chalcogenide custom glass developmentspie.org/documents/industry relations/ir standards... · 2012....
TRANSCRIPT
Business Unit/Department
Qualification of US-produced IG-series glasses
SCHOTT recently transferred the Vitron IG-series glasses to the US for production in
Duryea, PA.
• All properties for IG2-IG6 have been verified as being in-specification compared to Vitron.
• Physical and Thermal property measurements are straight-forward, refractive index is more
challenging.
• Refractive index guaranteed to
0.0005 RIU
• To date, all index measurements have been carried out by 3rd-party vendor M3MSI
• Current reproducibility of measurements seems to be
0.0002 RIU.
• dn/dT relative measurements carried out from -50 to 75
C in 5
C increments.
• SCHOTT is conducting per-piece inspection down to 0.1mm resolution in-house.
• IR interferometer under development for homogeneity/birefringeance measurements.
• Prism system for per-melt verification of index/dispersion online, and being upgraded to 0.0002 RIU
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Business Unit/Department
Reference mirrors (removable)
Pin hole
Beam splitter
(removable)
I
G Striae
Inclusions
Sub-surface
damage
3
IR Quality Inspection
Business Unit/Department
Slide 4
Refractive index measurments
• Index resolution
0.0002 RIU
• Absolute accuracy
0.0005 RIU
• Wavelength range: 405nm – 12.2 μm
4x Diode laser
405, 650, 785,1550 nm
Mirror
Beam Splitter (Uncoated Ge)
Polarizer (Glan-Taylor)
Beam Splitter (Pellicle)
Detector (Si/InGaAs)
Detector (HgCdTe)
IR Tunable laser
6-12 μm
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Business Unit/Department
Origin of the Sellmeier dispersion relation
5
0 2 4 6 8 10 12
2.76
2.78
2.80
2.82
2.84
2.86
2.88
2.90
2.92
2.94
Re
fra
ctiv
e I
nd
ex
Wavelength [ m]
Kramers-Heisenberg Q.M. Oscillator
-Formulated according to Drude model
Converted from frequency to wavelength
and constants collected
Refractive index of IG6 at 20
C
0.0002
Band-gap
50
Phonon frequency
• Incident light at a harmonic frequency is absorbed – results in an asymptote (pole) in refractive index
• Crystals are well-defined but complex, dispersion may be calculated from first principles
• Glasses have only short-range structure and not known a-priori – must be determined empirically.
• Minimum of 2 oscillators are needed:
• Activation of vibrations (phonon modes) by infrared radiation
• Promotion of electrons from conduction to valence band in UV-Vis (NIR).
Business Unit/Department
Residuals from refractive index data fitting.
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0 2 4 6 8 10 12-0.0012
-0.0010
-0.0008
-0.0006
-0.0004
-0.0002
0.0000
0.0002
0.0004
0.0006
Re
gu
lar
Re
sid
ua
l [R
IU]
Wavelength [ m]
0 2 4 6 8 10 12
-0.00004
-0.00003
-0.00002
-0.00001
0.00000
0.00001
0.00002
0.00003
0.00004
C2 = 0.3362
C2 = 0.5866
C2 = 0.8157
Re
gu
lar
resid
ua
l [R
IU]
Wavelength [ m]
Fitting residuals: 2-poles Fitting residuals: 3 poles
• Residuals too large 0.001
• Systematic trend shows poor reproduction of
dispersion shape.
Too few terms!
• Residuals below measurement uncertainty and
randomly distributed.
• C2 can be any value with equally low residuals
Equation over parameterized. Too many terms!
6-term equation usable but not optimal. How can we reconcile this?
Business Unit/Department
Refractive index has an extra component
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• Fitting residuals are as a low as for the 6-term version
• A unique value is now found for all terms.
• Each term has physical meaning, rather than being a simple fitting parameter
Philosophically satisfying result.
0 2 4 6 8 10 12
-0.00004
-0.00002
0.00000
0.00002
0.00004
0.00006
Re
gu
lar
Re
sid
ua
l [R
IU]
Wavelength [ m]
Index is derived from the real part of
the dielectric constant.
Dielectric constant exists even at ν = 0 (DC).
A fifth term must be added for
DC component (ε0)
We can use this form to look at systematic variations.
Business Unit/Department
Evolution of Thermo-optic coefficient with wavelength.
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0 2 4 6 8 10 12
30
40
50
60
70
80
Th
erm
o-o
ptic c
oe
ffic
ien
t [p
pm
/K]
Wavelength [ m]
dn/dT seems to follow expected trend.
Business Unit/Department 9
0 2 4 6 8 10 12
2.76
2.78
2.80
2.82
2.84
2.86
2.88
2.90
2.92
2.94
-50oC
-25oC
0oC
25oC
50oC
75oC
Re
fra
ctive
In
de
x
Wavelength ( m)
-60 -40 -20 0 20 40 60 803.94
3.96
3.98
4.00
4.02
4.04
4.06
4.08
A
B1
B2
Temperature
3.68
3.70
3.72
3.74
3.76
3.78
3.80
3.82
3.84
1.723
1.724
1.725
1.726
1.727
1.728
1.729
1.730
1.731
0 2 4 6 8 10 12
-0.00008
-0.00006
-0.00004
-0.00002
0.00000
0.00002
0.00004
0.00006
-50 oC
-25 oC
0 oC
25 oC
50 oC
75 oC
Re
gu
lar
Re
sid
ua
l [R
IU]
Wavelength [ m]
Variation of Sellmeier coefficients with temperature.
Effects of temperature on Sellmeier coefficients
are as predicted from dn/dT curve.
• A term has negative dependence, mostly due to
thermal expansion.
• Influence of B1 is larger and overcomes effect of
CTE on index.
Can also expect changes in dispersion.
• Impact of B2 is negligible.
Business Unit/Department
Change in dispersion with temperature (d2n/dλdT)
10
-60 -40 -20 0 20 40 60 80
-0.015
-0.010
-0.005
0.000
0.005
0.010
0.015
0.020
0.025
VSWIR
VMWIR
VLWIR
% C
ha
ng
e in
Ab
be
nu
mb
er
Temperature (oC)
-60 -40 -20 0 20 40 60 80
13.1
13.2
13.3
13.4
13.5
13.6
Ch
an
ge
in
Ab
be
nu
mb
er V
SWIR
VMWIR
VLWIR
Temperature (oC)
166
167
168
169
170
171
172
159.5
159.6
159.7
159.8
159.9
160.0
160.1
160.2
160.3
As expected from previous examples, dispersion varies with temperature.
• Difficult to compare in absolute terms due to different scales for various IR bands.
• When presented in % change (ie. Impact on chromaticity) trends become more clear.
10x Larger change in dispersion in SWIR/MWIR bands with little change in LWIR.
• Critical knowledge for designing athermal multi-spectral systems.
Business Unit/Department
A note on some new data
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-10 0 10 20 30 40 50 60 70 80
2.7760
2.7765
2.7770
2.7775
2.7780
2.7785
2.7790
2.7795
IG6 @ 10.6 m
Re
fra
ctive
In
de
x
Temperature (oC)
• dn/dT is clearly nonlinear, average value will depend on the temperature range used.
• Wavelength-dependence of the nonlinearity has not yet been determined.
• Importance of this effect is not yet known, but is expected to be most significant at extreme temperatures.
-10 0 10 20 30 40 50 60 70 80-0.0003
-0.0002
-0.0001
0.0000
0.0001
0.0002
0.0003
Lin
ea
r F
ittin
g r
esid
ua
lTemperature (
oC)
Business Unit/Department
Conclusions
US-produced chalcogenide glasses are now fully qualified.
Higher accuracy refractive index and thermo-optic measurements are critical for next-
generation multispectral and wide FOV imaging systems.
Systematic definition of the Sellmeier equation format from first principles allows the
systematic analysis of various influences on the glass (temperature, composition, etc.)
SCHOTT is committed to providing the highest quality materials available, and aims to be the
industry leader in metrology and quality control.
SCHOTT currently has conventional finishing, SPDT and coating capabilities online in the US.
New IR glass compositions are now under development.
Contact for all your IR materials and optics needs.
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