VOLUME-PHASE HOLOGRAPHIC GRATINGS FOR ASTRONOMICAL SPECTROGRAPHS James A. Arns, Willis S. Colburn, &...
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VOLUME-PHASE HOLOGRAPHIC GRATINGS FOR ASTRONOMICAL SPECTROGRAPHS James A. Arns, Willis S. Colburn, & Mark Benson (Kaiser Optical Systems, Inc.) Samuel C. Barden & Joel B. Williams (National Optical Astronomy Observatories*) by the Association of Universities for Research in Astronomy, Inc. ( agreement with the National Science Foundation.
VOLUME-PHASE HOLOGRAPHIC GRATINGS FOR ASTRONOMICAL SPECTROGRAPHS James A. Arns, Willis S. Colburn, & Mark Benson (Kaiser Optical Systems, Inc.) Samuel
VOLUME-PHASE HOLOGRAPHIC GRATINGS FOR ASTRONOMICAL
SPECTROGRAPHS James A. Arns, Willis S. Colburn, & Mark Benson
(Kaiser Optical Systems, Inc.) Samuel C. Barden & Joel B.
Williams (National Optical Astronomy Observatories*) * Operated by
the Association of Universities for Research in Astronomy, Inc.
(AURA) under cooperative agreement with the National Science
Foundation.
Slide 2
VPH Grating Physics (true holographic gratings) Diffraction due
to modulations in refractive index ( n ) of the grating material
rather than by surface structure. Light is diffracted at angles
according to the classical diffraction equation: m = sin( ) sin( )
The energy distribution is governed by the Bragg condition: e.g.
for a Littrow grating configuration - m = 2sin( ) Grating depth ( d
) and index modulation ( n ) define Bragg performance.
Slide 3
Theoretical Diffraction Efficiency ( ) Rigorous Coupled Wave
Analysis (RCWA) is typically required to model the effect of d and
n on the grating efficiency ( ) Approximate theories give closed
form solutions: e.g. in transmission gratings - RCWA efficiency for
3 transmis- sion gratings in 1st order. Where g and g are in the
grating volume.
Slide 4
Spectral Bragg envelope. FWHM / d cot ( g ) Bragg Envelopes
Angular Bragg envelope. FWHM d
Slide 5
VP Grating Configurations A. Littrow transmission
configuration. B. Non-Littrow transmission configuration. C.
Non-dispersive reflection (notch filter). D. Reflection grating
configuration.
Slide 6
VP Grating Structures Grating material: Dichromated Gelatin
Grating substrates: BK7, Fused Silica, etc. Anti-reflective
coatings on substrate-air surfaces. Encapsulated nature protects
gelatin from environment over a range in temperature and humidity.
Encapsulated nature allows surfaces to be cleaned without risk to
grating. Lifetimes of at least 20 years if properly handled.
Slide 7
Typical VP Grating Parameters Line density: 300 to 6000 l/mm
Index modulation ( n ): 0.02 to 0.10 Ave. index ( n ): 1.5 Grating
depth ( d ): 4 to 30 m Wavelength range: 0.4 to 1.5 m may be viable
from 0.3 to 2.8 m. Grating size: 75 by 100 mm limited by
holographic exposure system expandable to 500 by 700 mm
Slide 8
Dichromated Gelatin Transmittance of dichromated gelatin as a
function of wavelength for a 15 m thick layer which has been
uniformly exposed and processed. Good transmittance covers the
range from 0.3 to 2.8 m.
Slide 9
NSF Funded Study of VPH Gratings Eight gratings are being
fabricated and evaluated. 300 l/mm at 1064 nm 1200 l/mm at 532 nm
2400 l/mm at 532 nm 1200/1620 l/mm H /H multiplex grating 1200 l/mm
reflection grating 2400 l/mm at 1064 nm with prism substrates 4800
l/mm at 532 nm with prism substrates 300 l/mm 10th order at 532 nm
grating attempt Some gratings will be distributed to US community
at the end of this study. Gratings fabricated and evaluated.
Gratings fabricated. Grating not yet fabricated.
Slide 10
300 l/mm VPH Grating Measured absolute efficiency in
unpolarized light for the 300 l/mm VPH grating. Note the tunability
of this grating which shifts the blaze function to higher orders of
diffraction as the grating angle is increased.
Slide 11
1200 l/mm VPH Grating Measured efficiency in unpolarized light
for the 1200 l/mm VPH grating. The solid line shows the efficiency
at a grating angle of 19 deg. The dashed line shows the peak
efficiency curve as the grating is tuned to different grating
angles (the super blaze). The dotted line shows the efficiency for
a theoretical 1200 l/mm surface-relief grating with similar blaze
characteristics.
Slide 12
2400 l/mm 532 nm VPH Grating Measured efficiency in unpolarized
light for the 2400 l/mm VPH grating (optimized for 532 nm) at
grating angles of 27, 33, 37, and 46 degrees. The super blaze shows
the envelope of peak efficiency as the grating is tuned to
different grating angles.
Slide 13
H /H Multiplex Grating Measured Efficiency for the 1200/1620
l/mm multiplex grating at grating angles of 17, 23, and 33 degrees.
A multiplex grating contains two gratings within one unit. The
second grating operates near the minimum of the Bragg spectral
bandwidth of the first grating. In this case, the 1200 l/mm grating
diffracts H light while the 1620 l/mm grating component diffracts H
light to the same angle of diffraction.
Slide 14
Spectrum of 18th magnitude, blue compact galaxy obtained with
the H /H multiplex grating with a fiber feed on the 2.1-meter
telescope at Kitt Peak National Observatory.
Slide 15
RCWA Predicted Efficiency for VPH Reflection Grating This is
the RCWA predicted efficiency for the 1200 l/mm VPH reflection
grating. The bandwidths for reflection gratings are quite narrow.
Wider bandwidths are possible by warping the fringe structure
through processing techniques.
Slide 16
1200 l/mm VPH Reflection Grating The diffraction efficiency of
the 1200 l/mm reflection grating has not yet been directly
measured. This plot shows the measured transmissivity of the
grating at a tilt angle of 4 degrees. Except for absorption losses,
the transmittance should be equal to the inverse of the diffraction
efficiency.
Slide 17
Conclusions and Future Effort VPH grating technology will allow
the fabrication of novel new instruments for astronomical
spectroscopy. Several such instruments are already in the works at
such facilities as the Anglo-Australian Observatory, NOAO (see
related poster on NOAOs spectrograph for the 4-meter telescopes),
SOAR, and ESO. Future efforts will entail upgrading facilities to
produce gratings as large as 200 to 300 mm (and possibly larger)
and to continue to explore the unique capabilities of this
technology. This project is supported under Cooperative Agreement
AST- 9613615 awarded by the National Science Foundation.