Fibre Bragg gratings for high spectral and temporal resolution astronomical observations

Mon. Not. R. Astron. Soc. 421, 3641–3648 (2012) doi:10.1111/j.1365-2966.2012.20592.x Fibre Bragg gratings for high spectral and temporal resolution astronomical observations Geraldine Mariën,1,2,3 Nemanja Jovanović,1,2,3,4 Nick Cvetojević,1,2,3 Robert Williams,1,3,5 Roger Haynes,6 Jon Lawrence,1,2,3,4 Quentin Parker2,3,4 and Michael J. Withford1,2,3,5 1 MQ Photonics Research Centre, Macquarie University, NSW 2109, Australia 2 MQ Research Centre in Astronomy, Astrophysics and Astrophotonics, Macquarie University, NSW 2109, Australia 3 Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia 4 Australian Astronomical Observatory, NSW 2122, Australia 5 Centre of Ultrahigh Bandwidth Devices for Optical Systems (CUDOS), Macquarie University node, NSW 2109, Australia 6 innoFSPEC – Leibniz-Institut für Astrophysik Potsdam, 14482 Potsdam, Germany ABSTRACT Dynamic spectral analysis of astronomical events has the potential to deliver the information needed to clarify or complete important theoretical descriptions of astronomical phenomena. There is currently a lack of detailed sub-minute observations due to limitations in instruments and detectors. Here, we present an investigation into the feasibility of using fibre Bragg gratings (FBGs) as single-line spectral filters specifically for temporal spectral astronomy, attaining both a high spectral and fast temporal resolution simultaneously. We present the device concept and discuss it in the context of two readily available FBG profiles. We demonstrate that this instrument concept could resolve spectral shifts down to 0.02 nm (3.9 km s−1 ) with sub-second temporal resolution on a 4-m class telescope, which is far superior to existing techniques that attain resolutions of 0.05 nm over several minutes. Key words: instrumentation: miscellaneous – instrumentation: photometers – instrumentation: spectrographs – stars: variables: general. 1 I N T RO D U C T I O N The detection and analysis of spectral line-shifts is commonplace in astronomy, indeed our current insights into the expanding nature of the universe are underpinned by this relatively simple methodology. However, the detection of spectral line-shifts has predominantly been used to study steady-state stellar objects and consequently the opportunities for exploring dynamic systems have been mostly overlooked. A contributing factor is that current astronomical instrumentation readily permits observations on time-scales down to micro-seconds in the radio, X- and gamma-ray part of the spectrum (Shearer et al. 2010), but these systems are limited to detection times on the scale of minutes in the optical and the infrared (IR). New detection techniques combining both high spectral and fast (subsecond) temporal resolution are needed to expand our knowledge of dynamic astronomical phenomena such as cataclysmic variables, gamma-ray bursts, accreting binary systems, chemically peculiar stars and T-Tauri stars, which are all well known, but not well understood. Internal stellar processes like in-falling accretion streams  E-mail:  C 2012 The Authors C 2012 RAS Monthly Notices of the Royal Astronomical Society  or the interactions among chemical anomalies, magnetic fields and pulsations are believed to happen on sub-minute and subsecond time-scales and induce shifts in spectral lines by 0.002 to 0.3 nm (corresponding to shifts of 1.5 to 830 km s−1 in terms of radial velocities) (Cash, Howell & Rosen 1998; Cowley, Castelli & Hubrig 2005; Kurosawa, Harries & Symington 2005; Joshi et al. 2009). External stellar processes like the collision of X-ray bursts from one star in a binary system on to its companion star can result in intensity fluctuations in the companion, known as flickering. These fluctuations can be as large as 50 per cent of the magnitude of the spectral line in the visible and IR on time-scales up to 200 ms (Casella et al. 2010). An example of a variable line of interest is that of the Paβ line at 1281.81 nm in T-Tauri stars. T-Tauri stars are a prototype class of very young variable stars, which are still undergoing gravitational contraction and accrete mass from the large protoplanetary disc surrounding them (Appenzeller & Mundt 1989). Several models exist for the accretion system within T-Tauri stars, but no definitive model has been established yet. The Paβ line in the near infra-red (NIR) is particularly interesting for investigation into this subject as it allows for observations of accretion in strongly embedded objects such as T-Tauri stars, because it does not suffer from contamination by the Accepted 2012 January 18. Received 2012 January 12; in original form 2011 December 1 3642 G. Mariën et al. Figure 1. Schematic representation of a FBG where a broad-band spectrum is used as input, resulting in a transmitted broad-band output with a notch and a reflected narrow line. Figure 2. A schematic representation of a FBG stopband (dashed pink) with its Bragg wavelength λB centred on a spectral line of interest at λc (solid black). The shaded grey area shows the section of the spectral line that will be reflected by the overlapping stopband. As the line shifts to λc2 , a change in reflected power will be observed as demonstrated in the bottom graph. have very sharp edges, smaller shifts than currently measured could potentially be observed in the spectral lines. Moreover, FBGs can easily be interfaced with fast single-pixel photon-counting detectors. This permits a FBG-based instrument to attain both a high spectral and temporal response simultaneously. These integrated optical-fibre devices enable the miniaturization and simplification of the instrument without losing robustness and significantly reducing the instrument cost. These are advantages common to many other recent developments in the field of astrophotonics such as the miniature integrated photonic spectrograph (Cvetojevic et al. 2012), photonic beam combiners (Labadie et al. 2011) and on-chip pupil re-mapping for optical stellar interferometry (Jovanovic et al. 2011). FBGs are not new to the domain of astronomy. Their wavelength filtering capability has already been used on-telescope for the suppression of the forest of strong, tightly packed OH night-sky emission lines in the near-infrared (Ellis & Bland-Hawthorn 2008) that can be severely detrimental to astronomical observations. The concept of using FBGs for research in short time-scale transient astronomy presented here is, to the best of our knowledge, new. This paper presents a feasibility study of such a FBG-based device. We provide a simple technology demonstration, showing that FBGs are capable of achieving a combination of high spectral and temporal resolution which exceeds current achievements. The paper is divided in two main sections: the first gives an overview of the device concept and the second part describes the experimental set-up with laboratory results and presents a system simulation of the dev (...truncated)