The infrared imaging spectrograph (IRIS) for TMT: volume phase holographic grating performance testing and discussion

Abstract

Maximizing the grating efficiency is a key goal for the first light instrument IR.IS (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 60001/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 IR.IS as a potential dispersing element in the spectrograph. Our team has procured near-infrared gratings from t\¥0 separate vendors. We have t\¥0 gratings with the specifications needed for IR.IS current design: l.5 l-l.82µm (H-band) to produce a spectral resolution of 4000 and 1.19-1.3 7µm (J-band) to produce a spectral resolution of 8000. ' The center wavelengths for each grating are l.629µm and l.27µm, and the groove densities are 1771/mm and 4401/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 t\¥0 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 \Vhen 5° dev iation from the Bragg angle.

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

“…The large differences in efficiency with polarization are not ubiquitous, however the RCWA analysis predicts a wide variability with wavelength. On the contrary, the VPH gratings exhibit a lower dependence of the efficiencies on polarization than the RD gratings 5 .…”

“…Within each polarization state the efficiency decreases by less than 10% when off the bragg angle by 3°. In contrast, the VPH gratings can exhibit a decrease in efficiency of greater than 20% when the incident angle is similarly changed 5 . This is very important to the design of IRIS as the gratings will be required to accept an incident angle difference of up to 4.4°8.…”

“…0.1%). The stability of the laser was measured by recording the power over 6 hours 5 . The laser, despite having a thermal electric cooler, required more than a half hour in order to stabilize for measurements.…”

The Infrared Imaging Spectrograph (IRIS) for TMT: instrument overview

“…Throughput and polarization testing was completed at the Dunlap Institute for Astronomy and Astrophysics during 2013 and early 2014. Preliminary results of this trade study are presented at this meeting (Chen et al 17 , Meyer et al 18 ).…”

The Infrared Imaging Spectrograph (IRIS) for TMT: instrument overview

“…Two companies were used for each type of grating and were tested at infrared wavelengths with our selected field angles. 10,11 So the beams from the lenslet and slicer collimators are directed to strike a common rotating turret of ruled diffraction gratings at the same incident angles and thus are directed into the common camera optics to the detector.…”