FirstOrder OptoMechanical Considerations in High Power Applications

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First-Order Opto-Mechanical Considerations in
High Power Applications
Victor Villavicencio NGIT / Defense
Group Technology Integration Applications
Operating Unit Science and Engineering
Applications December 06
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OUTLINE

• Geometric versus Diffraction Limited Spot
  Diameter Approximations
• Optical Element Parameters
• Materials Properties
• Thermal Effects/ Athermalization Approximations
• Scattering Approximations
• Software Tools

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For M2 gt 6, use (Geometric) Spot Diameter
Approximations
RMS diameter 0.7 Dz/Fn

• For Diffraction Limited System, Spot Diameter is
  defined as 86 of encircled energy
• For Geometric System, Spot Diameter is defined as
• greater than 50 of encircled energy is within
  70 of the marginal ray diameter

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Rules of Thumb for Opto-mechanical Tolerances
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Thermal Effects on a Len Thickness and Radius of
Curvature
In isotropic materials, a temperature change
makes inside Dimensions scale as outside
Dimensions.
A
A
A DA A
B DB B
DA A a DT DB B a DT
Radius of Curvature, R, changes to R, using the
same thermal expansion equation.
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Thermal Stress, s
Use Superposition to Calculate stress due to
temperature change
For Glass, Do not exceed 1000 psi (7 MegaPascal)
in tensile stress 50,000 psi (350 MPa) in
compression stress
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Thermo-Optic Coefficients, n, and CTE Values of
Materials
n ( x 10-6/ Celsius)
Plastics - 20 thru 40
CTE of Common Materials , a (x 10-6 /Celsius)
Aluminum 6061 / Brass 23.4 416 Stainless
9.9 Invar35
0.6 Titanium
8.7 Glass
3 to 7 Plastics
50 80 Adhesives 40
1000 Infrared Glasses 2 thru 20
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Thermal Effects
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Total Integrated Scatter Measurement
TISb(s,l) ?s / ( ?s ?r) 1 e (4 p
?cos?i/l) ?
Calculation for Straylight for 10W, 1.3 micron,
4 Fresnel Reflection ( Powerreflected ) Eq.
1 (10 0.04) (4 Pi 0.08)2 0.54 Watts
backwards scattered.
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Back Scatter Approximation
The approximate backscatter TIS is shown below
TISb(s,l)
As shown above, the approximate TIS is good for s
lt ?/25. That is, for s(l)/l gt 0.04, the
approximation underestimates the exact
exponential form TIS.
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U.S. Opto-mechanical design between various
disciplines
Zernike Analysis
Optics
Zemax CodeV ASAP TracePro
4Sight Vision SigFit
Structures
NASTRAN ANSYS COSMOS
Other Disciplines
Databases and Translator Software
Fluid Mechanics Acoustics 3D Graphics
CAD/CAM
AutoCAD SolidWorks ProEngineer
Control Systems
Heat Transfer
Sinda TAP MITAS
Matlab/Simulink LabView/LabWindows
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Conclusion

• Thermal effects and scattering first order
  calculations for high power applications.
• In your experimental setup, use the incoherent
  rms spot size equation to determine spot size.
  This provides the largest (worst case) spot size.
• TIS calculations are always conservative since it
  deals with surface roughness scatter. Internal
  straie /inclusions/stress will only slightly
  increase this TIS calculations.
• To achieve geometrical approximations, thermal
  effects must be taken into account for plastics
  optics over DT 20 C or more or glass optics
  needing to operate over DT 40 C or more.

References

• OPTI521 Class Notes, Fall, 2006.
• Michael G. Dittman, Frank Grochocki, Kathleen
  Youngworth, No such thing as s flowdown and
  measurement of surface roughness requirements,
  Optical Systems Degradation, Contamination, and
  Stray Light Effects, Measurements, and Control
  II, edited by O. Manuel Uy, Sharon A. Straka,
  John C. Fleming, Michael G. Dittman, SPIE Vol.
  6291.
• Frank DeWitt IV, Georg Nadorff, Rigid Body
  Movements of Optical Elements due to
  Opto-Mechanical Factors Optical Modeling and
  Performance Predictions II, edited by Mark A.
  Kahan, SPIE Vol. 5867, (2005)

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Back up Slides are slides 14 thru 17.
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Diffraction Limited Approximations

• Applies to M2 lt 4 Laser Systems

Minimum Spot Diameter 2.44 l
F/ Depth of Focus /- 2 l (F/)2
F/ is the F-number
Singlet
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Various Mounting Techniques
a) Edge-mounted b) Surface-centered c)
Cell-mounted
Sag
R
r
Sag
z
R
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Calculating Tilt
Figure 7 Accounting for tilt of a edge mounted
element 3 Using a semidiameter (SD) of 5 mm,
R 162 mm and a gap of 0.7 mm , a tilt of 2
degrees was calculated. This tilt value is used
to determine effects on image quality using
Zemax.
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Calculating the Change in Focal Length for a
Plastic Singlet, f 25 mm, 10 mm diameter.
The focal length expands, due to the Dt 40 C
temperature rise, by