The infrared imaging spectrograph (IRIS) for TMT: reflective ruled diffraction grating performance testing and discussion

Creators: Meyer, Elliot; Chen, Shaojie; Wright, Shelley A.; Moore, Anna M.; Larkin, James E.; Simard, Luc; Maire, Jérôme; Mieda, Etsuko; Gordon, Jacob

Others:: Ramsay, Suzanne K.; McLean, Ian S.; Takami, Hideki

Abstract

We present the efficiency of near-infrared reflective ruled diffraction gratings designed for the InfraRed Imaging Spectrograph (IRIS). IRIS is a first light, integral field spectrograph and imager for the Thirty Meter Telescope (TMT) and narrow field infrared adaptive optics system (NFIRAOS). IRIS will operate across the near-infrared encompassing the ZYJHK bands (~0.84 - 2.4μm) with multiple spectral resolutions. We present our experimental setup and analysis of the efficiency of selected reflective diffraction gratings. These measurements are used as a comparison sample against selected candidate Volume Phase Holographic (VPH) gratings (see Chen et al., this conference). We investigate the efficiencies of five ruled gratings designed for IRIS from two separate vendors. Three of the gratings accept a bandpass of 1.19-1.37μm (J band) with ideal spectral resolutions of R=4000 and R=8000, groove densities of 249 and 516 lines/mm, and blaze angles of 9.86° and 20.54° respectively. The other two gratings accept a bandpass of 1.51-1.82μm (H Band) with an ideal spectral resolution of R=4000, groove density of 141 lines/mm, and blaze angle of 9.86°. The fraction of flux in each diffraction mode was compared to both a pure reflection mirror as well as the sum of the flux measured in all observable modes. We measure the efficiencies off blaze angle for all gratings and the efficiencies between the polarization transverse magnetic (TM) and transverse electric (TE) states. The peak reflective efficiencies are 98.90 ± 3.36% (TM) and 84.99 ± 2.74% (TM) for the H-band R=4000 and J-band R=4000 respectively. The peak reflective efficiency for the J-band R=8000 grating is 78.78 ± 2.54% (TE). We find that these ruled gratings do not exhibit a wide dependency on incident angle within ±3°. Our best-manufactured gratings were found to exhibit a dependency on the polarization state of the incident beam with a ~10-20% deviation, consistent with the theoretical efficiency predictions. This work will significantly contribute to the selection of the final grating type and vendor for the IRIS optical system, and are also pertinent to current and future near-infrared astronomical spectrographs.

Additional Information

© 2014 SPIE. The authors would like to thank the Changchun Institute of Optics, Fine Mechanics and Physics and the Bach Research Institute for their support and cooperation throughout this study. We thank the support and resources offered by the Dunlap Institute of Astronomy & Astrophysics at University of Toronto. The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto. The authors gratefully acknowledge the support of the TMT partner institutions. They are the Association of Canadian Universities for Research in Astronomy, California Institute of Technology, Department of Science and Technology India, National Astronomical Observatories of the Chinese Academy of Science, the National Astronomical Observatory of Japan, and the University of California. The TMT project is planning to build the telescope facilities on Mauna Kea, Hawaii. The authors wish to recognize the significant cultural role and reverence that the summit of Mauna Kea has always had with the indigenous Hawaiian community.

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