137 research outputs found
The Photonic Lantern
Photonic lanterns are made by adiabatically merging several single-mode cores
into one multimode core. They provide low-loss interfaces between single-mode
and multimode systems where the precise optical mapping between cores and
individual modes is unimportant.Comment: 45 pages; article unchanged, accepted for publication in Advances in
Optics and Photonic
Efficient injection from large telescopes into single-mode fibres: Enabling the era of ultra-precision astronomy
Photonic technologies offer numerous advantages for astronomical instruments
such as spectrographs and interferometers owing to their small footprints and
diverse range of functionalities. Operating at the diffraction-limit, it is
notoriously difficult to efficiently couple such devices directly with large
telescopes. We demonstrate that with careful control of both the non-ideal
pupil geometry of a telescope and residual wavefront errors, efficient coupling
with single-mode devices can indeed be realised. A fibre injection was built
within the Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) instrument.
Light was coupled into a single-mode fibre operating in the near-IR (J-H bands)
which was downstream of the extreme adaptive optics system and the pupil
apodising optics. A coupling efficiency of 86% of the theoretical maximum limit
was achieved at 1550 nm for a diffraction-limited beam in the laboratory, and
was linearly correlated with Strehl ratio. The coupling efficiency was constant
to within <30% in the range 1250-1600 nm. Preliminary on-sky data with a Strehl
ratio of 60% in the H-band produced a coupling efficiency into a single-mode
fibre of ~50%, consistent with expectations. The coupling was >40% for 84% of
the time and >50% for 41% of the time. The laboratory results allow us to
forecast that extreme adaptive optics levels of correction (Strehl ratio >90%
in H-band) would allow coupling of >67% (of the order of coupling to multimode
fibres currently). For Strehl ratios <20%, few-port photonic lanterns become a
superior choice but the signal-to-noise must be considered. These results
illustrate a clear path to efficient on-sky coupling into a single-mode fibre,
which could be used to realise modal-noise-free radial velocity machines,
very-long-baseline optical/near-IR interferometers and/or simply exploit
photonic technologies in future instrument design.Comment: 15 pages, 16 figures, 1 table, published in A&
Characterization of hexabundles: Initial results
New multi-core imaging fibre bundles -- hexabundles -- being developed at the
University of Sydney will provide simultaneous integral field spectroscopy for
hundreds of celestial sources across a wide angular field. These are a natural
progression from the use of single fibres in existing galaxy surveys.
Hexabundles will allow us to address fundamental questions in astronomy without
the biases introduced by a fixed entrance aperture. We have begun to consider
instrument concepts that exploit hundreds of hexabundles over the widest
possible field of view. To this end, we have compared the performance of a
61-core fully-fused hexabundle and 5 lightly-fused bundles with 7 cores each.
All fibres in the bundles have 100 micron cores. In the fully-fused bundle, the
cores are distorted from a circular shape in order to achieve a higher fill
fraction. The lightly-fused bundles have circular cores and five different
cladding thicknesses which affect the fill fraction. We compare the optical
performance of all 6 bundles and find that the advantage of smaller
interstitial holes (higher fill fraction) is outweighed by the increase in
modal coupling, cross-talk and the poor optical performance caused by the
deformation of the fibre cores. Uniformly high throughput and low cross-talk
are essential for imaging faint astronomical targets with sufficient resolution
to disentangle the dynamical structure. Devices already under development will
have between 100 and 200 lightly-fused cores, although larger formats are
feasible. The light-weight packaging of hexabundles is sufficiently flexible to
allow existing robotic positioners to make use of them.Comment: Accepted for publication in MNRAS. See also a complimentary paper on
the development of hexabundles - Bland-Hawthorn et al. 2011, Optics Express,
vol. 19, p. 2649
(http://www.opticsinfobase.org/abstract.cfm?URI=oe-19-3-2649
Nineteen-port photonic lantern with multimode delivery fiber
We demonstrate efficient multimode (MM) to single-mode (SM) conversion in a 19-port photonic lantern with a 50 μm core MM delivery fiber. The photonic lantern can be used within the field of astrophotonics for coupling MM starlight to an ensemble of SM fibers in order to perform fiber-Bragg-grating-based spectral filtering. An MM delivery fiber spliced to the photonic lantern offers the advantage that the delivery fiber guides the light from the focal plane of the telescope to the splitter. Therefore, it is no longer necessary to have the splitter mounted directly in the focal plane of the telescope. The coupling loss from a 50 μm core MM fiber to an ensemble of 19 SM fibers and back to a 50 μm core MM fiber is below 1.1 dB.3 page(s
Air-structured optical fiber drawn from a 3D-printed preform
© 2015 Optical Society of America. A structured optical fiber is drawn from a 3D-printed structured preform. Preforms containing a single ring of holes around the core are fabricated using filament made from a modified butadiene polymer. More broadly, 3D printers capable of processing soft glasses, silica, and other materials are likely to come on line in the not-so-distant future. 3D printing of optical preforms signals a new milestone in optical fiber manufacture
Integrated photonic building blocks for next-generation astronomical instrumentation I: the multimode waveguide
We report on the fabrication and characterization of composite multimode
waveguide structures that consist of a stack of single-mode waveguides
fabricated by ultrafast laser inscription. We explore 2 types of composite
structures; those that consist of overlapping single-mode waveguides which
offer the maximum effective index contrast and non overlapped structures which
support multiple modes via strong evanescent coupling. We demonstrate that both
types of waveguides have negligible propagation losses (to within experimental
uncertainty) for light injected with focal ratios >8, which corresponds to the
cutoff of the waveguides. We also show that right below cutoff, there is a
narrow region where the injected focal ratio is preserved (to within
experimental uncertainty) at the output. Finally, we outline the major
application of these highly efficient waveguides; in a device that is used to
reformat the light in the focal plane of a telescope to a slit, in order to
feed a diffraction-limited spectrograph.Comment: 15 pages, 11 figures, accepted to Optics Expres
Suppression of the near-infrared OH night sky lines with fibre Bragg gratings - first results
The background noise between 1 and 1.8 microns in ground-based instruments is
dominated by atmospheric emission from hydroxyl molecules. We have built and
commissioned a new instrument, GNOSIS, which suppresses 103 OH doublets between
1.47 - 1.7 microns by a factor of ~1000 with a resolving power of ~10,000. We
present the first results from the commissioning of GNOSIS using the IRIS2
spectrograph at the AAT. The combined throughput of the GNOSIS fore-optics,
grating unit and relay optics is ~36 per cent, but this could be improved to
~46 per cent with a more optimal design. We measure strong suppression of the
OH lines, confirming that OH suppression with fibre Bragg gratings will be a
powerful technology for low resolution spectroscopy. The integrated OH
suppressed background between 1.5 and 1.7 microns is reduced by a factor of 9
compared to a control spectrum using the same system without suppression. The
potential of low resolution OH suppressed spectroscopy is illustrated with
example observations.
The GNOSIS background is dominated by detector dark current below 1.67
microns and by thermal emission above 1.67 microns. After subtracting these we
detect an unidentified residual interline component of ~ 860 +/ 210
ph/s/m^2/micron/arcsec^2. This component is equally bright in the suppressed
and control spectra. We have investigated the possible source of the interline
component, but were unable to discriminate between a possible instrumental
artifact and intrinsic atmospheric emission. Resolving the source of this
emission is crucial for the design of fully optimised OH suppression
spectrographs. The next generation OH suppression spectrograph will be focussed
on resolving the source of the interline component, taking advantage of better
optimisation for a FBG feed. We quantify the necessary improvements for an
optimal OH suppressing fibre spectrograph design.Comment: Accepted for publication in MNRAS. 15 pages, 18 figure
GNOSIS: the first instrument to use fibre Bragg gratings for OH suppression
GNOSIS is a prototype astrophotonic instrument that utilizes OH suppression
fibres consisting of fibre Bragg gratings and photonic lanterns to suppress the
103 brightest atmospheric emission doublets between 1.47-1.7 microns. GNOSIS
was commissioned at the 3.9-meter Anglo-Australian Telescope with the IRIS2
spectrograph to demonstrate the potential of OH suppression fibres, but may be
potentially used with any telescope and spectrograph combination. Unlike
previous atmospheric suppression techniques GNOSIS suppresses the lines before
dispersion and in a manner that depends purely on wavelength. We present the
instrument design and report the results of laboratory and on-sky tests from
commissioning. While these tests demonstrated high throughput and excellent
suppression of the skylines by the OH suppression fibres, surprisingly GNOSIS
produced no significant reduction in the interline background and the
sensitivity of GNOSIS and IRIS2 is about the same as IRIS2. It is unclear
whether the lack of reduction in the interline background is due to physical
sources or systematic errors as the observations are detector noise-dominated.
OH suppression fibres could potentially impact ground-based astronomy at the
level of adaptive optics or greater. However, until a clear reduction in the
interline background and the corresponding increasing in sensitivity is
demonstrated optimized OH suppression fibres paired with a fibre-fed
spectrograph will at least provide a real benefits at low resolving powers.Comment: 15 pages, 13 figures, accepted to A
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