent color correction in
the objective lens as the spectrometer is designed to work over a broad spectral range.
Typical tolerances
The positioning tolerances of the mirrors in an Offner spectrometer can be fairly tight. Typically, the position between
the primary and secondary and between the secondary and tertiary should be held within about 10 microns or less.
Diamond turning the mirrors and mechanics has an advantage of being able to hold very tight positioning tolerances as
well as making the optical system insensitive to thermal variations.
DIAMOND TURNING EXAMPLES
A typical example of a common diamond machined optical component would be a reflective off-axis parabola,
machined into an aluminum substrate, shown in figure 5. This monolithic component enables the mechanical designer to
incorporate mounting features that will ensure precision alignment into a diamond machined housing of the same
material, assuring system level thermal stability7.
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Figure 5
200 mm Aperture Off-axis Parabola
The term “Snap-Together” construction has been coined to describe such techniques. One can imagine the
further downstream advantages with respect to assembly and test, as well as cost, which “snap-together” construction
offers. Examples are shown in figure 6.
Figure 6
Offner-like “Snap Together” Reflective Assembly
Recent advancements in multiple axis diamond machining equipment combined with proprietary techniques
and materials permit the direct machining of visible quality powered blazed gratings. These new processes allow great
flexibility in the grating design including variable blaze angle, variable period, and aspheric toroidal base curves to name
a few. An example is shown in figure 7. Measurements of a sample grating are shown in figure 8 and 9.
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Figure 7 Diamond machined diffraction grating
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Figure 8 Profile and roughness measurement of diamond machined grating
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Figure 9 Finish of blazed surface of grating
Along with the grating surface, Corning has also developed a manufacturing process for directly diamond
machining precision slits which serve as the spatial resolution element of Offner spectrometers. True knife edge slits
have been produced with widths as small as 5 microns. The technique also allows for precision mounting features
significantly simplifying the alignment process. An example of a diamond machined precision slit is shown in figure 10.
Figure 10
400X Image of 10um Slit
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These techniques permit the production of imaging spectrometers that incorporate aluminum based optical
components, including mirrors, gratings and slits, mounted into precision machined aluminum housing. The result is a
lightweight, mechanically robust, and thermally stable system. Figure 11 shows the measured wavefront of a complete
assembled Offner spectrometer. The system was measured on a HeNe interferometer operating in a double pass
configuration. The design residual was 0.25RMS (1.0 PV) waves, and the assembled Offner achieves 0.24 RMS (1.3
PV) waves.
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Figure11 Wavefront of Offner spectrometer
GRATING EFFICIENCY
The diffraction efficiency was calculated and measured for a blazed diamond machined diffraction grating. The
pitch was 25µm and the blaze angle was 0.69 degrees. This diffracted most of the light into the first diffraction order.
The grating efficiency was measured at 532nm and at 650nm. Figure 12 shows graphically the calculated and measured
efficiency.
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0.6
0.5
Efficiency
0.4
0.3
0.2
Reflectivity (TE) Calculated
Reflectivity (TM) Calculated
Efficiency, -1 order (TE) TEST
Efficiency, -1 order (TM) TEST
0.1
Efficiency, -1 order (TE) Calculated
Efficiency, -1 order (TM) Calculated
Nickel Grating, 25um period, 0.687deg blaze
0
400
500
600
700
Wavelength, nm
800
900
1000
Figure 12 Calculated and measured diffraction efficiency
CONCLUSION
Corning has developed unique fabrication processes to allow precision fabrication of critical
components, namely diffraction gratings and object slits, for Offner hyperspectral imagers. Gratings
with variable periods and blaze angles can be manufactured directly on flat, spherical, aspherical,
and even toroidal substrates offering unique design flexibility while simultaneously achieving high
diffraction efficiency. Precision slit assembles limiting spatial resolution of the spectrometer can be
machined with feature sizes as small as 5 microns. These components coupled with precision
housing and diamond-machined mirrors with reference datums yield “snap together” alignment
significantly reducing complexity and assembly time. Furthermore, as all components of the
spectrometer can be fabricated from the same material, the resulting hyperspectral imaging system is
thermally stable allowing high performance operation even in extreme thermal environments.
1
K. Lewotsky, “Hyperspectral Imaging: Evolution of Imaging Spectroscopy” OE Reports (1994)
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4
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5
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6
A. Offner, “unit power imaging catadioptric anastigmat” U.S. patent 3,748,015 (1973)
7
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