Transcript Opti521 - University of Arizona

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
1
OUTLINE
• Geometric versus Diffraction Limited Spot Diameter
Approximations
• Optical Element Parameters
•
Materials Properties
• Thermal Effects/ Athermalization Approximations
• Scattering Approximations
• Software Tools
2
For M2 > 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”
3
Rules of Thumb for Opto-mechanical Tolerances
4
Thermal Effects on a Len Thickness and Radius of Curvature
In isotropic materials, a temperature change makes inside Dimensions scale as
outside Dimensions.
B
A
B’
A’
A’ = DA + A
B’ = DB + B
Radius of Curvature, R, changes to
R’, using the same thermal expansion equation.
DA = A a DT
DB = B a DT
5
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
6
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
7
Thermal Effects
8
Total Integrated Scatter Measurement
TISb(s,)
= s / ( s + r) = 1 – e –(4 

 4s   





scos /)
i
2
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.
9
Back Scatter Approximation
The approximate backscatter
TIS is shown below:
TISb(s,) =  4s   
  
2
As shown above, the approximate TIS is good for σ < ~λ/25.
That is, for s()/ > 0.04, the approximation underestimates the exact exponential form TIS.
10
U.S. Opto-mechanical design between various disciplines
Optics
Zemax
CodeV
ASAP
TracePro
Other
Disciplines
Fluid Mechanics
Acoustics
3D Graphics
Heat Transfer
Sinda
TAP
MITAS
Zernike
Analysis
4Sight
Vision
SigFit
Databases and
Translator Software
Structures
NASTRAN
ANSYS
COSMOS
CAD/CAM
Control Systems
AutoCAD
SolidWorks
ProEngineer
Matlab/Simulink
LabView/LabWindows
11
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 σ – 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)
12
Back up Slides are slides 14 thru
17.
13
Diffraction Limited Approximations
Applies to M^2 < 4 Laser Systems
Minimum Spot
Diameter =
2.44  F/#
Depth of Focus = +/- 2  (F/#)2
F/# is the “F-number”
Singlet
14
Various Mounting Techniques
a) Edge-mounted
b) Surface-centered
c) Cell-mounted
Sag
R
r
r2
Sag 
2R
R
z
15
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.
16
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
Df = -(n + ahousing) f DT
Where n is the plastic thermo-optics coefficient
, a is the CTE, f is the focal length, and DT is the
temperature range.
From Table 2, 27 microns exceeds the 13 microns and
therefore is outside the diffraction limits. Thus, the forward
scatter calculation underestimates the scatter. Since the
Wrms is > 0.04 . adding actual surface roughness will only
further increase the s value.
Table 2 – Plastic Singlet Dimensional
Changes due to 40 C temperature rise.
Parameters
Temp. Celsius
Lens Element A
Radius R1 - mm
Sag for Radius 1
Radius R2 (mm)
Sag for Radius 2
Thickness- mm , t
Refractive Index
Aluminum Spacing
between Element B and
detector (mm)
at Initial
Temp.
0
15.582
at Dtemp
40
5
1.572
15.613
0.822
-162.625
-325.172
5.01
1.574
27.770
27.797
-162.300
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