Potting Optical Elements in Cells

Eric BooenDecember 8th, 2008

OPTI 521

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Objective

• Introduce Concept of Potting Optical Components

• Present Adhesive Material Properties• Describe Failure Modes of Adhesives• Suggest Design Criteria and Assembly

Processes

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Potted Optical Assemblies

• Glass Optic rests on machined surfaces

• Metal cell or mount• Adhesive Creates

Bond• Can reduces part

count, weight, cost, and ease of assembly when compared to mechanical mounts

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Applications

• Applications with small temperature ranges

• Optics with low LOS sensitivity (mrad/mrad)

• Tolerance Required• Additional testing

should be completed to verify assumption in tolerance analysis

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Adhesive Materials

• Epoxies– High to moderate

stiffness and strength– Rule of thumb: E = 150

ksi, Shear strength = 2000 psi

• Urethanes– Not recommended for use

at high temperature– Commonly used for

staking• Acrylics

– Quick bonds, high strength, UV cure on occasion

• Silicone Elastomers– Flexible, RTVs, high CTE

• For more information see References 2 and 3

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Bond Failure

• Adhesive Failure– Critical damage across

the bond interface1

– Decreased by surface preparation, use of primer, proper curing process

• Cohesive Failure– Critical damage to

bulk material of bond1

– How you design your bonds to fail

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Design

• CTE mismatch causes shear stress – See Reference 1

alternate stress calc.

• Bond size must also prevent shear due to shock loading– F = mgashock

=F/A

• Vibration environment may drive bond stiffness requirements

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Assembly

• Hole size tolerances should be determined based on gap requirement for adhesive material and optic tolerance– Gap tolerances

recommended by manufacture or based on experience

• Maintain gap through cure temperature– Compliant shims– Spacer Balls added to

adhesive• Cure adhesive as

recommended then temperature cycle through operating temperature range

• Cell is ready for system integration

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References

1. Blain Olbert, “Adhesive Selection and Characterization – What you don’t know can kill you,” USAO Engineering Seminar (Aug 2004)

2. Daniel Vukobratovich, “Introduction to Opto-Mechanical Design,” Notes, (2008).

3. Paul R. Yoder, Opto-Mechanical Systems Design, 3rd, CRC Press (2006).