WO2020153843A1 - Optique de modulation de polarisation - Google Patents

Optique de modulation de polarisation Download PDF

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Publication number
WO2020153843A1
WO2020153843A1 PCT/NL2020/050034 NL2020050034W WO2020153843A1 WO 2020153843 A1 WO2020153843 A1 WO 2020153843A1 NL 2020050034 W NL2020050034 W NL 2020050034W WO 2020153843 A1 WO2020153843 A1 WO 2020153843A1
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WIPO (PCT)
Prior art keywords
polarization
slit
optical
optics
polarization modulation
Prior art date
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PCT/NL2020/050034
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English (en)
Inventor
Aaldert Hidde VAN AMERONGEN
Jochen Ignace Geert CAMPO
Jeroen Henricus Hubertus RIETJENS
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Stichting Nederlandse Wetenschappelijk Onderzoek Instituten
Airbus Defence and Space Netherlands BV
Original Assignee
Stichting Nederlandse Wetenschappelijk Onderzoek Instituten
Airbus Defence and Space Netherlands BV
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Publication of WO2020153843A1 publication Critical patent/WO2020153843A1/fr
Anticipated expiration legal-status Critical
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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/28—Investigating the spectrum
    • G01J3/447—Polarisation spectrometry
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G01J3/021—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using plane or convex mirrors, parallel phase plates, or particular reflectors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0205—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows
    • G01J3/0229—Optical elements not provided otherwise, e.g. optical manifolds, diffusers, windows using masks, aperture plates, spatial light modulators or spatial filters, e.g. reflective filters
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01J—MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY
    • G01J3/00—Spectrometry; Spectrophotometry; Monochromators; Measuring colours
    • G01J3/02—Details
    • G01J3/0291—Housings; Spectrometer accessories; Spatial arrangement of elements, e.g. folded path arrangements

Definitions

  • the present disclosure relates to polarization modulation optics for use in spectral modulation, e.g. as part of spectropolarimeter designed to operate from an orbiting or in situ platform.
  • spectral modulation is based on encoding the full linear polarization properties of light in its spectrum.
  • Such spectral modulation is obtained with an optical train of an achromatic quarter- wave retarder, an athermal multiple-order retarder, and a polarizer.
  • TIR retarders With regards to the technical implementation of an achromatic quarter- wave retarder, Snik et al. notes a major advantage of total internal reflection (TIR) retarders is the achromaticity of the retardance, which only varies with the wavelength variation of the refractive index. Noted examples of TIR retarders are the Fresnel rhomb and K-prism.
  • a K-prism based on three TIRs can he made out of fused silica and has the additional advantage of the lack of lateral beam shift, but it yields a much thicker optical component than a Fresnel rhomb. Furthermore it is noted that the FOV behavior of both a Fresnel rhomb and a K-prism is very anamorphic. With regards to positioning, Snik et al. notes that, as with any polarimeter, the modulator is best located as early in the beam as possible in order to minimize the number of optical components that modify the polarization of the source under investigation or introduce instrumental polarization.
  • the spectral modulator is located in the entrance pupil of a spectropolarimetric instrument.
  • the objective lens(es) are then positioned after the polarizer or polarizing beam splitter to image the source onto the entrance (slit) of the spectrometer.
  • SPEX Spectropolarimeter for Planetary Exploration
  • the broad-band spectral polarization modulator needs not only to be compatible with a space environment, but also allowing for a very compact, high performing optical design.
  • the PMO comprises an optical element such as an achromatic quarter-wave retarder having its (ordinary and extraordinary) axes in the direction of Stokes parameter ⁇ Q to transform e.g. linearly (Stokes U) polarized light into circularly (Stokes V) polarized light.
  • An athermal multiple -order retarder having its ordinary and extraordinary axes in the Stokes ⁇ U direction may receive this light and project the polarization state of the light onto the spectrum by means of spectral modulation of the polarization state (from linear over elliptical to circular, and back over elliptical to linear).
  • a polarizing beam splitter with polarization directions in the Stokes ⁇ Q directions may receive this light and output spectrally intensity modulated, orthogonally polarized beams. Accordingly, the spectral polarization modulator may transform incident light into two spectrally modulated intensities, such that amplitude and phase of the modulation are proportional to the degree and angle of linear polarization, respectively.
  • a slit plate is arranged inside the PMO and forms at least one physical slit having its slit length direction parallel to each of the optical interfaces of the optical components.
  • the inventors find that placing the physical slit inside the PMO and parallel to the optical interfaces, may provide a relative compact design with good performance. For example, placing the entrance slit of the spectrometer inside the PMO may enable sufficiently tight alignment of the two images from the polarizing beam splitter for them to fit on the detector. A too large distance from the slit would lead to a too large beam divergence and thereby to a too large distance between the two images.
  • placing the slit inside the PMO may be beneficial to accommodate telescope back focal length and/or the spectrometer collimator front focal length.
  • placing the slit imaging plane between the optical components of the PMO preferably, at or near the center along the propagation direction of the PMO beam path, the maximum beam size can be kept relatively small.
  • FIG 1 illustrates a plan view of light beams traversing a polarization modulation optics
  • FIG 2 illustrates a perspective view of the polarization modulation optics
  • FIG 3 illustrates an exploded perspective view of the polarization modulation optics and corresponding frame
  • FIG 4 illustrates a perspective view of the polarization modulation optics and combining optics
  • FIG 5A schematically illustrates a satellite comprising a multi - angle spectropolarimeter
  • FIG 5B schematically illustrates a top view of an embodiment for a multi-angle imager
  • FIG 6A schematically illustrates a side view the multi-angle imager
  • FIG 6B schematically illustrates a perspective view of the multi - angle imager
  • FIG 7 illustrates a schematic view of a spectropolarimeter
  • FIG 8A illustrates an image of different view angles imaged at a slit plane onto a slit plate
  • FIG 8B illustrate an image of polarization modulated spectra projected at an imaging sensor
  • FIG 9A shows a graph of retardance measurements of the SPEX PMO Mooney Rhomb, as described herein;
  • FIG 9B shows graphs of total internal reflection phase change in a fused silica rhomb with double reflection
  • FIG 10 shows a graph of maximum ghost intensity due to double reflections in the PMO.
  • FIG 11 illustrates a table of polarimetric error budget for a spectral polarization modulator.
  • FIG 1 illustrates a plan view of light beams (La,Lb,Lc,Ld,Le) traversing a polarization modulation optics (PMO) 200.
  • PMO polarization modulation optics
  • the PMO comprises an (achromatic) quarter- wave retarder 10.
  • the quarter- wave retarder (10) having its ordinary and extraordinary axes in the direction of Stokes parameter ⁇ Q, has a first set of optical interfaces 11-14 configured to receive a first beam La of light in a wavelength range L, and output the light as a second beam Lb of light, wherein a Stokes U component of the light in the first beam La is transformed into a Stokes V component of the light in the second beam Lb.
  • the polarization is substantially independent of its wavelength l in the operating range L, e.g. visible wavelength range (385 - 770 nm) as described herein, or other wavelength range.
  • the PMO comprises an (athermal) multiple-order retarder 20 having its ordinary and extraordinary axes in the Stokes ⁇ U direction, with a second set of optical interfaces 21-24 configured to receive the second beam Lb and output a third beam Lc for which the polarization state of the light is projected onto the spectrum by means of spectral modulation of the polarization state (from linear over elliptical to circular, and back over elliptical to linear).
  • the PMO comprises a polarizing beam splitter 30 with polarization directions in the Stokes ⁇ Q directions, with a third set of optical interfaces 31-34 configured to receive the third beam Lc and output a fourth, spectrally intensity modulated beam Ld with a first polarization TM (p-polarized), and a fifth beam Le with a perpendicular second polarization TE (s-polarized).
  • the spectral polarization modulator may transform incident light into two spectrally modulated intensities, such that amplitude and phase of the modulation are proportional to the degree and angle of linear polarization, respectively.
  • the Stokes ⁇ Q parameter is parallel/perpendicular to the Z axis
  • the Stokes ⁇ U parameter is at a ⁇ 45 degree angle with the Z axis.
  • a slit plate 50 disposed in the third beam Lc.
  • the slit plate 50 forms at least one physical slit 51 having its slit length direction Z parallel to each of the said optical interfaces 11-14, 21-24, 31-34 of the optical components. So each optical interface that interacts with the light is in a respective plane parallel to the slit.
  • An optical interface of an optical component is generally understood as a plane or surface forming a common boundary between two optical media.
  • the optical interface may be formed at an outer surface of the component, or at an internal plane e.g. in case of two interconnected monolithic parts forming the component.
  • an optical interface as used herein may refer to an interface having an optical function in the system, e.g.
  • the optical interface is arranged in the beam path.
  • the optical interface has a function in the optical system of reflecting, transmitting, and/or refracting a light beam impinging the interface.
  • the optical interface may thus be distinguished from possibly other component surfaces or boundaries which do not have a specific optical function.
  • a non-optical surface has no function in the optical design of the system e.g. wherein the intended beam path according to the optical function of the design does not traverse the non-optical surface.
  • the beam entering the PMO is telecentric with angles of incidence on the transmissive optical components relatively small.
  • This may be expressed e.g. by the half angle a/2 between the central optical axis and rays of the beam, as shown.
  • this may be expressed as the opening angle a of the light beam at the physical slit 51. It may be noted that the angle a may relate to a deviation from the intended angle of incidence at the various optical interfaces.
  • an opening angle a of the third beam Lc between chief rays at the physical slit 51 is less than six degrees, more preferably less than four degrees, e.g. between one degree and two degrees. It will be appreciated that the effect of angular deviation can be alleviated by the advantageous selection of specific optical component, e.g. as shown for the Mooney rhomb in FIG 9B (middle, right). In any case it is preferably that the largest angle made by any ray is kept as small as possible while still providing a sufficient field of view.
  • the quarter-wave retarder 10 comprises a first set of optical interfaces including one or more transmitting interfaces, i.e. where the light beam is transmitted through the respective interface, such as the input port 11 and/or output port 12.
  • the transmitting interfaces 11,14 are provided with respective anti-reflection coatings 11a, 14a.
  • the interface of the input port 11 is substantially normal to the direction Y of the first beam La.
  • the interface of the exit port 14 is substantially normal to the direction of the second beam Lb.
  • the quarter-wave retarder 10 comprises a first set of optical interfaces including one or more reflecting interfaces, i.e. where the beam is partially or fully reflected off the
  • the reflection may affect a polarization state of the light such as the phase changing internal reflecting interfaces 12,13 shown here.
  • each reflection may introduce a relative phase retardance between the two polarization directions to achieve a total of ninety degree or quarter wave retardance.
  • the reflecting interfaces are provided with phase changing coatings 12a, 13a.
  • relative phase change is achieved by consecutive reflection off two non-parallel interfaces.
  • the reflecting interfaces are arranged such that the beam rotates twice in the same direction. In this way misalignment of the reflection on the first interface may be partly compensated by the reflection on the second interface.
  • the reflecting interfaces may have a relative (top) angle QT between forty and hundred-forty degrees (plane angle), more preferably between seventy and hundred-ten degrees, e.g. hundred-five degrees, as in the embodiment shown.
  • providing the top angle QT close to ninety degrees may provide back reflecting behavior which may increase compactness. Having QT slightly above ninety degrees may allow easier placement of components so they may be clear of the incoming beam.
  • the quarter-wave retarder 10 comprises a retarder based on total internal reflection (TIR), more specifically a monolithic piece of material wherein the polarization of the light is altered by total internal reflection in the material at one or more of its (internal) optical interfaces 12,13.
  • TIR total internal reflection
  • the achromatic quarter- wave retarder 10 is formed by a Mooney rhomb.
  • the monolithic piece of material comprises fused silica (Si02).
  • fused silica is a favorable material because of its low amount of
  • the internal reflection interfaces 12,13 of the Mooney rhomb are provided with a phase changing coating 12a, 13a.
  • the phase changing coating on e.g. a fused silica Mooney rhomb may yield more equal phase change over a wider wavelength band.
  • the coating may compensate dispersion in the monolithic piece of material.
  • a single layer phase change coating on Fresnel rhomb may thus improve achromatic performance.
  • the monolithic piece is provided with multiple layers of different coating materials to even further improve achromatic performance.
  • the monolithic pieces may be provided with two, three, four, five, or different coating layers.
  • two different material may be used in
  • One of the coating materials may be the same as the monolithic material.
  • possible coating materials on a fused silica rhomb may include Si02 and MgF2. Also other or additional materials may be used.
  • the achromatic quarter-wave retarder comprises a Mooney rhomb of fused silica with four coating layers having the following layer thicknesses and material compositions in sequence from the rhomb outward: 20 nm MgF2, 110 nm Si02, 33 nm MgF2, 200 nm Si02.
  • the inventors find this combination of layers provides particular good achromatic performance over the desired visible wavelength range as illustrated in FIG 9A.
  • the layer thickness e.g. plus-minus five percent deviation, preferably less than one percent. Such deviation may lead to less ideal but still acceptable performance depending on the application. While the inventors find that Si02 and MgF2 are particularly suitable for the present applications due to their high optical qualities as well as precise
  • the multiple -order retarder 20 is implemented as an athermal combination of a MgF 2 crystal with a first thickness Tl, and crystal quartz with a second thickness T2.
  • the inventors find these materials have relatively low birefringence so the costal thicknesses T1,T2 (along the beam direction) can be on the order of a millimeter making it relatively robust and easy to handle. At the same time, the inventors find that this subtractive
  • the first thickness Tl is more than one millimeter and a ratio of the first thickness divided by the second thickness T1/T2 is between one and half and three.
  • a thickness ratio T1/T2 of 2.25 can be chosen to have the athermal point in a desired visible wavelength slightly below 500 nm.
  • the multiple-order retarder 20 is oriented with its ordinary and
  • the polarizing beam splitter 30 is formed by an internal optical interface 32 inside a monolithic component.
  • the third set of optical interfaces 31-34 of the polarizing beam splitter 30 comprise one entry interface 31 and two exit interfaces 33,34, said interfaces arranged normal to the third beam Lc, fourth beam Ld, and fifth beam Le, respectively, wherein all non-optical interfaces such as the interface 35 and the (non-indicated) top and bottom interfaces of the polarizing beam splitter 30 are non-parallel to any of the optical interfaces 31-34.
  • This configuration may alleviate undesired stray reflections.
  • the polarizing beam splitter 30 comprises a pair of optically bonded fused silica prisms forming an internal optical interface 32 there between at an angle QB of fifty-seven degrees with respect to the entry interface 31, wherein the first exit interface 33 is parallel with the entry interface, and the second exit interface 34 is at an angle QE of sixty-six degrees with respect to the entry interface.
  • the polarizing beam splitter 30 comprises a pair of wire grid polarizers 36,37 aligned along the polarization directions outputted by the monolithic component. This may further improve polarization distinction.
  • the optical interfaces of the wire grid polarizers 36,37 are also parallel to the slit length direction Z, most preferably parallel to the corresponding exit optical interfaces 34,33 of the polarizing beam splitter 30.
  • FIG 2 illustrates a perspective view of the polarization
  • the figure illustrates a preferred embodiment, wherein the slit plate 50 has multiple sub slits as will be discussed later with reference to FIG 8A. Also it is illustrated that the slit plate may have a trapezoidal cross-section which is found advantageous in mounting the slit plate in a frame.
  • the spectrometer entrance slit is placed inside the PMO, just before the polarizing beam splitter. This allows for a very compact design of the combination of telescope PMO and spectrometer. Compactness is key for space applications because of the launch cost but compactness is also favorable for thermal stability.
  • FIG 3 illustrates an exploded perspective view of the polarization modulation optics 200 and corresponding frame 60 for mounting the various components.
  • the optical components 10,20,30 of the polarization modulation optics 200 are adhesively bonded to a monolithic frame 60.
  • the monolithic frame 60 comprises a titanium-aluminum alloy such as TiAlV6.
  • the monolithic frame is made using milling in combination with wire erosion. The inventors find this may enable micrometer accuracy of placement of optical parts. Angular tolerance of positioning of the flat optics is assured by adhesive bonding of the optical surface to the frame.
  • the optical components 10,20,30 are disposed in respective cavities 10a, 20a, 30a of the monolithic frame 60.
  • the respective cavities comprise a plurality of abutment or alignment surfaces (not indicated).
  • the optical interfaces of the optical components 10,20,30 can be bonded to the respective alignment surfaces.
  • the alignment surfaces comprise beam passages for passing the light beams between the respective components.
  • the passages may allow the first light beam La to pass into a first cavity 10a of the frame comprising the quarter-wave retarder 10.
  • the passages may allow to pass the second beam to a second cavity 20a comprising the multiple -order retarder 20.
  • the passages may allow to pass the third beam Lc to a third cavity 30a comprising the polarizing beam splitter 30.
  • the passages may allow to pass the fourth beam Ld and fifth beam Le out of the monolithic frame 60.
  • the optical parts are adhesively bonded, e.g. using 3M Scotch-Weld Epoxy Adhesive EC2216 applied with a syringe dispenser through dedicated holes that are made in the frame in open connection to the respective alignment surfaces, as shown.
  • the trapezoidal cross-section of the slit plate 50 is pushed in place using a blade spring (not shown).
  • the blade spring may ensure accurate and reproducible positioning. No gluing is needed. Post alignment in the long direction of the slit is easily achieved. Transverse adjustment is possible with shims.
  • no screws are employed. Screws need locking with glue and may provide irreproducible stress.
  • the monolithic frame 60 forms a plurality of leaf springs 61. This may improve robustness, e.g. during launch.
  • the polarization modulation optics preferably has mechanical mounting that is compatible with both the vibration and shock loads of a rocket launcher and the thermal environment in space.
  • a PMO concept has been designed and tested in which all optical components of the PMO are adhesively bonded into a monolithic titanium (TiA16V4) housing.
  • This housing is specifically designed to allow the subsequent adhesive bonding of the components to angles within 0.2 degrees with respect to the surface normal as designed .
  • Titanium was chosen for the housing material as this best fits the CTE mismatches with the three optical materials used: fused silica, crystal quartz and MgF2.
  • 3M Scotch-Weld Epoxy Adhesive EC2216 was chosen as adhesive, because of extensive heritage with this material for space use. Bond spots were sized to be both strong and flexible enough to hold the optical components in place under the required thermal and vibration conditions.
  • the titanium slit plate was mounted using a leaf spring that is sufficiently strong to prevent slip thereby maintaining the precise alignment
  • the breadboard was first subjected to thermal cycling in ambient conditions at proto-flight level (- 30°C up to +40°C) and was subsequently exposed to both sine and random vibration sweeps of increasing load level along three orthogonal axes (the random loads were applied for one minute per axis up to 14 g rms). The assembly was then post-vibe thermally tested at more severe temperatures (10°C beyond survival; -40°C up to +60°C; ambient).
  • This PMO breadboard was tested by means of a simple optical setup in which a light beam originating from a fiber coupled quartz-tungsten-halogen white light source is linearly polarized by means of a wire-grid polarizer and is then sent through the PMO components. The beam emerging from the unit is subsequently collected and analyzed by means of a fiber coupled miniature spectrometer, to register the spectral modulation pattern. This pattern was found to be in good agreement with the spectrum calculated based on the crystal retardances and thicknesses.
  • FIG 4 illustrates a perspective view of the polarization modulation optics 200 with combining optics 70.
  • the optical system further comprises combining optics 70 for receiving the fourth beam Ld with the first polarization TM (p-polarized) and receiving the fifth beam Le with the second polarization TE (s- polarized), and outputting a combined beam Lh in a common direction.
  • the combining optics 70 comprises a set of folding mirrors 71,72 for directing the fourth beam Ld with the first polarization TM (p-polarized) and the fifth beam Le with the second polarization TE (s-polarized) towards a roof mirror 73.
  • the roof mirror 73 is configured to receive the respective beams Lf from the folding mirrors (or directly from the PMO), and direct the beams Lg towards a combining mirror which sends them as the combined beam Lh to a next stage in the optical system, e.g. spectrometer.
  • FIG 5A schematically illustrates a satellite comprising a multi- angle spectropolarimeter 1000.
  • FIG 5B schematically illustrates a side view of an embodiment for a multi-angle imager 100, e.g. as part of the
  • FIG 6A schematically illustrates a different side view of the multi-angle imager 100.
  • FIG 6B schematically illustrates a perspective view of the multi-angle imager 100.
  • the index“i” as in“Si”,“Vi”,“cd”,“Li”,“Mil”, “Mi2” may be interpreted as indicating the number of the respective field of view, angle or imaging branch in the multi-angle imager 100.
  • the index has values 1,2, 3, 4, 5.
  • a second index“j” as in“Mij” may be used to differentiate the primary mirror (Mil) or secondary mirror (Mi2) in the respective imaging branches. There may also be more or fewer mirrors per imaging branch.
  • the multi-angle imager 100 comprises at least one entrance pupil A1 configured to pass through light beams Li of an object P0 to be imaged from multiple entry angles ai into the imager 100.
  • the imager may comprise multiple entrance pupils, e.g. one for each imaging branch.
  • multiple apertures can be used instead of a single aperture to minimize the angle of incidence on the Mi1 and Mi2 mirrors.
  • the entrance pupil can be described as the optical image of the physical aperture stop, as 'seen' through the front of the imaging system. It is typically, located at the vertex of the imager's angle of view.
  • an imaging array Mij is configured to receive the light beams Li via the one or more entrance pupils A1, e.g.
  • the imaging array Mij is configured to image subsections Si of the object P0 according to the distinct fields of view Vi onto an imaging plane.
  • an imaging plane P2 can be at the surface of a detector (not shown here).
  • an (intermediate) imaging plane Ps can be at a slit plane (not shown here).
  • an (intermediate) image plane of the imaging array Mij coincides with a slit 51 that is disposed between optical elements of the polarization modulation optics 200, e.g. at a slit plate 50 between the optical elements 20,30, as shown in the embodiments of any of FIGs 1-4.
  • the imaging array Mij comprises multiple imaging branches, e.g. indicated in FIG 6B by the references M1j,M2j, etc.
  • the imaging branches are configured to form respective optical paths for the light beams through the imager 100 for imaging the respective subsections of the object P0.
  • each imaging branch comprises a distinct set of optical elements, e.g. the mirrors M11,M12, etc. for imaging branch M1j.
  • each imaging branch is configured to receive the respective light beam along the respective entry angle a1 and/or redirect the respective light beam towards the imaging plane PI (or intermediate imaging plane).
  • the light beams L1,L2 from each of the multiple imaging branches M1j,M2j are redirected to travel in a common direction“ ⁇ ” between the imaging array Mij and the imaging plane P1. More preferably, the redirected light beams travel along substantially parallel paths in the common direction“Y”. For example, the paths are parallel to within ten degrees plane angle, or less, e.g. within five degrees, or even within one degree, or less than a tenth of a degree.
  • the imaging array Mij is configured to combine the incoming light beams Li to have a common effective focal plane, e.g. at an imaging sensor or slit (not shown here).
  • the imaging branches M1j,M2j, etc. may have optical elements each having different curvatures but the branches preferably have a focal length optimized to achieve a common size on the detector for each subsection Si.
  • the imaging branch for each angle may have its own distinct focal length in order to achieve common magnification of each strip on the detector
  • the plurality of beams can be effectively treated as a single extended beam.
  • This has the advantage that further optical elements in an optical path after the imaging array Mij can be combined or integrated.
  • a single polarization modulation optics (not shown here) can be used to modulate the multiple beams
  • a single grating (not shown here) can be used to spectrally resolve the multiple beams
  • one mirror or lens combination (not shown here) can be used to collimate or focus the multiple beams
  • single spectral filter single beam splitter, et cetera.
  • the light beams, e.g. L1,L2 from each of the multiple imaging branches M1j,M2j, etc. are stacked in a line or row along a direction“Z” transverse to their direction of travel“Y”.
  • the multiple imaging branches M1j,M2j, etc. are configured to stack their respective light beams one above the other along a direction“Z” transverse to a plane (X,Y) spanned by the multiple entry angles ai.
  • each imaging branch M1j has its own set of at least two optical elements M11,M12, etc.
  • the optical elements are distinct from any other imaging branch M2j.
  • each imaging branch M1j,M2j, etc. has a distinct primary mirror M11,M21, etc.
  • the primary mirrors Mil are distributed to receive the light beams Li from the one or more entrance pupils Al at a range of multiple entry angles ai.
  • the primary mirrors Mil have a concave reflecting surface to at least partially focus the reflected beams.
  • each imaging branch M1j,M2j, etc. has a distinct secondary mirror M12,M22, etc.
  • the secondary mirrors Mi2 are stacked in a row to receive the light beams Li reflected from the primary mirrors Mil and reflect parallel light beams in a common direction.
  • the secondary mirrors Mi2 have a concave reflecting surface e.g. to at least partially collimate or focus the reflected beams.
  • the secondary mirrors Mi2 can be convex in some designs, e.g. with three mirrors per imaging branch.
  • the directions of the primary, secondary, and optionally further mirrors of each of the imaging branches are distinctly oriented to provide a common direction of the light beams exiting the imaging array Mij.
  • the primary, secondary , and optionally further mirrors of each of the imaging branches are distinctly curved to provide a common back focal length for the different imaging branches.
  • this may provide a common imaging plane for each of the imaging branches at a common slit plane, e.g. in the PMO.
  • the corresponding mirrors for different imaging branches can have different radii of curvature but as a set provide the same effective back focal length.
  • a common back focal length can be achieved by adjusting mirror separation of the mirrors and/or their radius of curvature.
  • off-axis mirrors are used; alternatively, or in addition, also on-axis mirrors may be used.
  • the mirror surfaces can be described by a biconic design.
  • the biconic design can e.g. be arranged to provide a desired anamorphic ratio.
  • a pupil stop can be positioned at any (intermediate) focal point of the telescope system.
  • the optical elements adjacent imaging branches M1j,M2j, etc. are preferably distinct.
  • corresponding optical elements of different imaging branches M1j,M2j, etc. are separate from each other, e.g. spaced apart.
  • each optical surface provides a distinct optical function.
  • each optical surface has its own geometric definition.
  • the optical elements may be interconnected but with an optical discontinuity between the optical surfaces.
  • the stack of secondary mirrors can be embodied as a monolithic element with stepped reflective surfaces (not shown).
  • the combination of a concave primary mirror and concave secondary mirror may form a so-called Gregorian telescope.
  • each imaging branch can have a distinct (off-axis) Gregorian telescope design.
  • the combination of a concave primary mirror and convex secondary mirror may form a so-called Cassegrain telescope.
  • each imaging branch can have a distinct off-axis Cassegrain telescope design.
  • the same optical functionality as described herein can also be provided by more than two optical elements per imaging branch, e.g.
  • mirrors though at the cost of extra weight. Also other combinations of convex/concave elements can be used to provide similar optical results. Alternative, or in addition to mirrors, also lenses can be used. While the current embodiment shows five imaging branches, there can also be more or less, e.g. two, three, four, five, six, seven, eight, nine, or more.
  • FIG 7 schematically illustrates an embodiment of a multi- angle spectropolarimeter 1000.
  • the multi- angle spectropolarimeter 1000 e.g. as shown the
  • spectropolarimeter 1000 comprising a multi-angle imager 100, e.g. as described with reference to FIGs 5B,6A,6B.
  • a multi-angle imager 100 e.g. as described with reference to FIGs 5B,6A,6B.
  • the spectropolarimeter 1000 comprises a polarization modulation optics 200, e.g. as described with reference to FIGs 1-4.
  • the spectropolarimeter 1000 comprises a polarization modulation optics 200, e.g. as described with reference to FIGs 1-4.
  • the polarization modulation optics 200 e.g. as described with reference to FIGs 1-4.
  • spectropolarimeter 1000 comprises a spectrometer 300 which may include a spectral resolving element 310 and/or imaging sensor 320.
  • the optical elements M11,M12, etc. in each imaging branch M1j are configured to focus their respective light beam L1 onto a slit 51.
  • the multiple imaging branches M1j,M2j, etc. are configured to project respective images of the respective subsections S1,S2, etc. onto the slit 51.
  • the images are extended along a length direction“Z” of the slit 51 to provide spatial information of the respective subsections S1,S2, etc. at least along said length direction“Z”, while the width direction of the slit 51 can be used for e.g. subsequent spectral and/or polarization analysis.
  • the direction“Z” of the slit 51 is parallel with each of the optical interfaces of the polarization modulation optics 200, as described herein.
  • the subsections S1,S2, etc. of the object P0 to be imaged have a length that is greater than their width by at least a factor two, three, five, or more, e.g. a factor ten.
  • the system can be modified by adding (folding) mirrors, to relocate the aperture, slit or lens set. Possibly, this may further compactify the system.
  • the imager 100 is combined with polarization modulation optics 200 to form a multi-angle spectro- polarimeter.
  • the spectropolarimeter comprises a single integrated polarization modulator optics (PMO), preferably disposed in a plurality of light beams Li after the imaging array Mij.
  • the light beams can be stacked parallel and imaged at the slit 51.
  • the slit 51 is arranged between optical elements of the polarization modulation optics 200, most preferably between the multiple-order retarder 20 and polarizing beam splitter 30, as described herein.
  • the imager 100 is combined with a single spectrally resolving element 310 and/or imaging sensor 320 to form a multi- angle spectrometer 300.
  • the multi-angle spectrometer 300 has as a single grating or prism configured to resolve the plurality of light beams Li traveling in parallel and/or having common degree of collimation.
  • the spectrally resolved light beams Li are imaged onto a single detector surface, e.g. pixel array. While not all depicted in the schematic figure, typically a spectrometer comprises the following elements: collimator, spectrally resolving element, objective; these are typically followed by a single detector surface, e.g. pixel array.
  • the spectropolarimeter 1000 comprises one or more of the polarization modulation optics 200 as described herein, an imager 100, configured to project a distant object P0 onto the physical slit 51 in the polarization modulation optics 200; and a
  • spectrometer 300 with its entrance slit formed by the physical slit 51 in the polarization modulation optics 200.
  • FIG 8A illustrates an image of different view angles imaged at a slit plane Ps onto a slit plate 50.
  • the physical slit 50 comprises a number of distinct sub slits arranged in a sequence along the slit length direction Z. This may allow to use the same PMO for different view angles.
  • the physical slit 50 has an absorbing black coating between the openings. The inventors find by analysis that
  • reflections on the slit plate may have secondary reflections on other surfaces inside the PMO and may end up on the detector.
  • a black coating is added to the Titanium slit plate. The black coating may be compatible with space use.
  • FIG 8B illustrate an image of polarization modulated spectra S1- S5; SP-S5’ projected at an imaging sensor 320.
  • the spectra S1-S5 and S1- S5’ may correspond to the different view angles of the multi - angle imager 100.
  • the spectropolarimeter 1000 is configured to project sets of polarization modulated spectra S1-S5; SP-S5’ at an imaging sensor of the spectrometer 300.
  • the projection comprises a first set of spectra S1-S5 corresponding to the first polarization TM (p-polarized) and a second set of spectra corresponding to the second polarization TE (s-polarized).
  • the sets of spectra are projected along a spatial resolving direction Z’ corresponding to the slit direction Z, and projected along a spectral direction corresponding to their respective wavelengths l.
  • the spectral polarization modulator can be used to transform incident light into two spectrally modulated intensities, such that amplitude and phase of the modulation are proportional to the degree and angle of linear polarization respectively.
  • the spectral polarization modulator comprises an achromatic quarter- wave retarder, an athermal multiple -order retarder, and a polarizing beam splitter that each have a dedicated function.
  • the quarter- wave retarder having its ordinary and extraordinary axes in the direction of Stokes parameter ⁇ Q, transforms e.g. linearly (Stokes U) polarized light into circularly (Stokes V) polarized light (operation 1).
  • the multiple-order retarder having its ordinary and extraordinary axes in the Stokes ⁇ U direction, projects the polarization state of the light onto the spectrum by means of spectral modulation of the polarization state (from linear over elliptical to circular, and back over elliptical to linear (operation 2).
  • the polarizing beam splitter having polarization directions in the Stokes ⁇ Q directions, projects the polarization modulated spectrum onto a set of spectrally intensity modulated TM (p) and TE (s) polarized spectra
  • a polarimetric uncertainty budget has been composed for each of these three components, see FIG 11.
  • the PMO is placed in a telecentric beam.
  • the creation of instrumental polarization is found to be of the order of 10 -4 only and can be neglected.
  • the uncertainty budget therefore contains sources that result in a deviation from unity of the modulation efficiency, also called the polarimetric scaling parameter.
  • the polarimetric scaling parameter usually depends on the wavelength, temperature, angle of linear polarization, and time.
  • the correction accuracy has been assumed 90% for the on-ground calibration, 80% for the in-orbit vicarious calibration, and 50% when no dedicated vicarious calibration measurement can be defined to monitor the parameter and indirect data has to be used.
  • preferred properties of the PMO may be as follows:
  • polarimetric error budget has been used to derive preferred characteristics of the individual components and can be summarized by the following preferred embodiments of the various components, preferably used in combination for achieving the preferred properties of the PMO.
  • the quarter-wave retarder has one or more, preferably all of the following characteristics:
  • AR Anti Reflection
  • a QWR is manufactured from fused silica that has good transmission properties from the UV up to the IR wavelengths. This glass is compatible with the space radiation environment.
  • the QWR is designed for an angle of total internal reflection of 52.5 degrees. Therefore the top angle is 105 degrees. The angle between the entrance and exit surfaces and the surface of total internal reflection is 150 degrees.
  • a fused silica Mooney rhomb-type with a broadband phase changing coating is used as the QWR.
  • phase change coating can be used to increase the phase change to 45° with a single TIR at a nominal angle of incidence (or corresponding Mooney rhomb wedge angle) that is close the maximum phase change angle of incidence without coating, see FIG 9B (left). Therefore, the Mooney rhomb is much more insensitive to deviations from normal incidence on the entrance surface compared to e.g. an uncoated BK17 Fresnel rhomb.
  • FIG 9B (middle) shows the effect of deviation in the direction“Dq” perpendicular to the TIR-plane as indicated in FIG 1.
  • FIG 9B In the other direction“DF”, FIG 9B (right), the sensitivity to the angle of incidence is a factor two smaller compared to a BK7 Fresnel rhomb.
  • the grey shading in FIGs 9B, middle and right indicates the phase change from the nominal value caused by a deviation of plus-minus two degrees in either the“q” or“F” or direction.
  • the multiple-order retarder has one or more, preferably all of the following characteristics:
  • a multiple -order retarder can be made of any birefringent material.
  • the desire for a relatively low total retardance (e.g. less than 14 micron) and the desire for a low temperature sensitivity may limit the available materials.
  • the birefringence of the material is preferably low enough such that the crystal thickness can be of the order of a millimeter, which may simplify handling and mounting, and preferably has sufficient mechanical strength.
  • the inventors find that two crystals that can be well manufactured with high dimensional accuracy and have extremely predictable optical properties are MgF 2 and SiO 2.
  • a thickness ratio of 2.25 may be chosen in order to have the athermal point slightly below 500 nm.
  • a different combination of bi-refringent crystals may be used (e.g. MgF2 and sapphire) with a different thickness ratio such that the athermal point is located slightly below 500 nm.
  • the total retardance of the multiple-order retarder may be chosen much higher or much lower than 14 microns, e.g. in applications that require a much higher or much lower polarimetric spectral resolution.
  • the polarizing beam splitter has one or more, preferably all of the following characteristics:
  • the polarizing beam splitter is based on an off-the-shelf beam-splitter cube based on optically bonded fused silica prisms and a dedicated polarizing multilayer coating.
  • the inventors have customized the angle between the surface normal of the entrance- and exit surfaces and the normal to the internal beam splitting surface to reduce the angles of incidence on the entrance and exit ports, e.g. see FIG 1.
  • the inventors have truncated the unused fourth surface (37) so that it is not parallel to one of the exit ports (33,34), or the entry port (31). This may be useful e.g. to reduce reflections.
  • a fused silica beam splitter was chosen for its large transmission in the desired wavelength range, its resilience to space radiation and low dispersion. Also, a cube or cube-like polarizing
  • beamsplitter has the advantage that the splitting angle does not depend on wavelength, as is e.g. the case with a Wollaston type of beam splitter.
  • the relatively low extinction ratio compared to crystal based beam splitters can be mitigated by applying dielectric coatings at the beam splitting surface and using an additional pair of wire-grid polarizers.
  • the polarizers (e.g. following the PBS) have one or more, preferably all of the following characteristics:
  • two orthogonally oriented polarizers (36,37) can be placed after the corresponding exit windows of the polarizing beam splitter to further purify the polarization state.
  • the polarizers comprise fused silica wire-grid polarizers.
  • the wire-grid polarizers preferably have a contrast ratio of >650, and perform within specifications up to an angle of incidence of ⁇ 20°.
  • the SPEX Polarization Modulation Optics (PMO) module may be constructed such as shown e.g. in FIG 1.
  • the optical train comprises an achromatic quarter- wave retarder that is preferably implemented as a Mooney rhomb (10), a multiple -order retarder (20) that is preferably implemented as an athermal combination of MgF2 (21, 22) and crystal quartz (23, 24), and a polarizing beam splitter (30).
  • the polarizing beam splitter (30) is based on beam-splitter cube, that has been customized to reduce the angles of incidence on the entrance port 31 and exit ports (33,34). In combination with a set of wire-grid polarizers (36,37) the desired polarization purity >1000 may achieved.
  • the peak ghost intensity is smaller than 0.3%, while the spectrally averaged ghost intensity is of the order of 0.1%.
  • the actual intensity will be lower since most of the ghosts will be slightly defocused at the focal plane, reducing the intensity. Since all reflections occur close to an image plane (the slit), all ghosts will form close to the nominal image, preventing (ghost) stray light from one part of the image to another part relative far away. This is especially important since this prevents cross-talk between images from different telescopes and limits the impact of bright areas in an image on dark areas in the same image.

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  • Physics & Mathematics (AREA)
  • Spectroscopy & Molecular Physics (AREA)
  • General Physics & Mathematics (AREA)
  • Polarising Elements (AREA)

Abstract

L'invention concerne une optique de modulation de polarisation (200) comprenant un retardateur quart d'onde achromatique (10) conçu pour recevoir un premier faisceau (La) de lumière et pour émettre la lumière en tant que deuxième faisceau (Lb). Un retardateur d'ordre multiple athermique (20) est conçu pour recevoir le deuxième faisceau (Lb) et pour émettre un troisième faisceau (Lc) pour lequel l'état de polarisation de la lumière est projeté sur le spectre. Un diviseur de faisceau polarisant (30) est conçu pour recevoir le troisième faisceau (Lc) et pour émettre un quatrième faisceau (Ld) ayant une première polarisation (TM), et un cinquième faisceau (Le) ayant une seconde polarisation perpendiculaire (TE). Une plaque fendue (50) est disposée dans le troisième faisceau (Lc). La plaque fendue (50) forme au moins une fente physique (51) dont la direction de longueur de fente (Z) est parallèle à chacune des interfaces optiques (11-14, 21-24, 31-34) des composants.
PCT/NL2020/050034 2019-01-23 2020-01-23 Optique de modulation de polarisation Ceased WO2020153843A1 (fr)

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