The Infrared Imaging Spectrograph (IRIS) for TMT: Volume Phase Holographic Grating Performance Testing and Discussion

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

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

Abstract

Maximizing the grating efficiency is a key goal for the first light instrument IRIS (Infrared Imaging Spectrograph) currently being designed to sample the diffraction limit of the TMT (Thirty Meter Telescope). Volume Phase Holographic (VPH) gratings have been shown to offer extremely high efficiencies that approach 100% for high line frequencies (i.e., 600 to 6000l/mm), which has been applicable for astronomical optical spectrographs. However, VPH gratings have been less exploited in the near-infrared, particularly for gratings that have lower line frequencies. Given their potential to offer high throughputs and low scattered light, VPH gratings are being explored for IRIS as a potential dispersing element in the spectrograph. Our team has procured near-infrared gratings from two separate vendors. We have two gratings with the specifications needed for IRIS current design: 1.51-1.82μm (H-band) to produce a spectral resolution of 4000 and 1.19-1.37μm (J-band) to produce a spectral resolution of 8000. The center wavelengths for each grating are 1.629μm and 1.27μm, and the groove densities are 177l/mm and 440l/mm for H-band R=4000 and J-band R=8000, respectively. We directly measure the efficiencies in the lab and find that the peak efficiencies of these two types of gratings are quite good with a peak efficiency of ~88% at the Bragg angle in both TM and TE modes at H-band, and 90.23% in TM mode, 79.91% in TE mode at J-band for the best vendor. We determine the drop in efficiency off the Bragg angle, with a 20-23% decrease in efficiency at H-band when 2.5° deviation from the Bragg angle, and 25%-28% decrease at J-band when 5° deviation from the Bragg angle.

Additional Information

© 2014 SPIE. We want to thank Kaiser Optical System Inc. and Wasatch Photonics for working with us on procuring these gratings. Each company was incredibly helpful and responsive in the design and procurement of these challenging gratings. During the measurement, they gave us suggestions and feedback, which guaranteed the projects success. We thank the support and resources offered by the Dunlap Institute of Astronomy & Astrophysics at University of Toronto. Shaojie Chen is supported through a Dunlap Fellowship from the Dunlap Institute for Astronomy & Astrophysics, University of Toronto. The Dunlap Institute is funded through and 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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