<title>Astronomical applications of volume-phase holographic gratings</title>

Abstract

Volume-Phase Holographic (VPH) gratings provide unique capabilities over classical surface gratings which can be exploited in modern astronomical spectrographs. The peak diffraction efficiency of a VPH grating is tunable due to the nature of Bragg diffraction. This allows a single dispersing element to serve the function of several classical gratings. Additionally, VPH grating structures can be stacked to produce complex gratings capable of directing the diffracted light in ways that classical gratings can not. INTRODUCTIONA VPH grating diffracts light through the mechanism of Bragg diffraction inside a volume in which periodic modulation of the refractive index forms the grating structure. The intersection of these modulations with the surface of the element form a surface grating that disperses the light according to the classical grating equation. The energy envelope of the diffracted light is, however, controlled by the fringe modulation frequency and its orientation within the grating medium and provides peak diffraction efficiency at the wavelength that meets the Bragg condition. For a grating with fringes normal to its surface, the Bragg condition is given by mνλ = 2n sin(α) (1) where m is the Bragg order, ν is the fringe frequency, λ is the wavelength of light, n is the refractive index of the grating medium, and α is the diffraction angle within the grating medium.

Volume‐Phase Holographic Gratings and the Efficiency of Three Simple Volume‐Phase Holographic Gratings

“…KOSI has been fabricating VPH gratings for utilization in Raman spectroscopy for several years (Tedesco et al 1993 ;Owen et al 1995 ;Arns 1995). To evaluate the potential of VPH gratings for astronomical spectroscopy, Barden et al (1998) evaluated a 613 line mm~1 grating designed for optimal efficiency at 700 nm of 70%È80% with a bandwidth adequate to cover the spectral range of 500È900 nm. At a grating angle of 11¡È12¡, the grating delivers greater than 55% efficiency for unpolarized light across the 500È900 nm band with peak efficiency of nearly 80% between 600 and 700 nm.…”

“…obtained with VPH gratings. Barden et al (1998) presented the Ðrst known astronomical spectrum taken with the original 613 line mm~1 test VPH grating. The Ðrst science conducted with a VPH grating was performed at the Anglo-Australian Observatory, where they retroÐtted a 400 line mm~1 VPH grism into the LDSS]] instrument (Glazebrook et al 1998(Glazebrook et al , 2000.…”

“…This technology, on the other hand, has gone relatively unnoticed by the astronomical community until only recently. Barden, Arns, & Colburn (1998) Ðrst presented the performance and potential of volume-phase holographic (VPH) gratings for astronomical spectrographs through the analysis of a 613 line mm~1 VPH grating. The result of that work has spawned the development of several spectro-ÈÈÈÈÈÈÈÈÈÈÈÈÈÈÈ 1 Operated by the Association of Universities for Research in Astronomy (AURA), Inc., under cooperative agreement with the National Science Foundation.…”

“…As pointed out by Barden et al (1998), VPH gratings provide many possible gains in performance capability over the typical astronomical gratings that are currently in use. Amongst the beneÐts are the following :…”

VPH grating technology for the Thirty Meter Telescope instrumentation program

“…The VPHG and its application in astronomy is discussed by Barden [1], [2] . The reader is referred to these papers for background on the VPHG.…”