To control the spectral and angular bandwidths with high diffraction efficiency, it is important to investigate the characteristics of volume phase (VP) gratings. The relationships between the characteristics and the refractive index distributions (RIDs) inside the recording material of VP gratings are focused on. In the previous work, continuously changeable graded types were investigated. In this work, the functions expressed as continuously changeable trapezoid types of RIDs are proposed. The characteristics of trapezoid-type and graded-type of RIDs which change continuously from a triangular type to a rectangular type are simulated and compared using the rigorous coupled wave analysis. It becomes clear that the same bandwidths can exist for the different types of RIDs with the same refractive index modulation. They are the trapezoid types and the graded types of RIDs. Theoretically the characteristics of a certain angulated trapezoid type can be considered to be the same as that of a certain rounded graded type of RID. It also becomes clear that there are some gratings whose thickness and bandwidths are the same for various trapezoid types of RIDs with the different refractive index modulation, and they are controlled. The tendency is the same as various graded types of RIDs.
We had developed three types of large VPH grisms (110×106 mm2) for FOCAS of the 8.2 m Subaru Telescope with
high efficiency, high dispersion and small wavefront error in visible region. However, it has been highly difficult to
fabricate VPH gratings for longer wavelength due to thickness of the grating. In order to overcome this problem, by
optimizing exposure condition and introducing active phase control technique, we had successfully developed VPH
grating for optical communication wavelength (1550 nm) with diffraction efficiency over 90% (TE mode) and a high
refractive index modulation of 0.047. We extend these techniques to the device for astronomical observation, aiming at
the application of K band VPH gratings for MOIRCS of the Subaru Telescope. The resultant grating has attained high
diffraction efficiency of 91.5%, spectral bandwidth (FWHM) 320 nm, and small wavefront error 0.03 waves in r.m.s. at
2200 nm. This VPH grism is a promising dispersion device for astronomical observation in near-infrared region.
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