Next Generation Multi-Spectral Materials SCHOTT Advanced Optics

Next generation multi-spectral materials SCHOTT Advanced Optics

Qualification of US Produced IRG Series Glass

• All properties for IRG22 through • All index measurements have been IRG26 have been verified to published correlated with independent source specifications or better o Reproducibility of measurements • Standard physical and thermal to 0.0002 RIU property metrology o dn/dT relative measurements from -50 C T 75 C in 5 C • SCHOTT refractive index guaranteed ⁰ ≤ ≥ ⁰ ⁰ to +/-0.001 (405nm to 12.2 µm) increments

• SCHOTT provides individual sample inclusion inspection at 0.1mm resolution (in house) • IR interferometer under development for homogeneity and birefringence metrology • Prism system for melt verification of index / dispersion; 0.0002 resolution

© SCHOTT North America, Inc. SCHOTT Advanced Optics 3 Traditionally, optical designers use multiple lens materials to compensate for optical aberrations. • Spherical curves create spherical aberration. • A positive spherical aberration may be balanced by a negative spherical aberration on another lens surface. • Same effect is true for coma, astigmatism, etc. • Each “free” surface allows the correction of one aberration. • Chromatic aberration leads to defocus of shorter or longer wavelengths. • Is a function of the refractive index dispersion of the optical material.

• The sum of the products of lens powers (P) and dispersions (ν) must be zero. ∑ ν = 0 • Requires positive and negative elements of different refractive index and dispersion. • Thermo-optic effect causes defocus with temperature change (particularly for IR). • Similar techniques to correct chromatic effects used to balance thermal effects but with CTE and dn/dT instead of n and ν.

Origin of the SCHOTT glass catalog.

0.90 0.70 CdSe 0.85 Si Ge ZnSe KRS-5 IRG26 0.65 IRG23 ZnS GaP IRG24 0.80 CsBr CsI GaAs AgCl KRS-5 KBr TiO2 PbF 0.60 IRG22 CdSe 0.75 2 CsI Y O IRG25 2 3 0.55

R GaP I BaF R ZnSe 0.70 2 I CsBr W W S M P P AgCl 0.65 0.50 KBr PbF ZnS 2 CaF2 CaF Spinel BaF 2 0.60 MgO 0.45 2 Y2O3 Al O MgO 0.55 2 3 BeO 0.40 Spinel Al2O3 TiO 2BeO 0.50 0.35 80 70 60 50 40 30 20 10 0 500 400 300 200 100 0 V SWIR VMWIR 0.65

Ge Dramatic changes to Abbe diagram depending on which 0.60 band is examined

0.55  -250.0 R I -200.0 Typically, crown-type materials in LWIR become flints W L -150.0 CdSe AgCl P 0.50 GaAs/IRG25 ZnSe -100.0 in SWIR and vice-versa. IRG26 -50.00 KRS-5 KBr 0.000 IRG23 CsI PbF2 0.45 50.00 IRG24 BaF How can we choose pairs/triplets for systems covering 100.0 CsBr 2 IRG22 ZnS 150.0 GaP CaF2 0.40 multiple bands simultaneously? 800 700 600 500 400 300 200 100 0

VLWIR SCHOTT Advanced Optics 5 Instantaneous definitions from the Sellmeier

2 2 2 2 B1 B2 B3 n 1  2  2  2   C1   C2   C3

1000 2 GaAs 1.8 IRG24 1.6 KRS-5 1.4 ]

1 ZnSe - 1.2 100 m As2S3 u [ 1 CsBr r n e o i b

s BaF2 r

m 0.8 e u IRG11 p n

i IRG2 e 10 D b

0.6 l b a A i t r a P 0.4 1

1 10 1 10 Wavelength [m] Wavelength [m] 1 1 () 1 (λ) − 1 ′  = −  = − Instantaneous 2 ν() Instantaneous 2 4 ν() 2 2 Abbe Number Partial Dispersion d n BiCi Ci  3  dn 1  BiCi       n 2  2 3  2 d i   C d i   Ci   

Materials which share similar minimum dispersion wavelengths also share similar partial dispersion at all wavelengths – ideal from achromats. SCHOTT Advanced Optics 6 Redefining Abbe number for IR

10 9 -250.0 IRG24 IRG23 -225.0 8 -200.0 IRG25 IRG26 -175.0 7 -150.0 GaAs -125.0 KRS-5 h Several advantages to using t 6 -100.0 CdSe g IRG22 CsI -75.00 n instantaneous vs. standard Abbe.

e -50.00

l 5 -25.00 GaP e ZnSe CsBr • Trends between composition and

v 0.000 a 4 25.00 AgCl optical properties become 50.00 KBr W

CaLa S 75.00 2 4 ZnS

n apparent. 100.0 2 N-Type “Normal” -30.00 o i 125.0 -25.00

s 3 1.9 La-flints Glasses -20.00 • Fundamental to all materials rather

r 150.0 -15.00 e 1.8 -10.00 than varying with wavelength. p PbF -5.000 s Rutile (o) 2 i 1.7 Y O 0.000 • Any material may be included in D 2 3 5.000

2 IRG2 1.6 10.00 m BaF FK/PK the diagram. IRG11 2 u ZBLAN 1.5

m • Accentuates similarities between

i Oxide

n CaF 2 1.4 i Glass MgO materials rather than differences

M KZFS MgF MgAl O 2 1.3 Ba-flints / SSK 2 4 LiF Al O (o) 2 3 1 10 15 20 25 10 100 Instantaneous Abbe Number

• Comparing Oxide glasses to IR materials shows lack of available selection and very large range of properties (Particularly thermo-optical). • Significant improvement over using many simultaneous Abbe diagrams for multi-band designs. • All needed information is inside the Sellmeier equation:

IR Tunable laser Mirror Prism direct measurement 7-12 μm

IR Tunable laser 3-5 μm

Beam Splitter Detector (HgCdTe) Polarizer (Glan-Taylor)

ZnS Beam combiner Vis-NIR fiber lasers 405, 650, 785,1550 nm Detector (Si/InGaA(Si/InGaAs)s)

~10-30° wedge prism preferred (angle need not be precise). • 4 reflections measured: 2x first surface, 2x refracted second surface  1x prism angle + 2x index meas. • Equivalent to minimum deviation measurement (inherently symmetrical) , but with only 1 moving part. • Same sample as used for 3rd party minimum deviation measurement (M3MSI) – useful for validation. Environmental chamber for sample stabilization and dn/dT measurement: • Samples should be held at temp for ~5 minutes to stabilize. Limited to ±0.0002 by rotary stage repeatability • Sufficient for QA and optical designs, but non-ideal for dn/dT measurements Improved wavelength accuracy needed for Vis-NIR wavelengths. • 4 reflections detected: 2x first surface, 2x refracted second surface  1x prism angle + 2x index meas. • Equivalent to minimum deviation measurement, but with only 1 moving part. • Same sample as used for 3rd party minimum deviation measurement – useful for validation. SCHOTT Advanced Optics 8 Index measurements and fitting

2.6 Residuals from full fit ZnS Full data set 150 Residuals from reduced fit Reduced data set 2.5 100 ] m x p e p

d 2.4 [

n 50 I s l

a e u v i t d i c

2.3 s

a e

r 0 f R e

g R n

i t

2.2 t i -50 F

2.1 -100 0 2 4 6 8 10 12 14 0 2 4 6 8 10 12 14 Wavelength [m] Wavelength [m]

B1 B2 B3 C1 C1 C3 2 2 2 B1 B2 B3 2.07621 0.18252 0.53704 0.00915 0.07742 1022.3 n  2  2  2   C   C   C B1 B2 B3 C1 C1 C3 1 2 3 2.06227 0.19652 0.53667 0.00879 0.07583 1021.0 • Used minimum data to specify terms (6 points) and no reduction of quality of fit. • Large numbers of wavelengths improves immunity to errors, but are not required. • Placement of wavelengths is important: 2 per term pair, don’t extrapolate beyond available data.

 λ = 1-1.5x absorption edge (C1), 3-6x absorption edge (C2), 0.5-1x IR multi-phonon cut-off (C3). • Works out to about even spacing in terms of index (~0.8RIU for ZnS). • Wavelengths should be chosen based on material properties, not absolute. • Standardization is needed for useful comparisons of reference data. SCHOTT Advanced Optics 9 Index measurements continued

Ge )

Si 1

10 - GaAs

GaP m 200 -1  CdSe (

1cm wavelength error ZnS ) n

ZnSe 180 o

1 i m ]

1 IG6 s p - IG5 1 r p 160 e m (

IG4  p r [ IG3 s

o i

) 140 IG2 r 

0.1 r D (

IGX-A

E s

 IGX-B 120 u x d IGX-C / o e

n IGX-D e d 100 0.1 d KRS-5 n

0.01 n I n a AgCl t o

e 80

i KBr n v s i

CsBr a r t t

e CsI c

60 s p

CaF2 a 1E-3 n r I s i f BaF2 40 e

D PbF2

0.01 m R

MgO u Al2O3 20 m TiO2 1 10 i

1E-4 x Spinel a 1 10 BeO

Wavelength (m) M Wavelength [m] Y2O3

As seen before, large numbers of wavelengths are not needed, but wavelength must be well-known. 1 cm-1 or better wavelength precision recommended (±0.02nm @ λ = 500nm). • Calibrated gratings, scanning FP cavities, gas lamps, gas lasers, DFB or EC diode lasers. • Diode lasers can be modulated to allow high SNR and throughput. • Current system @ SCHOTT uses 20kHz modulation for < 1 hour full curve measurement. • Rotating prism sample itself forms a high precision monochromator. • Broadband excitation and detector arrays may possibly be a better solution. SCHOTT Advanced Optics 10

Improvements needed for dn/dT measurements

2.7795 300

2.7790 IG6 @ 10.6m

) 200 m

2.7785 p p (

x l 100 e a d 2.7780 u n d I i

e 0 e v 2.7775 r i

t g c n a i r t f 2.7770 t -100 i e F

R r

2.7765 a

e -200 n i 2.7760 L -300 -10 0 10 20 30 40 50 60 70 80 -10 0 10 20 30 40 50 60 70 80 o Temperature (oC) Temperature ( C)

• 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.

• Recommend use of high precision etalon / interferometry techniques versus index measurement over temperature.

• Should also be used for relaxation / index drop, ie. for molding applications. SCHOTT Advanced Optics 11 IR Quality Inspection

Beam splitter (removable)

Pin hole

I R G

Reference mirrors (removable)

Inclusions Striae Sub-surface damage Slide SCHOTT Advanced Optics 12 IR Quality Inspection - continued

Birefringence: • Polarizer and analyzer may be used in order to check residual stress. • 4-point bend stress-optic instrumentation has been constructed. • Currently going through validation measurements.

Working with multi-spectral glasses (IGX program) to define homogeneity grades H-1 through H-5 similar to traditional oxide glasses.

Full homogeneity measurements via IR interferometry have been requested.

IR adhesives have recently been developed at SCHOTT. • Thermal cure is required due to UV opaque optics and low IR transmission of polymers. • IR active alignment tools will be required along with process development.