Problem 1:
Vacuum Window
OPTI 523
Laura Coyle, TA
January 17, 2014
Design an optical window
to a vacuum chamber
• Requirements:
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2 in diameter clear aperture
< λ/2 rms wavefront distortion (λ = 633 nm)
1 atm pressure on the outside (10-6 Torr on the inside)
Transmit light from 0.6 – 5. µm
– Thermal: inside the chamber is ambient temperature
– Interface: Mate with a 6” diameter flat stainless steel flange:
• 3” ID
• 6” OD
• 5” diameter bolt circle, 8 bolts, 0.20” clearance holes
Project Schedule
• Write the requirements for window.
Due 1/24
• Perform preliminary design.
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Specify the window, investigate possibilities for procurement
Sketch the geometry
Perform initial calculations to verify design
Define the analysis and design degrees of freedom that must be chosen
• Document the preliminary design, but you do not need a complete
report
• Complete the design. Provide solution as a technical report. Due 1/31
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Design details including drawings for custom parts
Analysis demonstrating performance, survival
Bill of materials
Preliminary fabrication plan
Optical Materials
Optical Properties
• Wavelength range
• Refractive index
– Dispersion
– dn/dT
• Quality
– Bubbles
– Homogeneity
– Birefringence
Mechanical Properties
• CTE
• Young’s modulus (stiffness)
• Density
• Thermal conductivity
• Specific heat
• Hardness
• Climatic resistance
Glass Data Sheets
http://www.schott.com/advanced_optics/us/abbe_datasheets/schott_datasheet_all_us.pdf
IR vs. Visible Glasses
• IR glasses have significantly higher refractive indices
– Visible glass – n ranges between 1.45 and 2.0
– IR glasses – n ranges between 1.38 to 4.0
• Dispersion can be significantly lower (depending on spectral band)
– Visible glasses – V ranges from 20 to 80
– IR glasses – V ranges from 20 to 1000
• Many IR glasses are opaque in the visible
– And most visible glasses are opaque in the IR
• IR glasses are often heavier than visible glasses
• IR glasses have significantly higher dn/dT values
– Factor of 10 or more higher
• IR glasses cost more than visible glasses
– by 2 or more orders of magnitude
• Significantly fewer number of practical IR glasses than visible
glasses
The Glass Map - Visible
The Glass Map - IR
• Larger v, larger n
Designing with IR Glasses
Advantages
Disadvantages
• Refractive index is usually
higher, so fewer lenses are
needed to achieve diffractionlimited performance
• Most IR materials can be
diamond point machined, so
aspherics are commonly used
in designs
• The Airy disk size and
diffraction-limited depth of
focus are larger for the IR than
for the visible, so achieving
difraction-limited performance
is easier
• Small choice of glasses
• Materials are expensive
(~1$/gram)
– One-inch diameter BK7 lens $5
– One-inch diameter germanium lens
$500
– Five-inch diameter sapphire dome
= Priceless
• Some IR materials are difficult to
fabricate and/or antireflection
coat
• Fragile, soft, chip easily, low
thermal conductivity, etc.
• Most IR materials have large
dn/dT values, so athermalizing
can be difficult
Mounting Windows
• Windows are used in systems that require
protection/isolation from the environment
– Not supposed to have an effect on light passing
through
• Design Considerations
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Strength
Wedge angle
Surface figure
Sealing
Bowing to pressure/temp differentials
Climatic resistance (AR coating)
Mounting Windows
BOND IT
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One time assembly, difficult to take
apart
Easy, stiff in normal direction
Can be compliant in shear direction
CTE differences can cause large
stresses, especially for
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Large temperature ranges
Large CTE difference
Large dimensions
Use UV curing cement to allow
adjustment before curing
Can provide seal
Provides some compliance, mitigates
stress from shock loading
Requires careful preparation of surfaces
May require special jigs and procedures
Possibility of outgassing, affecting
coatings
Does not require preload
CLAMP IT
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Allows easy separation of constraint
and preload
Allows disassembly
Clamping forces should be controlled
Can cause distortions, affect
performance
Can cause large stresses, affect
survivability
Can allow for thermal expansion
Lens barrels with threaded retainers,
consider thread size, assembly torque
Clamping – Preload Force
• Apply force in line with constraint to limit stress, possible distortion,
and instability
• Analyze shock load:
– Force at constraint = Preload Force +/- mass * acceleration
• For shock, which is typically given in G’s :
– Use mass in kg, 1 G = 9.8 m/s2 get force in N
– 1 kg optic that sees 10G shock, Force = 98 kg m/s2= 98 N
• Stress = Force/Area
– Make sure that applied stress is not so large that it breaks the optic
– Rule of thumb for glass, limit short term compressive stress to 50,000
psi
– Special calculations to determine stress for point and line contacts
• Set preload to maintain contact between the optic and the
constraint
– Otherwise it can rattle, which causes very high local stress, p
Clamping - Stresses
• Calculate preload force
• Choose material and thickness to limit stress to <
25% of glass strength (SF = 4)
• Increases window thickness a bit doesn’t cost
much and provides extra margin
Clamping + o-rings
O-rings
• Good resource: Parker O-rings
– “O-ring Handbook”
• Advantages
– Don’t require large preload forces
– Seal over wide range of temp, pressure, materials
– Economical
• Considerations for vacuum applications
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Outgassing
Size of gland
Surface finish inside gland
Lubrication
Leak rate
O-rings cont’d
Wavefront Deformation Surface Irregularity
• Surface Irregularity weakly couples into
transmitted WFE
Wavefront Deformation Clamping
• Stress from clamping causes birefringence –
affects transmitted WFE when you care about
Wavefront Deformation –
Pressure Differential
• Simply mounted circular window with uniform
loading
Previous Solutions
Resources
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521 Notes
Schott website – glass data sheets
Parker o-ring handbook
Schwertz&Burge, Field Guide to
Optomechanical Design and Analysis
• Vukobratovich notes