Enabling the next generation of scientiﬁc discoveries by ...Enabling the next generation of scientiﬁc discoveries by embracing photonic technologies — 3/16 This was recently

Astro2020 APC White Paper: Technology Development Activity10 July 2019

Enabling the next generation of scientific discoveries byembracing photonic technologiesNemanja Jovanovic1**, Charles Beichman1,2,3, Cullen Blake4 Michael Bottom5, JeffreyChilcote6, Carl Coker2, Jonathan Crass6, Justin R. Crepp6, Nick Cvetojevic7, Miguel Daal8,Mario Dagenais9, Kristina Davis8, Richard Dekany1, Don Figer10, Michael P. Fitzgerald11,Pradip Gatkine9, Olivier Guyon12, Sam Halverson13, Robert J. Harris14, Philip M. Hinz15,David Hover1, Andrew W. Howard1, Rebecca Jensen-Clem15, Jeffrey Jewell2, ColbyJurgenson16, Stephanie Leifer2 Julien Lozi12, Stefan Martin2, Frantz Martinache7, DimitriMawet1,2, Ben Mazin8, Bertrand Mennesson2, Renan Moreira18, Jacklyn Pezzato1, PeterPlavchan19, Michael D. Porter1, Garreth Ruane2, David Redding2, Ananya Sahoo12,Christian Schwab20, Eugene Serabyn2, Warren Skidmore21, Andrew Skemer15, David VanBuren2, Gautam Vasisht2, Sylvain Veilleux9, Sebastien Vievard12, Jason Wang1 Ji Wang16

AbstractThe fields of Astronomy and Astrophysics are technology limited, where the advent and applicationof new technologies to astronomy usher in a flood of discoveries altering our understanding of theUniverse (e.g., recent cases include LIGO and the GRAVITY instrument at the VLTI). Currently,the field of astronomical spectroscopy is rapidly approaching an impasse: the size and cost ofinstruments, especially multi-object and integral field spectrographs for extremely large telescopes(ELTs), are pushing the limits of what is feasible, requiring optical components at the very edgeof achievable size and performance. For these reasons, astronomers are increasingly lookingfor innovative solutions like photonic technologies that promote instrument miniaturization andsimplification, while providing superior performance.Astronomers have long been aware of the potential of photonic technologies. Despite this, the fieldhas been slow to adopt photonics solutions for major facility instruments. There are four possibleexplanations:1) Difficulty of efficient optical coupling, 2) Cost and effort of re-engineering existingindustrial photonics for niche astronomy applications, 3) Lack of technological maturity of certainphotonic components and technologies, and 4) A conservative mentality amongst astronomicalinstrumentation designers, instrument review panels, and funding bodies.The goal of this white paper is to draw attention to key photonic technologies and developments overthe past two decades and demonstrate there is new momentum in this arena. We outline where themost critical efforts should be focused over the coming decade in order to move towards realizing afully photonic instrument. A relatively small investment in this technology will advance astronomicalphotonics to a level where it can reliably be used to solve challenging instrument design limitations.For the benefit of both ground and space borne instruments alike, an endorsement from the NationalAcademy of Sciences decadal survey will ensure that such solutions are set on a path to their fullscientific exploitation, which may one day address a broad range of science cases outlined in theKSPs.

1California Institute of Technology 2Jet Propulsion Laboratory 3NASA Exoplanet Science Center 4Universityof Pennsylvania 5University of Hawaii 6University of Notre Dame 7Observatoire de la Cote d’Azur 8Universityof California Santa Barbara 9University of Maryland 10Rochester Institute of Technology 11University ofCalifornia Los Angeles 12Subaru Telescope 13Massachusetts Institute of Technology 14Zentrum furAstronomie der Universitat Heidelberg 15University of California Santa Cruz 16Ohio State University17University of California Berkeley 18Ultra-low Loss Technologies 19George Mason University 20MacquarieUniversity 21Thirty Meter Telescope** E-mail, phone: [email protected], +1 (626) 395-1214

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1. An Introduction to PhotonicsIn order to understand the potential of photonictechnologies, it is important to first understandwhat photonics are and the properties that makethem favorable for astronomy. There is no sin-gle definition of what falls under the category ofphotonics, so we adopt the following commonlyaccepted definition here: Photonics are the classof components and devices that either makesuse of optical waveguides (optical fibers or in-tegrated circuits) or micro/nanostructured ma-terials (e.g. photonic crystals or metamateri-als) that can manipulate the optical propertiesof light (phase, direction, etc.).

In this paper, we confine our discussion towaveguiding photonic technologies. Waveguid-ing photonic components such as optical fibersand optical integrated circuits have been advancedover the past 30 years by significant investment bythe telecommunications industry. This has led tothe maturation of many technologies, with compo-nents now being widely used in many other fields,including astronomy. However, the overwhelm-ing majority of these components were optimizedfor operations in the telecommunications O- andC-band wavelength ranges (1260–1360 nm and1530–1565 nm) which does not address the needsof the broad astronomical community.

Light can be routed with a photonic waveguide(fiber or chip platform) thanks to the principle oftotal internal reflection (amongst others). Thisrequires that the core region, where the light isto be guided, have a higher refractive index thanthe surrounding material, known as the cladding.Depending on the core size, the index contrast be-tween the core/cladding, and core/ cladding struc-ture, the waveguide will fall into one of two cat-egories: multimode (MM) or single-mode (SM).SM devices are named as such because they canonly transmit light in a single spatial mode, whichis typically categorized by a near-Gaussian profileand a flat phase front. These devices have coresizes typically between 100 nm and 20 µm in sizeand operate at the diffraction limit. In contrast,MM devices support 100s-1000s of modes with

distinct irradiance and phase profiles in cores thatcan be as large as 1 mm in diameter.

1.1 Coupling to Photonic WaveguidesGiven their small spatial extents, efficiently cou-pling into waveguides is a key consideration whendeciding their applicability for astronomical ap-plications. The coupling efficiency depends onthe extent to with which one can match the inputbeam properties (irradiance and phase) to thoseof the modes supported by the waveguide. Theplethora of modes supported by a MM device al-lows even complex beam profiles to be efficientlycoupled, which is why they have been referred toas a “bucket for light” and been the fiber technol-ogy of choice for seeing-limited spectrographs.

On the other hand, matching the beam froma telescope to the requirements of a SM deviceis challenging. In the presence of atmosphericturbulence, the flat phase front requirement of themode can only be met for ground-based observa-tions through the use of an adaptive optics (AO)system. Significant effort has been invested in AOfiber injection over the past 20 years (Coude duForesto et al., 2000; Perrin et al., 2006; Mennes-son et al., 2010; Ghasempour et al., 2012; Bechteret al., 2016), but despite many successes, deliv-ered Strehl ratios of 30−50% currently availableat most telescopes limit SM fiber coupling effi-ciencies. More recently, several large observato-ries have commissioned “extreme” AO (ExAO)systems capable of delivering 90% Strehl ratiosin better-than-median seeing (Dekany et al., 2013;Macintosh et al., 2014; Jovanovic et al., 2015; Vi-gan, A. et al., 2016), well-suited for photonicsapplications.

High Strehl ratio, however, still leaves a beamshape mismatch between the Airy beam fromthe telescope and the Gaussian-like mode of aSM fiber. The theoretical coupling limit of ∼80% (Shaklan and Roddier, 1988) is reduced bythe introduction of a central obstruction, and con-tinues to drop as the central obstruction size in-creases. To prevent this beam mismatch, the pupilcan be apodized with optics in a lossless fash-ion to match the illumination profile of the fiber.

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Enabling the next generation of scientific discoveries by embracing photonic technologies —3/16

This was recently demonstrated by Jovanovic et al.(017a), who boosted the theoretical coupling effi-ciency to > 90% using this technique. Indeed, Jo-vanovic et al. (017a) were the first to demonstratethe simultaneous achievement of both require-ments outlined above using the SCExAO instru-ment on the Subaru Telescope (Jovanovic et al.,2015) and the subsequent realization of > 50%coupling efficiencies to SM fibers from large aper-tures on the ground. This milestone demonstrationhas validated that it is indeed possible to achievethe challenging combination of coupling light intoSM fibers from large-aperture ground-based tele-scopes. Although it relies on sophisticated AOsystems, this is increasingly common at all largeobservatories. With high coupling efficiencies forboth MM and SM fibers clearly established, ei-ther family of photonic waveguides can now beconsidered for collection and routing of light toan instrument.

1.2 Advantages of SM WaveguidesOwing to the single spatial mode supported by aSM waveguide, the output of such a device doesnot evolve with time or with input beam illumina-tion fluctuations, and so is extremely stable. Thiseffect is termed “spatial filtering” and is analogousto passing a beam through a very small pinhole.SM fiber use offers a clear advantage for instru-ments that require a stable instrument point-spreadfunction (PSF). For example, it would eliminatemodal noise as well as pointing-induced illumina-tion fluctuations common to MM fiber-fed radialvelocity (RV) spectrographs. A stable instrumentresponse function makes it possible to calibratedata to a greater precision improving SNR fora given observation. It also means that the per-formance of the instrument is entirely decoupledfrom the performance of the telescope, AO system,and injection, which can greatly relax constraintson the design of the telescope and upstream optics.This decoupling reduces both cost and complex-ity, powerful motivators that have led future spacemissions such as EarthFinder (Plavchan et al.,2018) to plan for use of SM fibers to detect andstudy Earth-analogs in the habitable zone around

G-stars from space.

Operating at the diffraction-limit also offerscritical benefits to the spectrograph design andultimately its size (Jovanovic et al., 2016). To un-derstand this, consider the basic equations govern-ing spectrograph performance. Equations 1 and 2show the resolution (δλ ) and resolving power (R)of a spectrograph, respectively (Schroeder, 1999).

δλ =rφDTel.

Adcol.(1)

R =λ

δλ=

λAdcol.

rφDTel.(2)

The parameters in the equations are defined asfollows: λ is the wavelength of light, r is theanamorphic factor, φ is the angular slit size, DTel.is the diameter of the telescope, A is the angulardispersion, and dcol. is the diameter of the colli-mated beam incident on the disperser. It can beseen that the resolving power is inversely depen-dent on the slit size and is maximized when theslit size is at a minimum (i.e. the diffraction-limit).It can also be seen that the resolving power willdecrease as the telescope diameter increases. Fora given wavelength, angular dispersion, slit size,and anamorphic factor, the diameter of the col-limator (and hence disperser and camera optic)needs to proportionally increase to maintain a cer-tain resolving power. To preserve the focal ratio ofthe collimator and the magnification of the spec-trograph, the focal length of both the collimatorand camera optics must also increase proportion-ally. This ultimately drives an increase in thevolume of the entire seeing-limited spectrograph.However, SMFs operate at the diffraction-limit,so the slit formed by the output beam of such afiber would have a size that can be expressed as

φ ≈ λ

DTel.. (3)

Substituting Equation 3 into Equations 1 and 2,we obtain

δλ =rλ

Adcol.(4)


and

R =λ

δλ=

Adcol.

r. (5)

In the diffraction-limited regime, it is clearthat both parameters are now independent of thetelescope diameter and are only a function of theoptics in the spectrograph itself (i.e., the angu-lar dispersion and collimated beam size). Thismeans that a high-resolution spectrograph whichexploits a diffraction-limited feed (e.g. a SMF)would have the same properties independent of theobservatory, and the instrument can be designedwithout consideration of the telescope. It alsomeans that that the volume of the spectrographno longer scales with telescope aperture for agiven resolving power, and very high resolutionspectrographs can be made extremely compact.Furthermore, thermal and vibration deflections,which scale with the cube of length of the instru-ment, are also significantly reduced, providingexceptional instrument stability. These advan-tages were first exploited by the RHEA (Fegeret al., 2016) and iLocator (Crepp et al., 2016)instruments and more recently the upcoming HIS-PEC/MODHIS spectrographs. HISPEC/MODHISis a SMF-fed spectrograph operating from y-Kband designed for operation at R > 100,000 atboth the Keck and TMT Observatories. By oper-ating at the diffraction limit this instrument willcost several times less than other first-light seeing-limited instruments under development for ELT’s.

1.3 Other Photonics PropertiesIn addition to spatial filtering, photonics technolo-gies support numerous other functions typicallyseen with classical bulk optics such as spectralfiltering, spectral dispersion, frequency comb gen-eration, beam combination and even interferomet-ric starlight nulling. When you combine theseproperties with the compact footprint of photonicdevices (optical fibers measure 250 µm in diame-ter and photonic chips of only a few square cen-timeters) and often monolithic (single component)platforms, waveguiding photonic devices offersignificant potential for the miniaturization of as-tronomical instrumentation.

In this paper we offer an overview of severalkey areas in astronomy photonics have completelytransformed, outline others where significant de-velopment has been undertaken and what the criti-cal next steps are. The goal is to highlight areaswhere development should be focused over thenext decade to enable the possibility for fully pho-tonic instruments in the future. These instrumentsoffer the opportunity for entirely new and trans-formative science. A complementary whitepaper(Van Buren et al., 2019) makes the case that pho-tonics applied at the observatory level may offer acost-effective approach to ever larger systems.

2. Areas Impacted by PhotonicsHere we describe two key areas that have mostwidely embraced and benefited from the applica-tion of photonics over the past two decades. Whilewe focus on facility-level instrumentation in thissection, it would be amiss not to mention the sub-stantial development of astrophotonic technologycurrently at the prototype/visitor instrument level,examples of which can be found in a recent specialissue of Optics Express (Bryant et al., 2017).

Large scale surveys enabled by MMF fedspectrographs: There are numerous advantagesto feeding a spectrograph with an optical fiber,including: 1) the ability to locate the spectrographin a more stable location without the need forcomplex beam steering optics, 2) the ability tomaximize the use of detector real estate in a multi-object spectrograph by aligning the output fibersvertically along a pseudo-slit, leading to muchhigher efficiency 3) the ability to feed light into avacuum vessel without needing a window and 4)the ability to control the input illumination to thespectrograph due to the fixed fiber positions alongthe slit or scrambling/homogenization. These ad-vantages were recognized early on, and in thelate 1970s, the first fiber fed spectrographs weredemonstrated (Hill et al., 1980; Hill, 1988). Thistechnical development promoted countless newinstruments with advanced capabilities over thenext 20 years. For example, by combining fiberswith a robotic fiber positioner, the 2dF Galaxy


Redshift Survey team increased the observationalefficiency of the AAOmega instrument by severalorders of magnitude, enabling them to produceone of the first high-redshift maps of galaxies inthe Universe (Colless et al., 2001; Croom et al.,2004) and paving the way for an era of very large-scale surveys (100,000+ objects).

Precision astrometry with interferometers:Interferometry offers the possibility to combinelight from multiple telescopes to increase the an-gular resolution beyond what is imposed by thesize of a single aperture. Once the light from thetelescopes is routed to a central room it is thencombined to form interference fringes which canbe analyzed to study the astrophysical object.

It was recognized early on that the spatial fil-tering property of a SM waveguide/fiber couldbe used to improve the contrast of the generatedfringes and hence maximize the SNR of the data(Coude du Foresto and Ridgway, 1992; Coudedu Foresto, 1994). The European Southern Ob-servatory (ESO) invested in many generations ofphotonic beam combiners (both fiber- and chip-based platforms including MIDI (Leinert et al.,2003), AMBER (Petrov, R. G. et al., 2007), andPIONIER (Le Bouquin, J.-B. et al., 2011)) cul-minating with the GRAVITY instrument (GravityCollaboration et al., 2017). GRAVITY utilizesa 4-channel integrated photonic beam combiningchip to combine the light of the 8-m telescopesat the VLT (Perraut et al., 2018) (see Figure 1 fordetails). The long baselines and exquisite calibra-tion of the interferometer enable micro-arcsecondprecision astrometry. This extreme performancehas been truly tranformative. The instrument hasgreatly improved the astrometry on the stars orbit-ing the supermassive black hole at the center ofthe Milky Way and have enabled the first observa-tion of flares from the accretion disk at the edge ofthe event horizon (GRAVITY Collaboration et al.,2018). In the exoplanets field, GRAVITY has de-livered a high resolution spectrum of HR8799eand the most precise astrometry for any imagedexoplanet to date (Gravity Collaboration et al.,2019).

3. The Road to PhotonicSpectroscopy - State-of-the-field

Despite the advances from photonic devices inthe areas of beam combination, nulling, and in-terferometry, the most promising underexploitedavenue for astrophotonics is spectroscopy. Whilecertain photonic technologies are already beingapplied to spectroscopy, in particular the use of op-tical fibers, the transformational potential of fullyphotonic spectrographs operating at the diffraction-limit, enabled by their size, simplicity, and lowunit cost has yet to be realized.

3.1 The Photonic Spectrograph:

An Integrated Photonic Spectrograph (IPS) is acompact device, typically measuring a few squarecentimeters, which consists of an optical circuitimprinted into a transmissive material on a wafer.An example of a device with an R of ∼ 7000operating around 1.55 µm is shown in Figure 2.Owing to the fact they are fed with a diffraction-limited SMF, these devices include the equivalentof a spectrograph collimator, disperser, and cam-era optics in a single monolithic element, whichfits in the palm of your hand, enabling an order ofmagnitude reduction in each of the 3 linear instru-ment dimensions (3 orders of magnitude smallervolume) with respect to a bulk-optic counterpart.

Several IPS variants exist including PlanarConcave Gratings and Arrayed Waveguide Grat-ings (AWGs) to name a few (please see Gatkineet al. (2019) for a review on the topic). Giventhat AWGs are the most mature versions of IPSs,development and testing for astronomical appli-cations has almost exclusively been reserved tothem. They were originally developed by thetelecommunications industry and are now usedin optical networks worldwide. Off-the-shelf de-vices like the one shown in Figure 2 have beenoptimized to operate around 1550 nm, with highthroughput (78%), a free-spectral range (FSR) of50 nm (defined as the wavelength span of a sin-gle order of the dispersion element), and mediumR ∼ 7000 (Cvetojevic et al., 012a). More re-cently there have been efforts to extend the oper-


Figure 1. (Left) An image of the integrated photonic beam combiner used by GRAVITY. The circuit which is used tocombine the light from the four 8-m telescopes at the VLT via a series of splitters and couplers can clearly be seen. Thewafer is approximately the size of two US quarters and monolithic (i.e. a single component) as opposed to classical bulkoptic beam combiners. This image was reproduced based on Figure 3 in Perraut et al. (2018). This device was designedand manufactured by CEA/LETI (Grenoble, France). (Right) the superior spectrum obtained for HR8799 e with GRAVITYcompared to previous results. This figure was reproduced from the Gravity Collaboration et al. (2019).

ating range of such devices for biomedical appli-cations (Geuzebroek et al., 2017). This milestonedemonstration successfully realized an AWG thatcould operate from 400–1700 nm in a single shotat low resolving power (R ∼ 100).

Over the past 10 years, AWG-based IPS tech-nologies have been tested for astronomical ap-plications. The off-the-shelf device mentionedabove was tested in several prototypical instru-ments on-sky. Cvetojevic et al. (2009) first usedthe device to generate a full H-band spectrumof night time airglow by pointing the collectionSMF towards the sky (no optics or telescope) andusing a bulk optic cross-disperser to unravel theorders. The team showed that it was possibleto use an AWG to process signals from multipleSMFs simultaneously in a single chip and recoverthem through cross-dispersion (Cvetojevic et al.,012a). This concept was tested at the AustralianAstronomical Telescope (AAT) (Cvetojevic et al.,012b). To take full advantage of the technology,the highly efficient SMF injection built behindSCExAO (outlined above) was used. With > 50%coupling efficiency from an 8-m telescope on theground directly into a SMF realized (Jovanovicet al., 017a), it was possible to take the ultimatestep and directly feed the IPS device. The teamused the spectrograph once again with externalcross-dispersion to enable operation across the Jand H-bands with a total throughput from fiberto detector of 42±3% and a resolving power of∼ 5000 (5% throughput from sky-to-detector as-

suming a 65% Strehl ratio; Jovanovic et al., 017d).The photonic instrument successfully recoveredspectra from several stellar sources. This demon-stration shows that it is now possible to consider afully photonic instrument that can be competitivewith traditional approaches.

Although this initial success is encouraging,the first demonstration device lacked the specifi-cations suited to most astronomical applications.Having proven the fundamental advantages, weneed to further the development into engineeredcomponents for astrophysics. NASA appreciatesthe importance of this development and under-stands this technology could revolutionize spacemissions and awarded an APRA to support someongoing efforts in this domain (Gatkine et al.,2017), but significantly investment is needed.

3.2 Enabling Photonic Spectroscopy:

The spectrograph is the single element that if re-placed with a photonic alternative could dramati-cally alter the course of astronomical instrumenta-tion. There are numerous photonic technologieswhich can be used to either couple the light froma telescope that does not have an ExAO system,realize an IFU that can feed an IPS, spectrallyfilter the light or enhance calibration. The nexttwo sections summarize some of the devices thatenable and enhance an IPS.

Photonic lanterns: these devices allow lightfrom a low Strehl ratio beam to be coupled effi-ciently at the focus of the telescope while provid-


Figure 2. (Top) An arrayed waveguide grating versionof an integrated photonic spectrograph device. Anoff-the-shelf device with a R = 7000, packaged andconnected to a SMF feed. The circuit on the chip is clearlyvisible (Cvetojevic et al., 2009). (Inset) An image for senseof scale. Disclaimer, to the best of the teams knowledge thehand used in this image is of average size. (Bottom) A J andH band spectrum of a lamp taken with the AWG prototypeinstrument in the laboratory (Jovanovic et al., 017d).

ing a diffraction limited input to a downstreamspectrograph (Leon-Saval et al., 2013; Birks et al.,2015). Originally, they were produced by taper-ing a bundle of SMFs inserted into a glass capil-lary (melting and drawing) until they combined toform a single MMF which could efficiently couplethe low Strehl beam at the focal plane. As longas the number of SMFs is equal to or exceedsthe number of modes supported in the MMF end,there will be an efficient transition from a seeinglimited beam to many diffraction limited beams

offering diffraction-limited performance to a spec-trograph on a poor AO/seeing-limited telescope(these devices can of course also be used on highStrehl ratio beams, but at high Strehl ratio thereare advantages of going directly with SMFs).

Photonic lantern devices based on fibers andintegrated photonic chips have been demonstrated(Leon-Saval et al., 2005; Thomson et al., 2011;Birks et al., 2012) and optimized for high effi-ciency in the H-band (Noordegraaf et al., 2009,2012; Jovanovic et al., 2012; Spaleniak et al.,2013). More recently, both types of devices weretested on-sky on seeing limited (Trinh et al., 2013)and AO equipped telescopes (MacLachlan et al.,2014; Harris et al., 2015; Cvetojevic et al., 2017;Jovanovic et al., 017a). The integrated photonicdemonstrations also reformatted the output of thedevice to a linear slit of SM waveguides, in prepa-ration to feed a spectrograph.

One interesting development in photonic lanterntechnology has been the invention of the mode se-lective lantern (Leon-Saval et al., 2014). Thisdevice allows for one particular spatial mode atthe MM end of the device to couple to only oneSMF at the output. By addressing spatial modesin this way it may be possible to exploit thesedevices for imaging or wavefront control as well.

Multicore fibers: we define a multi-core fiber(MCF) as any fiber, which supports numerouscores, be it SM of MM within a single claddingand is formed with tapering/fusing techniques (todistinguish from stacking fibers in a tube and glu-ing them in place) (this follows the discussionin Jovanovic et al. (017c). Astronomers recog-nized early on that a fiber with multiple corescould be used to transfer spatial information fromthe focal plane to the spectrograph, just like anintegral field unit (IFU).

A recent application involves using SMF basedMCFs as IFUs to study directly imaged exoplan-ets. An AO system and a coronagraph are firstused to suppress the bulk of the starlight, whichis many orders of magnitude brighter than theplanet. Then a SMF (or one core of a MCF) isplaced at the location of the known planet (Snellen


et al., 2015). Due to the strict requirements ofcoupling into a SMF, the unwanted starlight cou-pling is reduced by another factor of 2-3 (Mawetet al., 2017). The signal can then be dispersedin a spectograph and integrated upon. The otherfibers in the input focal plane can be used to trackthe stellar flux and collect a spectrum of the starwith telluric contamination contemporaneously,which can be used in the spectral extraction pro-cess (Wang et al., 2017). This method is currentlybeing explored by the SCExAO (Jovanovic et al.,017b) and KPIC instruments (Mawet et al., 2018).Recent simulations by Coker et al. (2019) haveshown that SMFs have the potential to expandthe usable spectral bandwidth over which highcontrast is maintained by a factor of 2-3 whenused in this manner. Another recently proposedinstrument concept combines a vector ApodizedPhase Plate (vAPP) with a micro-lens array and aMCF to realize the SCAR coronagraph (Por andHaffert, 2018; Haffert et al., 2018).

Endlessly single-mode fibers: these opticalfibers defeat the finite bandwidth limitations ofstandard SMFs and allow a fiber to perform as aSMF across the entire range of transmission andcould find future applications (Birks et al., 1997).

3.3 Spectral Filtering & CalibrationSpectral filtering can be divided into two maincategories: filtering science light or filtering a cal-ibration lamp to create reference lines. In addition,reference lines can also be created by a laser.

Filtering the science light: With the light col-lected by a SMF and routed to a spectrograph it ispossible to exploit Fiber Bragg Gratings (FBGs)to conduct spectral filtering of the light prior toinjecting it into the spectrograph. They are ver-satile and can be tailored to remove light fromparts of a spectrum with custom bandwidths andattenuation’s. These devices were explored forthe removal of OH-lines which are caused by therecombination of molecules in the atmosphere atnight. The FBGs fabricated for this applicationwere among the most complex spectral filters everrealized and attenuated over 100 lines across theH-band, with varying bandwidths and levels of

attenuation (Bland-Hawthorn et al., 2004; Bland-Hawthorn et al., 2011; Trinh et al., 2013). TheseFBGs were utilized in the GNOSIS instrument toeliminate the OH line contamination in the spec-trum of redshifted galaxies prior to injection intoa spectrograph (Bland-Hawthorn et al., 2011).

Bragg gratings have also been integrated ontomonolithic chips with photonic lanterns to openup the possibility for compactness and scaling infuture (Spaleniak et al., 2014).

Filtering the calibration light: Photonic tech-nologies can also be used to filter the spectrumof a broadband lamp to create a comb of linesused for spectral calibration. These devices donot have the stability and accuracy of laser fre-quency combs but offer a simple, compact andcost effective alternative.

These technologies revolve around using SMF-based interferometers to create a comb-like spec-trum for calibration. One version uses two simplefiber splitters/couplers to form a Mach–Zehnderinterferometer (Feger et al., 2014). Another isbased on a Fabry–Perot etalon formed by a veryshort length of SMF by using mirror coatings onthe ends (Halverson et al., 2014). An integrated-optic version could use micro ring-resonators (El-lis et al., 2012). They all deliver a comb of linesat the output when a broadband light source is in-jected into the input, the spacing of which can beoptimized by modifying the physical dimensionsof the devices. As demonstrated by Gurevich et al.(2014) and Schwab et al. (2015), the etalon ver-sion can be locked to an atomic transition as awavelength reference, resulting in long term sta-bility at the cm/s level from a very simple system.

Laser frequency combs (LFCs) In 2005 theNobel Prize in physics was awarded for contri-butions to the development of laser-based preci-sion spectroscopy, including the optical frequencycomb technique. The LFC, a photonic device us-ing nonlinear fibers or chip-based micro-resonators,produces a spectrum consisting of laser lines spacedregularly in frequency that can be employed forspectral calibration. Once phase locked and sta-bilized, the wavelength of each comb line is sta-


ble to better than one part in 1012, for years ordecades. LFCs address several key limitations inspectral calibration sources: they offer comb linesacross the entire spectrum of interest, many morelines for calibration than spectral lamps, and pro-vide more uniformity in the brightness of the lines.The first LFCs were validated on instruments likeHARPS and demonstrated distinct improvementscompared to gas lamp calibration sources.

New approaches to frequency comb genera-tion have been developed that offer performancemore optimally suited (spectral and temporal sta-bility, wavelength range of operation and inter-line spacing) to precision RV (PRV) studies (Yiet al., 2016; Obrzud et al., 2018; Metcalf et al.,2019), but further development is required. Con-siderable efforts are now being invested to de-velop compact, low power, chip-based combs suit-able for astronomical applications (Obrzud et al.,2019; Suh et al., 2019).

Both families of frequency combs describedhere could be fed into an IPS device contempora-neously with the science signal (in a second SMF)allowing for accurate calibration of the IPS data.

4. Technology DriversThe biggest roadblock to building compact, fullyphotonic, instruments is being able to customizeIPS’s for astronomical applications. Given thatspectrographs are becoming prohibitively expen-sive on ELTs, replacing a classical bulk opticspectrograph with an IPS is where the most sig-nificant gains can be made in reducing footprint,complexity, and cost and hence where the great-est community energy should be focused in thecoming decade.

4.1 Developing IPS TechnologiesGiven AWG technology is well understood andhas been tested and characterized in the contextof astronomy for the past 10 years, it is the idealchoice for the next stage of optimization (althoughother technologies should be explored in parallel).AWGs should be developed beyond the require-ments for the telecommunications industry to sup-

Figure 3. Concept of an AWG instrument (a) AWGbutted against a linear detector, (b) AWG with multipleinput fibers butted against a linear detector and (d) multipleAWGs butted against a 2D detector (Douglass et al., 2018).

port the wide ranging scope of astronomical appli-cations, by engineering them for higher spectralresolution (R > 7000) than is currently available,in entirely new wavebands spanning from 500 nmin the visible to 5.0 µm in the MIR for use in bothsingle and multi-object science cases with on-chipcross-dispersion implemented.

By developing custom devices a range of inno-vative instrument architectures can be considered.The most streamlined concept involves butting theoutput facet of an AWG directly against the de-tector, eliminating the need for reimaging optics.Since the output is formatted in a 1D line, a lineararray detector could be used reducing costs andcomplexity (Figure 3(a)). If the detector were anMKID array (Szypryt et al., 2017), its ability toresolve the energy of the incoming photon wouldeliminate the need for cross-dispersion on the chip(a recent NASA STTR was awarded for this de-velopment) (Cropper et al., 2003). To increase thenumber of objects that can be observed simultane-ously several AWGs could be stacked on top ofone another utilizing a 2D detector array instead(Figure 3(c)). This assumes that only a single FSRfrom each device needs to be imaged by the detec-tor. An alternative method to increase the numberof objects that can be studied is to feed the AWGs


with multiple input SMFs (see Figure 3(b)). Tounravel the signals from different sources or in-crease the bandwidth for a single object beyond asingle FSR, the orders need to be separated witha cross-disperser which has recently been shownon-chip as well. Here AWGs are cascaded so thatthe first device splits the light into coarse spec-tral channels and then a series of high dispersiondevices operate on narrower wavelength rangesdownstream. To realize these minimal instrumentconcepts however (i.e. butting chips to detectors),the optical properties of the devices need to firstbe investigated at the cryogenic temperatures thatdetectors operate at.

Organizations: There are many commercialfoundries that fabricate photonic devices regularlythat typically have better control of their processparameters (although University-based foundriescan equally play a key role). These foundriesoffer Photonic Design Kits (PDKs), software todesign standard devices compliant with their pro-cesses. In addition there are numerous commer-cially available photonic design suites out therefor more customized component design and anal-ysis. To develop astronomically relevant devices,researchers at Universities pursuing this area ofresearch such as Caltech, JPL, RIT, and U Mary-land, to name a few, will need to work closely withthese foundries to understand their processes andlimitations and use the design tools mentioned.A careful choice of material platform will playinto this but will most likely involve silica-on-silicon, silicon nitride, and/or silicon-on-insulator.In 10 years time we hope to have strong relation-ships and a well-developed community procure-ment process, with several world class fabricationfoundries capable of delivering IPSs on demand.

Schedule: The schedule for IPS developmentdepends on the specific science application. Ap-plications that are similar to current performancespecifications can be realized in a much shortertime frame. More ambitious specifications, R >100,000 over a wide bandwidth for example willtake time as they are a departure from the currentperformance capabilities. With a focused devel-

opment on simple devices, reasonably performingprototype devices could be produced within a 2 to3-year period (the scope of an NSF or NASA pro-posal). We expect it to take 5-10 years to build acomprehensive design suite capable of designingcustom modifications to IPSs, realizing deviceswith more challenging properties and understand-ing limitations of the technology.

Cost estimate: An investment of the orderof $10M in IPS technology over the next 5-10years would significantly advance the field andmake it possible to select IPS approaches for fu-ture instruments for both ELTs and space mis-sions. This estimate for the IPS development isbased on projections from recent costing exer-cises performed in early 2019 for NASA APRAand JPL RTD grants. It should be highlightedthat this development cost pales in comparison tocurrent cost estimate for a single ELT or spacemission spectrograph, but is currently consideredtoo risky for funded projects to embrace. Giventhe potential for cost reduction and performanceimprovement, such a public/private level of invest-ment would be a small price to pay.

4.2 Developing Supporting TechnologiesIn this section we briefly describe where we wouldlike to see further development of photonic de-vices that can enable and enhance photonic spec-troscopy over the next 5-10 years. Our findingsare summarized in Table 1.

Organization: These technologies need tobe developed with a combination of Universityresearchers and vendors. Ultimately we want thetechnology to transfer to commercial vendors whowill conduct the necessary engineering and be ableto deliver much more cost effective devices withshorter turn around times in the future.

Schedule: The devices listed in the table aboveshould be developed steadily over the next 5-10years consistent with available funding.

Cost estimate: To engage with industry andget them to develop some of these componentscommercially will cost more than if done withinresearch laboratories. We estimate that an invest-ment of $10M would be needed to advance all


Table 1. A list of photonic developments that we would like to see over the next 5-10 years to enable photonicspectroscopy. Most devices have thus far been developed in the telecommunications waveband. owing to laboratoryequipment availability. and will need to be optimized as part of this process for other wavebands.

Technology Development descriptionIPSs IPSs are available from many sources but do not meet astronomical requirements. IPSs should be

developed to have higher spectral resolution (R > 7000) in new wavebands (500–5000 nm); ableto support single/multi-object science cases, on-chip cross-dispersion, and concurrent calibrationsources; cryogenic testing of IPS-detector interface.

Photoniclanterns

Custom lanterns are only available through a single vendos. Ideally we would increase thisvendors ability so they become standard products. Develop new devices with high efficiencyacross various wavelength ranges (vis-MIR). Test mode selective lanterns for imaging andwavefront control purposes.

Multicorefibers

These devices are available through a few vendors optimized for the telecommunications bandonly. These devices should be developed for other wavelength ranges (vis-MIR) and tested tosee if they are suitable to be used for SMF IFUs.

Endlessly sin-gle mode fibers

Currently only available through a single vendor. These should be developed for applicationin the MIR, with simulations and experiments to determine their applicability for feedingspectrographs and/or in IFUs with micro-lenses.

Frequencycombs

Compact electro-optic and micro-combs are not commercially available. These should be furtherdeveloped and optimized to include a broader wavelength coverage in the NIR from y-K, devicesthat even span into the MIR, and a higher uniformity in brightness of the comb lines across thespectrum.

Beam combin-ers

There are many commercial vendors for beam combiner chips. Beam combining chips ifcorrectly designed could be used for nulling. Further development is needed into new waveband(vis to MIR), active phase delays to scan path lengths achromatically, and methods to calibratethese devices.

elements in the table above with commercial ven-dors (not including the IPSs discussed separatelyabove) to the point where at least the feasibilityof each development would be understood. De-pending on the difficulties with developing someof these components in new wavelength ranges,which may require new materials, process andequipment, further investments may be needed tofully complete the list.

5. Cost EstimatesCombining the cost estimates outlined for tech-nology development, photonic technologies couldbe significantly advanced with ≤$20 M. (smallground based funding category). More advancedcomponents will naturally get funding supportflowing down through instrument trade and riskmitigation studies, but less mature technologieswill require directed funding to address key issues

before the technology can be considered for suchstudies. At a fraction of the cost of an instrumentfor the ELTs or a space mission this investmentwill pay for itself rapidly.

Despite the fact that GRAVITY at the VLTIrequires four 8-m class telescopes, four high per-formance AO systems, extreme laser calibrationmetrology, and one of a kind integrated photonicbeam combiners, ESO guided by European sci-entists recognized the potential of their coura-geous, sustained investment and now have anglobally unrivaled science capability. A simi-lar US commitment to investment in integratedastrophotonics is needed over the next decadeto reestablish US astronomy competitiveness inthese forefront observational capabilities.


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