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Validation and extension of the PAWS Zemax model as a first step in the development of a Compact Arrayed Waveguide Stacked Multi-Object Spectrograph (CAWSMOS)

This paper outlines the development of a Compact Arrayed Waveguide Stacked Multi-Object Spectrograph (CAWSMOS) to miniaturize astronomical instruments, featuring a validated and extended Zemax model of the PAWS system that reveals specific positional accuracies and significant discrepancies in point spread function size and detector capacity.

Original authors: Fabienne Mangelsdorff, Andreas Stoll, Lars Zimmermann, Stefan Lischke, Riya M. Pattery, Kalaga Madhav, Martin M. Roth

Published 2026-08-26

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CC BY 4.0

Original authors: Fabienne Mangelsdorff, Andreas Stoll, Lars Zimmermann, Stefan Lischke, Riya M. Pattery, Kalaga Madhav, Martin M. Roth

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

Astronomers have long relied on massive instruments to split the light from distant stars into rainbows of color, revealing the chemical makeup and motion of the universe. These devices, called spectrographs, are traditionally built with large mirrors and lenses that grow in size alongside the telescopes they serve. As telescopes become larger to capture fainter light, the spectrographs attached to them risk becoming unwieldy and prohibitively expensive. To solve this, scientists are turning to a technology that shrinks these complex optical systems down to the size of a computer chip. By using tiny channels etched into glass to guide and separate light, researchers hope to build instruments that are compact, efficient, and capable of observing many objects at once. The challenge lies in proving that these microscopic components can perform as well as their giant, room-sized predecessors when stacked together to handle multiple streams of light simultaneously.

In a recent study, a team of researchers from Germany took a significant step toward this goal by testing a digital blueprint for a new type of compact spectrograph. Their work focuses on a device called PAWS, which uses a specialized chip known as an arrayed waveguide grating to disperse light from the H-band, a specific range of infrared wavelengths where the atmosphere is relatively transparent. The researchers wanted to see if they could expand this single-chip system into a multi-object instrument, capable of observing several stars or galaxies at the same time by stacking multiple chips on top of one another. To do this without building and rebuilding physical hardware repeatedly, they created a sophisticated computer simulation of the entire optical path, from the light leaving the chip to the moment it hits the detector. This digital model allowed them to predict how light would behave and to compare those predictions against real-world measurements taken from the existing PAWS instrument.

The team began by carefully aligning their computer model with the actual data collected from the PAWS spectrograph. They tracked where specific colors of light landed on the detector, comparing the simulated positions to the measured ones. They found that after adjusting for a slight shift in the setup, the computer model could predict the location of the light spots with remarkable accuracy, missing the mark by less than 28 pixels in one direction and 18 in the other. This level of precision confirmed that the digital twin was reliable for tracking where the light would go. However, when the researchers looked at the size and shape of the light spots, a major discrepancy emerged. The computer simulation predicted spots that were five times smaller than what was actually observed in the laboratory. This difference occurred because the model only accounted for the optics after the light left the chip, ignoring the imperfections and physical limitations inherent to the chip itself and the way light enters it.

Armed with a model that could accurately predict position but not size, the researchers tested the concept of stacking a second chip directly on top of the first one. In their simulation, they added a second source of light to represent the second chip and watched how the two sets of spectral lines would appear on the detector. The results showed that the two chips could indeed be used simultaneously, with the light from the second chip landing in a different vertical position on the detector, effectively creating a second layer of data. However, the simulation also revealed a physical constraint: the current detector is not large enough to capture the full range of wavelengths from both chips without cutting off the edges of the spectrum. Furthermore, because the light spots are larger in reality than the model suggested, there is a risk that the signals from the two chips could blur into one another at the extremes of the wavelength range.

The study concludes that while the digital model is a powerful tool for planning the layout of these compact instruments, it cannot yet fully predict the quality of the final image because it misses the subtle distortions introduced by the chip itself. The researchers determined that stacking two chips is physically possible with the current setup, but only partially, as the detector would need to be larger or the optical system modified to accommodate more chips in the future. This work serves as a crucial proof of concept, demonstrating that the path toward a compact, multi-object spectrograph is viable, but it also highlights the need for further physical experiments to refine the design and ensure that the stacked chips can deliver clear, distinct views of the cosmos without their signals overlapping.

Technical Summary: Validation and Extension of the PAWS Zemax Model for CAWSMOS Development

Problem Statement
The linear size of bulk optical astronomical spectrographs scales with the primary mirror diameter of the telescope. As modern telescopes increase in aperture, miniaturization becomes critical not only for multifunctional integration and photon efficiency but also for cost reduction. The Potsdam Arrayed Waveguide Spectrograph (PAWS) utilizes Arrayed Waveguide Grating (AWG) technology on a Silica-on-Silicon (SoS) platform to achieve compactness. However, to fully realize the potential of AWG technology for ground-based, airborne, and spaceborne applications, a new concept called the Compact Arrayed Waveguide Stacked Multi-Object Spectrograph (CAWSMOS) is being developed. This concept aims to stack multiple AWG chips and utilize multiple input channels to observe multiple objects simultaneously. A prerequisite for this development is a validated digital twin (Zemax model) of the existing PAWS setup to guide the design of the stacking frame, cross-dispersion optics, and detector requirements without requiring immediate physical prototyping.

Methodology
The study focuses on the validation and extension of the existing Zemax optical model of PAWS. The methodology involves three primary stages:

Data Preparation and Model Setup:
- Experimental Data: Calibration data was collected using a tunable laser source in 5 nm steps (1505 nm to 1665 nm) covering diffraction orders 29 to 32. Spot positions were determined by fitting 2D Gaussians to detector images, and traces were fitted to parabolas for each order.
- Zemax Model: The model includes all optical elements from the AWG exit slit to the detector (collimator, cryostat window, filters, lenses, mirrors, cold-stops, and cross-dispersion grating). The AWG chip serves as the primary dispersive element.
- Wavelength Mapping: To ensure physical accuracy, the model was adjusted so that specific wavelengths correspond to specific slit positions based on the grating equation and effective indices of the waveguide and slab materials, rather than launching all wavelengths from all fields.
Validation (Single Chip):
- Positional Alignment: The Zemax model's predicted spot positions were compared against measured data. A coordinate shift was applied to minimize the mean residual distance between the simulation and the experimental data.
- PSF Analysis: The Point Spread Function (PSF) size and shape were compared. The measured PSF was fitted with a 2D Gaussian to determine the Full Width Half Maximum (FWHM) and eccentricity, which were then compared to the Huygens PSF simulation in Zemax.
Model Extension (CAWSMOS Concept):
- Fiber Extension: The model was extended to include a single SMF-28 fiber butt-coupled to the chip, calculating the vertical offset based on fiber and chip dimensions.
- Second Chip Extension: A second AWG chip was modeled directly stacked on the first. The vertical separation was calculated based on the full thickness of the chip (core, cladding, buffer, and substrate). The resulting spot positions for the stacked configuration were analyzed to determine detector coverage requirements.

Key Contributions and Results

Positional Validation: The Zemax model successfully predicts the relative positions of spectral orders. After applying a global shift (−559 px in x, +308 px in y), the model aligns with measurement data within 28 pixels in the x-direction and 18 pixels in the y-direction.
- The residuals in the x-direction show no systematic deviation.
- The y-direction residuals suggest a tilt in the experimental cross-disperser not accounted for in the current Zemax model.
- Despite the shift, the model confirms that light from the edges of the AWG slit may miss the current detector area.
PSF Discrepancy: A significant discrepancy was found in the PSF size. The measured PSF is approximately five times larger than the Zemax simulation (Measured FWHM $\approx$ 13.17 px vs. Simulated FWHM $\approx$ 2.68 px).
- The authors attribute this to the Zemax model's limitation: it treats slit positions as point sources and does not model the AWG's internal spectral response, chip aberrations (e.g., defocal), the finite number of waveguides, or the fiber-to-chip coupling interface.
- Consequently, the model is insufficient for predicting signal quality or resolving power for stacked chips, though it remains useful for positional planning.
Stacking Feasibility: The extension of the model to a two-chip configuration reveals:
- The second chip's spectrum is shifted vertically by approximately 980 pixels relative to the first.
- The current detector size (2048 pixels) can only partially accommodate the second chip's spectrum.
- At the limits of the wavelength range, there is a risk of overlap between the orders of the two chips, exacerbated by the large measured PSF size.

Significance and Claims
The paper modestly claims that the Zemax model serves as a valid tool for assessing the positional mapping of PSFs on the detector but cannot currently predict the quality (size/shape) of the signals due to the unmodeled physical effects of the AWG chip and coupling.

The primary significance of this work lies in its role as a necessary first step for the CAWSMOS development:

It validates the optical path geometry, allowing for faster iterative learning without physical changes.
It identifies critical constraints for the next generation of the instrument: the current detector is too small for a fully stacked multi-chip configuration, and the cross-dispersion optics may require modification to correct for tilt and accommodate more chips.
It establishes that while stacking two chips is partially possible with the current setup, future designs must either increase detector size or map outputs to multiple detectors to fully utilize the stacked architecture.

The authors conclude that further experimental verification on the physical PAWS setup and modifications to the free-space optics are required before the full potential of the CAWSMOS concept can be realized.

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