Optimal fabrication of volume phase holographic grism with high efficiency and high dispersion, and its applications for astronomical observation

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.