Low bend loss waveguides enable compact,
efficient 3D photonic chips
Alexander Arriola,1,2,3* Simon Gross,1,2 Nemanja Jovanovic,1,4,5
Ned Charles,6 Peter G. Tuthill,6 Santiago M. Olaizola,3
Alexander Fuerbach,1,2 and Michael J. Withford1,2,4
1MQ Photonics Research Centre, Dept. of Physics and Astronomy, Macquarie University, NSW 2109, Australia
2Centre for Ultrahigh-bandwidth Devices for Optical Systems (CUDOS), Australia
3 CEIT and Tecnun, University of Navarra, Pº Manuel de Lardizabal 15,Donostia-San Sebastian, 20018, Spain
4Macquarie University Research Centre in Astronomy, Astrophysics and Astrophotonics, Dept. of Physics and
Astronomy, Macquarie University, NSW 2109, Australia
5 Australian Astronomical Observatory (AAO), PO Box 296, Epping NSW 1710, Australia
6Sydney Institute for Astronomy (SIFA), School of Physics, University of Sydney, 2006, Australia
Abstract: We present a novel method to fabricate low bend loss
femtosecond-laser written waveguides that exploits the differential thermal
stabilities of laser induced refractive index modifications. The technique
consists of a two-step process; the first involves fabricating large multimode
waveguides, while the second step consists of a thermal post-annealing
process, which erases the outer ring of the refractive index profile, enabling
single mode operation in the C-band. By using this procedure we
report waveguides with sharp bends (down to 16.6 mm radius) and high
(80%) normalized throughputs. This procedure was used to fabricate an
efficient 3D, photonic device known as a “pupil-remapper” with negligible
bend losses for the first time. The process will also allow for complex chips,
based on 10's - 100's of waveguides to be realized in a compact foot print
with short fabrication times.
References and links
K.M. Davis, K. Miura, N. Sugimoto, and K. Hirao, “Writing waveguides in glass with a femtosecond laser,”
Opt. Lett. 21, 1729-1731 (1996).
M. Ams, G. D. Marshall, P. Dekker, J. A. Piper, and M. J. Withford, “Ultrafast laser written active devices,”
Laser Photonics Rev. 3, No. 6, 535–544 (2009).
R. R. Gattass, and E. Mazur, “Femtosecond laser micromachining in transparent materials,” Nat. Photonics 2,
S. Nolte, M. Will, J. Burghoff, & A. Tuennermann, "Femtosecond waveguide writing: a new avenue to three-
dimensional integrated optics," Appl. Phys. A 77, 109–111 (2003).
R. Osellame, H. J. W. M. Hoekstra, G. Cerullo, and M. Pollnau, “Femtosecond laser microstructuring: an
enabling tool for optofluidic lab-on-chips,” Laser Photonics Rev. 5, No. 3, 442–463 (2011).
Y. Liao, J. Song, E. Li, Y. Luo, Y. Shen, D. Chen, Y. Cheng, Z. Xu, K. Sugiokad, and K. Midorikawa, “Rapid
prototyping of three-dimensional microfluidic mixers in glass by femtosecond laser direct writing,” Lab Chip
12, 746-749 (2012).
R. Osellame, V. Maselli, R. M. Vazquez, R. Ramponi, and G. Cerullo, “Integration of optical waveguides and
microfluidic channels both fabricated by femtosecond laser irradiation,” Appl. Phys. Lett. 90, 231118 (2007).
G. D. Marshall, A. Politi, J. C. F. Matthews, P. Dekker, M.Ams, M. J. Withford, and J. L. O’Brien, “Laser
written waveguide photonic quantum circuits,” Opt. Express 17, 12546–12554 (2009).
A. Crespi, R. Ramponi, R. Osellame, L. Sansoni, I. Bongioanni, F. Sciarrino, G. Vallone, and P. Mataloni,
“Integrated photonic quantum gates for polarization qubits,” Nat. Commun. 2, 566 (2011).
10. N. Jovanovic, P. G. Tuthill, B. Norris, S. Gross, P. Stewart, N. Charles, S. Lacour, M. Ams, J. S. Lawrence, A.
Lehmann, C. Neil, G. Robertson, G. Marshall, M. Ireland, A. Fuerbach, and M. J. Withford, “Starlight
demonstration of the dragonfly instrument: an integrated photonic pupil-remapping interferometer for high-
contrast imaging,” Mon. Not. R. Astron. Soc. 427, 806–815 (2012).
11. R. R. Thomson, A. K. Kar, and J. Allington-Smith, “Ultrafast laser inscription: an enabling technology for
astrophotonics,” Opt. Express 17, 1963-1969 (2009).
12. D. J. Little, M. Ams, S. Gross, P. Dekker, C. T. Miese, A. Fuerbach, and M. J. Withford, “Structural changes in
BK7 glass upon exposure to femtosecond laser pulses,” J. Raman Spectrosc. 42, 715–718 (2011).
13. S. M. Eaton, H. Zhang, P. Herman, F. Yoshino, L. Shah, J. Bovatsek, and A. Arai, “Heat accumulation effects
in femtosecond laser-written waveguides with variable repetition rate,” Opt. Express 13, 4708–4716 (2005).
14. N. Jovanovic, S. Gross, C. Miese, A. Fuerbach, J. Lawrence, and M. J. Withford, “Direct laser written
multimode waveguides for astronomical applications,” Proc. SPIE 7739, 773923 (2010).
15. S. M. Eaton, H. Zhang, M. Li Ng, J. Li, W-J. Chen, S. Ho, and P. Herman, “Transition from thermal diffusion to
heat accumulation in high repetition rate femtosecond laser writing of buried optical waveguides,” Opt. Express
16, 13, 9443-9458 (2008).
16. N. Charles, N. Jovanovic, S. Gross, P. Stewart, B. Norris, J. O’Byrne, J. S. Lawrence, M. J. Withford, and P. G.
Tuthill, “Design of optically path length matched, three-dimensional photonic circuits comprising uniquely
routed waveguides,” App. Opt. 51, 27, 6489-6497 (2012).
17. H. E. Hagy, “Fine annealing of optical glass for low residual stress and refractive index homogeneity,” App.
Opt. 7, 5, 833-835 (1968).
18. S. Kanehira, K. Miura, and K. Hirao, “Ion exchange in glass using femtosecond laser irradiation,” App. Phys.
Lett. 93, 2 (2008). http://dx.doi.org/10.1063/1.2959820.
19. M. Shimizu, M. Sakakura, M. Ohnishi, M. Yamaji, Y. Shimotsuma, K. Hirao, and K. Miura, “Three-
dimensional temperature distribution and modification mechanism in glass during ultrafast laser irradiation at
high repetition rates,” Opt. Express 20, 2, 934-940 (2012).
20. N. Jovanovic, I. Spaleniak, S. Gross, M. Ireland, J. S. Lawrence, C. Miese, A. Fuerbach, and M. J. Withford
“Integrated photonic building blocks for next-generation astronomical instrumentation I: the multimode
waveguide,” Opt. Express 20, 15, 17029-17043 (2012).
21. A. W. Snyder and J. D. Love, Optical waveguide theory (Chapman and Hall, 1983).
In 1996, Davis et al.  first demonstrated that a tightly focused femtosecond laser pulse
could induce a highly localized and permanent refractive index change in a transparent
material, which could then be used to fabricate waveguides. Since then the field of
femtosecond laser direct-writing [2-4] has received growing attention from a variety of real-
world applications including optofluidics [5-7], quantum optics [8,9] and astrophotonics [10-
11]. The femtosecond laser direct-write process can be divided into two different regimes.
Using low pulse repetition rates (kHz), a repetitive modification of the material occurs
typically creating a Gaussian-like refractive index profile in commonly used materials such as
fused silica . This refractive index profile is ideal for guiding light as its behavior is
similar to a step-index profile, but the drawback is that for complex devices long fabrication
times are required (1-100 hours) due to the low sample translation speeds (typically tens of
microns per second) involved. In contrast, by using high pulse repetition rates (MHz) an
accumulation of heat occurs because the inter-pulse spacing is shorter than the thermal
diffusion time of the material, resulting in local melting, strong heat diffusion followed by
rapid quenching of the material due to the high translation speeds (typically millimeters per
second). Under these conditions, fabrication times are greatly reduced. However the high
repetition rate regime typically creates complex refractive index profiles in multicomponent
silicate glasses (which are most commonly used in this regime [13-14]). Figure 1 shows a
micrograph image of a typical single-mode waveguide at 1550 nm written with high repetition
rates in alkaline earth boro-aluminosilicate glass (Corning Eagle2000). The mode field
diameter (MFD) is 10.7 × 10.0 µm which is a close match to the nominal 10.4 µm MFD of
Typically, such index profiles exhibit a Gaussian-like central region of modification with a
strong positive index contrast (bright white region in Fig. 1(a)) surrounded by a uniform ring
with a lower but still positive index contrast [13-15] and a depressed cladding region (i.e. a
region with a negative index change) in between (dark region in Fig. 1(a)). Such index
profiles work well for straight waveguides but exhibit large losses for tightly bent waveguides
compared to a step-index waveguide with similar MFD . The requirement of large bend
radii in order to obtain a high throughput imposes a limit on the complexity and the footprint
of the circuit that can be realized. This fundamental shortcoming motivated the research
leading to the solution presented here.
Fig. 1: (a) Micrograph image, (b) Mode field profile at 1550 nm and (c) Refractive index profile (at 635
nm) for a waveguide written with high repetition rates in Eagle2000 glass (pulse energy = 40 nJ).
To address the bend-loss limitations of high-repetition rate waveguides we have
augmented the fabrication process. The technique now consists of a two-step process: the
fabrication of large multimode waveguides (3-4 times larger in diameter than the usual single-
mode waveguide) and a subsequent annealing stage. In the second step of the process, we
exploit the different thermal stabilities between the various regions of the cumulative heating
written structure. These different thermal stabilities enable the erasure of the structure’s outer
ring and the removal of residual laser induced stress. As a result, we obtain a single modified
region of slightly smaller dimensions than that of a typical 40 nJ single-mode guide with two
critical improvements: firstly, as the initial multimode guide is written with higher pulse
energies, the remaining core has a larger index contrast (therefore, a smaller MFD can be
achieved to further reduce the bend losses) and secondly, as the outer ring of index change is
removed, the index profile post annealing becomes Gaussian-like in shape.
In 2008, Eaton et al.  reported the effect of thermal annealing on the physical
dimensions and MFDs of waveguides written in the cumulative heating regime. However, to
the best of our knowledge, there has not been any demonstration of improving waveguide
performance by exploiting such techniques (or the different thermal stability regions of the
glass). We demonstrate that by applying the process described above, waveguides with low
bend losses at small radii become possible, bringing rapid prototyping of efficient 3D
photonic devices closer to reality.
The paper is structured in the following way. In section 2 we will describe the fabrication
technique and subsequent annealing procedure in detail. Section 3 summarizes the
characterization of the waveguides and leads into a discussion while Section 4 shows a real
world application. Concluding remarks are made in Section 5.
The waveguides were inscribed using an ultrafast Ti:sapphire oscillator (800 nm center
wavelength) which emits laser pulses with a duration <50 fs at a repetition rate of 5.1 MHz
(FEMTOSOURCE XL 500, Femtolasers GmbH). The circularly polarized laser beam was
focused by a 100× oil immersion objective lens (Zeiss N-Achroplan, 1.25 Numerical
Aperture, 450 µm physical working distance) into an alkaline earth boro-aluminosilicate glass
substrate (Corning Eagle2000). The sample was placed on a set of 3-axis Aerotech air-bearing
translation stages to achieve smooth translation during inscription. To measure the bend
losses, arcs spanning 90° with radii ranging from 6.6 - 40 mm in eleven steps were inscribed
into a sample with the dimensions 40×40×1.1 mm as seen in Fig. 2(a). Furthermore, the
sample also contained 2 straight waveguides for propagation loss normalization purposes.
Figure 1 shows the layout of the chip.
Fig. 2: (a) Schematic of the chip layouts. (b) Temperature profile for the rate annealing process. The process was
done in 3 steps: first, heating the chip to 600 °C in 6 hours (rate: 100°C/h), then to 750°C in 2 additional hours
(75°C/h) and last cooling the chip down to 18°C in 120 hours (-6 °C/h).
In order to fabricate single-mode waveguides at 1550 nm, the typical fabrication
parameters are ~40 nJ pulse energy and 250-1500 mm/min translation velocities, resulting in
circularly cross-sectioned waveguides with physical diameters of 12 µm as shown in Fig. 1.
However, for the novel fabrication process presented here, waveguides were written with the
greater pulse energy of 90 nJ and a translation speed of 500 mm/min, which resulted in large
diameter (~30 µm) multimode waveguides. After fabrication the sample was ground and
polished to reveal the waveguide ends.
Once the sample was polished, we administered a thermal treatment in order to erase the
outer ring of the refractive index modification. A rate annealing process  was chosen for
this purpose (in an air environment and room pressure), which is based on initially heating the
sample above the transformation temperature in order to initiate an erasure and stabilization
process and is shown in Fig. 2(b). Once the maximum temperature has been reached (which is
above the annealing point: 722°C for Eagle2000), a very slow cooling gradient is applied until
the glass is cooled below the strain point or transformation temperature (666°C), in order to
ensure that the glass cools adiabatically. The key feature of this annealing process type is the
slow cooling rate as it allows for the stress and birefringence to be removed as well. The
profile we used consisted of heating the chip to 600 °C in 6 hours (rate: 100°C/h), then up to
750°C in an additional 2 hours (75°C/h) before cooling it down to 18°C in 120 hours (-6°C/h).
Figure 2(b) shows the corresponding temperature profile. It is worth mentioning that this
annealing procedure was administered at temperatures well below the softening point of the
glass which is 985°C for Eagle2000.
After the thermal annealing process, waveguides were imaged under a transmission
microscope (Olympus IX81) to analyze the physical changes produced. Bright field images
before and after annealing are shown in Fig. 3(a) and 3(b) respectively. It can clearly be seen
that the surrounding ring of index modification has been erased, leaving behind a waveguide
that consists of a positive refractive index core surrounded by a narrow depressed region (dark
ring) only. In order to determine whether the waveguides were truly single-mode, the mode
emanating from the guides was imaged on a camera while the injection was offset in an
attempt to excite higher order modes. Light at 1550 nm was launched into the waveguides
using a SMF-28 fiber and the mode fields were imaged with an IR camera (Spiricon
SP1550M). By using this technique it was determined that indeed the waveguides were
single-mode. Figure 3(c) shows an image of a mode from a representative waveguide with a
horizontal and vertical MFD of 8.5 and 10 µm, respectively (note that the subfigure has a
different horizontal and vertical scaling as compared for Fig. 3(a) and (b)). As can be seen all
the energy is confined within the central core of the waveguide. These MFDs result in a
mode-mismatch coupling loss of 0.25 dB to a SMF-28 fiber calculated by numerically
evaluating the mode overlap integral.
Fig. 3: (a) Bright field images of the waveguides before and (b) after annealing and the corresponding mode-
field for the annealed waveguides (8.5 × 10 µm MFD) (c). Note that a different scale bar has been used in Figure
3(c) for better visualization.
3. Waveguide characterization
In order to quantify the physical changes induced by the writing laser and the influence of
the differential post-annealing process, refractive index profiles were measured using a
refracted near-field profilometer (Rinck Elektronik) at 635 nm before and after annealing (see
Fig. 4). The initial refractive index profile exhibits a maximum index change of 9.7×10-3 in
the central region surrounded by a strip of ∆n=-1×10-3, which is enclosed by a cladding with a
nearly uniform index change of ~ 3.2 ×10-3. After annealing, it is clear that almost all traces of
the outer cladding are gone (Fig. 4(b)). Furthermore the peak refractive index of the central
region is reduced by 1.3×10-3 to a peak value of 8.4×10-3 along with the depressed region that
has increased in magnitude to ∆n=-2x10-3. The erasure of the outer refractive index
modifications while the core index contrast is almost entirely preserved indicates that the
structural changes of the glass network induced by the laser are of a different nature in the
core and in the outer regions. This is related to the fact that the different regions are exposed
to different thermal conditions during the fabrication process. Little et al. investigated the
femtosecond laser induced structural changes in a multicomponent silicate glass (Schott BK7)
in two different regimes . Firstly, at low repetition rates, which corresponds to a repetitive
modification of the glass network at lower temperatures and secondly at high repetition rates,
which induces higher temperatures due to cumulative heating of the irradiated volume. In both
regimes a positive refractive index change was found. However, at low repetition rates/low
temperatures the origin of refractive index change was related to a change in polarizability,
due to more ionic bonds, whereas at high repetition rates/high temperatures the origin of
refractive index change was related to densification and rarefraction of the glass network .
This change of density is associated with the redistribution of elements, where network
modifiers migrate out of the central region . However, the temperatures involved in the
post-annealing process are insufficient for reversing the elemental migrations  thereby
preserving the core of the refractive index modification. Conversely the removal of the outer
ring indicates that the associated index change is due to molar refractivity, which is
susceptible to erasure at the temperatures reached in this annealing process .
Fig. 4: Refractive index profiles of a 90 nJ waveguide measured before (a) and after annealing (b).
The throughputs of the chip were measured at 1550 nm by butt-coupling a single-mode
fiber to the input of each waveguide using immersion oil while the light was collected with a
power meter at the output end of the waveguide. A collection fiber was not used to route the
light to the power meter at the output because the output plane of the waveguides was at 90°
to the injection plane, which isolated the emerging signal from stray light in the block. The
transmitted powers were normalized against the injected power and losses due to coupling,
propagation and bulk glass absorption were subtracted in order to isolate the bend losses. The
coupling loss between the waveguides and SMF-28 fiber was calculated to be 6 ± 2% by
numerically evaluating the mode-overlap integral, and the absorption loss in Eagle2000 glass
was accounted for with an absorption coefficient of α = 0.0075 ± 0.0003 mm−1 at 1550 nm
. Finally, the bend losses were rescaled to a 30 mm length, which corresponds to the
practical length of a device and are plotted in Fig. 5 (red dots). For comparison we have also
replotted the data recently reported by Charles et al. , for a chip fabricated with identical
specifications (number of guides, bend radii, length of guides, etc) which utilized non-
annealed cumulative heating waveguides written at 40 nJ instead (black dots). The data from
Charles et al. was normalized using the same procedure.
It can be seen that, for small bend radii (<20 mm), the throughput of the annealed 90 nJ
waveguides is far superior to that of the non-annealed 40 nJ guides. In particular the 50%
transmission point is found at a radius of 23.3 and 13.3 mm for the 40 and 90 nJ guides,
respectively. We attribute this to two factors: Firstly, the central core region has a greater
index contrast due to the higher pulse energies used for inscription. Secondly, the index
profile now only consists of a Gaussian-like region, which has a similar performance to a
step-index profile which is known to guide well around bends . In addition for large bend
radii (>20 mm), it can be seen that the throughput for the annealed waveguides plateaus at
approximately 91%, which means that near-lossless soft bends can be used in circuit designs
with minimal penalty. This was clearly not the case for the non-annealed guides which show
greater losses even in this large bend radius regime.
To validate the trends in the data, bend loss simulations of the fabricated waveguides were
conducted. A commercial beam-propagation software, BEAMProp by RSOFT, was used. For
the simulations, the core radius of the waveguides was set to 4.85 µm, the wavelength to
1550 nm and the background refractive index of the glass to 1.4877 (the measured bulk
refractive index of Eagle2000 at 1550 nm). The measured refractive index profiles of the
annealed and non-annealed waveguides were then inserted into the software. The bend losses
were modeled by using the “simulated bends” function available in the beam propagation tool.
Results were then rescaled to a 30 mm length for comparison with the measured data. The
simulated throughputs for the annealed and non-annealed guides are displayed in Fig. 5 as a
solid red and black line respectively. For completeness we simulated a step-index
waveguide/fiber with the same core size and an index contrast of 5.3 × 10-3 (the index contrast
of SMF-28 fiber). It can be seen that modeled data fits the measurements reasonably well.
However, due to the complex nature of the index profile of the guides and the difficulty with
calculating bend losses with such tight bend radii, there are some minor deviations. Finally as
mentioned earlier the step-index profile offers the best bend loss performance, which the non-
annealed waveguide most closely approaches due to its near-Gaussian index profile core.
Fig. 5: Normalized throughput as a function of radius of curvature for a 30 mm long waveguide. Solid lines show
the result of BeamPROP simulations for the measured refractive index profiles. A step-index with a contrast of
5.3 ×10-3 was used for the single mode fiber with a core radius of 4.85 µm.
4. Real-world application: The integrated photonic pupil-remapper
To demonstrate the potential of these low-bend loss waveguides we applied them to the
fabrication of an integrated device known as a “pupil-remapper” [10,16]. A pupil-remapper is
a 3D photonic device that consists of a set of waveguides, which remap the light from a 2D
input plane to a linear array at the output of the chip . The purpose of the pupil remapper is to
reformat the light from the telescope pupil into a linear array so that all the light from the
pupil can be interferometrically recombined downstream. The interferometric data is then
used to reconstruct a diffraction-limited image of the stellar target despite of the presence of
atmospheric turbulence. In this way it will be used to image protoplantary disks around stars
in order to study large exoplanets during formation which will enhance our in our
understanding of the formation and evolution process.
This astrophotonic device is subject to numerous constraints, which complicate its design;
chief amongst these are maintaining a minimum distance between waveguides to avoid cross-
coupling and equalizing path-lengths for high contrast fringe formation. The guides are also
offset laterally from the injection position such that they do not overlap with the uncoupled
cone angle of light from the injection micro-lenses. We refer the interested reader to  and
 for full details about the design procedure and astronomical motivation. Recently,
Charles et al. fabricated an 8 waveguide prototype pupil-remapper based on non-annealed
cumulative heating waveguides , a diagram of which is depicted in Fig. 6(a). The guides
in the remapper were based on circular arcs, which have constant bend radii along their
lengths that ranged from 23-35 mm (See Fig. 6(c)). The prototype remapper was fabricated
using the same writing-laser as described above with 40 nJ pulse energies and 250 mm/minute
translation speeds without annealing. The device was 30 mm long and ~6.5 mm wide. The
throughputs for the 8 waveguides of the device reported by Charles et al. were between 5 and
47%. Although these values are good enough for preliminary astronomical experiments, the
dramatically unequal throughputs between pairs of waveguides used to form a given
interferometric fringe result in a degradation of fringe contrast. Such loss in signal fidelity has
a direct negative impact on the utility of such a device as a core component of a stellar
Fig. 6(a) Diagram of a 8 waveguide pupil-remapper (taken from ). The waveguides are spaced by 250 µm at
the output of the chip. (b) Bright field microscope image of the input facet of the annealed remapper fabricated in
this body of work. (c) Measured throughput and minimum bend radius of the individual waveguides within the
We fabricated the same device shown in Fig. 6, using the new fabrication method, namely by
inscribing 90 nJ waveguides and then subsequently applying the annealing process outlined
above. The sample was ground and polished prior to probing. A microscope image of the
input facet of the fabricated device is shown in Fig. 6(b). The 1550 nm laser probe was
injected into each guide with a SMF-28 fiber and collected at the output by another fiber
before being routed to a power meter. The throughput of a straight reference guide, embedded
in the block adjacent to the remapper, was also measured. All throughputs were normalized to Download full-text
two butt-coupled probe fibers without the pupil-remapping chip in between. The absolute
throughputs of the 8 waveguides of the remapper were between 70 and 73% (see Fig. 6c),
which constitutes a major improvement over the device reported by Charles et al. The
remaining losses are mainly attributed to bulk absorption in the 30 mm long Eagle2000 block
that contributed 20% (highlighted by a red box in Fig. 6) to the loss and two 6% losses (per
facet) due to the mode-mismatch between the fibers and the waveguide mode at each end of
the device. Indeed, when the 8 remapper throughputs were normalized with respect to the
straight guides (dashed red line), the throughputs ranged between 96 and 100%. This indicates
that there were negligible bend losses as is expected for 30 mm long waveguides with bend
radii between 23 and 35 mm from Fig. 5. As the bend losses have now been minimized, it
now becomes possible to redesign a remapper which has a smaller footprint (which will have
shorter tracks and hence lower absorption losses), by increasing the minimum bend radius
from the 23 mm used in the device discussed here towards 17 mm which sits at the edge of the
throughput curve in Fig. 5. There is a limit to this as well and we estimate that a gain of 5-8%
improvement in throughput can be made this way.
The vastly improved and equalized throughputs of the remapper will significantly increase
the fringe contrast of an interferometer based on this chip and allow for very precise
determination of the complex coherence of the incident radiation field which is key to high
fidelity astronomical imaging (with specific application to high contrast detection of faint
exoplanets against the glare of their host stars). However, measurements of the precision of
such an interferometer are deferred to future work. This illustrates a successful real-world
application of this technique.
In conclusion, we have identified different thermal stability regions in the direct-written
waveguides and have utilized a differential thermal annealing process to significantly decrease
the bend losses of waveguides written in the cumulative heating regime. The thermal
annealing process exploits the different thermal stability of different regions within the
induced refractive index profile. The heat treatment erases the outer ring, but leaves a very
high index contrast (8.4×10-3), Gaussian-like profile core behind. As a result, the waveguides
show dramatically superior bend loss characteristics as compared to non-annealed
waveguides. Indeed, we have demonstrated that this method has enabled the realization of an
efficient 8 waveguide 3D, pupil-remapping chip. This performance improvement will have far
reaching implications allowing rapid prototyping with MHz repetition rates (fabrication
speeds in the range of 500-2000 mm/minute), while retaining the ability to fabricate complex,
multi-element 3D photonic circuits.
This research was conducted by the Australian Research Council Centre of Excellence for
Ultrahigh bandwidth Devices for Optical Systems (project number CE110001018) and in part
performed at the Optofab node of the Australian National Fabrication Facility; a company
established under the National Collaborative Research Infrastructure Strategy to provide nano
and microfabrication facilities for Australia’s researchers.