Transcript An Introduction to Optical Windows Design

An Introduction to Optical Window Design
University of Arizona
Introductory Opto-Mechanical Engineering
Dan Willistein
December 14, 2006
Introduction to Optical Windows Design
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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
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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
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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
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ALON for IR windows
• ALON or polycrystalline aluminum
oxynitride (~Al23O27N5) 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
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Effect on OPD
• Vukobratovich3 presents equation used to approximate OPD
–
–
–
–
Simply supported, round window
Subjected to load DPW on entire clear aperture diameter, Aw
Young’s modulus, EG
Index, n

OPD  0.00889(n  1)DPW2 AW6 / EG2 tW5

– Allowable window thickness, tW
– Equation can be iteratively solved
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Effect on OPD
• Vukobratovich3 presents equation used to approximate OPD
– Can be extended for simple axial accelerations
– Substitute:
DPW  aG r G tW
– For DPW in first equation:

OPD  0.00889(n  1)DPW2 AW6 / EG2 tW5

– Where aG is acceleration, rG is material density
– Again, allowable window thickness, tW
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Mounting Windows
• Typical window mount
example from Yoder1
• 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. 6.11)
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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
Vs.
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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
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Pressure differentials
• Harris5 gives equation used to determine minimum window thickness,
tW, based on unsupported aperture diameter, AW (see Figure 3)
– Subject to pressure differential DPW

DPW 
tW  0.5 AW  K W f s

S
F 

1
2
– KW is support condition
• Clamped = 0.75
• Unclamped = 1.25 (need 67% more thickness)
– Typical value for factor of safety fs is 4
– Fracture strength SF is given for several IR window materials (next slide)
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Fracture strength
• Depends on:
Table 2 (Yoder 2006, Table 6.1)4
– surface finish
– fabrication method
• Best to check this value
with manufacturer

DPW 
tW  0.5 AW  K W f s

S
F 

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2
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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 thick6
– 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
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Figure 4. NaCl window for high vacuum IR system (Manuccia et. al., adapted by
Yoder, 2006)6
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Special sealing considerations
• Dunn et. al. studied conical window mounting interfaces vs. flat
interfaces
• Found that 90 degrees cone angle gave approximately same strength
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Fracture Toughness
•
Doyle and Kahan7 present Griffith’s law for stress intensity factor KI
calculation:
K I  Ys a
•
•
•
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
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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.
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References
1. Yoder, P.R., Opto-Mechanical Systems Design, 3rd 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).
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