An Introduction to Optical Window Design

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December 14, 2006

Introduction to Optical Windows Design

1

An Introduction to Optical Window Design

University of Arizona

Introductory Opto
-
Mechanical Engineering

Dan Willistein

December 14, 2006

Introduction to Optical Windows Design

2

Outline


Definition


Window materials


OPD affects due to different loadings,
uniform face loads, accelerations


Typical window mounts


High pressure differential windows


Fracture strength


Example of non
-
adhesive seal or gasket


Fracture toughness



December 14, 2006

Introduction to Optical Windows Design

3

Definition


What is an optical window?


Transmits the desired radiation
within allowable wavefront
deviation


Separates the two environments,
sometimes with gaskets


Resists high pressure differentials
and temperatures


Typically flat on both sides but can
be domed or conformal




December 14, 2006

Introduction to Optical Windows Design

4

Window Materials


Application dependent


BK7 common for visible
spectrum


CaFl for IR apps.


MgF2 for UV apps.






Table 1. adapted from Red Optronics,
www.redoptronics.com

2

December 14, 2006

Introduction to Optical Windows Design

5

ALON for IR windows


ALON or polycrystalline aluminum
oxynitride (~Al
23
O
27
N
5
) developed at
Raytheon in Lexington, MA


Optically equivalent to Sapphire (IR
transparent), with the same fracture
strength


4 times the strength and hardness as glass


Applications include forward looking IR
systems, missile domes, underwater
sensors, armor, scratchproof lenses.


Does not need to be grown as a single
crystal
-
> cheaper and faster


Can be made much larger more easily




December 14, 2006

Introduction to Optical Windows Design

6

Effect on OPD


Vukobratovich
3

presents equation used to approximate OPD


Simply supported, round window


Subjected to load
D
P
W

on entire clear aperture diameter, A
w


Young’s modulus, E
G


Index, n






Allowable window thickness, t
W


Equation can be iteratively solved



5
2
6
2
/
)
1
(
00889
.
0
W
G
W
W
t
E
A
P
n
OPD
D


December 14, 2006

Introduction to Optical Windows Design

7

Effect on OPD


Vukobratovich
3

presents equation used to approximate OPD


Can be extended for simple axial accelerations


Substitute:




For
D
P
W
in first equation:





Where a
G

is acceleration,
r
G

is material density


Again, allowable window thickness, t
W




5
2
6
2
/
)
1
(
00889
.
0
W
G
W
W
t
E
A
P
n
OPD
D


W
G
G
W
t
a
P
r

D
December 14, 2006

Introduction to Optical Windows Design

8

Mounting Windows


Typical window mount
example from Yoder
1


Glass window held with
adhesive into stainless steel
barrel


Windows do not have an
optical axis
-
> loose
diameter tolerances


Nominal 0.5mm clearance
between 50.8mm diameter
window and barrel inside
diameter


Figure 1. Bonded
-
in
-
place glass instrument window (Yoder 2006, Fig. 6.1
1
)


December 14, 2006

Introduction to Optical Windows Design

9

Mounting Windows


Clamped and unclamped windows


Unclamped (held with adhesive)


cheapest most simple method


Clamped window


requires additional parts but is less
subject to deflection from pressure
differentials, body forces and
accelerations.


Analogous to beam support
scenarios

Figure 3. Unclamped and clamped window configurations (Yoder 2006,
Fig. 6.29, adapted from Harris, D.C.,
Materials for Infrared Windows
and Domes, Properties and Performance
, SPIE Press, Bellingham, 1999.)
5


Vs.

December 14, 2006

Introduction to Optical Windows Design

10

Pressure differentials


Harris
5

gives equation used to determine minimum window thickness,
t
W
, based on unsupported aperture diameter, A
W

(see Figure 3)


Subject to pressure differential
D
P
W






K
W

is support condition


Clamped = 0.75


Unclamped = 1.25 (need 67% more thickness)


Typical value for factor of safety f
s

is 4


Fracture strength S
F

is given for several IR window materials (next slide)

2
1
5
.
0






D

F
W
s
W
W
W
S
P
f
K
A
t
December 14, 2006

Introduction to Optical Windows Design

11

Fracture strength


Depends on:


surface finish


fabrication method


Best to check this value
with manufacturer


Table 2 (Yoder 2006, Table 6.1)
4




2
1
5
.
0






D

F
W
s
W
W
W
S
P
f
K
A
t
December 14, 2006

Introduction to Optical Windows Design

12

Special sealing considerations


Adhesives not always permissible


Depends on requirements of application
-
>is outgasing a problem?


Example: 7.6 cm diameter NaCl window, 0.9 cm thick
6


High vacuum chamber used for laser irradiation of sample gases.


High thermal shock


Long term pressure loading





lead gasket is pressed
between NaCl and metal
housing forming seal


Belleville washers
provide axial preload


Free to move with
temperature changes
radially

Figure 4. NaCl window for high vacuum IR system (Manuccia et. al., adapted by
Yoder, 2006)
6

December 14, 2006

Introduction to Optical Windows Design

13

Special sealing considerations


Dunn et. al. studied conical window mounting interfaces vs. flat
interfaces


Found that 90 degrees cone angle gave approximately same strength




December 14, 2006

Introduction to Optical Windows Design

14

Fracture Toughness


Doyle and Kahan
7

present Griffith’s law for stress intensity factor K
I

calculation:




Y is a crack geometry factor,
s

id the nominal tensile stress, and a is the flaw
size.


Failure when the stress intensity factor exceeds fracture toughness


Fracture toughness for several optical window materials given:
7










a
Y
K
I
s

December 14, 2006

Introduction to Optical Windows Design

15

Conclusions

1.
Optical windows at the outset seem like a simple design task but when
they are used in applications with any amount of special requirements,
careful attention to details of the design is in order.

2.
Sealing optical windows can be accomplished with flexible silicone
based adhesives such as RTV and aided with clamps. However, when
requirements prohibit the use of conventional sealants, other methods
and materials must be used such as the lead gasket example discussed
here.

3.
Windows subjected to high pressures can be designed using guideline
formulas discussed here and appropriate safety factors.

4.
The strength of optical glass involves understanding not only the basic
material strength properties but also the surface quality, inclusions and
the loading cycle of the window.


December 14, 2006

Introduction to Optical Windows Design

16

References


1.
Yoder, P.R.,
Opto
-
Mechanical Systems Design, 3
rd

Ed
., CRC Press, 2006.

2.
Red Optronics in Mountain View, CA, website:
www.redoptronics.com

3.
Vukobratovich, D., Introduction to Opto
-
Mechanical Design,
SPIE Short
Course

SC014, 2003.

4.
Dunn, G. and Stachiw, J., Acrylic windows for underwater structures,
Proc.
SPIE
, 7, D
-
XX
-
1, 1966.

5.
Harris, D.C.,
Materials for Infrared Windows and Domes, Properties and
Performance
, SPIE Press, Bellingham, 1999.

6.
Manuccia, T.J., Peele, J.R., and Geosling, C.E., “High temperature ultrahigh
vacuum infrared window seal”,
Rev. Sci. Instum
., 52, 1857, 1981.

7.
K.B. Doyle, M.A. Kahan, “Design strength of optical glass,”
Optomechanics
2003
, Proc. SPIE 5176 (2003).