Sodern recent development in the design and verification of the passive polarization scramblers for space applications

Sodern recent development in the design and verification of the passive polarization scramblers for space applications M. Richert, G. Dubroca, D. Genestier, K. Ravel, M. Forget, J. Caron and J.L. Bézy 18 October 2016

Outline Purpose of polarization scrambler Scrambler principles and design Polarization performance test Result on actual scramblers SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 2

Outline Purpose of a polarization scrambler Scrambler principles and design Polarization performance test Result on actual scramblers SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 3

Radiometric accuracy Earth-observation imaging spectrometers strive to obtain ever better spatial and spectral radiometric accuracy. The spectral oscillating error and the polarization sensitivity must be minimized Major design driver of the instrument Mission requirements: Carbonsat: CO2 detection accuracy of 0.5ppm (atmospheric concentration ~400ppm) Trace gas may have only small spectral features Note that the NO2 oscillation to measured is about 0.5%! SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 4

Polarization of Earth radiance Rayleigh scattering is the strongest source of polarization in UV-VIS-NIR. In SWIR other effects start to play, e.g. Sunlight, ice clouds Degree of polarization (DOP) varies both spectrally and spatially spectral variations DOP strongest in absorption bands, as ground unpolarized contribution disappears spatial variations DOP depends on scattering angle, and may vary from 0 to 1 within the swath Spectrometer must be insensitive to polarization BUT this is impossible even with careful design (in particular of the grating) SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 5

Need for a depolarizer Today s space borne imaging spectrometers require low errors on radiometric accuracy errors due to polarization The wavelengths range from the UV to the SWIR Example: Sentinel 5 requirements 270nm 2385nm BAND UV1 UV2VIS NIR1 NIR2 SWIR1 SWIR3 Polar. sensitivity (%) 0.32 0.38 0.38 0.38 1.5 1.5 Rel. Spec Rad. Acc.(%) 0.06 0.06 0.06 0.06 0.15 0.15 Need for a depolarization device working from the UV to the IR Can be achieved by using assembly of crystalline birefringent plates SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 6

Scrambler principle A scrambler is made of stack of birefringent plates which works as a variable retarder across the pupil The principle is to equally distribute the intensity between polarization states mix of polarizations at the output The price to pay is a degradation of the image quality. In a standard Dual Babinet scrambler the beam is split into 4 spots. Slope direction Slope direction Dual Babinet arrangement Spot split of a Dual Babinet in the image plane SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 8

Scramblers realization Extensive heritage in the scrambler manufacturing and test (OLCI, Sentinel 3, Sentinel 4 BB/QM/FM, Microcarb BB) for space application Validated manufacturing process for space environment (thermal and vacuum environment, cleanliness constraints) Gluing assembly principle transparent from 250 nm to more than 2500 nm Sentinel 4 scramblers (QM, FM1&2 and BB type A ) 165 78 65 130 SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 9

Main Polarization requirements Polarization Sensitivity (PS) Relative Spectral Radiometric Accuracy (RSRA) SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 10

Design and software capabilities CodeV and Zemax can compute Müller matrices necessary to the polarimetric performance analysis of a scrambler BUT. The computation time is excessively long Sodern developed a C++-program MuellerCalc to calculate the Müller matrices, PS and the RSRA of a scrambler As we need a pupil sampling of 256x256 and a spectral sampling of 0.1 nm to have a converging of PS and RSRA results, the computation speeds of the different software options are: Matlab Code V Mueller Calc 1 wavelength ~ 165s ~ 15s ~ 20 ms Spectrometer band ~ 90h ~ 8h ~ 40s Thanks to MuellerCalc, it is possible to explore exhaustively the space solution and run Monte-Carlo simulations with more than 1000 runs SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 11

MuellerCalc capabilities MuellerCalc performs ray tracing calculations MuellerCalc is capable of calculate: Spatial Müller matrices at a given wavelength and a given ray of a stack of birefringent elements Spatial Müller average matrix over the pupil in function of wavelength PS and RSRA This can be done by specifying the beam and each scrambler surfaces : Beam parameters: Shape : rectangular or elliptical Collimated, converging or divergent (thanks to the F-number) Uniformity (thanks to a polynomial function) Scrambler surface parameters : Material : Quartz, MgF2, Sapphire, or air (mono or multi-material scrambler) Shape: plane, roof or spherical Optical axis : X, Y and Z angles Wedge angles and azimuthal orientation Wedge thickness Pre/Post Müller matrices Müller files of elements before of after the scrambler (to take into account the coating effect for instance) SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 12

MuellerCalc capabilities The predicted performances of a scrambler is very difficult to foresee in function of the wavelength and the geometric parameter of the scrambler Thanks to MuellerCalc, Sodern is able to perform a systematic search of the best geometric parameters of a scrambler Below an illustration of a scrambler design search for MicroCarb project. The scrambler is a classical Dual Babinet, which needs to depolarize the light for 3 narrow bands from about 750nm to 2100nm. SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 13

Test capabilities Customers require a guarantee of performance through tests. The OGSE s principle is simple BUT the challenge lays in the very high required accuracy! L2 L1 PD2 Mono. PD1 SW SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 15

Reconstruction algorithm As PS and RSRA are defined based on max and min operator. A straightforward processing will lead to bias errors. A better approach is to fit the data by the statistical model provided by the Mueller matrix physical model Simulation of the error propagation Gaussian noise SNR = 5000 RSRA(0 &90 ) / λ = 3nm Even with a SNR of 5000, a straightforward algorithm leads to increase the true RSRA value by 0,2%! Mueller base reconstruction algorithm tends to render to error propagation negligible SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 16

Source characteristic Presentation of the polarized OGSE FWHM (can be reduced or increased with a slit change) Wavelength step Wavelength accuracy Bandwidth Beam, alignment performances and SNR Beam diameter in collimated beam Beam centering (with respect to the scrambler reference frame) From 0.3nm down to 0.08nm 0.028nm 0.15nm 320nm-800nm ø100mm ±100µm Beam pointing (with respect to the scrambler reference frame) ±0.5 Beam diameter (on the scrambler entrance surface) ±100µm F number tolerance ± 0,05 SNR on the raw signal S = Vm/Vr 320nm-340nm 340nm-500nm 735nm-775nm > 2 000 > 5 000 > 10 000 SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 17

Impact of the aperture shape - Mask 400mm 2 Breadboard S4 scrambler Type A : Dual Babinet scrambler High qualitative and quantitative agreement with the simulation Square pupil can lead to a better or worse PS performance depending on the pupil orientation SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 19

RSRA example Breadboard S4 scrambler Type B : Dual Babinet scrambler Deviation of actual scrambler performances from nominal ones can be simulated by reverse engineering (red curve (nominal), blue (meas.), green (estimated built design) SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 20

Example of measurement on S4SW S4 QM scrambler in converging beam PS and RSRA results SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 21

Conclusions Sodern has an extensive means to design and optimize high performance scramblers. The scramblers development process is complete and fully validated : A software, optimized for the computation of Müller matrices of birefringent optical elements was developed and validated. Thanks to a computation speed ~1000X greater than CodeV it allows a fast optimization of the scrambler design and Monte-Carlo tolerance analyses OGSE validated for the high accuracy measurement of PS and RSRA from 340nm to 775nm Statistical errors have been eliminated Know how & good work on the bench minimizes physical sources of errors SODERN - 2016/10/18 ICSO 2016 SODERN RECENT DEVELOPMENT IN DESIGN AND VERIFICATION OF POLARIZATION SCRAMBLERS P. 22