arXiv subjects

Sergio Leon-Saval

Publications and source records attributed to Sergio Leon-Saval.

At least 19 recordsLinked to original sources

arXiv2026

Towards High-Throughput Visible Photonic Lanterns for the EMARCOT Project

Photonic EMARCOT is an innovative project involving Spanish, German and Australian research institutes that aims to integrate multiple Optical Tube Assemblies (OTAs) using photonic lanterns. The "Pathfinder" prototype, featuring seven OTAs with a 1.1-meter effective aperture, will feed a spectrograph at the Calar Alto Observatory, with first light expected in 2026. We report the fabrication and experimental evaluation of a custom 7x1 multi-mode photonic lantern (MMPL) developed for this framework, featuring seven 25 um core multi-mode inputs merging into a single 50 um core multi-mode output optimized for the visible wavelength range (400-700 nm). Optical characterization centered at 600 nm reveals exceptional channel-to-channel uniformity, with statistical variations close to zero across both bare-fiber and connectorized MMPL configurations. However, the total baseline throughput of this initial device was limited to below 4%. From the refractive index studies, this low throughput is attributed to severe refractive index mismatch between the internal fiber cladding geometry and the structural capillary, which suppresses total internal reflection during the tapering transition. This work establishes an important diagnostic baseline that highlights the necessary fabrication tolerances needed to improve future high-throughput manufacturing processes for precision radial-velocity astronomy.

astro-ph.IM↗

arXiv2026

The Photonic Lantern Nuller: from concept to laboratory and on-sky demonstrations

This thesis work presents the conceptual design and experimental characterization of the Photonic Lantern Nuller instrument, which uses a multimode-to-single-mode demultiplexing waveguide to cancel out starlight while maintaining planet light, allowing for the direct characterization of planets at a telescope's diffraction limit. The PLN was experimentally characterized in the lab, where it was further enhanced using common-path wavefront sensing and control techniques, and then demonstrated on sky at the Subaru Telescope. Highlights include measured in-lab null-depths of $\sim 10^{-4}$ in three out of four ports simultaneously and on-sky null-depths of approximately $\sim 10^{-1}$ (limited by jitter and atmospheric residuals). We provide an overview of these results and discuss avenues for future work.

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arXiv2026

Demonstration of a multimode-to-multimode photonic lantern for astronomy

Photonic lanterns have been widely used in astronomy as low-loss multiplexing devices, typically coupling light from a multimode input into several single-mode outputs. In this work, we present the first multimode-to-multimode photonic lantern specifically designed to combine light from several multimode fibers into a single multimode waveguide. We fabricated and characterized the devices at multiple wavelengths to evaluate the performance of the adiabatic multimode transition. The measured efficiencies exceed $90\ \%$, demonstrating low-loss multimode propagation and efficient modal transfer through the lantern structure. This architecture enables efficient multimode beam combination and represents a significant step toward scalable modular telescope concepts without requiring diffraction-limited injection.

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arXiv2026

FIRST-PL: Commissioning the first visible photonic lantern spectrograph for sub-diffraction-limit astronomy on Subaru/SCExAO

FIRST-PL (Fibered Imager foR a Single Telescope - Photonic Lantern) is a newly commissioned visible-light instrument on Subaru/SCExAO achieving spectroscopy below the diffraction limit. The instrument uses a Photonic Lantern (PL)-converting multimode fiber into 19 single-mode outputs-feeding a mid-resolution spectrograph (R 3000, 630-790 nm). On-sky performance demonstrates 40% injection efficiency at 680 nm (Strehl 30%) and 12x throughput improvement over single-mode fibers. Three operational modes enable spectro-astrometry (50 microarcseconds precision demonstrated on beta-CMi), image reconstruction, and high-contrast imaging. FIRST-PL represents a significant advancement in high-throughput photonic instrumentation.

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arXiv2026

Empirical verification of principal mode orthogonality and relative phase calibration in photonic lanterns

Photonic lanterns efficiently map input spatial modes to single-mode outputs for applications like high angular resolution imaging and nulling interferometry. However, manufacturing limits prevent full control over the device's mode transfer matrix at the design stage, making empirical characterisation essential. In this work we further analyse a dataset of direct measurements of a photonic lantern's principal modes using digital off-axis holography over a 73 nm range near 1550 nm. By analysing the electric field directly, we find that the principal modes are significantly more orthogonal than random vectors in a space of the same size, as expected for near-adiabatic devices. We propose metrics for quantifying this effect, noting that mode converters with orthogonal principal modes provide better conditioned inverse solvers. We also simulate additional measurements that characterisation systems could take, where the orthogonality would be leveraged to determine the relative phase between principal modes.

physics.optics↗

arXiv2026

Seidr update: photonic 'black magic' for high-contrast interferometry using kernel-nulling and photonic lanterns

Seidr is a new interferometric beam combiner within the Asgard Suite, utilizing infrastructure common to the BIFROST instrument at the Very Large Telescope Interferometer. Seidr combines hybrid mode-selective photonic lantern injection modules with a kernel-nulling photonic chip backend to enable deep H-band nulling for high-contrast studies of exoplanets, exomoons, and circumstellar dust. This instrument update summarizes Seidr's current design maturity and recent simulations of the point source - to - lantern outputs. We also outline progress on our neural network-based wavefront estimation scheme, which uses the photonic lantern outputs to sense phase fluctuations, designed to feed back to Baldr's deformable mirror, and improve nuller light injection.

physics.optics↗

arXiv2026

Overcoming the low signal-to-noise problem for hybrid mode-selective photonic lantern-based wavefront correction using machine learning

Hybrid mode-selective photonic lanterns transform an input complex point-spread function into several single-mode outputs, where a selected core feeds the fundamental mode to a photonic science instrument, while the remaining cores are used for wavefront sensing in a closed-loop adaptive optics system. A neural network maps the intensities of the wavefront sensing cores to an estimated wavefront correction, which is applied to an upstream deformable mirror. However, there exists a trade between maximizing the amount of light reserved for the photonic instrument and the reduced signal-to-noise ratios for the wavefront sensing cores. We explore wavefront correction for the Seidr instrument, a part of the Asgard Suite for the Very Large Telescope Interferometer. We evaluate different neural network architectures, comparing wavefront estimation performance for different wavefront error types, as a first step toward addressing the signal-to-noise trade-off. Results show transformer neural networks as a promising solution for temporal photonic lantern-based wavefront estimation.

physics.optics↗

arXiv2026

PEEPSS: Photonic-Enabled ExoPlanet Spectroscopic Sensor for the Habitable Worlds Observatory

The next few years will be critical for technology development for Habitable Worlds Observatory (HWO) in its mission to search for and characterize extrasolar planets. To achieve its stated goals with contrasts of one part in ten billion, HWO will require outstanding stability and precision, particularly in measuring and controlling the wavefront of the light propagate through the telescope and coronagraph system. We present simulations for the Photonic-Enabled ExoPlanet Spectroscopic Sensor (PEEPSS), which uses a set of photonic lanterns to efficiently couple light from the "dark hole" in the coronograph focal plane (where the exoplanets are expected to lie) into single-mode fibers and the main spectrograph. PEEPSS uses rejected host star light from the region interior to the dark hole to aid in the wavefront sensing; this has the advantage of doing the sensing in the coronograph focal plane, eliminating non-common-path errors between the wavefront sensing and science channels. The photonics lanterns allow us to combine our science channel and wavefront sensor into a single system. PEEPSS will be particularly advantageous provide in the near-infrared (NIR) bandpass, which is of particular interest for HWO. Because the limiting inner working angle (IWA) of a coronagraph scales as wavelength over diameter, exoplanet imaging in the NIR becomes a major challenge as the IWA can exceed the exoplanet orbital radius. PEEPSS will enable NIR coronagraphic observations at smaller IWA than other approaches, increasing the observational parameter space HWO can probe in the search for exoplanets.

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arXiv2026

Wavelength-Dependent Evolution of Full-Field Transfer Matrices in Photonic Lanterns

A fiber-based photonic lantern can couple an array of single-mode optical fibers to the guided modes of a multimode fiber, with the mapping between the single-mode fibers and guided modes fully described by a complex-valued transfer matrix. Recent experimental studies have reported strong wavelength-dependent evolution of this matrix in non-mode-selective photonic lanterns, yet a quantitative physical explanation for this behavior has not previously been demonstrated. Here, we present direct measurements of the wavelength-dependent encoding transfer matrix of a photonic lantern across the range 1525 nm to 1575 nm using off-axis holographic imaging, enabling high-fidelity recovery of both amplitude and phase. Beyond measurement, we introduce a physically grounded propagation model and numerical simulation that quantitatively reproduces the observed wavelength evolution and provides a unified physical explanation for behavior reported in prior experimental work. The model identifies differential modal phase accumulation in the multimode section as the dominant mechanism governing spectral evolution and shows that increasing the length of the multimode end systematically accelerates the phase evolution of the transfer matrix with wavelength. These results establish a direct and predictive link between photonic lantern geometry and spectral response, providing a design framework for tailoring lanterns either to enhance sensitivity to closely spaced wavelengths or to enforce uniform response over broad bandwidths for spectroscopic and imaging applications.

physics.optics↗

arXiv2026

Combining spectroscopy and wavefront control at deep contrast with photonic lanterns

HWO aims to directly image objects orbiting Sun-like stars, using a 6-m telescope capable of high-contrast imaging ($10^{-10}$) and spectroscopy to search for biosignatures in planets located in the habitable zone. Recent laboratory demonstrations and ground-based telescope projects have shown the effectiveness of SMFs in spectroscopy, paving the way for SMF-fed spectrographs in future space missions like HWO. SMFs enhance spectral stability and reduce modal noise. HWO spectroscopy will need extended integration times, potentially lasting weeks. During these observations, the wavefront must be precisely measured and maintained to achieve the deep contrast and robust calibration of starlight contamination necessary for exoplanet characterization. We show that photonic lanterns (PLs) are ideally suited to meet these requirements. PLs are compact devices that couple light over a broader angular range than SMFs, ensuring higher throughput, converting a multimode input into multiple single-mode outputs. Positioned at the focal plane, they measure the complex amplitude of the coherent starlight within $\sim$ 2 l/D of the planet image, acting as compact wavefront sensors. Among the different variants of PLs that have emerged, the Hybrid-Mode Selective Photonic Lantern (HMSPL) is particularly attractive, as it directs object light into a central SMF feeding a mid-R spectrograph for exoplanet spectroscopy, while the adjacent SMFs route surrounding speckle light to a low-R spectrograph for rapid wavefront sensing. This dual function eliminates non-common path aberrations, optimizing injection efficiency and background suppression. We introduce HMSPL's dual role and planned tests at UTSA's high-contrast imaging lab and at SCExAO at the Subaru Telescope.

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arXiv2025

Illuminating the lantern: coherent, spectro-polarimetric characterisation of a multimode converter

While photonic lanterns efficiently and uniquely map a set of input modes to single-mode outputs (or vice versa), the optical mode transfer matrix of any particular fabricated device cannot be constrained at the design stage due to manufacturing imperfections. Accurate knowledge of the mapping enables complex sensing or beam control applications that leverage multimode conversion. In this work, we present a characterisation system to directly measure the electric field from a photonic lantern using digital off-axis holography, following its evolution over a 73 nm range near 1550 nm and in two orthogonal, linear polarisations. We provide the first multi-wavelength, polarisation decomposed characterisation of the principal modes of a photonic lantern. Performance of our testbed is validated on a single-mode fibre then harnessed to characterise a 19-port, multicore fibre fed photonic lantern. We uncover the typical wavelength scale at which the modal mapping evolves and measure the relative dispersion in the device, finding significant differences with idealised simulations. In addition to detailing the system, we also share the empirical mode transfer matrices, enabling future work in astrophotonic design, computational imaging, device fabrication feedback loops and beam shaping.

physics.optics↗

arXiv2025

On-sky Demonstration of Subdiffraction-limited Astronomical Measurement Using a Photonic Lantern

Resolving fine details of astronomical objects provides critical insights into their underlying physical processes. This drives in part the desire to construct ever-larger telescopes and interferometer arrays and to observe at shorter wavelength to lower the diffraction limit of angular resolution. Alternatively, one can aim to overcome the diffraction limit by extracting more information from a single telescope's aperture. A promising way to do this is spatial mode-based imaging, which projects focal-plane field onto a set of spatial modes before detection, retaining focal-plane phase information crucial at small angular scales but typically lost in intensity imaging. However, the practical implementation of mode-based imaging in astronomy from the ground has been challenged by atmospheric turbulence. Here, we present the first on-sky demonstration of a subdiffraction-limited, mode-based measurement using a photonic lantern (PL)-fed spectrometer installed on the SCExAO instrument at the Subaru Telescope. We introduce a novel calibration strategy that mitigates time-varying wavefront error and misalignment effects, leveraging simultaneously recorded focal-plane images and using a spectral-differential technique that self-calibrates the data. Observing the classical Be star $β$ CMi, we detected spectral-differential spatial signals and reconstructed images of its H$α$-emitting disk. We achieved an unprecedented H$α$ photocenter precision of 50$μ$as in about 10-minute observation with a single telescope, measuring the disk's near-far side asymmetry for the first time. This work demonstrates the high precision, efficiency, and practicality of photonic mode-based imaging techniques to recover subdiffraction-limited information, opening new avenues for high angular resolution spectroscopic studies in astronomy.

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arXiv2025

Experimental and on-sky demonstration of spectrally dispersed wavefront sensing using a photonic lantern

Adaptive optics systems are critical in any application where highly resolved imaging or beam control must be performed through a dynamic medium. Such applications include astronomy and free-space optical communications, where light propagates through the atmosphere, as well as medical microscopy and vision science, where light propagates through biological tissue. Recent works have demonstrated common-path wavefront sensors for adaptive optics using the photonic lantern, a slowly varying waveguide that can efficiently couple multi-moded light into single-mode fibers. We use the SCExAO astrophotonics platform at the 8-m Subaru Telescope to show that spectral dispersion of lantern outputs can improve correction fidelity, culminating with an on-sky demonstration of real-time wavefront control. To our best knowledge, this is the first such result for either a spectrally dispersed or a photonic lantern wavefront sensor. Combined with the benefits offered by lanterns in precision spectroscopy, our results suggest the future possibility of a unified wavefront sensing spectrograph using compact photonic devices.

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arXiv2025

Implicit Electric Field Conjugation with the Photonic Lantern Nuller

The Photonic Lantern Nuller (PLN) is an instrument concept designed to characterize exoplanets within a single beam-width from its host star. The PLN leverages the spatial symmetry of a mode-selective photonic lantern (MSPL) to create nulled ports, which cancel out on-axis starlight but allow off-axis exoplanet light to couple. The null-depths are limited by wavefront aberrations in the system as well as by imperfections in the lantern. We show that the implicit electric field conjugation algorithm can be used to reduce the stellar coupling through the PLN by orders of magnitude while maintaining the majority of the off-axis light, leading to deeper null depths (~10^{-4}) and thus higher sensitivity to potential planet signals. We discuss a theory for the tradeoff we observed between the different ports, where iEFC improves the nulls of some ports at the expense of others, and show that targeting one port alone can lead to deeper starlight rejection through that port than when targeting all ports at once. We also observe different levels of stability depending on the port and discuss the implications for practically implementing this technique for science observations.

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arXiv2024

Efficient, ultra-high attenuation fiber Bragg grating filter for photon noise suppression

Precision optical filters are key components for current and future photonic technologies. Here, we demonstrate a low loss spectral filter consisting of an ultrasteep bandpass feature with a maximum gradient of (90.6$\pm$0.7) dB/GHz, centred within a notch filter with (128$\pm$6) dB of suppression. The filter consists of a fiber Bragg grating with multiple $π$-phase discontinuities inscribed into a single mode photosensitive fiber. The measured performance closely matches the simulated spectrum calculated from the design parameters indicating a high degree of confidence in the repeatability and manufacture of such devices. These filters show great promise for applications reliant on high-frequency resolution noise suppression, such as quantum networking, and highlight the opportunities for the versatility, efficiency, and extreme suppression offered by high-performance fiber Bragg grating devices.

physics.optics↗

arXiv2024

Spectroscopy using a visible photonic lantern at the Subaru telescope: Laboratory characterization and first on-sky demonstration on Ikiiki (α Leo) and `Aua (α Ori)

Photonic lanterns are waveguide devices enabling high throughput single mode spectroscopy and high angular resolution. We aim to present the first on-sky demonstration of a photonic lantern (PL) operating in visible light, to measure its throughput and assess its potential for high-resolution spectroscopy of compact objects. We used the SCExAO instrument (a double stage extreme AO system installed at the Subaru telescope) and FIRST mid-resolution spectrograph (R 3000) to test the visible capabilities of the PL on internal source and on-sky observations. The best averaged coupling efficiency over the PL field of view was measured at 51% +/- 10% with a peak at 80%. We also investigate the relationship between coupling efficiency and the Strehl ratio for a PL, comparing them with those of a single-mode fiber (SMF). Findings show that in the AO regime, a PL offers better coupling efficiency performance than a SMF, especially in the presence of low spatial frequency aberrations. We observed Ikiiki (alpha Leo - mR = 1.37) and `Aua (alpha Ori - mR = -1.17) at a frame rate of 200 Hz. Under median seeing conditions (about 1 arcsec measured in H band) and large tip/tilt residuals (over 20 mas), we estimated an average light coupling efficiency of 14.5% +/- 7.4%, with a maximum of 42.8% at 680 nm. We were able to reconstruct both star's spectra, containing various absorption lines. The successful demonstration of this device opens new possibilities in terms of high throughput single-mode fiber-fed spectroscopy in the Visible. The demonstrated on-sky coupling efficiency performance would not have been achievable with a single SMF injection setup under similar conditions, partly because the residual tip/tilt alone exceeded the field of view of a visible SMF (18 mas at 700 nm). Thus emphasizing the enhanced resilience of PL technology to such atmospheric disturbances. The additional

astro-ph.IM↗

arXiv2024

Spectral characterization of a 3-port photonic lantern for application to spectroastrometry

Spectroastrometry, which measures wavelength-dependent shifts in the center of light, is well-suited for studying objects whose morphology changes with wavelength at very high angular resolutions. Photonic lantern (PL)-fed spectrometers have potential to enable measurement of spectroastrometric signals because the relative intensities between the PL output SMFs contain spatial information on the input scene. In order to use PL output spectra for spectroastrometric measurements, it is important to understand the wavelength-dependent behaviors of PL outputs and develop methods to calibrate the effects of time-varying wavefront errors in ground-based observations. We present experimental characterizations of the 3-port PL on the SCExAO testbed at the Subaru Telescope. We develop spectral response models of the PL and verify the behaviors with lab experiments. We find sinusoidal behavior of astrometric sensitivity of the 3-port PL as a function of wavelength, as expected from numerical simulations. Furthermore, we compare experimental and numerically simulated coupling maps and discuss their potential use for offsetting pointing errors. We then present a method of building PL spectral response models (solving for the transfer matrices as a function of wavelength) using coupling maps, which can be used for further calibration strategies.

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arXiv2024

On the Potential of Spectroastrometry with Photonic Lanterns

We investigate the potential of photonic lantern (PL) fiber fed spectrometers for two-dimensional spectroastrometry. Spectroastrometry, a technique for studying small angular scales by measuring centroid shifts as a function of wavelength, is typically conducted using long-slit spectrographs. However, slit-based spectroastrometry requires observations with multiple position angles to measure two-dimensional spectroastrometric signals. In a typical configuration of PL-fed spectrometers, light from the focal plane is coupled into the few-moded PL, which is then split into several single-mode outputs, with the relative intensities containing astrometric information. The single-moded beams can be fed into a high-resolution spectrometer to measure wavelength-dependent centroid shifts. We perform numerical simulations of a standard 6-port PL and demonstrate its capability of measuring spectroastrometric signals. The effects of photon noise, wavefront errors, and chromaticity are investigated. When the PL is designed to have large linear responses to tip-tilts at the wavelengths of interest, the centroid shifts can be efficiently measured. Furthermore, we provide mock observations of detecting accreting protoplanets. PL spectroastrometry is potentially a simple and efficient technique for detecting spectroastrometric signals.

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