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# The path to detecting extraterrestrial life with astrophotonics

Nemanja Jovanovic  Department of Astronomy, California Institute of
Technology, 1200 E California Blvd, Pasadena, CA, 91125, USA  Yinzi Xin
Department of Astronomy, California Institute of Technology, 1200 E California
Blvd, Pasadena, CA, 91125, USA  Michael P. Fitzgerald  Department of Physics &
Astronomy, 430 Portola Plaza, University of California, Los Angeles, CA 90095,
USA  Olivier Guyon  National Astronomical Observatory of Japan, Subaru
Telescope, 650 North Aohoku Place, Hilo, HI 96720, U.S.A.  Astrobiology
Center, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588, Japan  Peter Tuthill  Sydney
Astrophotonic Instrumentation Laboratory, School of Physics, The University of
Sydney, Sydney, NSW 2006, Australia  Sydney Institute for Astronomy, School of
Physics, Physics Road, The University of Sydney, NSW 2006, Australia  Barnaby
Norris  Sydney Astrophotonic Instrumentation Laboratory, School of Physics,
The University of Sydney, Sydney, NSW 2006, Australia  Sydney Institute for
Astronomy, School of Physics, Physics Road, The University of Sydney, NSW
2006, Australia  Pradip Gatkine  Department of Astronomy, California Institute
of Technology, 1200 E California Blvd, Pasadena, CA, 91125, USA  Greg Sercel
Department of Astronomy, California Institute of Technology, 1200 E California
Blvd, Pasadena, CA, 91125, USA  Svarun Soda  Department of Astronomy,
California Institute of Technology, 1200 E California Blvd, Pasadena, CA,
91125, USA  Yoo Jung Kim  Department of Physics & Astronomy, 430 Portola
Plaza, University of California, Los Angeles, CA 90095, USA  Jonathan Lin
Department of Physics & Astronomy, 430 Portola Plaza, University of
California, Los Angeles, CA 90095, USA  Sergio Leon-Saval  Sydney
Astrophotonic Instrumentation Laboratory, School of Physics, The University of
Sydney, Sydney, NSW 2006, Australia  Sydney Institute for Astronomy, School of
Physics, Physics Road, The University of Sydney, NSW 2006, Australia  Rodrigo
Amezcua-Correa  The College of Optics and Photonics, University of Central
Florida, 4304 Scorpius St, Orlando, FL 32816  Stephanos Yerolatsitis  The
College of Optics and Photonics, University of Central Florida, 4304 Scorpius
St, Orlando, FL 32816  Julien Lozi  National Astronomical Observatory of
Japan, Subaru Telescope, 650 North Aohoku Place, Hilo, HI 96720, U.S.A.
Sébastien Vievard  National Astronomical Observatory of Japan, Subaru
Telescope, 650 North Aohoku Place, Hilo, HI 96720, U.S.A.  Astrobiology
Center, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588, Japan  Chris Betters  Sydney
Astrophotonic Instrumentation Laboratory, School of Physics, The University of
Sydney, Sydney, NSW 2006, Australia  Sydney Institute for Astronomy, School of
Physics, Physics Road, The University of Sydney, NSW 2006, Australia  Steph
Sallum  Department of Physics & Astronomy, University of California, Irvine,
4129 Frederick Reines Hall, Irvine, CA 92697 USA  Daniel Levinstein
Department of Physics & Astronomy, University of California, Irvine, 4129
Frederick Reines Hall, Irvine, CA 92697 USA  Dimitri Mawet  Department of
Astronomy, California Institute of Technology, 1200 E California Blvd,
Pasadena, CA, 91125, USA  Jet Propulsion Laboratory, California Institute of
Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109, USA  Jeffrey Jewell
Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove
Drive, Pasadena, CA, 91109, USA  J. Kent Wallace  Jet Propulsion Laboratory,
California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA, 91109,
USA  Nick Cvetojevic  Laboratoire Lagrange, Observatoire de la Côte d’Azur,
Université Côte d’Azur, 06304 Nice, France

###### Abstract

Astrophysical research into exoplanets has delivered thousands of confirmed
planets orbiting distant stars. These planets span a wide ranges of size and
composition, with diversity also being the hallmark of system configurations,
the great majority of which do not resemble our own solar system.
Unfortunately, only a handful of the known planets have been characterized
spectroscopically thus far, leaving a gaping void in our understanding of
planetary formation processes and planetary types. To make progress,
astronomers studying exoplanets will need new and innovative technical
solutions. Astrophotonics – an emerging field focused on the application of
photonic technologies to observational astronomy – provides one promising
avenue forward. In this paper we discuss various astrophotonic technologies
that could aid in the detection and subsequent characterization of planets and
in particular themes leading towards the detection of extraterrestrial life.

###### keywords:

Exoplanets, astrophotonics, integrated photonics, photonic lanterns, beam
combiners, spectrographs

##  1 INTRODUCTION

There have been over 5400 exoplanets confirmed to date. Figure 1 shows the
mass vs orbital period distribution of the known planets. Data is color coded
to highlight the detection technique used. For the vast bulk of this
population of planets, very little is known. Spectroscopy of the exoplanet
atmosphere is critical to revealing a wealth of information (composition and
abundance, spin rate, weather patterns, etc). The field of exoplanetary
sciences is now focusing efforts on characterization of these systems by for
example providing direct exoplanet spectoscopy [1, 2]. By understanding more
about the known exoplanets, we can refine planetary formation and evolution
models and better understand where life is likely to exist.

Figure 1:  5400+ confirmed exoplanets as of the 11th of August 2023 on a
Jupiter mass vs orbital period plot. The Earth is shown on the plot to
indicate where an Earth-like planet around a sun-like star would appear. The
blue region highlights the terrestrial regime. Very few Earth mass/size
planets have been detected thus far. Only a handful are in the habitable zone
of their host star. Data taken from exoplanetsarchive.ipac.caltech.edu

The blue region in Fig. 1 highlights the terrestrial planet regime (planets
with Earth sizes/masses). Only a handful of terrestrial mass planets have thus
far been detected. The Earth is overlaid highlighting that we don’t currently
know of an Earth/Sun analog, which would constitute a primary candidate in the
search for life. Of the planets that have been detected in this regime, very
few are in the habitable zone of their host planets. To detect
extraterrestrial life, we must first detect more terrestrial like planets in
the habitable zones of their host stars. In particular, finding planets around
solar type stars (G stars) is critical for studying systems similar to our own
to understand our place in the Universe.

##  2 Detecting terrestrial planets

The transit and radial velocity techniques have detected the bulk of the known
exoplanets thus far. Although both techniques will continue to add more
terrestrial candidates around M type stars, the radial velocity technique is
the most likely technique to detect Earth-like planets around G type stars,
needed to form the target list for the Habitable Worlds Observatory (HWO)
mission. This will require measuring a velocity change in the speed of light
of $<$50 cm/s and more likely 10-20 cm/s (1 part in $3\times 10^{9}$) [3]. And
if the Earth-like planet is in the habitable zone, this velocity change would
occur over half a year, so the rate of change is extremely small. For this
reason this endeavour is known as extreme precision radial velocity (EPRV).

###  2.1 Laser Frequency Combs

To be able to measure such a small velocity change, it is critical to be able
to calibrate out any instrument drifts to better than 1 cm/s. This requires an
extremely stable spectral calibration source which is known as the laser
frequency comb (LFC). An LFC emits a series of ultra-stable, uniformly spaced
lines across a broad spectrum that can be used to calibrate an instrument over
many years and even decades.

There are numerous ways to generate the comb. A mode-locked laser was
originally used for this application. It generates an extremely densely packed
comb (comb line spacing’s are in the MHz) [4]. At such small line spacing’s an
R 100,000 spectrograph, typically applied to EPRV can not separate the lines
out. Fabry-Perot Etalon filter cavities are therefore used downstream to
remove the bulk of the lines and establish line spacing’s of 10-30 GHz, which
are better suited for astronomical spectrometers [5, 6].

Electro-optic combs rely on electo-optic modulators to generate side bands on
a continuous-wave laser line [7]. This allows the comb to be formed with the
adequate line spacing from the outset. The line spacing can be stabilized by
locking it to a clock reference for example.

Finally, LFCs have also been realized on a chip [8]. These systems are still
not quite turn key, but once they are matured may be applicable to many
applications that have tight space constraints, like those in space.

The preliminary spectrum of the LFC is then sent through a non-linear medium,
such as a highly nonlinear fiber or crystal to broaden it. In this way its
possible to realize combs that span more than 1 octave.

LFCs can be locked to clock references and also to one another allowing for
the frequency uncertainty to be easily reduced to well below those needed for
EPRV. These locking loops also ensure that the frequencies of the lines can be
maintained for decades [9], which will be critical to detecting Earth like
planets around G stars.

###  2.2 Spectral flatteners-on-a-chip

The amplitude across an LFC can vary by orders of magnitude. The precision of
the wavelength solution scales with the square root of the number of lines
used to derive it, and the signal-to-noise of a line scales with the square
root of the number of photons in a line. To optimize the wavelength solution
the lines would have uniform amplitude with high signal-to-noise ratio in a
given exposure time.

To achieve this with an LFC, the spectrum must first be flattened.
Traditionally a flattener consists of a bulk optic set up that collimates the
light out of a single mode fiber (SMF), disperses it with a diffraction
grating and bounces it off a spatial light modulator (SLM) which can control
the reflected amplitude of the beam before it is spectrally recombined and
injected into another SMF to be sent to the science instrument [10]. A compact
spectrometer is also needed to measure the actual spectrum to drive the SLM to
flatten the output. This system is bulky, costly and unstable.

Recently on-chip flatteners have been investigated [11]. These consist of the
same basic elements on a chip. An arrayed-waveguide grating (AWG) is used to
disperse the light into a series of discrete output channels before light is
sent through Mach-Zehnder interferometers (MZI) which can be used to adjust
the amplitude of the channel by thermally tuning the phase of one of the arms
of the MZI before a thermo-optic phase modulator is used to re-phase all the
spectral channels before a final recombination of the entire spectrum in
another AWG. The prototype device developed operated on a single polarization,
over a 400 nm range in the astronomical H band and offered $\sim 38$ dB of
amplitude tuning.

Spectral flatteners on a chip are useful for ground based LFCs but will be
critical to any future mission that requires spectral shaping in flight.

LFCs combined with spectral flatteners could play a key role in enabling
state-of-the-art spectrometer calibration and enable terrestrial planet
detection over the next decade.

##  3 Characterizing Terrestrial Planets

The most effective way to characterize an exoplanet is to collect a spectrum.
A spectrum can indicate the presence of molecules needed to sustain life, like
water and oxygen, as well as ozone which might protect the planet from UV and
greenhouse gases like carbon monoxide, carbon dioxide and/or methane. This is
the most effective way to determine if a planet could harbor life.

To take a spectrum of an Earth-like planet in the habitable zone we must first
reduce the glare from the host star, which could be $10^{10}\times$ brighter.
Given the small angular separation to the orbital distance of the habitable
zone planet, this will require a large telescope ($>6$ m sized aperture) and
advanced high contrast imaging techniques. These consist of wavefront sensing
and control to eliminate aberrations and improve contrast, followed by
starlight suppression from for example a coronagraph. Although there has been
tremendous progress in coronagraph performance over the past few decades,
these systems have still not achieved the requirement of $10^{10}$ contrast
across a 10-20$\%$ bandwidth needed to take a spectrum.

Photonics can be used to support the wavefront sensing and control and
starlight suppression aspects of the high contrast imaging system as well as
for the science instrument as outlined below.

###  3.1 Photonic Lantern Wavefront Sensing & Control

Photonic technologies offer the ability to coherently mix light, necessary for
generating signals for wavefront sensing. Although there are several possible
approaches, photonic lanterns provide a convenient avenue. Photonic lanterns
are waveguide devices that convert a multimoded input into several single mode
outputs via an adiabatic transition. As long as the number modes at the single
mode end is greater than the number of modes in the multimode end, the
transition will be efficient [12].

When a lantern is placed in a focal plane, the input beam is coupled amongst
the modes the lantern supports and traverses the transition to the output
array of SMFs. Owing to their few moded nature (3, 6, 19, 61 mode counts are
typical), lanterns have a greater collecting efficiency as compared to SMFs.
At the output, the flux distribution across the ports uniquely encodes the
information about the complex field of the incident beam. Therefore, a lantern
could be placed in a focal plane to collect light for a downstream instrument
and provide the ability to do wavefront sensing. Focal plane wavefront sensors
of this nature can eliminate non-common path and chromatic errors.

It is possible to use a neural net to map the input electric field to output
intensity distributions to maximize the dynamic range of the lantern [13] and
use this mapping to reconstruct the input complex field. Operating the lantern
in the linear regime has also been modelled [14] and recently demonstrated on
the SCExAO testbed [15]. This test was done around 1550 nm with a 19 port
photonic lantern and demonstrated the successful closed loop control of 5+ of
the low order Zernike modes off-sky. The outcome of this work is discussed in
another paper in the same conference [16].

An interesting prospect is to use a hybrid lantern - a lantern that can
transport light injected into the LP01 mode to one isolated output, while the
other ports consist of a combination of light from each of the modes (i.e. are
coherently mixed) [17]. This concept allows for light to be routed directly to
a science instrument while also providing on-board wavefront sensing which can
allow for improved coupling to the lantern. It was recently proposed and
simulated but has not been demonstrated.

###  3.2 Photonic Nulling

Starlight cancellation is extremely important to reduce the photon noise,
which would otherwise dominate a spectrum collected from the planet. There are
numerous technologies/approaches that could be used to suppress starlight
using photonics.

GLINT is a photonic instrument in operation on the SCExAO testbed at Subaru
telescope [18]. It relies on segmenting the pupil and injecting each of the
beamlets into a pupil remapping chip realized with ultrafast laser
inscription. The beams are routed in 3D inside the photonic component and
tapped via splitters for the purpose of photometric monitoring. Flux in the
main channel is combined pairwise in photonic couplers with beams from other
parts of the pupil. Carefully arranging the relative phases between combined
beams generates nulled signals. For our case this can be done across many
baselines simultaneously. To improve Fourier coverage, nulls on numerous
baselines need to be obtained simultaneously. Scaling the number of input
channels is possible with this approach [19], but getting achromatic nulls
across multiple baselines at once remains challenging. Solutions are being
proposed to design more achromatic circuits[20].

Photonic lanterns can also be used for nulling. Specifically, mode-selective
lanterns - lanterns that map LP modes to unique single-mode outputs naturally
provide a nulling capability. For a 6 port lantern, LP11a, LP11b, LP21a, and
LP21b all have phase inversions on axis prohibiting light in a pure even-
symmetry mode to couple into them [21, 22]. Therefore, at the output SMFs
corresponding to these modes, the starlight is suppressed to some extent while
planet light is coupled. This has recently been demonstrated in the laboratory
with a 6 port mode selective lantern operating around 1550 nm [23].
Preliminary results show monochromatic and 10% bandwidth polychromatic null
depths ranging from 10-3-10-2 across the 4 nulled outputs, which were limited
by the finite cross-talk between the modes in the device. Next steps include
developing lower cross-talk devices. Nonetheless, this passive component
provides a simple avenue to improve contrast between 0.5-2 $\lambda/D$. A
possible extension of this concept would be to realize a hybrid lantern that
allows the two LP11 and two LP21 modes to be separated into unique SMF
outputs, while keeping all other modes coherently mixed together. If this were
possible, it would allow for only the channels used in nulling to be separated
and used for this application, while all of the rest of the light from the
planet would be used for focal plane wavefront control. This could improve the
contrast and stabilize it as well.

Similar to the GLINT concept, its also possible to either segment the focal
plane into an integral field unit that subtends the inner 2$\lambda/D$ or use
a standard lantern to collect the light. At the output, the beams from the
various parts of the focal plane, or the ports of the lantern can be
interfered pair wise using directional or multi-mode interference couplers.
Indeed, several stages of beam combination with fine path length adjustments
could be used to reduce any light leak from an upstream coupler due to for
example imperfections in the coupler or difficulties in phasing, to get to
deeper nulls [24]. This circuit could also be combined with the mode-selective
nuller above as a second stage of nulling. For a mission like the HWO, the
contrast is extreme and photon rates in the final stage will be low, so it may
be difficult to generate sufficient signals to phase the circuit
appropriately. This is something that would need to be investigated. Kernel
nulling self-calibration can also be applied to analyze the output of such
circuits and will also provide a boost in the contrast [25].

###  3.3 Photonic Spectroscopy

As outlined above, collecting a spectrum of the planet itself is critical to
confirming that the planet can host life. Photonic spectrometers in the form
of AWGs form an ideal solution as they are 1) compact and can be flown on the
HWO, 2) consist of a single-monolithic component with no moving parts that can
be readily thermally stabilized, and 3) offer a discretized output that can be
routed to a detector located elsewhere.

At such extreme contrasts, even on 6-m apertures, the photon rates from the
exoplanets are very low. Therefore, lower resolving powers ranging from R$\sim
50$ and up to $1000$ are being considered for the NIR characterization
channel[26]. Typical commercial AWGs operate at R$\sim$7000 [27], so this
represents a reduction in resolution. However, larger bandwidths than those
typically used in telecommunications will be needed. A minimum bandwidth of
$10\%$ (150 nm at 1550 nm) will be needed and more likely even broader
bandwidths. When operating at low resolutions its possible to indeed broaden
the free spectral range (FSR) as well as the bandwidth of the device.

We have recently developed several low resolution devices between Caltech/JPL
in both SiN and Silica photonics. The SiN device was optimized for a FSR of
500 nm while the Silica device was optimized for a FSR of 220 nm. Both devices
were designed for a channel spacing of 8 nm at 1600 nm, corresponding to an
R$\sim$200\. The devices have recently been characterized and the results will
be presented in detail in an upcoming publication. But its worth noting that
the Silica device had an end-to-end efficiency $>70\%$ including fiber
coupling to and from the chip. This demonstrates the viability of optimizing
AWGs for exo-Earth characterization on the future HWO mission.

##  4 Characterization Instrument Architecture

There are numerous possible architectures that a photonic-based instrument
could take. However, given the extreme contrast needed to detect and
characterize and Earth-like planet around a sun-like star, its unlikely
photonics will do it alone. A more likely scenario is to use a coronagraph at
moderately high contrast (10-6-10-7 at 2 $\lambda/D$) and then use photonic
components down stream to build on this. Several possible architectures are
shown in Fig. 2.

One architecture could consist of a GLINT like nuller injecting light in the
same pupil plane as the Lyot stop of the coronagraph (see bottom panel of Fig.
2). Another possibility is to use the mode-selective lantern nuller in a
downstream focal plane with a back end beam combiner chip optimized for Kernel
nulling (see top panel in Fig. 2). This approach has the added advantage that
that mode-selective lantern is a passive component offering some starlight
suppression without the need for sensing and active control. The Kernel nuller
however will need some active control, which again will be challenging at the
low photon rates expected after such extreme starlight suppression. Another
approach would be to segment the pupil into two apertures, inject each into
individual mode selective photonic lanterns and then combine the outputs in a
chip to realize a double Bracewell architecture [22].

Figure 2:  Two potential photonic-based planet detection and characterization
instrument architectures for the HWO. Light is captured by the telescope, and
then bounces off the 2 deformable mirrors used for wavefront control before
passing through a coronagraph (shown as a focal plane mask) before being
filtered at the downstream Lyot stop. (Top) The light is injected into a
photonic lantern and routed to a downstream beam combiner before being
dispersed in AWGs and sent to a detector. (Bottom) Light is captured at the
pupil plane immediately following the Lyot stop by a 3D pupil remapper and
beam combiner chip. The outputs are dispersed and sent to a detector.

In addition to the coronagraph and photonics, the use of masks (vortex, phase
knife, etc) and/or phase plates (applied to the deformable mirrors or a
separate plate) need to also be considered in the architecture.

Photonic spectrographs will be critical to characterize the planet. However,
if the beam combiner chips can not be made to provide deep nulls over broad
bandwidths, then AWGs might need to be used immediately after light
collection. In this way the spectral channels are split early on and then
downstream beam combination is conducted on a spectral channel by spectral
channel basis with narrower overall band passes. This is similar to the layout
of the prototype spectrum shaper on a chip [11]. This increases the complexity
of the circuit as well as the total number of degrees of freedom, which should
ultimately allow for a broader null to be achieved.

The challenge’s that need to be addressed include:

  * •

Defining if and what sort of coronagraph is ideally suited to work in tandem
with a photonic nuller.

  * •

Understanding the chromatic behavior of the various photonic nulling options
and if pre-dispersion can be used as a viable pathway to broaden the null and

  * •

Investigating how to control and calibrate photonic components used with low
photon rates far downsteam in the starlight suppression system.

##  5 Summary

We have outlined how the functionalities of photonic technologies could be
exploited to detect and characterize terrestrial planets in the habitable
zones of sun-like stars. From the ground, giant segmented mirror telescopes
could be used to detect similar planets around cooler M stars using similar
technical solutions. Technologies like LFCs are already at technology
readiness level (TRL) 9 and are being widely used at ground based
observatories. Nulling and wavefront control technologies range from TRL 2-5
depending on the approach. There is a lot of work to be done to advance all
these concepts to a sufficient readiness level to be able to properly evaluate
their potential for application to the HWO.

This work closely follows the 2023 astrophotonics roadmap [28]. For more
details, please consult the roadmap.

###### Acknowledgements.

This work has been supported by the National Science Foundation under Grant
No. 2109231. Y.X acknowledges support from the National Science Foundation
Graduate Research Fellowship under Grant No. 1122374. This work was supported
by the Wilf Family Discovery Fund in Space and Planetary Science, funded by
the Wilf Family Foundation. This research was carried out in part at the
California Institute of Technology and the Jet Propulsion Laboratory under a
contract with the National Aeronautics and Space Administration (NASA).
Support for P Gatkine was provided by NASA through the David & Ellen Lee Prize
Postdoctoral Fellowship and NASA Hubble Fellowship Grant HST-HF2-51478.001-A
awarded by the Space Telescope Science Institute, which is operated by the
Association of Universities for Research in Astronomy, Incorporated, under
NASA Contract NAS5-26555.

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