Fibre Bragg gratings have been demonstrated to be a powerful tool with which to filter atmospheric emission lines from astronomical spectra. Multicore fibre technology has the potential to simplify the fabrication of fibre Bragg gratings, since all cores can be inscribed simultaneously rather than individually which is both time consuming and expensive to do. Solving the multicore challenge has fundamental implications for many fields outside of astrophotonics. To realise a working multicore fibre Bragg grating (MCFBG), all cores must be written with identical gratings providing uniform depth, Bragg wavelength and bandpass. However, to date, all multicore fibre Bragg gratings display a variation in the Bragg wavelength of the central cores compared to the outer cores. This seems to be a property of the multicore fibre itself, and is not due to the Bragg grating writing process.<p> </p> We investigate the origin of these core-to-core variations using finite difference time domain and finite element simulations, combined with analysis of fabricated multicore fibre. We find that the ellipticity of the core, the size of the core, and the coupling between cores all affect the propagation constants. However, the dependence on ellipticity is very weak, and cores would have to be highly deformed in the manufacturing process for this to be a concern. A variation in radius of ~ 2:5% could account for the observed variation in propagation constants. However, the measured variation in the fabricated MCF is too small and does not display any radial trend. The coupling between cores is too small to change the propagation constants significantly, but even if it were significant any effect would be expected increase the Bragg wavelengths of the central cores, the opposite of what is observed.
The problem of atmospheric emission from OH molecules is a long standing problem for near-infrared astronomy. PRAXIS is a unique spectrograph which is fed by fibres that remove the OH background and is optimised specifically to benefit from OH-Suppression. The OH suppression is achieved with fibre Bragg gratings, which were tested successfully on the GNOSIS instrument. PRAXIS uses the same fibre Bragg gratings as GNOSIS in its first implementation, and will exploit new, cheaper and more efficient, multicore fibre Bragg gratings in the second implementation. The OH lines are suppressed by a factor of ∼ 1000, and the expected increase in the
signal-to-noise in the interline regions compared to GNOSIS is a factor of ∼ 9 with the GNOSIS gratings and a
factor of ∼ 17 with the new gratings.
PRAXIS will enable the full exploitation of OH suppression for the first time, which was not achieved by GNOSIS (a retrofit to an existing instrument that was not OH-Suppression optimised) due to high thermal emission, low spectrograph transmission and detector noise. PRAXIS has extremely low thermal emission, through the cooling of all significantly emitting parts, including the fore-optics, the fibre Bragg gratings, a long length of fibre, and the fibre slit, and an optical design that minimises leaks of thermal emission from outside the spectrograph. PRAXIS has low detector noise through the use of a Hawaii-2RG detector, and a high throughput through a efficient VPH based spectrograph. PRAXIS will determine the absolute level of the interline continuum and enable observations of individual objects via an IFU. In this paper we give a status update and report on acceptance tests.
Atmospheric emission from OH molecules is a long standing problem for near-infrared astronomy. PRAXIS is a unique spectrograph, currently in the build-phase, which is fed by a fibre array that removes the OH background. The OH suppression is achieved with fibre Bragg gratings, which were tested successfully on the GNOSIS instrument. PRAXIS will use the same fibre Bragg gratings as GNOSIS in the first implementation, and new, less expensive and more efficient, multicore fibre Bragg gratings in the second implementation. The OH lines are suppressed by a factor of ~1000, and the expected increase in the signal-to-noise in the interline regions compared to GNOSIS is a factor of ~ 9 with the GNOSIS gratings and a factor of ~ 17 with the new gratings. PRAXIS will enable the full exploitation of OH suppression for the first time, which was not achieved by GNOSIS due to high thermal emission, low spectrograph transmission, and detector noise. PRAXIS will have extremely low thermal emission, through the cooling of all significantly emitting parts, including the fore-optics, the fibre Bragg gratings, a long length of fibre, and a fibre slit, and an optical design that minimises leaks of thermal emission from outside the spectrograph. PRAXIS will achieve low detector noise through the use of a Hawaii-2RG detector, and a high throughput through an efficient VPH based spectrograph. The scientific aims of the instrument are to determine the absolute level of the interline continuum and to enable observations of individual objects via an IFU. PRAXIS will first be installed on the AAT, then later on an 8m class telescope.
For the past forty years, optical fibres have found widespread use in ground-based and space-based instruments. In most applications, these fibres are used in conjunction with conventional optics to transport light. But photonics offers a huge range of optical manipulations beyond light transport that were rarely exploited before 2001. The fundamental obstacle to the broader use of photonics is the difficulty of achieving photonic action in a multimode fibre. The first step towards a general solution was the invention of the photonic lantern<sup>1</sup> in 2004 and the delivery of high-efficiency devices (< 1 dB loss) five years on<sup>2</sup>. Multicore fibres (MCF), used in conjunction with lanterns, are now enabling an even bigger leap towards multimode photonics. Until recently, the single-moded cores in MCFs were not sufficiently uniform to achieve telecom (SMF-28) performance. Now that high-quality MCFs have been realized, we turn our attention to printing complex functions (e.g. Bragg gratings for OH suppression) into their <i>N</i> cores. Our first work in this direction used a Mach-Zehnder interferometer (near-field phase mask) but this approach was only adequate for <i>N</i>=7 MCFs as measured by the grating uniformity<sup>3</sup>. We have now built a Sagnac interferometer that gives a three-fold increase in the depth of field sufficient to print across <i>N</i> ≥ 127 cores. We achieved first light this year with our 500mW Sabre FRED laser. These are sophisticated and complex interferometers. We report on our progress to date and summarize our first-year goals which include multimode OH suppression fibres for the Anglo-Australian Telescope/PRAXIS instrument and the Discovery Channel Telescope/MOHSIS instrument under development at the University of Maryland.
Fiber Bragg gratings are used in astronomy for their ability to suppress narrow atmospheric emission lines of temporally varying brightness before the light is dispersed. These gratings can only operate in a single-mode fiber as the suppressed wavelength depends on mode velocity in the core. Recent experiments with fibers containing multiple single-moded cores have demonstrated the potential for inscribing identical gratings across all cores in a single pass. We have already improved the uniformity of gratings in 7-core fibers via modifications to the writing process; further progress can be achieved by tuning the gratings of the outer and inner cores relative to one another. Our eventual goal is to make the entire fiber suppress one wavelength to a depth of 30 dB or greater. By coating the fiber in a heat-conductive material with a high expansion coefficient, we can examine the effects of temperature and strain on the spectral response of each core. In this paper we present methods and results from experiments concerning the post-write tuning of gratings in multicore fibers.
Multi-core fiber Bragg gratings (MCFBGs) will be a valuable tool not only in communications but also various
astronomical, sensing and industry applications. In this paper we address some of the technical challenges of fabricating
effective multi-core gratings by simulating improvements to the writing method. These methods allow a system designed
for inscribing single-core fibers to cope with MCFBG fabrication with only minor, passive changes to the writing
process. Using a capillary tube that was polished on one side, the field entering the fiber was flattened which improved
the coverage and uniformity of all cores.
PRAXIS is a second generation instrument that follows on from GNOSIS, which was the first instrument using fibre
Bragg gratings for OH suppression to be deployed on a telescope. The Bragg gratings reflect the NIR OH lines while
being transparent to the light between the lines. This gives in principle a much higher signal-noise ratio at low resolution
spectroscopy but also at higher resolutions by removing the scattered wings of the OH lines. The specifications call for
high throughput and very low thermal and detector noise so that PRAXIS will remain sky noise limited even with the
low sky background levels remaining after OH suppression. The optical and mechanical designs are presented. The
optical train starts with fore-optics that image the telescope focal plane on an IFU which has 19 hexagonal microlenses
each feeding a multi-mode fibre. Seven of these fibres are attached to a fibre Bragg grating OH suppression system while
the others are reference/acquisition fibres. The light from each of the seven OH suppression fibres is then split by a
photonic lantern into many single mode fibres where the Bragg gratings are imprinted. Another lantern recombines the
light from the single mode fibres into a multi-mode fibre. A trade-off was made in the design of the IFU between field of
view and transmission to maximize the signal-noise ratio for observations of faint, compact objects under typical seeing.
GNOSIS used the pre-existing IRIS2 spectrograph while PRAXIS will use a new spectrograph specifically designed for
the fibre Bragg grating OH suppression and optimised for 1.47 μm to 1.7 μm (it can also be used in the 1.09 μm to 1.26
μm band by changing the grating and refocussing). This results in a significantly higher transmission due to high
efficiency coatings, a VPH grating at low incident angle and optimized for our small bandwidth, and low absorption
glasses. The detector noise will also be lower thanks to the use of a current generation HAWAII-2RG detector.
Throughout the PRAXIS design, from the fore-optics to the detector enclosure, special care was taken at every step along
the optical path to reduce thermal emission or stop it leaking into the system. The spectrograph design itself was
particularly challenging in this aspect because practical constraints required that the detector and the spectrograph
enclosures be physically separate with air at ambient temperature between them. At present, the instrument uses the
GNOSIS fibre Bragg grating OH suppression unit. We intend to soon use a new OH suppression unit based on multicore
fibre Bragg gratings which will allow an increased field of view per fibre. Theoretical calculations show that the gain in
interline sky background signal-noise ratio over GNOSIS may very well be as high as 9 with the GNOSIS OH
suppression unit and 17 with the multicore fibre OH suppression unit.
Ground based near-infrared observations have long been plagued by poor sensitivity when compared to visible
observations as a result of the bright narrow line emission from atmospheric OH molecules. The GNOSIS instrument
recently commissioned at the Australian Astronomical Observatory uses Photonic Lanterns in combination with
individually printed single mode fibre Bragg gratings to filter out the brightest OH-emission lines between 1.47 and
1.70μm. GNOSIS, reported in a separate paper in this conference, demonstrates excellent OH-suppression, providing
very “clean” filtering of the lines. It represents a major step forward in the goal to improve the sensitivity of ground
based near-infrared observation to that possible at visible wavelengths, however, the filter units are relatively bulky and
costly to produce.
The 2nd generation fibre OH-Suppression filters based on multicore fibres are currently under development. The
development aims to produce high quality, cost effective, compact and robust OH-Suppression units in a single optical
fibre with numerous isolated single mode cores that replicate the function and performance of the current generation of
“conventional” photonic lantern based devices. In this paper we present the early results from the multicore fibre
development and multicore fibre Bragg grating imprinting process.
We discuss the development of multi-core fiber Bragg gratings (FBGs) to be applied to astrophotonics, more specifically
to near-infrared spectroscopy for ground-based instruments. The multi-core FBGs require over 100 notches to reject the
OH lines in a broad wavelength range (160 nm). The number of cores of the fiber should correspond to the mode number
in the multi-mode fibers and should be large enough to be able to capture a sufficient amount of light from the telescope.
A phase-mask based technique is used to fabricate the multi-core FBGs.
In the paper, we report experimental results of the effect of UV-light exposure on the profile of thermal poling induced
second-order optical nonlinearity (SON) in twin-hole optical fibers. Before UV-exposure, uniform thermal poling at
320°C and 3.5 kV along a 5-7 cm long section of twin-hole fiber produced a uniform SON of 0.263 pm/V in the fiber
core. A focused light beam from a frequency-doubled Ar+ laser operating at 244 nm was used to locally erase the
induced nonlinearity in the fibers to create a periodic structure for quasi-phase matching applications. The UV-exposed
fibers were then observed under a second-harmonic microscope to characterize the distribution profile of any residual
nonlinearity after exposure. Effects of scanning speeds of the laser beam on the length of nonlinearity-erased section and
amplitude of the residual nonlinearity were investigated. It was found that the required UV-light fluence to fully erase
the induced nonlinearity is only ~0.5% of that typically required for fabricating fiber Bragg gratings.