Our research uses photonic techniques and devices to improve the light collection and processing, ultimately enabling us to observe and understand the universe.
The astrophotonics group is comprised of Astralis-USyd (Director, Professor Julia Bryant) and Sydney Astrophotonics Instrumentation Laboratories (SAIL; Director, Professor Sergio Leon-Saval) to focus on developing new photonic technologies for both astronomical instrumentation and for uses in a broad range of industries. The astrophotonics group was initially established by Professor Joss Bland-Hawthorn. The group collaborates with other groups in Australia including The Institute of Photonic and Optical Science (IPOS) and the other two nodes of the Astralis consortium - Astralis-AAO and Astralis-AITC, as well as research groups around the world.
Featured image: The field plate at the focus of the Anglo-Australian Telescope (AAT) where the Hector instrument is mounted. (Image credit: Dr Adeline Wang)
Astronomy and/or astrophysics is dedicated to the study of the universe - using observational, theoretical and computational techniques to understand the physical properties, origin, history and fate of celestial objects and the universe itself.
Photonics is the use of materials to manipulate light, involving the emission, transmission, processing and detection of light. The telecommunication and information technology revolution of recent decades is a direct result of advances in optics that have allowed higher information bandwidths to be transmitted over longer distances. This in turn has ushered in the Information Age.
Astrophotonics is where these areas meet. Astrophotonics is the use of photonic techniques and devices to manipulate our collection and processing of light for the purpose of improving our ability to probe and hence understand the universe. It has many applications and new technologies are constantly being developed.
The rate of advance in astrophotonics in the last decade have been astonishing, with clear impact on astronomical instruments. The next 10 years will see the development of concepts that are currently in their infancy such as space photonics, integration of photonic spectrographs into large high resolution wide bandwidth replicated spectrographs, arrayed waveguides acting as dispersers but with a much smaller footprint than classical dispersive elements (diffraction gratings), further development of OH suppression into a commonplace major instrument, fibre scramblers to stabilise the illumination from a fibre for precise radial velocity work (e.g. exoplanet detection), frequency combs, optical circulators, ring resonator filters, forked gratings, sub-lambda gratings, spatial light modulators, and more.
The biggest advances over the next decade and beyond will be driven by the need for smaller instrument solutions for the next generation of extremely large telescopes. And while the astronomical applications are many and varied, many of the devices developed through astrophotonics have applications in the wider world, in applications ranging from communications to medicine.
A 169-fibre-core photonic hexabundle with a typical galaxy overlaid. (Image credit: Dr Adeline Wang)
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LinkHistorically, photonics' primary application has been in the telecommunications industry. Astrophotonics takes photonics such as optical fibres and planar waveguides and uses them for astronomical purposes. Many of the technologies that have been developed by astrophotonics researchers are a direct result of a problem faced by the astronomical and astrophysics community. As a result, the fields are intimately intertwined.
The key two drivers behind photonics technologies in Astronomy are the increasing cost associated with increasing telescope size; and implementing photonic functions to new instrument concepts. To detect fainter or more distant targets at ever-higher spatial and spectral resolution, telescopes are built to be larger, requiring larger, more expensive optics and components. Astrophotonics can break the cost cycle by miniaturising instruments, enabling multiplexing on a whole new scale, and at the same time enabling technological advances and photonic functions not previously possible in astronomical instrumentation.
The Anglo-Australian telescope is a 4m-class optical facility in Australia. (Image credit: Dr Adeline Wang)
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LinkWe are now in an era where multi-object wide-field surveys, which traditionally use single fibres to observe just one spectrum on each of many galaxies simultaneously, can now exploit new optical fibre imaging bundles as integral field units in place of the single fibres. Such photonic devices, called "hexabundles" were first demonstrated on the SAMI instrument on the Anglo-Australian Telescope (AAT), leading to the largest integral field galaxy survey at the time. 3000 galaxies were observed, leading to over 100 refereed published papers.
SAMI led to a duplication of the idea in the USA with an instrument called MANGA, but with lower fill-fraction fibre bundles. It successfully imaged 10,000 galaxies. Both SAMI and MANGA lacked the higher spectral resolution needed to measure stellar kinematics across galaxies of all masses. This is important in understanding how galaxies formed and grew their mass and angular momentum.
To address that science, new generation larger-size higher throughput hexabundles were incorporated with a higher resolution spectrograph and a novel new robotic positioner, to build the Hector instrument on the AAT. Hector is currently carrying out the Hector Galaxy Survey which aims to observe 15,000 galaxies to address fundamental questions about how galaxies grow their mass and angular momentum, how gas accretion and outflows from AGN or starburst-driven winds controls star formation, and how the environment around galaxies controls this growth and delivers the different morphologies seen in galaxies in the local Universe.
The hydroxyl radical is a molecule that exists in the Earths atmosphere. However, hydroxyl emission lines account for a vast majority (approximately 98%) of the near IR (NIR) background in the night sky over the wavelength range 0.9-2.0 mm: right across the J and H infrared bands where many optical and infrared telescopes operate. The hydroxyl is produced from a reaction between ozone and hydrogen at high altitudes (~87 kilometres). The emission results from the vibrational decay of the excited OH molecule.
Detection of emission lines (such as H-alpha for measurements of star formation rates) in high redshift galaxies is limited in the NIR by the OH emission - 1000 times brighter than in the optical bands. Solving the NIR sky background problem is a critical challenge for high redshift astronomy. The background cannot be subtracted as it is highly variable and the contaminating lines scatter in the spectrograph, adding contamination between the OH lines, and therefore need to be blocked before entering the spectrograph.
A novel astrophotonic solution came from combining fibre Bragg gratings and photonic lanterns into the GNOSIS instrument – an H-band feed for the NIR IRIS2 instrument on the AAT. GNOSIS has the advantage of photonically-filtering OH sky lines, but still maintaining the light-collecting advantage of multi-mode fibres. OH suppression technology has now been extended into the J-band. This will leave a background continuum in the H and J-bands that is close to that seen in the optical, which allows the detection of emission lines from faint high redshift galaxies, opening up a host of galaxy evolution studies not previously possible from the ground.
Interferometry is the technique of using an array of telescopes in conjunction to observe and determine the location of an object. Photonic interferometry uses on-chip manipulation of a beam as well as photonic lantern technologies to combine light from many telescopes. New technologies in this space are now being implemented on ESO’s Very Large Telescope (VLT) within the Asgard suite. The Heimdallr and Seidr instruments for the Asgard suite were built at Astralis-USyd.
Some astrophotonics technology is being incorporated in micro-satellites.
Astrophotonics research at the University of Sydney, in collaboration with other groups, has led to the development of a series of technologies and instruments.
Fibre Bragg gratings (FBGs) are optical devices used in optical fibres to filter certain wavelengths propagating along the fibre. This is achieved when the light is made to reflect at a refractive modulation index on the fibre’s core. As a result the grating can reflect light with high efficiency over a narrow wavelength band. This technology has many applications within the field of Astrophotonics, as well as other fields such as communications and optical fibre sensory equipment.
The Fibre Bragg Grating (FBG) team create high-precision in-fibre filters for a range of astrophotonic applications. Gratings developed in our lab at the University formed the backbone of the GNOSIS instrument, where they removed over 100 atmospheric emission lines from near-infrared observations of the night sky. We are currently developing gratings in multi-core fibres, which if successful will have applications anywhere spectroscopy is used.
Fibre Bragg gratings operate using the principal of Fresnel reflection. This is the phenomenon that occurs when light, upon moving in one medium moves to another with a different refractive index, both reflects and refracts. The reflected fraction of the light is called the Bragg wavelength, denoted as λB. The Bragg wavelength is related to the refractive index of the core (n) and the grating’s period (Λ) by λB = 2nΛ. Therefore, by modifying these two properties, the wavelength can be carefully selected. Peak reflectivity (RP) is directly related to the amplitude of the grating (κ) and the length of the grating (L) by RP = tan2hκL, and amplitude is in turn related to Δn along the axis of the fibre (z) by κ(z) = pΔn(z)/(2Λ(n + )) where <Δn> is the average change in the refractive index in the core.
Using a GeSiO2 fibre, the grating can be printed directly onto the core. This is achieved by exposing the fibre to a specific pattern of UV light. When below λ = 300nm, the Si-O bonds break down. Therefore the refractive index varies microscopically along the length of the exposed section.
Gratings are written into fibres using a 244nm laser (left photo) and a phase mask (right). The control system is capable of applying various user-defined apodisation and chirp functions. We are then able to characterise the wavelength response of each grating using an optical spectrum analyser and an infrared camera. The diagram below shows a sample of the notches written into a fibre containing 120 single-mode cores.
Hexabundles are an optical fibre imaging device with an hexagonal fibre arrangement. Simply packing fibres together gives a poor throughput because of Focal Ratio Degradation (FRD) and low filling factor (the fraction of the hexabundle face that contains active core area). We developed a process in our glass fibre processing facility in which we produce the highest fill fraction fibre imaging bundles, which have no losses from FRD beyond that of the raw fibre material.
Hexabundle fibres are (usually) hexagonal optical fibre bundles with independent cores. This means that the each component fibre individually collects light, which can then be fed into a spectrograph. The bundles are composed up a number of multimode fibres, ranging from 7 to 169 or more fibres per bundle. Each fibre gives a spectrum for the component of the galaxy it aligns to, and therefore a “3-D” image of a galaxy can be formed.
At the University of Sydney, we have developed hexabundles with custom configurations including different sizes with circular cores, and using square-core fibres. The miniature size of hexabundles allows them to be positioned across a focal plane at a telescope. Each fibre in a hexabundle is only 100 microns diameter – about the thickness of a human hair – and these are crafted using sophisticated techniques, into devices with typically 61 to 169 cores that have diameters of only 1-2 mm. They are then coupled into miniature optical systems. These devices are now being used for multi-object spectroscopy on wide-field telescopes and have been fundamental to enabling the SAMI Galaxy Survey and the Hector Galaxy Survey.
One approach to the problem of rising cost and complexity in telescopes as their size increases is to feed the fibres into individual spectrographs rather than one huge (and expensive) spectrograph.
As astronomers design and build increasingly large and complex telescopes, a new device with a low cost and small size is needed if extremely large telescopes are to be built. If current technology is simply up-scaled, the expense and complexity become limiting factors in the creation of ELTs. These devices would target multi-object spectroscopy
As a result, Astrophotonics looks at developing a solution: an optical device with a variety of functions that can be produced on a small (tens of millimetres in size) chip. Currently, several successful single mode IO systems have been shown to work with existing interferometers. The benefits of these systems are many and varied: easy installation, better interferometer resolution, increased flexibility, better instrument stability, and reduced overall cost and complexity.
Systems operating with multimode fibres are currently being developed by various groups around the world. Advances in 3D optical circuitry would overcome some of the drawbacks associated with planar optics in the current systems. Another consideration is the integration of OH suppressed fibres into the input fibre. Researchers envisage devices with resolutions R = 250, 500, 1000, 2000, possibly integrated into an IR array. While currently the research is largely focused on spectroscopy, multi-purpose devices are the next step in integrated optics technology.
The devices under development at the University of Sydney are built around an array waveguide grating (AWG). The other primary potential technology is the photonic echelle grating, but difficulties encountered in production have made AWGs the primary focus. Both PEGs and AWGs originate in the telecommunications industry, but astronomy requires different manufacturing specifications. The AWG acts as a multiplexor (mux) feeding light from an input fibre (single mode, usually) into an optical phased array waveguide (which acts like the grating on a conventional spectrograph). This connects to another AWG, the demultiplexor (demux) which produces the spectra, imaged through output fibres. While the devices may be expensive individually, when taken as whole, the cost decreases exponentially, as each spectrograph potentially performs a variety of functions and can be easily rewired into a number of configurations.
Orbital Angular Momentum (OAM) has recently been investigated as a fundamental property of photons and has already found a number of applications. In astrophysics, it has been shown that by detecting the OAM of photons scattered from rotating objects, the rate of rotation can be measured.
The photonic lantern has been developed as a solution to the problem of working with single and multimode fibres in conjunction. This is a technology with many potential applications, not only in astrophysics and astronomy.
In order to use many emerging and existing technologies (such as fibre Bragg gratings) effectively within an optical system, a method for going from single to multimode fibres and vice versa was needed. Multimode fibres (MMFs), along which multiple wavelengths can propagate, are used to collect incoming light, while single mode fibres (SMFs) are needed to isolate individual wavelengths to feed into a spectrometer. However, many optical devices (such as FBGs) used successfully in single-mode fibres exhibit very poor performance when applied to multimode fibres.
A new device has been developed that is able to split multimode fibres into a large number of single mode fibres. This has been dubbed the photonic lantern, and has many possible applications in communications and sensory equipment as well as astronomy and astrophysics.
These waveguides combine the signals from the component telescopes of an interferometry array, which are connected by an optical network.
Frequency combs allow astronomers to detect extrasolar planets by comparing the oscillation of the observed star’s line spectrum with a fixed predicted spectrum, mapped using a laser.
The images of most objects picked up by telescopes are badly distorted by the medium in between the source and the receiver. Adaptive optics attempts to correct this.
A close by reference point is required in order to use adaptive optics technology. If a bright celestial object is nearby, this makes observations much simpler, but normally an artificial “guide star” is required, and is generated by a powerful laser exciting molecules in the upper atmosphere.
Liquid crystal polymers are used in adaptive optics as the “adaptive” part, as they can be deformed depending on the nature of the correction being made by the system.
Research continues to go into the robotic fibre positioning systems used to move fibres around on the spectroscopy plates to observe the desired area of sky. Latest fibre positioning robotic instruments, such as built for Hector by Astralis-USyd, now position in 3-D, not just 2-D, on the field plate at the focus of the telescope. This means they can correct for telecentricity - an optical effect that changes the angle of light across the field plate - so that up to 20% more light can be imaged through the fibres.
