# 2023 Astrophotonics Roadmap: pathways to realizing multi-functional integrated astrophotonic instruments

Nemanja Jovanovic, Pradip Gatkine, Narsireddy Anugu, Rodrigo Amezcua-Correa, Ritoban Basu Thakur, Charles Beichman, Chad Bender, Jean-Philippe Berger, Azzurra Bigioli, Joss Bland-Hawthorn, et al.

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## To cite this version:

Nemanja Jovanovic, Pradip Gatkine, Narsireddy Anugu, Rodrigo Amezcua-Correa, Ritoban Basu Thakur, et al.. 2023 Astrophotonics Roadmap: pathways to realizing multi-functional integrated astrophotonic instruments. JPhys Photon., 2023, 5 (4), pp.042501. 10.1088/2515-7647/ace869. hal-04268523

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ROADMAP • OPEN ACCESS

2023 Astrophotonics Roadmap: pathways to realizing multi-functional integrated astrophotonic instruments

To cite this article: Nemanja Jovanovic et al 2023 *J. Phys. Photonics* 5 042501

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ROADMAP

2023 Astrophotonics Roadmap: pathways to realizing
multi-functional integrated astrophotonic instruments

Nemanja Jovanovic¹,⁵,⁷,* ID, Pradip Gatkine¹,⁵,⁷,* ID, Narsireddy Anugu² ID, Rodrigo Amezcua-Correa³, Ritoban Basu Thakur¹⁰,⁵⁰ ID, Charles Beichman⁴, Chad F. Bender⁵ ID, Jean-Philippe Berger⁶ ID, Azzurra Bigioli⁷ ID, Joss Bland-Hawthorn⁸ ID, Guillaume Bourdarot⁹ ID, Charles M Bradford¹⁰ ID, Ronald Broeke¹¹, Julia Bryant⁸ ID, Kevin Bundy¹² ID, Ross Cheriton¹³ ID, Nick Cvetojevic¹⁴ ID, Momen Diab¹⁵ ID, Scott A Diddams¹⁶ ID, Aline N Dinkelaker¹⁷ ID, Jeroen Duis¹⁸ ID, Stephen Eikenberry³ ID, Simon Ellis¹⁹ ID, Akira Endo²⁰ ID, Donald F Figer²¹ ID, Michael P. Fitzgerald²² ID, Itandehui Gris-Sanchez²³ ID, Simon Gross²⁴ ID, Ludovic Grossard²⁵ ID, Olivier Guyon⁵,²⁶,²⁷,²⁸ ID, Sebastiaan Y Haffert⁵ ID, Samuel Halverson¹⁰ ID, Robert J Harris²⁹,³⁰ ID, Jinping He³¹,³² ID, Tobias Herr³³ ID, Philipp Hottinger³⁴ ID, Elsa Huby³⁵, Michael Ireland³⁶ ID, Rebecca Jenson-Clem¹², Jeffrey Jewell¹⁰, Laurent Jocou³⁷ ID, Stefan Kraus³⁸ ID, Lucas Labadie³⁹ ID, Sylvestre Lacour³⁵ ID, Romain Laugier⁷ ID, Katarzyna Ławniczuk¹¹, Jonathan Lin²² ID, Stephanie Leifer⁴⁰ ID, Sergio Leon-Saval⁵⁶ ID, Guillermo Martin³⁷ ID, Frantz Martinache¹⁴, Marc-Antoine Martinod⁷ ID, Benjamin A Mazin⁴¹ ID, Stefano Minardi⁴², John D Monnier⁴³ ID, Reinan Moreira⁴⁴, Denis Mourard¹⁴ ID, Abani Shankar Nayak⁴⁵ ID, Barnaby Norris⁸, Ewelina Obrzud⁴⁶ ID, Karine Perraut³⁷ ID, François Reynaud²⁵ ID, Steph Sallum⁴⁷ ID, David Schiminovich⁴⁸ ID, Christian Schwab⁴⁹ ID, Eugene Serbayn¹⁰, Sherif Soliman¹⁸ ID, Andreas Stoll¹⁷ ID, Liang Tang³¹,³², Peter Tuthill⁸ ID, Kerry Vahala¹⁰, Gautam Vasisht¹⁰ ID, Sylvain Veilleux⁵¹ ID, Alexander B Walter¹⁰ ID, Edward J Wollack⁵² ID, Yinzi Xin¹ ID, Zongyin Yang⁵³ ID, Stephanos Yerolatsitis³ ID, Yang Zhang⁵⁴ ID and Chang-Ling Zou⁵⁵ ID

¹ Department of Astronomy, California Institute of Technology, Pasadena, CA, United States of America

² The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, Altadena, CA 91203, United States of America

³ CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, FL, United States of America

⁴ IPAC/NASA Exoplanet Science Institute, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America

⁵ Steward Observatory, University of Arizona, Tucson, AZ, United States of America

⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

⁷ Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

⁸ Sydney Institute for Astronomy (SIFA), School of Physics, The University of Sydney, Sydney, Australia

⁹ Max Planck Institute for Extraterrestrial Physics, Garching, Germany

¹⁰ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America

¹¹ Bright Photonics BV, Eindhoven, The Netherlands

¹² Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, United States of America

¹³ Advanced Electronics and Photonics Research Centre, National Research Council Canada, Ottawa, Canada

¹⁴ Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France

¹⁵ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, Toronto, Canada

¹⁶ Electrical, Computer and Energy Engineering and Department of Physics, University of Colorado, Boulder, CO, United States of America

¹⁷ Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, Germany

¹⁸ PHIX Photonics Assembly, Enschede, The Netherlands

¹⁹ Australian Astronomical Optics, Astrophysics and Space Technologies Research Centre, Macquarie University, North Ryde, NSW, Australia

²⁰ Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology, Delft, The Netherlands

²¹ Center for Detectors, Rochester Institute of Technology, Rochester, NY, United States of America

²² Department of Physics and Astronomy, University of California, Los Angeles, CA, United States of America

²³ ITEAM Research Institute, Universitat Politècnica de València, Valencia, Spain

²⁴ MQ Photonics Research Centre, School of Engineering, Macquarie University, North Ryde, NSW, Australia

²⁵ Limoges, CNRS, XLIM, UMR 7252, F-87000 Limoges, France

²⁶ Subaru Telescope, National Astronomical Observatory of Japan, National Institute of Natural Sciences, Hilo, HI, United States of America

²⁷ Astrobiology Center of NINS, Osawa, Mitaka, Tokyo, Japan

²⁸ College of Optical Sciences, University of Arizona, Tucson, AZ, United States of America

²⁹ Max-Planck-Institute for Astronomy, Heidelberg, Germany

³⁰ Department of Physics, Durham University, Durham, United Kingdom

³¹ National Astronomical Observatories, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing, People's Republic of China
32 Key Laboratory of Astronomical Optics & Technology, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing, People's Republic of China

33 Deutsches Elektronen-Synchrotron DESY, Germany and Universität Hamburg, Hamburg, Germany

34 Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Heidelberg, Germany

35 LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, Meudon, France

36 The Australian National University, Canberra, Australia

37 Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France

38 Department of Physics and Astronomy, University of Exeter, Exeter, United Kingdom

39 I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Cologne, Germany

40 Department of Applied Physics, California Institute of Technology, Pasadena, CA, United States of America

41 Department of Physics, University of California, Santa Barbara, CA, United States of America

42 Ams-OSRAM, Jena, Germany

43 Department of Astronomy, University of Michigan, Ann Arbor, MI, United States of America

44 Ultra-Low Loss Technologies, Santa Barbara, CA, United States of America

45 Institut für Angewandte Physik, Friedrich-Schiller-Universität Jena, Jena, Germany

46 Centre Suisse d'Electronique et de Microtechnique, Neuchâtel, Switzerland

47 Department of Physics and Astronomy, University of California, Irvine, CA, United States of America

48 Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, New York, NY, United States of America

49 School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW, Australia

50 Department of Physics, California Institute of Technology, Pasadena, CA, United States of America

51 Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD, United States of America

52 NASA Goddard Space Flight Center, Greenbelt, MD, United States of America

53 College of Information Science and Electronic Engineering, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou, People's Republic of China

54 Electrical and Computer Engineering Department, University of Maryland, College Park, MD, United States of America

55 CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui, People's Republic of China

56 Sydney Astrophotonics Instrumentation Laboratory (SAIL), School of Physics, The University of Sydney, Sydney, Australia

57 Guest editors of the Roadmap.

* Authors to whom any correspondence should be addressed.

E-mail: nem@caltech.edu and pgatkine@astro.ucla.edu

Keywords: astrophotonics, spectrograph, lanterns, detectors, PICs, hybridization, integration

Abstract

Photonic technologies offer numerous functionalities that can be used to realize astrophotonic instruments. The most spectacular example to date is the ESO Gravity instrument at the Very Large Telescope in Chile that combines the light-gathering power of four 8 m telescopes through a complex photonic interferometer. Fully integrated astrophotonic devices stand to offer critical advantages for instrument development, including extreme miniaturization when operating at the diffraction-limit, as well as integration, superior thermal and mechanical stabilization owing to the small footprint, and high replicability offering significant cost savings. Numerous astrophotonic technologies have been developed to address shortcomings of conventional instruments to date, including for example the development of photonic lanterns to convert from multimode inputs to single mode outputs, complex aperiodic fiber Bragg gratings to filter OH emission from the atmosphere, complex beam combiners to enable long baseline interferometry with for example, ESO Gravity, and laser frequency combs for high precision spectral calibration of spectrometers. Despite these successes, the facility implementation of photonic solutions in astronomical instrumentation is currently limited because of (1) low throughputs from coupling to fibers, coupling fibers to chips, propagation and bend losses, device losses, etc, (2) difficulties with scaling to large channel count devices needed for large bandwidths and high resolutions, and (3) efficient integration of photonics with detectors, to name a few. In this roadmap, we identify 24 key areas that need further development. We outline the challenges and advances needed across those areas covering design tools, simulation capabilities, fabrication processes, the need for entirely new components, integration and hybridization and the characterization of devices. To realize these advances the astrophotonics community will have to work cooperatively with industrial partners who have more advanced manufacturing capabilities. With the advances described herein, multi-functional integrated instruments will be realized leading to novel observing capabilities for both ground and space based platforms, enabling new scientific studies and discoveries.
Contents

<table><tr><td>1. Introduction</td><td>4</td></tr><tr><td>2. Symbiosis between adaptive optics and photonics: the path to fully integrated instruments</td><td>10</td></tr><tr><td>3. Photonic lantern wavefront sensing and control</td><td>14</td></tr><tr><td>4. Spectroscopic applications enabled by photonic lanterns</td><td>18</td></tr><tr><td>5. New optical fibers for astrophotonics</td><td>22</td></tr><tr><td>6. Getting on-chip arrayed waveguide grating spectrographs ready for astronomy</td><td>26</td></tr><tr><td>7. New directions in photonic spectrograph architectures</td><td>31</td></tr><tr><td>8. The emergence of UV astrophotonics</td><td>37</td></tr><tr><td>9. Advances in mid-IR astrophotonics</td><td>41</td></tr><tr><td>10. Integral field units for THz astrophysics</td><td>45</td></tr><tr><td>11. Astrophotonic spectral filtering</td><td>49</td></tr><tr><td>12. Measure what you need with optical correlation spectroscopy</td><td>55</td></tr><tr><td>13. Laser frequency combs</td><td>59</td></tr><tr><td>14. Spectral flattening of laser frequency combs on-a-chip</td><td>63</td></tr><tr><td>15. Fabry-Perot Etalons as precision wavelength calibrators</td><td>67</td></tr><tr><td>16. Future of photonic beam combining technologies for interferometers</td><td>71</td></tr><tr><td>17. Future prospects of discrete beam combination techniques</td><td>75</td></tr><tr><td>18. Realizing extremely long-baseline interferometers by exploiting photonic technologies</td><td>78</td></tr><tr><td>19. Nulling interferometry with on-chip beam combination</td><td>82</td></tr><tr><td>20. Kernel nulling self-calibration</td><td>86</td></tr><tr><td>21. Nulling interferometry with optical fibers and photonic lanterns</td><td>90</td></tr><tr><td>22. Heterodyne interferometry and frequency mixing techniques</td><td>93</td></tr><tr><td>23. Efficient hybrid photonic and electronic integration to realize multi-functional instruments</td><td>97</td></tr><tr><td>24. Semiconductor detectors and integrated photonics</td><td>101</td></tr><tr><td>25. Integrated superconducting detectors for optical and IR astrophotonics</td><td>105</td></tr><tr><td>Data availability statement</td><td>108</td></tr><tr><td>References</td><td>109</td></tr></table>
# 1. Introduction

Nemanja Jovanovic and Pradip Gatkine

Department of Astronomy, California Institute of Technology, Pasadena, CA, United States of America

## Status

Astrophotonics is simply defined as the application of photonic technologies to astronomy. Like many fields of technology-driven science, astrophotonics directly benefits from the multi-decade and multi-billion dollar investments in photonics by industry, especially by telecommunications. Photonic technologies are appealing because they can provide many avenues for controlling and manipulating light, including spectral dispersion, spectral, spatial, and polarization filtering, phase and amplitude modulation, light generation, frequency shifting, and light detection to name a few. In addition, operating at the diffraction limit of the input telescope means that the instrument can have the smallest possible footprint for a given set of specifications, reducing volume, mass and cost [1, 2]. Guiding the light in fibers or waveguides in a photonic integrated circuit (PIC) allows further reductions in instrument size by enabling flexible integration of many functions optimized for a specific science case and the elimination of bulk optics in most cases leading to extreme miniaturization, which can more readily be thermally and mechanically stabilized. Unlike bulk optics, photonic technologies can be highly replicable, offering dramatic cost savings once produced in volume. A highly replicable diffraction-limited instrument can be readily deployed to numerous observatories and achieve the same performance, potentially providing cost-savings by eliminating the prototypical nature of current astronomical instruments.

With such favorable properties, photonic technologies stand to impact many areas of astronomy. Leveraging the replicability for example, means that low cost massively multiplexed spectroscopic surveys could be enabled in the future. These would allow the measurement of large samples of spectroscopic redshifts, constrain galaxy evolution through tracers of star formation, outflows, etc, and enable detailed compositional study of stellar populations to name a few. On the other hand, the diffraction-limited nature of photonics makes them directly applicable to point-source-like targets that typically rely on adaptive optics (AO) observations. These include the study of exoplanets and disks, young stellar objects, evolved stars, active galactic nuclei, and nuclear star cluster kinematics for example.

Despite its origin in the late 1970s, and pioneering overviews about the prospects of photonics in the 90's [3], the field of astrophotonics was first recognized as a sub-field of astronomical instrumentation in 2009, when the editors of Optics Express [4] solicited a special issue on the topic for the first time. Although then already a thriving field, it has grown over the last 14 years as shown in figure 1, which highlights some of the various technical developments resulting in two further special issues being developed in 2017 and 2021 respectively [5, 6].

Numerous technologies have been advanced over the past decade. Here we briefly highlight a few success stories. The photonic lantern (PL) was invented to exploit the coupling advantages of multimode (MM) fibers and the stability of single-mode fibers (SMFs) for use in astronomy [7]. This mode converting device has since undergone extensive technical development [13–21] and now a device that converts a MM input into 19 SM outputs can routinely do this with a throughput >95% at 1550 nm. The lantern device was leveraged to realize a complex spectral filter that could suppress the telluric OH emission [9, 22]. This was achieved by inscribing complex fiber Bragg gratings (FBGs) and attaching them to each of the SM outputs of a lantern, before recombining the filtered signal into a MM fiber and feeding a downstream spectrograph [23–25]. This spectral filtering capability enabled accurate sky-subtraction in the near-infrared (NIR), key to advancing faint observations in that region. Regarding calibration, laser frequency combs (LFCs) provide an ultra-stable set of highly dense comb lines across large bandwidths [26, 27], making them the perfect tool for calibrating a spectrograph [28, 29]. These tools have been optimized for astronomy for the past decade [30, 31] and are now becoming commonplace for use with precision radial velocity (RV) spectrographs [32]. Finally, in the field of interferometry, the GRAVITY instrument is a long-baseline interferometer that can combine the four 8 m telescopes at the VLT in the NIR K-band centered at 2.2 µm. It does this using a photonic beam combiner chip [33] and provides milli arcsec spatial resolution, microarcsecond astrometric precision, and high throughput thanks to the large telescopes and AO systems, which have enabled many transformative discoveries [34–37].

With these successes and a long list of possible technologies and functions to explore, astrophotonics has a huge potential to impact astronomy. This potential was recently outlined in several whitepapers [38, 39] solicited by the National Academy of Sciences for the US Decadal survey and later explicitly called out in the survey itself [40]. Specifically the survey stated ‘The possibility of obtaining extremely high-precision radial velocities, of the order of a 10 cm s⁻¹ or better, as well as direct imaging of exoplanets may largely rely on the maturity of SMFs and on-chip nulling interferometers’.
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Top left: a plot showing the growing trajectory of research in the field of astrophotonics, compiled using NASA ADS search engine. The surrounding insets show examples of various technologies representing the sub-fields in astrophotonics, as they matured for on-sky demonstration. These examples include: photonic lanterns. Reproduced with permission from [7] © The Optical Society. Photonic beam combiners for interferometry with GRAVITY. Reproduced with permission from [8]. Fiber Bragg gratings for atmospheric OH emission suppression. Reproduced with permission from [9]. Arrayed waveguide gratings for spectroscopy. Reproduced with permission from [10] © The Optical Society. Fiber-based Fabry-Perot interferometer for spectral calibration. Reproduced from [11]. © IOP Publishing Ltd. All rights reserved. And an on-chip nulling interferometer. Reproduced with permission from [12]. This is just a small subset of a large family of astrophotonic technologies that have been developed and demonstrated over the last two decades.</figcaption></figure>

Importantly, astronomy driven technology developments have broader applicability which could improve funding for such developments. For example, the all-photonic flattener on a chip [41, 42] may find application beyond astronomy for gain flattening [43, 44], temporal pulse shaping [45, 46] as well as for targeted excitation of particular molecular species [47]. In addition, PLs are being considered for spatial division multiplexing in telecommunications [48] as well for high efficiency free-space optical communications [49]: the latter is critical to ensuring high data rates for future astronomy and planetary science missions and may also find application in microscopy. These are just a few examples of the potential of astrophotonics to impact society more generally.

Despite these successes, astrophotonic technologies require further developments in nearly all cases to advance them to facility-class instrument science readiness and/or expand capabilities. With this roadmap, we aim to outline some key areas development should be focused on to advance the field and allow for truly integrated, multi-functional instruments to be realized that advance scientific investigation. The choice of development areas outlined in this work was driven by (1) relevant technologies demonstrated by the community with the potential for further growth and (2) promising opportunities given recent technological advances to address outstanding goals. The areas of development covered in this article are non-exhaustive and other areas should also be considered.

## Roadmap organization and goals

The roadmap explores the status of currently used technologies and outlines other promising choices that should be investigated. The aim is to highlight the potential astronomical applications of a range of technologies and provide guidance on the specific developmental path to realize that promise including, developing more advanced design tools, prototyping, fabricating, characterizing, packaging, integrating, field testing, and science demonstration. The roadmap is broken up into five key thematic areas which span all of astrophotonics as follows:

### Light injection, wavefront control, and light transport

The efficient coupling of light into photonic technologies is the first step in being able to utilize them for astronomical instruments. Given photonic devices generally operate at or near the diffraction limit, the ability to couple light from a telescope into such a device relies on either an AO system to correct for the wavefront in the optical/infrared [50] (common to most large 5 m+ telescopes) or a mode-matching solution, be it a with a PL [18, 20], an integral field unit (IFU), or a series of small telescopes more closely matched to the Fried parameter, $r_0$ [51]. In addition, with advances in AO systems that can now generate reasonable correction across large fields (MCAO; ~25% Strehl over ~2 arcminute fields-of-view [52]) and
references therein, when combined with low-mode-count PLs, efficient multi-object (MO) photonic instruments can be realized.

Photonic technologies can also be used to produce signals critical to driving the AO system, and offer numerous advantages including eliminating non-common path and chromatic errors. Next-generation instrument architectures will merge photonic wavefront sensing and science instruments and result in integrated instruments with superior performance.

Once light is coupled to a photonic system, light transport is the next critical aspect to ensure the light can be efficiently routed to the science instrument. Optical fibers are ideally suited to this and here we explore a range of new silica fiber architectures that enable photonic applications to currently un-explored wavelength ranges with advantageous properties (dispersion, polarization, bandwidth, few-mode vs SM, etc.).

### Spectroscopy and spectral filtering

Spectroscopy is a key analytical tool in an astronomers toolbox. It can be used in several ways: to disperse the light for scientific measurement as well as for spectral filtering applications. Photonic dispersing elements in the form of arrayed waveguide gratings (AWGs) have been tested for their suitability for astronomy, but need further developments to optimize throughput, bandwidth, polarization response as well as to scale to higher resolution before wide astronomical application. Apart from AWGs, other photonic technologies provide a plethora of novel approaches to the dispersion problem including dispersed Fourier transform interferometers consisting of Mach–Zehnder interferometers (MZIs) and AWGs, ring-resonator enhanced spectrometers and integrated serpentine grating spectrometers to name a few. These technologies need to be explored to ascertain their scientific potential and demonstrate the technical advantages and to assess challenges to implementation.

Critical to exploiting photonic spectroscopy is being able to realize the devices in wavebands other than the NIR, where telecommunications has already invested heavily. Photonics is evolving in the visible, so we outline photonic efforts to develop platforms, both photonic integrated circuits (PICs) and fiber based, that are promising at more extreme wavelengths like the UV (<400 nm) and in the MIR (>2.5 µm). AWGs for example have already been built and demonstrated efficiently at sub-mm wavelengths, and in the roadmap we look at further advances needed to optimize these devices, as well as a pathway to large field-of-view instruments.

Spectral filtering is a critical capability. Technologies like FBGs are very mature, but require further development to be applicable to wavebands other than the NIR. Micro-ring resonators, which form tiny resonant cavities and waveguide Bragg gratings, are less mature and need more significant advancements, including utilizing smaller feature sizes, enabling complex filters and reducing cladding mode coupling, but offer the potential to integrate them with other components on PICs.

The applications of filtering devices are growing as well. Optical cross-correlation for example looks at introducing a paradigm shift in how instruments are built, eliminating the need for extremely costly high pixel count arrays, if efficient and flexible filters can be realized to process the light optically. This would be transformative for instruments designed for extremely large telescopes.

### Spectral calibration

Key to extracting physical quantities via spectroscopy is the ability to calibrate the spectrograph. Photonics offers the ultimate wavelength calibration tool: the LFC. An LFC can be stabilized to extreme levels over decades [26, 27], meeting even the most demanding science needs in astronomy (e.g. extreme precision radial velocity (EPRV)). Although LFCs are becoming common at many observatories, they are still complex and costly, and lack reliable spectral coverage in the blue, as needed for the most precise RV measurements of Sun-like stars. In addition, LFCs offer the potential for non-spectroscopic calibration applications, including extreme time keeping for very long baseline interferometers as well as metrology.

To maximize the benefit to spectroscopy, the spectral profile of the comb lines in an LFC spectrum must be flattened, or be made more uniform to fit within the spectrometer's dynamic range. On-chip flattener technology has only recently been explored for astronomy and requires further development. In particular, investigation is needed into the optimal device design given the LFCs optical properties, the number of channels that can be realized on a single chip, how to build devices that span several astronomical bands, and the prospects for devices in the visible region.

An alternative calibration technology that offers a compact, portable, cost effective solution to wavelength calibration is the Fabry–Perot etalons. In recent years, these devices have been commissioned at several observatories with precision RV instruments (e.g. ESPRESSO, HARPS, NEID, HPF, SPIROU, Maroon-X). Their spectrum can be readily engineered to provide very broad wavelength coverage including the UV range, and linewidth and spacing precisely matched to the spectrograph. While etalons have demonstrated very reliable operation, and excellent short term stability, they exhibit long term drifts that are
wavelength dependent, likely due to aging effects in the mirror coatings, necessitating the use of other, absolute calibrations sources to periodically recalibrate the etalon spectrum and track the long term drift. The mechanism responsible for the chromatic drifts needs to be identified and the coating effects mitigated before Etalons can become stand-alone calibrators at extreme precisions.

Interferometry

Interferometry is a powerful approach to reach angular resolutions beyond that of single telescopes so one can study the environment in the immediate vicinity of the host target. While GRAVITY has led to groundbreaking results in the NIR with its classical ABCD beam combiner approach, several technical advancements can be undertaken to extend the scientific reach including improving the throughput in several spectral ranges, increasing the Fourier coverage by combining more telescopes, and integrating active fringe tracking capabilities onto the beam combining chips. As an alternative, discrete beam combiner (DBC) technologies are a promising approach that may simplify beam combination circuits when >4 telescopes need to be combined due to their implementation of straight SM waveguides in a lattice. But, the technology also requires their operational bandwidths to be extended and at the same time, realize a smart algorithm for finding the best input configurations and the geometry of the lattice when the number of input telescopes increases. Given their substantial differences, we discuss classical ABCD based beam combiners and DBCs in separate chapters.

In parallel to advancing beam combiner technologies, developing the components to allow kilometric baselines by using optical fibers is also critical. This requires the development of low loss fibers across the NIR and MIR regions (1–17 µm), a careful understanding of the dispersion properties of fibers and how to compensate for delays with fibers as well as FBGs, and to determine if photonic fringe tracking, via piezo stretched fibers or thermal or electro-optic (EO) phase shifters on-a-chip will be sufficient to replace bulk optic delay lines. Birefringence and diattenuation will also need to be closely studied.

For high-contrast applications, nulling interferometry is the desired approach and the roadmap outlines the components (tri-couplers, achromatic phase shifters, etc) needed to realize high performance circuits in future at both NIR and MIR wavelengths. To eliminate any stellar leakage the circuit architecture also needs further development, with approaches like kernel nulling showing promise, but careful evaluation is needed. Fiber and PL nulling rely on a fiber or a mode-selective photonic lantern (MSPL) to be positioned directly in the telescope focal plane. Although they are simple to implement, the contrast will be limited without further investigations into (1) the ability of the cladding to suppress the rejected stellar light and (2) the limitations of cross-coupling in MSPLs. A PIC-based beam combiner is a promising solution to calibrate the leaked light of a PL nuller to enhance the null, but needs to be studied in detail to determine the optimum architecture of the circuit.

Finally we can consider other techniques used in radio astronomy, including heterodyning and frequency conversion. The latter allows the thermal IR photons to be converted to the visible or the near infrared where detectors are abundant and thermal background is no longer the limiting factor. Moreover the combination of the heterodyne or frequency conversion and the use of photonic technologies borrowed from the telecommunication world offers the possibility of linking the telescopes of an array to the central interferometric correlator without the need for costly and hard-to-maintain infrastructure. Interestingly, the combination of a heterodyne approach and single-photon quantum technologies may allow highly efficient interferometric schemes. However, to realize functioning instruments in the MIR one needs broader bandwidths using faster mid-infrared (MIR) detectors, phase-stable mid-IR LFCs and broadband nonlinear conversion crystals.

Realizing efficient, multi-functional instruments

The ultimate goal is to route and process the light collected from the telescope completely in photonics to the detectors (see figure 2 for an example of an integrated photonic instrument concept). Photonics offers many useful functions to achieve this, each optimized across a broad range of disparate materials and platforms. To advance towards realizing instruments on-a-chip, efficient hybridization will be necessary. Mastering repeatable high-efficiency packaging to deliver ultra-low loss coupling from fiber arrays to PICs, and between disparate PIC platforms optimized for various functions (silica-on-silicon (SOS), ion-exchange waveguides in glass, silicon nitride, silicon on insulator, etc) will be necessary. In addition, fan-out devices, needed to go from the output of multi-core fibers (MCFs) to PICs will need to be further developed to reduce losses. Active circuits will also require electrical integration and possibly laser sources for calibration and/or metrology.

Repeatable and robust packaging—integration of various technologies to make a single functioning device will be critical. Fabrication and packaging of most photonic devices is still a niche industry or done on an individual basis, we need to better utilize the expertise of the broader integrated photonics industry and for their capabilities to evolve and mature (e.g.: multi-project wafer (MPW) runs, detector integration, fiber
<figure><img src="image_6.png" /><figcaption>**Figure 2.** (Top) The five main subcategories of the roadmap are shown with example technologies that can be used in each, as well as an example potential implementation. Some of the technologies shown here include: fiber injection (with photonic lanterns [20], lantern-based wavefront sensing (Reproduced from [53]. CC BY 4.0.) and 3D-printed microlens arrays on multi-core fibers (Reproduced with permission from [54))), spectral filtering and spectroscopy (using complex waveguide Bragg gratings (Reproduced with permission from [55] © The Optical Society) and arrayed waveguide gratings [56]), spectral calibration (with on-chip laser frequency combs (Reproduced with permission from [57))), interferometry (using Kernel-phase nulling [58]), and detection integration. Reproduced from [59]. CC BY 4.0. (Bottom) A future vision for an integrated photonic high-contrast imaging instrument. Micro-lenses segment the pupil plane, a pupil remapper routes the light to a nulling beam combiner chip which has active control for fringe tracking, before light is injected into an arrayed waveguide grating (AWG) to disperse the light and then onto a detector (image taken from [60]).</figcaption></figure>

to chip to free space/detector packaging). Standardization within the photonic industry will dramatically reduce costs for future devices.

Detector integration with PIC or fiber devices will be key to realizing an integrated instrument. Semiconductor detectors operating at temperatures closer to where photonics are typically operated could be integrated via single pixel photo-diodes on the chip, on an active chip which is flip chipped onto the passive device, via linear arrays which are edge coupled to the PIC or 2D arrays that image the beams ejected out of the top surface of the PICs, via grating or vertical couplers. These options need to be studied in more detail to understand the relative pros and cons and for example at which pixel count to transition from single pixel
photodiodes to arrays. Losses of the various outcoupling options (edge, vertical, grating couplers) need to be minimized over broadbands as well. The role of micro-optics needs to be better understood.

To push the sensitivity of detection, photonics could be integrated with superconducting detectors as well. Both microwave kinetic inductance detectors (MKIDs) and superconducting nanowire single photon detectors (SNSPDs) run at much lower temperatures (<4 K), but provide extreme sensitivity photon counting capabilities, free from dark current and read noise. The mismatch in materials between those used for PICs and detectors means further studies are needed to determine the optimum route to integrate them. In addition, superconducting detectors and circuit structures should be optimized for IFUs in the THz as well.

### Community development

In addition to the technical developments outlined above, the community needs development as well. Firstly, there is a lack of astrophotonics experts primarily because universities nurture astronomers or photonics technologists. Although specialists are needed, the field also requires interdisciplinary experts that can understand the astrophysics science cases, formulate requirements and then design and realize photonic instruments. New bridges between workforce training centers and new pipelines to bring people into the field are needed. In addition, multidisciplinary centers such as innoFSPEC (Germany) fuel innovation in astrophotonics by bringing together expertise from different fields.

Secondly, the community is not representative of society at large as is the case of the parent field—astronomical instrumentation. As a first step towards addressing the latter point, the community should leverage the initiatives developed by professional societies like SPIE, Optica and AAS, who frequently have discussions, training and workshops on various topics about diversity, equity and inclusivity at conferences, to name a few. At the local level, revising admissions and hiring, retention and advancement practices to eliminate biases and increase diversity and equity are critical.

Realizing the potential of astrophotonics is contingent upon developing a talented, robust and diverse workforce. This work force will not be developed without significant and directed attention and effort by the community.

### Concluding remarks

The series of papers that follow elaborate on these five technical themes, each outlining the status, current and future challenges, and advances in the science and technology to meet the challenges for 24 key areas of the astrophotonics field. The technical developments specified in the roadmap will require development of the workforce, adding new interdisciplinary specialists and creating a more diverse and a robust community. Through close collaboration between academia, which is better positioned to advance concepts, designs and qualify devices, and industry partners who are better positioned to provide fabrication and packaging, the technical developments outlined can most efficiently be realized. While astrophotonics has great science potential, currently the field is not funded at a level that can sustain large infrastructure upgrades for PIC, fiber, and packaging advancements. Instead, the field will benefit from continued investments by industries such as telecommunications, AR/VR headsets, photonics for satellites, and LIDAR for autonomous vehicles. Conversely, those fields will also benefit from astrophotonic driven developments. Despite this, the astronomical community must stay engaged with industry partners to provide them with guidance on the needs of the community as well as to be inspired by advances in vendor capability.

### Acknowledgments

This work was supported by the National Science Foundation under Grant No. 2109232 and support for P Gatkine was provided by NASA through the NASA Hubble Fellowship Grant HST-HF2-51478.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA Contract NAS5-26555.
## 2. Symbiosis between adaptive optics and photonics: the path to fully integrated instruments

Philipp Hottinger<fnref n="1" />, Olivier Guyon<fnref n="2" />,<fnref n="3" />,<fnref n="4" />,<fnref n="5" /> and Rebecca Jenson-Clem<fnref n="6" />

### Status

Most 8–10 meter class telescopes are equipped with AO to compensate for atmospheric turbulence. All AO systems rely on wavefront sensor(s) (WFS) to measure optical aberrations, deformable mirror(s) (DMs) for optical correction, and computational methods for linking WFS measurements to DM commands. Yet their design details vary considerably depending on the telescope environment and the intended science application.

Laser guide stars (LGS) assisted wavefront sensing enables AO-corrected observations of fields with no bright star, such as the galactic center [61, 62]. The AO-corrected field of view (FoV) can be extended by using multiple DMs and WFSs [52] to support, for example, crowded-field astrometry or MO spectroscopy (e.g. Gemini's GEMS and GNAO, VLT's MAVIS [63] and Subaru's ULTIMATE [64]).

Extreme-AO systems (ExAO), on the other hand, are optimized for performance over a small field-of-view around bright natural guide stars, integrating high actuator count DMs with advanced wavefront control methods. Several such systems have been deployed for imaging exoplanets, including Gemini/GPI [65], VLT/SPHERE [66], Subaru/SCExAO [67], and Magellan/MagAO-X [68]. Focal plane WFSs can address remaining wavefront distortions such as non-common path aberrations (NCPA) and thermally-induced phase discontinuities [69], using a wide range of algorithms and approaches [70–72].

Thanks to excellent AO correction in the (near-)IR over a small FoV of ~50" on large telescopes, starlight can now efficiently be coupled into SMF for high angular and spectral resolution spectroscopy, with an efficiency closely linked to Strehl ratio (SR). Jovanovic et al [50] achieved coupling efficiencies of over 50% with SRs of 60% in H-Band at Subaru/SCExAO, Crass et al [73] >35% in Y- and J-Band with LBTi/iLocater, and Delorme et al [74] coupling efficiencies of 60% aim to reach coupling efficiencies of 60% in K- and L-Band with Keck2/KPIC. Photonic SM components extend SMF use to multiple telescopes [75], and compact integral-field spectroscopy with approaches including hexabundles [76], SCAR [77, 78], and 3D-M3 [54].

With the intermediate SRs of 10%–20% delivered in near-IR by wide FoV (~100") AO systems, coupling into a single SMF becomes less efficient. Modal conversion with photonic lanterns (PLs) [7] can couple light to multiple SMFs to feed downstream SMF-base instrumentation such as a fiber-bragg gratings (FBG) for airglow suppression of OH emission lines in the H-Band [23]. Alternatively, few-mode fibers can make use of this partial AO correction like NIRPS [79] but these are prone to the negative impact of modal noise.

AO subsystems could individually be replaced by maturing photonic technologies, offering identical or enhanced functionalities in a miniaturized and integrated footprint. The manufacturing processes often allow in-situ alignment [80] that reduces operational complexity and increases optical stability. One of the most promising applications is the use of PLs as focal plane WFS with the potential to supplement well-established pupil plane WFSs as compact additions to sense residual aberrations (see section 3 for more details). Large systems would benefit from the reduced complexity and smaller footprint of these sensors, enabling better scalability for Multi-Object-AO and Multi-Conjugate-AO systems requiring multiple WFSs and thus making it easier to achieve better AO performance over a wide FOV. Goodwin et al [81] introduced the concept of a miniaturized Shack-Hartmann WFS with similar benefits.

To optimally exploit photonic technologies, they should not simply replace individual conventional components but rather aim to be integrated as part of the science instrument and the AO system. This type of hybridization will make telescope optics more resource efficient as it reduces optical and mechanical footprint and complexity. One partially integrated approach has been proposed by Dietrich et al [82] for reconstructing tip-tilt with a multi-core SMF equipped with an 3D-printed lenslet array and tested on-sky with a refined design utilizing a micro-lens ring tip-tilt sensor (MLR-TT sensor) by Hottinger et al [83].

<footnote n="1">¹ Landessternwarte, Zentrum für Astronomie der Universität Heidelberg, Heidelberg, Germany</footnote>

<footnote n="2">² Subaru Telescope, National Astronomical Observatory of Japan, National Institute of Natural Sciences, Hilo, HI, United States of America</footnote>

<footnote n="3">³ Astrobiology Center of NINS, Osawa, Mitaka, Tokyo, Japan</footnote>

<footnote n="4">⁴ Steward Observatory, University of Arizona, Tucson, AZ, United States of America</footnote>

<footnote n="5">⁵ College of Optical Sciences, University of Arizona, Tucson, AZ, United States of America</footnote>

<footnote n="6">⁶ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, United States of America</footnote>
There, wavefront sensing is integrated into a vital part of the science instrument, in this case with simultaneous SMF coupling. While tip-tilt sensing is only a limited functionality, it shows the advantages such an integrated approach can have as it reduces complexity by replacing multiple bulk optic components while almost completely eliminating NCPA.

This interplay between AO and instrument has led to demand and existence of test environments that allow transition of the development from laboratory to on-sky performance in order to mature existing concepts. These are core objectives of the SCExAO testbench at Subaru [67] and Canary at WHT [84].

## Current and future challenges

**Efficient injection of astronomical light into photonic devices** requires exquisite wavefront quality and stability, beyond what is achieved with AO on large telescopes. Reliable, efficient and stable coupling will require further improvements in AO performance, with particular attention to vibration control and atmospheric dispersion correction. Proper optical matching of the telescope beam to the fundamental mode of the SMF is also important [85]; phased induced amplitude apodization has already been shown to have the potential to significantly increase single mode (SM) fiber coupling [50, 86].

**Modern AO systems' demanding wavefront quality requirements** on large telescopes drive them to employ fast, sensitive and accurate WFSs driving high-count DMs, with higher optical complexity and use of advanced wavefront control algorithms. Precise calibration of system components (WFS, DM) and control of residual NCPA is becoming increasingly critical. This is especially essential in HCI applications requiring starlight suppression by nulling or coronagraphy to support the direct imaging and spectroscopic characterization of exoplanets. Photonic technologies can provide the high performance interferometric wavefront sensing solutions to meet this challenge, combining high sensitivity with large dynamical range, and providing wavelength diversity. This interferometric approach to wavefront sensing is well-suited for a second-stage high-precision WFS/C following a more conventional AO correction so that starlight is efficiently coupled to the photonic device(s). These benefits have recently been demonstrated on-sky with the PL [87] and the GLINT instrument [88, 89]. To fully realize this potential, the interferometric WFS output signals will need to be used for real-time active wavefront control, either by DM actuation or in-chip phase modulation. Wavefront information is then extracted by direct measurement of interferometric phases, as is done in multi-telescope interferometers.

**Manufacturing capabilities** of photonic components are often still insufficient for astronomy applications, with most new developments driven by the much more broadly funded telecommunication industry which usually has less demanding requirements on optical throughput, pathlength control, and broadband performance. The astronomical community has the chance to contribute in these new areas, expanding the range of applications of photonic devices. When astrophotonics aims to integrate or replace full-scale AO systems, scaling of these devices also becomes an issue as the number of coherent elements scales with ~($r_0/D^2$) and so will the required number of individual sensor readout and pathlength control units. In the near future, photonics-based AO systems could be deployed as small units downstream of AO systems using bulk optics, while full-scale photonics AO systems on large telescopes will require more development.

**Practical challenges** of astrophotonic technologies need to be considered as these are often a new class of manufactured devices that need special attention. For small scale refraction components, 3D-printed microlenses fabricated using two-photon lithography allow high flexibility in free-form shapes and enable the possibility for *in-situ* printing providing integrated alignment [80]. But this also means that care needs to be taken for handling these devices during transportation and employment as their structural integrity and the bond joint between components is vital to their performance. SMFs feature a mode-field diameter (MFD) that varies slightly between manufacturing batches. For maximum coupling between separate fibers, the MFD needs to be matched. Ultra-fast laser inscription [90, 91] can be used for precise waveguide design and is continuing to mature but many manufacturing challenges such as reliable waveguide printing, limited refractive index contrast, and high attenuation still remain. In particular, these considerations will also be essential when astrophotonic devices are to be employed at space based observatories as the environmental impact regarding vibrations, temperature, pressures and background radiation pose an additional challenge.

## Advances in science and technology to meet challenges

There are two main roads towards an increased symbiosis between astrophotonic instruments and AO. Firstly, individual photonic components can be employed to augment conventional systems, for example by providing a second stage WFS, or by performing a science measurement well suited for photonic approaches downstream of high performance AO correction. In the long term, photonic devices can potentially also replace parts of conventional AO systems, for example as the main WFS. Secondly, photonic components will be tightly integrated together as a system performing a wide range of functions, leading to more compact,
<figure><img src="image_7.png" /></figure>

less complex instrument systems that enable high automation both in manufacturing as well as in operation. Some concepts enable new optical approaches potentially surpassing the performance and capability of conventional systems. Figure 3 brings together these points, showing the existing individual photonic components that correspond to or can supplement conventional systems (center column) as well as integrated concepts where photonic components are part of a larger instrument (right column).

A general challenge in the development of astrophotonic components is the adaptation of photonic components to the requirements of astronomical observations. Three requirements that are significantly less stringent in telecommunication application play a central role in astrophotonics. Firstly, the wavelength performance needs to be enlarged to broader bands as both instrument sensitivity and science goals require throughput and optical performance over a larger wavelength range. Secondly, throughput of astrophotonic devices has only a very small loss budget as most scientific observations are performed in the photon starved regime. Thirdly, environmental stability cannot always be guaranteed in observatories, imposing higher tolerances on temperature, humidity, and mechanical constraints.

The deployment of the photonic beam combiner at GRAVITY [75] has indeed shown that photonics can already individually replace conventional components and uniquely enable high precision astrophysical measurements. In general, evolution for these components is slow as these concepts need to prove their optical performance and optical efficiency before consideration for large scale investments. Adoption of astrophotonic solutions in major astronomical instrumentation projects relies on successful proof-of-concept demonstrations, preferably performed on-sky. To boost development, access to and support from state-of-the-art AO instrumentation on large telescopes should be prioritized.

As outlined in the previous sections, astrophotonic technologies are already bringing advances in wavefront sensing, although they currently support a moderate number of wavefront modes. Two main approaches are on the horizon. Firstly, interferometric wavefront sensing between multiple telescopes for facilities like VLTI/GRAVITY and CHARA or within the same large telescope aperture as demonstrated in FIRST [95] and GLINT [12], offering nanometer-level phase precision simultaneously with spectral resolution thanks to wavelength-dispersed fringes. Secondly, PLs and MCFs can reduce optical elements and thus provide a simple high-throughput alternative to bulk optics slit formatting, light transport, etc. Additionally, in these applications, the photonic device can perform low-order wavefront sensing of a few
modes and operates as a final-stage WFS downstream of the high-order AO system. In the near future, these few-mode devices could be integrated with the kHz-speed AO correction by feeding back into the upstream AO system.

For WFS control, communication between different subsystems must be established. The use of machine learning (ML) algorithms is explored in a number of projects and shows potential to solve some of the challenges associated with analysis of complex sensor data as was demonstrated in PL-WFS demonstration by [53]. The use of these algorithms is already widespread in many real time applications outside astronomy, and will need to be accommodated in observatories as well. The astronomical context will require operation in the kHz regime with a wide variety of input data from both optical and environmental sensors such as accelerometers, temperature sensors, and seeing measurements. The computational infrastructure must be expanded to fulfill this need, including the corresponding telemetry data filter and storage which is needed for both analysis and future algorithm training as well as specialized ML inference hardware. Linear and higher order reconstruction methods have also been explored by [96].

Photonic devices can integrate wavefront correction as on-chip phase shifters, a highly miniaturized equivalent to optical delay lines used in interferometers. While current interferometer systems use mechanically driven fiber delay lines, photonics provides several solutions that can potentially offer control authority into the kHz regime, including thermoacoustic devices [97], thermo-coupled [92], EO phase shifters [98] and piezo-actuated phase shifters [99]. Additionally to classical wavefront correction, fast photonic-based pathlength modulation can reveal incoherent exoplanet light within the coherent starlight residual when performing high-contrast imaging (HCI).

Photonic components can both transport and modify the transported light. This enables combinations of these functions where a fraction of the guided photons is used for sensing. Such integration of AO and science acquisition in a single compact photonic device will allow for new self-calibrating and self-tuning capabilities to be a core feature of the instrument design. This architecture is particularly attractive for HCI, where a photonic nulling chip could iteratively reconfigure its internal phase delays [100] or drive external DM(s) to maintain optimal broadband null depths. By integrating science measurement and WFS functions within the same photonic chips, the stable relationship between WFS and science outputs can be measured and leveraged for self-calibration of science measurements down to the photon noise residual [88].

## Concluding remarks

Given the rapid pace of development of photonics applications to AO and astronomical instrumentation over the last decade, and the growing number of successful on-sky demonstrations, photonic technologies and devices are poised to play an increasing role in astronomy. Progress in this area is driven by advances in underlying core technologies, maturation of instrument designs incorporating emerging photonics solutions, and access to increasingly powerful AO systems providing the suitable wavefront quality on large astronomical telescopes.

Integration of AO and astronomical measurement functions in a single photonics-based system will be particularly enabling, providing in a compact format a rich set of functionalities with high stability and accurate calibration of science measurements. In order to realize this potential, further developments will need to include both lab prototyping activities and on-sky prototyping demonstrations. In this paper we have summarized the vast variety of different components involved, hinting at the challenges that lie ahead when all these systems are aimed to be combined. Sustained collaborative efforts between phononics experts and on-sky AO-equipped development platforms and photonics-capable instruments have been, and will continue to be particularly instrumental in maturing photonics systems for astronomy.

## Acknowledgments

We thank R J Harris for the fruitful discussions on the topic. P H is supported by the Deutsche Forschungsgemeinschaft (DFG) through Project 326946494, ‘Novel Astronomical Instrumentation through Photonic Reformatting’. O G acknowledges support from NASA Grant 80NSSC19K0336 and JSPS Grant 21H04998 supporting development of advanced WFS techniques including photonic devices. O G and R J-C acknowledge support from the Heising-Simons foundation.
### 3. Photonic lantern wavefront sensing and control

Barnaby Norris<fnref n="1" />, Jonathan Lin<fnref n="2" />, Robert J Harris<fnref n="3" />,<fnref n="4" />, Aline N Dinkelaker<fnref n="5" /> and Momen Diab<fnref n="6" />

#### Status

Astronomy has been revolutionized by the adoption of AO [101], where the wavefront of light, corrupted by Earth's turbulent atmosphere, is sensed, analyzed and corrected. This allows diffraction-limited, HCI of targets such as exoplanets, and is key to enabling the full potential of the upcoming Extremely Large Telescopes. Existing AO systems generally measure the wavefront error at the pupil plane, using WFSs such as a Shack-Hartmann or pyramid sensor [102]. These sensors are blind to highly problematic modes such as petaling (phase shear at the pupil plane around the telescope spiders [103]), and suffer from NCPA with respect to the science focal plane. In a standard imaging system, the focal plane image alone cannot be used to sense the wavefront since it contains only intensity (not phase) information about the PSF.

Placing a PL at the focal plane, allows the complex amplitude of the PSF, and hence the wavefront, to be directly measured (see figure 4 for examples). A PL is a passive device that consists of sets of waveguides with different numbers of modes, between which light is transferred [7, 20], see also section 4. Of particular interest is when the MM waveguide at the input tapers into several SM waveguides at the output, the number of which matches or exceeds the number of spatial modes at the input. The excitation of modes in the MM end of the PL is a direct function of the spatially-dependent complex amplitude of the injected PSF. If the transfer function of the PL is known, the wavefront can be reconstructed from the PL's SM outputs. But this transfer function is only known by measurement post-fabrication, since the fabrication process is not deterministic.

The PL-WFS offers several advantages: it is sensitive to any mode which affects the PSF, including modes to which pupil-plane WFSs are blind, it makes optimal use of detector pixels (one pixel per mode) minimizing detector noise, and the entire device fits within a standard fiber connector making it ideal for MO systems. The output can also be spectrally dispersed via a prism or grating (see sections 6 and 7), to achieve wavelength-resolved wavefront sensing, useful in breaking phase-wrapping degeneracy (such as seen in petaling) and to measure atmospheric scintillation. It can also be used for PSF reconstruction. Perhaps most significantly, a fully photonic device has high stability and is ideal for optimally injecting light into a SM fiber (for subsequent spectroscopy [104], interferometry [12], etc) wherein it offers a truly zero NCPA WFS (see *hybrid MSPL* below).

The relationship between low-order wavefront modes (such as Zernike aberrations) and PL outputs has been shown [105] as well as laboratory and on-sky demonstrations of wavefront reconstruction. These include a 19-mode PL-WFS and a neural network reconstructor using a multi-core-fiber based PL [53] and a pupil plane low-order WFS [106].

Research is underway to evaluate the efficacy of different PL types and algorithms for WFSing. Both optical-fiber-based [7] and ultrafast laser inscribed (ULI) lanterns [107] are being investigated, as well as MSPLs [108, 109]. While in a standard PL each SM output is a complex linear combination of the complex input mode amplitudes, in a MSPL a one-to-one mapping between the modes excited at the MM port and the SMFs can be engineered [19]. This can ideally match the modes coupled to those in the PSF [110]. However, the number of modes that MSPLs can multiplex is limited by device length and core diameter, with the length of the required taper proportional to the square of the number of modes [111], and therefore MSPLs are not expected to be effective in WFSing applications [96].

A so-called *hybrid* MSPL can circumvent this limitation; here, all light propagating in the fundamental (LP₀₁) mode is routed to a single output, and higher order modes are used for WFSing. In this case, the higher order outputs drive the AO system to maximize coupling in the LP₀₁ mode, which is then routed to the SM science instrument. Alternative modes could be made selective instead if desired. Importantly, this selectivity is maintained over a broad bandwidth [112], such as the 1.5 µm–1.8 µm range demonstrated in this reference.

With ongoing developments and diverse configurations, PLs have the potential to provide solutions for a broad range of WFS applications (see figure 5 for an example of PL WFS integration).

<footnote n="1">¹ Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, Sydney, Australia</footnote>

<footnote n="2">² Department of Physics and Astronomy, University of California, Los Angeles, CA, United States of America</footnote>

<footnote n="3">³ Max-Planck-Institute for Astronomy, Heidelberg, Germany</footnote>

<footnote n="4">⁴ Department of Physics, Durham University, Durham, United Kingdom</footnote>

<footnote n="5">⁵ Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, Germany</footnote>

<footnote n="6">⁶ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, Toronto, Canada</footnote>
Science Cases

<figure><img src="image_8.png" /></figure>

Wavefront sensors

<figure><img src="image_9.png" /></figure>

**Figure 4.** Different photonic lantern wavefront sensor designs (bottom) and their potential applications (top): the highest level of correction/narrow field-of-view on the left, and lower level of correction/wide field-of-view on the right. Blue indicates focal plane and red indicates pupil plane WFS. Image credits for the insets (top row, 'Science Cases'): 'Exoplanets': Jason Wang (Northwestern)/William Thompson (UVic)/Christian Marois (NRC Herzberg)/Quinn Konopacky (UCSD), 'Stars/AGN': ESO/WFI (Optical); MPIfR/ESO/APEX/A.Weiss et al (Submillimetre); NASA/CXC/CfA/R.Kraft et al (x-ray), 'Spectroastrometry': ESO, 'Galaxy surveys': M. Blanton and SDSS. Insets for the wavefront sensors (bottom row): (a) Reproduced from [53], CC BY 4.0. (b) Reproduced with permission from [105]. (c) Reprinted with permission from [113] © The Optical Society. (d) Reproduced from [106], CC BY 4.0. The source images for the 'Science Cases' insets and references [53, 106] are licensed under a Creative Commons Attribution 4.0 International License (https://creativecommons.org/licenses/by/4.0/).

<figure><img src="image_10.png" /></figure>

**Figure 5.** Overview of a PL-WFS system. Light arrives at the telescope distorted from the atmosphere. After the telescope mirrors, the light is guided to a photonic lantern, where it is distributed between different channels and exits at multiple outputs. Here, it can be spectrally dispersed and is then detected. The data can be used directly for scientific analysis as well as wavefront reconstruction (e.g. using a neural network), with the results feeding back to a deformable mirror earlier in the optics chain for wavefront correction.

Current and future challenges

One difference to overcome between the PL-WFS and traditional WFSs is that PL-WFSs are non-linear. Since the measured quantity is the SM output intensity (the square of the complex amplitude), the transfer function mapping wavefront phase to output flux is non-linear (and non-monotonic for large phase errors [53], since the coupling is cycling in a sinusoid-like fashion between constructive and destructive interference as phase-difference increases). Therefore, for wavefront reconstruction (e.g. for closed-loop AO) the standard linear reconstruction algorithms only work over small wavefront error ranges, where the transfer function is locally linear. One option is to use a higher order (e.g. quadratic) reconstruction algorithm [96] or to use a fully non-linear inference model (e.g. neural network [53]). An alternative approach is to instead
optimize for PL linearity, either altering the PL itself or through additional optics such as phase masks and beam recombiners.

While measurements of on-sky wavefronts have been demonstrated [87], the next step is to deploy a PL-WFS in a closed loop configuration on-sky. The current challenge is deploying a suitable (e.g. non-linear) wavefront reconstruction algorithm that can operate with sub-millisecond latencies and integrating it into existing AO software, while running in parallel with existing high-order WFSs. Additionally, while the compact size of a PL-WFS makes it ideal for MO WFSing, algorithms and instruments to exploit this are yet to be developed.

To date, only low-order (<20 modes) PL-WFSs have been demonstrated. For high-order AO applications such as speckle suppression many more modes would be advantageous. PLs with many 100s of modes have been produced (e.g. using a cascaded design [21]), but not yet routinely. The difficulty in implementing algorithms for 100 or even 1000 mode PL WFsing is yet to be determined. An alternative could be to use microlens arrays to reduce the number of modes in each lantern, such as the WFS proposed by [106].

A high mode-count PL becomes especially important for imaging, wherein each mode corresponds to one spatial element of the image. This is an important goal, as obtaining a full complex image of the science target would allow speckles and the science object to be optimally disambiguated—currently one of the greatest challenges in HCI [114]. A suitable architecture (and algorithm) to measure this, along with the coherence properties of the source, is an active area of research. One possibility is to interferometrically recombine the outputs of the PL to measure their complex amplitudes [113, 115]. Alternatively, the interferometry could take place within the PL itself, with the number of PL outputs being made greater than the number of input modes, thus oversampling, and encoding phase, amplitude and spatial coherence directly into the output intensities.

While the design parameters of fiber-based PLs can be broadly specified, the tapering-based fabrication process means that the exact transfer function cannot be specified, and tapering parameters are set empirically [116]. A precise, deterministic fabrication method would enable more specialized and optimized designs. Deterministic fabrication would address the mismatch between the ‘ideal’ PLs assumed in simulations, and real PLs, which feature slightly mispositioned cores, non-circular claddings, and other defects. The simulation process also faces other challenges, such as the high computational cost and approximations of standard beam propagation algorithms (such as the beam propagation method (BPM)), reducing the accuracy of future high mode-count models.

One promising application is the use of the PL-WFS (possibly with hybrid MSPL fiber injection) on arrays of small telescopes. Their small apertures offer high SRs with only low-order correction such as that provided by a MSPL. Light from multiple telescopes could then be efficiently injected into SMFs and (in)coherently combined for diffraction-limited spectroscopy, or interferometry. A general challenge, however, is the overall lower flux collected by these smaller telescopes, requiring the PL-WFS system to match the brightness limit of any other WFS in the system.

### Advances in science and technology to meet challenges

More accurate, deterministic control of PL parameters during fabrication would enable devices to be manufactured whose properties match those in the simulated design. Most mature photonic fabrication techniques such as photolithography are limited to two-dimensional structures, making them unsuitable for PLs. Current fiber-based PLs are produced via tapering on fusion splicing glass processing machines, making reliable and repeatable fabrication difficult. Increased demand for PLs in industry (e.g. telecommunications) may lead to PL-oriented glass processing machines. ULI is a promising technology, allowing the use of more arbitrary structures to construct the PL. This technology is still developing, and generally has lower refractive index contrast and worse control than fiber-tapering methods, but the field is evolving rapidly [90, 91].

For any fabrication technique, accurate simulations require a better understanding of how errors in the manufacturing process propagate to actual WFS behavior. For end-to-end simulations, the source of wavefront error represented should be refined to incorporate errors from a wider range of on-sky and instrumental sources. Advances in numerical simulation algorithms are also needed, both for increases in accuracy (e.g. adopting a fully vectorial BPM technique or finite-difference time-domain method) and speed. Fully differential numerical models would also be extremely useful to enhance the optimization processes. Together these will allow production of devices with precisely optimized imaging/wavefront sampling.

Advancements in AO algorithms are required, including optimization of non-linear algorithms (e.g. neural networks) at low-latency kilohertz rates for closed-loop operation. The performance of these algorithms under a range of conditions must be carefully characterized to achieve the reliable, routine operation demonstrated by standard AO algorithms. Beyond WFSing, performing full coherent imaging—simultaneously measuring the image and wavefront, and its spatial coherence properties—will require innovation in analysis and image reconstruction. New detector technologies, especially
photon-counting, wavelength-resolved detectors such as MKIDS (see section 25), would also offer a huge
advantage as they have effectively zero read-noise and would allow high spectral resolution (acting as
cross-dispersion for an Echelle spectrograph), but are currently limited to low photon-count rates.

Due to their stability, compact form factor and high sensitivity due to optimal detector usage, PL WFSing
may become invaluable for space-based telescopes. Since they operate in the low wavefront error regime, a
high mode-count PL could be the only WFS (both for speckle control and mirror phasing), relaxing the need
for separate pupil-plane WFS and optics. As highlighted by the Roman space telescope coronagraph
technology demonstration [117] the success of future high resolution space telescopes will be contingent on
WFSing and control. WFSing and control is also critical for optical communications between space and
ground. Before PLs can be deployed in these contexts, these components will have to be space qualified by
means of environmental testing and technology demonstrator missions using small satellites.

Concluding remarks

The use of PLs for wavefront sensing is a novel application and has already seen a surge of interest. Advantages include the ability to perform direct measurements of both phase and amplitude at the focal or pupil plane, inherent compactness and stability, optimal use of detector pixels, and the ability to perform optimal injection of starlight into one SMF (acting as a zero non-common-path WFS). While the concept has been demonstrated in laboratory and in open loop on-sky experiments, further development is required to enable widespread adoption. This includes implementing low-latency non-linear control algorithms and developing deterministic, accurate fabrication methods coupled with precise simulations. Full coherent imaging (wavefront and image measurement) is also on the horizon. There is a strong research effort by multiple groups worldwide to investigate these aspects, and programs to test PLs on major telescopes are already underway.

Acknowledgments

B R M N acknowledges support from the Australian Research Council, Discovery Early Career Researcher Award (DE210100953).

R J H would like to thank the Deutsche Forschungsgemeinschaft (DFG) for their support through Project 326946494, ‘Novel Astronomical Instrumentation through photonic Reformatting’.

A N D acknowledges support by the Bundesministerium für Bildung und Forschung (BMBF) through the Project 03Z22AN11.

This material is based upon work supported by the National Science Foundation Graduate Research Fellowship Program under Grant No. DGE-2034835. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation. This work was also supported by the National Science Foundation under Grant No. 2109232.
## 4. Spectroscopic applications enabled by photonic lanterns

Sergio-Leon Saval<fnref n="1" />, Steph Sallum<fnref n="2" />, Stephen Eikenberry<fnref n="3" /> and Kevin Bundy<fnref n="4" />

### Status

Conventional astronomical fiber-fed spectrographs use MM optical fibers to feed telescope light to a disperser. These fibers can be sized to match the width of the point-spread function (PSF), providing more efficient light capture compared to their telecom counterpart, SMFs [118]. However, these spectrographs are larger and limited in spectral resolution compared to diffraction-limited platforms fed with SM fibers. These have recently been demonstrated for precision RV measurements [119–121] for example, but require high performance AO systems to prevent significant fiber coupling losses [50, 122]. PLs offer a powerful solution for efficient diffraction-limited spectroscopy by converting MM inputs to SM outputs (figure 6(A) and (C)). This reduces AO performance requirements, enabling near-diffraction-limited spectroscopy at the outputs [123–125]. As a result, PLs have recently emerged as a viable technology for the following applications: spectro-astrometric imaging; efficient combination of light from small telescope arrays; and massively-multiplexed spectroscopy for wide fields of view.

Spectroastrometry (SA [126]); involves using spectral centroid shifts to infer the presence of wavelength-dependent morphology within the diffraction limit (as depicted in figure 7(C)). Traditionally carried out with slit spectrographs both with and without AO (e.g. [127]), conventional SA's astrometric precision is set by the PSF width and signal-to-noise ratio, with precision <1 mas for the best AO experiments [128]. Even AO-corrected SA can suffer from PSF variability (e.g. [129]), and PLs have recently been demonstrated in simulation as an avenue for recovering SA signals in the presence of variable correction [130] (figure 7(C)). For example, hydrogen line signals from accreting planets can be recovered for achievable tip-tilt jitters of 0.1 λ/D. Furthermore, since SM output intensities encode pupil plane phase variations, PL spectro-astrometry does not suffer from position angle degeneracies present in conventional approaches, making it a promising technique for complex scenes such as resolved circumstellar disks and outflows.

A second PL application involves combining light from arrays of incoherently-linked smaller telescopes. Such arrays have recently emerged as a reduced-cost (by ~10× per unit area) method of increasing telescope collecting area for spectroscopy of faint sources [131–134]. In the PolyOculus implementation (figure 7(A)), packs of semi-autonomous, small (25–40 cm), off-the-shelf telescopes comprise the array [132]. PLs that incoherently combine several multi-/few-mode fibers into a single MM fiber offer particular advantages for combining the light, because they preserve etendue without oversized downstream pupils [132]. PolyOculus has demonstrated ‘7-pack’ PL light combiners with >90% efficiency across the visible [135], producing a collecting area equivalent to a ~0.9 m telescope (figure 7(B)). Furthermore, using multiple seven-pack arrays, the PL outputs can be spliced to optical fibers which then feed into another PL combiner. This approach can be extended, providing a hierarchical scaling to build collecting areas equivalent to the 30–40 m diameter mirrors of upcoming Extremely Large Telescopes.

Finally the ability of astronomical spectrometers to measure multiple target spectra simultaneously (i.e. the multiplex; e.g. SDSS, DESI, SAMI; [136–138]), has enabled major progress in cosmology, galaxy formation, and stellar astrophysics. The state-of-the-art is the DESI facility, which collects 5000 simultaneous fiber spectra across an eight square degree field on a 4 m telescope. Future instruments with 4–10 times greater multiplex on larger telescopes are already under consideration. With a conventional design, such an instrument would be impractical, exceeding the cost of the telescope itself. Mass-produced photonic spectrometers, however, offer a path toward greatly reducing the ‘cost per spectrum’ (e.g. a ‘spectrometer-on-a-chip’; sections 6 and 7). Because these devices typically demand single- or few-mode input, PLs will play an important role by extracting the modal components of potentially thousands of on-sky sources.

### Current and future challenges

Currently, PLs have achieved high performance with transmission efficiency of >90%. However, lanterns have been constrained in terms of the number of inputs and outputs. PLs with up to 19 output SMs are fairly standard, but larger port number devices would be desirable. They would enable more efficient combiners

<footnote n="1">¹ Sydney Astrophotonics Instrumentation Laboratory (SAIL), School of Physics, The University of Sydney, Sydney, Australia</footnote>

<footnote n="2">² Department of Physics and Astronomy, University of California, Irvine, CA, United States of America</footnote>

<footnote n="3">³ CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, FL, United States of America</footnote>

<footnote n="4">⁴ Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA, United States of America</footnote>
<figure><img src="image_11.png" /></figure>

for telescope arrays, more versatile reformatting for integral field and multiplexed spectroscopy, and more detailed recovery of asymmetries via spectro-astrometry.

Increasing the number of PL modes relies on increasing the number of fibers or cores in a multicore (figure 6(A)). Scaling to a large number of fibers is challenging due to manufacturing limitations related to handling hundreds of individual fibers and limitations on the maximum diameter handled by glass processing equipment (e.g. splicing, tapering). However, MCFs reduce some manufacturing limitations and could offer hundreds to thousands of cores. To date, the devices with the highest port counts constructed from individual SMFs are 1 × 61 [14], and 1 × 88 with reduced SMF cladding diameters [20]. The largest single multi-core PL with SM cores have 121 cores [139]. A 600 µm-diameter, 511-core multi-core lantern was reported in Birks 2015. However, this PL, intended for fiber scrambling, contained a mix of SM and few-mode cores.

The need for high-performance, wide-field AO presents an additional challenge for high-multiplex instruments with large fields-of-view (e.g. >1° diameter; [136–138]). Diffraction-limited performance on large ground-based telescopes across several arcminutes is currently infeasible, although modest corrections are possible with ground-layer adaptive optics (0.3" FWHM over 8 arcminute fields; [140]) and multi-conjugate adaptive optics (MCAO; ~25% Strehl over ~2 arcminute fields-of-view [52]; and references therein). For instruments with fields-of-view comparable to MCAO correction, the architectural goal would be to optimize coupling efficiency, number of modes, and field-of-view, to achieve acceptable coupling efficiency without prohibitively high requirements on the number of lanterns, outputs, and instruments. While the AO system can reduce the number of modes, high-performance low-mode-count (<19) lanterns will be needed to take full advantage. Although such devices have been demonstrated, reliable reproduction and connectorization of such lanterns remains a challenge. For wider fields—i.e. where Strehl remains low—the challenge lies in identifying and filtering the most valuable SMs from the low-Strehl, multi-mode light. For example, a PL ‘speckle spectrograph’ (where individual speckles are spatially directed to a spectrograph; [141]) may reduce sky noise and boost signal-to-noise, compensating for ignoring many fainter, more spatially-distributed modes.

### Advances in science and technology to meet challenges

One solution to increase the desired number of modes (i.e. inputs/outputs) in PLs systems may be to implement ‘hierarchical’ PLs [21]. In this concept, a large field-of-view PL efficiently separates the light into a large (but manufacturable and manageable) number of MM output fibers. Each multi-mode output feeds into another PL that further subdivides the light. This cascade can be repeated until each output reaches the
<figure><img src="image_12.png" /><figcaption>**Figure 7.** Spectroscopic photonic lantern applications. (A) Schematic layout of the PolyOculus telescope array concept. An array of small autonomous telescopes point to the same target on the sky, with fiber optic feeds coupled via high-efficiency photonic lanterns into a single fiber, which feeds into a scientific detector. (B) Illustration of the state of the art in massively-multiplexed spectrographs, future instruments enabled by photonic lanterns and chip spectrometer arrays, and their anticipated science outcomes. (C) Simulation of photonic lantern spectroastrometry. Companions with excess flux relative to the primary star at specific wavelengths (left) can be detected within the classical diffraction limit by measuring spectral centroid shifts. These signals can be modeled as Zernike modes, illustrated in the right panels showing an injected and recovered protoplanet with Pa-β excess emission (adapted from [130]). Reprinted with permission from [Xanadu X8 quantum photonic processor: Xanadu]. Reproduced from [130].</figcaption></figure>

desired modal content—be it single-, few-, or (smaller) multi-mode. With this approach, lanterns with relatively modest output numbers (~19) could approach ten-thousand-mode outputs with only three hierarchical levels.

Generally, fabricating complex lanterns as described above is possible, with limitations imposed by the system's scientific and/or technical applications. For example, limitations to realizing practical, high-order lanterns are imposed by the need to potentially share light between a science instrument and a WFS, in which case some cores would physically be split off from others. These requirements will impact both the manufacturability and functionality of the lantern. Additionally, manufacturing and engineering challenges will need to be overcome to achieve integration and connectorization of these systems for rugged use in observatory environments.

Hybrid PL devices may soon allow for increasing the number of modes while improving AO performance [87]. Hybrid PLs are made with dissimilar fibers, and can divide light from an MM fiber into separate science and wavefront sensing fiber arrays. These could potentially bring the best of both worlds: single- or few-mode fiber coupling to a science instrument, and simultaneous wavefront sensing without non-common path errors (as discussed in section 3). This would, for example, improve spectroastrometric precision by providing a better, more stable point spread function, and by allowing for selection of science fibers that optimally recover asymmetries in the target. Furthermore, deploying many of these devices could improve AO correction for large fields-of-view, highly-multiplexed instruments, enabling more efficient spectroscopy of faint, distant objects.

## Concluding remarks

PLs present an opportunity for efficient, diffraction-limited spectroscopy using both single-object and wide-field, highly-multiplexed instruments. While these devices have been demonstrated to provide high coupling efficiency for a small number of inputs/outputs, their versatility for astronomical observations would be improved by: (1) manufacturing devices with larger numbers of inputs/outputs, increasing the number of PL modes, (2) developing techniques to extract the most valuable modes from low-Strehl light, and (3) manufacturing devices where outputs can be physically separated for simultaneous applications
(i.e. science and wavefront sensing, which would improve AO performance for both narrow and wide
fields-of-view). Future applications of these enhanced PLs will lead to new observational constraints for a
wide range of sub-fields, from cosmology, to galactic astrophysics, to stellar and planetary formation and
evolution.

Acknowledgments

This work was also supported by the National Science Foundation under Grant No. 2109232.
## 5. New optical fibers for astrophotonics

Stephanos Yerolatsitis¹, Rodrigo Amezcua-Correa¹, Itandehui Gris-Sanchez² and Julia Bryant³

¹ CREOL, The College of Optics and Photonics, University of Central Florida, Orlando, FL, United States of America

² Institute of Telecommunications and Multimedia, Universitat Politècnica de València, Valencia, Spain

³ Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, Sydney, Australia

### Status

Fiber optics have been used in several diverse areas of astronomy since the early 1980s [142–145] and are nowadays commonly deployed at a large number of telescopes. They have been routinely used to transport light from the telescope’s focal plane to a spectrograph and to optically multiplex telescopes, linking remote instruments to enable interferometric observations [143, 146, 147]. In this regard, fiber optics has become a powerful tool used to simultaneously observe thousands of astronomical objects, allowing the realization of massive spectroscopic surveys for the first time (2dFGRS: [148], SDSS: [149]) and integral field spectroscopic surveys (SAMI: [136], MANGA: [137]) Most of these developments in astronomical instrumentation have so far relied almost exclusively on using optical fibers as efficient light pipes. However, processing light in an optical fiber system can provide unprecedented opportunities in many areas of astronomical instrumentation where current approaches are still in need of a solution. In practice, fibers are already providing some degree of signal processing in telescope settings, for example, they are used as spatial [150, 151] and spectral [24] filters, efficient mode scramblers [17], to feed light into PICs [152, 153] and in the development of PLs to mention a few [20, 154]. The difficulty of implementing new fiber technologies and PICs in general for astronomical instruments arises from the MM nature of the light being collected at the telescope’s focal plane. This demands fibers and fiber devices supporting 1000s of spatial modes with low loss, broadband operation, and for some applications without significant mode mixing. High performance adaptive optic systems can help reduce the number of modes that need to be supported, but are limited to narrow fields of view (1–2 arcminutes) [52].

In the last decade, the demand to increase capacity in optical communication systems ensued a flurry of activities on space division multiplexing that utilizes spatial modes in MM and multicore fibers in addition to conventional multiplexing techniques. These research activities not only led to new classes of MM and multicore optical fibers but also new components to tailor and control MM light [48, 155–157]. These optical fiber-technology advances greatly overlap with and can drive the development of specialty tools for astronomical instrumentation.

This paper focuses on silica fibers, but we note that fibers for mid-IR wavelengths based on other glass materials have evolved tremendously and are also important for astrophotonics instrumentation. Here, we outline opportunities with new fiber designs and describe their challenges and necessary advances for their successful employment in current and future astronomical instruments.

### Current and future challenges

Few Mode, MM, and Multicore Fibers are any fiber supporting more than one spatial mode, including graded-index, flat-top, and fibers with unique refractive index profiles.

Graded index fibers exhibit less mode coupling compared to step-index fibers, due to their refractive index profiles. Therefore, graded-index fibers or fibers with specially tailored refractive index profiles can be used to reduce focal ratio degradation (FRD). On the other hand, flat-top fibers are specially designed to scramble the mode content propagating in their core, by tailoring the mode power distribution. They can thus achieve a complete mixing between the modes of the fiber, enabling strong mode-scrambling of the output pattern of the fiber. These fibers can be used to mitigate modal noise which can cause inaccuracies in the measured spectra for RV observations [158]. Multicore fibers with graded index cores or flat-top cores can also be realized. Different types of multicore fibers have been extensively investigated in telecommunication applications as a way to increase transmission capacity. We show some examples in figure 8. It is important to highlight complementary technologies such as fan-ins [157] and 3D-printed microlenses [82, 159, 160] to couple light to each core/channel and to increase coupling efficiency and decrease the gaps in the IFU have seen tremendous advances. Nevertheless, developing broadband low-loss complementary technologies with a large-number of spatial channels remains a challenge and an active area of research.

**Fibers with unique index profile:** By tailoring the refractive index profile of solid core fibers, fibers with unique performance capabilities can be realized. Here, we discuss two such examples. The first is an Airy fiber: a fiber featuring a complex refractive index profile (shown in figure 9) that allows the efficient coupling of an Airy pattern mode, increasing the coupling efficiency from the telescope to the fiber up to 93.7%. A
<figure><img src="image_13.png" /><figcaption>**Figure 8.** (a) Scanning electron image of an AR-HCF. The hollow-core is surrounded by an antiresonant cladding structure which consists of nine thin silica tubes. (b)–(f) Micrographs of multicore fibers featuring different index profiles and wavelength operation range.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Figure 9.** (top left) Optical micrograph of the Airy fiber (bottom left) physical refractive index profiles of the designed Airy (red), and of that of the PCVD preform (blue). Reproduced from I. Gris-Sánchez et al [161], (CC-BY 4.0). (top right) 2D theoretical refractive index profile of a fiber with logarithmic profile.(bottom right) 1D refractive index profile of the same fiber (relative to undoped silica). Reproduced from [161]. CC BY 4.0.</figcaption></figure>

significant improvement over the theoretical limit (80%) of coupling a beam from a circular aperture to the fundamental mode (Gaussian-like) of an ordinary step-index SMF. This was achieved by inversely designing the fiber to support an approximate Airy pattern [161]. Nevertheless, due to the complex refractive index profile, the fiber guides more than one mode, while the airy mode is not the lowest order mode of the structure.

The second example is a fiber featuring a logarithmic index profile [162]. In many applications including astronomical instrumentation, it is desirable to effectively combine light from different sources. This is usually done by fusing and tapering gradually many fibers together. In this process, the main limiting factor on the number of fibers that can be combined is the fulfillment of the adiabatic criterion which in turn dictates the requirement of a minimum transition length. In contrast, an optical fiber with a logarithmic refractive index profile can be adiabatically tapered over any length, however short, enabling the development of a new class of fiber components, including couplers and PLs. The fiber's mode field distribution is independent of the fiber's size and therefore does not change as the fiber is tapered down (i.e. it remains the same along the transition). Nevertheless, developing low-loss PLs using such fibers that support a large number of modes (>1000) still remains a significant challenge that needs to be addressed. The required transition length for adiabatic propagation increases with increasing number of supported modes and can become the limiting factor for realistic implementations.
**Hollow core fibers (HCFs):** Recent advances in the design of HCFs led to the development of a new class of antiresonant HCFs with remarkably low losses [163, 164]. In their simplest form, these fibers consist of a hollow core with a ring of anti-resonant cladding tubes surrounding the core as shown in figure 8. A unique feature of HCFs is that most of the light (>99.99%) can be guided in the central air-core with only a tiny fraction of light overlapping with the surrounding glass structure, hence increasing the optical damage threshold, and reducing material absorption significantly. In addition, these fibers are simpler to manufacture, omitting the complicated structure of previous HCFs. By precisely designing the structure of this fiber to match specific conditions (such as the core-to-cladding tubes ratio), SM behavior can be achieved. Due to their remarkable transmission properties, recent interest led to the first experimental realization of AR-HCFs with extremely low loss (0.174 dB km⁻¹ at 1550 nm which is on the same scale as SMF) and improved guidance [163]. This recent spark of interest opens up possibilities in developing novel HCFs for guidance in wavelengths that silica fibers cannot operate such as UV (even extreme-UV) and mid-IR. In turn, these wavelengths can result in spectroscopic measurements that are currently prohibited due to the use of conventional fibers. For example, the UV spectrum can be utilized to interrogate the planetary atmospheres for specific molecular species. In addition, UV LGS are interesting alternatives to Rayleigh guide star AO applications [165, 166]. HCFs offer the prospect of being able to mount the laser in a stable environment off-telescope and transport the light to the launch telescope via fibers, improving safety at the observatory as compared to mirror relays as well as providing improved stability for the launched laser. In the mid-IR (>2.0 µm), there is currently only one fiber fed spectrograph in operation [74] and it utilizes ZBLAN fibers. These fibers are significantly more lossy than HCFs and therefore HCFs could enable a wave of fiber-fed spectrographs operating at the diffraction-limit in the mid-IR [121]. Such spectrographs could be utilized for RV measurements, the direct characterization of exoplanets and brown dwarfs [167] and for studying highly redshifted light of distant galaxies, newly forming stars, and faintly visible comets. Finally, HCF fibers could also be used to realize very long baseline (km scale) MIR interferometers, which are not possible with current technologies (see section 18 for details). Processes leading to the routine connectorization and termination of AR-HCFs need also to be investigated. This in turn will enable the successful incorporation of AR-HCFs to existing photonic technologies.

AR-HCFs are inherently MM although they suffer from mode-dependent loss; higher-order modes tend to attenuate quicker compared to the fundamental mode. Nevertheless, AR-HCFs can be designed such that the higher-order modes have comparable losses to the fundamental mode. In order to achieve this and maintain low-loss guidance, additional cladding elements are utilized [168–170]. Similar to standard solid-core fiber, we can increase the number of guided modes by increasing the HCF’s core size. MM AR-HCFs can significantly increase the collection efficiency of any system at wavelengths (such as visible/UV and mid-IR) where the performance of silica is hindered due to photodarkening/solarization and where other multicomponent glass fibers have not reached the required performance levels. Current fabrication processes prohibit the realization of continuous long lengths of AR-HCFs, especially for mid-IR applications where the core size and the surrounding structure of the HCF need to be significantly larger to achieve the desired low-loss guidance compared to near-IR/visible wavelengths. It is important to mention technologies such as mode adapters/reformatters, fiber combiners and splitters also need to be developed to effectively incorporate these fibers with current technologies [171].

**Fiber materials other than silica:** Alternative materials such as ZBLAN and chalcogenide are also used to construct optical fibers and have been used in astronomy [74], and silicon core fibers with low-loss optical windows upto 3.3 µm [172] However, the former technologies are extremely brittle. This not only makes handling them difficult, but also limits the maximum length of a single piece to the order of 70–100 m. Although there are efforts being made to improve losses, these fibers still have a long way to go to reach the maturity and applicability of silica fibers.

### Advances in science and technology to meet challenges

In addition to the fiber technologies outlined above, complementary technologies like fiber components need to advance for these fibers to be applicable to astronomy. For future applications, we envision the development of a range of in-line fiber technologies based on these novel fibers. These can include broadband fiber splitters, polarization beam splitters, large-channel fan outs, etc. New functionalities can be incorporated to the fibers, for example, separating the cores from a MCF to perform different measurements. These devices will require new fabrication techniques and may need other materials.

HCFs with polarization-maintaining properties and HCF devices such as beam-splitters could have a major impact on future astronomical instruments. Nevertheless, as mentioned above, to achieve HCFs with improved performance and low-loss guidance in the mid-IR and UV region new fabrication techniques need to be exploited. At these wavelengths, any non-uniformities at the final structure of the fiber will have
significant implications in their optical performance. Precise control of the drawing process of these fibers needs to be exploited.

A significant challenge that needs to be addressed is the scalability of the technology for the next-generation of fibers that scramble or prevent scrambling of the modes; i.e. to achieve the desired performance with an increased number of modes. To accomplish this, designs with complex refractive index profiles need to be exploited as increasing the number of modes (by increasing the core size) of the current fiber designs will result in degraded performance. Artificial intelligence (AI) techniques can be utilized for this purpose, enabling for the first time the realization of fibers with unique refractive index profiles designed specifically for astronomical applications.

Current MCF technologies have enabled significant advances in various areas of astronomy. Nevertheless, there is a constant desire to increase the number of cores as well as the packing efficiency (>90%) of these MCFs. Minimizing the gaps between cores enables more efficient collection of astronomical light and eliminates the need of lens arrays at the input to these fibers. Nevertheless, the core-to-core coupling increases significantly with decreasing separation. A significant challenge that needs to be overcome is minimizing the core-to-core coupling and therefore maintaining the uncoupled nature of the fiber. One way to increase the packing efficiency while minimizing the coupling between cores is by using cores with different refractive-index profiles including complex profiles. This is again an area where AI can be of significant value. In addition, it is important to mention that developing fan-outs for MCFs with many cores is not straightforward. Nevertheless, complementary technologies such as ultra-fast laser inscription can be utilized for this to develop high-precision fan-out systems [173].

## Concluding remarks

In recent years, fiber optic technology has seen tremendous advances which resulted in the development of unique fiber designs. These advances can feed the development of a new class of astronomical instruments. Fibers with unique profiles can be used either to effectively scramble the light or to minimize FRD. Novel fiber designs such as the log fiber and Airy fiber can be utilized in the fabrication of fiber devices for feeding light to the next generation of spectrographs. Furthermore, hollow-core fibers hold great promise in advancing the field of astronomical instrumentation. Low-loss hollow-core fibers for visible/UV and mid-IR guidance can be employed for new astronomical surveys, collecting light at wavelengths where previously we could not imagine that it would have been possible.
# 6. Getting on-chip arrayed waveguide grating spectrographs ready for astronomy

Pradip Gatkine<fnref n="1" />, Andreas Stoll<fnref n="2" /> and Yang Zhang<fnref n="3" />

## Status

The volume, mass, cost, and complexity of seeing-limited spectrographs grow as the square of the telescope diameter. Building on-chip astrophotonic spectrographs is a promising approach for both ground- and space-based telescopes in order to achieve compactness (given the SMF feed), flexibility (with the ability to manipulate the amplitude, phase, and polarization in waveguides), thermo-mechanical stability (given the small size and no moving parts), and cost-effective replicability. They are ideally suited for capturing the adaptive-optics-corrected light and realizing various science cases, such as characterizing exoplanet atmospheres, precision RV measurements, stellar chemistry/kinematics, planetary missions, and remote sensing. Multi-object spectroscopy can also be achieved by leveraging the stackability and replicability of the chips. The benefits of astrophotonic spectrographs go well beyond astronomy including biomedical sensors, industrial chemical detection, etc [174].

AWGs and photonic echelle gratings (PEGs) are well-known and mature technologies used in photonic wavelength-division-multiplexing [175]. However, astronomical spectroscopy poses unique and stringent specifications including high efficiency, broadband (eg: J and H bands), high resolving powers ($\lambda/\Delta\lambda > 10\,000$), and polarization independence [56, 176]. While PEGs can deliver high resolving powers, achieving smooth on-chip reflecting surfaces is challenging due to fabrication limitations [177]. AWGs, on the other hand, require simpler fabrication schemes and can deliver high efficiency (>75%), which makes them more suitable for astrophotonic spectroscopy [10]. Hence, we focus on improvements needed in the AWG architecture to make it science-ready. Cascaded dispersive architectures (two or more on-chip dispersers) are also widely discussed offering various benefits, such as on-chip order separation, adaptable component design, and potential tunability (eg: micro-ring resonator + AWG [178, 179] or cascaded AWG [180, 181]). Since AWGs require long path delays with bends, low-loss material platforms are desirable, with the most promising being Silica and silicon nitride.

## Silica

Silica-based passive integrated photonic device technology is an attractive choice for astrophotonics due to the high coupling efficiency to fibers and excellent transmission characteristics from the visible to the NIR (J, H, and K-bands). Astrophotonic AWGs on a 2% refractive index contrast silica platform with $R \sim 10\,000-36\,000$ and efficiencies of up to 72% in the H-band have been recently demonstrated experimentally, using three-stigmatic-point geometry for focal field flattening [182].

## SiN and high-contrast AWGs

AWGs need curved waveguides to introduce the path differences. Smaller radii of curvatures can help achieve smaller device footprints, and hence lower susceptibility to material/process variation (and thus, the effective index variation) across the chip. The higher contrast ratio offered by the silicon nitride ($Si_3N_4$) material platform allows low-loss radii of curvature as small as 100 µm, depending on the thickness of $Si_3N_4$ [183]. Further, ultra-low propagation loss has been demonstrated in $Si_3N_4$ over a broad band (400–2400 nm) [184], making it suitable for astronomy [185]. This has encouraged several implementations of $Si_3N_4$ AWGs [186–188].

## Current and future challenges

For astrophotonic spectrographs, the primary losses occur at the interfaces between the telescope focal plane and optical fibers, between fibers and photonic chips due to imperfect mode matching, and within the PICs due to propagation losses and scattering at discontinuities of the effective refractive index. Efficient coupling of starlight to SMFs becomes increasingly challenging with growing telescope diameter and focal spot size. However, coupling to SMFs with AO on large telescopes (>4 m) is becoming more common now [189] and is discussed in section 2. The current challenges in making the photonic spectrographs science-ready are discussed below.

<footnote n="1">¹ Department of Astronomy, California Institute of Technology, Pasadena, CA, United States of America</footnote>

<footnote n="2">² Leibniz Institute for Astrophysics Potsdam (AIP), Potsdam, Germany</footnote>

<footnote n="3">³ Electrical and Computer Engineering Department, University of Maryland, College Park, MD, United States of America</footnote>
<figure><img src="image_15.png" /><figcaption>**Figure 10.** *Left:* the schematic of a conventional AWG spectrograph from telescope to detector (with bulk optics cross-dispersion), and that of a cascaded AWG architecture with on-chip order separation and an edge-coupled detector array. *Right:* three-stigmatic-point AWG architecture in a folded configuration.</figcaption></figure>

### AWG loss

The biggest source of insertion loss in an AWG is the waveguide-slab interface [187]. Interface losses occur mostly during the two transitions between the waveguide array and the free propagation slabs. Hence, achieving the highest efficiency across these transitions is crucial. Further, coupling of light into and out of the chip is a key contributor to the loss due to mode mismatch and Fresnel reflection, particularly for high-contrast integrated platforms such as Si₃N₄. Recent developments in low-loss on-chip spot-size converters/tapers [190] and off-chip interposers have demonstrated high fiber-chip coupling efficiency [191].

### Phase errors

High-resolution AWGs ($R > 20\,000$) require a larger number and longer length of arrayed waveguides, leading to a significant increase in the footprint of the device. Phase errors arise due to material/process variations across the chip, causing systematic and random variations in the effective refractive indices of individual waveguides. They become more significant as AWG size increases [192, 193]. Phase errors degrade the AWG performance by decreasing power in the main lobe and increasing the cross-talk. For instance, an AWG at $R \gtrsim 24\,000$ requires precise control of the optical path lengths of the order of ~10 ppm to ensure sharp constructive-interference peaks with minimal crosstalk [194, 195]. Fabrication of such high-R AWGs becomes increasingly difficult with a larger ($N > 300$) number of arrayed waveguides due to path length accumulation of several centimeters. Correcting the phase errors using EO, thermo-optic effect or piezoelectric modulation is possible [196]. However, incorporating a phase shifter (~1 mm × 1 mm size) for every waveguide ($N \sim$ hundreds) can prohibitively increase the chip size and cost (see section 23 for more details).

### Cross-dispersion

The free spectral range (FSR) of high-resolution AWG devices varies between 10 and 20 nm which necessitates spectral order separation by cross-dispersion to cover a range of several hundred nanometers or an astronomical band. Semi-integrated spectrographs achieve order separation using bulk-optics cross-dispersion, which limits the potential for miniaturization [152, 197]. Truly integrated spectrographs require full integration of imaging optics and detectors on the chip [198, 199]. Alternatively, on-chip order separation using tandem/cascaded AWG architectures [181] will help achieve direct edge-coupling with off-chip detector arrays (figure 10). Implementation of ultra-low-loss single-stage AWGs and minimizing the inter-channel spectral dropout in the coarse AWG stage are essential to enable low-loss tandem AWGs.

### AWG focal plane

The AWG focal plane is curved (Rowland circle). In conventional AWGs, the output spectrum is discretely sampled by the waveguides in discrete spectral channels, leading to inter-channel gaps. However, in astronomy, a continuous, gapless spectrum is desired. To achieve that, the curved focal plane of the AWG needs to be imaged, which poses challenges in cleaving along the curved surface and imaging a curved plane on a flat detector [200]. The three-stigmatic-point technique can help achieve a flat focal plane (figure 10(b)), thus alleviating these challenges [182, 201–203]. The flat focal plane minimizes the defocusing aberration of off-centered channels and allows a direct bonding of the detector to the focal plane of the chip. Accurately etching/cleaving/polishing the chip-edge to expose the exact focal plane is important to minimize defocusing.
## Polarization dependence

Most photonic waveguides are birefringent (i.e. different effective indices for TE and TM modes) due to imperfect symmetry of the waveguide cross-section shape. At high resolving powers ($R > 20\,000$), waveguide birefringence causes significant relative separation in the TE- and TM-mode spectral channels, resulting in broadening of the composite channels and reduced spectral resolution in the unpolarized regime. High-resolution AWGs are therefore restricted to fully polarized light unless waveguide birefringence can be adequately compensated for. One solution is to split the polarization using low-loss free-space or fiber-based or on-chip polarization splitters (e.g. [204]), and then rotate the TM-polarized output (e.g. [205, 206]) to feed two copies of the TE-optimized AWG. Alternatively, birefringence-compensating designs such as waveguide cross-section engineering [207], varying waveguide width within the array [208], or slanted FPR-waveguide array interface [209] can be used.

## Advances in science and technology to meet challenges

To achieve the vision of compact, fully integrated, high-resolution/broadband, and massively replicable astrophotonic spectrographs/IFUs for ground- and space-based telescopes, the following advances are critical, addressing the challenges of throughput, phase errors, and imaging of the dispersed spectrum.

1. **Innovative AWG component designs for low on-chip loss:** Tapers have proven effective in minimizing FPR-waveguide interface loss. Specific taper geometries can achieve an adiabatic transition. Sub-wavelength structures [210, 211], multi-mode interferometers (MMIs), and inverse-design tapers [212] can also be used to eliminate interface losses. UV-lithography limits the closest separation between the tapers (due to minimum feature size ~200 nm), thus creating gaps, which lead to losses. E-beam lithography is well-suited to fill these gaps (minimum feature size ~5 nm), however, it is not scalable. Hence, novel designs (e.g: with thinner waveguide cores) are needed that can offer ultra-low loss with the scalable stepper-lithography process [213, 214].

2. **Phase-error correction:** Current state-of-the-art AWGs use EO, piezoelectric [215], or thermo-optic phase shifters integrated on the arrayed waveguides for post-fabrication phase-error correction [196, 216]. However, the elimination of phase errors at the source using tighter fabrication process control and minimal-phase-error designs [217] is the ideal solution for achieving high-throughput, high-resolution AWGs. Phase error control techniques for silica AWG devices require post-fabrication phase error measurements and treatment by UV trimming [218], which is not feasible for mass production unless automated post-processing facilities are implemented. An alternative novel method of phase control can be implemented via programmable photonics [219], allowing in-situ phase adjustments on individual optical paths, which opens the possibility of mass-produced self-correcting AWGs.

A novel architecture called reusable-delay line AWG utilizes an optimized array of directional couplers to distribute the input signal from the delay line into the free propagation region (figure 11.(4)). It is a potential solution to reduce the impact of fabrication imperfections and non-uniformity for ultra-high resolution ($R > 50\,000$) AWGs [220]. This architecture can be ~100 times more compact than the traditional AWGs.

3. **On-chip order separation:** Tandem AWGs can be designed to separate the spectral orders to cover broad bands uniformly (figure 10). For the micro-ring resonators + AWG concept (figure 11), the quality factor of the resonators is the source of the high spectral resolving power. It is challenging to choose the radius of the rings such that the set of peaks is shifted very slightly to achieve uniform coverage. However, the peaks can be tuned after fabrication using thermo-optic modulators.

4. **Efficient coupling of light into and out of the chip:** The light can be efficiently injected from SMFs to waveguides using lensed fibers and/or adiabatic inverted tapers (~95%) [190]. For certain material platforms, inverse designs [221, 222] or low-loss interposers are more appropriate [223]. Getting the light out of the chips efficiently is challenging for high-index-contrast materials (Si₃N₄, SOI). Currently, high-NA microscope objectives are used to image the AWG focal plane onto a detector. However, they have a small Fov (~1 mm). With the goal of moving away from bulk optics, one could consider solutions such as NA-matched microlens arrays with 100% fill factor [224–226] or 3D-printed lenses on chip [80, 227] need to be explored for better integration.

5. **Detector integration:** Several approaches are being tried to achieve true integration of photonic spectrographs + detectors, with no moving parts. Integration of energy-resolving MKIDs as cross-dispersing detectors is a promising approach (see section 25). Alternatively, tandem AWGs perform on-chip cross-dispersion and allow direct integration of detectors via heterogeneous material platforms (eg: Si + Ge or Si + InP) [228,229,234]. Small-pitch linear detector arrays need to be developed for directly butt-coupling with the output FPR of cascaded AWGs (figure 10(a)). Other approaches include sending the light upwards from the chip using efficient grating couplers/metamaterial optimized for
<figure><img src="image_16.png" /><figcaption>**Figure 11.** Top-left: fiber-based frequency domain interferometer setup to measure AWG path-length errors; Top-right: measured optical path length error between adjacent waveguides in a silica AWG. Reproduced from [233]. CC BY 4.0. © [2022] IEEE. Bottom-left: an integrated photonic chip with a cascaded array of ring resonators (thermally tunable) and AWGs. Bottom-right: a schematic of the arrayed waveguide grating with reusable delay lines (RDL-AWG). Reprinted, with permission, from [220].</figcaption></figure>

different wavelengths [230] (for discrete AWGs) or an inclined reflective edge or micro-mirror arrays [231] (for both discrete and continuous AWGs) (sections 24 and 25). It is much simpler to image the discrete waveguide outputs as opposed to a continuous AWG focal plane onto a detector. New design efforts are needed in FPR-waveguide tapers (at output) to achieve a discrete-output AWG that can still sample the complete spectrum gaplessly.

6. **Beyond near-IR:** The scientific scope of photonic spectrographs can be dramatically increased by extending to wavelengths ranging from near-ultraviolet to MIR. Further developments are needed in new materials such as AlN-on-Sapphire for UV [232] and chalcogenide glass or Si/Ge for developing low-loss AWGs in the mid-IR. The specific challenges and advances are discussed in sections 8 and 9.

7. **Space readiness:** A stable, durable, and crack-free fiber-chip bond is of particular importance for applications that require operation at cryogenic temperatures, as well as space applications in hard vacuum. Refractive index matching gap fillers/gels/oils are not suitable for these conditions due to solidification at low temperatures and potential outgassing. The impacts of radiation-induced degradation and temperature-induced core-cladding stresses need to be assessed on the overall throughput.

8. **AWG spectrograph throughput outlook:** The main throughputs in a photonic spectrograph are SMF-to-chip, on-chip, and chip-to-detector throughputs. We present the current state-of-the-art and optimized throughputs in the future to give a near-term throughput outlook.
<table><caption>AWG spectrographs: Current overall throughput and near-term outlook.</caption><thead><tr><th>Component</th><th>Current throughput</th><th>Optimized throughput</th></tr></thead><tbody><tr><td>SMF-to-chip efficiency</td><td>95%</td><td>95%</td></tr><tr><td>AWG throughput</td><td>70% (Silica AWG)</td><td>85% (optimized tapers)</td></tr><tr><td>AWG to free-space</td><td>80%</td><td>95% (with AR coating + output facet optimization)</td></tr><tr><td>Relay optics + cross dispersion</td><td>50% (prism + lenses)</td><td>70% (optimized cascaded AWG)</td></tr><tr><td><strong>Total throughput</strong></td><td>~26%</td><td>~53%</td></tr></tbody></table>

## Concluding remarks

Astrophotonic spectrographs offer significant advantages for both ground- and space-based telescopes, particularly at high spectral resolution. Multi-pronged developmental work is currently being pursued with several novel photonic technologies to achieve the vision of compact, high-throughput, fully integrated (with fibers and detectors), and highly replicable photonic spectrographs to augment both ground- and space-based astronomy.

## Acknowledgments

Support for P Gatkine was provided by NASA through the NASA Hubble Fellowship Grant HST-HF2-51478.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA Contract NAS5-26555. P Gatkine would further like to acknowledge support from the Wilf Family Discovery Fund in Space and Planetary Science, funded by the Wilf Family Foundation, as well as the support from Keck Institute for Space Studies at Caltech. Some of this research was carried out at Caltech and the Jet Propulsion Laboratory and funded through the President's and Director's Research & Development Fund program. Y Zhang is supported by National Science Foundation (NSF) under Grant 1711377 and the National Aeronautics and Space Administration (NASA) under Grant 16-APRA 16-0064.
# 7. New directions in photonic spectrograph architectures

Pradip Gatkine¹, Chang-Ling Zou², Zongyin Yang³, Jinping He⁴,⁵ and Ross Cheriton⁶

¹ Department of Astronomy, California Institute of Technology, Pasadena, CA, United States of America

² CAS Key Laboratory of Quantum Information, University of Science and Technology of China, Hefei, Anhui, People's Republic of China

³ College of Information Science and Electronic Engineering, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou, People's Republic of China

⁴ National Astronomical Observatories, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing, People's Republic of China

⁵ Key Laboratory of Astronomical Optics & Technology, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing, People's Republic of China

⁶ Advanced Electronics and Photonics Research Centre, National Research Council Canada, Ottawa, Canada

## Status

Bulk-optic, seeing-limited astronomical spectrographs have limitations like high complexity, large size, mass, and high cost, which scale quadratically with the telescope diameter. Integrated photonic spectrographs can address these concerns by enabling high-stability, high-precision dispersion spectral processing for many science cases. Astronomical spectrographs require high efficiency, large spectral bandwidth (e.g. J-H bands: 1200–1700 nm), low or high resolving powers ($R = \lambda/\Delta\lambda \sim 1000–100 000$), and in most cases, polarization insensitivity, which each present a different set of challenges. Some of the potential use cases include: (a) high-precision radial velocity (RV) measurements of stars to constrain exoplanet masses ($R > 80 000$, RV precision $\sim 10$ cm s⁻¹), (b) high-resolution spectroscopy to characterize exoplanet atmospheres composition, measuring stellar composition and kinematics ($R \sim 30 000$), (c) diffraction-limited MO spectroscopy of crowded fields to measure stellar properties in extreme environments ($R > 10 000$), (d) low-cost spectrographs for rapid spectroscopy of a large number of transients on small telescopes ($R \sim 3000$), and (e) ultra-compact spectrographs for space-based telescopes and planetary missions.

Currently, AWGs are the most prominent candidate architecture for astrophotonic spectroscopy [2, 10, 182]. Conventional high-resolution AWG spectrographs require bulk-optic cross-dispersion to achieve broadband coverage. However, we envision fully integrated on-chip spectrographs that would lead to ultra-stable monolithic instruments. Challenges with the AWGs and potential solutions are discussed in section 6. While astrophotonic AWG spectrographs have been demonstrated over the last decade, the phase errors in the AWG chips pose a significant barrier for $R > 10 000$ [195, 233]. This paper therefore focuses on emerging photonic architectures to expand to higher resolving powers ($R > 10 000$).

Several novel photonic spectrograph architectures [56, 176] are on the horizon with the potential to overcome the limitations of AWGs and enable improved integration with detectors. In this paper, we present a non-exhaustive list of promising new spectrograph architectures, their unique advantages, and the developments needed to make them science-ready.

## Current and future challenges

Here, we discuss the specific architectures, the advantages they offer, and the challenges that need to be addressed in the future to make them suitable for astronomy.

### Ring resonator-enhanced spectrographs

Achieving a truly integrated broadband photonic spectrograph requires an on-chip separation of the overlapping spectral orders of the AWG. One solution is to use an AWG with high-resolution and low FSR (Eg: $R \sim 30 000$, FSR $\sim 10$ nm) followed by an array of microring resonators (MRRs) as filters to separate the spectral orders [235, figure 12(a)]. MRRs can also be placed after a coarse AWG to achieve high spectral resolution within each AWG output channel. Cascaded AWG-MRR spectrographs have also been produced with detectors on a single chip [178, 236], creating a fully integrated spectrograph.

The resonance wavelengths of MRRs are highly sensitive to fabrication errors and temperature fluctuations without active thermal tuning. In addition, to achieve a broadband, high-resolution AWG + MRR spectrograph (figure 12(a)), we need:

$$ \text{Total \# of MRRs} = (\text{\# of AWG channels}) \times (\text{\# of AWG spectral orders}). $$

This implementation can quickly result in thousands of MRRs and tuning elements, presenting a serious integration challenge. Advanced routing and packaging strategies, such as the use of flip chip bonding with vias, may be necessary to integrate these components.
<figure><img src="image_17.png" /><figcaption>**Figure 12.** (a) A schematic showing the concept of high-resolution AWG with MRRs at the output channels to separate the overlapping spectral orders. (b) The concept of a serpentine integrated grating using a serpentine optical phased array (SOPA), providing high-resolution dispersion along the y-axis (with long cumulative phases), and low-resolution separation along the x-axis. Reprinted with permission from [244] © The Optical Society. (c) A schematic showing the concept of switchable path delays in an FTS and the AWG dispersing the FTS fringes to effectively create a dispersed FTS architecture. Reproduced from [242]. CC BY 4.0.</figcaption></figure>

### Fourier-transform spectrographs

Fourier-transform spectrographs (FTSs) are an architecture that can offer ultra-high-resolution spectroscopy for bright sources [237] by deducing the optical spectrum from an interference pattern generated as a function of the optical path length difference. This method enables the use of a single-channel detector that can be directly integrated into a photonic FTS chip. However, broadband photonic FTSs are limited by photon noise and waveguide dispersion, which degrades the fringe quality, and the throughput limit [238]. Stationary wave integrated FTS (SWIFTS), which reconstructs the spectrum by sampling the standing wave created by forward and reflected waves within a waveguide-mirror configuration, faces a similar narrow-band challenge due to the undersampling of the standing waves [239, 240]. However, a dispersed FTS architecture (e.g. FTS + AWG) could resolve these issues [241].

An FTS + AWG concept (figure 12(c)) effectively multiplies the resolving power of low-resolution, broadband AWGs (e.g. FSR = 100 nm, $R = 5000$) using an upstream FTS with switch-selectable path delays. Kita et al [242] experimentally demonstrated the switchable FTS concept with a 64-channel FTS (figure 12(c)) and a single photodiode. This concept can be adapted to multiply the resolving power of the AWG by a factor of $2^k - 1$, where $k$ is the number of phase-shift units incorporated in the circuit. The switchable FTS modulates the spectrum as we switch through the path delays. This allows us to reconstruct the spectrum at a much finer wavelength step within each AWG channel, effectively multiplying the AWG resolving power, while maintaining a high FSR.

Various challenges need to be addressed to make FTS-AWG architectures suitable for astronomy. It requires high-precision measurement of the output power and detailed high-resolution calibration to reliably retrieve the spectrum at high resolving power. Hence, a stable, low-noise detector array is highly desirable. Implementation of integrated photodetectors (PDs) will enable a highly compact, and stable high-resolution photonic spectrograph. Furthermore, this architecture requires broadband 50–50 splitters and thermally-controlled photonic switches (to switch between path delays) that offer ultra-low loss and minimal crosstalk to enable easier spectral reconstruction. Achieving ultra-low propagation losses ($<0.1 \text{ dB cm}^{-1}$) is critical to minimize the impact of the long path delays.
## Serpentine integrated grating spectrograph

Current high-resolution AWGs require cross-dispersion to separate the spectral orders and prevent spectral overlap in each channel. While cascaded AWGs can help resolve this issue, they can be too large for fabrication on a single chip and would require very long (>1000 pixels) linear detector arrays to sample the spectral elements. The serpentine integrated grating (SIG, figure 12(b)) can help alleviate this problem by naturally allowing a 2D sampling of the spectrum. This architecture uses a long folded delay line with grating couplers to create a large optical delay path along two dimensions (high-dispersion along the column and 'order separation' along the serpentine row) in a compact integrated device footprint [243]. The grating couplers send the light upward, thus achieving two goals—high spectral resolution and easy imaging of the 2D spectrum from above the chip. This method requires high index-contrast waveguides, such silicon-on-insulator (SOI) and silicon-nitride (SiN) platforms, which allow sharp low-loss bends (with radii as small as ~10 µm), to enable several centimeters of path delay in a small footprint (~few mm^2). An SIG spectrograph with R ~ 100 000 and ~6750 spectral bins over the 1540–1650 nm range has recently been demonstrated on the SOI platform [244].

High losses of the device in the SIG demonstration (~36 dB) need to be addressed to make it viable for astronomical spectroscopy. The main contributors to the loss were: straight-to-bend waveguide tapers (8.4 dB = 0.07 dB per taper × 120 tapers), input fiber-waveguide edge couplers (7 dB due to mode-mismatch), and the on-chip grating couplers (16 dB). Optimized transition tapers can offer ultra-low-loss U-bends ([245], 0.0022 dB per taper). Similarly, high-efficiency broadband edge-couplers need to be engineered (e.g. [246, 247]: >90% efficiency). High-efficiency grating couplers are challenging to realize. However, apodized designs [248] and dimension reduction techniques can reduce the design parameter-space and device optimization time [249]. Alternatively, low-loss 3D circuit fabrication will allow easier integration of the device [250].

## Microcomb-based spectrometer

The ultimate resolution of an on-chip disperser is limited by the total optical path difference (OPD) in the dispersive elements. Further spectral resolution increases could be obtained by converting the optical frequency to electronic RF frequency by measuring the heterodyne beating signal between the signal light and a local oscillator (LO). The microcomb source, which is generated by dissipative Kerr solitons in a laser-driven integrated microresonator [251], provides an excellent LO for an integrated spectrometer (for bright objects), as it delivers a series of comb lines with a precise and fixed FSR. Innovations such as battery-operated integrated comb generators [252], octave-spanning microcombs [57, 253], and frequency stabilization [215, 254] have recently been demonstrated experimentally.

By mixing the input signal with the microcomb, the optical signal frequency can be converted to an RF signal due to the beating between the input light and a nearby comb line. Using an integrated silica microresonator to generate a microcomb with a 22 GHz repetition rate (i.e. frequency spacing between comb lines), a spectrometer has been demonstrated with a frequency uncertainty of around 4 MHz [255].

This spectrometer architecture faces various challenges: (i) the resolution of the frequency measurement is limited by the frequency instability (~MHz) of the comb lines of an unlocked microcomb. The frequency instability mainly comes from two sources, the instability of the drive laser, and the thermal stability of the MRR. (ii) There is ambiguity in determining the frequency and power of the heterodyne signal due to ambiguity in determining the closest comb line. This ambiguity could be resolved by simultaneously beating the signal with two independent comb sources with different repetition rates, and the optical signal frequency could be determined by comparing the two beating frequencies (dual-comb spectroscopy). Alternatively, the two comb sources could be provided by a single microresonator by either employing different mode families [255] or switching the repetition rate in real time [256].

## Computational spectroscopy

A significant departure from the traditional dispersive approaches is the use of computational techniques to decode spectral information from precalibrated encoded information to produce an enhanced spectrum beyond the limitations of gratings/filters. So far, complex spectral-to-spatial [257, 258] and multiplexed spectral response [259, 260] strategies have been proposed for encoding spectral information (figure 13(a)). In the complex spectral-to-spatial encoding process, incident light is dispersed by random media, such as disordered materials or MM fibers to form an overlay of encoding patterns created by individual wavelengths. The multiplexed spectral response encoding is an alternative to spatially dispersed wavebands. This encoding can be achieved in the filters or detectors themselves. A unique precalibrated wavelength-responsivity library is then used for decoding the incident spectrum.

Both encoding strategies can offer high throughput compared to dispersive spectrometers. Moreover, a higher spectral resolution can be achieved in the same device area compared to traditional photonic
<figure><img src="image_18.png" /><figcaption>Spectral response</figcaption></figure>

<figure><img src="image_19.png" /><figcaption>b)</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>**Figure 13.** (a) Two encoding schemes of computational spectrographs are shown here. The top part shows the pattern-to-spectrum reconstruction for random dispersive media, and the bottom part shows the reconstruction from a calibrated spectral response library. Both require advanced reconstruction algorithms for robust reconstruction. (b) The concept of an optical signal mixed with a microcomb to create an RF beat signal for high-resolution spectral reconstruction.</figcaption></figure>

spectrographs, since the scattering pattern is not restricted by the effective index or path lengths as in traditional architectures. Emerging strategies use inverse design to specify the desired output spectrum, and iterative steps to generate non-intuitive, disordered geometries for computational spectroscopy that can meet the requirements of the output spectrum [257]. The compactness of the geometry means shorter propagation lengths and smaller chips, leading to lower phase noise and higher thermal stability.

Another computational architecture photonic cross-correlation [261] uses tunable spectral filters to process the entire spectrum for a set of specific spectral lines, enabling higher throughputs and the use of a single detector channel (detailed discussion in section 12).

However, computational spectrometers face certain challenges in reconstructing robust results. The results are accurate with sparse spectra, but if the input is a broadband spectrum, the result is vulnerable to distortion. Furthermore, significant computational resources are required for both the spectrograph design generation and spectrum-interpretation algorithms to achieve high performance. Another challenge is their limited spectral bandwidths. This can be mitigated by appending compact computational/inverse-designed spectrographs to AWG output channels to perform broadband, high-resolution spectroscopy.

### Advances in science and technology to meet challenges

Significant efforts are needed in several directions including computational reconstruction, heterogeneous material platforms and fabrication, inverse design, precision calibration, light routing schemes, and integration of photonics, electronics, and detectors, to achieve fully integrated high-resolution, broadband photonic spectrographs for ground- and space-based telescopes. These advances are summarized in table 1. We can classify key advances in three categories:

a. **Design and fabrication of low-loss components:** Fully integrated cascaded spectrograph concepts are viable only with low-loss, broadband individual stages. Major losses occur at interfaces with mismatched refractive indices. Highly efficient taper designs are required for waveguide to SMF, waveguide to free space, SM to MM, and straight-to-bend waveguide transitions. Similarly, ultra-low loss and broadband splitters are essential for all interferometric architectures (Eg: FTS + AWG). For SIGs, a significant effort
<table><caption>**Table 1.** Spectrograph architectures, challenges, and future advances.</caption><thead><tr><th></th><th>New architectures (with main advantages)</th><th>Challenges</th><th>Advancements required</th></tr></thead><tbody><tr><td>1</td><td>Ring resonator-enhanced (high resolution and bandwidth)</td><td><ul><li>Thermal cross-talk</li><li>Complex electrical integration</li></ul></td><td><ul><li>High-throughput AWG designs</li><li>Thermal stability when driving MRRs</li><li>Scalable thermo-optic control of large arrays of MRRs</li></ul></td></tr><tr><td>2</td><td>FTS + AWG hybrid (multiplies the resolving power of the AWG)</td><td><ul><li>High-precision output power measurement</li><li>Loss in the delay lines</li><li>Broadband splitters</li></ul></td><td><ul><li>Integrated detector arrays</li><li>Ultra-low propagation loss</li><li>Ultra-broadband splitters/switches (sub-wavelength inverse-design can help)</li></ul></td></tr><tr><td>3</td><td>Serpentine integrated spectrograph (easy integration with detectors and R ~ 100 000)</td><td><ul><li>Throughput</li><li>Straight-to-bend taper losses</li><li>Fiber-waveguide losses</li><li>Grating coupler losses</li></ul></td><td><ul><li>Cross-section optimization</li><li>Inverse design</li><li>Broadband grating couplers optimized for the specific waveband of the spectrograph</li></ul></td></tr><tr><td>4</td><td>Microcomb-based (ultra-high R > 100 000)</td><td><ul><li>Comb line stability</li><li>Ambiguity in determining the relevant comb frequency</li></ul></td><td><ul><li>Self-referenced comb</li><li>Advanced on-chip resonator stabilization</li><li>Fast/accurate repetition rate switching</li></ul></td></tr><tr><td>5</td><td>Disorder modes (no FSR overlap, compactness)</td><td><ul><li>Spectral bandwidth</li><li>Fabrication quality to enable higher resolution and throughput</li></ul></td><td><ul><li>Large device areas to enable greater performance</li><li>Greater computing resources for larger 3D structure simulations</li><li>Novel metasurface realizations</li></ul></td></tr><tr><td>6</td><td>Computational Spectrometers (mitigates architecture-dependent performance limitations)</td><td><ul><li>Low stability</li><li>Noise sensitivity</li><li>Reliability over larger bandwidths</li></ul></td><td><ul><li>Accurate and fine coding strategies</li><li>Powerful reconstruction algorithms</li><li>Computational techniques to complement traditional dispersive spectrometers such as AWGs</li></ul></td></tr></tbody></table>

is required in developing high-efficiency vertical grating couplers for launching the dispersed light perpendicularly for simpler 2D detector integration [262].

b. **Computational advances:** Computational spectroscopy and dispersed FTS implementations will greatly benefit from high-precision spectral calibration, and state-of-the-art spectral reconstruction techniques (eg: ML-based elastic-D1 regularization [242]). Further, novel algorithms are needed for forward modeling and inverse-design optimizations (bandwidth, throughput) on large photonic components in various spectrograph architectures [263, 264]. Fast approximate solvers for scattered fields are critical to optimizing large surfaces that can be thousands of wavelengths in diameter [265].

c. **Integration with detectors and electronics:** Future implementations should aim to integrate the detection array monolithically into the photonic chip itself or couple the light onto a nearby external detector with minimal optics for low cost, compactness, and improved throughput. Efforts are needed to develop reliable fabrication recipes in heterogeneous material platforms (eg: InGaAs-SiN [266], NbN/WSi/MoSi-SiN [267]) allowing a large number (thousands) of on-chip pixels. Ongoing efforts in kilopixel SNSPD arrays [268] are promising with improvements expected in efficiency and dark current. Industrial foundries are well-placed to offer these to the broader photonics community (see sections 24 and 25).

## Conclusion

The new directions in astrophotonic spectrographs have the potential to overcome many of the limitations with the prevalent AWG architectures. Further developments in photonic fabrication, computation, complexity and complete integration need to be leveraged for building compact, efficient, and broadband astrophotonic spectrographs.
Acknowledgments

Support for P Gatkine was provided by NASA through the NASA Hubble Fellowship Grant HST-HF2-51478.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA Contract NAS5-26555. Some of this research was carried out at the California Institute of Technology and the Jet Propulsion Laboratory and funded through the President's and Director's Research & Development Fund and the Center for Innovation Fund programs.
## 8. The emergence of UV astrophotonics

David Schiminovich<fnref n="1" />, Simon Gross<fnref n="2" />, Lucas Labadie<fnref n="3" />, Reinan Moreira<fnref n="4" /> and Pradip Gatkine<fnref n="5" />

### Status

Although early ultraviolet (UV) photonic devices were developed nearly 50 years ago (e.g. [269]), progress towards widespread application has been slow. A limited number of materials can be used to fabricate UV (<400 nm) optoelectronic devices, particularly those with high index, and scattering, losses and damage susceptibility can severely reduce performance. Below ~200 nm, O₂ in the atmosphere is highly absorbing, necessitating operation in vacuum. When viewing the night sky from the ground, the ozone layer (O₃) absorbs incoming photons shortward of 320 nm; astrophysical telescopes and instruments operating in the near-UV and shortward must be situated in—and qualified for—space.

The astrophysical benefits of a space-based UV observatory have long been recognized [270] efficient and solar-blind sensor technologies for astronomy. The Hubble Space Telescope (HST) has been transformative, particularly for its moderate-to-high UV spectroscopic capabilities that probe numerous atomic and molecular lines. HST has obtained spectra of hot young and evolved stars and white dwarfs, active galactic nuclei, star-forming galaxies, galactic nebulae, supernova remnants, and protoplanetary disks, and absorption (and emission) from the multiphase interstellar, circumgalactic and the intergalactic media. The next leap forward, in the form of the Habitable Worlds Observatory (HWO; based in part on LUVOIR and HabEx Concepts), could target the same UV O₂ and O₃ absorption signatures that confound UV observations on Earth, but that might serve as candidate biosignatures on distant exoplanets (see figure 14). Such flagship mission concepts are currently developed around relatively mature UV technologies although UV photonics could indeed play a role on the HWO and pathfinder missions.

UV astrophotonics has yet to demonstrate the 'on-sky' maturity achieved by the NIR counterparts discussed extensively in this roadmap. But substantial progress in UV photonics exists elsewhere. Early developments in UV nonlinear optics, harmonic generation, low-loss optical fibers, damage mitigation, and materials selection have led to recent successes. Novel material platforms have been developed that enable photonic operation at blue/UV wavelengths. These are mainly driven by life-science and quantum optics applications, sensing, spectroscopy and optical clocks (see [271] for a comprehensive review). In light of these developments, the nascent field of UV astrophotonics may finally be emerging.

### Current and future challenges

*Photonic technologies for the UV domain*

Early developments in UV nonlinear optics, harmonic generation, low-loss optical fibers, damage mitigation, and materials selection have enabled recent efforts to demonstrate the potential and feasibility for a broad suite of photonic technologies in the UV.

With photon energies exceeding the Si bandgap, UV/visible photonics pose distinct materials, design and fabrication challenges as compared with the NIR. Issues include limited material, dispersion and photon source selection; increased absorption and scattering losses; decreased performance due to material impurities and defects. High energy photons and high intensities can induce defects that increase losses, an effect known as photodarkening [272], which leads to a degradation of performance over time. UV PICs require smaller structures compared to their established NIR counterparts. This puts the required feature sizes closer to the resolution limit of wafer-scale lithography which in turn reduces yield. Nevertheless, some of these challenges and issues may ultimately be exploited for the design of sensitive probes and sensors, and engineered quantum emitters.

*Overcoming the challenges in the UV may enable astrophotonic solutions for optimal telescope-instrument coupling, foreground suppression (e.g. geocoronal Lyman alpha), interferometry and very high resolution spectroscopy, and light sources. Space-based UV instruments also stand to substantially benefit from new technologies that reduce volume, mass and power, and number of reflective and transmissive surfaces.*

*Material choices and fabrication platforms*

The bandgap of an optical material ultimately provides a lower wavelength-bound for its suitability as a waveguide substrate (see figures 14(b) and 15(b)). Deposited thin films that serve as guiding layers often

<footnote n="1">¹ Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, New York, NY, United States of America</footnote>

<footnote n="2">² MQ Photonics Research Centre, School of Engineering, Macquarie University, North Ryde, NSW, Australia</footnote>

<footnote n="3">³ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Cologne, Germany</footnote>

<footnote n="4">⁴ Ultra-Low Loss Technologies, Santa Barbara, CA, United States of America</footnote>

<footnote n="5">⁵ California Institute of Technology, Pasadena, CA, United States of America</footnote>
<figure><img src="image_21.png" /></figure>

cannot achieve the same optical quality as their bulk counterparts. Impurities in the deposited films lead to energy levels within the bandgap that reduces their UV wavelength cut-off. At short wavelengths, Rayleigh scattering becomes a dominant loss mechanism where light scatters at grain boundaries of polycrystalline materials as well as scattering from the residual sidewall roughness of the patterning process used to define the waveguides [276]. Such losses can be mitigated by using single-crystalline materials and improved photolithography.

At UV and visible wavelengths [271], identifies materials for PIC and wafer-scale blue and UV photonic applications, highlighting silicon nitride Si₃N₄ (>400 nm), tantalum pentoxide Ta₂O₃ (>300 nm), amorphous aluminum oxide Al₂O₃ (>250 nm) and aluminum nitride AlN (>200 nm) as leading material platforms. Alternatively to wafer-scale lithography, UV and blue wavelength waveguides have been demonstrated in polymers [277], with losses <1 dB cm⁻¹ at 266 nm, as well as in glass substrates [278, 279] using ultrafast laser inscription (ULI) [91, 280]. Looking to the future, these will be supplemented by other 3D and 2D wide and ultra-wide bandgap materials (e.g. III-N, AlGaN, Ga₂O₃, hexagonal boron nitride, silicon carbide, diamond), metamaterials. Many of these materials have already seen application in the development of UV-efficient and solar-blind sensor technologies for astronomy [281, 301]. A novel advance is a 2nd-harmonic-generating and focusing metalens using ZnO resonators for the vacuum UV [282] and a first XUV metalens [283]. See figures 14(b), (c) and 15(b) for an overview of potential UV astrophotonics materials.

Although UV advances have yet to achieve theoretical performance limits, recent successes include a crystalline, wurzite AlN-on-Sapphire ([232]; cf roadmap section 6) PIC integrated platform/waveguide with high-Q into the UV, targeting control and connectivity for quantum computing (see also [284]). Other examples include SiN on Al₂O₃ down to 372 nm [302] as well as low-loss UV waveguides made using atomic layer deposition of Al₂O₃ (see figures 15(a) and (b)). Potential issues with sapphire and other materials are the lattice mismatch (see e.g. [275, 303]). Another promising platform is an AlGaN waveguide on SiC substrate which, as an example, has been used to achieve the shortest wavelength LED (210 nm) to date [285]. Deep UV AlGaN LEDs (~250 nm) have been life-tested for the Laser Interferometer Space Antenna (LISA) mission, allowing low-power contactless discharge of test masses [286, 287].
<figure><img src="image_22.png" /></figure>

Future progress in materials and platforms requires mitigating sidewall roughness, mode and interface engineering and improved crystal growth. A future challenge for UV astrophotonics is to clearly highlight the dominant materials and an optimal platform (or platforms); this will facilitate incorporation into novel instrument design solutions.

## Fiber Optics -Losses, Coupling and Interfacing

Although UV-transmitting fiber optics have been available for decades, future gains can be made by extending to shorter wavelengths (<200 nm), reducing transmission losses and increasing solarization resistance. Current technologies seeing continued development include transmissive step-index fiber (UV grade silica [288, 289], fluoride and flurophosphate [290]) and solarization-free anti-resonant hollow core fiber technologies [291–293]. Photonic crystal and hollow-core fibers have also been used for the development of UV-visible photonic sources (e.g. harmonic generation, frequency comb [294], and supercontinuum source [295]). The FIREBall stratospheric balloon experiment [296] flew 200 nm-transmitting high-OH fused silica fibers. New instrument concepts are also incorporating hollow core fibers for the deep UV ([297] also see figure 15).

UV PICs suffer from the same mode-mismatch challenges between PIC waveguide and optical fiber as their NIR counterparts. In the NIR, low-loss coupling between fibers and PIC uses mode-matching tapers that utilize a lower refractive index contrast material than the PIC itself to expand the MFD and match to commercial fibers. At UV wavelengths, no such tapers have been demonstrated yet. Liu et al [284] reviewed several methods of fiber-to-chip interfacing for AlN platforms in the UV, including laser cleaving and ion beam milling (see their table 2). In the UV, smaller waveguide dimensions and thus MFDs make this task more difficult. Single-mode optical fibers for the UV are preferably designed for a large MFD to reduce the intensity in the core and avoid solarization [298]. Solarisation can be avoided entirely by moving to a hollow-core fiber design [299]. Yet hollow-core fibers feature an even larger core and hence mode-field mismatch to the PIC.

The most common method to attach fibers to PICs is using UV-curing adhesive (Ref to section 23). These adhesives should feature a high transparency to avoid absorption losses, yet UV curing requires sensitization to UV light which induces absorption bands, in particular at 365 nm. This contradicts the requirement for high UV transparency. When using photonic crystal fibers [298] or hollow core fibers [299],
the adhesive can infiltrate the fiber via capillary forces which influence the guiding properties of the fiber and induce additional losses if the fiber ends are not collapsed.

*A challenge remains to determine the optimum way to mode match in the UV and identify the ideal solution to connecting fibers to PICs for maximum efficiency.*

**Advances in science and technology to meet challenges**

UV PIC development is likely to include further progress towards robust, low loss, higher-Q devices. With Rayleigh scattering increasing into the blue/UV, improvements such as those seen in CMOS foundry-based devices with low-roughness waveguides patterned on Si₃N₄ [304] are needed. Recent developments include the fabrication of UV ring resonators [305], UV AWGs [235] with high bandgap photonic materials coupled with UV/VIS transmitting substrates.

Finally, radiation hardening will become an increasingly important challenge in the mid-term future for space applications of astrophotonics, regardless of the frequency range. This is also relevant for other fields such as satellite-based quantum technologies where efforts in this direction have already been made [306] (targeting predominantly the VIS/NIR domain.)

An interesting direction to guide future UV astrophotonic development is consideration of the design requirements of the HWO. Despite its large aperture (e.g. 6–10 m), it is unlikely to be diffraction-limited in the far and near-UV. UV astrophotonic technologies may provide new avenues for reaching the highest possible resolution and sensitivity limits, with more compact instruments. And we may soon see pathfinding UV astrophotonic instruments more routinely deployed on suborbital and smallsat missions. These advances will enable the study of exoplanet atmospheres and biosignatures, and high resolution absorption line studies of massive, faint baryonic structures observed throughout cosmic history.

**Concluding remarks**

The field of UV photonics has shown limited growth until the past decade. Recent advances driven by non-astronomical applications have demonstrated the promise of new materials, device platforms and interfacing technologies. As efficiency increases, and losses decrease, astronomical UV instruments stand to benefit from serious consideration of photonic design solutions. A key challenge will be converging on a suite of materials and a core toolkit of UV astrophotonic technologies that should then be qualified for the space environment. An important 5–10 year goal will be the realization of pathfinding UV photonic devices in a suborbital or orbital telescope.

**Acknowledgments**

L L acknowledges support from the German Ministry of Education and Research (BMBF, ALSI Project 05A14PK2) and the DFG (NAIR Project, Grant 326946494) in the field of mid-infrared astrophotonics. S G acknowledges funding by the Australian Research Council (FT200100590). Support for P Gatkine was provided by NASA through the NASA Hubble Fellowship Grant HST-HF2-51478.001-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA Contract NAS5-26555.
## 9. Advances in mid-IR astrophotonics

Lucas Labadie<fnref n="1" />, Simon Gross<fnref n="2" />, Reinan Moreira<fnref n="3" /> and David Schiminovich<fnref n="4" />

### Status

The development of PICs is driven by optical communication applications at wavelengths of 1.3 and 1.5 µm, utilizing silicon and its oxide form, SiO₂, such as SOS and Si/SiN-on-insulator (SOI) technologies [307–309]. These optical communication wavelengths overlap with the astronomical NIR wavelength band of 1–3 µm. In the context of astronomy, the GRAVITY instrument [75], operating at wavelengths of 2.0–2.4 µm and utilizing mature SOS technology, is an excellent example of the astrophysical yield of photonics in the NIR (cf section 15). Other mature NIR photonic platforms include lithium niobate for instance, which is employed for signal modulation in data transmission and integrated quantum photonics [310, 311]. More recently, three-dimensional material structuring by the means of ULI has emerged as an alternative to lithographic fabrication to offer new opportunities for manufacturing photonic devices in the NIR and beyond [280, 312].

From an astrophysical perspective that needs panchromatic information, there is great interest in extending integrated photonics towards the MIR range beyond ~3 µm [313]. At MIR wavelengths, the fields of young stellar objects, the spectral characterization of nearby exoplanets, or the study of the circumnuclear regions of active galaxies fully justify the extension of PICs into the 3–20 µm MIR wavelength range (figure 16).

To date, MIR photonics have yet to demonstrate the ‘on-sky’ maturity achieved by their NIR counterparts. In the MIR, the TISIS experiment at IOTA using a fiber X-coupler in the astronomical L-band (~3.8 µm) for interferometric beam combination is the only ‘on-sky’ astrophotonic experiment undertaken thus far [314]. Most MIR integrated photonics and fiber demonstrations are laboratory-based and focused on the characterization of new devices and platforms. Originally motivated by the DARWIN/TPF mission to build a space interferometer, the effort has moved from simple waveguides and fibers [315–318] to more elaborate optical functions [319–323] such as couplers, splitters and beam combination circuits for ground-based astronomy. In the near future, the NOTT instrument at the VLTI [324] will eventually reach a new stage in on-sky use of MIR astrophotonics by employing a beam combiner for the 3.5–4.0 µm wavelength band.

### Current and future challenges

**Identifying priorities in the spectral bands of operation of MIR astrophotonics:** Most astronomers interested in photonics tend to broadly define the MIR as the 3–20 µm range, which is difficult to cover with a single photonic technology/platform. Furthermore, PICs usually require SM behavior, in particular for interferometric applications. Extending the usable wavelength bandwidth of SM waveguides beyond one octave requires careful design to avoid higher order modes at short wavelengths and excessive bend losses at long wavelengths. One could isolate sub-bands of operation, such as matching the typical atmospheric transmission windows. In recent years, the 3–5 µm spectral range has seen the largest progress (figure 16, left), for instance, with significant advances in beam combination capabilities for interferometry using ULI [325]. In the 8–12 µm spectral range, a proof-of-concept of astronomical functions has been achieved [320, 326]. Recently, a silicon-germanium integrated Fourier transform spectrometer [327] was demonstrated at 7.7 µm using thermo-optic modulation (figure 16, center). To date, the only attempt to cover the ultrabroad MIR range 6–20 µm with a single technology and waveguide geometry used telluride (Te)-rich chalcogenide SM waveguides (figure 16, right), albeit at the cost of high propagation losses [328].

*A future challenge resides in dividing the astrophotonic MIR range into several narrower spectral regions over which the overall performance can be maximized.*

**Material choices and fabrication platforms:** Highly transparent materials in the MIR (chalcogenides, heavy metal oxides, fluorides, III–V materials, Ge/Si, ...) are often more difficult to handle and process than the well-established Si/SiN/SiO₂ systems. MIR materials are generally less compatible with established fabrication techniques. For lithography, highly pure and defect-free thin-films are required and etch processes with minimal sidewall roughness. Suitable cladding materials are required to protect the

<footnote n="1">¹ I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Cologne, Germany</footnote>

<footnote n="2">² MQ Photonics Research Centre, School of Engineering, Macquarie University, North Ryde, NSW, Australia</footnote>

<footnote n="3">³ Ultra-Low Loss Technologies, Santa Barbara, CA, United States of America</footnote>

<footnote n="4">⁴ Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, New York, NY, United States of America</footnote>
<figure><img src="image_23.png" /><figcaption>**Figure 16.** Global schematic view illustrating the links between some astrophysical objectives and the photonic technologies that require to be developed in the wavelength range addressed by this paper, namely the mid-infrared range beyond ~3 µm. The top panels show two science cases that can particularly benefit from observations in the mid-IR: the spectroscopic characterization of exoplanet atmospheres in the fingerprint region where C/N/O based molecules can be detected and the mapping of planet-forming disks to understand the formation and evolution of young extrasolar systems [courtesy Chester Harman and ESO]. The chemical fingerprint region is linked to possible material platforms relevant for photonics in the top-central figure]. Reproduced from [wikipedia commons/chester harman]. CC BY-SA 4.0. Reproduced with permission from [332]. The middle panel shows that, next to the visible and near-IR range, the mid-IR range shows specific atmospheric transparency windows that can be technologically exploited for ground-based astronomy. The three bottom panels illustrate with examples the current yield of astrophotonics in three different accessible bands; Panel 3–5 µm range: *Top*: micrograph of a chalcogenide-GLS multimode interferometer fabricated by lithographic techniques. Reproduced from [eso0827a]. CC BY 4.0. Reprinted with permission from [330] © The Optical Society. *Center*: image of a ZBLAN coupler’s end face. Reprinted with permission from [312] © The Optical Society. *Bottom*: MIR interferogram obtained with a ULI coupler in GLS glass delivering high interferometric contrast. Reproduced with permission from [322] © ESO. Panel 7–13 µm: *Top*: view of the front-face and coupling region of a ULI coupler in GeAsSe chalcogenide glass. Reproduced from [337]. CC BY 4.0. Reprinted with permission from (Vigreux 2011) © The Optical Society. *Center*: SEM picture of an MZI array using the Si:Ge platform for integrated Fourier transform spectroscopy at 8 µm. *Bottom*: reconstructed spectrum with a resolution R ~ 100 at 1300 cm⁻¹. Reproduced from [327]. CC BY 4.0. Panel >15 µm: SEM picture (bottom) of a chalcogenide rib waveguide showing continuous transparency from 4 to 20 µm [338]. The blue arrows illustrate the cycle that links the mid-infrared technologies to possible astrophysical science cases.</figcaption></figure>

waveguides and a careful choice of carrier materials is also needed to avoid delamination due to thermal expansion coefficient differences. In contrast, ULI can directly modify highly pure bulk material, but generally cannot reach the high index contrasts of lithographic platforms, resulting in less compact waveguide circuits that are several centimeters in size rather than just a few millimeters, as commonly achieved with lithographic platforms. Hence the waveguide propagation losses are of great importance in ULI to maintain an overall high throughput. With several platforms/materials now tested (e.g. SiN/Si, SOI, Ti:LiNbO₃, Si/Al₂O₃, GLS, Ge/Si.), the current status suggests that ULI in chalcogenide glasses (e.g. GLS, As₂Se₃, Ge₃₃As₁₂Se₅₅) is a promising combination for the spectral extension beyond 3.5 µm of interferometric beam combiners, owing to the relative simplicity and versatility of the manufacturing process
[326, 329]. Unlike lithography, ULI does not require any photomask and is a single-step fabrication process and thus enables rapid design iteration. Its ability to create 3D waveguides avoids waveguide crossings and thus the associated crosstalk that can be detrimental for high contrast interferometry. Lithographic fabrication of PICs, owing to its origins from the electronics industry, can provide a much higher fabrication throughput, yet to date astronomical applications typically only require a handful of devices. The photolithography and etching of chalcogenide films, already proposed for channel waveguides [316], is being studied to develop MM interference couplers [330]. Silicon- and silicon/germanium-based integrated photonics [331–333] as well as III–V semiconductors [334] are attractive for the field, but no dedicated astrophotonics device has been demonstrated yet.

A future challenge for MIR astrophotonics is therefore to clearly highlight a dominant and optimal platform (or platforms) in terms of device quality and broadband performance for identified sub-ranges. Furthermore, the availability of materials and substrates with excellent purity and batch-to-batch repeatability is a key aspect for manufacturing MIR PICs using different platforms.

**Transparency/Losses (including Fresnel losses):** Low loss PICs are a major requirement due to the intrinsic weakness of the distant astronomical sources being observed. The propagation losses for MIR SM waveguides are in the 0.1 dB cm⁻¹–3 dB cm⁻¹ range (or 98% to 50% throughput over 1 cm) [332, 335, 336] within the wavelength sub-band of 3–5 µm, an order of magnitude larger than for NIR silica waveguides (~0.01–0.05 dB cm⁻¹, or down to 99% throughput per cm).

MIR materials with indices of ~2.5 exhibit ~15% Fresnel reflection losses per facet, and silicon or germanium even >30% per facet. Removing these losses requires broadband anti-reflection coatings on the small-sized input/output facets. Alternatively, materials with n ~ 1.5 and thus lower Fresnel losses of only 4% have also been investigated, such as ZBLAN glass [312] for instance, albeit their transparency is limited to <~4 µm.

Reliably and reproducibly processing high purity and defect-free MIR materials for creating waveguides, waveguide bends and devices with <1 dB cm⁻¹ losses are important future challenges for the progress of MIR astrophotonics. Furthermore, the ability to deposit broadband AR coatings on the chip/waveguide facets are of equal importance to maximize the throughput.

**Coupling and Interfacing with fibers:** Pigtailing PICs components for astronomical applications is an important aspect at all wavelengths, including in the MIR. Typically, MFD of MIR SM devices range from a few microns in high index contrast platforms to a few tens of microns for low field-confinement waveguides. The low-loss interfacing of MIR integrated photonics to commercial MIR fibers (e.g. fluoride, chalcogenide, silver halide or hollow core fibers) requires close matching in MFDs, as well as a good level of index matching or anti-reflection coatings to avoid reflection losses. In the NIR, well-developed and low-loss mode-matching taper designs are readily available to interface common PIC platforms to optical fibers. At MIR wavelengths, the diversity in optical, mechanical and thermal properties of the used materials as well as the high likelihood of the PIC and fiber being of dissimilar materials e.g. low-index fluoride fibers and high-index chalcogenide PIC, results in additional challenges. This is in contrast to NIR wavelengths where both interfaced ends are often made of silica glass. Moreover, the diversity in the fiber design (solid-core, hollow core, microstructured fibers; see section 5) driven by the requirement for increased wavelength transparency does add challenges as well as general handling, stripping and cleaving of MIR fibers. Also, polarization becomes important in high index contrast PIC systems, which exhibit a strong waveguide birefringence and polarization dependence in mode-field profile.

For the development of pigtailed MIR PICs, a critical challenge is to optimize the mode-matching between fibers and the integrated optic waveguides, at least for specific spectral sub-bands. An additional challenge is to identify a suitable, robust and stable method to attach MIR fibers to MIR PICs. This requires for instance a suitable IR-transparent glue with well characterized thermal behavior, or a thermal fusion process, in order to permanently attach optical fibers to the chips.

## Advances in science and technology to meet challenges

MIR integrated photonics have focused on wavelengths around or below 10 µm. The 10–20 µm range, of interest for space missions such as the proposed LIFE interferometer [339], remains largely unexplored. The propagation losses are expected to be larger than a few dB cm⁻¹, at least in the SM regime [328]. While SM Hollow Core Waveguides were proposed to circumvent the limitation of dielectric materials, they were found
to strongly underperform [340] as has the alternative InGaAs/InP technology [341]. It has been estimated
that a Germanium-Tin-Silicon platform could cover the 3–19 µm range with ~2 dB cm⁻¹ losses [342].
However, this remains to be proven experimentally.

*The optimization of a 10–20 µm platform for astronomy could be expensive and would need to be supported by a long-term technology program justified by a major astrophysical initiative.*

A critical aspect for MIR astrophotonics is the testing of operation at low or cryogenic temperatures, required to minimize thermal background. The GRAVITY experience has proven the operation of 2.0–2.4 µm pigtailed germanium-doped silica beam combiners at –80 °C [33], but MIR photonics may have to operate at lower temperature as, for instance, for the NOTT instrument.

Finally, radiation hardening will become an increasingly important challenge in the mid-term future for space applications of astrophotonics, regardless of the frequency range. This is also relevant for other fields such as satellite-based quantum technologies where efforts in this direction have already been made [306], targeting predominantly the VIS/NIR domain. In the future, since radiation hardness strongly depends on the material, specific tests for MIR integrated photonics will have to be implemented.

**Concluding remarks**

We have discussed the opportunities to extend the operation range of astrophotonics beyond the traditional VIS/NIR range into the MIR. The choice of material and compatible waveguide fabrication technologies is limited. Generally, MIR waveguides compared to their NIR counterparts exhibit larger losses and have to operate at extreme temperatures, which adds to the challenges. However, MIR photonics (waveguide and advanced devices such as on-chip spectrometers) have progressed significantly over the past 10 years, driven by other applications such as sensing, which the astrophotonics community can benefit from.

**Acknowledgments**

L L acknowledges support from the German Ministry of Education and Research (BMBF, ALSI Project 05A14PK2) and the DFG (NAIR project, Grant 326946494) in the field of mid-infrared astrophotonics. S G acknowledges funding by the Australian Research Council (FT200100590).
# 10. Integral field units for THz astrophysics

Akira Endo<fnref n="1" />, Charles M Bradford<fnref n="2" />, Ritoban Basu Thakur<fnref n="2" />,<fnref n="3" /> and Edward J Wollack<fnref n="4" />

## Status

Superconducting astrophotonics will revolutionize submillimeter/millimeter astronomy ($f \sim 0.1–1$ THz, $\lambda \sim 0.3–3$ mm) by enabling massive IFUs, that can simultaneously measure the wideband spectrum of all points in a 2D image. A wide-field and wideband spectrometer in the far-infrared offers a sensitive and unique 3D approach to topics which bridge cosmology and astrophysics, in particular the earliest galaxies as they are born in the reionization epoch [343, 344]. Mm/submm wave-IFUs have yet to be realized because they are out of reach of existing technology such as heterodyne receivers, direct-detection cameras, and quasioptical spectrometers [345]. The physical wavelength scale makes it impractical to design quasioptical IFUs with many spatial pixels (spaxels) and spectral channels (voxels). The photonic architecture addresses this challenge by shrinking the optical path into equivalent guided waves along SM superconducting transmission lines and metamaterial structures [346]. For example, analogues of a high-order optical grating spectrometer and an FTS have been integrated on a chip, as shown in figures 17(e) and (f), respectively. The advantages of superconducting astrophotonic instruments (compared to their quasioptical equivalents [345]) are mechanism-free, compact, lightweight, and ultimately scalable designs. The utility of the superconducting photonic devices goes hand-in-hand with the rapid advances in far-IR/submm/mm detector array format and sensitivity, particularly for kinetic inductance detectors (KIDs) [347]. A full discussion of KIDs is beyond the scope here, but we refer the interested reader to articles by [348, 349], and references therein. KID-based instruments with total pixel counts of $10^5$ or more are now conceivable, and photonic techniques are required to make optimal use of these large formats. For related developments at optical wavelength see section 24 (Mazin, Walter and Zou).

The development of superconducting astrophotonics to date has been largely focused on ultra-wideband filterbank spectrometers (figure 17(a)) or pseudo-grating spectrometers (figure 17(b)), which sort the light from a single sky mode into an array of spectral channels for detection. Examples of these implementations can be found in [350, 351], respectively. These dispersive approaches offer high sensitivity to individual sources, and are being used for detecting [CII] and CO lines in dusty star-forming galaxies at high redshift [352]. These spectrometers typically use a guiding structure created with patterned superconducting films (often niobium or NbTiN) and a thin dielectric (typically silicon nitride or silicon). For example, the on-chip FTS presented in figure 17(f) employs NbTiN/SiN/NbTiN microstrip lines. This architecture allows operation up to the Nb (or NbTiN) gap frequency of 690 GHz (1100 GHz), with spectral resolution generally limited by the loss in the dielectric (currently $R = f/\Delta f \sim 300–500$). Examples of on-chip filterbank spectrometers are Deep Spectroscopic High-redshift Mapper (DESHIMA) [350] that has seen astronomical first light on the Atacama Submillimeter Telescope Experiment 10 m, and SuperSpec [353, 354] which will soon be deployed on the 50 m Large Millimeter Telescope. Both use microstrip lines, with half-wave filters to define channel bandpasses. The Micro-Spec device employs a phased-array approach in which interference creates spectral channels via a lithographically patterned 2D multi-mode region [351, 355].

Another complementary approach which is emerging is an on-chip Fourier-transform spectrometer (FTS), called superconducting on-chip Fourier transform spectrometers (SOFTS), which employs an electrically tunable wave speed to modulate the phase delay [356]. See figure 17(c) for a schematic representation and operational principle. Each spatial mode of the spectrum is encoded in time and detected by a pair of detectors, enabling improved spectral recovery [357]. This allows spatial multiplexing at the expense of sensitivity in each spectral channel (due to the full band photon load on the detector). Another virtue of the SOFTS is that, like a classical FTS, the working resolving power is selectable in operation by simply adjusting the amount of phase delay used. SOFTS prototypes have been demonstrated at 10 GHz, with expectations for adaptation to higher frequencies.

## Current and future challenges

The first generation of on-chip spectrometers (DESHIMA, SuperSpec) are offering photon-noise-limited performance, and others are coming soon (e.g. EXperiment for Cryogenic Large-Aperture Intensity Mapping [355], South Pole Telescope Summertime Line Intensity Mapper [354], SOFTS [357], Cambridge Emission

<footnote n="1">¹ Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology, Delft, The Netherlands</footnote>

<footnote n="2">² Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America</footnote>

<footnote n="3">³ Department of Physics, California Institute of Technology, Pasadena, CA, United States of America</footnote>

<footnote n="4">⁴ NASA Goddard Space Flight Center, Greenbelt, MD, United States of America</footnote>
<figure><img src="image_24.png" /></figure>

Line Surveyor [358]. These demonstrations pave the way for larger, more powerful instruments. In considering approaches to the next generation instruments for wide-field science, a first-order figure of merit is simply the number of detectors that can be fielded (see figure 18). Whether they are multiplexed primarily spectrally as in the dispersive filter banks and pseudo-gratings, or spatially as in the SOFTS, the net spatial-spectral survey speed scales as the number of detectors, provided the detectors are photon-noise limited at their respective backgrounds [345]. While the legacy submm telescopes have modest field-of-view (e.g. Caltech Submillimeter Observatory/Leighton Chajnantor Telescope [359] with 15 arcmin giving 170 feeds at 220 GHz), new wide-field telescopes under development offer ample etendu (e.g. Fred Young Submillimeter Telescope which could feed some 10 000 devices at 220 GHz). As a result, in the near term the SOFTS and dispersive approaches could offer comparable survey speed. Assuming continued development in KID format to 10⁵ pixels, the dispersive approach offers the ability to field more total detectors, and has the potential to become the preferred solution.

In any case, scaling to larger and dual-polarization array formats presents packaging challenges. For full
spatial packing, ultimately the spectrometers must be either miniaturized greatly (so that each spectrometer
unit cell is only a few wavelengths on a side and they can be positioned between the antennas in focal plane),
or they need to be arrayed on chips which extend behind the telescope focal plane. The latter approach will
require new antenna/coupling approaches.

**Advances in science and technology to meet challenges**

We observe that there are compelling scientific opportunities to be realized by pushing astrophotonic
spectrometers to higher resolving power (R > 10³), higher frequency (>1 THz) and total sensitivity. See
figure 18 for a graphical summary of the achieved total spatial pixels (spaxels) and spectral channels (voxels)
number. Expanding upon these implementations will require careful attention to dielectric material loss,
potentially higher-gap superconductor materials, and either improving lithographic precision (down to tens
<figure><img src="image_25.png" /><figcaption>**Figure 18.** Current astrophotonic (imaging) spectrometers. The dashed lines represent constant total number of detectors $N_{\text{detectors}}$, which is the product of the number of spaxels (horizontal axis) and the number of detectors per spaxel (vertical axis). The mapping speed is proportional to $N_{\text{detectors}}$, regardless of whether the photonic device is a filterbank (○), an FTS (×), or a 2D grating (★).</figcaption></figure>

of nanometers: [360]) or development of new designs. The challenges in the photonic elements (filters, delay lines, etc) reside in the direction of higher frequencies and higher spectral resolution, combined with a high optical efficiency on a system level. Because the resolving power scales with pathlength in the medium, higher frequency resolution requires low-loss transmission lines in the mm/submm frequency range. Another frontier is wider (>one octave) bandwidth. This could be particularly useful for study of individual point sources for which full-band spectra desired. For mapping however, arraying ultra-wide bandwidth SM photonic devices will ultimately be limited in either in beam aperture efficiency or focal plane filling factor, relative to ~octave-band devices, which can be arrayed with good efficiency.

To improve the frequency resolution and optical efficiency of the photonic elements, dielectric materials with low losses are being investigated. Because coplanar waveguides suffer from large radiation losses, microstrip lines are a preferred option at mm/submm wavelengths. One possibility is to take a crystalline Si membrane from a SOI wafer and process on both sides [346]. Another approach is to deposit a dielectric film, which is more convenient for processing but has the disadvantage that amorphous dielectrics (e.g. SiNx with $Q_{\text{loss}} \sim 1 \times 10^3$ at 235 GHz [361], Si with $Q_{\text{loss}} \sim 3 \times 10^4$ at 350 GHz [362], and SiCx with $Q_{\text{loss}} \sim 1 \times 10^4$ at 270 GHz [363], at cryogenic temperatures) exhibit larger losses than crystals. It is notable that the physical loss mechanisms of amorphous and poly-crystalline dielectrics in the mm/submm are not well understood at cryogenic temperatures [364] (because experiments sensitive enough to measure losses at these low values are only recently being developed and applied, either optically [365] or on-chip [362, 363] and present an opportunity for potential improved performance.

To push beyond the gap frequency of NbTiN (1.1 THz), higher gap superconductors (e.g. $MgB_2$) must be used. Dielectric losses also increase with frequency (i.e. due to the presence of IR absorption bands wings) and may represent the next limiting factor [365]. In scaling to larger superconducting integrated photonic device formats control over transmission line losses, inter-element crosstalk [366] and unintended modal coupling [367] will become more pressing design considerations. These challenges are seen as manageable, however, will ultimately influence the device topology, layout, and packaging strategies.

An exciting platform for superconducting astrophotonics is space observatories. Here it is encouraging that KIDs have recently shown excellent noise equivalent power down to $3 \times 10^{-20} \text{ W Hz}^{-0.5}$ [368]. The next challenge is to integrate this type of KID in an astrophotonic spectrometer. For space applications, mitigation of cosmic rays is also important [369]. Recent advances in wideband planar lens-antennas can be leveraged to meet the need for wideband optical coupling. Using these large-format IFUs strategies to achieve wide-band and field-of-view optical systems represents an additional challenge.

## Concluding remarks

Similar to astrophotonic implementations in the optical, superconducting IFUs employ low-loss guided-wave structures, but also utilize the unique properties of superconductors to enable a variety of functions. It is a young and growing field of research, with multiple architectures being explored in parallel.
The overarching goal of enabling 3D volumetric surveys in the millimeter and submillimeter band. A range of astronomical and cosmological applications stand to benefit, from 3D tomographic line intensity mapping to study evolving galaxy populations [355], and cosmological 3D structure [370], to distortions and recombination imprints in the cosmic microwave background spectrum [371]. We encourage the participation of experts from the related fields, including astronomy, solid-state physics, electrical engineering, and data science to collaborate in bringing this technology to its full potential.

## Acknowledgments

A portion of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). A E was supported by the European Union (ERC Consolidator Grant No. 101043486 TIFUUN). Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or the European Research Council Executive Agency. Neither the European Union nor the granting authority can be held responsible for them.
# 11. Astrophotonic spectral filtering

Simon Ellis<fnref n="1" />, Sylvain Veilleux<fnref n="2" /> and Joss Bland-Hawthorn<fnref n="3" />

## Status

The frontiers of astrophysical knowledge inevitably lie at the limits of detection of the most advanced telescopes and instruments. To advance beyond these limits requires increasing the signal-to-noise of detections, via (i) larger telescopes, (ii) more efficient instruments, (iii) isolating the signal and reducing the sources of noise, or (iv) probing new properties of light, either different wavelengths or higher order moments (e.g. polarization). Astrophotonic filters are aimed at (iii), either by selectively filtering and measuring the signal of interest, or by removing the sources of noise.

Filters (absorptive, interference, holographic, etc) have long been a part of observational astronomy, but astrophotonic filters (i.e. filters embedded into waveguides) offer much more complex filtering with the ability to isolate multiple (>100) specific signals in a single waveguide. The first astrophotonic filters were FBGs, first developed in 2004 [22] to selectively filter the emission from atmospheric OH lines to reduce the background noise in NIR spectroscopy. FBGs have their origins in telecommunications, where typically one fiber can filter a single wavelength. A breakthrough in their application for astronomy was the ability to print *aperiodic* FBGs which can filter >100 wavelengths in a single fiber [9, 372]. Since FBGs require SM behavior, the PL (see sections 3 and 4) was developed in order to incorporate FBGs into the MM fibers necessary for efficient coupling with seeing-limited astronomical telescopes. Further development and refinement of FBGs led to on-sky experiments, GNOSIS [24, 25] and PRAXIS [23], which have demonstrated their efficacy, suppressing 103 OH doublets from 1.47 to 1.7 µm by factors of up to 40 dB (see figure 21), and resulting in a reduction of the integrated background by a factor 9. PRAXIS achieved an end-to-end throughput of ≈18% [23]. Figure 19 shows a sketch of the use of FBGs within an astronomical instrument; similar schemes can be used for other technologies and for multiplexed systems.

To date, SM FBGs in PLs are the only astrophotonic filters to have been incorporated into an astronomical instrument, but there has been considerable attention given to other platforms and other applications. Table 2 lists the different platforms (see figure 20) currently under development, as well as an estimate of their TRL (using NASA's TRL scale of 1–9) and comments on their advantages and challenges. These will be discussed more fully in the next section.

A different use for astrophotonic filters is to isolate the desired signal, for example using the reflected light in an FBG [373] or the drop port of an add-drop ring-resonator [374, 375] or cascaded MMIs [55]. Not only can a specific signal be selected this way, but multiple features can be selected and combined *photonically* in the same waveguide, with no associated electronic noise penalty. For example, exoplanets could be detected by selecting and combining multiple stellar spectral lines (see section 12), and monitoring the rise and fall in intensity (after photometric calibration using e.g. an 90/10 splitter to monitor the variable coupling due to changes of seeing etc) as the spectral features are Doppler shifted into and out of the filter bandpass. This technique could potentially dispense with the need for Doppler-spectroscopy, and the subsequent loss of signal-to-noise, as well as the requirement to observe parts of the spectrum not useful to the analysis, thereby reducing the need for expensive large 2D detector arrays.

## Current and future challenges

The main challenges for astrophotonic filtering in general are (i) scaling the technologies to increase the number of modes, and thereby accessible field-of-view and multiplex, (ii) extending the wavelength range of the filters, (iii) integrating the technologies with other photonic components, and with astronomical instruments. Astrophotonic filtering with asymmetric waveguides has the added challenge (iv) of being polarization (TE/TM) dependent. Specific solutions also have their own particular challenges. We discuss these challenges in turn.

(i) *Multi-mode filtering.* Photonic filtering is fundamentally a SM process, since it requires precise control of the phase of the light propagating in the waveguide to produce interference effects, whereas the modes of a MM waveguide will necessarily propagate with different phase velocities making this control impossible. In the diffraction-limited case, e.g. when using AO with high performance correction, it is possible to feed SM waveguides directly [50]. However, the input beam in seeing-limited astronomy is multi-mode and therefore has to be split into SMs without incurring significant losses. Furthermore, this requires high numbers of

<footnote n="1">¹ Australian Astronomical Optics, Astrophysics and Space Technologies Research Centre, Macquarie University, North Ryde, NSW, Australia</footnote>

<footnote n="2">² Department of Astronomy and Joint Space-Science Institute, University of Maryland, College Park, MD, United States of America</footnote>

<footnote n="3">³ Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, Sydney, Australia</footnote>
<figure><img src="image_26.png" /></figure>

replicated devices, all of which must achieve nearly identical performance (requiring high yield fabrication, or low cost). These must then be packaged into the astronomical instrument. For multiplexed observations typical of fiber optic spectroscopy this problem is likewise multiplied.

(ii) *Broad-band filtering*. To date, most astrophotonic filters have been centered on the telecom C band (1530–1565 nm), in order to exploit the R&D and fabrication processes already existing. A key challenge is to extend these processes to other wavelength ranges—ideally across the full UV-visible-NIR spectrum available for use with ground-based optical telescopes. The particular challenges in achieving this are very platform specific. However, some common difficulties are the SM cut-off limiting the bandwidth of a specific waveguide; the transmission of different types of waveguides ($0.3 \ \mu\text{m} < \text{SiO}_2 < 1.8 \ \mu\text{m}$; SOI > 1 $\mu\text{m}$) requiring the use of more exotic or doped glasses; the higher mode count at shorter wavelengths requiring narrower waveguides; the size of required structures scales with wavelength range (e.g. longer Bragg gratings, higher numbers of ring resonators); limitations in the FSR of periodic structures; precision of UV photolithography for UV/visible filtering; scattering errors from waveguide sidewall irregularities; stitching errors between e-beam fields. If accessing the reflected light, as for bandpass filters, then any optical circulators or coupling to drop-port waveguides also need to be broad-band and low-loss.

(iii) *Integration*. Integration has two equivalent challenges, coupling light from the telescope into the waveguides, and coupling light from the waveguides into the instrument. In the first place, there is the difficulty of coupling light into SMF already discussed in (i); see also section 2. This problem is exacerbated when using high-index contrast lithographic waveguides. For example, SOI may have a ~300 × 400 nm profile, leading to a mode-mismatch with SMF-28 causing ~13 dB of loss. For this reason, other lower contrast waveguides such as Si₃N₄ or SOS are often preferred. It is usually preferable to minimize the number of channels imaged onto the detector to avoid spreading the light over an unnecessarily large number of pixels increasing the electronic noise. If a MMF feed is used then it will be necessary to recombine the multiple SM waveguides back into an MM feed. If using an extreme-AO feed this problem can be avoided with direct coupling into SMF at reasonable (50%) efficiency.
<table><caption>**Table 2.** Techniques of astrophotonic filters, along with an estimate of their technology readiness level (TRL) and comments on their advantages (+) and challenges (−).</caption><thead><tr><th>Technique</th><th>Fibers</th><th>Comments</th></tr></thead><tbody><tr><td>FBGs</td><td>8</td><td>+ on-sky demonstration [23–25]<br>+ complex phase masks for routine manufacture<br>– single-mode usage only<br>– backward cladding-mode coupling<br>– inexpedient to scale to large numbers of modes</td></tr><tr><td>Direct write FBGs</td><td>4</td><td>+ does not require phase masks and photosensitized fibers like standard FBGs [376]<br>– single-mode usage only<br>– not yet demonstrated for complex filtering (e.g. 100 notches)</td></tr><tr><td>MCFBGs</td><td>3</td><td>+ multi-mode usage<br>– uniformity across the multiple cores [377]</td></tr><tr><td>Ultra-deep ultra-narrow filters (UDUN)</td><td>3</td><td>+ very deep (&gt;100 dB) notches<br>+ very narrow (&lt;100 MHz)<br>– currently single notch only</td></tr><tr><td colspan="3">Direct write waveguides</td></tr><tr><td>Direct write Bragg gratings</td><td>3</td><td>+ integrated structure reduces # of interfaces [377, 378]<br>– depth and precision of notch profiles<br>– not yet demonstrated for complex filtering (e.g. 100 notches)</td></tr><tr><td colspan="3">Integrated photonic circuits</td></tr><tr><td>Complex waveguide Bragg gratings (CWBGs)</td><td>4</td><td>+ compact design reduces stitching errors [90]<br>+ end-to-end throughput comparable to that of FBGs when used in combination with fiber-waveguide couplers with adiabatic tapers [90, 379]<br>+ ease of integration with other photonic components on a chip<br>+ thermo-mechanical stability for precision applications<br>+ lithographic printing enables replication<br>+ can be incorporated with MMIs to provide add-drop filtering [375]<br>– single-mode usage only<br>– backward cladding-mode coupling [90, 380]<br>– polarization dependence</td></tr><tr><td>Ring resonators</td><td>3</td><td>+ optimized for single notch filtering [374, 375]<br>+ tunable using on-chip micro-heaters or stress-optic actuators<br>+ lithographic printing enables replication<br>– scalability to complex filtering is not yet demonstrated<br>– small FSR limits bandpass<br>– single-mode usage only<br>– polarization dependence</td></tr></tbody></table>

(iv) *Polarization dependence of asymmetric waveguides.* Waveguides on wafers are inherently polarization dependent, even when the cross-section of the waveguides is symmetric (square). Although there are some polarization independent waveguides these can only work over a relatively narrow wavelength range.

**Advances in science and technology to meet challenges**

We now discuss the possible solutions and necessary advances in technology needed to address the challenges described in the previous section.

(i) *MM filtering.* PLs were invented as a direct solution to this problem [7], allowing the conversion between MM and SMF and vice versa. Direct-write waveguide versions of this concept have also been proven [90, 378]. Scaling PLs to very high numbers of modes is difficult, and ‘divide-and-conquer’ methods [21] may need to be employed. Beyond this, multicore fibers offer an attractive solution, allowing the inscription of multiple Bragg gratings at the same time. Devices with Bragg gratings across 121 cores have been demonstrated, but so far all suffer from a variation in Bragg wavelength as a function of the distance from the