Transcript VOLUME-PHASE HOLOGRAPHIC GRATINGS FOR …

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.
VPH Grating Physics
(true holographic gratings)
• Diffraction due to modulations in refractive index (Dn) of the
grating material rather than by surface structure.
• Light is diffracted at angles according to the classical
diffraction equation:
mnl = sin(a) - sin(b)
• The energy distribution is governed by the Bragg condition:
e.g. for a Littrow grating configuration mnl = 2sin(a)
• Grating depth (d) and index modulation (Dn) define Bragg
performance.
Theoretical Diffraction Efficiency (h)
• Rigorous Coupled Wave
Analysis (RCWA) is typically
required to model the effect
of d and Dn on the grating
efficiency (h).
• Approximate theories give
closed form solutions: e.g. in
transmission gratings 2
Dnd 
hs  sin 

 l cosag  
hp  hs cosag  bg 
Where ag and bg are in the grating
volume.
RCWA efficiency for 3 transmission gratings in 1st order.
Bragg Envelopes
1.0
1.0
Bragg Angle for 500 nm = 17.5 degrees
0.8
0.8
HG-T-532-19
1200 l/mm
l = 500 nm
m=1
0.6
EFFICIENCY (h)
EFFICIENCY (h)
HG-T-532-19
17 degree angle of incidence
1200 l/mm
m=1
0.4
0.6
0.4
0.2
0.2
0.0
0
5
10
15
20
25
30
35
INCIDENT ANGLE (DEGREES)
Angular Bragg envelope.
DaFWHM ~ L/d
40
0.0
300
400
500
600
700
800
900
1000
1100
WAVELENGTH (nm)
Spectral Bragg envelope.
DlFWHM / l ~ L/d cot(ag)
VP Grating Configurations
A. Littrow transmission configuration.
B. Non-Littrow transmission configuration.
C. Non-dispersive reflection (notch filter). D. Reflection grating configuration.
VP Grating Structures
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•
•
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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.
Typical VP Grating Parameters
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•
•
•
•
Line density:
Index modulation (Dn):
Ave. index (n):
Grating depth (d):
Wavelength range:
300 to 6000 l/mm
0.02 to 0.10
1.5
4 to 30 mm
0.4 to 1.5 mm
– may be viable from 0.3 to 2.8 mm.
• Grating size:
75 by 100 mm
– limited by holographic exposure system
– expandable to 500 by 700 mm
Dichromated Gelatin
Transmittance of
dichromated gelatin as a
function of wavelength for a 15
mm thick layer which has been
uniformly exposed and
processed.
Good transmittance covers
the range from 0.3 to 2.8 mm.
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 Ha/Hb
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
Gratings fabricated and evaluated.  Grating not yet fabricated.
 Gratings fabricated.
Some gratings will be distributed to US community at the end of this study.
1.0
300 l/mm VPH grating
a = 6 degrees
m=1
0.9
0.8
300 l/mm VPH Grating
m=2
0.7
0.6
0.5
0.4
0.3
0.2
Measured absolute efficiency
in unpolarized light for the 300
l/mm VPH grating.
0.1
0.0
1.0
a = 9 degrees
0.9
EFFICIENCY
0.8
m=2
0.7
m=1
m=3
0.6
0.5
0.4
0.3
0.2
Note the tunability of this
grating which shifts the blaze
function to higher orders of
diffraction as the grating angle
is increased.
0.1
0.0
1.0
a = 12 degrees
0.9
0.8
0.7
m=2
m=3
0.6
m=1
m=4
0.5
0.4
0.3
0.2
0.1
0.0
300
400
500
600
700
800
WAVELENGTH (nm)
900
1000
1100
HG-T-532-19
1200 l/mm VPH Grating
Absolute Efficiency Measured
1.0
m = 1 at 19 degrees
Super Blaze
Surface Grating Model
EFFICIENCY (h)
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
900
1000
WAVELENGTH (nm)
1200 l/mm VPH Grating
1100
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.
2400 l/mm 532 nm VPH Grating
HG-T-532-40 2400 l/mm, 532 nm
Blaze Profiles
1.0
a = 33
o
0.9
a = 37o
EFFICIENCY (h)
0.8
Super Blaze
0.7
0.6
0.5
0.4
a = 27o
a = 46o
0.3
0.2
0.1
0.0
350
400
450
500
550
600
WAVELENGTH (nm)
650
700
750
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.
Ha/Hb Multiplex Grating
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 Ha light while the 1620 l/mm grating
component diffracts Hb light to the same angle of diffraction.
Ha/Hb Multiplex Grating
Measured Efficiency for 1200 l/mm Component
1.0
Ha/Hb Multiplex Grating
Measured Efficiency for 1620 l/mm Component
1.0
Measured Super-Blaze
0.9
0.9
RCWA Predicted
Super-Blaze
0.7
0.6
a = 23o
0.5
a = 33o
0.4
0.3
RCWA Predicted Super-Blaze
0.8
EFFICIENCY (h)
EFFICIENCY (h)
0.8
0.7
0.6
0.5
a = 17o
a = 23o
Measured Super-Blaze
0.4
a = 33o
0.3
a = 17o
0.2
0.2
0.1
0.1
0.0
0.0
300
400
500
600
700
800
WAVELENGTH (nm)
900
1000
1100
300
400
500
600
700
800
900
1000
1100
WAVELENGTH (nm)
Measured Efficiency for the 1200/1620 l/mm multiplex grating at grating angles of 17, 23,
and 33 degrees.
Ha Channel Spectrum
Ha
14000
12000
Night Sky Emission
10000
SIGNAL
Ha
8000
6000
4000
2000
[SII]
0
650
660
670
680
690
700
WAVELENGTH (nm)
[OIII]
Hb Channel Spectrum
Hb
[OIII]
SIGNAL
20000
15000
10000
[OIII]
Hb
5000
0
480
490
500
510
WAVELENGTH (nm)
Spectrum of 18th magnitude, blue compact galaxy obtained with the
Ha/Hb multiplex grating with a fiber feed on the 2.1-meter telescope
at Kitt Peak National Observatory.
520
RCWA Predicted Efficiency for VPH
Reflection Grating
Predicted Efficiency for 1200 l/mm VPH Reflection Grating
100
EFFICIENCY (%)
80
60
40
20
0
400
420
440
460
480
500
520
WAVELENGTH (nm)
540
560
580
600
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.
1200 l/mm VPH Reflection Grating
HG-R-532-4/34 1200 l/mm 532 nm
Reflection Grating
PERCENT TRANSMITTED
100
80
60
40
20
0
400
450
500
WAVELENGTH (nm)
550
600
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.
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
NOAO’s 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 AST9613615 awarded by the National Science Foundation.