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Dimpled scalar vortex coronagraph laboratory demonstration

This paper presents laboratory demonstrations of second-generation dimpled scalar vortex coronagraphs that achieve near testbed-limited high-contrast performance across narrow and broadband spectral ranges, validating their potential as polarization-independent candidates for the Habitable Worlds Observatory.

Original authors: Niyati Desai, Garreth Ruane, Susan Redmond, Dimitri Mawet, Eugene Serabyn, Bertrand Mennesson

Published 2026-03-25

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CC BY 4.0

Original authors: Niyati Desai, Garreth Ruane, Susan Redmond, Dimitri Mawet, Eugene Serabyn, Bertrand Mennesson

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are trying to take a photo of a tiny, glowing firefly sitting right next to a blindingly bright lighthouse. If you point your camera directly at the lighthouse, the glare will wash out the entire picture, and you'll never see the firefly.

In astronomy, this is the ultimate challenge: trying to see exoplanets (the fireflies) orbiting sun-like stars (the lighthouses). The star is billions of times brighter than the planet, and they are very close together in the sky.

This paper describes a new, clever trick scientists at NASA's Jet Propulsion Laboratory (JPL) used to build a better "sunshade" for their telescopes, called a coronagraph. Here is the story of how they did it, explained simply.

The Problem: The "Rainbow" Glare

To see these planets, astronomers use a special mask in the telescope to block the star's light. One popular type of mask is called a Scalar Vortex Coronagraph.

Think of this mask like a swirling vortex or a tornado made of light. It twists the starlight so that it spirals away from the center, leaving a dark hole in the middle where the planet can be seen.

However, there was a major flaw with the old versions of these masks: Chromaticity.

The Analogy: Imagine the mask is a prism. If you shine white light (which contains all colors) through it, the mask works perfectly for one specific color (say, green). But for red light, the twist isn't quite right, and for blue light, it's twisted too much.
The Result: The "swirl" gets messy when you try to use a wide range of colors (broadband light). The starlight leaks through the gaps in the swirl, creating a foggy glare that hides the planet. This is called "chromatic leakage."

The Solution: The "Dimple"

The team realized they needed to fix the swirl so it worked for all colors at once. They came up with a new design: the Dimpled Scalar Vortex.

The Analogy: Imagine the old mask was a smooth, perfect funnel. The new mask is the same funnel, but they added a tiny dimple (a small depression) right in the very center.
How it works: This dimple acts like a "reset button" for the light. It shifts the phase of the light in the center just enough to cancel out the messy leakage that happens when different colors try to pass through. It's like tuning a guitar string; the dimple tightens the tension so the note (the light suppression) stays perfect across the whole song (the spectrum of colors).

The Experiment: The "Dark Room" Test

The scientists built these new masks out of fused silica (a type of glass) and tested them in a high-tech lab at JPL called the In-Air Coronagraph Testbed.

Think of this testbed as a simulated space environment on a table. They used a laser to mimic a star and tried to block its light to see how dark they could make the "sky" around it.

They used a smart computer algorithm called Electric Field Conjugation (EFC).

The Analogy: Imagine you are trying to smooth out ripples in a pond. You have a robotic hand (a deformable mirror) that can push and pull the water surface. The computer calculates exactly where to push to cancel out the ripples caused by the star's glare.

The Results: A Record-Breaking Success

The results were impressive:

Narrow Band (One Color): When they tested with just a tiny slice of color (2% bandwidth), the new dimpled mask blocked the starlight almost perfectly, reaching a contrast of 1 in 100 million ( $10^{-8}$ ). This is nearly as good as the testbed itself could possibly get.
Broad Band (Many Colors): This is the big win. When they opened the floodgates to use a wide range of colors (10% and even 18% bandwidth), the new mask still blocked the starlight incredibly well.
- The old masks let a lot of light leak through in these wide ranges.
- The new "dimpled" masks improved the performance by 10 times compared to the old designs.

Why This Matters for the Future

The ultimate goal is the Habitable Worlds Observatory (HWO), a future space telescope designed to find Earth-like planets. To do this, it needs to block starlight with a contrast of 1 in 10 billion ( $10^{-10}$ ).

The Takeaway: While this lab test didn't reach the final $10^{-10}$ goal yet, it proved that the "dimpled" design works. It showed that we can fix the "rainbow glare" problem without needing complex, expensive polarization filters that cut the amount of light we collect in half.
The Future: This is a massive step forward. It proves that we can build a "sunshade" that works for all colors of light, is simple to manufacture, and doesn't waste light. This brings us much closer to actually taking a picture of an Earth twin orbiting another star.

In short: The scientists added a tiny "dimple" to a light-blocking mask, which stopped the star's glare from leaking through when looking at many colors at once. This makes the mask much better at finding hidden planets, bringing us one step closer to answering the question: "Are we alone?"

1. Problem Statement

The primary science goal of NASA's Habitable Worlds Observatory (HWO) is to directly image and characterize Earth-like exoplanets. To detect biosignatures in reflected light, HWO requires a coronagraph capable of achieving a contrast of $10^{-10}$ at small angular separations ( $\sim 3\lambda/D$ ) across a broad spectral bandwidth ( $\ge 20\%$ ).

Current coronagraph designs face a trade-off:

Vector Vortex Coronagraphs (VVCs): Offer broadband performance but are polarization-sensitive. They suffer from manufacturing-induced polarization leakage, often requiring filtering that reduces throughput by 50%.
Scalar Vortex Coronagraphs (SVCs): Offer polarization-independent operation and high throughput but are inherently chromatic. The phase delay in SVCs scales with wavelength, causing "chromatic leakage" (starlight not suppressed) when operating over broad bands.

The specific challenge addressed in this work is overcoming the chromatic limitations of SVCs to achieve broadband high-contrast performance without sacrificing polarization independence or throughput.

2. Methodology

The authors developed and tested a new generation of scalar vortex masks featuring a "dimpled" topography.

Design Innovation:
- Traditional SVCs use an azimuthal phase ramp (e.g., a sawtooth profile).
- The new design integrates a central radial phase dimple (a $\pi$ phase shift in the center) into the azimuthal sawtooth vortex.
- Theoretical Basis: Based on modal decomposition, the central dimple targets the 0th-order mode (zeroth-order leakage), which represents light passing straight through the vortex without acquiring the necessary phase shift. The dimple phase-shifts roughly half the PSF energy in the central region to cancel this leakage term.
- Optimization: The dimple radius was optimized to $0.621 \lambda/D$ to maximize broadband contrast when combined with the sawtooth vortex.
Fabrication:
- Masks were fabricated on fused silica substrates ( $15 \times 15 \times 1$ mm).
- The vortex pattern was confined to a $14 \times 14$ mm central region.
- Manufacturing utilized direct laser writing and reactive ion etching (RIE).
- The central dimple diameter was measured at $\sim 68.5$ microns (target $\sim 69$ microns), well within tolerances.
- Anti-reflective coatings were applied to minimize reflection ( $<0.5\%$ ).
Experimental Setup:
- Testing was conducted on the In-Air Coronagraph Testbed (IACT) at JPL.
- The setup included a tunable broadband light source, a 7.6 mm pupil, and a single deformable mirror (DM) with 34 $\times$ 34 actuators.
- Wavefront Control: The team used Pairwise Probing (PWP) and Electric Field Conjugation (EFC) to correct wavefront errors and create "dark holes" (regions of suppressed starlight).
- Testing Protocol: Experiments were run in three spectral configurations:
  1. Narrowband: 2% bandwidth.
  2. Broadband: 10% bandwidth (using 5 subbands).
  3. Extended Broadband: 18% bandwidth (to assess chromatic stability).

3. Key Contributions

Novel Topography: First experimental validation of the "dimpled" scalar vortex design, which combines azimuthal and radial phase modulation to mitigate chromatic leakage.
Performance Benchmarking: Direct comparison of the new dimpled masks against previous-generation sawtooth scalar masks under identical laboratory conditions.
Model Validation: Demonstration that the predicted bench-limited contrast (based on simulations and testbed floor measurements) agrees with measured results within a factor of two, validating the theoretical models used for design optimization.

4. Results

The dimpled scalar vortex prototypes achieved record-breaking performance for scalar vortex masks in an air-based testbed:

Narrowband (2%):
- Achieved an average contrast of $1.00 \times 10^{-8}$ (3–10 $\lambda/D$ ).
- This represents a $\sim 2\times$ improvement over the best first-generation sawtooth masks and indicates performance near the testbed's physical limit.
Broadband (10%):
- Achieved an average contrast of $3.76 \times 10^{-8}$ .
- Compared to the previous generation ( $3.24 \times 10^{-7}$ ), this is an order of magnitude ( $\sim 10\times$ ) improvement.
Extended Broadband (18%):
- Achieved an average contrast of $5.27 \times 10^{-8}$ .
- The contrast degradation from 10% to 18% bandwidth was less than a factor of two, demonstrating excellent chromatic stability.
Comparison to Limits:
- The measured broadband contrasts were within a factor of two of the predicted "bench-limited" performance (the theoretical limit imposed by the testbed's noise floor).
- The results confirm that the dimpled design successfully mitigates chromatic leakage, allowing the system to approach the limits of the current hardware rather than being limited by the mask's chromaticity.

5. Significance

Path to HWO Requirements: These results mark a significant step toward polarization-independent coronagraphs capable of meeting HWO's stringent requirements. By solving the chromaticity issue in SVCs, the design offers a viable alternative to polarization-sensitive VVCs, potentially doubling throughput.
Ground-Based Applicability: The demonstrated contrast levels ( $\sim 10^{-8}$ ) are already sufficient for next-generation ground-based observatories like the Thirty Meter Telescope (TMT) and the Extremely Large Telescope (ELT).
Future Roadmap: The success of the dimpled design establishes a clear path for future space-based missions. The authors note that further improvements (e.g., adding a second DM for amplitude/chromatic control, vacuum testing, and multi-material approaches) are necessary to reach the ultimate $10^{-10}$ contrast goal.
Technological Maturity: This work advances the Technology Readiness Level (TRL) of scalar vortex coronagraphs, proving that topographically achromatized masks can perform near theoretical limits in realistic laboratory environments.