Ultrafast Laser Inscription of a 121-Waveguide
Fan-Out for Astrophotonics
R. R. Thomson1*, R. J. Harris2, T. A. Birks3, G. Brown1, J. Allington-Smith2 and J. Bland-
1Scottish Universities Physics Alliance (SUPA), School of Engineering and Physical Sciences, Department of Physics,
Heriot-Watt University, Riccarton Campus, Edinburgh, EH14 4AS, UK
2Centre for Advanced Instrumentation, Physics Department, Durham University, Durham DH1 3LE, UK
3Department of Physics, University of Bath, Claverton Down, Bath, BA2 7AY, UK
4Institute of Photonics and Optical Science, School of Physics, University of Sydney, NSW 2006, Australia
5Sydney Institute for Astronomy, School of Physics, University of Sydney, NSW 2006, Australia
*Corresponding author: R.R.Thomson@hw.ac.uk
Using ultrafast laser inscription, we report the fabrication of a prototype three-dimensional 121-waveguide fan-out device
capable of reformatting the output of a 120 core multicore fiber (MCF) into a one-dimensional linear array. When used in
conjunction with an actual MCF, we demonstrate that the reformatting function using this prototype would result in an
overall throughput loss of ≈ 7.0 dB. However, if perfect coupling from the MCF into the fan-out could be achieved, the
reformatting function would result in an overall loss of only ≈ 1.7 dB. With adequate development, similar devices could
efficiently reformat the output of so-called “photonic lanterns” fabricated using highly multicore fibers.
OCIS Codes: (130.3120) Integrated optics devices; (140.3390) Laser materials processing.
Astrophotonics is the field where photonic principles are
applied to instrumentation for astronomy. The aim of this
field is to reduce the cost, size and weight of instruments,
while increasing their functionality and performance.
Astrophotonic technologies currently under development
include integrated beam
interferometry  and photonic spectrometers for
applications in multi-object spectroscopy .
One area where astrophotonics may make a
significant impact is in the suppression of the fluorescence
lines at around ≈1.5 µm generated by OH-radicals in the
upper atmosphere . Observations in this spectral region
(the astronomical J- and H-bands) are of key interest to
many areas of modern astronomy e.g. high-redshift
observations for studies of dark matter and dark energy.
Several OH-line suppression methods have been
explored, including the use of interference filters  and
gratings masks , but each of these suffers from severe
drawbacks. Currently, the most promising OH-line
suppression route is using complex single mode (SM) fiber
Bragg-gratings (FBGs) [6, 7]. Through careful design,
such FBGs can, in principle, reflect >100 individual lines
at high resolution (R = λ/∆λ ≈ 10,000) , while preserving
the transmission of the dark inter-line continuum of
At first sight, however, there is a problem with
the astrophotonic approach to OH-line suppression; in
order to achieve acceptable coupling efficiencies from the
telescope into an optical fiber it is necessary to use highly
multimode (MM) fibers which are not suitable for FBG
applications. To facilitate the use of FBGs for OH-line
suppression, it is therefore necessary to couple the light
from the MM fiber into SM-FBGs. However, the
brightness theorem (fundamentally the second law of
thermodynamics) prevents the efficient coupling of light
combiners for stellar
from an arbitrarily excited MM fiber into a single SM
fiber. To address this issue, the so-called photonic lantern
(PL) was developed [9-12] – a remarkable guided wave
transition which facilitates the efficient adiabatic coupling
of light between a MM fiber (or waveguide) and an array
of SM fibers (or waveguides).
Initially, PLs were fabricated using stack-and-
draw techniques [9, 10] which are labor intensive and
expensive – a considerable drawback if PLs are to be
mass-produced for instruments on future Extremely
Large Telescopes (ELTs). Recently, however, efficient PLs
have also been demonstrated using two additional routes;
the first using three-dimensional (3D) ultrafast laser
inscription , the second using tapered multicore fibers
(MCFs) consisting of many SM guiding cores – such as the
120-core MCF shown in Fig. 1(a) .
Although the ultrafast laser inscription route is
particularly promising for certain applications, including
high mode count PLs and PLs for regions of the spectrum
where silica is unsuitable, the MCF route has already
been shown to facilitate low-loss lantern transitions
involving > 100 modes. Unfortunately, however, the SM’s
generated by the MCF-lantern are necessarily arranged
into a two-dimensional array, which is undesirable for
many applications. For example, the PIMMS instrument
concept  is being developed for applications in multi-
object spectroscopy. In this case, a PL is used to couple
light from the telescope into a planar arrayed waveguide
grating for spectral analysis. It is clear, therefore, that a
photonic interconnect is required which is capable of
reformatting the two-dimensional array of SM’s generated
by the MCF-PL into a linear array of SM’s.
As a step towards properly addressing the
interconnect issue, we have fabricated a prototype 3D
integrated optical waveguide fan-out device - a conceptual
sketch of which is shown in Fig. 2. The axes shown in Fig.
1(b) and Fig. 2 are consistent, and will be referred to in
the rest of the paper. The fan-out consists of a 3D network
of 121 optical waveguides. At one end, the waveguides are
arranged to match the core geometry of the 120 core MCF
shown in Fig. 1(a) (with an additional spare core). At the
opposite end, the cores are arranged into a linear array
with an inter-waveguide spacing of 50 µm.
The fan-out was fabricated using ultrafast laser
inscription (ULI) . Structures were inscribed using an
ultrafast Yb-doped fiber laser system (IMRA FCPA
µ−Jewel D400). The system supplied 340 fs pulses of 1047
nm radiation at a pulse repetition frequency of 500 kHz.
The substrate material (Corning EAGLE2000) was
mounted on x-y-z air−bearing translation stages (Aerotech
ABL1000) which facilitated the smooth and precise
translation of the sample through the laser focus. The
polarization of the laser beam was adjusted to be circular
and was focused below the surface of the substrate
material using an aspheric lens with a numerical
aperture of 0.6. All waveguides were inscribed using a
translation velocity of 8.0 mm.s-1 and 210 nJ pulses. The
cross section of the waveguides was controlled using the
well-known multiscan technique . Consequently, each
waveguide was fabricated using 20 scans of the material
through the laser focus, with each scan offset from the
previous scan by 0.4 µm in the x-axis.
As shown in Fig. 1(b), the MCF end of the fan-out
consisted of 23 columns of waveguides. The paths of the
waveguides were controlled such that adjacent columns
became adjacent sections of the linear array, and such
that adjacent waveguides in each column remained
adjacent in the linear array. Waveguides further up each
column (more positive z-axis position) were moved into
lower numbered positions in the linear array (see Fig. 2).
Starting from the linear array end of the fan-out, each
waveguide first moved into its final z-axis position, by
moving only in the z-y plane, and then into its final x-axis
position, by moving only in the x-y plane. These
movements were performed using two connected S-bends,
each of which was formed using two arcs of 40 mm radius.
For all of the waveguides, the S-bends were initiated at
the same y-axis position on the sample. After inscription,
the fan-out ends were ground and polished to expose the
The fan-out was characterized using 1.55 µm
light. In all cases the light was injected into the fan-out
using a Corning SMF-28 fiber butt-coupled to the
waveguide facet. Initially, the light was coupled into the
MCF coupling end of the fan-out. Near field imaging
indicated that the waveguides were strongly coupled, and
that they each supported a single transverse mode with
1/e2 mode field diameters (MFDs) of 11 ± 0.1 µm and 7 ±
0.1 µm in the x- and z-axis respectively. The strong
coupling prevented us from measuring the MFDs of the
waveguides at the MCF coupling end of the fan-out.
To measure the performance of the fan-out
waveguides quantitatively, light was coupled into each
waveguide individually at the linear array end, and the
transmitted light was collected at the MCF coupling end
using a highly MM, 600 µm core diameter fiber directly
butt-coupled to the fan-out. This large core diameter fiber
was used in order to capture all the light guided by the
coupled waveguide array. The light collected by the MM
fiber was measured at the other end of the MM fiber
using a germanium detector. Index matching gel was
used at all fiber-waveguide interfaces in order to reduce
Fresnel reflections. For our purposes, the insertion loss of
each waveguide was defined as the difference in signal
measured when the SMF-28 fiber and MM fiber were
directly coupled to each other, compared to when they
were each coupled to the fan-out.
Fig. 3 presents the results of the insertion losses
measurements. It can be seen that the insertion losses of
the waveguides vary between 1.6 dB and 3.26 dB.
Summing across the measurements for all waveguides,
the fan-out exhibits a total throughput loss of 2.0 dB.
Using the Gaussian field approximation , the
minimum coupling loss due to mode-mismatch between
the fan-out waveguides and the SMF-28 fiber (which
exhibits a MFD of 10.4 ± 0.8 µm) is ≈ 0.35 ± 0.15 dB.
These results indicate that if negligible coupling losses
could be achieved between the MCF and the fan-out, this
fan-out would exhibit a total throughput loss of ≈ 1.7 dB.
To connect the MCF to the fan-out, the MCF was
glued into a glass V-groove and polished back. To align
the MCF with the fan-out, we first illuminated all cores of
the MCF using white light coupled into the MCF at the
opposite end from the V-groove. We then butt-coupled the
MCF+V-groove to the fan-out, and viewed the light
transmitted by the fan-out as the rotation and position of
the MCF was adjusted. Once an apparently optimal
position of the MCF had been achieved, we secured the
MCF in place using UV curing epoxy.
The insertion loss
combination was tested by coupling light into each fan-out
waveguide at the linear array end. We defined the
insertion loss in this case as the difference in signal
measured when the input coupling SMF-28 fiber was
coupled directly to the detector, compared to when the
light was coupled into the fan-out and the light emerging
from the unconnected end of the MCF was measured. The
results of these measurements are also shown in Fig. 3,
where it can be seen that the insertion loss of the fan-
out+MCF varies considerably between 5.0 and 13.9 dB.
Summing across the measurements for all waveguide
inputs, we calculate that the fan-out+MCF combination
exhibits an overall throughput loss of 7.0 dB.
Clearly, the coupling losses between the fan-out
and the MCF are responsible for the lowered throughput
of the fan-out+MCF combination. The MCF cores were
known to support rotationally symmetric single guided
modes with a 1/e2 diameter of ≈ 6.6 µm. Again, using the
Gaussian field approximation , we estimate that the
minimum coupling loss due to mode-mismatch between
the fan-out waveguides and the MCF cores is ≈ 0.6 dB.
The mode-mismatch is, therefore, insufficient on its own
to explain the lowered throughput of the fan-out+MCF
combination, and we conclude that alignment accuracy
between the fan-out waveguides and the MCF cores is
primarily responsible. From Fig. 3, for example, it can be
seen that 23 of the cores exhibit an insertion loss > 10 dB.
Of these 23 cores, 22 are in the bottom (lowest z position)
four rows of cores in the MCF coupling end of the fan-out.
of the fan-out+MCF
This is clear evidence that spatial alignment error is
primarily responsible for the low throughput, which can
be readily addressed in the future through the
implementation of improved alignment techniques.
Finally, for the final OH-line suppression
application we require total throughput losses < 1.0 dB.
Currently, the individual waveguides are close to
achieving this level of performance, but the alignment
between the MCF and the fan-out must be improved
significantly. To conclude, we have demonstrated that
ultrafast laser inscribed fan-out devices are a promising
way for reformatting the output of MCF-PLs into a one-
dimensional array of single modes. The full development
of such devices is therefore crucial to future applications of
mass-producible MCF-PLs in astronomy.
This work was funded by the UK – STFC through RRT’s
Advanced Fellowship (ST/H005595/1) and by the UK –
EPSRC (EP/G030227/1). TAB thanks the Leverhulme
Trust for a Research Fellowship, Brian Mangan for help
in fiber fabrication and the Astrophysikalisches Institut
Potsdam for providing materials. JBH is supported by an
Australian Research Council Federation Fellowship. RRT
thanks A. K. Kar for access to the ULI facility.
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Fig. 1 Optical micrographs of (a) The cleaved end-facet of
the 120 core multicore fiber. (b) The multicore fiber
coupling end of the three-dimensional waveguide fan-out.
Fig. 2 Sketch of the three-dimensional waveguide fan-out
device. The position of waveguide 1 (WG1) is indicated in
the sketch. The waveguide number then increases
sequentially along the linear array.
Fig. 3 Insertion losses measured for the individual fan-out
waveguides and the fanout+MCF combination.