Cheapest nuller in the World: Crossed beamsplitter cubes
Cheapest nuller in the World: Crossed beamsplitter cubes François Hénault Institut de Planétologie et d’Astrophysique de Grenoble, Université Joseph Fourier, CNRS, B.P. 53, 38041 Grenoble – France Alain Spang Laboratoire Lagrange, Université de Nice-Sophia Antipolis, CNRS, Observatoire de la Côte d’Azur, B.P. 4229, 06304 Nice – France
Conference 9146 - Optical and Infrared Interferometry IV
Montréal, 26 juin 2014
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Cheapest nuller in the World: Crossed beamsplitter cubes
Design • General view of the crossed-cubes nuller
(Zemax artwork)
Cube 1
Focusing optics
Cube 2 Y’
X’ Z O"
Conference 9146 - Optical and Infrared Interferometry IV
Montréal, 26 juin 2014
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Cheapest nuller in the World: Crossed beamsplitter cubes
Principle • Two “crossed” beamsplitter cubes have their semireflective (SR) perpendicular one to the other • The input beams propagates parallel to both cubes SR layers • It is spitted into four parallel beams, being recombined axially • A null is created at the focal plane centre • It is independent of polarization orientation and of wavelength (at least theoretically) • This is actually an Achromatic phase shifter
Conference 9146 - Optical and Infrared Interferometry IV
Montréal, 26 juin 2014
3
Cheapest nuller in the World: Crossed beamsplitter cubes
Cubes polarization model Cube 2 Cube 1 Beam 1
Beam 2 Beam 4
Beam 3
X’ Polarization
Y’ Polarization
Y’
Y’
Y’
Y’ X’
SubPupil 2
SubPupil 1
Z
0 X’
SubPupil 3
SubPupil 4
Conference 9146 - Optical and Infrared Interferometry IV
π
0
0
X’
π
π Montréal, 26 juin 2014
X’
π
0 4
Cheapest nuller in the World: Crossed beamsplitter cubes
Used as a coronagraph Secondary Mirror Monolithic telescope (P)
• •
High throughput Output baseline B’ can be adjusted via cubes translation
(
B' = A (1 − tan θ ) − 2h = A 1 − 1
Primary Mirror
Relay optics Wavefront sensor
)
2n 2 (λ ) − 1 − 2h
θ
h A
Crossedcubes nuller
•
O’ P’2
(P’) F’
B’ O”
Focal plane
B’
Very small Inner working angle IWA ≈
Multi-axial P’1 combiner
θ
1 λF' 1 N' λ ≈ 4 B' F 4 N D
F Telescope focal length D Telescope diameter N Telescope F/D number N’ Focusing optics F/D number
Z Conference 9146 - Optical and Infrared Interferometry IV
Montréal, 26 juin 2014
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Cheapest nuller in the World: Crossed beamsplitter cubes
Used as a nulling combiner From collecting telescopes B
B
O
(P)
Telescope 1
Telescope 2
Relay optics 1
Crossedcubes nuller
Relay optics 2
Metrology beams 1-4
Metrology beams 2-3
Metrology beams 1-4
Crossedcubes nuller
Metrology beams 2.3
B’ Metrology beams 14-23
(P’) O’
More photons Narrower IWA
O’
(P’) F’
F’
O”
Focal plane
Focal plane
Z
Z Conference 9146 - Optical and Infrared Interferometry IV
Montréal, 26 juin 2014
6
Cheapest nuller in the World: Crossed beamsplitter cubes
Used as a nulling combiner • Fizeau interferometers Input and output baselines B and B’ are matched by sizing the cube hypotenuse A
(
B' = A (1 − tan θ ) − B = A 1 − 1
)
2n 2 (λ ) − 1 − B
• Other configurations are possible (e.g. X-array) • Maximal densification is also feasible better for nulling interferometers, cf. paper 9146-89 θ θ
θ
B
A
θ
B’
Conference 9146 - Optical and Infrared Interferometry IV
B
B’
A
Montréal, 26 juin 2014
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Cheapest nuller in the World: Crossed beamsplitter cubes
Nulling maps at image plane V
V
Y’ Polarization
U
1 arcsec
1 arcsec
X’ Polarization
U
1 arcsec
1 arcsec V
1 arcsec
Unpolarized
U
1 arcsec
Conference 9146 - Optical and Infrared Interferometry IV
Montréal, 26 juin 2014
8
Cheapest nuller in the World: Crossed beamsplitter cubes
Preliminary manufacturing requirements • If OPDs are compensated for by optical delay lines, there remains one tight specification: Flux balance < 0.1 % PARAMETER Operating wavelength Spectral range Semi-reflective layer (SR) Transmission factor Reflection factor Flux mismatch Anti-reflective coating (AR) Geometrical parameters Cube hypotenuse Transmitted pathlength in glass Reflected pathlength in glass Pathlength difference in glass Angular errors Wavefront error
REQUIRED VALUE
EQUIVALENT NULLING RATE
REMARKS
λ = 10 µm 8-12 µm
Depending on science requirements Depending on science requirements
50 ± 0.1 % 50 ± 0.1 % < 0.1 % Standard
On full spectral band On full spectral band On full spectral band λ/4 AR coating
75.5 ± 0.1 mm 21.4 ± 0.1 mm 21.4 ± 0.1 mm < 0.005 µm < 3 arcmin < λ/4 PTV Total Null (RMS sum)
Conference 9146 - Optical and Infrared Interferometry IV
1.0E-06
9.8E-06 7.6E-07 0.0E+00
Case of ZnSe material Case of ZnSe material Case of ZnSe material Only applicable to coronagraph For both SR/AR faces, including pyramid For both transmitted and reflected beams, on each sub-pupil
4.6E-06
Montréal, 26 juin 2014
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Cheapest nuller in the World: Crossed beamsplitter cubes
Main advantages • • • • •
Simple, compact, low mass and volume Reasonable manufacturing tolerances Potentially not expensive High throughput, close to maximum Can be implemented into a nulling coronagraph telescope or a sparse-aperture nulling interferometer • Very small Inner working angle (IWA) when used as a coronagraph • Capacity for fringes rotation and baseline modulation • Good candidate for future space missions characterizing extra-solar planets atmospheres
Conference 9146 - Optical and Infrared Interferometry IV
Montréal, 26 juin 2014
10
Cheapest nuller in the World: Crossed beamsplitter cubes
Conclusion • “Cheapest nuller in the World” ? Yes, for producing “quick and dirty nulls” (e.g. student courses) • Probably a little bit more expensive if very deep nulls are required • Alternative title: “PERSEE interferometer in a nutshell” Star and Planet Simulator
Axial combination (modified Mach-Zehnder interferometer)
Entrance sub-pupils OPD and tip-tilt injection errors
Spatial filtering (mono-mode optical fibers)
Periscope achromatic π-phase-shifter Fringe sensor
Delay lines
Tip-tilt sensor
Conference 9146 - Optical and Infrared Interferometry IV
Montréal, 26 juin 2014
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