David J. Robertson

Refractive elements for the measurement of the orbital angular momentum of a single photon

We have developed a mode transformer comprising two custom refractive optical elements which convert orbital angular momentum states into transverse momentum states. This transformation allows for an efficient measurement of the orbital angular momentum content of an input light beam. We characterise the channel capacity of the system for 50 input modes, giving a maximum value of 3.46 bits per photon. Using an electron multiplying CCD (EMCCD) camera with a laser source attenuated such that on average there is less than one photon present within the system per measurement period, we demonstrate that the elements are efficient for the use in single photon experiments.

Influence of atmospheric turbulence on states of light carrying orbital angular momentum

We have experimentally studied the degradation of mode purity for light beams carrying orbital angular momentum (OAM) propagating through simulated atmospheric turbulence. The turbulence is modeled as a randomly varying phase aberration, which obeys statistics postulated by Kolmogorov turbulence theory. We introduce this simulated turbulence through the use of a phase-only spatial light modulator. Once the turbulence is introduced, the degradation in mode quality results in crosstalk between OAM modes. We study this crosstalk in OAM for 11 modes, showing that turbulence uniformly degrades the purity of all the modes within this range, irrespective of mode number.

Efficient measurement of an optical orbital-angular-momentum spectrum comprising more than 50 states

A light beam may be separated into its orbital-angular momentum (OAM) components by a geometric optical transformation that converts each OAM component into a plane wave with a transverse phase gradient. Subsequent focusing produces a spot the lateral position of which is proportional to the input OAM state (Lavery et al 2012 Opt. Express 20 3). In this paper, we improve this approach, extending the measurement bandwidth to >50 OAM states and showing a simultaneous measurement of the radial coordinate.

Fiber multi-object spectrograph (FMOS) for the Subaru Telescope

Design concept of the fiber multi-object spectrograph (FMOS) for Subaru Telescope together with innovative ideas of optical and structural components is presented. Main features are; i) wide field coverage of 30 arcmin in diameter, ii) 400 target multiplicity, iii) 0.9 to 1.8 micrometers near-IR wavelengths, and iv) OH-airglow suppression capability. The instrument is proposed to be built under the Japan-UK-Australia international collaboration scheme.

Integral field unit for the Gemini near-infrared spectrograph

The Gemini Near IR Spectrograph (GNIRS) currently under development at NOAO and scheduled for delivery in the summer of 2002, will include a powerful and innovative Integral Field Spectroscopy (IFS) capability. The design, integration and test of the GNIRS Integral Field Unit (IFU) are the responsibility of the University of Durhams Astronomical Instrumentation Group. The Critical Design Review is scheduled during the second quarter of the year 2000. Its design is based on the Advanced Image Slicer concept developed as a result of research conducted under the auspices of the Durham Instrumentation R and D Program. A slicer-based system has many advantages over fiber-based designs, especially for cryogenic instruments. The GNIRS IFU consists of two self-contained modules mounted inside the GNIRS slit slide mechanism. This slide mechanism is employed to select the required spectroscopy mode by sliding the respective module into the instruments optical path. The low resolution option provides a field of view of 3.2 inch X 4.4 inch with a sampling resolution of 0.15 inch over 625 spatial elements and a spectrum length of 1024 pixels, whereas the high resolution optic provides a field of view of 1.0 inch by 1.5 inch with a sampling resolution of 0.04 inch over 972 spatial elements and a spectrum length of 1024 pixels. This paper gives an overview of the IFUs optical design, which has been optimized to take full advantage of the excellent image quality provided by the Gemini telescopes, and the mechanical design.

GIRMOS: an infrared multi-object spectrograph for Gemini

Gemini have funded a design study to investigate the technologies needed in a versatile multi-object spectrograph for IR astronomy. We report on our investigations into wide- field spectroscopy using multiple integral-field units (MIFUs) to match particular areas of interest to the available detector(s). Such technologies enable integral field spectroscopy of several targets over a much wider field than can be covered with a single IFU. A brief overview of the scientific rationale for a multipel0IFU capability matched to multi-conjugate adaptive optics, and with its wider uncorrected field, on Gemini is given. A proposed method of deploying MIFUs is then described along with the optical consequences of the method.

Status of the KMOS multi-object near-infrared integral field spectrograph

KMOS is a multi-object near-infrared integral field spectrograph being built by a consortium of UK and German institutes. We report on the final integration and test phases of KMOS, and its performance verification, prior to commissioning on the ESO VLT later this year.

ESA NGST integral field and multiobject spectrograph slicer system

An Integral Field and Multiobject Spectrograph (IFMOS) for NGST has been studied for the European Space Agency by a European consortium. This paper describes the design of the integral field unit (IFU), the optical system which divides up the 2D field and reformats in into one or more slits. The IFU uses the Advanced Image Slicer concept, which has many advantages over other designs of IFU and is particularly well suited to space applications.

Integral field spectroscopy with the Gemini Near-Infrared Spectrograph

The Astronomical Instrumentation Group (AIG) of the University of Durham has recently completed an integral field unit (IFU) for use on the Gemini-South telescope with the Gemini Near-Infrared Spectrograph (GNIRS) built by the National Optical Astronomy Observatories (NOAO, USA). When the IFU is deployed remotely inside the instrument cryostat, GNIRS is converted into an integral field spectrograph with a field of 5 × 3 arcsec2 and spatial sampling of 0.15 × 0.15 arcsec2, optimised for 1-2.5μm but operable up to 5μm. We present summaries of the design and construction and results from laboratory testing. We also show results obtained at the telescope where a throughput of 90% was measured at 2.5μm, and show that this is consistent with predictions of a simple model where surface scattering is the dominant loss mechanism. The throughput data are well fit by the roughness measured in the laboratory. Finally, we show a few examples of astrophysical data from the commissioning run in April 2004.

Developments on the UK FMOS project for the Subaru Telescope

We describe the UK participation in the FMOS project to provide multi-object IR spectroscopy for the Subaru telescope. The UK is working on the design of an OH suppression IR spectrograph, this work comprises the optical design, the opto-mechanical layout, spectrograph thermal environment and cryogenics and detector control system. We give a progress report on the current design work.