PCB Division
Lens Mounting Methods’ Effect on Optical Surface DeformationBoris Greenberg, Guy Gross, Regina Kour, Ram OronOptical Engineering ConferenceJuly 2011
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Laser Beam Requirements
Some optical systems depend strongly on laser beam quality for achieving the desired effect
‘Top Hat’ and Gaussian beams with pre-defined and achievable parameters are required on the target
In order to achieve this goal not only premium quality components are important but also their mounting method
Parago
nEmera
ldPerFix
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Flat Mounting Methods Mechanical methods Pros
Very short preparation time; Low stress on the optical part achievable if properly implemented
ConsMight be complicated
Adhesion methods Pros
Simple; Cheap Cons
Requires time and methods to cure; might deform the optics
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Flexture
Adhesive
Set ScrewBack Surface Adhesion
Peripheral Adhesion
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Lens Mounting Methods
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LensRetaining Nut
3 Point Mechanical Retaining Nut Adhesive
The choice of an optimal lens mounting method is defined by various parametersChiefly: Centering tolerance; Allowed lens deformation; Budget; Procedure convenience
25mm
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Experiment
Device under test [DUT] Flats Lenses
Tested with Zygo interferometer Parameters that were observed were: power and irregularity
Each unit was tested before and after mounting
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All values are in waves [632nm]Cells marked in red are out of specification
Selected tests resultMounting Irregularity Power
Item Method Required Measured Required
Measured
UnMounted
Mounted
UnMounted
Mounted
Flats1_M Flexture 0.2 0.0578 0.1263 0.5 0.0696 0.10672_M Flexture 0.2 0.0731 0.0731 0.5 0.0927 0.09273_M Set screw 0.2 0.0892 0.1298 0.25 0.0864 0.108
4_M Peripheral adhesion 0.1 0.11 0.2235 0.2 0.1162 0.1983
5_M Peripheral adhesion 0.125 0.1134 0.4845 0.5 0.0696 0.5008
Lenses1_L Retaining nut 0.125 0.1176 0.6107 2_L Retaining nut 0.125 0.1244 2.358 3_L Retaining nut 0.25 0.0918 0.6326
X19 times
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Deformation Implications on Beam Diameter
The simulation was performed with Zernike polynomials from Zygo analysis in Zemax optical simulation softwareOptical component deformation does not only affect the minimum spot size on the target but also its uniformity over the scan area
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FSM 0° 2_L 0λω0=4.3µm
FSM 2° 2_L 0λω0=4.45µm
FSM 0°2_L 2.3λω0=5.2µm
FSM 2°2_L 2.3λω0=6.4µm
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Mechanical Simulation Resultsof Retaining Nut Lens Mounting
A nut closed with torque of 0.6Nm Vertical pressure creates deformation of <0.25λ Shear forces create additional <0.3λ deformation
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Vertical Forces Shear Forces
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Device Under Test – 2_L
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2_L
LensRetaining ring
2_L had strongest deformationImprovement will be most visibleMounting methods
Adhesives RTV 3145 RTV 3140 RTV 5910 OP-29-GEL 621-GEL
Retaining nut
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Adhesive Application Methods
3 radial points The adhesive was applied via 3 radial holes in the lens's
housing Uniform ring
The adhesive was uniformly applied around the lens
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Lens
Adhesive
Holder Lens
Adhesive
Holder
Schematic top view
3 point adhesion
Uniform adhesion
LensHousing
Lens
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Retaining nut method
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In this method, the element is positioned in a lens holder and fastened with a retaining nut. Contact closure – the retaining ring was fastened manually until first contact with the lens. To fix the retaining ring OR-LACK® is applied between the housing and the ring at two or three points. To check the method dependency on the person fastening the ring and its repeatability the mounting was performed by two users and one DUT was re-mounted (opened and re-closed).
Torque meter fastening – the retaining ring was fastened by torque meter to enable closure with different torque values
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Optical Power Distortion vs. Mounting Method
Method Details Obtained optical irregularity [λ]
Reference lens 1, un-mounted --- ~ 0.1
Reference lens 2, un-mounted --- ~ 0.1
Mounting method: Adhesion AdhesiveRing 3145, RTV, 24h curing 0.08
3 points 3145, RTV, 24h curing 0.13 points 3140, RTV, 24h curing 0.08
Ring 3140, RTV, 24h curing 0.083 points 5910, RTV, 24h curing 0.1
Ring 5910, RTV, 24h curing 0.083 points OP-29-GEL, UV, 2min curing 0.076
Ring OP-29-GEL, UV, 2min curing 0.23 points 621-GEL, UV, 2min curing 0.25
Mounting method: retaining nut Torque [cNm]
contact closure, user 1 <5 0.142contact re-closure <5 0.142
contact closure, user 2 <5 0.1255 0.310 0.3420 0.660 1.1
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Within the spec Out of specSpec ≤ 0.125 λ
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Optical Power Distortion vs. Torque
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The pressure is not distributed evenly on the entire element perimeterThere is a good agreement with Hooke’s law which confirms the elastic nature of the fixture
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Conclusions1.The best results obtained
UV adhesive with "3 points" adhesion The "3 point" adhesion method is preferable for the UV adhesives. As during curing the adhesive is squeezed and pulls the optical element. The "Ring" method causes stronger optical distortions in elements
For RTV adhesives we found no difference between adhesion methods All tested RTV adhesives showed excellent results Adhesive strength is the main aspect of adhesive choice Outgassing and long term degradation are other considerations for such an adhesive choice
2.The choice between RTV and UV curing adhesives should be guided by allowed curing time and centering tolerance need as well as long term stability
3.The retaining nut contact closure method has a relatively good repeatability but low performance
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Thank You!
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