Experimental Investigation of
Secondary Back-Reflection
Aaron Spector - University of Florida
NASA Grants - NNX11AL16H and NNH10ZDA001N
LISA X - Gainesville, Fl
May 22, 2014
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
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On-Axis Telescope
Telescope transmits and receives lasers between SC
On-axis quadpod (Cassegrain) design
Larger aperture to weight ratio than off-axis designs
5% of light lost to shadows from Secondary support and struts
Symmetric shadows on quadrant photodetectors
Material stability confirmed to meet requirements
Thermal and dimensional stability tested at
UF with GSFC using a SiC test spacer
J Sanjuan et al.
Potential Problem:
Back-reflection from the
secondary mirror
Secondary
Primary
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
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Telescope Back-Reflection
Secondary and primary mirrors axially aligned
Outgoing beam incident normal to secondary mirror
Light reflected from secondary back to the optical bench
Distance between OB and telescope not stabilized
Motion between the OB and telescope introduces phase noise to the BR light
Couples into measurement signal via small vector phase noise
δφ
Local Oscillator:
Back-Reflection:
δx
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
Back-reflected Phase Noise
LISA pm phase noise requirements < 10-6 cycles/rtHz
BR phase noise �
proportional to spatial overlap (η) of the far-field and BR field
δφ = η
Po δx
χPl λ
−6 cycles
δφ < 10
√
Hz
→ η < 5 × 10−5
variable
description
value
Po
outgoing power
�2W
Pl
local oscillator power
� 500 µW
λ
laser wavelength
1064 √
nm
δx
distance changes between OB and telescope 10 nm/ Hz
χ
extinction ratio of polarizing beam splitter
1000
η
spatial overlap between recieved and BR fields < 5 × 10−5
Technical noise sources must be a factor of 10 below the LISA requirements:
η<
-6
5×10
Unmitigated spatial overlap is 3×10-4
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On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
Lotus Shape
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Must cast shadow along optical axis
Use dark spot in the center of the Highly Reflective (HR) coating on the secondary
Initial simulations used a circular hole in the HR coating
Produces a bright spot along the optical axis (spot of Arago) η = 8 ×10-5
Must use irregular shape to break spatial coherence of the edge of the hole
Simulations:
10 shapes investigated
Huygen’s integral and FFT propagation codes
‘Lotus’ shape chosen to investigate further
Axial power suppression limited by:
Petal tips minimum feature size (MFS)
Anti-Reflective (AR) coating reflectivity
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
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Simulated Distributions
Rough Circle
Lotus
−15
0
−10
−10
−10
−0.5
−0.5
−10
−0.5
−1
−1
−5
−5
−5
−1.5
−1.5
000
−2
−2
555
−2.5
−2.5
−3
−3
10
10
10
Vertical Axis (mm)
00
Vertical Axis (mm)
Vertical Axis (mm)
−15
−15
−15
−1
−5
−1.5
0
−2
5
−2.5
−3
10
−3.5
−3.5
15
15
15
−3.5
15
−15
−15
−15
−10
−10
−10
−5
0
5
−5
−5
00
55
Hoizontal Axis (mm)
Hoizontal Axis (mm)
10
10
10
15
15
−4
−4
−15
−10
−5
0
5
Hoizontal Axis (mm)
10
15
Lotus is an improvement over the rough circle
Do not meet requirements with 2 micron petal tips (η = 1.1 to 3.8 ×10-5)
−4
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
Secondary Prototypes
Deposited dielectric HR coating over
curved substrate
16 mm radius of curvature
1mm diameter hole secondaries
30 micron chipping region
‘Lotus’ secondaries manufactured
using photo-lithographic techniques
180 micron gold HR layer deposited over curved
substrate (convex)
AR coating has reflectivity of 2.5×10-3
2 mm diameter pattern etched into gold
Minimum feature size in petal tips of 2 microns
1 micron undercut at the edge of the structure
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On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
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Experimental Testbed
−3
−5
Laser
λ/2
Laser
λ/2
−3
data
PZT
λ/4
Secondary
−4
x 10
Telescope
Aperture
Secondary prototypes tested on optical bench
−2
Telescope aperture
beam shaping optics
−1
recreate simulated
received field
0
Amplitude
of
interference signal
1
between
BR and
received field
measured
on PD
2
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
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Preliminary Results
Results for Secondary Prototypes
Goal: η < 5 ×10-6
Measured unmitigated BR η = 3.2 ×10-4 (theory: 3.0 ×10-4)
Rough circular hole: η = 4.1 to 6.2 ×10-5 (theory: 4.8 ± 0.4 ×10-5)
Lotus: η = 0.8 to 2.2 ×10-5 (theory: 1.1 to 3.8 ×10-5)
Shape
No Hole
Circular Hole 1
Circular Hole 2
Circular Hole 3
Lotus 1
Lotus 1 (DL)
Lotus 2
Lotus 2 (DL)
Lotus 3
Lotus 3 (DL)
Lotus 4
Lotus 4 (DL)
Radius of Curvature
16 mm
16 mm
16 mm
16 mm
39 mm
39 mm
45 mm
45 mm
51 mm
51 mm
77 mm
77 mm
Max η (Experiment)
3.2×10−4
4.1×10−5
5.4×10−5
6.2×10−5
2.2×10−5
1.5×10−5
9.8×10−6
8.4×10−6
1.1×10−5
1.2×10−5
1.7×10−5
1.7×10−5
Max η (Simulation)
3.0×10−4
4.8±0.4×10−5
4.8±0.4×10−4
4.8±0.4×10−4
1.7×10−5
1.3×10−5
2.9×10−5
1.1×10−5
2.2×10−5
1.4×10−5
3.7×10−5
3.8×10−5
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
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Preliminary Results
Results for Secondary Prototypes
Goal: η < 5 ×10-6
Measured unmitigated BR η = 3.2 ×10-4 (theory: 3.0 ×10-4)
Rough circular hole: η = 4.1 to 6.2 ×10-5 (theory: 4.8 ± 0.4 ×10-5)
Lotus: η = 0.8 to 2.2 ×10-5 (theory: 1.1 to 3.8 ×10-5)
−50
No Hole
Amplitude (dBV)
−60
Circular Hole 3
Lotus 2
−70
−80
−90
−100
−110
−120
9.97
9.98
9.99
10
10.01
Frequency (MHz)
10.02
10.03
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Future Work
Apodized Shapes
90
0.001
120
60
0.0008
Etch sub-wavelength features into gold coating
0.0006
150
30
Forms a tunable effective index of refraction
0.0004
0.0002
Allows for smooth transition from AR to HR
180
0
Reduces light scattered due to diffraction effects
For ideal AR coating η = 1 to 4 ×10-7
210
Au
Reflectivity
For practical AR coating (R = 0.25%) η = 2 to 4 ×10-6
| |
�
�2
1
�
�
1 − NAu
1 − NSi iφ �
�
R = �SAu
+ (1 − SAu )
e �
0.8
1 + NAu
1 + NSi
330
240
300
270
0.6
0.4
0.2
Si
0
0
0.5
1
Radius (mm)
1.5
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
Conclusions
Results: We are close but not quite there
Best Lotus pattern etching within a factor of 1.7 of the requirements
Different results for different telescope designs
Simulations: limited by minimum feature size in the petal tips
Future: Apodized coatings present a possible solution
Simulated back-reflection below spatial overlap requirements
Work with practical AR coatings
Have procedure to manufacture coatings at UF, with help from the NRF
Thank You!
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On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
extra
Experimental Testbed
Telescope
Aperture
λ/2
!
Laser
λ/2
~
PZT
! + 90ο
λ/4
Clock
10 MHz
!
!
f2
10 MHz
+
Laser
Output
f2
64 MHz
~ ~
Secondary prototypes tested on optical bench
2 Lasers phase locked at 10 MHz with clocks synchronized to acromag card
Telescope aperture beam shaping optics recreate simulated received field
λ/4 wave plate and PBS used to separate back-reflected and incident beams
PZT mirror scans incident beam over mirror positions
Interference signal of far-field and BR field sent from PD to acromag card
Amplitude of both quadratures measured to eliminate path length noise
Amplitude of the signal is calibrated to give the spatial overlap
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
extra
Lotus Shape
Average Reflectivity for a given radial length
Average Radial Reflectivity
1
0.8
0.6
0.4
0 MFS
2 µm MFS
0.2
10 µm MFS
0
0
0.5
1
1.5
Average Radial Reflectivity
r (mm)
1.04
1.02
1
0.98
0.96
0.94
0 MFS
2 µm MFS
0.92
10 µm MFS
0.92
0.94
0.96
0.98
r (mm)
1
1.02
On-Axis Telescope Simulations Experimental Investigation Future Work & Conclusions
extra
Spiral Simulations
−2.5
−2.5
−2
−5
−2
−1.5
−3
−1
−0.5
0
−3.5
−5.5
−1.5
−1
−6
−0.5
0.5
1
−4
1.5
0
−6.5
0.5
2
2.5
−2.5
−2
−1
0
1
2
−2.5
−4.5
3
−2
−1.5
−7
1
1.5
2
2
1
2.5
−7.5
−1
−0.5
0
−2
−1
0
1
2
−8
0
0.5
−1
1
1.5
−2
2
2.5
−3
−2
−1
0
1
2
For ideal AR coating η = 1 to 4 ×10-7
For practical AR coating η = 2 to 4 ×10-6