Potting Optical Elements in Cells Eric Booen December 8 th, 2008 OPTI 521

2 Objective Introduce Concept of Potting Optical Components Present Adhesive Material Properties Describe Failure Modes of Adhesives Suggest Design Criteria and Assembly Processes

3 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

4 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

5 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

6 Bond Failure Adhesive Failure –Critical damage across the bond interface 1 –Decreased by surface preparation, use of primer, proper curing process Cohesive Failure –Critical damage to bulk material of bond 1 –How you design your bonds to fail

7 Design CTE mismatch causes shear stress –See Reference 1 alternate stress calc. Bond size must also prevent shear due to shock loading –F = mga shock –  =F/A Vibration environment may drive bond stiffness requirements

8 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

9 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).