# 3D-M3: High-spatial resolution spectroscopy with extreme AO and 3D printed
micro-lenslets

Theodoros Anagnos  Department of Physics and Astronomy, Macquarie University,
NSW 2109, Australia  MQ Photonics Research Centre, Department of Physics and
Astronomy, Macquarie University, NSW 2109, Australia  Landessternwarte,
Zentrum für Astronomie der Universität Heidelberg, Königstuhl 12, 69117
Heidelberg, Germany  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117,
Heidelberg, Germany  ©2021 Optical Society of America]. One print or
electronic copy may be made for personal use only. Systematic reproduction and
distribution, duplication of any material in this paper for a fee or for
commercial purposes, or modifications of the content of this paper are
prohibited.  Mareike Trappen  Institute of Microstructure Technology (IMT),
Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344
Eggenstein-Leopoldshafen, Germany  Institute of Photonics and Quantum
Electronics (IPQ), Karlsruhe Institute of Technology (KIT), Engesserstr. 5,
76131 Karlsruhe  Blaise C. Kuo Tiong  Department of Physics and Astronomy,
Macquarie University, NSW 2109, Australia  MQ Photonics Research Centre,
Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia
Tobias Feger  Department of Physics and Astronomy, Macquarie University, NSW
2109, Australia  Redback Systems Pty Ltd, Sydney, Australia  Stephanos
Yerolatsitis  Department of Physics, University of Bath, Claverton Down, Bath,
BA2 7AY, UK  Robert J. Harris  Landessternwarte, Zentrum für Astronomie der
Universität Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany  Julien Lozi
National Institutes of Natural Sciences, Subaru Telescope, National
Astronomical Observatory of Japan, Hilo, Hawaii, United States  Nemanja
Jovanovic  California Institute of Technology, 1200 E. California Blvd.,
Pasadena CA, 91125, USA  Tim A. Birks  Department of Physics, University of
Bath, Claverton Down, Bath, BA2 7AY, UK  Sébastien Vievard  National
Institutes of Natural Sciences, Subaru Telescope, National Astronomical
Observatory of Japan, Hilo, Hawaii, United States  Olivier Guyon  National
Institutes of Natural Sciences, Subaru Telescope, National Astronomical
Observatory of Japan, Hilo, Hawaii, United States  Itandehui Gris-Sánchez
Department of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, UK
currently at ITEAM Research Institute, Universitat Politècnica de València,
Camino de Vera, 46022 Valencia, Spain  Sergio G. Leon-Saval  University of
Sydney, Sydney Astrophotonics Instrumentation Laboratory, School of Physics,
Camperdown, Australia  University of Sydney, Sydney Institute for Astronomy,
School of Physics, Camperdown, Australia  Barnaby Norris  University of
Sydney, Sydney Astrophotonics Instrumentation Laboratory, School of Physics,
Camperdown, Australia  University of Sydney, Sydney Institute for Astronomy,
School of Physics, Camperdown, Australia  Sebastiaan Y. Haffert  Leiden
Observatory, Leiden University, PO Box 9513, Niels Bohrweg 2, 2300 RA Leiden,
The Netherlands  Steward Observatory, University of Arizona, 933 North Cherry
Avenue, Tucson, Arizona  Phillip Hottinger  Landessternwarte, Zentrum für
Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg,
Germany  Matthias Blaicher  Institute of Microstructure Technology (IMT),
Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344
Eggenstein-Leopoldshafen, Germany  Institute of Photonics and Quantum
Electronics (IPQ), Karlsruhe Institute of Technology (KIT), Engesserstr. 5,
76131 Karlsruhe  Yilin Xu  Institute of Microstructure Technology (IMT),
Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344
Eggenstein-Leopoldshafen, Germany  Institute of Photonics and Quantum
Electronics (IPQ), Karlsruhe Institute of Technology (KIT), Engesserstr. 5,
76131 Karlsruhe  Christopher H. Betters  University of Sydney, Sydney
Astrophotonics Instrumentation Laboratory, School of Physics, Camperdown,
Australia  University of Sydney, Sydney Institute for Astronomy, School of
Physics, Camperdown, Australia  Christian Koos  Institute of Microstructure
Technology (IMT), Karlsruhe Institute of Technology (KIT), Hermann-von-
Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany  Institute of
Photonics and Quantum Electronics (IPQ), Karlsruhe Institute of Technology
(KIT), Engesserstr. 5, 76131 Karlsruhe  Vanguard Photonics GmbH, Hermann-von-
Helmholtz-Platz 1,76344 Eggenstein-Leopoldshafen, 76227 Karlsruhe  David W.
Coutts  Department of Physics and Astronomy, Macquarie University, NSW 2109,
Australia  MQ Photonics Research Centre, Department of Physics and Astronomy,
Macquarie University, NSW 2109, Australia  Christian Schwab  Department of
Physics and Astronomy, Macquarie University, NSW 2109, Australia  MQ Photonics
Research Centre, Department of Physics and Astronomy, Macquarie University,
NSW 2109, Australia  Andreas Quirrenbach  Landessternwarte, Zentrum für
Astronomie der Universität Heidelberg, Königstuhl 12, 69117 Heidelberg,
Germany

###### Abstract

By combining integral field spectroscopy with extreme adaptive optics we are
now able to resolve objects close to the diffraction-limit of large
telescopes, exploring new science cases. We introduce an integral field unit
designed to couple light with a minimal platescale from the SCExAO facility at
NIR wavelengths to a single-mode spectrograph. The integral field unit has a
3D-printed micro-lens array on top of a custom single-mode multi-core fiber,
to optimize the coupling of light into the fiber cores. We demonstrate the
potential of the instrument via initial results from the first on-sky runs at
the 8.2 m Subaru Telescope with a spectrograph using off-the-shelf optics,
allowing for rapid development with low cost.

††journal: ao

##  1 Introduction

A plethora of intrinsic and extrinsic information can be collected through
astronomical spectroscopy, such as distance, motion properties, chemical
characteristics as well as the existence of nearby celestial bodies [1]. New
techniques are rapidly advancing to extract more information from the spectral
data, which translates to discoveries about celestial phenomena.

Almost half a century ago, the first fiber-fed instruments began supplementing
existing astronomical spectrographs (e.g., Ref. [2, 3, 4]). With the help of
optical fibers, it became possible to guide starlight from the telescope to
remotely located instruments. This relaxed the mechanical constraints due to
varying gravity vectors affecting the spectrograph, and the measurement
precision increased as the instrument could be stabilized and controlled far
better in a stable environment as opposed to that in the telescope dome. The
fibers in these instruments are typically large in diameter multi-mode fibers,
in order to couple as much light as possible from a seeing-limited image [5,
6].

A contemporary development of fiber spectrographs was the integral field unit
(IFU). A fibered IFU makes use of several fibers to provide separate spatial
samples, to investigate small regions of the image plane (e.g. adjacent or
extended objects) [7]. The spectra from different locations of the image plane
are positioned on the same detector, but physically displaced to avoid cross
contamination. The use of fibers for integral field spectroscopy (IFS)
provides spatial information of the target object, as well as improved
flexibility to reformat the obtained spectra to fit the limited detector
space.

Using multiple MMFs with relatively large core diameters of greater than
$\sim$50 $\upmu$m to form an IFU that couples seeing-limited starlight, is
today a relatively standard practice (e.g., Ref. [8, 9, 10, 11]), which has
enabled many discoveries. However, these instruments are large, due to
conservation of étendue, which means they are costly and difficult to
stabilize. The ability to reduce the size would open up new avenues and reduce
cost. One solution which has been proposed for several types of instruments
(e.g., Ref. [12, 13, 14, 15, 16, 17, 18]), is to use fibers with smaller core
diameter, or single-mode fibers. The main roadblock is that coupling starlight
into SMFs from telescopes with a 4 m or larger primary diameter with a seeing-
limited output, scale that is larger than the Fried parameter $r_{0}$, is
extremely inefficient.

Various efforts have been undertaken to increase the coupling efficiency to
SMFs, with great success in recent years (e.g., Ref. [19, 20, 21, 22, 23, 17,
24, 25, 26, 27]). This is in large part due to adaptive optics (AO) systems,
which reduce the point spread function (PSF) size and wavefront deformation to
acceptable levels. In particular, extreme adaptive optics (ExAO) systems have
demonstrated over $90\%$ Strehl ratio correction (restoring the PSF close to
the diffraction-limit) in the H-band (1.5-1.8 $\upmu$m) (e.g., Ref. [28, 29,
30, 31, 32, 33]), allowing close to optimum coupling. ExAO systems are also
able to maintain a constant PSF position, in better than median seeing on-sky
conditions, which is essential for efficient coupling [34].

Using SMFs also enables astronomers to take advantage of the diffraction-
limited imaging provided by AO. Together these can open up science cases not
available to conventional instruments. Examples include spatially resolving
the surface of barely resolved stellar photospheres as well as their chemical
composition of their close molecular layer known as MOLsphere. The Subaru
Coronagraphic Extreme Adaptive Optics (SCExAO) system at the Subaru Telescope
is an ideal platform for combining such approaches [31]. The list of stars
that could be imaged includes (in an angular diameter size order), Betelgeuse
($\alpha$ Orionis) $\sim$50 milli-arcseconds (mas), Mira ($o$ Ceti) $\sim$50
mas, W Hya $\sim$42 mas, Antares ($\alpha$ Scorpii) $\sim$40 mas, $\alpha$ Her
$\sim$36 mas, R Leo $\sim$31 mas, Arcturus ($\alpha$ Bootis) $\sim$21 mas,
$\mu$ Cep $\sim$21 mas, $\gamma$ Her $\sim$21 mas and Aldebaran ($\alpha$ Tau)
$\sim$19 mas. These are mostly giants with complex mass loss through a cool
($\sim$2000 K) molecular wind ($\sim$10 km $\mathrm{s^{-1}}$) and/or a hot
corona ($\sim$10,000 K) [35]. A spectrograph with a resolving power of several
tens of thousand is enough to distinguish the differential spectral velocities
of the surface that are of order a few km $\mathrm{s^{-1}}$ and to probe the
atmospheric structure by measuring the wavelength-dependence of the stellar
diameter [36]. A single observation will provide useful information regarding
the photosphere of the star in high resolution, while a sequence of
observations can inform on the differential rotation of the surface features,
probing the magnetic field properties and the mass loss of the target (e.g.,
Ref. [37, 38, 39, 40, 41, 42]). Further science applications involve the study
of Mira variables where the mass loss kinematics are of special importance as
well as their dust and wind properties (e.g., Ref. [43]), and investigation of
the orbital parameters of sub-arcsecond companions such as the unevenly bright
yet well resolved Mira AB system, which is the closest wind accretion binary
(e.g., Ref. [44, 45, 46]). Additionally, with a high resolution spectrograph
the stellar chromospheres can be studied by observing chromospherically
sensitive lines, such as the Paschen lines in the infrared (IR) regime and the
He I IR triplet lines at 1083 nm, which are valuable indicators of
chromospheric activity [47]. Lastly, stabilizing the environment of the
instrument will enable precise radial velocity (RV) measurements.

Whilst single-mode (SM) IFUs enable high-spatial resolution spectroscopy and
open these new science cases, they suffer from an issue with fill fraction.
Due to the need for a cladding region around the central core, fiber IFUs can
only sample a certain percentage of the light at the focal plane. Originally
the percentage of sampled light was of the order of 60-65% for MMF-IFUs,
though recent developments have increased this to 73% [9]. To increase the
coupling efficiency, microlenses can be attached to the IFU, increasing the
fill fraction (e.g., Ref. [48]). However, aligning these arrays and fiber
bundles is difficult as the alignment tolerances are of the order of a few
microns.

To reduce these problems, an alternative and more versatile type of fiber that
can act as an IFU, i.e. the multi-core fiber (MCF) could be utilized. This is
a combination of at least two fiber cores forming an individual fiber using a
common cladding. The cores of a MCF can sample light across the image plane
making it ideal for constructing an IFU. By 3D printing lens structures on top
of its cores using lithography techniques, we save time and avoid alignment
issues, dramatically improving the free-space coupling of light into the fiber
and relaxing the requirement for precise alignment of the input (e.g., Ref.
[49, 50, 51, 52, 53]).

Currently, the replicable high-resolution exoplanet and asteroseismology
spectrograph (RHEA) at Subaru [18, 54] is one of the few IFU configured
instruments using SMFs [17, 55, 56]. It uses the visual arm for wavelengths
below 900 nm, of the SCExAO system, but suffers from low coupling efficiency
and the presence of modal noise. The low coupling efficiency is likely due to
mis-alignments in the commercial off-the-shelf micro-lens array (MLA) and
anamorphic prisms to couple the light into the cores of the bundle of the
fibers (stacked and glued together), which are sensitive to alignment,
illustrating the difficulties of applying conventional approaches to SM
technology. Following a similar approach as detailed below, the IFU of RHEA
was upgraded recently with 3D-printed MLAs on a MCF in the visual band [57].

In this paper, we show the first demonstration of a custom MCF with in-situ 3D
printed MLA on top of its 19 cores using two-photon polymerization
lithography, in combination with a high resolving power compact SM
spectrograph optimized for the 900 - 1100 nm wavelength range, further on
referred to as 3D-printed mono-mode multi-core fiber spectrograph (3D-M3).
This spectrograph takes design cues from RHEA@Subaru [18] for a wavelength
range in the near-infrared (NIR) and the multicore fiber Photonic TIGER
Échelle spectrograph [13, 15]. The instrument is designed for operation in the
NIR arm of the SCExAO facility, making 3D-M3 the first instrument to take
advantage of the phase-induced amplitude apodization (PIAA) optics in that
path. PIAA are custom optics [58] that induce a softer apodization on the
collimated beam, forming a near-Gaussian profile PSF output in the image plane
[25]. This makes the coupling of starlight more efficient as the PIAA beam
intensity profile is close to the mode-field profile of an SMF. The setup was
tested on-sky using SCExAO at the 8.2 m Subaru Telescope; it covers an
instantaneous 54 mas field of view (FOV). The custom MCF achieves high
throughput coupling by using the 3D printed MLA on top of each of its cores,
which provide a very high fill factor of the fiber surface and are extremely
efficient for off-axis field injection while acting as an IFU. Combined with
the SM spectrometer and SCExAO, the instrument shows potential towards a
precise high-resolution IFS with astrophysical sources due to its inherent SMF
stability and absence of modal noise as the MCF is designed to exhibit SM
behavior in the wavelength regime of operation.

In Section 2 we discuss the conceptual design and parameters, followed by a
description of the experimental setup to characterize performance. Then in
Section 3 we present the laboratory and on-sky results. Finally, we summarize
in Section 4.

##  2 Methods

To efficiently inject light from an 8-m class telescope into the cores of a
MCF and a diffraction-limited spectrograph, the following elements are
required: a high performance AO system, an IFU with micro-lenses to dissect
the focal plane image and direct light to the individual cores efficiently,
the MCF itself, and the spectrograph, which must be able to handle a multi-
core input In this section these elements will be discussed in detail.

###  2.1 Instrument architecture

To simulate the instrument, we compile component specifications from the
resulting beam output of the telescope down to the detector. Starting with the
8-m Subaru Telescope, starlight continues through the complex set of the AO
systems at Subaru, namely AO188 and SCExAO, where the intensity distribution
of the resulting output beam is close to a Gaussian profile.

To efficiently couple light into the SM cores of the MCF, a custom fiber
injection unit is required. This system has to be carefully optimized to match
the parameters of the MCF, which has a 5.3 $\upmu$m mode-field diameter (MFD)
at 980 nm ($\mathrm{1/e^{2}}$) with a SM cut-off at $\sim$800 nm with an upper
working wavelength limit of 1400 nm (a microscope image of the cross-section
of the fiber is presented in Fig. 1). The core-to-core spacing (pitch) of the
MCF is 40 $\upmu$m. This is important to make sure that the cross-coupling of
light between the cores is negligible, as they are physically sufficiently
separated with a ratio of pitch/MFD of 7.5:1, resulting in a theoretical
cross-coupling less than few percent. To maximize the amount of light coupled
to the fiber cores, micro-optics are 3D printed on top of the cores using a
custom lithography system described below. To demonstrate the potential of
such an injection unit system, a compact spectrograph was built with off-the-
shelf optics detailed in the following sections.

Figure 1: Microscope image of the polished multi-core fiber in its ferrule.
The cores are visible in a hexagonal configuration with a pitch of 40
$\upmu$m. The fiber has a 5.3 $\upmu$m mode-field diameter at 980 nm
($\mathrm{1/e^{2}}$) and a cladding diameter of $\sim$ 220 $\upmu$m. Here, the
cores are back-illuminated with a white light source. Numbers in red are used
for core referencing below.

To spatially resolve a stellar surface without compromising the throughput,
the selected plate scale was $0.45^{\prime\prime}$/mm. That means that each
fiber core subtends 18 mas on the sky. As a reference, the Subaru Telescope
with the SCExAO facility is able to spatially resolve the surface of stars
with an angular diameter of $\sim$50 mas, which is the apparent size of a
handful of giant stars, such as, for example, Betelgeuse ($\alpha$ Ori), or
Mira ($o$ Ceti). The required focal ratio to achieve the
$0.45^{\prime\prime}$/mm plate scale is $f/57.5$. The diffraction limit of the
Subaru Telescope at 980 nm is 25 mas. The plate scale of
$0.45^{\prime\prime}$/mm was selected for a sub-diffraction limit sampling in
order to seek for the sweet spot between spatially resolve the stellar surface
and perform decently in terms of throughput.

###  2.2 The SCExAO infrastructure

SCExAO is installed at the NIR Nasmyth focus of the 8.2 m Subaru Telescope.
Detailed information and schematic layouts are provided in [31]. Initially the
starlight from the 8.2 m Subaru Telescope enters the AO188 facility, which
offers 30-40% Strehl PSF correction in the H-band under moderate atmospheric
seeing conditions [59, 60, 61]. After a series of components in the optical
train for manipulation of the beam, the light undergoes further wavefront
corrections of higher-order spatial and temporal modes caused by the
atmosphere. Next, it is filtered using a dichroic filter and split in two
paths, the visible channel ($<$900 nm) and the NIR ($>$900 nm) channel.
However, the wavelength response of the dichroic filter does not have a sharp
cut-off profile, and there is a slight overlap of wavelengths between both
channels. After that, the starlight is focused using a gold-coated off-axis
parabola (OAP) with $f$ = 519 mm onto the 3D-printed MLA surface. The
3D-printed end of the MCF was installed on the NIR bench of SCExAO (see Fig.
2).

Figure 2: Schematic illustration of the NIR bench of the SCExAO facility. The
3D-printed fiber is shown at top left. The beam from the AO188 to the IFU is
represented with purple and red color.

###  2.3 Simulations

To simulate the output beam, the profile of the Subaru Telescope and the key
parameters of SCExAO from the literature (e.g. Ref. [31, 25]) were used.

The physical-optics propagation (POP) module of the commercial Zemax [62] ray
tracing software was used to simulate the output beam profile of the Subaru
Telescope, including the effect of the PIAA optics [58] installed in the NIR
optical train of SCExAO, where the fiber is positioned. This output was used
as an input for optimizing the performance of the MLA structure that forms the
IFU.

To optimize the coupling efficiency of the SCExAO beam output at its focal
plane, the physical-optics propagation module was used again to model the
surface shape of the MLA structure. Even polynomials with coefficients up to
eighth order were used to model the aspheric lens surfaces of the MLA, forming
a hexagonal aperture at the top end of the lenslets where they are merged
together leaving no space in between. Since the 3D printing method is able to
achieve surface roughness with an root-mean-square (RMS) of 37 nm [52], the
roughness of the structure is expected to be better than $\lambda/20$ at the
wavelength of operation. This resulted to a total coupling efficiency of 48.8%
(coupling efficiency 56.4%, transmission through the MLA 86.4%, total coupling
48.8%).

###  2.4 Micro-optic manufacturing process

The structure of the MLA as detailed in Section 2.2.3 was directly printed in
a single block using two-photon lithography into the commercially available
negative-tone photoresist IP-Dip [63]. The MLA was printed on the cleaved
facet of the MCF, which had been manually glued into an FC-PC connector and
then polished to achieve a flat surface to enable straight-forward printing on
the front face of the fiber. A lithography machine built in house [50, 51, 52,
64] was used to generate the lenses. The system is equipped with a 780 nm
femtosecond laser [65] and a 40$\times$ Zeiss oil immersed objective lens with
numerical aperture (NA) = 1.4. For high-precision alignment and writing with
high shape fidelity, machine control software developed in house was used.

The fiber was back-illuminated with a red LED, which was used together with
machine vision to detect the 19 cores of the MCF and align the individual
lenses to each core. To compensate for any slight location and pitch variation
of the individual cores of the MCF, the full 3D-model is generated only after
core detection. Prior to printing, the individual models of the lenses are
merged by a Boolean operation to avoid unnecessary double illumination.
Automated detection of the fiber end-face tilt is employed and the structures
are corrected accordingly. The writing distances between subsequent lines and
layers, i.e., both hatching and slicing distance, were set to 100 nm. The
fabricated structure is developed in propylene-glycol-methyl-ether-acetate
(PGMEA), flushed with isopropanol, and subsequently blow dried. In the next
stage, scanning electron microscopy (SEM) and vertically-scanned white-light
interferometry (VSI) images of the structure were acquired to check the
quality of the manufacturing process (see Fig. 3).

Figure 3: Scanning electron microscope image of the 3D printed micro-lens
array structure on top of the multi-core fiber ferrule. All 19 cores were 3D
printed, though only 7 were used for the on-sky run.

###  2.5 Fiber injection unit

In order to achieve precise repeatable positioning of the injection fiber
across the PSF of the target, a 5-axis remotely controlled stage was used. The
5-axis stage (Newport, M-562-XYZ & 562F-TILT) was equipped with computer
controlled stepper motors (Zaber, T-NA08A25), which allowed a minimum step
translation of $\sim$50 nm and $<$ 1 $\upmu$m of unidirectional repeatability
(see Fig. 4). To align the fiber for tip-tilt, adjustments were performed
manually at the L-bracket on which the fibers were mounted. The alignment of
the fiber to the optical axis was achieved by adjusting the Gaussian
illumination at the pupil to be centered while the fiber was back-illuminated
with a HeNe laser. Immediately in front of the L-bracket, a $-$48 mm focal
length plano-concave lens (Edmunds #67-995) was installed to re-adjust the
injected $f/\\#$ to match that of the fiber. This was accomplished by using
rails to translate the plano-concave lens and the 5-axis mount together and
independently of each other. Using this arrangement a range of $f/\\#$ from 28
to 60 could be achieved.

Figure 4: Illustration of the fiber-injection opto-mechanic setup on the
SCExAO NIR bench. The MCF (blue tubing) with the 3D-printed MLA is attached to
a fiber socket and inserted in the beam path. An MMF (orange tubing) can be
inserted in the beam for calibration of the system.

The L-bracket was designed to support 2 fibers, the MCF and another fiber with
SMA connector. On the SMA slot, a MMF was installed (365 $\upmu$m core, NA =
0.22, Thorlabs - FG365LEC) to calibrate the throughput measurements as the
5-axis stage moves laterally (sideways).

In order to achieve $f/\\#$ of 57.5 (see Section 2.2.1), the spacing between
the focusing OAP (f = 519 mm), the concave lens and the fiber was adjusted.
Zemax simulations served as a reference starting point, and then further
optimized using a detector imaging the near field output of the MCF to confirm
the required $f/\\#$.

###  2.6 Throughput measurement setup

To measure the total throughput performance of the custom IFU, a throughput
test experiment was constructed. A schematic illustration of this setup is
presented in Fig. 5. The light from each of the fibers was collimated using a
set of microscope lenses; L1 - Thorlabs RMS20X-PF for the MMF and L2 -
Thorlabs RMS4X-PF for the MCF. Both beams were directed towards a 50:50 non-
polarizing beamsplitter (BS) (Thorlabs CM1-BS014). After the BS, a 50 mm
achromatic lens (L3 - Thorlabs AC254-050-B-ML) was used to re-focus the light
on the detector (D - ASI-120MM-S mono). To measure the total throughput
(including the coupling losses) of the MCF, the throughput of the MMF was used
to calibrate the absolute flux. This was achieved by translating the 5-axis
stage at the telescope focus to couple it to each core of the MCF input, and
then normalizing each collected signal by the flux measured in the MMF.

Figure 5: Throughput test setup for measuring the efficiency of the 3D printed
micro-lens array. Starlight from SCExAO is sampled by one of the fibers at any
time. The calibration of the throughput is achieved using the multi-mode
fiber, lenses (L1-L2-L3) for collimation and focusing of the beam,
beamsplitter (BS) and CMOS detector (D) for sampling the light output.

###  2.7 Spectroscopic setup

To show the potential of using a 3D printed MLA in conjunction with an MCF, a
spectrograph similar to the RHEA at Subaru design [66, 18] and the MCF
Photonic TIGER Échelle spectrograph [13, 15] was built. The benchtop
instrument is a compact diffraction-limited échelle spectrograph using only
off-the-shelf components.

As illustrated in Fig. 6, the spectrograph is composed of the MCF with the 3D
printed MLA (P1), a combination of optical lenses (P2) to collimate the beam
(EO#49-656-12mm, EO#47-655-INK-36mm, Thorlabs TTL200-A) an échelle R2 grating
(P3) (Thorlabs GE2550-0363) for dispersion, then cross-dispersion using a
transmission grating (P4) ($2^{\prime\prime}$ 200 l/mm Baader Planetarium) and
finally a lens (P5) (Thorlabs TTL200-B) to focus the beam onto the detector
(P6) (ASI-183MM-pro mono).

3D-M3 makes use of only the 7 inner cores of the MCF out of 19 in total in
order to avoid overlapping of cores on the detector. A custom aperture to mask
off the outer ring of 12 cores was positioned in between the MCF exit and the
FC/PC connector mounted on the spectrograph side. The MCF was rotated around
its central core until all the spectra from the individual cores were well
separated and equidistant (see Fig. 13 bottom panel and Ref. [13, 15]). The
resolving power of the spectrograph was measured as 30,000 at 1 $\upmu$m,
while the footprint of the spectrograph is $300\times 600$ mm. It was
installed on the Nasmyth NIR platform of the Subaru Telescope.

Figure 6: Illustration of the spectroscopic setup of the 3D-M3 using only off-
the-shelf components. The footprint of the breadboard is $300\times 600$ mm.
The light injected from the MCF (P1) is collimated by a combination of optical
lenses (P2) before it is chromatically dispersed by an échelle grating (P3),
cross-dispersed using a transmission grating (P4) and focused down using an
optic (P5) on to the detector (P6). For a detailed description of the parts
see Section 2.2.7.

##  3 Results

###  3.1 Laboratory throughput results

To characterize the performance of the injection system prior to on-sky
operation, we performed throughput measurements in lab conditions as described
in Section 2.2.6. The results are presented here.

SCExAO is equipped with an internal calibration system using a Fianium
supercontinuum source to inject light using an endlessly- single-mode fiber
(SMF) delivering a broadband (from visible to K-band) diffraction-limited PSF
into the facility following the same optical path as the light entering from
the Subaru Telescope and the AO188 facility. This source was used to inject
light into SCExAO (see Fig. 4) reaching the 3D printed MLA facet. Frames with
exposure times of a fraction of a second were acquired with both fibers, the
MCF and the large MMF used as a reference, using the experiment apparatus
described in Section 2.2.6. Dark frames were also recorded, which were
averaged and subtracted from the data frames before further processing.

Two different experiments were performed to calculate the throughput
performance of the IFU; in the first the maximum coupling of light into each
core was calculated by measuring the optimal throughput after centering the
incoming light on the fiber core, and in the second the sensitivity of the
coupling efficiency to mis-alignment of the injected beam was calculated by
measuring the throughput of the central core of the MCF as the injected beam
was laterally displaced by steps of 5 $\upmu$m (9% of the each lenslet
diameter) in order to evaluate the performance of the fiber in a more
realistic scenario, where the target affected by atmospheric perturbations
will be moving.

Through simulation we determined that the MMF coupled all of the injected
light. Therefore, we could use the power coupled into the MMF to normalize the
flux from the MCF. To this end, we scanned the 5-axis stage to optimize the
coupling into each core.

The results were derived using the PIAA optics of SCExAO. The outcome of the
first experiment are shown in Fig. 7. The average throughput was 35.8 $\pm$
1.6% with a maximum equal to 40.7 $\pm$ 2%. From the calculation of the
average the low throughput of the fiber core #2 (12.3 $\pm$ 2%) was excluded
as it was not representative of the rest of the lenslet structure due to
manufacturing errors. The maximum achieved throughput corresponds to 83 per
cent of the theoretically expected value (48.8%) according to Zemax
calculations (see Section 2.2.3). The throughput across all the 7 lenslets
when illuminated by a single, unresolved star was measured to be 70 $\pm$ 3%
while the simulated is 83% (84.8% of the theoretical value The residual
throughput loss of $\sim$8% can be attributed to Fresnel reflections
($\sim$4%), impurities of the imperfectly polished fiber facets (less than few
percent estimated based on past lab experience), and mode-field mismatch at
the focus (3-5%).

Figure 7: Throughput efficiency for each of the 7 cores of the fiber (see Fig.
1 for the correspondence of core# to position on the fiber). The injected
$f/\\#$ was fixed to $f/$57.5 by positioning the 5-axis mount and the fiber to
the desired location.

The results of the second experiment are shown in Fig. 8, which shows that the
mis-alignment tolerances of the injected beam are relaxed using the micro-
lenses, maintaining the throughput to within 35% of the peak even for a
lateral $\sim$20 $\upmu$m off-axis injection.

Figure 8: Throughput efficiency of the central core as a function of lateral
displacement for SCExAO data compared to simulations with Zemax. Results are
normalized to the maximum throughput and error-bars are smaller than the data
points.

###  3.2 Turbulence impact on the throughput

Figures 7 and 8 represent the throughput efficiency of ideal atmospheric
conditions with a nearly perfect wavefront. However, under realistic
atmospheric conditions the performance is reduced as a function of the
injected wavefront errors post AO correction. In order to assess the coupling
efficiency of the starlight into the IFU system the throughput across all of
the 7 cores of the MCF was measured, representative of an unresolved target.
The measurements were performed using the turbulence simulator of SCExAO in
laboratory conditions. A detailed description of the turbulence simulator can
be found in Ref. [25, 31].

The turbulence simulator was configured to produce a range of atmospheric
turbulence amplitudes to replicate a variety of on-sky conditions ranging from
0-200 nm RMS wavefront errors with variable steps of 12.5 and 25 nm. This was
done to address the coupling performance of the IFU under different strehl
ratios. Three types of data frames were captured: 1) PSF with the PIAA, 2) PSF
without the PIAA and 3) dark data images for the background subtraction of the
previous two data frames. Following the data reduction, the SR as a function
of the turbulence amplitude was calculated. Using the data frames without the
PIAA optics, the reduced data frames were compared with a simulated y-band
(960-1080 nm) PSF in order to work out the absolute SR as a sanity check. The
maximum measured SR was found to be $\sim$90% as a result of the image
sampling that is close to the Nyquist criterion in addition to uncorrected
low-order errors left on the optical train. After that, the measured PSF was
used as our reference for the SR derivation with the PIAA. The ten variations
of injected PSFs as well as the calculated SRs as a function of the turbulence
amplitude are presented in Fig. 9 as captured by the internal camera of the
SCExAO. On the top sub-figure of Fig. 9 from its top to middle, the ten
injected PSFs are shown without the PIAA optics while in the middle to bottom
frame the corresponding to those acquired with the usage of PIAA optics on
train.

Figure 9: Top panel: Images of the injected beam at the focal plane of the
internal camera at the SCExAO bench as a function of turbulence amplitude.
This is shown without (top to middle) and with (middle to bottom) the phase-
induced amplitude apodization optics in the optical train. Bottom panel:
Calculated strehl ratio as a function of turbulence amplitude using the phase-
induced amplitude apodization optics (green) and without them for the absolute
(blue) and relative (red) strehl ratio measurements.

The output flux of the IFU was measured using the above mentioned setup in
Section 2.2.6. The results from this experiment are presented in Fig. 10. This
figure shows the relation of the normalized throughput efficiency as a
function of the measured SR. A linear fit was performed to highlight the
linear response of the throughput as a function of the SR. The mathematical
equation of the linear fit at 980 nm is given by $\eta=\acs{SR}\times
1.07-0.01(\%)$, where $\eta$ is the coupling efficiency. This means
theoretically that the coupling efficiency using the SCExAO can be as high as
24-33% provided that a SR of 60-80% is achieved for the fine platescale of
18mas. As the fiber was found to be damaged near the non 3D-printed end, the
following measurements are normalized. This then neglects the damage to the
fiber, whilst showing the performance of the MLAs.

Figure 10: Normalized throughput efficiency as a function of strehl ratio,
calculated using laboratory data in the y-band (960-1080 nm). A linear fit is
overplotted onto the data points and the seeing limited, AO188 and SCExAO
correction regimes are shown.

###  3.3 On-sky photometric results

The on-sky performance of the IFU system was tested on the night of 7th of
October 2020 during a SCExAO engineering run. Consequently, the true
performance of the system was measured under real observing conditions.
However, as mentioned above the IFU cable was damaged and thus the maximum
expected throughput performance was not achieved.

The starlight follows the optical path as described in Section 2.2.2.
Similarly to the laboratory experiments, the PIAA optics were used to apodize
the injected beam.

In order to evaluate the on-sky conditions, right after the end of the
observation, data of the injected PSF was gathered for $\sim$4 min. These data
are from a non-common focal plane with the IFU (see bottom left of Fig. 2).

The photometric image data were acquired using the setup described in Section
2.2.6, (see Fig. 5). The image data were taken with exposure times of a
fraction of a second. Three types of on-sky data frames were gathered: 1)
output data images of the focal plane of the IFU, 2) dark data images for the
background subtraction of the output data images and 3) bias frames for pixel-
to-pixel variation correction. Following that, the averaged bias and dark data
images were subtracted from the output images as well as abnormal pixel values
(hot, dead).

Two different targets were selected to be observed in order to characterize
the efficiency of the IFU system, a target resolved by the Subaru Telescope
and an unresolved one. The selected targets were Mira ($o$ Ceti), as a
representative of resolved target with an angular dimension of $\sim$50 mas
and $\delta$ Psc, an unresolved target with an angular diameter of about
$\sim$4 mas. The atmospheric conditions during the night of observation were
within acceptable values (mean Seeing 0.42′′, mean humidity 30%,
http://mkwc.ifa.hawaii.edu/current/seeing/images/20201007.meteogram.jpg)
resulting in a mean SR of 0.4 in the y-band as calculated.

Numerous image data sets were captured on the photometer on both targets and
the best overall throughput summed from all the 7 cores was 11.9 $\pm$ 2.5%
for the $o$ Ceti and 10.9 $\pm$ 3% for $\delta$ Psc. As mentioned in Section
3.3.1 the laboratory measured throughput across all the 7 lenslets for the
case of a single un-resolved star is 70%. The coupling results in the
experiment were limited by the above mentioned damaged fiber. The photometric
results are presented in Fig. 11. From the figure it can be seen that the
dispersion around the maximum throughput highlights the MLA performance in
keeping the coupling uniform even with the fine platescale of 18 mas. That
produces uniform spectral traces of the individual MCF cores on the
spectrograph detector for a resolved target, while a great portion of the
light of an unresolved target is sampled mostly by the central core of the
fiber.

Figure 11: Calculated on-sky throughput results as a function of the
occurrence rate for $o$ Ceti and $\delta$ Psc.

###  3.4 On-sky spectroscopic results

To evaluate the on-sky performance of the 3D-M3 instrument, it was tested on
the night of 16th of October 2019 during a SCExAO engineering run. Thus, the
true performance of the 3D-M3 was measured under real observing conditions.
However, due to restrictions in available on-sky time, only one exposure of 8
min duration was captured at 6:00 (UTC-11) (see Fig. 12).

Figure 12: Illustration of time vs. airmass/altitude of the observation at
6:01 UTC -10 on the 17th of Oct. 2019 from Mauna Kea Observatory (green
vertical region), for $\upgamma$ Geminorum (solid blue line) and the moon (red
dashed). Exposure started at the very end of observing time, close to sunrise.

As explained in Section 2.2, starlight from the Subaru Telescope enters
through the AO188 [61] into the SCExAO instrument. With the intent of
maximizing the starlight coupling into the IFU, the wavefront sensor (WFS)
loop was closed and the PIAA optics deployed. Specifically, additional
software modules were used to improve the sensitivity by predictive control
applied on the WFS to remove resonances [67] and enhance the wavefront
correction. In addition, PIAA optics apodize the injected beam as was the case
for the laboratory experiments.

After the end of the observation, PSF data of the injected beam was collected
for $\sim$4 min in order to assess the on-sky conditions. However, the
acquired data were from a non-common focal plane with the IFU, so there were
non-common path errors as expected (see bottom left of Fig. 2).

Figure 13: Top panel: Spectra of $\upgamma$ Geminorum from the 7 individual
cores of the MCF in the wavelength range of 925-935 nm where the many telluric
absorption features are visible. Each spectrum is self-normalized and offset
with respect to its intensity for clarity. Bottom panel: 2D detector image of
a part of a spectral order from $\upgamma$ Geminorum. All 7 cores are visible
with sampled starlight. Notice the rotated pattern of the MCF formed by
absorption features marked in red circles. On both axes numbers represent the
detector pixels 4.8$\times$4.8 $\upmu$m.

The spectroscopic image data were acquired with the ASI-183MM-pro mono CMOS
detector with 8 min of exposure time. For the spectroscopic analysis four
types of on-sky data images were collected: 1) output data images were taken
at the focal plane of the spectrograph, 2) dark frames that were used for the
background subtraction of the the output data images, 3) bias frames for
correction of the pixel-to-pixel variations and 4) flat frames for the output
normalization. Following the acquisition of data images, the averaged dark and
bias frames were subtracted from the output images, abnormal (hot, dead)
pixels were removed, and the averaged flat frames were used to normalize the
final spectrum image data. The reduced frame is presented in Fig. 15.

The target was $\upgamma$ Geminorum, which is an A1.5IV+ sub-giant star
expected to be almost free of absorption features, except for broad hydrogen
lines. The resulting low signal to noise ratio (SNR) spectrum (SNR$\sim$9)
from this observation is shown in Fig. 13. In this figure, only the spectrum
over the range of 925-935 nm is presented (908-968 nm is the full range),
focusing on a narrow region where the spectral absorption features that are
caused by the Earth’s atmosphere are most intense. The figure shows the
spectra for all of the 7 cores individually. In the bottom panel of the
figure, the 2D image of a part of the full spectrum is included for visual
clarity of the geometry. The hexagonal pattern of the MCF at optimal rotation
appears as 7 lines for each order.

In the next step the spectra from each of the 7 cores were summed together in
order to further increase the SNR, representative of the case of a single
unresolved star spectrum observation. The resultant combined spectrum is
presented in Fig. 14. The observed spectrum is represented with black color,
with blue color the model spectrum for an $\upgamma$ Geminorum-like star from
the PHOENIX spectral library [68] without the effect of telluric absorption,
and with orange color the PHOENIX model spectrum with telluric absorption as
calculated using ESO’s SkyCalc [69, 70].

Figure 14: Full spectrum of $\upgamma$ Geminorum derived from combining all
the fiber cores overplotted (in blue) with a PHOENIX synthetic star spectrum
[68], and also (in orange) including telluric absorption estimated with ESO’s
SkyCalc [69, 70]. The model spectra were convolved down to R$\sim$30,000 for
the comparison. The two broad features in the stellar spectrum are the Paschen
8 and 9 lines. Figure 15: Image of the spectrograph detector of the $\upgamma$
Geminorum.

###  3.5 Adaptive optics performance

To quantify and evaluate the performance of the AO correction, the on-sky PSF
profile data were captured as explained in Section 3.3.4. The PSF was recorded
in Flexible Image Transport System (FITS) format, with data cubes composed of
1000 instantaneous frames each, where the total exposure time of each data
cube was $\sim$0.83 sec in total. For calibration purposes, additional PSF
data cubes were captured off-sky as well using the internal calibration source
of SCExAO, which follows the same optical path as the starlight (see center
left of Fig. 2) and represents the case of an ideally shaped and stabilized
target. Lastly, dark frames were collected for background subtraction.

Initially, each of the 1000 frames in the data cubes, for off and on-sky data,
were dark current subtracted and averaged. After this, the ratio of the
maximum intensity ($I\mathrm{max}$) to 90% of corresponding flux ($Flux$) was
calculated and stored for all of the data. To approximate the Strehl ratio
during the observation time, the stored maximum to flux ratios for each data-
set (off and on-sky) were divided; this results in

${S=\frac{I\mathrm{max_{on-sky}}}{Flux_{\mathrm{on-
sky}}}}{\frac{Flux_{\mathrm{off-sky}}}{I\mathrm{max_{off-sky}}}}.$ (1)

The result of the calculation was $S$ = 0.2, representing an estimation of the
Strehl ratio on-sky. From this analysis we confirm the visually observed low
Strehl for the spectroscopic observation and highlight the reduced Strehl with
respect to the night when the photometry was collected.

In the next step, the averaged intensity of a data cube was plotted along with
a Gaussian fit. This was done for the laboratory PSF data as well. The
resultant profile and fit are presented in Fig. 16. In the top panel of the
figure the normalized intensity profile of the averaged on-sky data along one
axis, its Gaussian fit and the profile from the laboratory ideal beam
measurements are plotted together. In the bottom panels the 2D image of the
on-sky PSF is plotted in linear and logarithmic scale for better clarity.

Figure 16: Top panel: Intensity profile of the apodized injected point spread
function normalized to its peak using the phase-induced amplitude apodization
optics, for the on-sky and laboratory data. A Gaussian fit was performed to
the on-sky data. Bottom panel: 2D image of the PSF on-sky in linear (left) and
logarithmic (right) color scale for better clarity. The exposure time was
$\sim$0.83 sec.

Following that, the 1000 data images from each data cube of the on-sky data,
were averaged per data cube. Afterwards, the center of mass for each of them
was calculated. In addition, the ratio of the on-sky to the laboratory beam
full width at half maximum (FWHM) was calculated as a function of time through
the exposure to visualize the beam diameter changes as experienced on-sky.
Results are presented in Fig. 17; they suggest that the center of mass shift
of the on-sky beam was rather big. Furthermore the size of the on-sky beam was
twice as large compared to the laboratory one as illustrated in the bottom
panel of the figure. Consequently, the position stability of the PSF was not
representative of the typical SCExAO system performance but it was a
consequence of the atmospheric conditions mentioned in Section 3.3.4 and the
time of the observation.

Figure 17: Top panel: Center of mass shift of the on-sky beam during the
observation. Over-plotted with two circles are the FWHM of the on-sky beam and
the laboratory beam while the dashed hexagon is the effective area of the
central micro-lens. Bottom panel: The ratio of on-sky to the laboratory FWHM
for both detector axes for the same time duration.

##  4 Discussion

###  4.1 Laboratory throughput limitations

One of the crucial benefits of the 3D-printed MLAs is to significantly reduce
the off-axis mis-alignments of the injected beam to a SMF, and as a
consequence the light loss during the free-space coupling is reduced.

The results of the intended lateral beam displacements as shown in Fig. 8
suggest that there is a difference between the expected and the laboratory
measured performance. The difference ranges between 0 and $\sim$27% depending
on the position of the injected beam. This is caused by several factors
sorting them by their importance are, the high sensitivity on the the PIAA
lenses alignment that apodize the beam, the surface roughness of the MLA
structure that is limited by the manufacturing process at the moment and any
mismatch of the injected $f/\\#$.

The actions to reduce the influence of these factors involve, the more
accurate alignment of the PIAA lenses regularly to confirm the position as
well as the shape of the apodized beam. An additional step is the validation
of the injected $f/\\#$ of the beam by imaging it before its entrance in the
IFU. Moreover, a better physical protection layer of the fiber will be
implemented to avoid any future damaged to the fiber when it is exposed in the
telescope environment.

###  4.2 On-sky observation limitations

As explained in Sections 3.3.4 and 3.3.5, the atmospheric conditions
influenced the observation (mean Seeing 0.62′′, high humidity over 70%,
http://mkwc.ifa.hawaii.edu/current/seeing/?night=20191017&lastURLnight=20191017).
Additionally, at the time of the observation (sunrise) the sky brightness was
not negligible and thus it was driving the WFS close to its saturation levels,
leading to an under-performing AO system. Furthermore, the wind was rather
high in strength and variable in direction (16 km/h), inducing vibrations to
the structure of the telescope. Consequently, the stabilization and the shape
of the beam coupled into the MLA facet was far from ideal as shown in Fig. 17
[71]. All the factors mentioned above are represented in the data. In more
detail, the target under observation, which is unresolved by the Subaru
Telescope, and the majority of its light should have been sampled by the
central core of the MCF as inferred from Section 2.2.3. However, as appears in
Fig. 15, the illumination of the fiber cores is almost uniform, resulting in a
loss of spatial information of the input. A likely explanation of this is the
very poor AO performance in correcting the shape and the stabilization of the
injected beam. On the other hand, this demonstrates that our IFU can couple
light from a point source rather efficiently into an SM spectrograph, even
under poor conditions.

The expected performance of the IFU system encountered on-sky can be
reproduced by Zemax simulations. In more detail, the POP module was used to
simulate the performance of one lenslet out of the 7, as the lenslets are
considered homogeneous. Two simulations were performed to calculate the
expected flux output at the end facet of the MCF; in the first one,
representative of the ideal on-sky conditions, we used a Gaussian beam with a
72 $\upmu$m MFD, which is a good approximation of the output beam profile of
the Subaru Telescope, in combination with the SCExAO and PIAA optics, as
described in Section 2.2.1. In the second one, we used a 2$\times$ bigger beam
(144 $\upmu$m) in MFD size, as the input to our simulations (as observed in
Section 3.3.5), which had also a lateral displacement term, relative to the
central on-axis fiber core equal to 11 $\upmu$m (as observed in Fig. 17).

The apparent magnitude of $\upgamma$ Geminorum is taken from the literature
and is approximately equal to 1.88 in NIR wavelengths. The transmission
properties of the sky at Maunakea as well as the performance of all of the
optical components of the telescope down to the injection fiber (without the
spectrograph), for the corresponding optical band, were adapted from the
literature and combined. The throughput of the IFU system was taken from
Section 3.3.1. The resulting maximum flux derived from the simulations for the
first experiment was found to be $1.63\times 10^{9}$ photons/sec in the Y
photometric band (970-1007 nm) where a bandwidth of 104 nm of the
spectroscopic band were used. The resulting throughput for the second
experiment was found to be equal to 2.4% corresponding to an average flux of
$1.04\times 10^{8}$ photons/sec. This is shown in Fig. 18, where the flux
performance of the system up to the IFU level corresponds to 6% of the value
measured in laboratory.

Figure 18: Simulated performance of the system from the star down to the IFU
level, for ideal and the approximated on-sky conditions. Despite the relative
low performance, the IFU manifested decent coupling from a point source into
an SM spectrograph, even under poor conditions.

The analysis above provides a solid explanation of the observed on-sky
performance. Moreover, the on-sky results are useful for predicting the
expected performance of the system for future reference. To extrapolate the
acquired results, given a photometric sky and median seeing conditions when
Strehl ratios are over 60%, the required exposure time for achieving similar
levels of SNR as experienced on-sky, would be 48 secs instead of 8 min.

##  5 Conclusions

In this study we accomplished the first technology demonstration of a high
resolution diffraction-limited integral field spectrograph, 3D-M3. It utilizes
a custom MCF with 3D printed micro-optics on top of its cores in order to
increase the coupling efficiency of starlight from SCExAO at the 8.2 m Subaru
Telescope while the the spatial information of the input is retained.

The injection IFU system is optimized for an on-sky angular dimension of 54
mas, using the output beam profile of SCExAO that makes use of PIAA optics
shaping the output PSF to a near Gaussian profile.

The MLA was directly 3D printed onto the MCF using two-photon lithography,
considerably improving the coupling of light to each of the fiber cores from a
few per cent levels up to a theoretical maximum of 48.8% with the MLA. In
addition to this, the mis-alignment tolerances of the injected beam were
relaxed with this method, performing decently even for a lateral $\sim$20
$\upmu$m off-axis injection.

The throughput results on SCExAO confirm the theoretical expectation. A
maximum of 40.7 $\pm$ 2% of throughput per core was achieved (corresponding to
$\sim$83% of the theoretical value) reaching an average of 35.8 $\pm$ 1.6% per
core. The throughput performance of the IFU system across all the 7 lenslets,
representing the case of a single unresolved target was measured to be 70
$\pm$ 3% (corresponding to $\sim$84.8% of the theoretical value). The
difference between the theoretical and measured values can be attributed to
the Fresnel losses at the interfaces of the MLA with the MCF and to their
surface roughness. These results were obtained with SCExAO and an artificial
light source, which closely represents the performance of the facility under
good observing conditions. A short on-sky observation suffered from windshake
and bad seeing, resulting in considerably lower throughout of less than 2%
compared to the laboratory measurements during the spectrum acquisition.

The on-sky throughput results on SCExAO did not reach the expected performance
as measured in the laboratory due to technical problems. However, demonstrate
the principle and will serve as a baseline for future reference. A maximum
throughput of 10.9 $\pm$ 3% was achieved for the unresolved target while 11.9
$\pm$ 2.5% was measured for the case of the resolved target.

Our system met the expectations and performed reasonably well given the
conditions; the results highlight the importance of good on-sky conditions
allowing optimal performance. If normal photometric conditions are met, the
required exposure time for an observation with an SNR of 20 is reduced by a
factor of ten. Compared to conventional high resolution IFS [72], the achieved
SNR is still low due to the performance limitations of the optical components,
mostly the detector efficiency in this wavelength regime.

Aims for future work include further optimization of the optical design by
performing more simulations using Zemax, replacing the spectrometer optics
with custom built ones using appropriate coatings for the wavelength range
used, and investing in the stabilization of the instrument with enclosures and
controls.

Observation aims for the future involve a list with a variety of targets
(resolvable star, non-resolvable, double star system, massive confirmed
exoplanets, spectroscopic standard stars) for evaluating its scientific
potential.

##  6 Backmatter

Acknowledgments T.A. is a fellow of the International Max Planck Research
School for Astronomy and Cosmic Physics at the University of Heidelberg
(IMPRS-HD) and is supported by the Cotutelle International Macquarie
University Research Excellence Scholarship. M.T., M.B., Y.X. and C.K. are
supported by Bundesministerium für Bildung und Forschung (BMBF), joint project
PRIMA (13N14630), the Helmholtz International Research School for Teratronics
(HIRST), Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)
under Germany’s Excellence Strategy via the Excellence Cluster 3D Matter Made
to Order (EXC2082/1-390761711). R.J.H., P.H. and A.Q. are supported by the
Deutsche Forschungsgemeinschaft (DFG) through project 326946494, ’Novel
Astronomical Instrumentation through photonic Reformatting’. T.B. & S.Y. are
supported from the European Union’s Horizon 2020 grant 730890, and from the UK
Science and Technology Facilities Council grant ST/N000544/1. S.Y.H. is
supported by 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. The development of SCExAO was supported by the JSPS (Grant-in-Aid
for Research #23340051, #26220704 #23103002), the Astrobiology Center (ABC) of
the National Institutes of Natural Sciences, Japan, the Mt Cuba Foundation and
the directors contingency fund at Subaru Telescope, and the OptoFab node of
the Australian National Fabrication Facility. The authors wish to recognize
and acknowledge the very significant cultural role and reverence that the
summit of Mauna Kea has always had within the indigenous Hawaiian community.
We are most fortunate to have the opportunity to conduct observations from
this mountain. This research made use of Astropy, a community-developed core
Python package for Astronomy [73, 74], Numpy [75] and Matplotlib [76].

Disclosures The authors declare no conflicts of interest.

Data Availability Statement Data underlying the results presented in this
paper are not publicly available at this time but may be obtained from the
authors upon reasonable request.

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  *[SCExAO]: Subaru Coronagraphic Extreme Adaptive Optics
  *[NIR]: near-infrared
  *[IFU]: integral field unit
  *[IFS]: integral field spectroscopy
  *[MMFs]: multi-mode fiber
  *[SMFs]: single-mode fiber
  *[AO]: adaptive optics
  *[PSF]: point spread function
  *[ExAO]: extreme adaptive optics
  *[mas]: milli-arcseconds
  *[RV]: radial velocity
  *[SM]: single-mode
  *[IFUs]: integral field unit
  *[MMF]: multi-mode fiber
  *[MCF]: multi-core fiber
  *[RHEA]: replicable high-resolution exoplanet and asteroseismology spectrograph
  *[MLA]: micro-lens array
  *[MLAs]: micro-lens array
  *[3D-M3]: 3D-printed mono-mode multi-core fiber spectrograph
  *[PIAA]: phase-induced amplitude apodization
  *[SMF]: single-mode fiber
  *[FOV]: field of view
  *[MFD]: mode-field diameter
  *[OAP]: off-axis parabola
  *[POP]: physical-optics propagation
  *[RMS]: root-mean-square
  *[NA]: numerical aperture
  *[LED]: light-emitting diode
  *[PGMEA]: propylene-glycol-methyl-ether-acetate
  *[SEM]: scanning electron microscopy
  *[VSI]: vertically-scanned white-light interferometry
  *[SR]: strehl ratio
  *[PSFs]: point spread function
  *[SRs]: strehl ratio
  *[WFS]: wavefront sensor
  *[SNR]: signal to noise ratio
  *[FITS]: Flexible Image Transport System
  *[FWHM]: full width at half maximum