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Abstract
We demonstrate a new design high-contrast optics suitable for use on a stray-light reduction. Two-dimensional distribution of diffracted light was numerically analyzed and confirmed by a laboratory measurement to determine its characteristics. Laboratory measurements confirm stray-light performance with ${\sim}{{10}^{- 8}}$ rejection viewing as close as 1.0° to a bright-light source. The present improvement replaces the conventional normal-edge vane by an edge with microscopic Gaussian-shaped structures. This permits simplified fabrication, provides even more weight reduction, and delivers improved performance for a given size.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. INTRODUCTION
It is believed that Mars once hosted a habitable environment and lost it in the course of its evolution. The cumulative effect of the atmospheric erosion to the external forcing is regarded as one of the plausible candidates for the drastic climate change from a warm and wet to cold and dry environment. The solar wind-induced cold ion outflow from the ionosphere is identified as having a potentially significant contribution to total atmospheric escape from Mars [1], while it involves substantial uncertainties according to previous measurements and theoretical studies [2–4].
Ionospheric flows driven by momentum transfer from the solar wind to the ionosphere are considered as one of the important drivers of cold ion outflows from Mars. To eventually pull out the molecular ions to interplanetary space, the momentum transfer should reach to the lower ionosphere, where molecular ions are abundant. The outflow rate of ions thus depends largely on how deeply into the ionosphere the solar wind momentum and magnetic field penetration can reach. It is necessary to measure the spatial distribution of the momentum transfer and particle acceleration regions for quantitative evaluation. The viscous-type interaction between the shocked solar wind and the ionospheric plasma has been considered as one of the plausible candidate mechanisms to cause the penetration [5–7]. Terada et al. [5] pointed out that the interplanetary magnetic field (IMF) orientation could control the development of the Kelvin–Helmholtz instability (KHI) and cause large undulations of the ionopause. However, a lack of measurements prevents us from clarifying the mechanism of the formation of the instabilities as well as the penetration of the magnetosheath plasma. Thus, a breakthrough in addressing ionospheric flows will be achieved with ionospheric imaging of spatial structures from Martian orbit.
The goals of the Mars Aqueous-environment and space Climate Orbiter (MACO) mission are to understand necessary conditions for sustainability of habitable environments of terrestrial planets in the context of space weather and climate and to acquire necessary technology and knowledge for future landing on and human exploration to Mars. The MACO proposes to measure the global two-dimensional imaging of the Martian ionopause, where atmospheric escape is triggered, by a combination of a UV–visible (UV–VIS) camera system and a high-contrast baffle to reduce a strong sunlight reflected from the Mars surface. Although the detailed mission design of the MACO has not been decided yet, the instrumental requirement for the Martian ionospheric imager has been discussed.
The ionopause, which is the boundary between the draping magnetosheath plasma (induced magnetosphere) and ionosphere, is usually located above 300 km altitude [8]. After more than 10 years of observations by the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) instrument onboard the Mars Express orbiter, Chu et al. [9] find that the ionopause is located on average at an altitude ${363}\pm{65}\;{\rm km}$, which has a weak dependence on solar zenith angle and varies with the solar extreme ultraviolet flux on annual and solar cycle time scales. This region is determined by complicated solar wind interaction and electromagnetic fields [4,10,11]. Ionospheric composition at this altitude range has been measured by the Viking probes [12] and the Neutral Gas and Ion Mass Spectrometer (NGIMS) instrument onboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft [13–15]. ${\rm O}_2^ +$ and ${{\rm O }^ +}$ are the dominant at above 300 km. In contrast, ${\rm CO}_2^ +$ is about one order less than ${\rm O}_2^ +$ at 300 km altitude [14].
We have chosen the wavelength region of 561 nm for ${\rm O}_2^ +$ 1 N (1-0). The visible range of 561 nm is the optimum frequency region to observe the most abundant composition of ${\rm O}_2^ +$ above 300 km. Although we have experience with Earth/Venus observations at this domain, ${\rm O}_2^ +$ 1 N (1-0) emission has never been detected in the Mars atmosphere. The prominent ${\rm CO}_2^ +$ emission feature at the Fox–Duffendack–Barker (FDB) band in the UV region is another candidate, which was first observed by Mariner 6 [16]. It is an advantage that the UV regime has lower albedo of the Martian surface to prevent ionospheric imaging. Table 1 represents the estimated photon counts and required reduction rate against the reflected sunlight from the surface at the tentative observation wavelength regions for ${\rm O}_2^ +$ 1 N (1-0). The solar flux applied here is cited from the 2000 ASTM Standard Extraterrestrial Spectrum Reference E-490-00 [17]. The albedo applied here is assumed to be 0.17 at 561 nm [18]. The Einstein coefficient is cited in Shemansky and Jones [[19]. The emission rate of solar reflectance is estimated assuming bandwidths of 0.5 nm at 561 nm as the typical number for each regime. We recognize that the difficulty in the mission to perform the Martian ionospheric imaging is the strong sunlight reflected from the Mars surface of about one 7th magnitude spread over the Martian disk in the field of view.
Table 1. Photon Counts and Required Reduction Rate at Tentative Observation Wavelengtha
The exact orbit of the MACO is still under discussion, but one of the possible orbits being discussed is a highly elliptical polar orbit with a periapsis of ${\sim}{150}\;{\rm km}$ and an apoapsis of three Martian radii. The local time is preferably fixed at the terminator to observe the cross section of Martian ionopause in a ${y} {\text -} {z}$ plane in the Mars-centered solar orbital (MSO) coordinate system. The MACO instrument will observe the ionospheric emissions from the apoapsis at three Martian radii for continuous monitoring. The angular distance between the surface and the ionopause is ${\sim}{1}^\circ$. A substantial reduction rate larger than ${{10}^7}$ is required within ${\sim}{1}^\circ$ to achieve the ionopause imaging. This is a critical point in instrument development.
High-contrast technology has been especially developed in the fields of exoplanet exploration [20–22] and heliosphere science [23–26]. For the extended light source, the earlier technique used nested vanes with each vane top lying just within the shadow of its next outer neighbor [23]. Multiple vanes build up an exponential light rejection as a function of the deflection angle. Buffington et al. [23] demonstrated that the three-edge measurement achieved the contrast of ${\sim}{{10}^{- 8}}$ at the angular distance of 1.5°. Buffington [23] presents a significant improvement of the contrast by a smooth curved surface, with ${\sim}{{10}^{- 14}}$ rejection at 3° to a bright-light source. However, our requirement for the contrast to be more than ${{10}^{- 7}}$ within 1° is still a technical challenge.
We propose to solve the mission requirement by applying our dedicated new apodization by microscopic Gaussian-shaped structure, based on the history of the coronagraph technique [22]. The performance test of the element demonstrates a substantial improvement of the contrast to satisfy the requirement [27]. Here we numerically show an influence of the two-dimensional distribution of the diffracted light by the Gaussian-shaped apodizer. The laboratory measurements confirm the prediction of the basic design.
2. DESIGN AND MANUFACTURING OF THE GAUSSIAN-SHAPED STRUCTURE
The history of development in exoplanet science is directed to this study for its concept, design, high-precision manufacturing of an optical device, and high-contrast experiments. We present a concept and laboratory demonstration for a baffle in which high contrast is produced via apodization based on diffraction optics. The apodization in this study is characterized by an edge with microscopic Gaussian-shaped structures. The Gaussian-shaped structure has a preferably scattering light parallel to the direction along the vane. As the result, this structure substantially reduces a scattering light perpendicular to the orthogonal direction. The extraction direction is here oriented perpendicular to the vane axis [27].
The applied baffle substrate design and the expanded view of the Gaussian-shaped pattern of the manufactured baffle as observed from an optical micrograph are shown in Fig. 1. We selected to produce the microscopic Gaussian-shaped pattern made of thin aluminum film on one surface of the transparent substrate, based on its feasibility. The substrate is made of fused silica with a physical diameter of 100 mm and thickness of 1 mm. Broadband anti-reflection coating for the visible wavelength region is applied for both sides of the transparent area. The requirement of the thickness of the aluminum pattern is greater than 200 nm. For a comparative study, we designed a part of the edge of the aluminum pattern with the Gaussian-shaped pattern and a part with the straight edge (i.e., without apodization). The expanded views of the Gaussian-shaped pattern of the manufactured baffle as observed from a scanning electron microscope are shown in Fig. 2. We find that the Gaussian-shaped pattern is well manufactured with the accuracy of ${\sim}{100}\;{\rm nm}$, as is designed.
Fig. 1. Baffle substrate design (left). The expanded view (corresponding to the region of the red rectangle in the left panel) of the manufactured baffle as observed from an optical microscope (right). The outer diameter is 100 mm. The thickness is 1 mm. The red dashed circle in the left panel represents the effective area. The material of the transparent area is made of fused silica with the broadband anti-reflection coating for visible wavelength region for both sides. The opaque area is made of aluminum.
Fig. 2. Expanded view of the manufactured baffle as observed from a scanning electron microscope. The upper panels represent the convex part of the Gaussian-shaped pattern. The lower panels represent the concave part. The right panels correspond to the images with the magnification of 10.
3. NUMERICAL SIMULATION
Prior to the manufacturing of a conceptual device and laboratory experiment, we performed a numerical simulation to demonstrate the concept of the high-contrast apodized baffle and to study the distribution of the diffracted light by the baffle to be manufactured using physical optics software VirtualLab Fusion. The light source of the optical system was an ideal Gaussian beam with a wavelength of 632.8 nm and a ${{1/e}^2}$ beam system of 5 mm. The light source was followed by a Gaussian-shaped apodizer or a simple straight edge that caused diffraction. The thickness of these edges was ideally zero. The calculation area of the electric field passing through these edges was 4.8 mm square and the sampling resolution was 100 nm. To suppress the influence of pseudo diffraction generated at the edge of the calculation area, the electric field strength was smoothly truncated at the edge of the calculation area. Two-dimensional angular distribution at infinity was calculated by plane-wave expansion of the electric field strength passing through these edges. Because the plane-wave expansion method is a rigorous calculation method that satisfies Maxwell’s equations, more accurate results can be obtained when compared with the Fresnel diffraction integral.
Fig. 3. Two-dimensional distribution of diffracted intensity predicted by simulations for apertures with Gaussian-edge apodizers and without apodizers.
The results of the two simulations are presented in Figs. 3 and 4. Figure 3 shows the intensity maps for the case with the simple straight edge and with the Gaussian-shaped apodizer. For the case without the Gaussian-shaped apodizer, diffracted light is directed perpendicular to the edge as normally expected. However, in the case with the Gaussian-shaped apodizer, light is diffracted is directed parallel to the edge and perpendicularly directed light is suppressed. In addition, we can see an interference fringe pattern along the vane edge at a regular interval. The fringe pattern gradually stretched perpendicular to the edge with the distance from the intercepted point.
Figure 4 shows the fractional intensity as a function of the angular distance from a bright-light source. For the comparison, the fractional intensity without apodizer is also shown as the dotted line. The Gaussian-shaped apodizer works to produce much higher contrast than the structure with a straight edge with no apodization. The predicted contrast with the Gaussian-shaped apodizer is better than ${{10}^{- 8}}$ at the angular distance of 1°, which satisfies the requirement described in the previous section. In contrast, the predicted contrast without apodizer reaches only to ${{10}^{- 5}}$. The contrast between the two is significant, about ${{10}^3}$. It is also remarkable that the predicted contrast with the Gaussian-shaped apodizer can provide almost ${{10}^{- 8}}$ at the angular distance of 0.5°. Such rapid reduction within a short angular distance is the most important characteristic of the microscopic Gaussian-shaped apodizer.
4. LABORATORY MEASUREMENT
The laboratory setup illuminated the Gaussian-shaped structure array under test with a 5 mm-wide collimated He–Ne laser beam (${\rm wavelength} = 632.8\;{\rm nm}$). Narrowing the widths of both the illuminating light beam and the aperture at the rear of the light baffle (1.0 mm) further reduced the scattered-light background. The diffracted light by the Gaussian-shaped structure was measured with a movable light detector placed 1770 mm behind the Gaussian-shaped structure array. Use of neutral-density filters and variation of the exposure time roughly doubled the four decades of intrinsic dynamic range of the detector. Here we used three neutral-density filters, as ND30, ND50, and ND80 for 0.1%, ${{10}^{- 3}}\%$, and ${{10}^{- 6}}\%$, respectively. ND00 corresponds to a direct signal without neutral filter. This setup’s limit at ${{10}^{- 8}}$ was caused by scattering (Mie from suspended dust and Rayleigh from air) from the undiffracted beam directly into the detector. Making the measurements in a high-efficiency-particle-absorbing-filtered clean room further reduced the scattered-light background. These ideas to improve the technique permitted the wide dynamic range, and reducing the effect of scattered-light background has been demonstrated thanks to previous studies [24].
Figure 5 illustrates the setup. The laser illuminated the detector aperture tangent to the vane. The laser beam was positioned so that approximately half of its beam was intercepted and scattered by the Gaussian-shaped structure. The detector module baffle disposed of unwanted scattered light near the detector aperture and blocked direct view of most of the room. Dust particles in the air were minimized by the clean room. As a comparison with no apodization, the laser was shifted along the vane edge. Exposure time and neutral-density filters placed in the detector module were adjusted to keep measurements within the detector dynamic range.
Fig. 4. Predicted fractional intensity as a function of the angular distance from a bright-light source along a horizontal line, extracted from Fig. 3. Two simulations for apertures with Gaussian-edge apodizers (solid line) and without apodizers (dotted line) are compared.
Fig. 5. Schematic plan diagram of the stray-light measurement setup within the clean room. HEPA, high-efficiency particulate air; NDs, neutral-density filters; 2-D, two-dimensional.
Figure 6 shows the diffracted intensity map for the case with the Gaussian-shaped apodizer. The diffracted light is directed parallel to the edge, and perpendicularly directed light is suppressed, as predicted by the prior simulation described in the previous section. Interestingly, the measured intensity map reproduced the interference fringe pattern along the vane edge well, as seen in the simulation. This match tells us that this fringe pattern can be purely explained by the Fresnel propagation.
Fig. 6. Diffracted light by the Gaussian-shaped structure behind the sensor module in the laboratory setup.
Figure 7 shows measured light reduction as a function of the angular distance from a bright-light source with the Gaussian-shaped apodizer. Laboratory measurements demonstrate a stray-light performance with ${\sim}{{10}^{- 8}}$ rejection viewing as close as 1° to a bright-light source. Laboratory measurements basically show a very good match with the numerical prediction of two-dimensional light map by Fresnel propagation simulation. Measurement with the Gaussian-shaped apodizer accomplished ${\sim}{{10}^{- 7}}$ contrast at the distance of 0.5°. The measured contrast at the distance of 1.0° suggests slightly above ${{10}^{- 8}}$. The discrepancy between the measurement and the prediction beyond 1° might be caused by the setup’s limit at ${{10}^{- 8}}$ by scattering (Mie from suspended dust and Rayleigh from air). We cannot explain another discrepancy caused by the bump between 0.1° and 0.2°, which might be due to the stray light from the undiffracted beam directly into the detector. The possible sources of stray light might originate from (i) a part of the edge of the aluminum pattern with the Gaussian-shaped structure array, and/or (ii) the light baffle for the aperture at the rear, illuminated by the light beam. The multi-scattered light by the curtain of the clean room could also be the source.
Fig. 7. Relative intensity of light passing through the vanes at the detector, as a function of angular deviation along a horizontal line in the diffracted light map. Measurements are normalized on this figure, with ${\rm theta} = 0$ placed at approximately the intensity of the direct laser beam, correct with (and without) the microscopic Gaussian-shaped structure. Relative intensity is measured by combination of the use of four neutral-density filters (four colored solid lines). Predicted intensity by numerical simulation is also shown for comparison (dotted line).
For comparison, Fig. 8 shows measured light reduction as a function of the angular distance from a bright-light source without the Gaussian-shaped apodizer (which means diffracted light by a normal edge). Measurement without the Gaussian-shaped apodizer shows ${\sim}{{10}^{- 4}}$ contrast at the distance of 0.5°. The measured contrast at the distance of 1.0° suggests slightly better than ${{10}^{- 4}}$. The improvement along the distance is not significant. Laboratory measurements basically demonstrate a similar trend with the numerical prediction by Fresnel propagation simulation, though the measured values are systematically larger than the predicted values by almost one order.
Fig. 8. Same as Fig. 7 except for the intensity, correct without apodizers. Relative intensity is measured by combination of the use of three neutral-density filters (three colored solid lines). Predicted intensity by numerical simulation is also shown for comparison (dotted line).
We concluded that the present improvement by Gaussian-shaped apodizer replaces the conventional normal-edge vane by ${\sim}{{10}^{- 3}}$ at 0.5° and 1.0° for each. This permits simplified fabrication, provides even more weight reduction, and delivers improved performance for a given size.
Our result first demonstrated a two-dimensional map of diffraction light by the microscopic Gaussian-shaped structures. Figures 9 and 10 show measured light reduction as a function of the angular distance from a bright-light source along vertical lines in the light map with the Gaussian-shaped apodizer at ${x} = 0.0\;{\rm mm}$ and ${x} = 5.0\;{\rm mm}$, respectively. These figures capture the repetitious peaks in the plane parallel to the vane edge well, as seen in the simulated prediction in Fig. 3. There is no explanation for such strong features. However, this should be explained by the Fresnel propagation process as predicted by the simulation. These might be likely to be interference fringe caused by the orderly located Gaussian-shaped structure. Further investigation is required to clarify the origin.
Fig. 9. Same as Fig. 7 except for a vertical line in the diffracted light map at ${x} = 0.0\;{\rm mm}$ (zero offset from the intercepted point of the laser).
Fig. 10. Same as Fig. 7 except for a vertical line in the diffracted light map at ${x} = 5.0\;{\rm mm}$ (offset from the intercepted point of the laser).
Figure 10 shows an expanded feature of the fringes along the perpendicular to the vane edge, especially at the distance from a bright-light source. Enhancement of the peak intensity of fringes is obvious at ${x} = 5.0\;{\rm mm}$ beyond 1.0°. This result indicates that the diffracted light can potentially penetrate into the unwanted area, perpendicular to the vane edge at a certain distance from the point source of the light. Caution must be taken because the sunlight reflectance by the Martian surface is assumed to be an extended light source (i.e., not a point source), which interacts with the Gaussian-shaped apodizer with more complexity.
5. SUMMARY AND CONCLUSIONS
We have demonstrated a high microscopic Gaussian-shaped structure contrast optics, which is developed by using an edge with microscopic Gaussian-shaped structure. We have numerically investigated the influence of the two-dimensional distribution of the diffracted light. The laboratory measurements confirmed that the performance of an array of microscopic Gaussian-shaped structure provides significant improvement compared with conventional normal vane. Measurement with Gaussian-shaped apodizer accomplished ${\sim}{{10}^{- 7}}$ contrast at the distance of 0.5°. The measured contrast at the distance of 1.0° suggests slightly above ${{10}^{- 8}}$. The measured contrast suggests a significant improvement by ${\sim}{{10}^{- 3}}$. The fringe-like features have been also identified in the plane parallel to the vane with the Gaussian-shaped apodizer in both laboratory measurement and simulated prediction. Further investigation is required using an extended light source.
Funding
Institute of Space and Astronautical Science.
Acknowledgment
This work was supported by the joint research program of the Planetary Plasma and Atmospheric Research Center (PPARC), Tohoku University, and by the Payload Basic Development Program of the Institute of Space and Astronautical Science (ISAS), Japan Aerospace and Exploration Agency (JAXA).
Disclosures
The authors declare no conflicts of interest.
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