<figure><img src="image_27.png" /><figcaption>**Figure 20.** Sketch of the different methods of astrophotonic filtering. The top panel shows a fiber Bragg grating, in which there is a variation in the refractive index of the core, causing light of specific wavelengths to be rejected. The middle panel shows a complex waveguide Bragg grating, which works on the same principle, but the variations in effective index are achieved by varying the waveguide width. The bottom panel shows a ring resonator, in which light with a wavelength which is a unit fraction of the optical path length of the ring is filtered from the input bus waveguide. These same techniques can be applied to different platforms, e.g. fibers, planar waveguides, direct write ultrafast laser inscription etc, with different manufacturing techniques.</figcaption></figure>

center of the core to the center of the fiber, which is not fully understood [381]. Direct-write PLs may offer a solution to this dilemma since then the Bragg gratings are written independently into each waveguide. A totally different approach is the use of extreme AO correction to feed SM waveguides at the diffraction limit [50, 74], with a theoretical coupling efficiency of ~80%, with the best coupling-efficiency to date of >40% [50]. This technique will be even more apposite in the future era of ELTs with very high AO correction, and very small diffraction limited PSFs, see for example the MODHIS instrument [382].

(ii) *Broad-band filtering*. Part of the solution to this challenge rests in adapting current techniques to new materials, e.g. using fluoride fiber to access wavelengths >1.7 µm, or chalcogenide glasses to access wavelengths up to 20 µm, or UV-fused silica to extend to shorter wavelengths (>195 nm). New recipes for fabricating filters in these different glasses will need to be perfected. Even with the adoption of new materials, the wavelength range will be limited by mode-count, detector sensitivity and FSR, possibly requiring multiple divisions into different wavebands employing differently optimized filters. To extend lithographic filters to shorter wavelengths will require higher precision next-generation nanolithography (e.g. EUV, x-ray, e-beam, focused ion-beam, nanoimprint lithography) to create nm-scale structures and reduce scattering off sidewall irregularities. Losses at wavelengths >1 µm due to N-H and O-H absorption features in Si₃N₄ waveguides can be reduced by annealing *before* patterning to provide wider passbands (0.4-2.4 µm) [383].

(iii) *Integration*. There are several proposed solutions for matching fiber and waveguide modes. Lensed fibers can reduce the mode field diameter to <3 µm, while inverted taper waveguides can expand the waveguide modes to 3.5 µm. These solutions are now becoming mature, with commercial off-the-shelf (COTS) interposer chips available which provide an efficient interface between the chip and a fiber array (see section 23). Moreover, these can work over ultra-broad bands [190], unlike some other telecom devices. Other solutions such as grating couplers are wavelength dependent and therefore operate over limited passbands.

The coupling from the waveguide filters into instruments could be significantly improved with customized detectors. For example, directly bonding photonic devices to detectors with a small pixel pitch that properly samples the output of the photonic device would eliminate the free-space interfaces between the filters and detector and make devices more robust and stable; likewise custom formats of linear arrays or rectangular pixels would allow for more efficient integration in some case; energy resolving sensors (e.g. MKIDS; see sections 23 and 24) could help to alleviate the low FSR of certain types of filters such as ring-resonators.
<figure><img src="image_28.png" /><figcaption>**Figure 21.** The measured transmission (black) of the FBGs used in the GNOSIS and PRAXIS experiments [24], which consists of two FBGs in series covering 1.47–1.58 µm and 1.58–1.7 µm respectively and filtering 103 OH doublets in total. Also shown is a model of the night sky OH spectrum (red). Note that the FBG response matches the wavelengths, line strength, and OH doublet separation perfectly. This remains the state-of-the-art in astrophotonic filtering, although significant progress is being made with other technologies, see table 2.</figcaption></figure>
(iv) *Polarization dependence of asymmetric waveguides.* It is extremely challenging to make a polarization-independent waveguide that can work over a wide wavelength range. A more promising approach is to exploit the modular and replicable nature of lithographic photonics and use broad-band polarization splitters based on MMIs [384] or directional couplers [385], or to avoid the problem with fiber-based solutions or direct-write ULI of symmetric waveguides in fused silica.

In addition to the specific challenges described above there is a general need to streamline the fabrication of science-ready astrophotonic devices through dedicated access to facilities and expertise in fabrication, characterization, and packaging [38]. This access is becoming easier due to the availability of commercial foundries, especially those offering MPW fabrication services which significantly reduce the cost for making and testing prototypes.

**Concluding remarks**

Astrophotonic filters have already been shown to have significant promise for astronomy, most notably for NIR spectroscopy using FBGs to filter atmospheric emission lines. So far the science benefits of photonic filters have only been partially realized, and there are many science cases other than OH suppression which can profit from the complex filtering enabled by the devices discussed, including Doppler planet searches, emission line diagnostics, and wavelength calibration. Beyond this, there are several promising technology platforms being investigated, including novel ways to inscribe Bragg gratings in multicore fibers and direct-write waveguides which will help to enable a scaling-up of this technology for larger fields-of-view and higher multiplexing capability. A significant, but challenging, development is the integration of photonic filters on lithographic photonic circuits. This has the potential for extremely miniaturized devices, which are modular and easily integratable with other photonic devices such as arrayed-waveguide gratings (sections 6 and 7) and photonic beam combiners (sections 16, 17 and 20) with the ultimate aim of fully photonic instruments.
# 12. Measure what you need with optical correlation spectroscopy

Sebastiaan Y Haffert¹ and Peter Tuthill²

¹ Steward Observatory, University of Arizona, Tucson, AZ, United States of America

² Sydney Institute for Astronomy (SIfA), School of Physics, The University of Sydney, Sydney, Australia

## Status

Technological advances driving ever larger telescope apertures have also increased the size of the instruments [386]. Competitive astrophysics demands ever higher spectral resolution, more simultaneous spatial points and larger bandwidths. In particular, high spectral resolving power is needed for identification of atomic and molecular gasses in astronomical objects as their spectral fingerprints may be distinguished at high resolution. Spectral differentiation between species is now used as a successful post-processing technique to search for exoplanets with direct imaging instruments [387, 388]. Applying such techniques to large fields would allow us to map the chemical compositions of many objects simultaneously, like stars in dense stellar clusters, or extended objects like the Orion Nebula. However, the competing requirements are difficult to fulfill for IFU s and MO spectrographs that require many simultaneously sampled spatial points. Conventional spectrographs—which must be built with real-world limitations on the size of the optics, number of pixels, and data rates—are therefore forced into compromises between spatial sampling, spectral resolving power and bandwidth. Optical cross-correlation offers a pathway to avoid this compromise based on the realization that acquiring the full spectrum may not be necessary for all science applications. We may require, say, only a measurement of the elemental composition, without details of all the lines present. Distinguishing information at high spectral resolution need not mandate the recovery of every single spectral channel.

Such measurements can be achieved through optical correlation spectroscopy (OCS) in which light passes through a spectral filter that partially mimics the spectrum of the element of interest [389] (see figure 22). The simplest example of a spectral filter might be a template tailored to match the response of one specific molecule, whose presence is then indicated by measuring the total intensity passing through. Most light passes through if the same lines are present in the analyzed spectrum and the filter. However, a lower intensity is measured if there are different lines present in the filter and the spectrum. This means that a single intensity measurement is sensitive to the presence or absence of specific species. Obtaining a high spectral specificity in a single sensor element breaks the trade-off above between resolution, spectral bandwidth, and density of spatial sampling. While a single intensity measurement is enough in theory, flux variations require two measurements (with and without the spectral filter) to normalize the incoming amount of light.

There are several different ways to implement an OCS. The earliest methods used gas-cells that contained the gas of interest [389]. These may be considered ideal spectral filters as they contain exactly all the lines of interest. A downside to the gas-cell correlators is that not all elements can be contained in a cell at astronomical temperatures and/or pressures. A tunable optical filter that replaces the gas-cell is preferred for OCS in astronomy where elements occur under conditions that cannot be reproduced on Earth, and also noting that astronomical objects are not observed in the same rest frame causing lines to undergo Doppler shifts [390].

## Current and future challenges

OCS recovers a compressed data stream so generally requires simpler input optics, detectors and output signal train. The key challenge for implementation of OCS is realization of the spectral filter. To date this has been accomplished with several different implementations (see figure 23).

### Interferometric filters

Interferometric filters use path length differences to create regularly spaced absorption peaks through constructive and destructive interference. This is often done with a Lyot Filter [391], Fabry-Perot [392, 393] or a Michelson [394] interferometer. More recently, photonic implementations use ring resonators [261] to create a regular pattern. While velocity shifts of the template can be achieved by adjusting the path length, this approach is unable to form a tailored match to spectra of arbitrary line spacing and complexity.

### FBGs

While interferometric methods such as etalons can successfully exploit regularities in the spectral response of many gasses, the use of FBGs offers dramatically greater flexibility. In particular it is possible with modern aperiodic grating designs to obtain an excellent match even for complicated sets of irregularly spaced atomic or molecular lines. Such bespoke gratings may be tailored to target gas species with high specificity [395] and can be fabricated from the visible through the NIR. Such an optic may be integrated into a working sensor
<figure><img src="image_29.png" /><figcaption>**Figure 22.** Light from two astrophysical sources (a star and a planet in this case) are fed to an optical cross-correlator spectrograph. The OSC passes the light of the objects through a spectral filter that is matched to a particular element in the planet's atmosphere. The planet has a strong correlation, while the star has no correlation signal at all. This allows the OCS to search for particular elements.</figcaption></figure>

<figure><img src="image_30.png" /><figcaption>**Figure 23.** An overview of different ways to implement the optical correlation filter. The systems on the left create notches regularly spaced in frequency (diagrams at the top). The systems on the right create irregular notches by multi-step modulation. Figure 22 shows an example of irregular notches.</figcaption></figure>

system [396] by imposing a variable filter velocity response (potentially by way of a fiber stretcher, or tuning the response with temperature), and ideally facilitates recovery of both the reflected and transmitted beams to optimize capability for accurate calibration. Current FBGs have reached a spectral resolution close to 100 000 which is more than sufficient for scientific observations. FBGs require that the light passes through a SMF. However, spatially coherent sources (such as stars) need high performing AO correction (see section 2) for efficient coupling into SMF [50], which is very challenging towards shorter wavelengths. Extreme AO systems deliver much higher performance at wavelengths upwards of 2.0 µm, but at those wavelengths the SMF needs different materials as silica is not transmissive anymore. FBGs have not yet been demonstrated at longer wavelengths.

### Multiplexed Volume Bragg gratings

Volume Bragg gratings (VBGs) are thick transparent gratings that have a periodic modulation of the refractive index. The VBG diffracts only a narrow wavelength range around the Bragg wavelength. Multiple gratings can be created in a VBG at the same time if the refractive index profile is a superposition of the
individual grating profiles [397]. Such a grating is called a multiplexed VBG (MVBG). Each grating then diffracts its specific spectral line in the same direction (angle) where they incoherently combine on the same detector pixel. Effectively creating the OCS signal. An advantage of this method is that it creates a tiny spectrum around the diffracted wavelength. This tiny spectrum is the velocity correlation spectrum. The presence and velocity of the template gas species can be measured at the same time, which is an advantage over most other methods.

FBGs have recently been made for spectra with very complicated profiles, such as for OH-suppression filters [22]. The FBGs are inscribed inside the core of silica fibers by using holography and phase masks [398]. This fabrication approach fails for MVBGs due to their thickness: instead a method in which refractive index structures are laser engraved within a substrate (so-called ‘direct write’) [90] is used. However grating-based approaches have a strict tolerance on the written grating period: the relative precision is the inverse of the spectral resolving power. For MVBGs the precision has to be better than $1 \times 10^{-5}$ to meet a design specification of $R = 100\,000$ and unfortunately current direct-write methods are not yet at the level where this is possible over sufficiently large areas ($30 \times 30$ mm) due to aberrations in the writing systems [399].

### Advances in science and technology to meet challenges

Most OCS instruments deployed to date have either been based on the gas-cell method or interferometric approaches due to simplicity of the technology and ease of implementation. They have demonstrated most success for molecules with rovibrational transitions that create lines with relatively regular spacing. However in reality lines are never exactly periodic, imposing a limitation on the interferometric approach to a small part of the spectrum (and consequently to bright objects). Many other species have complex line patterns that are not regular at all. Filters with regular spacings between the notches cannot create the required spectral templates. Complex filters with aperiodic notches are required for most cases. The multiplexed VBG and the aperiodic FBG are therefore the most promising methods for OCS as these provide the most flexibility in template spectra. However, as discussed before several challenges have to be solved.

Integral-field OCS with FBGs present challenges due to the SMF injection step. Single-mode IFUs, often exploiting a micro-lens array to inject into a matched array of fibers, have been successfully demonstrated in the past few years [78]. Nevertheless, the stringent spatial beam requirements for efficient SM injection drive challenges for IFU throughput and efficiency. Few-moded fibers in concert with lanterns may present one pathway for FBG based integral-field OCS to boost efficiency [21, 400]. However such arrays of low mode-count PLs have not been developed, with further innovations needed to couple them to banks of FBGs (see section 4).

A fundamental issue for the FBG is that at each moment in time only a single correlation match to the template is made, so that time modulation of the response is required to recover velocity information. At the cost of escalating optical complexity, a scheme might be envisaged whereby light from a first-stage FBG is passed to a second FBG tuned to a slightly different velocity. A cascade of such FBGs (where reflected light from one FBG is sent to the next FBG in the cascade and the transmitted light from all the FBGs is measured) can be used to create the velocity spectrum instantaneously with no need for a time sweep. The FBG alters the trade space from spectrograph real estate to telescope observing time and spectral templates. Measurements have to be taken sequentially if several species or Doppler shifts have to be measured. One possible scheme may implement a different FBG filter (tuned for various molecular species) that tiles some region of spatial extent in the image plane. Some scheme of dithering or field rotation would then bring different spatial elements into alignment with different FBG filters, over time sampling the whole scene with templates matched to all species.

The MVBG can provide access to multiple species and velocity channels in a single shot. However, the complexity of the grating increases with every additional information point. This requires very accurate control over the grating pattern which is not available with passive direct write methods. AO techniques are deployed to create stable and high precision direct-write setups and have shown promise in improving the write quality [401]. Acousto-optical gratings [397, 402], in which sound waves act to modulate the refractive index, are also being explored to provide an alternative. Multiplexed gratings can be realized by sending complex waveforms through the material. With active control of the acoustic synthesizer, the demanding requirements to implement an MVBG can be met. There are two technical challenges to overcome. The high-spectral resolution of $R = 100\,000$ requires a large piece of EO crystal with high homogeneity, which is difficult and expensive to source. Furthermore, the required acoustic power scales with $\lambda^4$. Large gratings require injection of significant acoustic power for lines at longer wavelengths which generates waste heat in the crystal that needs to be extracted.
Concluding remarks

Optical correlation spectroscopy is a promising technique to map gasses and elements, distilling information at high spectral resolution over significant fields-of-view. This technique allows us to think in a different way about observing. There are many advantages to compressing the information content of the data optically, before the light reaches the sensor, recovering the imprint of the chemical content in the spectrum, rather than the wealth of lines themselves. The method could accomplish our science with fewer targeted measurements, offering relief where constraints from instrumentation or data volume hamper progress. However, it is still a relatively new technique for astronomy and there remain hurdles to be overcome before working systems can be deployed to modern observatories.

Acknowledgments

Support for SYH was provided by NASA through the NASA Hubble Fellowship Grant #HST-HF2-51436.001-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.
## 13. Laser frequency combs

Chad F. Bender¹, Scott A Diddams², Tobias Herr³, Stephanie Leifer⁵, Ewelina Obrzud⁴ and Kerry Vahala⁵

¹ Steward Observatory, University of Arizona, Tucson, AZ, United States of America

² Electrical, Computer and Energy Engineering and Department of Physics, University of Colorado, Boulder, CO, United States of America

³ Deutsches Elektronen-Synchrotron DESY, Germany and Universität Hamburg, Hamburg, Germany

⁴ Centre Suisse d'Electronique et de Microtechnique, Neuchâtel, Switzerland

⁵ Department of Applied Physics, California Institute of Technology, Pasadena, CA, United States of America

### Status

LFC, the Nobel prize-winning technology recognized in 2005 [26, 27], have revolutionized time and frequency metrology. The LFC or, alternately, optical frequency comb (OFC) is an optical spectrum produced by a laser that consists of an array of delta-function-like frequency modes with perfectly uniform spacing determined by the laser's RF-domain pulse repetition rate, $f_{\text{rep}}$. All modes share a common frequency offset, $f_0$, that can be measured and controlled with the technique of $f-2f$ self-referencing [403]. In this way the frequency of each mode of the comb, $f_n$, is given by $f_n = nf_{\text{rep}} + f_0$, where n is an integer and $f_0$ and $f_{\text{rep}}$ can be referenced to an atomic clock with uncertainty given by the clock itself. For example, when referenced to the clocks in the global navigation satellite system, the LFC inherits a typical fractional uncertainty of $10^{-12}$ or $10^{-13}$. It is accordingly a very precise 'spectral ruler' for measuring any frequency of light. In precision astronomical spectroscopy, LFCs are ideal calibration sources with performance that surpasses the systematic uncertainty introduced by existing spectrographs and astronomical processes such as stellar activity. On the other hand, the intrinsic noise in the LFC supports frequency synthesis at and below the $10^{-21}$ level [404]. As such, it also serves as the clockwork that links RF and optical domains to implement optical atomic clocks with present frequency uncertainty at the $10^{-18}$ level [405], which is two orders-of-magnitude more precise than current time standards. Thus, LFCs can be a critical tool for studying a wide range of challenging questions in physics and astronomy that involve precision frequency measurements.

There are a variety of methods of generating LFCs, and the resultant combs can be characterized by their mode spacing ($f_{\text{rep}}$), spectral bandwidth (visible through MIR), and optical power. The most mature comb technology, in terms of commercialization, is the fiber laser comb composed of all-optical-fiber components that form a passively mode-locked laser cavity in sequence with a fiber amplifier, supercontinuum generator, and $f-2f$ interferometer [406]. These low repetition rate combs ($f_{\text{rep}} \sim 100$ MHz), operating in the NIR (center wavelength of 1550 nm), are used to form an essential component of a new generation of optical atomic clocks. A space-based network of optical atomic clocks that utilize LFCs as the clockwork with $10^{-18}$ accuracy has been proposed for gravitational wave detection [407, 408], relativistic geodesy, dark matter experiments [409], opportunities to search for violations of general relativity [410], the standard model, and the Einstein equivalence principle, and to provide the potential to discover new physics [411]. When used for femtosecond-level time transfer or synchronization, LFC technology could enable synchronization of distant sites such as those used for the Event Horizon Telescope [412], a global network of radio telescopes that can provide very high angular resolution imaging, thereby drastically reducing extensive, compute-intensive post processing needed to correctly align the constituent signals.

An LFC referenced to an atomic clock and the SI second has become an essential wavelength calibration tool. As the LO in a heterodyne radiometer [413], they could provide higher resolution Doppler maps of the sun than those produced by NASA's Solar Dynamics Observatory [414]. But in the astronomy community, LFCs have found their greatest application in ground-based Precision Radial Velocity (PRV) measurements, and when specialized for this role, are dubbed 'astrocombs' [28, 29, 415]. With absolute stability at the $10^{-12}$ level or better, corresponding to the Doppler shift of <1 cm s⁻¹, astrocombs provide sufficient precision in spectral calibrations for PRV spectrographs that aim to measure <10 cm s⁻¹ Doppler shifts in absorption features of G-type stars, and in so doing, enable characterization of earth-like exoplanets. There are currently more than a dozen astrocombs demonstrated or in operation at major optical and NIR facilities that are pursuing this and related RV measurements [30, 31, 416]. They cover a range of wavelengths from ~500 nm to greater than 2 µm, and have been customized to provide spectra with mode spacing appropriate for the unique spectrographs they calibrate (10–30 GHz). This flexibility makes astrocombs attractive calibration sources even for non-PRV spectrographs as a replacement for traditional hollow-cathode lamps that provide inferior calibration spectra due to a nonuniform line distribution over the spectral span, high dynamic range and line blending. Moreover, in some cases, such lamps are difficult to procure due to changing environmental and geopolitical restrictions. Figure 24 depicts comb generation by various methods, as well as various applications in astronomy as a function of RV precision and wavelength.
<figure><img src="image_31.png" /></figure>

## Current and future challenges

The promise of astrocombs often belies coupled scientific and technical challenges that limit the widespread application of these highly complex systems. When one considers the details of frequency combs, there are many challenges that must be addressed including comb spectral flatness, low temporal variability, uniform coupling to MM spectrographs, and polarization effects in SM spectrographs. But the most critical issues at this point in time can be essentially described as: (1) overall complexity which is coupled to size, weight, need for maintenance, operation over many years, environmental stability, remote operability, and cost; and (2) full spectral coverage that matches that of stellar spectra, with particular shortcomings on the blue side of the frequency comb spectrum.

Frequency combs are significantly more complicated and costly than traditional calibration sources such as hollow-cathode discharge lamps or gas cells. This complication arises in part from the fact that they are built on active lasers, but it is compounded by the coupled requirements of large mode spacing ($f_{rep} \sim 10-30$ GHz) and broad spectral bandwidth (as large as $\Delta\lambda = 350-2500$ nm). To achieve both of these at the same time requires combinations of average power and pulse duration that are not found in any common LFC. The canonical approach, shown in figure 24(a), involves creation of a high repetition rate pulse train (a comb with narrow spectral bandwidth), whose short pulses along with amplification permit spectral broadening through nonlinear optical processes. Finally, the spectral envelope of the broadened spectra is flattened to avoid over- and under-exposure of the detector, and detector effects such as the 'brighter-fatter' effect [32] that can induce erroneous RV signatures.

A first challenge is to identify a pulse source that permits operation at high pulse repetition rates. As shown in figure 24, comb technologies that are being explored include: (a) low repetition rate lasers that are subsequently filtered to provide large mode spacing [28, 29, 417, 418], (b) EO frequency combs that
intrinsically generate 10 GHz combs [419–421] (c) high-repetition rate active lasers [422, 423], and (d) microresonator frequency combs, also known as microcombs [57, 424]. There are tradeoffs that must be made in all of these technological approaches, e.g. wavelength of operation, permissible comb line spacing, size, complexity, cost and ease of operation.

Regardless of the pulse source, a fundamental challenge is that 10 GHz combs have low pulse energy which restricts the nonlinear optics required to generate broad spectral bandwidth, such that maximal RV information can be obtained. The most straightforward way to address this challenge is by increasing the average optical power to the multi-Watt level, but this comes with challenges of thermal management and damage in the LFC system and degradation of components. Such factors further drive the price and complexity, which restricts reliability. Presently, this is most challenging for frequency combs aimed at RV measurements on Sun-like G stars where the desired spectral coverage is ~380 nm to beyond 800 nm, or more than 400 THz and 40 000 comb lines at 10 GHz comb spacing. Nonlinear spectral broadening in silica fibers has been developed to cover this spectral range. However, the reliability of such an approach is limited due to rapid damage of the fiber upon UV exposure [425]. Thus, reliable operation is only routinely achieved at wavelengths longer than approximately 500 nm, excluding nearly 200 THz of valuable spectral bandwidth. Continued research efforts are needed to provide reliable solutions for continuous nightly spectral LFC calibration down to the shortest blue wavelengths.

### Advances in science and technology to meet challenges

Major technological advancement can be expected from new materials and their potential to be monolithically integrated on photonic chips for efficient nonlinear optics, compactness and cost reduction. The integration of comb systems requires a range of optical technologies to be co-located on a common silicon substrate. In recent years, the narrow bandwidth comb sources themselves have undergone rapid development based on two technologies: soliton microcombs and thin-film lithium niobate EO modulators [251, 426]. Moreover, integrated nonlinear waveguides have been fabricated showing dramatic spectral broadening and opportunity for spectral tailoring through optimized geometries potentially beyond what is possible in current nonlinear silica fibers [427–431]. Each of these platforms has created opportunities for the complete integration of future comb systems.

Soliton microcombs are miniature mode-locked parametric oscillators fabricated from high-Q optical microcavities [251]. They can be self-referenced and have been used to demonstrate all comb functions including optical clocks and optical synthesizers. While early microcombs were demonstrated as discrete devices, significant progress in optical loss reduction of CMOS-friendly materials such as silicon nitride has made possible microfabrication of more complex microcomb systems. These combine integrated waveguides as well as soliton repetition-rate tuning control. Also, these systems have been heterogeneously integrated with III–V semiconductor lasers for comb pumping. Surprisingly, the pumping of microcombs can be configured to simplify comb activation while also eliminating difficult-to-integrate functions like optical isolators. Compact, butterfly-packaged microcomb systems with optical pump and tuning controls have resulted [173, 252].

While microcombs are transitioning conventional table-top mode-locked combs to a semiconductor chip, there is also a renaissance in the world of EO combs. Here, the advent of the material system thin-film-LiNO₃ on silicon brings the key features of this photonic workhorse material (harmonic generation and EO modulation) into the realm of semiconductor microfabrication. The resulting capability to engineer dispersion and precisely control waveguide dimensions has enhanced the performance of devices like EO-modulators by lowering their V_pi values. It is also dramatically expanding the ways LiNO₃ can be fashioned into optical devices and complex systems on-a-chip. Miniature resonant modulators and even electro-optically controlled microcombs are two examples [426, 432].

With this rapid progress in microcomb and EO-comb technology, a pathway exists to compact, integrated astrocomb systems. However, before this can happen, efficient nanophotonic spectral broadening must also be integrated with the comb generator. Again, a challenge here is the low pulse energy that is available from the integrated micro and EO comb generators. Presently off-chip fiber amplifiers must be employed to boost peak power to a sufficient level (~10–100 pJ in 100 fs) to achieve the required spectral broadening. Nonetheless, there is significant progress in the exploration of new materials that offer multi-order-of-magnitude enhancements to optical nonlinearity that are critical for broad spectrum generation. Material platforms of UV transparent media like MgO-doped LiNO₃ [428, 429, 433], AlN [430, 434] and diamond [435] show some promise based on efficient spectral broadening and harmonic generation including at the high-repetition rates of astrocombs [436]. Their nanophotonic integration offers new opportunities for tailored spectra, however, more research and development is required to achieve high-power handling capability and sufficient flux at short wavelengths.
Concluding remarks

Future LFC precision calibrators will need to balance mature and emerging technologies to address conflicting requirements of broadband calibration and high reliability at reduced cost. Integrated photonics will play an important part in this development and hold prospects not only for improved performance but also for low-complexity, robust monolithic systems at lower cost; in case of device failure, such systems could potentially be swapped like a light bulb. On a practical note, the development cost of these miniature systems will be shared with other application areas, including miniaturized precision timing and navigation systems, airborne radar systems, and field-deployable precision chemical sensors. To leverage the full potential of LFCs, dedicated data pipelines and instrumental interfaces (including spectral flatteners—see section 14) will need to be developed. Additional developments in AO and SMF-fed spectrographs may bring a significant advance when calibrated with LFCs. And finally, the reduction of size, weight and power consumption creates opportunities for space-based EPRV observations as well as unperturbed studies of planetary atmospheres.

Acknowledgments

S Diddams acknowledges support from NIST, JPL, and NSF Grants AST-1310875 and AST-2009982.

T Herr acknowledges support by the Helmholtz YIG VH-NG-1404 and the EU ERC StG 853564.
# 14. Spectral flattening of laser frequency combs on-a-chip

Nemanja Jovanovic<fnref n="1" />, Stephanie Leifer<fnref n="2" /> and Charles Beichman<fnref n="3" />

## Status

The PRV technique is critical to the detection and characterization of exoplanets and relies on the measurement of Doppler shifts of the host star's spectral features [437]. Key to the success of PRV observations is an ultrastable wavelength standard, ultimately requiring sub-cm s⁻<fnref n="1" /> long-term precision and accuracy to enable the detection of terrestrial planets in the habitable zones of solar-type stars, which induce shifts as small as ~9 cm s⁻<fnref n="1" /> over a year [416]. LFCs offer exquisite long term stability making them ideal for this application (see section 13 in the roadmap). However, due to the nonlinear broadening processes that generate broad comb spectra, the amplitude of the comb lines can vary by many orders of magnitude (see figure 25). As Doppler precision scales with the square root of the number of lines used to derive the wavelength solution, it is critical to exploit the maximum number of lines when developing a wavelength solution. The limited dynamic range of astronomical detectors imposes limits on the amplitude variation that can be tolerated. In addition, the signal-to-noise per comb line scales with the square root of the number of photons per line. To achieve high precision with short integrations, it is clear that a flat, bright spectrum where each line is not saturating the detector would be ideal. In addition, the comb line amplitudes should be extremely stable, as changes to the comb profile of the spectrum can masquerade as erroneous Doppler shifts. To meet these requirements, a device called a 'flattener' is often used, downstream of the comb. Commonly, flatteners first collimate the LFC light from an optical fiber, disperse it with a grating, and then use a liquid crystal on silicon spatial light modulator (SLM) to control the amplitude of each spectral channel before recombining the spectrum and injecting it into another fiber [438]. These flatteners are large complex assemblies (see figure 25) requiring careful alignment, and the dynamic range afforded by the SLMs (20–30 dB) is often inadequate to flatten the spectrum produced by an LFC. Further, they are tailored for specific spectral bands and require developmental effort to move into new wavebands.

A photonic solution offers the possibility for a compact, portable, replicable, and inherently stable package that may even enable space-based applications. One such solution is an all-photonic spectral flattener based on SiN waveguides consisting of an AWG which disperses the light, MZIs to actively control the amplitude in each channel, thermo-optic phase modulators (TOPMs) to re-phase the channels, and a second AWG used in reverse to recombine the spectrum. Our group has recently demonstrated such a device with 20 channels providing ~40 dB of dynamic amplitude modulation range for a linearly polarized input source [41, 42]. It was capable of flattening a temporally incoherent source to a residual amplitude variation of 3 dB, and an LFC spectrum to ~5 dB where both sources started with >25 dB of variation over 250 nm (1400–1650 nm). The device has a millisecond response time (kHz) to make corrections and can handle optical power levels of hundreds of milliWatts. The current device operates in the near-IR, suitable for instruments designed to study planets orbiting cool, mid- to late M stars. Future devices could provide spectral flattening at optical wavelengths appropriate for instrumentation studying warmer, solar-type stars.

Another method for producing broad, relatively flat comb spectra is to accomplish it through engineering the appropriate combination of dispersion and geometry in the nonlinear photonic waveguides used for supercontinuum generation [439]. This would eliminate or reduce the need for an external flattener on-a-chip. Very broad spectra have been demonstrated in, for example, SiN [440], magnesium oxide doped lithium niobate thin films on silicon [428, 433], tantalum [431, 441], and aluminum nitride structures [434].

Beyond LFC flattening, spectral shaping devices can find applications in other fields including add/drop multiplexing [442], gain flattening [43, 44], temporal pulse shaping [45, 46] as well as for targeted excitation of particular molecular species [47].

## Current and future challenges

To advance all-photonic flattening technologies, requirements for specific science cases must be set with an understanding of how Doppler precision relates to the power stability of the comb spectrum. This has not been rigorously explored to date and requires simulations to understand how variations in the comb spectrum propagate to Doppler errors. Once the power stability requirement is determined for a specific science case, the device parameters (resolution, number of channels, overall bandwidth etc) can be established. Furthermore, other photonic devices, such as FBGs could be used to compensate for static, large

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

<footnote n="2">² Department of Applied Physics, California Institute of Technology, Pasadena, CA, United States of America</footnote>

<footnote n="3">³ IPAC/NASA Exoplanet Science Institute, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America</footnote>
<figure><img src="image_32.png" /><figcaption>**Figure 25.** (Top) (Left) a spectrum of the Palomar Radial Velocity Instrument (PARVI) LFC showing the strong intensity modulations across the spectrum. (Right) PARVI's bulk optic flattener. The flattener occupies a volume of $1 \times 0.5 \times 0.5 \text{ m}^3$. (Bottom) An image of the generation one all-photonic flattener device tested in the laboratory. Reprinted with permission from [42] © The Optical Society.</figcaption></figure>

amplitude power variations such as those around the pump region of the comb. The division of flattening capability between these photonic solutions needs to be determined through careful characterization of the specific comb to be used.

Technical challenges include sufficient AWG channel spacing to correct for narrowband features around the pump region. Devices with higher channel counts will be necessary, requiring the addition of more MZIs and TOPMs, and their associated electrodes and driver electronics. These devices are non-negligible in size ($2-3 \times 1 \text{ mm using SiN}$) and will eventually push the circuit size beyond a single reticle. Devices can be made across multiple reticles, but there could be stitching errors, although these are improving over time with advances in manufacturing processes. In addition, narrower channels may be desirable in some regions of the spectrum and could be accomplished with a cascaded approach [180], where a low resolution device is used over a broad band and narrower devices are used as a secondary stage (see figure 26 for an example of this concept). Adding more AWGs to the chip or increasing the resolving power of an AWG with a fixed bandwidth both increase the chip size and is limited by either reticle size (limit for a single device) or total usable area on the wafer. In addition, making high resolution, large FSR AWGs is plagued with many issues in itself (see sections 6 and 7 in this roadmap). Although the number of channels can be increased from the first generation device demonstrated above, it is clear that with current technology and lithographic approaches it would be challenging to scale devices to 1000s of channels, which is the pixel count of commercially available SLMs.

Another challenge is the bandwidth of the device. LFCs are currently used across several astronomical bands ranging from 350 nm to 2.5 µm [418, 421]. Photonic devices have a limited range over which they operate with a single guided mode with acceptable bend losses. The prototype outlined above spans only a single astronomical band (H). The two primary options for broader spectral coverage are: (1) making a series of parallel devices optimized for different wavebands, or (2) making broadband devices in a single platform. This technology could have the greatest impact in the visible band where most PRV spectrographs currently operate. Developing efficient SM circuits with the necessary bandwidth could be challenging. LioniX International offers visible waveguides (400–700 nm) in SiN through their MPW offerings, with very low loss ($<0.1 \text{ dB cm}^{-1}$) [443]. The lifetime of visible flattening devices would need to be assessed given the presence of UV photons generated by the combs.
<figure><img src="image_33.png" /></figure>

Finally, the detection scheme used to drive the flattening algorithm could also be integrated onto the chip. Currently, an external, dedicated spectrograph is used to monitor and adjust the flatness of the spectrum. An alternative is to use the light rejected by the MZIs as probes to drive the flattener. This can be done by sensing the signal in the ports with a linear array detector or integrating individual photodiodes onto the chip (see section 24 for details). Routing the probe channels to the edge of the chip would incur a large number of waveguide crossings (see figure 26), which could compromise the performance of the device and/or use of the dropped ports as probes due to losses and crosstalk. Integrating photodiodes in place circumvents this issue. Alternatively, the beams could be ejected vertically upward out of the chip via vertical couplers and/or grating couplers onto a two-dimensional array above or a secondary chip with photodiodes flip-chipped bonded to the top. This option is also challenging as vertical and grating couplers are difficult to manufacture without imperfections and grating couplers are large. The concept may also require the use of 2 × 3 MM interference coupler at the output of the MZIs, so two rejected ports can be tracked to disambiguate amplitude changes in a given channel with other sources of drift. It would also require careful calibration of the non-common chromatic losses between the output of the MZIs and the final spectrograph.

### Advances in science and technology to meet challenges

To advance the technology, all-photonic flattening devices should be designed, fabricated and characterized in various wavebands ranging from 350 to 2400 nm. Although SiN is transparent and applicable to nearly this entire region, other material platforms like SOS or ion-exchanged waveguides [444] could be considered. While the lower index contrast of these platforms would result in larger devices, they offer better mode matching to fibers and in the case of ion-exchanged waveguides offer better transparency beyond 2.2 µm, where SiN starts to become opaque.

Simulations and detailed design work will help to reveal the limitations on properties such as bandwidth, channel count, and losses across large bands, and channel-to-channel crosstalk as well as the potential of different material platforms. In addition, they will provide insight into how large and complex a device can be
realized within the limitations of the reticle and/or wafer size. Fabrication and testing will reveal limitations imposed by the manufacturing process including the impact of stitching errors at reticle boundaries.

With the maximum bandwidth of a single device established, several devices could be combined in parallel to span the full wavelength range of the comb. These solutions would require broadband beamsplitters, ideally based on photonics or highly miniaturized technologies. Such devices should be developed and tested in conjunction with photonic flatteners to demonstrate large spectral coverage.

Closed-loop control of these devices is a critical step to evaluating the technology and could soon be demonstrated in the field with the PARVI instrument [120], by utilizing a separate monitoring spectrometer. In a similar vein, the ability of the devices to create flat and reproducible spectra over the long term needs to be explored. Questions like do the high power levels impact the coupling losses from fiber to chip or lead to device degradation need to be understood. Ultimately, to make the flattener independent of system-specific architectures and portable, approaches that explore integrating the sensing with the chip should be tested as outlined in the section above.

Finally, other approaches or technologies should also be considered for all-photonic flattening as they may offer advantages in terms of channel count scaling. Section 7 of the roadmap presents several innovative approaches to dispersing the light that might be suitable for this application.

## Concluding remarks

All-photonic spectral shapers provide an avenue to extreme miniaturization and simplification of flattening devices for intensity modulation across the output spectrum of an LFC. These could be useful at current ground based PRV facilities but will really be beneficial for future astrocomb applications in space (not necessarily for PRV science). To become useful across a broad range of applications, these devices will need to undergo further developments to understand bandwidth limitations, demonstrate operability in other wavebands, understand channel number and channel bandwidth scaling limitations, as well as closed loop demonstrations and studies into the optimum architectures for closed loop control as well as field testing.

## Acknowledgments

The authors acknowledge the Keck Institute for Space Studies for funding and BRIGHT Photonics for their design and simulation efforts of the prototype devices presented herein. The research was carried out in part at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). We would like to acknowledge technical contributions by Pradip Gatkine, Boquang Shen, Maodong Gao, Nick Cvetojevic, Jeffery Jewell and Gautam Vasisht during the development of the first generation all-photonic flattener described above.
## 15. Fabry-Perot Etalons as precision wavelength calibrators

Samuel Halverson<fnref n="1" />, Liang Tang<fnref n="2" />,<fnref n="3" /> and Christian Schwab<fnref n="4" />

### Status

Fabry-Perot Etalons (FPEs) have a rich history in the field of optical physics [445]. At its core, an etalon can be as simple as a pair of planar mirrors, which form an interferometric cavity that acts as a spectral filter. The cavity transmits a discrete set of modes, with wavelengths and line widths dictated by basic properties of the cavity such as mirror spacing, mirror reflectivity, and refractive index of the cavity medium. It is these transmitted modes that can serve as calibration features for a variety of spectroscopic applications, ranging from laser stabilization [446] to stellar spectroscopy [447–449]. In astronomy, FPEs have most recently been used as highly precise wavelength calibration references for high resolution, broadband Echelle spectrometers, as they are capable of producing sharp spectral features across wide wavelength ranges.

One of the most demanding spectroscopic applications in astronomy is exoplanet detection via high precision RV measurements, which inherently relies on exquisite calibration of high resolution spectrometers to measure the minute Doppler reflex motion in stellar spectra due to orbiting planets. The field of EPRV measurements is aiming for cm s⁻<fnref n="1" /> level spectroscopic measurements, equivalent to a ~10⁻¹⁰ fractional wavelength measurement across wide wavelength ranges (hundreds of nanometers) [416]. This precision is orders of magnitudes smaller than the line widths in the stellar spectra; this goal is driven by the velocity signature of an Earth-like planet orbiting a Sun-like star, which imparts a ~9 cm s⁻<fnref n="1" /> (3 × 10⁻¹⁰) velocity signal. Achieving such precisions necessitates the use of novel calibration sources that provide a rich density of spectral features and are intrinsically very stable over many years.

These two requirements have driven the development of broadband ‘astro-comb’ LFC systems [28–30] (see section 13 in the roadmap), which combine unparalleled frequency accuracy with a high density of stable emission features. The primary drawbacks to these astro-comb systems are their intrinsic complexity (leading to limited reliability), limited wavelength coverage in particular in the blue part of the spectrum, and overall cost. These systems require significant upfront investment (> $1 M), and incur long-term costs due to maintenance and consumables [425].

Etalons, being intrinsically optically simple, are attractive for astronomical spectroscopy as they can produce a dense, comb-like spectrum similar to an LFC at a fraction of the cost and complexity. Additionally, the design flexibility of FPEs is high, with the FSR, i.e. spectral distance between adjacent peaks, finesse (ratio between the FSR and the full width at half maximum of the spectral peaks), and operating bandpass easily customizable for different spectrometers. These benefits have made etalons attractive calibration sources for the highest precision measurements, and most EPRV facilities now use etalons as part of their calibration strategy. Existing etalon systems include air-gap planar-planar cavities fed by SM or multi-mode optical fibers [450–455], entirely-fiber-based interferometer systems [11, 456], as well as microresonator-based systems [457]. Each system has distinct benefits and drawbacks, though all require technological improvements to reach the stability levels required for future EPRV instruments (see figure 27).

### Current and future challenges

Etalon calibrators are typically intended as a simultaneous reference observed alongside with the stellar spectrum to track the instrumental drift during the exposure. To anchor the line positions of the etalon itself, an absolute reference like a thorium-argon (ThAr) hollow cathode lamp or an LFC can be used to calibrate the FPE periodically. Another solution is to concurrently monitor the wavelength of one or several peaks of the etalon to a very high precision by comparing it to an atomic frequency standard, e.g. a well-known optical transition; this technique tracks the drift of the etalon in real time [456]. In all cases, the etalon requires the relative peak position to be stable to a very high degree. As fundamentally passive optical elements (no innate frequency locking to an atomic standard), these ‘astro-etalons’ require excellent thermomechanical control to stabilize the output spectrum at the levels required for EPRV applications (~10 × 10⁻¹⁰). While real-time monitoring of a single peak, as described above, can relax this requirement somewhat, this is unlikely to track any chromatic effects. This represents a technical challenge, especially for cavities designed around bulk materials such as SiO₂ (in the case of fiber-based etalons) or crystalline materials such as CaF₂ or MgF₂ (used in whispering-gallery-mode resonators) which have high coefficients

<footnote n="1">¹ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America</footnote>

<footnote n="2">² National Astronomical Observatories, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing, People's Republic of China</footnote>

<footnote n="3">³ Key Laboratory of Astronomical Optics & Technology, Nanjing Institute of Astronomical Optics & Technology, Chinese Academy of Sciences, Nanjing, People's Republic of China</footnote>

<footnote n="4">⁴ School of Mathematical and Physical Sciences, Macquarie University, Sydney, NSW, Australia</footnote>
<table><caption>**Figure 27.** Examples of Fabry-Perot-Etalon (FPEs) used in astronomical spectroscopy. FPEs are valuable tools for precise wavelength calibration of high resolution spectrometers. In all cases, the interferometric cavities act as sharp filters of a continuum spectrum, providing a dense series of lines for high precision calibration. Reprinted with permission from [458] © The Optical Society. Reprinted with permission from [459] © The Optical Society.</caption><thead><tr><th>Etalon type</th><th>Cavity Material</th><th>Advancements needed</th></tr></thead><tbody><tr><td>Fiber-fed air-gap Fabry-Perot</td><td>Air/Vacuum</td><td>Improved mirror dispersion control, stability, independent absolute referencing, decreased cost and package size.</td></tr><tr><td>Fiber-based Fabry-Perot</td><td>Fused silica fiber</td><td>Polarization stabilization, cavities using endlessly-single-mode fiber architectures, stabilized chromatic dispersion.</td></tr><tr><td>Whispering gallery mode resonator</td><td>MgF2, CaF2</td><td>Fully-single-mode operation, stabilized injection, lower CTE and dn/dT waveguide materials, improved thermal control.</td></tr></tbody></table>

<figure><img src="image_34.png" /></figure>

<figure><img src="image_35.png" /></figure>

<figure><img src="image_36.png" /></figure>

<figure><img src="image_37.png" /></figure>

<figure><img src="image_38.png" /></figure>

<figure><img src="image_39.png" /></figure>

of thermal expansion (CTE) and refractive index thermal coefficients (dn/dT). Even 'air-gap' etalon designs, which typically have spacers manufactured out of low-expansion materials such as Zerodur or ULE [460–462], still require thermal isolation and precise temperature control to maintain spectral stability (though not at the level of more standard glass materials). At the required level of control, even changes in the amount of injected light must be considered to avoid absorption-driven thermal variations.

Free-space or air-gap FPEs have been used extensively in RV applications [450–455] over the past ten years. These systems are optically simple, and revolve around a planar-planar cavity design fed by SM optical fibers. However, these systems require careful engineering to properly stabilize the cavity, including precise vacuum and temperature control, and to ensure stable illumination [463]. The structure of the etalon and the optical alignment require specialized techniques to maintain stability (optical contacting, athermal mount design, etc). Recently, it was shown that, in addition to an overall drift, bulk etalons show structured systematic drifts that vary as a function of wavelength (see figure 28) [454, 464]. The physical mechanism for this chromatic drift has yet to be identified, though it is likely associated with some variation in the FPE mirror stack properties. Careful investigations into the long term behavior of the optical dielectric coatings under realistic conditions (temperature stability, illumination, vacuum level) is needed to understand and mitigate these effects.

Fiber-based etalons have the benefit of being compact and easy to thermally stabilize, though birefringence issues can easily dominate the spectral stability noise floor. Unlike bulk etalons with a vacuum cavity, SM-fiber-based etalons exhibit birefringence due to inherent properties of the fiber that forms the cavity (see figure 28). Crucially, fiber cavities based on non-polarization-maintaining fiber [11, 456, 458] produce two distinct resonance modes, one for each polarization. The spectral displacement between these two modes is set by the cavity birefringence, and the relative amplitude between transmitted polarization modes is determined by the polarization conditions of the light source [459]. While careful polarization control can mitigate these problems to a high degree, this instability still limits the utility of fiber-based FPEs when aiming for the highest precision, as typical astronomical spectrometers will not be able to easily resolve the separated transmission modes. The superposition of the two modes may manifest as a single peak in the spectrometer, but the effective line centroid will drift should the polarization conditions change.
<figure><img src="image_40.png" /><figcaption>**Figure 28.** Left: Chromatic drift of an air-gapped 'astro-etalon' system, showing systematic structure as a function of wavelength [454]. This is a key area of research for future etalon systems that aim for the most precise ($10 \times 10^{-10}$ or lower) long term performance. Right: Observed birefringence in fiber-based etalon system, highlighting the two widely-separated polarization modes. These modes vary with incident polarization conditions, which are highly sensitive to input fiber geometry and thermomechanical drifts [463].</figcaption></figure>

Whispering Gallery Modes (WGM)-based etalons [457] show great promise as compact spectral filters that produce a wide bandwidth of sharp features (see section 10 of the roadmap). WGM etalons are dielectric cavities with curved surfaces where light is trapped by total internal reflection from the dielectric boundary along which it travels; no optical coatings are required. Constructive interference occurs when the round-trip path length along the curved surface in the cavity is an integral of the wavelength. WGM resonators operate at any wavelength as long as the dielectric is transparent and the resonator surface is well-polished to reduce surface scattering losses.

Unlike ULE glass or Zerodur FP etalons, WGM etalons fabricated from optical crystals like $MgF_2$ and $CaF_2$ have significant thermal expansion that must be compensated to achieve a frequency stability suitable for PRV measurements. This has been attempted by engineering a composite WGM etalon that balances thermal expansion and thermo-refractive behavior [465].

### Advances in science and technology to meet challenges

In the coming decade, the priority must be to develop stabilized systems that support broadband operation (100's of nm), with extreme stability ($<10 \times 10^{-10}$ fractional stability), and which are polarization insensitive. To meet these requirements, from the context of EPRV applications, multiple advancements must be made in a number of areas:

In the case of classical air-gap etalons, a more thorough understanding of interferometer mirror relaxation, dispersion, and coating stability are needed to better contextualize the observed behavior of current FPE systems (see figure 28 [454, 464]). A deeper exploration of manufacturing techniques and athermal materials, with a keen eye on maximizing long-term (months-years) stability is also needed to improve upon the current state-of-the-art, if FPEs are ever to become stand-alone EPRV calibrators. Advancements in these styles of FPEs will hinge on improved dispersion stability in mirror coatings, and advanced methods for optically contacting mirror substrates to etalon spacers. In both the air-gap and fiber-based FPE systems, further investigation of 'hard', low-dispersion mirror coatings is also crucial for improving stability at the cm s⁻¹ level chromatically.

In fiber-based systems, migrating towards using polarization maintaining (PM), endlessly SM-fibers [466] as the cavity material may offer a solution to the current limitations of these systems (polarization modes, narrow bandwidth operation). However, to establish a fully PM system, the alignment of optical systems will have to be strictly controlled, especially during construction of the etalon cavity. Additional technical challenges are present if broadband performance is to be guaranteed. Air-gapped fiber etalons and MM-fiber-based versions are worth exploring in the future. Fiber-based systems have the benefit of very small thermal mass, which makes them quick to respond to thermal tuning. This facilitates driving a servo loop to lock an etalon line on external laser sources.

Advancements in WGM-based designs will require exploration of waveguide materials that support hundreds of nm of bandwidth with pure SM operation, as is required for broadband spectroscopic applications. WGM etalons have yet to be demonstrated in an observatory environment, and will likely require a significant amount of engineering and testing to be implemented in a complete system ready for on-sky operation.

Improvements in white-light sources to take full advantage of etalon bandwidth is foreseeable with technical advances in broadband-light sources. Current supercontinuum sources only go down to ~420 nm and the significantly lower output power towards shorter wavelength poses a challenge for broadband spectral flattening. Alternatively, laser driven light sources could offer broadband coverage down to the UV
band. Yet, high efficiency coupling into a SM device remains somewhat challenging. Spectral energy distribution and flux jitter of the feed source, as well as the illumination stability, which could introduce error when determining line centroids of the transmitted peaks [463], should be deliberated.

For any etalon architecture, a system that is referenced to an atomic standard (or standards) at one or even multiple wavelengths provides potentially much higher precision than a passive system. The number of wavelength ‘anchors’ required to fully characterize the etalon dispersion drift is unknown, though recent studies have shown that the chromatic drift can be highly structured [454, 464]. This indicates that the chromatic drift due to slow changes in the mirror coatings must be calibrated regularly against an absolute, broad-band reference (like a ThAr lamp or LFC), while faster drifts (within a night) can be corrected with the frequency reference. There is also room for a better understanding of the transient behavior, e.g. the approach to equilibrium after illumination of the system.

Concluding remarks

The simplicity and rigidity of the FPEs sets them apart as practical systems for generating evenly spaced spectral peaks for calibration purposes. Calibrators based on passively-stabilized, bulk-optics, vacuum-gap FPEs have already demonstrated <10 cm s⁻¹ stability performance, when used in combination with an absolute reference. To operate independently from any other calibration source, the absolute wavelengths of the FPEs' transmitted peaks have to be anchored. One possible way is to laser-lock the etalon to spectroscopic references. The ultimate achievable broadband accuracy of such devices remains unknown; in particular, the chromatic behavior of the locked etalons has to be studied more extensively.

Photonic Etalons, such as fiber-based FPEs and chip-based microresonators have much smaller footprints compared with their bulk counterparts, and can be thermally tuned easily. The compactness of these photonic etalons enables new calibration applications, such as in space-borne missions, where traditional vacuum-enclosed solutions are not viable due to dimension or weight limitations.

The key technical challenges going forward include material limitations, thermal stability, polarization control, and dispersion behavior.
# 16. Future of photonic beam combining technologies for interferometers

Karine Perraut<fnref n="1" />, Guillermo Martin<fnref n="1" />, Laurent Jocou<fnref n="1" />, Elsa Huby<fnref n="2" /> and Denis Mourard<fnref n="3" />

## Status

Optical long-baseline interferometry (OLBI) combines signals from separate telescopes and provides reconstructed images at the exquisite spatial resolution of the synthetic mirror whose aperture is sized by the separation between the telescopes. At visible and NIR wavelengths (e.g. in V, R, J, H, K bands; [0.5-2.5] µm), OLBI is routinely in operation at the Very Large Telescope Interferometer (VLTI, Chile), at the CHARA Array (California), and at the NPOI and LBTI arrays (Arizona), giving access to angular resolutions smaller than 1 millisecond of arc. Such an exquisite resolution opens the road for studying complex environments and determining accurate angular diameters of thousands of stars over the Hertzsprung-Russell diagram [467] which is invaluable in the era of Gaia and space missions like PLATO or ARIEL.

While this technique has been for a long time hampered by its limited sensitivity (light is not collected by the giant synthetic mirror), GRAVITY at the VLTI [75] has revolutionized NIR interferometry by providing milli-arcsecond resolution imaging up to K ~ 19th magnitude and 20–50 µ arcsec astrometry, in addition to polarimetry and spectroscopy capabilities. GRAVITY has led to groundbreaking results covering the broad range of astrophysics which was targeted by OLBI long ago: the precision tests of Einstein’s theory of general relativity and notably the strong experimental evidence that the compact mass in the Galactic Center is a Schwarzschild-Kerr hole [36, 37]; direct observations of the broad line regions surrounding quasars at sub-parsec scale [468]; high-resolution K-band spectra of several exoplanets [35]; the first resolved microlensed images [469]; and the resolution of several dozens of protoplanetary disks at a spatial scale where planets form [470]. For all these fields, zooming in with OLBI is of the utmost importance to reveal and constrain the physical phenomena at play in these complex and time-variable environments.

The GRAVITY breakthrough comes from a combination of fringe tracking, cooled optics, high-quality metrology, infrared AO, low-noise detectors, the upgrade of the VLTI infrastructure, and SM beam-combination. Focusing on the four-telescope (4 T) beam-combiners at the heart of the instrument, following the pioneering integrated optics (IO) components in the H band installed at IOTA [471] and in the PIONIER instrument at the VLTI [472], the design of a pairwise static ABCD combiner has been optimized for the K band. The main challenges of the GRAVITY developments are the throughput in this specific spectral range where silica is no longer highly transmissive, operation in vacuum at a temperature as low as 200 K, and a back-illumination with a 1.908 µm metrology laser. Huge efforts were undertaken to optimize the technological process: doping process to increase the refractive index difference, annealing at 1000 °C to remove the OH bonds present in the silica, manufacturing the 20 mm x 50 mm combiners in three stitched photolithographic reticles, as well as mastering the stress inside the component to limit the birefringence. All the challenges have been successfully tackled, leading to the most complex IO beam combiner (figure 29) in this very specific spectral range [33].

## Current and future challenges

**Increasing the number of beams.** Snapshot imaging of variable and complex environments makes the recombination of more than four telescopes a key driver for the next photonic developments, with challenges about the length, the throughput, and the complexity of the combiners. As a prerequisite, the fringe coding, i.e. the design of the circuit and the achieved signal-to-noise ratio, should be optimized. As an example, the 6 T ABCD beam-combiner designed for the fringe-tracker of the visible SPICA instrument at the CHARA array optimizes the coding of the 15 fringe patterns. This 82-mm long chip exhibits a throughput higher than 50% for both polarizations over the whole H band [474]. Similar developments are ongoing for interferometers deployed on monolithic telescopes, such as pupil remapper instruments. Currently, a visible 9 T-combiner is being optimized for the FIRST instrument [475], and a complementary infrared multi-beam nuller interferometer is being developed for the GLINT instrument [12] (see section 19 for more details). Increasing the number of input beams to enhance the imaging capabilities of future instruments will come at the expense of complexity and a loss in signal-to-noise ratio. For more and more numerous input beams, within the context of limited wafer sizes, one future challenge would be to connect several chips together, which would make the interface, the optical path balancing, and the integration more complex.

**Improving the beam-combiner transmission.** Improving the OLBI sensitivity requires pushing the sensitivity of the beam-combiners by improving the intrinsic transmission of the materials, the coupling of

<footnote n="1">¹ Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France</footnote>

<footnote n="2">² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, Meudon, France</footnote>

<footnote n="3">³ Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France</footnote>
<figure><img src="image_41.png" /><figcaption>**Figure 29.** Left. Four-telescope beam-combiner of GRAVITY operating in K-band, designed and manufactured by photolithography of doped-silica on-silicon waveguides by CEA-LETI. Reproduced from [33]. **CC BY 4.0.** Right. Design (by VLC Photonics-OCA based on [473]) of the SPICA six-telescope fringe-sensor operating in H band. Reproduced with permission from [474]. Each beam is divided in five equal parts thanks to a smart succession of 60/40 (3), 50/50 (2), and 66/33 (4) couplers. Each of the 15 possible pairs is combined inside an ABCD circuit (1), sampling the fringes with fixed phase shifts of 0, $\pi/2$, $\pi$, and $3\pi/2$. This permits the instantaneous measurements of the phase-delay. Optical paths are equalized to avoid any bias in closure phase measurements.</figcaption></figure>

the light into the waveguides, and the light collection at the output. Using larger refractive index contrast waveguides allows one to better confine the light, implement smaller radii bends and thus realize shorter optical functions, leading to more compact chips. This tackles the potential wafer size challenge. As a by-product, more compact chips could be more easily packaged and cooled down, notably for cryogenic and space applications. On top of that, more compact optical functions open up the possibility to integrate more capabilities on a single-chip (detector and/or metrology laser hybridation [476], phase control, see section 23 for more details) assuming manufacturing technologies are compatible.

**Mastering spectral bandwidth and birefringence.** As several astrophysical diagnoses are based on spectral analysis, operating in different bandwidths (V, R in visible; J, H, K in NIR; L, M, N in MIR (section 9)) is fundamental. As a consequence, efforts have to be made to optimize these technologies for a given band. Routine observations with precision astrometry and polarimetry requires mastering metrology propagation (not necessarily in the spectral range of the operation wavelength of the instrument), and birefringence as well as diattenuation inside the beam-combiner. As long as they are not time-variable at the observation timescale and calibratable through day-time procedures, none of these effects are insurmountable and it might even be possible to compensate for some at a system level (e.g. internal birefringence can be compensated with external $LiNbO_3$ waveplates).

### Advances in science and technology to meet challenges

**Towards more compact combiners.** For greater than eight telescopes, planar lithographic chips require wafer sizes exceeding current technological limits when using low index difference waveguides (typ. 0.001). One option is to use high refractive index contrast waveguides (e.g. $SiN/SiO_2$ technologies with $\Delta n = 0.5$), where high confinement enables sharper bends (radii of tens of microns), and therefore compact devices with multiple optical functions (splitting, directional coupling, wavelength multiplexing) [477]. The issue that arises is that high numerical aperture (NA) waveguides require tapers to maximize the coupling with input/output fibers. These tapers have a highly polarization-dependent transmission, which requires specific efforts like the development of polarization independent mode converters [478] achieving high NA to low NA adiabatic transitions.

**Enabling complex multi-channel and multi-band designs.** Thanks to the ultrafast laser inscription (ULI [479]) technique, the refractive index of a substrate can be locally modified, in particular at different depths into the substrate. Interestingly, this technique can be applied to a large assortment of materials, thus allowing for optimized devices for different wavelength bands. The versatility of ULI allows for 3D waveguide architecture, expanding the variety of possible designs, with a large number of inputs and avoiding in-plane crossings, thus reducing crosstalk and propagation losses. Applications include combining light with waveguide lattices (see section 17 for more details), MM to singlemode conversion devices (like PLs (see section 3)), pupil remappers, converting a 2D input matrix into a 1D array (Dragonfly [480], GLINT [12]) and 3D directional couplers [481]. Challenges in ULI include the stabilization of the laser power during waveguide fabrication, and especially for writing waveguides deep inside the substrate, as power and focusing have to be finely tuned to ensure homogeneity of the refractive index modification.

**On-chip integration of injection, modulation, and detection.** The densification of the optical chip with new functions and many baselines is part of the efforts deployed to achieve fully integrated beam-combiners. Active functions based on EO [482], thermo-optic [483] or piezo-optic [99] materials are being developed to allow for modifications of the waveguide's effective refractive index, and thus phase control, at rates reaching
<figure><img src="image_42.png" /><figcaption>**Figure 30.** Examples of some ULI beam combiners; (a) image of a 3D 3 T beam combiner, with sampling grooves at the output waveguides. Reproduced with permission from [482]. (b) Scheme of a thermo-optic directional coupler. Reproduced from [483]. CC BY 4.0. Both examples (a) and (b) are singlemode in the near infrared. (c) Scheme of a visible 5 T 3D beam combiner for FIRST and white light image of the ten outputs. Reproduced with permission from [487]. And (d) mid-infrared waveguides and surface gratings written in LiNbO₃. Reproduced with permission from [488].</figcaption></figure>

the MHz. This kind of hybrid device not only enables on-chip phase modulation but also fringe or null tracking [482, 484]. To increase the global throughput by optimal direct injection into the waveguide, microlenses can be directly integrated on the input/output facets of the chip thanks to 3D-printing [82]. In addition, novel approaches of detection consisting of growing single pixels directly over the waveguides increase readout time and output coupling efficiency [485]. This can be coupled to on-chip spectrometry using ULI (SWIFTS [486]) to obtain a fully integrated spectro-interferometer.

Different examples of the solutions discussed are shown in figure 30.

## Concluding remarks

In recent years, astronomy has greatly benefited from the coming of age of photonic technologies for beam combination, opening new opportunities for optical long-baseline interferometry. The initiation and strengthening of collaborations between research teams and technology centers now allows for the routine development of components with mature technologies, and for a dynamic synergy to tackle the development and/or optimization of specific circuits (especially for non-standard telecom spectral bands), including new functions. Notably, the advent of high density pupil-remapping projects (full 30 inputs of FIRST at SUBARU, SPIDER project with 518 sub-apertures [489]) requiring input facets with tens of waveguides in a mm-size 2D distribution will be challenging, as refractive index modification must be homogeneous whatever the depth on the substrate. The requirements for the next generation of combiners will necessitate the exploration of new technologies as the substrate size and the yield of planar photonic circuits become limiting. The ULI process is probably one of the most promising alternatives as it allows for 3D-mapping, it optimizes the circuit length, produces 3D tapers and waveguides that are in principle independent of polarization, and it is well suited for waveguide fabrication in active materials and materials having transparency windows where classical waveguide fabrication techniques are not functional.
Acknowledgments

GRAVITY has been developed in a collaboration by the Max Planck Institute for Extraterrestrial Physics, LESIA of the Paris Observatory and IPAG of Université Grenoble Alpes/CNRS, the Max Planck Institute for Astronomy, the University of Cologne, the Centro Multidisciplinar de Astrofísica Lisbon and Porto, and the European Southern Observatory. We acknowledge the funding support of CNRS/INSU, Agence Nationale de la Recherche Contract #ANR-06-BLAN-0421, LabEx OSUG@2020 (Investissements d'avenir ANR10LABX56), LabEx FOCUS ANR-11-LABX-0013, Action Spécifique ASHRA of CNRS/INSU and CNES, and Observatoire des Sciences de l'Univers de Grenoble. SPICA-FT has been developed thanks to fundings from CNRS/INSU, OCA, UCA, Idex Jedi, and Région Sud. This work has been partially supported by the National Research Agency (ANR) through the French' Recherche Technologique de Base' Programme. The FIRST project has received support from Action Spécifique Haute Résolution Angulaire (ASHRA) of CNRS/INSU co-funded by CNES and from the French National Research Agency (ANR-21-CE31-0005).
# 17. Future prospects of discrete beam combination techniques

Abani Shankar Nayak¹, Stefano Minardi² and Stefan Kraus³

¹ Institut für Angewandte Physik, Friedrich-Schiller-Universität Jena, Jena, Germany

² Ams-OSRAM, Jena, Germany

³ Department of Physics and Astronomy, University of Exeter, Exeter, United Kingdom

## Status

OLBI is an important tool of modern astrophysics, enabling high angular resolution imaging of stellar surfaces, tight binaries and proto-planetary/accretion disks. OLBI facilities allow the interferometric combination of light collected by telescopes tens to hundreds of meters apart and are typically complex structures, where misalignment and vibration of the optical components can compromise the visibility of interference fringes. To counter the drawbacks of bulk optics at least in the beam combination instrument, Kern et al [490] proposed the use of IO components, which was successfully validated in the IONIC [491] and IONIC3 [492] instruments, and their successors, the PIONIER [472, 493] and GRAVITY [75] instruments at the VLTI facility that are currently delivering impressive high-angular observations of a multitude of objects [35, 494]. IO technologies (see section 16 for more details) are able to miniaturize complex optical setups on a few centimeter long chip that consists of several photonic functions such as waveguides, Y-splitters, X-couplers, and phase-shifter. Besides the reduction of footprint, IO devices offer much higher thermo-mechanical stability and a reduction of the maintenance costs as their bulk optics counterparts. More relevant to science is the opportunity to integrate on a small chip, an instrument for the simultaneous combination of many telescopes and deliver precise measurements of visibility amplitudes and closure phases (CP), two quantities required for the interferometric synthesis of images. IO devices are usually manufactured on a variety of silicon-based materials by means of photolithographic processes, which constrain the waveguide-network to a plane. Due to the planar topology, multi-telescope beam combiners feature waveguide crossovers, which are sources of crosstalk [495] between the interferometric baselines and reduce the precision of the measured visibilities.

A possible solution to this limitation of planar PICs is to exploit 3D manufacturing such as ultra-fast laser inscription (ULI—ultrashort laser pulses are used to locally modify the optical properties of a substrate material [90, 91, 479]) and avoid waveguide crossovers [496]. Therefore, a 3D-photonics approach to multi-telescope beam combination is the DBC (see figure 31), a "photonic lattice" consisting of a periodic array of parallel SM waveguides (WGs) that interact through the evanescent field of individual waveguide modes. The name DBC derives from the term "discrete optics", indicating optical systems where light is mainly bound to a discrete set of evanescently coupled WGs [497]. Initially conceived by Minardi & Pertsch [498] for OLBI applications, the DBCs have been the focus of R&D studies aiming at measuring visibilities and CPs in multi-channel interferometers. These studies were not limited to numerical modeling [499], but included also the characterization of DBC prototypes operating at astronomical R- [500], J- [501], H- [502], L- [323]- bands and combining up to 3- [500], 4- [323, 503], and 6 [501]- telescopes. Recently, a 4-channel DBC was tested on-sky at the William Herschel Telescope during a pupil remapping experiment [502].

Compared to pairwise IO combiners [472], DBCs have a very simple design and employ only straight, relatively short SM waveguides, thus avoiding bending losses and have low propagation losses. Therefore, the main losses are derived from the coupling of starlight in the device, which could feature a pupil remapper [480] or a coherent reformatter [504] that has inherent bending or transition losses. Retaining high throughput or sensitivity is important for any interferometer, especially for facilities that combine many apertures for high-fidelity imaging, including proposed next-generation facilities such as the 'Planet Formation Imager' (PFI [505, 506]) or the Magdalena Ridge Optical Interferometer (MROI [507]). ULI inherently allows low cost manufacturing of the components in a variety of materials (see sections 8 and 9 for details) with the transparency windows extending from the visible to the MIR, the latter being crucial for the exploration of science cases related to dusty objects such as protoplanetary disks or active galactic nuclei. One important science objective, both for VLTI and CHARA, is to enable interferometry at very high resolving power >20 000 to study molecular composition and kinematics from spectrally resolved lines (see figure 31). Other science cases, where DBCs could be potentially used are the characterization of exoplanet atmospheres [35], the study of the gas kinematics in the accretion/outflow launch region of young stars [508–510], and the measurement of spin-orbit alignment of exoplanet host stars [513]. Beyond OLBI applications, DBCs coupled to a PL may enable wavefront sensing for astronomical AO systems (see section 3 for details).
<figure><img src="image_43.png" /><figcaption>**Figure 31.** The science cases potentially addressable by a DBC-based interferometric combiner are shown in (a), (b) and (c). (a) Artist illustration of the environment around Active Galactic Nuclei (credit: ESO/M. Kornmesser and L. Calcada). (b) A model showing the magnetic field topology and velocity structure in the accretion region around a T Tauri star (credit: Romanova and Owocki). Reproduced with permission from [512]. (c) Radiative transfer simulations of a protoplanetary disk with embedded protoplanets, computed for wavelength of 10 µm (credit: Kraus *et al*). Reproduced with permission from [505]. (d) The IO chip used for on-sky tests at the William Herschel Telescope consists of pupil remappers, DBC (highlighted in red area) and coherent reformatters. Note that several DBC devices are contained in the red area (credit: AIP/A. Dinkelaker). (e) Design of current DBC with identical coupling coefficients across the width of the device. The red and blue cylinders are the input and output waveguides, respectively. (f) Design of a possible non-uniform DBC that has apodized coupling coefficients to enable broadband light measurements and precise estimation of visibilities.</figcaption></figure>

## Current and future challenges

Science cases requiring high-fidelity interferometric imaging need a dense sampling of the coherence function (uv-plane coverage) of the starlight. Usually this is achieved by collecting visibility data across many different baselines by changing the location of the combined telescopes and/or exploiting earth rotation. This approach is possible only assuming the stability over time of the target itself. Fast varying astronomical targets (e.g. novae, variable stars, dynamical inner-disk environments, orbital motion of companions/planets) but also the truthful imaging of extended structures like protoplanetary disks requires the simultaneous combination of larger arrays of telescopes and enable a better coverage of the uv-plane within a snapshot. For instance, for a hypothetical planet at 1 au around a solar-mass star in the Taurus star-forming region, the orbital motion is up to 0.12 mas per day, which is comparable to the beam size in a PFI-like facility with maximal baselines of $B = 1.2$ km at observing wavelengths of $\lambda = 1$ µm ($\lambda/2B = 0.09$ mas). Therefore, such an array needs to achieve sufficient uv-coverage for imaging in a single night. The DBC offers a straightforward approach for scaling the beam combiner to larger arrays of telescopes and the resulting PIC is far simpler and less prone to fabrication errors than pairwise IO combiners that have ABCD structures [472]. However scaling up DBCs to larger telescope arrays will face challenges such as: (1) the factorial scaling of the algorithm currently used to select the configuration of the input sites of the DBC, (2) the photometric loss due to the quadratic scaling of the number of output waveguides (a general problem for all-in-one combiners), and (3) the decreased throughput due to the requirement to have longer devices. To date, a 6-input DBC has already been fabricated and tested in the lab [501], but the design of e.g. an 8-input DBC would require a smarter approach to meet these challenges.

Another key requirement for any OLBI beam combiner is high instrumental fringe contrast, throughput, and broadband operation, which determines the device sensitivity. If we compare the performance metrics of a 4-input DBC (see figure 31) fabricated using ULI technology with its counterpart, the 4-input pairwise combiner [472] of the PIONIER instrument [493], we find that a DBC used with narrow-band light source (CW laser at 1600 nm) has an instrumental visibility (99%) and exhibit a closure phase accuracy (0.5°) comparable to that of the PIONIER combiner. However, astronomical applications need to combine light over a bandwidth comparable to an atmospheric transparency window (10% of carrier frequency). Because
of the strong chromatic dependence of the evanescent mode coupling, the challenge of current generation DBC is to achieve high instrumental visibility with a broadband light source. Laboratory measurements and simulations have shown that the chromatic nature of the DBC's transfer function reduces the accuracy of the retrieved visibilities with increasing bandwidth [500, 503]. A solution to this problem is to disperse the output light with a resolving power >50 and calibrate each individual color channel, as shown by Saviauk et al [500].

A further requirement from astronomy is to improve the precision of the visibility measurements and achieve a better dynamic range of the reconstructed image. This is crucial for the observation of exoplanets and protoplanetary disks, where the contrast between the colder object and the bright central star is huge. The SNR of visibility amplitude and phase would require ratios >1000:1. Besides coupling the DBC to low noise detectors, the control of depolarization effects in the waveguides [511] and the improvement of the transfer matrix calibration procedure are further challenges to be addressed.

### Advances in science and technology to meet challenges

**Broadband light extension:** Achieving high accuracy in the measurement of visibilities across a broad spectrum is still an issue for DBCs. A compromise between chromatic dispersion control, bend losses and coupling strength may be achieved by building arrays of periodically bent and/or apodized waveguides [514], which could increase the broadband operation of the DBC (see figure 31 for a schematic of a nonuniform DBC). Alternatively, DBC can be used in spectro-interferometric settings, e.g. by employing a hybrid integration approach by bonding the DBC chip to an IO-based AWG (see section 6 for details) or an Echelle grating [515].

**Polarization:** Depolarization effects in waveguide interferometers decrease the interferometric contrast and reduce the accuracy of the retrieved visibilities and CPs. While low birefringence has already been achieved in Eagleglass ULI waveguides operating at J- and H-band, this is not the case for ULI on other substrates (e.g. Infrasil [516], GLS [323], IG2 [326]) suitable for other astronomical bands. In principle it may be possible to optimize processes also on these glasses, e.g. by introducing appropriate annealing processes [517].

**Scalability:** An analytical approach has to be developed to solve the issue of factorial scaling of the current algorithm used to select the input sites of the DBC. This may be facilitated by relying on a supermode model rather than on the coupled mode approximation. However, from sensitivity considerations, all-in-one combiners perform worse when the number of combined telescope increases, but a cluster approach to combine subsets (3- or 4-) of the telescope array may be an effective strategy for imaging interferometry methods.

**Calibration:** An accurate calibration procedure of the DBC transfer matrix is essential to obtain good results in the on-sky observations [502]. Thus future implementations of DBC components would require special care in the development of a suitable calibration test-bench with low noise detectors. For instance, an AO system [518] could be included in the optical characterization setup for a more robust determination of the DBC's transfer matrix.

### Concluding remarks

The above works have demonstrated the viability of DBC techniques and in particular, as compared to concurring IO beam combiner design, its potential for the scalability of the beam combination to large arrays of telescopes. However, the development of a full instrument based on DBC requires further technological research to address issues like the chromatic dispersion, the algorithm for the input site choice and, on a second level, the tackling of depolarization effects in the device and the improvement of the transfer matrix calibration procedures and test-benches. Research on these issues is currently ongoing and the future years may bring a DBC component to sky, triggering new instrument development for instance for VLTI, CHARA, NPOI, MROI, or aperture masking interferometry. Such devices could also help enable the next generation of interferometric facilities and promise to solve some of the major challenges facing modern observational astrophysics.

### Acknowledgments

A S N acknowledges the combined support from Deutsche Forschungsgemeinschaft (326946494) and Bundesministerium für Bildung und Forschung (03Z22AN11). S K acknowledges support from an European Research Council (ERC) Consolidator Grant (Grant Agreement ID 101003096). The authors would like to thank Dr Aline Dinkelaker for her valuable comments that improved the clarity of the paper.
## 18. Realizing extremely long-baseline interferometers by exploiting photonic technologies

Narsireddy Anugu<fnref n="1" />, Gautam Vasisht<fnref n="2" /> and Ludovic Grossard<fnref n="3" />

### Status

The next leap in understanding important astrophysical phenomena e.g. protoplanetary disks and planet formation, young and mature exoplanets, environments of supermassive black-holes, requires optical/IR measurements on 10–100 microarcsecond (µas) angular scales; this is equivalent to an array with multiple km baselines in the near-IR (0.8–5 µm) [37, 506]. For these reasons, km-scale fiber linked interferometers have been discussed since the 1980s for both ground- and space-based applications [519]. The OHANA project [520] in the 2000s was the first attempt to fiber-link the large telescopes on Mauna Kea, and succeeded in obtaining fringes between the two Kecks.

On km baselines fiber light transport becomes quite attractive relative to classical beam trains as one largely avoids: (i) higher infrastructural expense ($\propto$ baseline $B$; larger optics, vacuum pipes, pumping systems, etc) and large transport optics (diameter $\propto \sqrt{\lambda B}$) for minimizing diffraction losses (ii) site and environmental constraints due to the need for straight beam passage between telescopes and the laboratory (see figure 32). This constraint can block expansion of existing arrays, for example, both CHARA ($B$ < 330 m) and VLTI ($B$ < 200 m) are limited by site topography, and (iii) operational complexity, as it is harder to maintain the pupil and field from the telescope to the laboratory.

The alternative use of fibers was first discussed by Froehly in 1981 [519], when it became apparent that SMFs could offer significant advantages for both beam transport and combination. In a ground-based fiber interferometer, starlight collected by the telescope passes through a corrective AO system before injection into SMFs [85, 520–524]. Fibers from each telescope transport light to the combiner laboratory, where path lengths are equalized using internal delays. Ideally all delay equalization would be in-fiber, however gross km-scale delays are not easily implemented in traditional fiber because of strong material dispersion and so effective dispersion management needs to be realized [525–530]. Fine delay equalization, however, can be done in fiber by stretching fiber segments [525–528] prior to beam combination [520, 523]. Alternatively, as is done today, gross and fine delay equalization can be in free-space using mirror arrangements and moving delay line carts [520, 523, 526, 527], but this breaks the realization of an all fiber, all photonic interferometer.

### Current and future challenges

The main challenges in km-baseline fiber interferometer can be summarized upfront:

1. Transmission losses in fiber transport across various astronomical bands in the infrared are high. Currently only silica fibers in H-band can compete with mirror transport over km distances.

2. In-fiber delay compensation remains a major current and future challenge due to dispersion

3. Polarization and OPD properties of the transport fibers could be actively monitored via laser or comb metrology. A challenge remains in demonstrating this over long distances.

Although fibers offer clear simplifications over distance, they add challenges due to their intrinsic properties (table 3)—material absorption, chromatic dispersion [525–530], polarization birefringence, and thermal and vibration induced OPD [531, 532]. Fibers are dispersive media, and dispersion due to fiber length mismatch, and fiber bending/stretching causes arbitrary phase delay as a function of the wavelength, which leads to a shift in the zero-order fringes and effectively loss of fringe visibility for broadband operation [525]. To minimize dispersion in the first order, all the static fiber lengths need to be equalized. Upstream air dispersion can be compensated with a double wedge glass system, translating one wedge with respect to the other to adjust thickness as implemented for CHARA [533]. Second-order differential chromatic dispersion can be compensated using a fiber stretcher or glass-based double wedge [525–530]. Unless dispersion can be minimized or controlled, in-fiber delay compensation is the major future challenge.

Due to their opto-elastic properties, the E-field vectors emergent from fibers are modified (unpredictably, chromatically) because of the bending and twisting, strongly reducing the fringe visibility. This can be remedied by splitting polarization states upstream, and using PM fiber transport, with any upstream differential birefringence corrected by adding controlled amounts using LiNbO₃ plates [534, 535]. Fibers also induce total optical path change due to thermal-mechanical stresses. Differential OPD is introduced by

<footnote n="1">¹ The CHARA Array of Georgia State University, Mount Wilson Observatory, Mount Wilson, Altadena, CA 91203, United States of America</footnote>

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

<footnote n="3">³ Limoges, CNRS, XLIM, UMR 7252, F-87000 Limoges, France</footnote>
<figure><img src="image_44.png" /><figcaption>**Figure 32.** (Left) A conceptual future extremely long baseline interferometer (E-LBI) that can be implemented with fiber beam transport on uneven terrain. (Right) A compact proposed fiber stabilization and delay line system.</figcaption></figure>

unequal longitudinal thermal gradients (OPD change of ~1 mm km⁻¹ K⁻¹), or fast differential vibrations [531, 532] which smear fringes on millisecond times or longer. While OPD errors can be reduced by careful passive insulation [523, 531, 532] and controlling the vibration environment, the OPD of fibers (and possibly the polarization state) can be monitored by using laser metrology and servo stabilized [531, 532].

As part of the CHARA Michelson Array/ALOHA project [147], this passive and active OPD stabilization of fibers was achieved with an accuracy better than 4 nm RMS [531, 532] for fiber lengths of 200 m. In this, a single metrology laser source operating at 1064 nm was split in two, injected into the same SMF used for the on-sky light transportation, and the signals from the two fibers were combined in the beam combiner laboratory and used to measure the differential OPDs by detecting fringe motion caused by the temperature fluctuations and vibrations. Next, these instrumental OPDs were corrected using fiber stretchers [531, 532]. In 2022, on-sky fringes [priv comm] at 810 nm were achieved at the CHARA Array between the South1 and South2 telescopes with fiber beam transport and with a first active fiber OPD stabilization system. However, we note that the fringes were obtained using the free space delay lines of CHARA, at a narrow bandwidth of 4 nm centered around 810 nm, and using a single polarization. Using a fiber for each polarization and making fringes separately in each polarization is interesting as it also brings science opportunities, e.g. for studying dust around AGNs. Perhaps in-band OFC metrology could be conceived to monitor the state of the fiber. These on-sky results are very encouraging. However, several challenges have to be overcome in obtaining fringes at broadband wavelengths and without splitting the polarizations.

### Advances in science and technology to meet the challenges

An extremely long baseline (>1 km) interferometer with multiple telescopes (>12) is conceivably the next 'big project' after the completion of the extremely large telescopes. Realizing an all-fiber, all-photonic interferometer can make such an ambitious instrument quite cost-effective. We relist the challenges but herein offer potential solutions for development in the coming several years.

**Transmission:** A critical challenge is the efficiency and therefore sensitivity of the fiber interferometer, key to accessing faint primary targets ($K \ge 10$) which in turn impacts sky coverage. Although the transmission of silica fibers in the J-H bands is good at ~0.3–0.5 dB km⁻¹, and that of ZBLAN fibers at K is tolerable, losses are much higher for other bands and are extreme in the mid-IR (5–13 µm) (table 3 and see section 5). Since mid-IR fibers have widespread uses, there is considerable research [536, 537] towards improving transmission with different materials and variations such as hollow-core fibers. Heterodyne interferometry and frequency mixing techniques offer alternatives by shifting the thermal wavelengths into the radio and NIR wavelengths respectively (see section 22). Currently, these techniques are limited to a narrow spectral bandwidth and lower sensitivity but they remain interesting for the future.

**Wavelength coverage:** Another challenge is maximizing broadband wavelength transportation while keeping the SMF properties and maximum fiber-coupling. For shorter wavelengths, the waveguide can become multimoded. For broadband wavelengths, the input beam size may not match the mode field diameter of the SMF leading to non-optimal fiber beam coupling. A promising alternative is the use of PM photonic crystal fiber, which are SM over the entire transparency range of silica (0.3–2 µm). The linear losses of this type of fiber are currently too high (>10 dB km⁻¹) to consider for the km scale baselines. Still this technology remains attractive in the future if losses and costs are reduced.

**Fiber stabilization:** Despite the availability of a state-of-the-art solution for the fiber stabilization for a two-fiber interferometer using a dedicated metrology laser [531, 532], we need a solution for multiple
<table><caption>**Table 3.** A compilation of Fiber optic materials that may be used in optical/IR interferometry. Hollow core fibers (HCF) and hollow core photonic bandgap fibers (HC-PBF) are not explicitly listed but have been demonstrated in silica and chalcogenide glasses. They are an active area of research [537, 539].</caption><thead><tr><th>Fiber Type</th><th>Transmittance</th><th>Advantages</th><th>Disadvantages</th></tr></thead><tbody><tr><td>Silica</td><td>High transmittance optical through H band<br>0.3 dB km<sup>-1</sup> (1.2–2.0 µm)<br>5 dB km<sup>-1</sup> (0.5–1.5 µm)</td><td>Non-toxic<br>Non-hygroscopic<br>Rugged, stable<br>Photonic crystals and PMF available<br>HC-PBF in rapid development</td><td>Brittle without proper coating</td></tr><tr><td>Fluoride:<br>ZrF4 (ZBLAN)<br>InF4</td><td>High transmittance in K-L bands<br>1 dB km<sup>-1</sup> at 2.5 µm<br>3–4 dB km<sup>-1</sup> at 3.5 µm</td><td>Non-toxic<br>Stable with temperature<br>Low dispersion<br>Solid Core Photonic structures developed</td><td>Brittle without proper coating<br>ZBLAN hygroscopic</td></tr><tr><td>Chalcogenide:<br>As-S<br>Ge-As-Se-Te (GAST)<br>Ge-As-S-Se (GASS)</td><td>Transmittance in 0.7–10 µm<br>1 dB m<sup>-1</sup> at 4.0–6.0 µm<br>(As-S)<br>0.2 dB m<sup>-1</sup> (8.0–11 µm<br>GASS)<br>Theoretical expectation for HC-PBF is 1 dB km<sup>-1</sup></td><td>Non hygroscopic<br>HC-PBF in development<br>SC Photonic structures developed</td><td>High refractive index (n)<br>High dn/dT<br>Toxic<br>Fragile</td></tr><tr><td>Polycrystalline IR fiber<br>(silver halide)</td><td>Transmittance in 3–17 µm<br>0.2 dB m<sup>-1</sup> (9–13 µm)</td><td>Non-hygroscopic<br>Non-toxic</td><td>Long term photodarkening<br>SMF difficult to manufacture</td></tr></tbody></table>

<figure><img src="image_45.png" /><figcaption>**Figure 33.** A schematic diagram of three stage delay lines: fiber stretcher, fiber delay lines with mechanical switches and free space delay. Since the additional delay is in glass, we would have to correct for dispersion differences to match the vacuum path in other telescope arms. This dispersion correction can potentially be realized with a custom dispersion compensating fiber Bragg grating (FBG) for each static fiber length. A combination of (i) longer static optical delays in fibers with fiber switches and small delays using (ii) fiber stretching or (iii) even free space delays of a few meters that address fast fringe tracking correction could be a solution to the current limitation of entirely free space delay lines.</figcaption></figure>

telescopes, e.g. six telescopes of CHARA [85]. The main challenge would be after splitting the coherent metrology laser beam, how to send it to multiple telescopes separated on km scales and recombine the beams by building a multi-beam combiner to detect differential OPDs compared with a reference fiber.

**Fiber delay lines:** Existing interferometers use either free space air (VLTI/CHARA/NPOI) or vacuum-pipe delay lines (MROI) employing movable optomechanical trolleys. The delay lines are complex, and a low cost all-fiber solution would be attractive for a fiber interferometer. Only a few tens of cm of OPD [524–526] have been demonstrated by fiber stretching. However, fiber stretching can only come with side effects such as additional polarization, dispersion, and transmission issues, all of which depend on the stretch-state and wavelength [526–528]. Large fixed delays could be provided based on mechanical switches, e.g. micro-optoelectromechanical, that can allow the addition/removal of static fibers with a library of fixed lengths to the optical path similar to pipes of pan mirrors at CHARA [538] (see figure 33). Photonic
dispersion via an AWG spectrometer (see section 6), would allow for an input beam delivered by the fiber delay lines to be dispersed into discrete bands of interest, with each spectral channel coupled to a unique output fiber. In this way, the corresponding spectral channels from different telescopes could be combined with channel specific beam combiners, rather than broadband combiners, allowing for more optimal devices to be used. In addition, fringe tracking, via phase shifters on the beam combiner or fiber stretchers, could be more effectively used to compensate over the narrower bands of each channel. The approaches and technologies described here would require a comprehensive maturation program of technology development and demonstration before they can benefit long baseline interferometry.

**Concluding remarks**

For extreme long-baseline interferometry, beam transportation with SMF offers a unique advantage. Recent improvements in the transmission, wavelength coverage, and fiber control of the SMFs may already provide partial solutions to some existing challenges. However, active technology development in this area is required to maximize the transmission, wavelength coverage, and usage of fiber delays. These breakthrough technology developments could deliver transformative science results by expanding the baseline size beyond the current <1 km barrier.

**Acknowledgments**

We wish to thank Robert Ligon and François Reynaud for fruitful discussions. We would like to acknowledge suggestions about dispersion compensating FBGs and AWGs made by Nemanja Jovanovic.
# 19. Nulling interferometry with on-chip beam combination

Marc-Antoine Martinod<fnref n="1" />, Romain Laugier<fnref n="1" />, Michael Ireland<fnref n="2" /> and Jeffrey Jewell<fnref n="3" />

## Status

Exoplanetary science is amongst the most active fields in astronomy. While thousands of exoplanets have been discovered, few have been characterized, and even fewer have been detected from the habitable zone to the ice line. These regions are key to understanding planetary formation [540] and studying the exoplanets' features such as the surface [541], the atmosphere [542] or spectral biosignatures [543]. Such studies require careful measurement of the orbital parameters as well as the spectrum of the exoplanets, particularly in the infrared, ranging from 1.5 µm for close-in or young giant planets, up to 18.5 µm for characterizing temperate planets from space [339]. However, these observations are challenged by the small angular separation between the planet and its host star and the high contrast needed to discriminate the faint light of the planet from the overwhelming glare of the star. Although relevant planetary contrasts in the literature range from 10³ to 10¹⁰, the post-calibration contrast range for infrared nulling interferometry ranges from 10⁴ for 50 Myr giant planets or typical hot Jupiters [544] to 10⁹ for 4 µm thermal emission from an Earth-like planet around a solar-type star [339].

Nulling interferometry produces interference so that the on-axis light is nulled out (and redirected to a separate path), while the faint, off-axis source's light can be gathered and sent to a sensor or spectrograph [545]. This technique provides all the capabilities to address these scientific questions and tackle the challenges: resolving and revealing the star's surroundings at high contrast, measuring the relative position of companions, and spectrally analyzing their light. To maximize the impact of the nulling, we need stable suppression and a diversity of spatial information to improve observing efficiency. Doing so in the MIR enables compelling observations with less difficulty to meet the contrast requirement than at shorter wavelengths.

Nulling can equally be performed by combining the light from several telescopes as in long-baseline interferometry or by segmenting a single dish like aperture masking and using a photonic integrated-circuit (PIC). In all cases, the technical challenges for PIC developments remain similar. To date, the only attempt to observe on sky with an on-chip nulling beam combiner was by the GLINT instrument (figure 34), using integrated-optics to combine four sub-apertures from a monolithic telescope simultaneously with modal filtering [12], and spectroscopy [546]. The upcoming ASGARD/NOTT instrument (formerly Hi-5) operating in the L-band (3.5–4.0 µm) will use a PIC to implement the latest developments of nulling with the long baselines and large apertures of the VLTI [324]. Although they are currently lossy, photonic technologies offer several assets for nulling including: modal filtering, flexibility in the circuit design, stability, compactness and low mass. The last two are also attractive for the prospect of space missions [547]. These assets make the PIC an ideal platform to combine at least five apertures simultaneously and provide a minimum contrast of 10⁵ (to get imaging capability). To achieve such a threshold, PICs will deliver most of the suppression but innovative data processing techniques can help enhance the contrast by an additional order of magnitude, such as numerical self-calibration [548] or ML techniques [89]. In the next sections, we will explore the challenges of using photonics for nulling and how we can address them over the next decade.

## Current and future challenges

The most obvious challenge is the injection of the light into a SM waveguide. A high quality wavefront and stable PSF, either from a space telescope or from an AO system, is required to reach the best coupling (section 2). Any light failing to couple in the waveguide propagates through the bulk of the chip and can re-couple into the waveguides or contribute to a variable incoherent background. Using a sidestep on the input waveguides to get out of the stray light cone solves this issue [480] (figures 34(c) and 35(b)). Other solutions to investigate could be adding baffles inside the chip or modifying the surface of the chip to prevent total internal reflections.

Temporal fluctuations of the light that gets coupled into the waveguide leads to imbalance in the recombination, and additional light leakage. For this reason, input waveguides must feature photometric taps to sample the light of each beam [549] for real time monitoring and post-processing. However, they do not allow an active optimization of the injection against variable environmental conditions like vibrations or atmospheric turbulence (next section).

Once in the chip, the light of the on-axis source can be nulled. However, any instrumental deviation from the correct coherencing of the combined inputs leads to a leakage of the starlight in the nulled output that is

<footnote n="1">¹Institute of Astronomy, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium</footnote>

<footnote n="2">²The Australian National University, Canberra, Australia</footnote>

<footnote n="3">³Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America</footnote>
<figure><img src="image_46.png" /><figcaption>**Figure 34.** Schematics of GLINT and its integrated-optics chip. (a) Global schematic of GLINT and its integration within the Subaru Telescope and SCExAO. GLINT optics consist of the image rotator (IMR), a pupil mask, a segmented mirror (MEMS), a shutter and a linear polarizing beamsplitter (POLA). The science beam is then injected by a microlens array (MLA) into the photonic chip while the dichroic beamsplitter delivers light to cameras viewing both the image plane and pupil plane. The processed beams coming from the photonic chip are spectrally dispersed in the GLINT spectrograph then imaged on the science detector (C-Red 2). (b) Conceptual layout of GLINT showing the inputs (a pupil segmented into four sub-apertures), the sidestep, the remapping, the splitters, the directional couplers and the photometric paths, and the outputs. (c) Schematics of the pupil remapper of the chip, coherently transforming the 2D configuration of the inputs (on the left) matching the desired pupil sampling pattern into a 1D configuration (on the right). The waveguide paths have been optimized to match their optical path lengths despite taking different routes. (d) Perspective view of the beam combiner of the chip. Credit for (a), (c), (d). Reproduced from [546]. CC BY 4.0.</figcaption></figure>

<figure><img src="image_47.png" /></figure>

<figure><img src="image_48.png" /><figcaption>**Figure 35.** Diagram of the photonic chip of Nott. (a) Conceptual diagram showing the inputs, the sidesteps, the dual stage coupling (double-Bracewell) featuring tapered directional couplers to get achromatic coupling ratios, the photometric taps and the outputs. (b) Top-view of schematics of the chip.</figcaption></figure>
difficult to disentangle from the planet's signal. Therefore, it is critical to control the amplitude and the phase of the light being combined. An AO system and a fringe tracker are necessary to handle these but they are insufficient. Simple combiners, like the Bracewell configuration, are sensitive to such light leakage. Combiners that are less sensitive to such effects and solutions providing a balanced and broadband coupling of light are needed (next section).

Additionally, the astrometry of the observed planet requires more diversity in the sampling of the UV plane; hence it is a driver for more complex nulling chips to simultaneously perform nulling on multiple baselines. GLINT pioneered it by using beamsplitters to attempt the simultaneous nulling of all six baselines [546]. However, differing optical paths inside the chip make nulling on more than two baselines simultaneously impossible. While such imperfections can be calibrated in an instrument like GRAVITY, they must be minimized for nulling devices to efficiently suppress the on-axis starlight.

However, front-end optics remain critical to correct biases such as alignment or beam layout to prepare for the injection. They are also useful to improve the nuller's capabilities (e.g. image rotator (figure 34(a)) or beam-switcher for a better sampling of the UV-plane).

Fabrication with the ULI technique in borosilicate and gallium lanthanum sulfide glass also face challenges. The low refractive index contrast delivered by this fabrication technique limits compactness (hence scalability and interest for space missions), especially to implement the aforementioned sidestep. Lithography provides higher index contrast but it does not offer the ability to realize circuits in three-dimensions like balanced 3D tricouplers (next section). In addition, nulling the maximum numbers of baselines simultaneously requires an accurate match of the optical paths between one input and the fed baselines. To date, there is no such criterion during the fabrication.

While the fabrication and the choice of the material are mature for chips used at 1.5 µm, there is little experience at longer wavelengths. Some possible materials have been identified (Si, Al2O3, Ge, GLS) but lithography and ULI are not routinely used for nulling chips working at wavelengths >3 µm. Efforts are required in the fabrication process to deliver cost-effective couplers and splitters that meet the requirements.

### Advances in science and technology to meet challenges

All the challenges above can be overcome. The PL could be a promising solution to provide active control for injection (section 3). Such devices have been directly inscribed inside ULI-fabricated chips in the past [107] but their use for active injection has still to be demonstrated. In addition, photonic beam combiners are especially interesting for space-borne instruments but the injection of starlight in space-borne photonic devices is still to be demonstrated (e.g. the PICsat mission [550] that was lost). It would open the door to low-cost instrument concepts for space observatories.

In order to provide the possibility of building up to more complicated solutions utilizing photonics technologies, characterization should provide not just the coupling ratio but also the required wavelength-dependent relative input phase in order to achieve a deep null, and how polarized light propagates through the chip. Such characteristics are rarely available in the literature, except in the case of simulations.

Active control for on-chip tuning (e.g. photoelectric [551], thermo-optical [552], piezo-electric systems [99]) can relieve the requirement on the optical path match between the waveguides upstream of the couplers by allowing for tuning of the input phase. Careful configuration of the baselines and sophisticated combiners could help to reduce the leakage due to the spatial coherence of the source or the atmospheric turbulence. The main illustrations of these configurations and combiners include the double-Bracewell [553] and the kernel-nulling [554, section 20] variants. Another part of the difficulty in perfecting optical path control lies in the non-common path between the fringe tracker and the nuller. Three-dimensional tricouplers offer a combination of two bright outputs instead of one, allowing a measurement of the residual phase of the combination. Furthermore, symmetry in this 3D design is favorable to achieving a broadband null [481].

However, the integration of the tricoupler leads to the challenge of the dynamic range of the spectrograph's detector. Different sensors, providing short integration time and low latency for the bright channels and longer integration time and low dark current for the dark outputs could be used; such setups have yet to be tested, and are also required for the use of photonics lanterns for assisting with the active injection. A broadband nuller also requires an achromatic phase shift that simple air-delay lines cannot provide. A promising concept of an achromatic phase shifter of 180° made in ULI and placed upstream of a tricoupler has been explored [555]. Its association with a tricoupler enables the scalability of broadband nullers and is compatible with complex circuits. Last but not least, perfecting such phase shifters is crucial to enable nulling with new photonics platforms.

The push toward longer wavelengths demands operation at cryogenic temperatures. Temperatures below 100 K are required to make the background emission of the chip negligible compared to the telescope's thermal emission at 3.5 µm [554]. This constraint is a challenge for devices using glues or other bonding
techniques between multiple chips, V-grooves, or microlenses (sections 23 and 24) and may impact the refractive index contrast. The nulling instrument NOTT [324] will pioneer the development of photonics that can withstand these operating conditions. This nuller features a sidestep, an achromatic double-Bracewell combiner and photometric outputs embedded in a single chip with ULI technique (figure 35).

## Concluding remarks

The prospect of nulling puts additional constraints on the requirement of interferometric beam combination. The need for precise matching of input phase and amplitude throughout the operation imposes a thorough characterization of the solutions in both phase and amplitude. While their host stars are bright, the planets are faint, leading to a need for throughput that encourages finding solutions to injection, achromatic coupling strategies and efficient on-axis light suppression.

Observations in the infrared region above 3.5 µm, require further development of circuitry with the mentioned requirements in a cryogenic environment for more ambitious ground and space projects, like the detection of different molecular signatures in the atmospheres of giant young planets. While the simple Bracewell combination is not expected to be sufficient for more ambitious goals, the challenges mentioned here pave the way towards efficient and sensitive nulling interferometers.

The single telescope nuller architectures described above are applicable to the HWO, which would address the main science focus. In addition, synergies between a long baseline space nulling interferometer and the HWO should also be considered and explored.

## Acknowledgments

This work has received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement CoG—866070) and from the European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 101004719.
## 20. Kernel nulling self-calibration

Nick Cvetojevic<fnref n="1" />, Frantz Martinache<fnref n="1" /> and Sylvestre Lacour<fnref n="2" />

### Status

The use of interferometric nulling for the direct characterization of extrasolar planets is an exciting prospect, but one that faces many practical challenges when deployed on telescopes. Despite the success of nulling interferometry [556], most photonic implementations thus far have been of the pairwise Bracewell type, and face considerable difficulties when implemented on-sky. With the key requirement of nulling interferometry being the stable destructive interference of two or more incoming interferometer arms, an exceptionally high level of up-stream wavefront control and pathlength matching is necessary to achieve this. This is needed to stabilize the injection into the SM waveguides, and provide high-speed correction of the differential piston between the arms which directly impacts the photometric leakage term in the null channel. With the key scientific driver of this kind of interferometry being high-contrast detections close to the stellar host, a large and time-varying photometric leakage is undesirable.

Various numerical self-calibration methods [12, 546, 548, 557] have been demonstrated to retrieve the astrophysical null in the presence of wavefront residuals, by exploiting the statistical distribution properties of the signal and various noise sources. However, these approaches have some limitations, with the model-fitting being fairly complex and scaling dramatically with more inputs, and the need for large datasets to obtain enough measurements to build accurate statistical models. Non-nulling interferometry typically makes extensive use of the production of self-calibrated observables, like CP [558, 559], and their generalized form, kernel phases [560], to sidestep the limitations brought about by wavefront residuals. Bringing together the benefits of self-calibration of interferometric observables and the photon-noise suppression of nulling is a powerful and exciting tool as it opens a previously unexploited parameter space.

The concept of creating a directly self-calibrated nulling observable capable of removing the effect of residual wavefront errors has been proposed in the past, by using a double-Bracewell architecture [553, 561, 562], and by exploiting the measurement of fringes in the leaked light of the nulled channel [563]. However, a more versatile solution exists where specially designed beam combiners can produce nulled outputs whose linear combinations produce self-calibrated observables [564]. These observables, called kernel-nulls, are robust to upstream differential pistons or phase errors. In general, kernel-nulls can be created by subtracting the measured intensities of two nulled outputs, provided the electric field in the two outputs are complex conjugates, with one possible 3-input configuration shown in figure 36. In practice, this is easily achieved in photonic interferometers by constructing a beam-combination architecture where input beams are recombined by mirroring phase offsets. It is also possible to create kennel nulling beam-combinations using bulk-optics [565]. Most of the techniques would use a number of cascaded beam-splitter cubes and delay lines to create the $\pm 2\pi/3$ phase delays in the different interferometer arms. However, because every output is a simultaneous combination of all the inputs, the number of optical elements scales, and the ultimate stability of the entire interferometer becomes more difficult to control. It is this ability to create complex beam combinations out of small integrated components that photonic technologies provide a distinct advantage.

Recent laboratory work has demonstrated the successful creation of kernel-nulls in 3-input [477] and 4-input [58] photonic beam combiners. These initial combiners avoided the use of classic pairwise combination using directional (evanescent field) couplers, and instead integrated the beam splitting and recombination into a single photonic component. This component is the MMI, a planar MM waveguide section (often rectangular) where multiple incoming waveguides are allowed to interfere spatially to form the required intensity profile at the MMI end face. By varying specific parameters (such as the length and width of the MM waveguide section), the exact beam-combination required to achieve kernel-nulling can be directly designed into the MMI.

### Current and future challenges

In addition to the challenges shared with most astrophotonic interferometers, the unique beam-combination techniques required for kernel nulling raise additional technological challenges. The key requirement faced by any implementation of kernel nulling is the stable, accurate, and broadband pathlength control of the incoming beams prior to a beam combiner. While traditional pairwise Bracewell nulling also faces this challenge, kernel nulling often necessitates second, or third stage beam combiners, all of which must be accurately path-matched to an accuracy of a few degrees in phase. While bulk-optic discrete designs are being explored for kernel nulling, particularly in the context of space interferometers [339], the required stability

<footnote n="1">¹ Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France</footnote>

<footnote n="2">² LESIA, l'Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, Meudon, France</footnote>
<figure><img src="image_49.png" /><figcaption>**Figure 36.** The electric field is represented by colored phasors on a complex polar plot, with the length of the vector being the amplitude and the angle showing the phase. For three in phase inputs, the Kernel Nuller creates three outputs. One bright channel where the input E-fields are in phase creating constructive interference, and two nulled channels where the input beams are given a ±2π/3 phase offset before being combined. In these channels the E-fields interfere destructively creating a null. Figure is adapted from figure 1. Reproduced from [477]. CC BY 4.0.</figcaption></figure>

and accuracy are challenging to achieve. Integrated photonic solutions are far more accommodating in this respect, but add complexity beyond what is seen in standard nullers. Because kernel nullers cannot function in a pairwise fashion (nulling one interferometric baseline at a time), and must interfere all incoming beams together in the null channels, any design using two-by-two beam combiners must be cascaded into multiple subsequent recombinations.

An alternative method to cascading 2-beam couplers is to instead collapse all the combinations into a single MMI. While utilizing MMIs as the primary beam combination component has distinct advantages (such as simplicity, robustness to manufacturing errors, and scalability), it does come with its own set of drawbacks. The primary challenge arises from the MM nature of the interference inside the MMI, which is inherently wavelength-dependent. While more tolerant than its directional-coupler counterparts, this unwanted chromatic behavior can further be exacerbated by manufacturing errors, particularly in high refractive index contrast materials. While traditional directional couplers are also chromatic, the methods of creating broadband MMIs are different and need to be developed further in an astronomical context. Additionally, kernel-nulling imposes further constraints on the level of phase offset accuracy, and photometric balance in the splitting, as the self-calibration degrades the further they deviate from the ideal. Unlike traditional architectures, where each splitter and coupler are individual components on a chip and can be developed independently, utilizing an MMI for both tasks requires a more thorough design procedure to ensure it maintains performance across a wide wavelength band.

While a 4-input kernel nuller has been demonstrated in the laboratory using a single 4 × 4 MMI, this method does not produce all the possible kernel null observables. To make a complete kernel nuller for four input beams requires a second stage of beam-recombination after the initial nulling stage, shown in figure 37. The practical impact of this is that any processes used upstream to phase up the interferometer arms (delay lines, DMs) can no longer be used without dephasing the first stage. Thus, either the pathlength-matching of the waveguides in the second stage must be made to very high tolerance, or the use of on-chip phase control is paramount. Thermo-optic phase actuators (TOPAs) integrated into the photonic chip are capable of correcting pathlength errors due to manufacturing or environmental drift, in a reproducible and stable manner. However, because the phase delay is created by a change in the effective refractive index of the waveguide, it is wavelength dependent. While the chromaticity is negligible for small TOPA strokes (less than ten degrees of phase), correcting larger path lengths of a few $\pi$ becomes difficult to do achromatically. As these devices will be required to work over large wavelength bands the underlying waveguide structure inside the TOPAs need to also be made as achromatic as possible. This is especially true if correcting for OPD errors that arise before injection into the photonic chip, when attempting to stabilize an atmospherically perturbed wavefront for example.

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

To do efficient astrophysical nulling one should sample as many baselines as the telescope arrangement allows, and create the complete set of kernel nulls for that number of inputs. With state of the art optical long baseline interferometers having four (VLTI) or six (CHARA) telescopes, one can create efficient Kernel
<figure><img src="image_50.png" /><figcaption>**Figure 37.** A schematic of a 4-input Kernel nuller device (top) achieved by the use of traditional waveguides, 2 × 2 directional couplers, and 1 × 2 splitters (green). TOPAs (colored) are used to control the internal path length matching of the interferometer arms inside the chip and correct for any error coming from either outside the chip (pink), inside the first stage nulling (green), and the beam-recombination that creates the kernel nulls. Microscope image (bottom) of the fabricated device, on a silicon nitride platform. Here the TOPAs are colored to their corresponding function.</figcaption></figure>

nulling devices with a small number of inputs. This is similar for long baseline concepts in space, which would likely have four or five inputs. However, in the case of the ELTs a pupil segmentation regime would likely require many more inputs (30 or 40) to make full use of the aperture area. While the exact trade-offs regarding the ideal number of apertures to get the best sensitivity are being studied, it is likely that future on-sky devices will require efficient broadband operation over at least one wavelength band, and potentially handle many more inputs simultaneously.

To date, only 3-input [477] and 4-input [58] kernel nullers have been realized. However, it is possible to create a kernel nuller with an arbitrary number of inputs [566], which can probe a much greater UV coverage on-sky. The drawback is the waveguide routing becomes very complex so new design work needs to be undertaken to realize this in a low-loss manner. Ultra low-loss waveguide crossings, routing, and pathlength matching are critical to achieving this.

To make broadband MMIs, a more complex design is required than those typically used as standard for telecommunication applications. The internal wavelength-dependency can be compensated for by creating a non-uniform effective refractive index inside the MM waveguide [567], either by changing the width of the MMI along the propagation direction, or by inserting more complex structures inside the cavity. The use of sub-wavelength grating structures inside the MMI has been shown to greatly increase the operating bandpass [568], and recent development in inverse design using deep neural networks makes the design of these complex structures far easier to derive [569]. To enable this, either high accuracy fabrication methods capable of achieving the required feature sizes need to be available for commercial foundries, or post processing using E-beam/Ion-beam lithography. Furthermore, robust modeling and design tools need to be developed to reliably and rapidly design prototypes for astronomical use. Similarly, the process of making broadband TOPAs requires modification of the waveguide core and thus has similar requirements. However, a more powerful technique to create true integrated dispersion compensation is the integration of low resolution wavelength dispersion, such as by AWGs (see section 7 for more details), in combination with traditional TOPAs. In this concept an AWG would disperse each input into a number of wavelength channels (few 10's of nanometers wide), with each one having a TOPA phase actuator to correct the phase individually, or a Mach-Zehnder configuration for amplitude control, before being recombined into a single waveguide by another AWG. This would enable direct control of the amplitude and phase at various wavelength channels over a broad wavelength band (see section 14 for details). To enable this, ultra low-loss waveguide crossings must be developed because of the extra routing required after the AWGs, along with active control systems for a much larger number of TOPAs.
Concluding remarks

The self-calibration of nulling observables enabled by kernel nulling can greatly reduce the impact of upstream wavefront residuals on the performance of nulling interferometers. This relaxes the requirements for upstream optics such as delay-lines, fringe-trackers, and DMs, and enables the use of nulling interferometry in non-ideal conditions. While the fundamentals have been demonstrated at NIR wavelengths, to truly exploit this technology for future astronomical instrumentation the photonic components that underpin a kernel-nuller should be developed to work over a wide wavelength band and at different wavelength regimes. Furthermore, the use of active-photonics in the form of on-chip phase control, enables the practical use of more complex designs that have thus far been beyond the reach of astrophotonic interferometry.

Acknowledgments

The authors acknowledge the funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant Agreement CoG—683029).
## 21. Nulling interferometry with optical fibers and photonic lanterns

Yinzi Xin¹, Sergio Leon-Saval² and Eugene Serabyn³

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

² Sydney Astrophotonics Instrumentation Laboratory (SAIL), School of Physics, The University of Sydney, Sydney, Australia

³ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, United States of America

### Status

The combination of diffraction due to a telescope's finite pupil diameter and scattering due to imperfect wavefronts leads to bright halos of light surrounding typical stellar images. This severely limits a telescope's ability to detect faint sources near bright stars, such as that from exoplanets and circumstellar dust. As a result, optical techniques that enable high contrast observations at the smallest possible angular separations are of great interest, with compelling applications on the ground as well as on future space telescopes such as the HWO [40]. Interfering the light arriving at different parts of a telescope aperture (sub-apertures) can potentially yield improved resolution, and appropriately phasing the sub-apertures can interferometrically suppress (null) starlight. This technique of nulling interferometry [570] across a telescope's aperture can enable observations closer to a star than classical full-aperture coronagraphic imaging, providing access to separations where exoplanets are expected to be brighter and more numerous. Nulling interferometry is increasingly turning to photonic devices to reach deep stellar rejection. In this work, we focus on approaches that inject an interferometrically-combined beam into a photonic device (e.g. a SMF or PL [7]), exploiting the interplay of the beam's phase with the spatial mode structure of the device, as well as leveraging the spatial filtering the device provides. These systems rely on appropriately phasing the different parts of the telescope pupil prior to beam combination, which results in a class of nulling interferometry instruments architecturally distinct from those employing pairwise beam-combination.

The first photonic nuller was the Palomar Fiber Nuller (PFN) [557], which combined the light from a pair of sub-apertures on the Hale Telescope by focusing them in anti-phase (π radians out of phase) onto the tip of an SMF. Anti-phasing provides an asymmetric stellar point-spread-function (PSF) that cannot propagate within a symmetric spatial mode, thus excluding the on-axis starlight [571, 572]. At the same time, SM operation also serves to greatly reduce the sensitivity of the null to telescope aberrations [573]. While the PFN was based on multi-axial focal-plane beam-combination at an SMF, co-axial pupil-plane beam combination could also be used prior to SMF injection, either with a standard beam splitter, or by focusing a pair of sub-aperture beams from opposite sides of the pupil onto a focal-plane grating to superpose their +1 and -1 orders [574]. However, combining light only from small pupil sub-apertures implies an efficiency loss. This can be obviated (figure 38) by applying a relevant phase pattern to the entire pupil, such as a vortex-phase or simple π radian phase-step (i.e. 'phase-knife') pattern [575-577], which yield respectively, circular and linear fringes. Such a Vortex Fiber Nuller (VFN), has recently been deployed at the Keck Observatory [74, 578]. A Photonic Lantern Nuller (PLN) [579] leverages the same principles, but replaces the SMF with a MSPL [19], which maps specific LP modes into individual SM outputs [580], resulting in multiple ports that null on-axis starlight but have different spatial sensitivities to off-axis light sources. The operating principles of the PLN are summarized in figure 39. While such techniques largely eliminate the need for free-space beam-combiner optics, one can go a step further, and carry out both beam combination and phase shifting within a photonic chip (see sections 15, 16 and 18). Additional layers of beam combination may also be useful for reducing residual stellar leakage in the null that result from device imperfections. Here we discuss the current and future challenges of fiber and PL-based approaches and the technology advances needed to meet the challenge of deep stellar nulling.

### Current and future challenges

Deep nulling requires an exquisite balancing of beam parameters, with any imbalance in amplitude or phase resulting in increased stellar leakage [570, 581]. With commercial SMFs, multi-axial fiber-nullers have reached differing null depths as a function of wavelength and bandpass, with raw laboratory null depths ~10⁻⁶ with a 633 nm laser [572] and ~7 × 10⁻⁵ for broadband NIR (~1.65 µm) light [582]. Meanwhile, laboratory tests of the VFN have achieved null depths of ~6 × 10⁻⁵ with a 635 nm laser [576]. This null depth is consistent with known static aberrations in the optical testbed, but the VFN will also eventually be limited by fiber-related performance limitations in systems with better wavefront quality.

These limitations can be attributed to a number of fiber imperfections that do not usually receive much attention, but will need to be understood for further progress, such as the deviation of the fiber beam profile from the theoretically symmetric SM shape (which can lead to a small coupling to asymmetric input modes), or light leaking into the fiber core from the bright starlight originally entering the fiber cladding (which can
<figure><img src="image_51.png" /></figure>

limit stellar rejection). At longer MIR wavelengths (~10 µm), there are fewer materials available to make fibers, and fiber lengths are limited [583, 584], both potentially limiting the approach to true SM rejection. SMFs are also sensitive to environmental (i.e. thermal) changes, but they are not sensitive when used in the common-mode configuration used for fiber nulling, where the SM fiber is essentially used in the combined beam.

Nulling with PLs will also be limited by practical limitations in PL design and manufacturing. The PLN relies on the symmetry properties of LP modes, as nulls are generated by the orthogonality of the stellar (on-axis) electric field with specific LP modes. Imperfections in mode shape caused by cross-talk amongst modes may break these symmetries, which will cause on-axis light to leak into the nulled channels. The limit of cross-talk currently achievable in MSPLs is around ~1%, limiting achievable contrast ratios to the 10⁻² level. Moreover, the modal structure of a PL varies with wavelength, deviating from perfect LP modes as the wavelength diverges from the design wavelength. Significant theoretical and experimental work is necessary to understand the fundamental limits of PLs, effectively characterize them in the lab, and implement them in a nulling configuration.

### Advances in science and technology to meet challenges

In the past, optical fiber-based nulling beam combiners have used a standard SM step-index fiber. However, the active optical fiber community has developed a myriad of new and improved optical fiber types, e.g. extremely low-loss (low scattering) graded index fibers [585], photonic crystal fibers (a.k.a. microstructured fibers), hollow-core fibers [586], and low-loss mid-IR (1–18 µm) optical fibers [587]. We envision that exploring these new SMF technologies for fiber nulling could provide extended wavelength ranges (e.g. SM hollow core fibers in the infrared), improved null depths by increasing cladding mode suppression and reducing mismatch with the fundamental mode (e.g. cladding mode suppressed photosensitive fibers), and improved fundamental mode input selectivity (e.g. using SM graded index fibers with a larger effective index separation between the fundamental mode and the second mode, which leads to a large acceptance angle separation).

Some limitations of PLs have already been studied and improved in devices for other applications. For example, new telecom MSPL devices provide a broadband operation (more than 600 nm of bandwidth in the near-IR) and highly circularly symmetric mode structure outputs (resembling quasi-perfect LP modes) [155]. However, the need to push input modal cross-talk to its limit is unique to high contrast astronomy, since a cross-talk of 10–20 dB is considered sufficient in telecommunications applications [156]. Significant work via both simulations and fabrication and testing is needed to understand the intrinsic limitations of
<figure><img src="image_52.png" /><figcaption>**Figure 39.** Left: Schematics of a 6-port mode-selective photonic lantern spatial-multiplexer fiber system. Each LP mode at the few mode fiber (FMF) face is mapped to one of the six single-mode ports of the SMF face, such that light with an LP mode shape at the FMF side will result in flux in the corresponding SMF core. The device is bi-directional, so light injected into one of the SMF ports will propagate into the LP mode corresponding to that port at the FMF face. Middle: The field amplitudes of the first six LP modes, corresponding to the ideal modes of six-port MSPL. Right: Coupling efficiency for each port as a function of off-axis separation when used with a charge 1 vortex phase mask. Note that the coupling efficiency is not circularly symmetric for all ports, and that x-axis cross-sections are shown here subfigures reprinted from [579].</figcaption></figure>

broadband mode selectivity and cross-talk in MSPLs, as well as the steps needed to push designs towards those limits. Additionally, robust methods for experimentally measuring cross-talk have yet to be fully established. Current characterization techniques include measuring modal purity through near and far field mode profiles [48], or leveraging refractive index differences to disperse the modes [585]. However, these approaches do not comprehensively measure the PL's structure. Other techniques, such as interferometry, phase diversity, or neural network methods need to be explored in order to fully characterize a PL's complex modal structure.

New geometric structures and waveguide properties may further improve the multi-mode-to-SMF mapping, hence increasing on-axis starlight rejection and exoplanet spatial information. In the meantime, the VFN and the PLN are relatively new instrument designs, and new instrument structures may also open different approaches. For instance, the use of hybrid MSPL [112] behind a vortex could potentially bring the best of both worlds: SMF nulling, wavefront sensing, and coupling optimization on a single device without introducing non-common paths. Techniques to remove residual stellar leakage, such as wavefront control or back-end beam-combination, will need to be explored. Criteria that future PLs must meet for viable broadband PLN usage still need to be established, and the performance of PLN designs with currently manufacturable PLs still need to be experimentally characterized. In parallel, however, many of the known and predicted limitations can be addressed by advances in photonics technology.

## Concluding remarks

The injection of an interferometrically-combined beam into an optical fiber or a PL is a promising approach to nulling interferometry, encompassing instrument configurations such as the PFN, VFN, the PLN, and the recently proposed Phase-knife Nuller. Besides the fiber itself, several of these approaches also rely on specialized pupil-plane phase masks, which can generally be manufactured with existing technologies such as spatially variant liquid crystal polymers or microstructures. Real-time configurable phase masks would be a further step forward. However, the stellar null can be degraded by fiber and PL imperfections, such as modal shape impurity, cross-talk, and cladding leakage, which will need to be mitigated in the future. Recent advances in the design and fabrication of fibers and lanterns may already provide solutions to some of these limitations, but further research will be necessary to understand them and push the limits of these techniques for higher contrast astronomy applications.

## Acknowledgments

This work was supported by the National Science Foundation under Grant No. 2109232, and by the National Science Foundation Graduate Research Fellowship under Grant No. 1122374. Part of this work was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA (80NM0018D0004).
## 22. Heterodyne interferometry and frequency mixing techniques

Jean-Philippe Berger<fnref n="1" />, Azzurra Bigioli<fnref n="2" />, Guillaume Bourdarot<fnref n="3" />, Ludovic Grossard<fnref n="4" />, John Monnier<fnref n="5" />
and François Reynaud<fnref n="4" />

### Status

Several important fields of astronomy such as planet formation or black-hole environments studies request observations at angular resolutions that lie far beyond the performances of current facilities. Optical interferometers offer the potential of reaching resolutions of a few tens of micro-arcseconds. Yet, the technologies at the heart of existing interferometers cannot be simply extrapolated to future arrays with kilometric baselines and tens of telescopes since they are costly, complex and require high maintenance (see e.g. [506]). One of the major simplifications of such infrastructures consists in replacing the free-space optical trains by photonic links. In the visible and near infrared, fiber links can be envisioned (see section 18). Unfortunately, such technologies are much less mature in the mid infrared domain (e.g. from 3 to 20 µm) where several key astrophysical processes such as planet formation, or dust formation and evolution, leave an important radiative imprint (see section 5 for early but interesting developments of specialty fibers and sections 8 and 9 for new photonics technologies). Newly developed heterodyne and frequency mixing techniques have the potential to expand to thermal wavelengths the concept of optical links, i.e. systems that avoid most of the free-space propagation from the telescopes to the central interferometric correlation unit. Moreover, the advent of quantum optics coupled with heterodyne technologies allows potentially revolutionary concepts to be explored.

**Heterodyne** interferometry uses the interference beat between the celestial signal and a local oscillator (LO) synchronized in phase between all telescopes to generate radio-frequency signals whose correlation will allow image of the brightness distribution of the object to be reconstructed (see figure 40). This approach allows the multiple splitting of the signal requested to correlate the electromagnetic field of one telescope with all the others to be compensated by amplification without loss of signal. The Berkeley Infrared Spatial Interferometer was a complete MIR (10 µm) infrastructure that demonstrated, through valuable astronomical observations, the feasibility of astronomical heterodyne interferometry albeit with some sensitivity limitations [588]. It used two CO₂ lasers synchronized in phase as LOs that were propagated to the telescopes and interfered with the celestial signal on ~3 GHz HgCdTe detectors.

**Frequency mixing techniques** use the non-linear sum frequency generation effect to shift the spectrum of the collected light from the thermal infrared to the near infrared or visible as close as possible to the telescope focus where the signal is extracted from the surrounding thermal noise. It thus allows the coherent propagation of the converted MIR light with silica optical fibers or guided optics (figure 41). The nonlinear interaction between the astronomical signal and a highly coherent laser beam occurs in a second order nonlinear waveguide. In 2015, first interference fringes on the sky were obtained using periodically poled lithium niobate (PPLN) nonlinear ridge waveguides with a 1550 nm to 630 nm frequency up-conversion interferometer installed on the CHARA telescope array [589]. On-sky tests were also performed at 3.5 µm on a single interferometric arm. Starlight up to magnitude $L_{mag} = 2.8$ at 3.5 µm was successfully detected after conversion to 820 nm in a PPLN ridge waveguide [590].

**Heterodyne interferometry with entangled photons** has been proposed to mitigate the LO photon shot noise that adds in quadrature at each output. This background acts as a substantial noise floor for wavelengths shorter than ~50 µm. The use of $N = 1$ quantum states or heralded single photons via entanglement (e.g. from spontaneous parametric down-conversion) promises to suppress shot noise and was explored for the case of heterodyne interferometry by Gottesman et al [591]. When such a quantum local oscillator (QLO) is split, sent to two telescopes, then mixed with starlight, the photon counts can be scrutinized for coincidence at the combiners. Two detected photons at the same time in different telescopes means the QLO and a stellar photon mixed and non-local interference can be recovered. A preliminary laboratory proof of principle was presented in [592]. There is still a 50% theoretical loss in efficiency compared to 'direct detection' as sometimes the two photons show up at the same telescope, but the improvement over shot-noise limited operation can be dramatic.

<footnote n="1">¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France</footnote>

<footnote n="2">² Katholieke Universiteit Leuven, Belgium</footnote>

<footnote n="3">³ Max Planck Institute for Extraterrestrial Physics, Garching, Germany</footnote>

<footnote n="4">⁴ Limoges, CNRS, XLIM, UMR 7252, F-87000 Limoges, France</footnote>

<footnote n="5">⁵ Department of Astronomy, University of Michigan, Ann Arbor, MI, United States of America</footnote>
<figure><img src="image_53.png" /></figure>

## Current and future challenges

Both heterodyne and frequency mixing techniques suffer from significant sensitivity limitations with respect to 'classical', direct interferometry. Both have intrinsic narrow spectral bandpasses caused respectively by the detector bandpass and the non-linear up-conversion process with a narrow-band laser.

In heterodyne interferometry the beating of a highly coherent LO with celestial thermal radiation adds a corresponding shot noise penalty that severely limits its practical use in astronomy in the MIR. Improving the sensitivity of the measurement chain requires increasing the overall spectral bandpass. This is achieved by two means: augmenting the detector temporal bandwidth (and thus, spectral bandpass) and using several spectral channels. First, detector bandwidths of the order of 30 GHz would be highly desirable (from current ~3 GHz commercial devices). This characteristic, at 30 THz (~10 µm), corresponds to a 10 nm spectral bandwidth. Secondly, in order to span a significant fraction of the astronomical N [8–13 µm] and Q [15–27 µm] bands, one should aim at developing focal planes combining a high number of such detectors. This would allow adjacent spectral bins to be sampled by dispersing the celestial light onto the detectors and having it interfere with a MIR LFC in which each line is tuned on the desired pixel central wavelength (see figure 40(a)). This would require MIR frequency combs with repetition rates of the order of several tens of GHz capable of producing of the order of 100 lines in order to provide several hundred nanometers of spectral coverage (see section 4 for details). Moreover, LOs at each telescope have to be synchronized in phase to allow the correlation. Therefore, a dedicated synchronization scheme between each oscillator is required. Finally, since classical digital radio correlation techniques with such high bandwidth signals request
<figure><img src="image_54.png" /><figcaption>**Figure 41.** Schematic of a two telescope interferometer with coherent upconversion of the celestial signal through a nonlinear crystal (ALOHA project). Reproduced from [589].</figcaption></figure>

prohibitive digitization and computing power [593], technologies capable of correlating high bandwidth signals have to be demonstrated.

**Frequency conversion**'s limited bandpass has to be tackled as well. For example, the current bandwidth with a single line pump laser at 3.5 µm is only 37 nm for a 2 cm long piece of PPLN. Two possible strategies can be envisioned: (1) injecting several pumps, for example using a comb, in the same crystal and demultiplexing through a multichannel spectral mode [598] or (2) multiplexing different signals on different crystals (with different periodicity to ensure wavelength sampling) which presents the advantage of allowing the conversion of large bandwidth signals with only one monochromatic pump laser. A second important challenge related to sensitivity is to improve the overall efficiency of the conversion chain which requires better mode overlap between the astronomical and pump fields in the nonlinear ridge waveguide, coupling efficiency at the input and output of the waveguide, conversion efficiency and possibly noise-free detection in order to compensate for the losses.

The **heterodyne interferometry with entangled photons** technique offers the unique promise of operating at all wavelengths. Yet, it is difficult to implement as the entangled photons still need to be coherently distributed in real time with near unity transmission, though future advances in quantum communications may make this practical [599]. Similarly, as for heterodyne and frequency up-conversion schemes, there is a similar limitation in bandwidth. Therefore, a dense spectral-multiplexing should be envisioned to cover a broad astronomical bandpass.

## Advances in science and technology to meet challenges

### High bandwidth detectors

N-band heterodyne interferometry requires high dynamic range, fast and sensitive single-pixel detectors. To overcome the limited speed (<3 GHz) of the HgCdTe detector, the most advanced alternative for the 10 µm spectral range is provided by quantum well infrared photodetectors (QWIPs), and quantum cascade detectors (QCDs) in unbiased mode. Thanks to their unipolar electronic transport (no hole mobility is involved), they can achieve flat responses up to more than 70 GHz [594] and saturation intensity up to kW cm⁻² [595] (see figures 40(b)–(d)). A 20 GHz–3 dB QCD for 4.6 µm radiation has entered the market as of 2021. Following the integration of a QWIP into a metamaterial acting as a micro-antenna and a cavity [600], the research on photonic-enhanced QWIPs and QCDs is increasingly promising [601]. An optimized photonic architecture could combine speed with a Quantum Efficiency of at least 40%, allowing the electromagnetic field confinement in the material to be maximized without increasing the doping density and thus the dark current [602]. Other fast technologies have been proposed, namely using graphene [603].
However, their short dynamic range, with a response saturation for power <10 µW, prohibits the regime where the laser shot noise dominates over the detector noise contributions.

### (Synchronized) Mid-infrared LFCs

Although LFCs have already been used for astronomical precision spectroscopy at short wavelengths [417] such technologies in the MIR are still in their infancy and have never been used in an actual instrument. In order to cover a sufficiently large spectral bandpass of the astronomical MIR filters, LFC are needed to act as LOs. They should span several 100 GHz with repetition rates or comb line spacings corresponding to the spectral bandwidth of each detector (e.g. several 10's of GHz). This can be achieved by difference frequency generation in quadratic nonlinear media [604]. Quantum Cascade Lasers are another promising technology since they were demonstrated to operate as frequency combs [605]. Further research could consider quantum cascade laser chips that include several emitting heads tuned to study specific lines and continuum. Ultrafast optical parametric oscillators with several GHz repetition rates have been demonstrated in the near infrared and [606], this could be one way forward in the MIR. Beyond that, LOs need to be synchronized in-phase across the array. One could find inspiration for this in the demonstration of the stabilization of a MIR (10.6 µm) laser to an atomic clock through a 43 km fiber link. A near infrared comb is locked onto a primary frequency reference and propagated on a phase-stabilized link to each telescope station. At the telescope level it is upconverted to the MIR to provide the LO locking signal [604].

### High bandwidth correlation

The correlation of high bandwidth signals requested by heterodyning could be done numerically at the price of huge computing power [593]. However, analog photonic correlation schemes using amplitude modulators have proven to be a robust way of propagating and correlating very large bandwidth (up to 100 GHz) heterodyne signals over telecom fibers (see e.g. [596, 597]). They should be investigated further.

### Reliable nonlinear crystals

The deployment of parametric conversion schemes requires the manufacturing of reliable nonlinear crystals. This in turns begs for robust industrial processes that allow a proper control of the spectral acceptance, waveguide structure and the management and reduction of the noise due to spontaneous parametric down conversion.

### Miscellaneous developments

Both heterodyne and frequency upconversion techniques would dramatically benefit from compact high transmission discrete or continuous kilometric delay lines. The first one because it needs an external co-phasing instrument capable of measuring atmospheric phase fluctuations, the later because it is intrinsically a direct interferometry technique that requires optical path compensation. Finally single-photon LOs are required as well as fast time-stamped photon-counting detectors in the search for the optimal quantum protocol to achieve the best possible visible and NIR telescope resolution of faint astronomical objects. More generally, the availability of true quantum networks, developed in other industrial contexts, would represent a major advance for this application.

### Concluding remarks

The expansion of optical astronomical interferometric arrays requires an increase in their imaging capabilities, angular resolution and sensitivity. This cannot be simply extrapolated from current technologies. Technological breakthroughs in the field of photonics and quantum technologies present new opportunities to envision radically different array architectures. Heterodyne interferometry, both classical and with single-photon LOs, as well as frequency up-conversion offer the perspective of arrays with infrastructure free links thus dramatically reducing their complexity.
## 23. Efficient hybrid photonic and electronic integration to realize multi-functional instruments

Sherif Soliman¹, Katarzyna Ławniczuk², Nemanja Jovanovic³, Jeroen Duis¹ and Ronald Broeke²

¹ PHIX Photonics Assembly, Enschede, Netherlands

² Bright Photonics BV, Eindhoven, The Netherlands

³ Department of Astronomy, California Institute of Technology, Pasadena, CA, United States of America

### Status

The ultimate goal for astrophotonics is to be able to realize multi-functional and efficient integrated photonic instruments. Given that various functions, such as dispersion, spectral filtering, switching, etc, have been developed and optimized over a range of material platforms (SiN, SOI, InP, etc), it will be necessary to exploit hybrid integration—the integration of disparate technologies, materials and chiplets—to be able to assemble these and realize multi-functional instruments.

Four of the most critical aspects which are currently preventing the mainstream adoption of astrophotonic technologies are (1) low throughputs (fiber to chip, propagation and bend losses, device losses, grating/vertical couplers, etc), (2) difficulties with scaling to large channel count devices needed for large bandwidths and high resolutions, (3) efficient integration with detectors and (4) issues with feeding large-port-count fiber arrays to planar photonic chips. Astrophotonic technologies can also be used to more generally support observations by enabling high bandwidth data links driven by the extremely high data rates of the square-kilometer array (SKA) (0.1–1 TB s⁻¹), for example [607], or for free space laser communications systems, critical to future astrophysics space missions. In addition, numerous astronomy applications require wide optical bandwidths, which imposes selection criteria on transparency and absorption in different photonic material platforms. For these reasons, developing efficient hybrid integration processes is key to the success of astrophotonic technologies.

To address issues related to coupling to high index contrast PICs from low index contrast fiber arrays, interposer devices can be used. These consist of tapered waveguides that provide better mode matching between technologies which can also augment the pitch between the waveguides simultaneously. For example, the Teem photonics interposer solution, referred to as the waveguide array to fiber transposers (WAFTs), allows for efficient coupling (<0.7 dB) between Si based PICs and silica-based optical fiber arrays [608, 609]. However, they are now also being used with SiN and InP PICs as well. These WAFTs consist of ion exchange waveguides imprinted into glass-based PICs. The company provides three versions of the WAFT, including (1) an edge coupling version which combines fiber spacing concentrators (FSC) with spot size converters/tapers (SSCs) (2) a top coupling variant which adds a mirror at the tip of the structure and combines it with FSCs that provides wafer level and grating coupler compatibility (see in figure 42(a)) and (3) an evanescent coupling version [608].

In a similar vein, being able to couple the light efficiently from 2D fiber arrays, (e.g. an MCF) and a PIC is also crucial. Two possible solutions exist for this currently including using a 2D array of grating couplers on the chip to inject the light from above [610] (see figure 42(c)) and/or utilizing fan-out structures that remap a 2D array of waveguides to a linear array [611]. However, neither option is efficient currently, with the former suffering from losses associated with the grating couplers due to manufacturing imperfections, their narrow bands of operation and the stringent requirement of the angle of incidence of the input beam for efficient coupling and the latter limited by realizing mode-matched waveguides and tapers with the ULI process. These technologies both require further development.

Integrating detectors, be it 2D or 1D arrays or single pixel devices with PICs is also critical to eliminating losses, minimizing cross-talk and realizing integrated instruments. We refer the reader to a full discussion in section 24.

Hybrid lasers based on III–V materials like GaAs or InP can also be integrated onto a Si or SOI chip [612] and could be used for either calibration or metrology via say soliton microcombs [613].

To drive active components, including lasers, detectors, phase shifters, etc, electronic integration is necessary. Typical electronic integration involves mounting PICs onto PCBs and using micro-wire bonding. But this does not scale when hundreds or thousands of active components are needed. Innovative solutions are being explored where the electronic integrated circuits (EIC) are etched into the PIC for transceiver applications. Different technologies are being proposed to meet the PIC and EIC integration requirements like advanced packaging substrates and new fiber-to-chip connectivity solutions [614]. Regardless of how the electronics are integrated, astronomical applications will demand low noise characteristics.

Photonic instruments may also require micro-optics, either micro-transfer printed [615, 616], 3D printed [80, 617] or metasurfaces etched into the PIC [618, 619] to feed flip-chipped PICs (PICs mounted above the primary chip) and/or detectors [620].
<figure><img src="image_55.png" /></figure>

While hybrid integration refers to integrating two or more PICs-devices into a single package, heterogeneous integration involves combining two or more material technologies into a single PIC chip, for instance, III-V materials for lasers or detectors onto Si/SiN PICs [621–623]. Some of the key approaches to achieving heterogeneous integration include flip-chip assembly, heterogenous bonding, micro-transfer printing, and epitaxial growth. Each technology is at a different level of maturity and has different alignment accuracy requirements (summarized in table 1 in [616]). These technologies offer a more compact alternative to ‘hybrid integration’, but have their own limitations in integrating material platforms such as high alignment accuracy (for all), complex back-end process flow (for heterogeneous bonding), and lattice matching (for epitaxial growth) [616].

## Current and future challenges

Optical mode matching is critical to efficiently couple the light between different photonic components realized in different material platforms. As light may be delivered to the PIC instrument via an optical fiber or may need to pass to other optical components located on another chip, mode matching is key to ensure maximum throughput. This is achieved with SSCs that relax the tolerances on in- and out- coupling requirements. SSCs are often realized as a taper, including lateral and inverted tapers, segmented and 3D tapers. Inverted tapers are prevalent but are typically highly polarizing (birefringence and diattenuation) and when large exit beams are needed (i.e. 10 µm for example), require a very small and precise exit taper width, making precise mode size selection challenging. Segmented tapers are more tolerant, but require extremely small feature sizes, i.e. sub-wavelength to prevent diffractive effects. This requires special processing (e.g. e-beam lithography), which can be very challenging for mass production. Finally, 3D tapers can mitigate the polarization sensitivities and deliver a circular output beam, but are harder to implement and are only offered by a handful of foundries (e.g. LioniX [443], Fraunhofer [626]) as part of their MPW or custom runs.

Due to the limitations in SSC performance, SSCs can be combined with interposer devices, like WAFTs. The WAFTs outlined above can generate circularly symmetric modes that can be tapered from 3 to 10 µm, with high efficiency, and insertion losses of <1.5 dB [608]. This reduces the requirement on the more complex and limited on-chip SSCs, and by combining the two, efficient transitions could be realized. Despite the existence of these technologies, optimizing each to maximize overall performance, especially over broadbands still remains a challenge.

Another loss that comes about from hybrid integration is that of Fresnel reflection brought about by the different indices of the materials used. In some applications, index matching glues are used between devices. To minimize the residual Fresnel reflections between the chips and the glue, anti-reflection coatings are placed on the end faces of the two chips. AR coatings range from single wavelength operation (e.g. for narrowband lasers) to coatings functioning over very broad spectral bands of >100 nm wide. Challenges
<figure><img src="image_56.png" /></figure>

remain with respect to finding suitable bonding adhesives with high transmission across all spectral regions of interest to astronomy applications (<380 nm up to >5 µm), as well as implementing low loss coatings across various material platforms and broader bandwidths.

There are multiple challenges and approaches towards integrating laser sources onto PICs [616]. Among the existing methods are: monolithic integration of lasers using InP-based material platforms, hetero-epitaxial growth of lasers on Si substrates, as well as hybrid integration approaches via micro-transfer-printing, flip-chip, vertical coupling, and lensed coupling, as well as die-to-wafer and wafer-to-wafer bonding, with some approaches schematically shown in figure 43(a) [627]. The challenges for hybrid integration are high alignment accuracy required in case of e.g. flip-chip, vertical coupling, and lensed coupling; as well as throughput, reliability, scalability, yield, and process and test complexity in case of e.g. die-to-wafer and wafer-to-wafer bonding. The main challenge in case of monolithic integration, in the context of astrophotonics instruments, is limited material transparency and optical gain bandwidth, typically covering telecommunication and data-communication C- and O-bands, as well as insufficient throughput (high losses).

When many active devices are needed on the chip, be it PDs, lasers, amplitude or phase shifters, it is necessary to also miniaturize the electronics. Etching the EIC into the PIC is a solution that allows scaling to hundreds or thousands of active devices and is been demonstrated [628] and is being offered commercially [629], but is challenging due to the mismatch between EIC and PIC lithographic technologies and not offered on a large range of material platforms. Further, even if this was solved, the number of devices that can
be placed on a chip will eventually be limited by the finite size of the photonic components, rather than the electronics.

Hybrid integration of different photonic components is challenging since the components are typically more fragile than electrical components and have extremely tight optical alignment tolerances, often <1 µm to minimize losses between devices/chips [630]. This is exacerbated by CTE mismatches as low noise detection in astronomy is typically conducted at 70 K for near infrared observations. In the same vein, more general space qualification (e.g. radiation hardening and vibration/shock) testing for hybrid integrated devices needs to be undertaken to prepare them for any future missions.

### Advances in science and technology to meet challenges

The goal is to reach fiber-to-waveguide and chip-to-chip coupling efficiencies of >95% per facet (<0.4 dB). This has been demonstrated in fiber-to-SiN waveguides over a broad band (1450–1650 nm) using inverted tapers [190]. However, this required lithographic processing with very small feature sizes, which is not offered as part of standard contact/stepper lithography fabrication and hence is not currently scalable. To enable this, more foundries need to offer new and improved processing techniques which allow for smaller feature sizes (necessary for inverted and segmented tapers) as well as 3D tapers as part of their standard services. Design tools also need to be advanced to optimize the SSCs given these new limitations for broadband applications.

The ion exchange technology for WAFTs could potentially be used to realize 3D circuits in future. Fanout devices that not only route the output of a MCF to a PIC, but also provide optimal mode matching between the disparate technologies could be realized in this way for the first time.

Alternative technologies such as SU8 polymer interposers or tapered waveguides (eg: bilayered inverse taper in lithium niobate [635]) could also be used in place of WAFTs. In addition, there are a host of other fiber tapers including capped adiabatic tapered fibers, lensed fibers [636], or cascaded tapers and cantilever tapers [637]. Interconnects are also critical to connect different photonic devices. One example is shown in figure 42(d), where a photonic wire bond is used to transmit light from a laser to a waveguide [625]. Polymer interconnects such as those using ZPU12 polymer additionally provide the required flexibility for interconnects [638]. The optical losses of PolyBoard PICs were reported to be ~0.7 db cm⁻¹ at 1550 nm. The fabrication process of the PolyBoard platform provides the added advantage of achieving multilayer waveguide structures by using alternating waveguide and cladding layers and a semi-automated assembly process has been demonstrated using commercially available equipment which eases the scaling from prototypes to production. A completely different approach utilizes 3D printed micro-optics to achieve mode-matching between PICs (figure 42(b)), beam shaping, and PIC-to-fiber-array efficiencies as high as 88% [80], albeit without the flexibility of interconnect approaches.

Finally, advances in heterogeneous integration are critical to minimize the losses, attain high stability and scalability, and thereby, maximize the integration advantage in astrophotonics. The key challenges of heterogeneous integration approaches are the necessity of high alignment accuracy (<0.5 µm) and the high pressures (200 mg/bump) that may be needed for bonding [630, 639]. However, new approaches such as mass-reflow of copper bumps have demonstrated a final self-alignment with ~0.5 µm accuracy in flip-chip assembly of SOI grating couplers and a microlens die, with an initial misalignment tolerance of 10 µm with only 5.5 mg/bump of applied pressure [630]. To achieve the highest possible coupling efficiencies with small waveguide modes (~3 µm) in high-contrast materials, alignment accuracies of ~0.1 µm will be needed. Further advances are also required in making such precision assembly processes scalable for various other material platform combinations.

### Concluding remarks

Processes such as heterogeneous integration and hybrid integration are critical to realizing compact all-photonic instruments for astronomy. Developments in regards to advancing SSCs, developing 3D interposers, scaling processes to integrate many detectors with chips, enabling efficient and cross-talk free waveguide crossings, improving processes to integrate micro-optics, and advancing the design and performance of vertical and grating couplers are all critical to realizing efficient astrophotonic instruments.

### Acknowledgments

We would like to acknowledge informative technical discussions with Pradip Gatkine.
## 24. Semiconductor detectors and integrated photonics

Donald F Figer

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

### Status

Semiconductor detectors are central to astronomical discovery and have been the principal tool used in the vast majority of high impact discoveries in the field over the past 40 years, such as the discovery of dark energy, dark matter, exoplanets, and the identifications of black holes in the centers of galaxies, including our own. These detectors convert photons of sufficient energy into charge carriers that they then integrate in a storage well that is often formed as a pn junction. They are the detector of choice for a large range of wavelengths spanning x-rays through MIR wavebands (~0.1–30 000 nm). Their application includes imaging and spectroscopy, and they are the primary detectors in the HST, James Webb Space Telescope, and the vast majority of telescopes on the ground. In general, they are integrated at the focal planes of bulk optical systems. They are almost always implemented as two-dimensional arrays of PDs.

The most commonly-used arrays are in charge-coupled devices (CCDs) and CMOS active pixel sensors. CCDs move integrated charge to a readout circuit. CMOS devices convert charge to voltage within a pixel and use a readout integrated circuit (ROIC) to transfer the voltage to the output. Simple ROICs consist of a source-follower per pixel, row/clock-select circuits, and a buffer that connects to external electronics. Complex ROICs perform operations such as multiple sample correlated reads. Very low noise semiconductor detectors can now count single photons and resolve photon numbers at optical [640] and infrared [641] wavelengths. Some other single photon detectors use Geiger-mode avalanche gain, such as SPADs (single-photon avalanche diodes) [642], but those generally have significantly lower fill factor and cannot count photon number. Single-element detectors (PDs) are rarely used in astronomy applications, but they are useful in photonic circuits [234]. A number of groups have implemented PDs using a variety of fabrication processes and material systems]. For example [643], present a recent PD architecture using InP waveguides with monolithic p-i-n PDs integrated in a two-step organometallic vapor phase epitaxy process. Also [644], present a Ge PD integrated on an SIO based waveguide platform.

A new class of CMOS detectors enable applications where it is important to detect, and sometimes count, individual photons. These devices have very low read noise, typically less than 0.5 e⁻. Examples include devices from Gigajot Technology [640], Hamamatsu [645], and BAE Systems [646]. In all cases, the small capacitance of the sense node produces a very large conversion gain, on the order of hundreds of microvolts per electron. This technique will work for individual detector elements integrated in a PIC, but it requires a PIC process that provides a small design node and integration of complex electronics, two things that are not common at the present time. A short-term solution could be to use these imaging arrays with ad hoc coupling of photonic elements, such as fibers, or to use micro-lenses. Other types of low noise detectors, such as EMCCDs, MKIDs, SNSPDs, etc could have some utility, and are discussed elsewhere in this volume.

The ultra-low read noise of emerging CMOS array detectors presents enormous opportunities for photonic circuit applications. As an example, one can implement a circuit which splits light in a multi-mode fiber into SMFs and detect that light without degrading the signal-to-noise ratio as would be the case with a detector that cannot detect single photons. Similarly, noiseless rebinning provides flexibility when pixel sizes do not perfectly match waveguides and the beam is oversampled.

### Current and future challenges

While semiconductor detectors for astronomy have evolved primarily into 2D formats, such as for imaging applications, integrated photonic implementations may benefit from other formats because the detection no longer needs to be made at a focal plane. In fact, it is challenging to integrate 2D detector formats into planar photonic circuits, making it difficult to take advantage of their near-gigapixel array formats. One-dimensional arrays are more easily integrated into planar architectures through edge-coupling, but they also present challenges when attempting to connect many optical channels from the chip to the array because of routing complexities to avoid waveguide crossings, which introduce loss and cross-talk. Finally, the most flexible format is single PDs that can have arbitrary physical relationships to each other.

Regardless of dimensional format, there are several challenges to realize the potential of integrated photonics when used with semiconductor detectors, i.e.: (1) materials, (2) routing and coupling, (3) form factor, (4) fabrication and (5) temperature control. In several important ways, dimensional format dictates the nature of the challenges in the aforementioned categories. For instance, coupling for 2D arrays requires some way for the PIC to emit light orthogonal to its surface, whereas the light can be in-plane for 1D and PD formats.
<figure><img src="image_57.png" /><figcaption>**Figure 44.** The panels show a linear array of 1D detectors in an interrogator circuit (top, Reproduced from [59] CC BY 4.0) and a photonic spectrograph coupled to a 2D array of detectors on the bottom left. Once the housing is closed on to the photodetector array, light enters through the grating coupler and travels through the planar waveguides, after which it is evanescently-coupled to the resonating emitters that vertically eject the light into the 2D array of detectors. The resulting transmission spectrum is on the bottom right. Reproduced from [648].</figcaption></figure>

Detector materials are chosen for their wavelengths of detection, and their compatibility with PIC materials, so they can be either deposited or pick-and-placed on the PIC (Si, SOI, SiN, InP). The detector materials detect light with wavelengths shorter than their bandgap wavelengths, which for silicon is ~1.1 µm, allowing visible applications. Ge, InP, InGaAs, HgCdTe, and InSb have suitable band gaps to detect infrared light, and they have generally been developed to deliver high performance. Wide bandgap semiconductor materials, such as AlGaN and AlN, are promising for UV wavelengths; although they are not as well developed as delta-doped silicon and some non-semiconductor designs that mate scintillating material or micro-channel plates with silicon. Silicon is again the material of choice for even shorter wavelengths in the x-ray regime up to energies of 10 keV, above which materials with higher atomic mass, such as CdTe, have better quantum efficiency. For wavelengths shorter than visible, Lattice mismatch presents serious challenges, limiting the combinations of detector and PIC materials that can be used. Coupling and fabrication techniques vary widely for each combination of materials.

Routing light inside a PIC to a detector pixel needs to be carefully considered to prevent excess loss and cross-talk. This becomes extremely challenging to achieve when thousands of pixels are needed. In this case, ejecting the light orthogonal to the plane of the chip as soon as it is ready, reduces the complexity of maintaining many low loss, cross-talk free channels inside the chip. It also provides a natural way to use a 2D array or a second PIC with PDs with a planar PIC device. This vertical ejection could be achieved with grating or vertical couplers [647]. Both of these devices are extremely challenging to realize efficiently currently and are not offered as part of the cost-effective multi-project-wafer (MPW) fabrication runs offered by foundries. Other challenges of this approach include mode-matching, which may require micro-optics and anti-reflection coatings to minimize losses. To exploit 1D edge-coupled arrays, low loss, cross-talk free routings become the primary challenge. Directly integrated PDs on the PIC can help circumvent the routing issue but are still susceptible to Fresnel reflection losses if not carefully engineered. Figure 44 shows examples of an integrated 1D array of PDs and vertical ejection into a 2D array of detectors.

The form factor of PDs is one limitation when considering direct integration onto the PIC. For example, PDs can be integrated via direct growth or bonding after picking-and-placing the PD and can be coupled to in a number of ways, including via evanescent coupling (see figure 45 for details and also [649]) and butt-coupling. The architecture in the figure for example needs relatively long pathlengths for the evanescent coupling to be efficient. It is also labor intensive to produce, and thus relatively expensive. On the other hand,
<figure><img src="image_58.png" /></figure>

coupling efficiencies can approach ~100%. The PDs footprint will depend on the type of optical coupling and the materials used. Fitting a large number of PDs directly on the chip while avoiding excessive crosstalk from waveguides is challenging.

Fabrication might be the principal future challenge in realizing the promise of PICs with detectors. Ideally, the fabrication process would provide a menu of elements and materials that are manufacturable and integrable in a foundry setting, such as described in a process design kit. A central challenge is the cost required for all the specialized tooling that is needed for growth and integration. Typically, the tools cannot be used for multiple material systems due to risk of contamination.

Temperature can be a relevant environmental challenge in several different ways. One, some semiconductor detectors require low temperatures, e.g. $T \sim 70$ K, in order to function with low dark current. This requirement implies an architecture that is very bulky and resource-intensive, requiring cryogenic dewars and complex readout electronics. Another way in which temperature is important is in architectures where elements need very stable temperatures. One example is the fine tuning of frequency that is commonly implemented for ring resonators (whispering gallery modes). If these elements are near a detector, and the detector generates transient heating, particularly through clocking lines, then there will be a coupling between the two elements that will require careful thermal management.

### Advances in science and technology to meet challenges

Although 2D detector arrays offer a large number of pixels, they can cost 5–20× the cost of a custom PIC in the IR. Despite the fact that PICs can be replicated to reduce cost, highly replicable astrophotonic instruments could be limited by the cost of IR detectors. There is a strong demand for low-cost, high sensitivity IR arrays to address this limitation and enable massively replicated instrument designs.

The materials mismatch challenge can be reduced by developing custom anti-reflection coatings and choosing materials with better index-matching to minimize losses due to coupling. On-die integration is a common solution to reduce these losses and also minimize path-length loses in waveguides/fiber, but that then makes fabrication and integration more complicated for non-silicon-based devices that require significant processing outside the standard CMOS foundry flow and more complex heterogeneous integration, such as flip-chip processing [620, 650]. See also [649] for monolithic heterogeneous integration. Another method to reduce the effects of materials mismatch is to use intermediate buffer layers. This reduces defect growth and dark current [651]. Foundries would ideally offer some of these newer processes and the photonic design kit (PDK) must accommodate mismatched materials in the design; very few combinations are now supported.

Several advances are needed to eject light vertically out of the PIC to PDs or detector arrays efficiently [620]. Firstly, vertical and grating coupler design and performance needs to be improved to realize the efficient ejection of a high quality beam over a broad wavelength range. Secondly, foundries need to offer these specialized components as part of MPW fabrication runs and provide them in standard PDKs. Cross-talk free waveguide crossings are needed to exploit butt-coupling of the edge of a PIC with a 1D or 2D detector array with low loss. Multilayer integration schemes that rely on evanescent coupling between layers have been demonstrated [652, 653], but they need further development to extend the bandwidth. Gratings are also used to eject light out of the circuit plane [654]. An example uses ring resonators to inject resonant photons [653] into a two-dimensional detector array. Another example is the use of a dielectric antenna [655]. One could use these kinds of devices to obtain a spectrum at approximately a thousand spatial locations for a megapixel array. This might be considered to be an ‘intermediate’ solution while fully integrated architectures mature. In addition to losses in couples, it is often the case that the optical elements are larger than the pixel spacing, which in turn introduces another inefficiency in the system. New foundry capabilities are addressing the fabrication challenge through technology transfer from academic and
government labs, such as in AIM Photonics which can accommodate simple PDs within PICs in both Si and Ge. Integrating other materials is more of a challenge and outside of the current process. Europe has foundry capability for integrating InP and Si [650].

Another geometry that is difficult to accommodate with a flat array of detectors is a curved focal plane, for example as would be produced by the Rowland circle of a photonic Echelle grating or an AWG. In these cases, it may be more convenient to use PDs arranged along an arc.

Finally, progress is needed in the arena of single photon counting and photon number resolving detectors in PIC format. Some photonic techniques use small beams, ~1 µm, which implies small pixel size for the optimum coupling. This is fortuitous because the majority of semiconductor foundry capacity is optimized for the small pixels in smart phone cameras. Indeed, the Gigajot devices have pixels of roughly the same size and single photon sensitivity.

## Concluding remarks

Semiconductor detectors are the workhorse for astrophysics applications over a broad range of wavelengths, and they are likely to become key elements in PIC implementations for those applications in the future. They have already achieved single photon sensing and photon number resolving capability at room temperature, making them an attractive option for integration into PICs. While there are challenges to using them, several research groups have made progress in implementing them, especially in single-detector (PD) formats. To realize much higher detector numbers, one must decide whether it is more cost-effective to implement currently-available high pixel count 2D and 1D arrays, such as in formats having many pixels in arrays, or to use lower dimensional form factors.