We are developing fabrication methods of a volume binary (VB) grating, trapezoid grating and reflector facet transmission (RFT) grating. The VB grating can achieve a larger angular dispersion and higher diffraction efficiency than conventional surface-relief transmission gratings with step shaped grooves, it can be achieved a wider spectral bandwidth than a volume phase holographic (VPH) grating whose refractive index is sinusoidally modulated. The trapezoid grating can bring the spectral characteristics of s- and p-polarization closer to each other than a VB grating, so it can further improve the peak diffraction efficiency.
Conventional reflection echelle gratings of surface relief (SR) type have significantly different spectral characteristics for s- and p-polarizations. Another problem was the phenomenon of an anomaly, in which the efficiency of p-polarization of a specific diffraction order is greatly reduced due to the influence of surface plasmons. The reflector facet transmission (RFT) grating is a new echelle grating of transmission type in which the incident light from one side facet of the sawtoothshaped grating is reflected by the other side facet, and the diffracted light is exited from the flat back side of the grating. The RFT grating achieves a large angular dispersion, higher diffraction efficiency and wider bandwidth than SR echelle gratings. Furthermore, since the RFT grating can be placed near the collimator and camera (imaging) optical system, the astronomical instrument can be made smaller than the reflective echelle grating. We prototyped a RFT grating with hard resin by ultra-precision cutting using a diamond tool. As a result of measurement of the diffraction efficiency, it demonstrates that a RFT grating is highly efficient and has small scattering losses.
A grism (direct-vision diffraction grating) combines a diffraction grating and prism to make any order and wavelength go straight. By setting a grism into the collimating beam (necessary for an astronomical instrument to place pupil mask, filters, etc.) of an imaging instrument, it becomes possible to easily perform spectroscopic observations. The prism array is a device aimed at increasing the angular dispersion (resolving power) of a grism within the limited size of the collimating beam part of the instrument.
Currently we are developing a VB grating for an echelle grism of the Subaru Multi-Object InfraRed Camera and Spectograph (MOIRCS) and a VB grating for the Advanced Lunar Imaging Spectrometer (ALIS) of the Lunar Polar Exploration Mission (LUPEX). The shapes of gratings are optimized to achieve high diffraction efficiency and a wide spectral bandwidth by performing numerical calculations of the rigorous coupled wave analysis (RCWA). Based on the calculated results, we are developing variety of gratings using MEMS technologies. In addition, we have deployed new high-dispersion grisms of J and H band of MOIRCS with transmission gratings fabricated by LightSmyth. The transmission grating is kind of a VB grating which ridges are composed by three kinds of dielectric layers.
We introduce novel gratings for next generation instruments of the TMT (Thirty Meter Telescope), the 8.2 m Subaru telescope, other ground-based and space-borne telescopes. The reflector facet transmission (RFT) grating which is a surface relief grating with sawtooth shaped grating lattice of an acute vertex angle, is developed for the WFOS of the TMT. The hybrid grism (direct vision grating) for the MOIRCSof the 8.2m Subaru Telescope is developed as a prototype of the RFT grating. The volume binary grating is developed for a high-dispersion echelle grism of the nuMOIRCS as the first light instrument of the ULTIMATE Subaru. We also developing a silicon grism for the MIMIZUKU of the 6.5m telescope of the University of Tokyo Atacama Observatory in Chile and a quasi-Bragg (QB) immersion grating.
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