Athermalization Techniques in Infrared S ystems

1. Athermalization Techniques in Infrared Systems OPTI 521 – Fall 2010 Tutorial Jeffrey T Daiker

2. Outline • Effect of temperature on focus • Athermalization of focus • Optically passive • Mechanically passive • Electromechanically active • Conclusion • References OPTI 521 – Fall 2010 Daiker Tutorial 2

3. Effect of temperature on focus • Defocus with temperature change due to the nature of IR lens materials • gthe thermo-optical coefficient of the lens is positive for most IR lens materials and indicates a negative change in focal length with temperature OPTI 521 – Fall 2010 Daiker Tutorial 3

4. Tolerable temperature change OPTI 521 – Fall 2010 Daiker Tutorial 4

5. Athermalization – optically passive • Combine suitably chosen lens materials which together compensate for thermal defocus • Athermalachromat • Total power • Achromatism • Athermalization OPTI 521 – Fall 2010 Daiker Tutorial 5

6. Athermalachromat • Example: Si, Ge, ZnS • Optimum three-material solution -20⁰C +20⁰C +60⁰C OPTI 521 – Fall 2010 Daiker Tutorial 6

7. Multiple Lens Systems • Optically passive athermalization very complex • Zoom lens is extreme case where completely passive athermalization generally not possible • Utilize optical design software OPTI 521 – Fall 2010 Daiker Tutorial 7

8. Thermal Modeling in ZEMAX • Before doing any thermal modeling • Set TCE of air spaces OPTI 521 – Fall 2010 Daiker Tutorial 8

9. Thermal Modeling in ZEMAX • Built-in tool in the Multi-Configuration Editor • Add different temperature configurations OPTI 521 – Fall 2010 Daiker Tutorial 9

10. Thermal Modeling in ZEMAX • MCE should look something like this OPTI 521 – Fall 2010 Daiker Tutorial 10

11. Athermalization in ZEMAX • Carefully construct merit function • Minimize RMS wavefront between two temperatures, for example • Optimization using glass substitution • Create a glass catalog from a short list (Si, Ge, ZnS, ZnSe, MgO, KRS5, AMTIR1, CaF2 for 3-5 microns) • No guarantee solution is global optimum • Use design practices, experience, and resources OPTI 521 – Fall 2010 Daiker Tutorial 11

12. Athermalization – mechancially passive • Involves some method of moving a lens element or elements by an amount that compensates for thermal defocus • By using two different materials with very different TCE arranged as either differential expansion cylinders or rods, it is possible to move the compensating element directly • Rods or cylinders must be of sufficient length OPTI 521 – Fall 2010 Daiker Tutorial 12

13. Differential expansion • Combine spacers of length L1 and L2 with TCE a1 and a2 respectively, then to athermalize over distance L • Using materials with a > 0 requires L < 0 OPTI 521 – Fall 2010 Daiker Tutorial 13

14. Athermalization – electromechanically active • Relies on compensator elements driven in a temperature controlled manner using information from separate temperature sensors • Brute force solution • Most suitable for complex systems such as zoom lenses where an electomechanical focus mechanism already exists OPTI 521 – Fall 2010 Daiker Tutorial 14

15. Avoidance of the Problem • Example: all-reflective system consisting of aluminum mirrors in an aluminum housing OPTI 521 – Fall 2010 Daiker Tutorial 15

16. Conclusion OPTI 521 – Fall 2010 Daiker Tutorial 16

17. References • Povey, V. “Athermalisation Techniques in Infra Red Systems,” Proc. of SPIE Vol. 0655, Optical System Design, Analysis, and Production for Advanced Technology Systems, ed. Fischer, Rogers (Apr 1986) • Rogers, P. “Athermalization of IR Optical Systems,” Critical Review Vol. CR38, Infrared Optical Design and Fabrication, ed. R. Hartmann, W.J. Smith (Apr 1991) • Jamieson, T. “Athermalization of optical instruments from the optomechanical viewpoint,” Critical Review Vol. CR43, Optomechanical Design, ed. P.R. Yoder, Jr. (July 1992) • Rayces, J. “Thermal compensation of infrared achromatic objectives with three optical materials,” SPIE Vol. 1354, International Lens Design Conference (1990) • Riedl, M. “Optical Design Fundamentals for Infrared Systems,” tutorial texts in optical engineering; v.TT20, SPIE • Rogers, P. “Thermal Compensation Techniques,” Handbook of Optics, Volume I, Part 9, Chapter 39. McGraw-Hill, 1995. • Zemax Knowledge Base. 2010. http://www.zemax.com/kb/articles/106/1/How-to-Model-Thermal-Effects-using-ZEMAX/ OPTI 521 – Fall 2010 Daiker Tutorial 17